L T t C t lli ti U it fLow Temperature Crystallization Unit for

Sulphate & Perchlorate RemovalFrom a Closed System Sodium Chlorate PlantFrom a Closed System Sodium Chlorate Plant

Kevin SundquistKevin Mamer

Allyse Kreiser

Low Temperature Crystallization Unit For Sulphate and Perchlorate Removal

From a Closed System Sodium Chlorate Plant

Report Prepared by: Allyse Kreiser Kevin Mamer

Kevin Sundquist

Department of Chemical Engineering University of Saskatchewan

2007-2008

i

EXECUTIVESUMMARY The objective of the project was to design a process to remove sulphate and perchlorate.

Experiments were performed to confirm that the low temperature crystallization unit

(LTCU) will effectively remove 70% of the sulphate from the slurry, although the LTCU

was only designed to remove 50%. Additional experiments showed that the lowest

concentration of sodium perchlorate that can be achieved through precipitation with

potassium chloride is 13 - 16 g/L, which is above ERCOs current perchlorate levels but

after the expansion the LTCU will effectively remove the perchlorate.

A small slip stream of the chlorate slurry will be fed to an agitated mix tank with

potassium chloride to facilitate perchlorate removal and hydrochloric acid to achieve a

pH of 4.5 to facilitate sulphate crystallization. This is then pumped to a crystallizer

where the slurry is cooled to a temperature of -5oC by calcium chloride brine flowing

through a cooling jacket. The crystals are then separated from the liquor by a centrifuge

and brine washed. The crystals are added to the existing chlorate dryer and the liquor

returns to the mother liquor tank at reduced sulphate levels of 10 g/L.

The expansion plans at ERCO make the LTCU a necessity, as the reduced sulphate levels

will improve the efficiency of ERCOs electrolysis process. As well, this process will

allow ERCO to become self-sufficient, and reduce its environmental footprint of the mud

disposal for the present purging mitigation. The production expansion increased revenue,

averted purge costs, power savings and the use of existing equipment make this project

economically successful, estimated to have a 42% discounted cash flow rate of return on

an initial investment of $1 949 000.

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ACKNOWLEDGEMENTS

We would like to thank the following individuals for their support throughout this

project:

Mr. Tim Barnstable, P.Eng ERCO Worldwide, Saskatoon - for all of the help

gathering resources and providing the experiment samples

Mr. Yuri Omelchenko ERCO Worldwide, Toronto - for additional support on

experiments and for testing the samples

Dr. Gordon Hill Chemical Engineering Professor - for all of his guidance

throughout the course and for the use of the glycol cooler

Mr. Chuck Edwards, P.Eng Engineer in Residence - for his guidance and

feedback obtained in the meetings

Dr. Richard Evitts Chemical Engineering Professor - for his help with the

understanding and implementing of control schemes

Mr. Dale Claude Chemical Engineering Laboratory Coordinator - for the use of

his lab and equipment

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TABLEOFCONTENTS

EXECUTIVE SUMMARY ................................................................................................. iACKNOWLEDGEMENTS ................................................................................................ iiLIST OF TABLES ............................................................................................................. viLIST OF FIGURES ......................................................................................................... viiiNOMENCLATURE .......................................................................................................... ix1. INTRODUCTION ...................................................................................................... 1

1.1. Background ......................................................................................................... 11.2. Expansion Plans .................................................................................................. 51.3. Current Mitigation .............................................................................................. 61.4. Project Deliverables ............................................................................................ 7

2. DESIGN ALTERNATIVES ....................................................................................... 92.1. Low Temperature Crystallization of Sulphate .................................................... 92.2. Low Temperature Crystallization of Sulphate and Perchlorate with the Addition of Potassium Chloride ................................................................................................... 102.3. Precipitation of Barium Sulphate by the Addition of Barium Chloride ........... 122.4. Sulphate Ion Exchange Resin ........................................................................... 13

3. EXPERIMENTS ....................................................................................................... 143.1. Bulk Density of Slurry Experiment .................................................................. 143.2. Preliminary Experiment .................................................................................... 163.3. Secondary Experiment ...................................................................................... 213.4. Additional Experimental Work Performed by ERCO ...................................... 26

4. DETAILED QUALITATIVE PROCESS DESCRIPTION ...................................... 284.1. Process Description ........................................................................................... 284.2. Control Scheme ................................................................................................. 324.3. Simulation ......................................................................................................... 334.4. Plot Space Plan ................................................................................................. 34

5. EQUIPMENT SIZING ............................................................................................. 375.1. Mix Tank ........................................................................................................... 375.2. Pump Material Selection ................................................................................... 375.3. Crystallizer Feed Pump ..................................................................................... 385.4. Crystallizer ........................................................................................................ 395.5. Centrifuge Feed Pump ...................................................................................... 485.6. Centrifuge ......................................................................................................... 495.7. Refrigeration Skid ............................................................................................. 515.8. Calcium Chloride Pump .................................................................................... 525.9. Brine Wash Heat Exchanger ............................................................................. 545.10. Pipe Sizing ........................................................................................................ 55

6. ECONOMIC ANALYSIS ........................................................................................ 586.1. Capital Costs ..................................................................................................... 586.1.1. Existing Equipment ....................................................................................... 586.1.2. Crystallizer Feed Pump ................................................................................. 596.1.3. Centrifuge Feed Pump .................................................................................. 59

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6.1.4. Refrigeration Skid ......................................................................................... 606.1.5. Calcium Chloride Pump ................................................................................ 606.1.6. Brine Wash Heat Exchanger ......................................................................... 616.1.7. Controllers and Valves .................................................................................. 616.1.8. Piping and Structural ..................................................................................... 626.1.9. Expansion Capital Costs ............................................................................... 636.1.10. Total Capital Costs and Implementation Time ............................................. 636.2. Operating Costs ................................................................................................. 646.2.1. Sulphate/Perchlorate Removal System ......................................................... 646.2.2. Expansion ...................................................................................................... 646.2.3. Total Operating Costs ................................................................................... 666.3. Revenues ........................................................................................................... 666.4. Cash Flow Analysis .......................................................................................... 676.4.1. Cash Flow with the Use of Existing Equipment ........................................... 676.4.2. Cash Flow without the Use of Existing Equipment ...................................... 706.4.2.1. Mix Tank ....................................................................................................... 706.4.2.2. Crystallizer .................................................................................................... 716.4.2.3. Pusher Centrifuge .......................................................................................... 716.4.3. Cash Flow with Sodium Chlorate Price Sensitivity ...................................... 72

7. PLANT SAFETY ...................................................................................................... 737.1. Material Safety Data Sheets .............................................................................. 737.2. Dow Fire & Explosion Index ............................................................................ 747.3. Hazard and Operability Study ........................................................................... 757.3.1. pH Conditioning of Slurry Node ................................................................... 777.3.2. Low Temperature Crystallization Node ........................................................ 817.3.3. Refrigeration Skid Node ............................................................................... 827.4. Alarms ............................................................................................................... 837.5. Interlocks ........................................................................................................... 847.6. Standard Operating Procedures ......................................................................... 85

8. CONCLUSIONS ....................................................................................................... 879. RECOMMENDATIONS .......................................................................................... 8910. REFERENCES ..................................................................................................... 90

Appendix A Project Description ................................................................................ 91Appendix B Literature Solubility Data ...................................................................... 93Appendix C Preliminary Experiment Sample Analysis Request ............................... 96Appendix D Preliminary Experiment Results for Processed Slurry Samples ........... 99Appendix E Secondary Experiment Procedure ........................................................ 101Appendix F Crystallizer Feed Pump Calculations ................................................... 104Appendix G CAPO Reactor Drawing ...................................................................... 106Appendix H Crystallizer .......................................................................................... 107Appendix I Centrifuge Feed Pump Calculations ..................................................... 115Appendix J CIMCO Refrigeration Skid Proposal ................................................... 117Appendix K Calcium Chloride Pump Calculations ................................................. 118Appendix L Goulds Pumps Calcium Chloride Pump Proposal ............................... 120Appendix M Brine Wash Heat Exchanger Sizing Calculations .............................. 121Appendix N Piping Calculations ............................................................................. 122

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Appendix O Process Flow Diagram ........................................................................ 126Appendix P Low Temperature Crystallization Unit P&ID ...................................... 127Appendix Q ERCO P&ID 07-07100 ....................................................................... 128Appendix R ERCO P&ID 07-07130 ........................................................................ 129Appendix S Patents .................................................................................................. 130Appendix T Material Safety Data Sheets ................................................................. 131Appendix U Dow Fire and Explosion Index ............................................................ 132Appendix V Hazard and Operability Analysis ........................................................ 133Appendix W Sodium Chlorate Decomposition Reactions ....................................... 134

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LISTOFTABLES

TABLE 3. 1: DATA COLLECTED FOR THE DETERMINATION OF THE RELATIONSHIP BETWEEN BULK DENSITY OF SLURRY AND % SLURRY .............................................................. 14

TABLE 3. 2: PRELIMINARY EXPERIMENT SAMPLE PREPARATIONS ..................................... 19TABLE 3. 3: PRELIMINARY EXPERIMENT RESULTS FOR DECANTED LIQUOR OF PROCESSED

SAMPLES .................................................................................................................... 20TABLE 3. 4: SECOND EXPERIMENT SAMPLE PREPARATIONS .............................................. 22TABLE 3. 5: SECOND EXPERIMENT RESULTS FOR DECANTED LIQUOR OF PROCESSED

SAMPLES .................................................................................................................... 23TABLE 6. 1: EXISTING EQUIPMENT INSTALLATION COSTS ................................................. 58TABLE 6. 2: CONTROLLERS AND VALVES COSTS ............................................................... 62TABLE 6. 3: PIPING AND STRUCTURAL COSTS .................................................................... 62TABLE 6. 4: EXPANSION COSTS .......................................................................................... 63TABLE 6. 5: ERCO PURGE COST ........................................................................................ 66TABLE 7. 1: SAFETY PROPERTIES ....................................................................................... 73 TABLE 8. 1: MAJOR EQUIPMENT SIZING & COST ............................................................... 88 TABLE B- 1: SOLUBILITY DATA FOR POSSIBLE PRODUCTS ................................................ 93 TABLE D- 1: PRELIMINARY EXPERIMENT SAMPLE PREPARATIONS .................................... 99TABLE D- 2: PRELIMINARY EXPERIMENT RESULTS FOR DECANTED LIQUOR OF PROCESSED

SAMPLES .................................................................................................................... 99TABLE D- 3: PRELIMINARY EXPERIMENT RESULTS FOR WET CRYSTALS OF PROCESSED

SAMPLES .................................................................................................................... 99TABLE D- 4: PRELIMINARY EXPERIMENT RESULTS FOR DRIED CRYSTALS OF PROCESSED

SAMPLES .................................................................................................................. 100TABLE D- 5: PRELIMINARY EXPERIMENT RESULTS FOR OTHER DRIED CRYSTALS OF

PROCESSED SAMPLES ............................................................................................... 100 TABLE E- 1: SECOND EXPERIMENT SAMPLES ................................................................... 103 TABLE F- 1: CRYSTALLIZER FEED PUMP SYSTEM ............................................................ 104 TABLE H- 1: NACLO3 SLURRY PROPERTIES ..................................................................... 108TABLE H- 2: TUBE SIDE CALCULATION RESULTS ............................................................ 108TABLE H- 3: CACL2 BRINE PROPERTIES ........................................................................... 109TABLE H- 4: TUBE ARRANGEMENT CALCULATIONS ........................................................ 110 TABLE N- 1: CALCULATION SUMMARY ........................................................................... 122TABLE N- 2: MAIN SLURRY FLOW CALCULATIONS ......................................................... 123

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TABLE N- 3: POTASSIUM CHLORIDE FLOW CALCULATIONS ............................................. 123TABLE N- 4: HYDROCHLORIC ACID FLOW CALCULATIONS ............................................. 124TABLE N- 5: SODIUM CHLORIDE BRINE WASH CALCULATIONS ...................................... 124TABLE N- 6: CALCIUM CHLORIDE BRINE CALCULATIONS ............................................... 125

viii

LISTOFFIGURES

FIGURE 1. 1: SIMPLE CHLORATE CELL ................................................................................. 2 FIGURE 4. 1: DIAGRAM OF A PUSHER CENTRIFUGE ............................................................ 31FIGURE 4. 2: PLOT SPACE FOR F-100 CHLORATE MIX TANK ............................................. 35FIGURE 4. 3: PLOT SPACE FOR D-120 CRYSTALLIZER ........................................................ 36 FIGURE 5. 1: TOP OF CAPO REACTOR ............................................................................... 40FIGURE 5. 2: BOTTOM OF CAPO REACTOR ........................................................................ 41FIGURE 5. 3: INTERIOR OF CAPO REACTOR ....................................................................... 44FIGURE 5. 4: DIMENSIONS OF HEAT EXCHANGER JACKET OF CAPO REACTOR

(CRYSTALLIZER) VESSEL, (A) TOP VIEW: TUBES RUN THROUGH ANNULUS; (B) SIDE VIEW: 5 BAFFLES EQUALLY SPACED ......................................................................... 46

FIGURE 5. 5: EXISTING PUSHER CENTRIFUGE ..................................................................... 50 FIGURE B- 1: SOLUBILITY DATA FOR POSSIBLE PRODUCTS ............................................... 94FIGURE B- 2: SODIUM PERCHLORATE AS A FUNCTION OF THE POTASSIUM CHLORIDE IN THE

SYSTEM ...................................................................................................................... 95 FIGURE I- 1: CENTRIFUGE FEED PUMP SYSTEM ............................................................... 115 FIGURE K- 1: CALCIUM CHLORIDE PUMP SYSTEM ........................................................... 118 FIGURE O- 1: PROCESS FLOW DIAGRAM .......................................................................... 126

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NOMENCLATURE

Symbol Definition Units

A Surface Area m2

AD Annual Depreciation $

ADCF Annual Discounted Cash Flow $

ADCFT Cumulative Annual Discounted Cash Flow $

AI Annual Investment $

AIT Annual Income tax $

ALM Log-mean Surface Area m2

ANCI Annual Net Cash Income $

ANCIT Cumulative Annual Net Cash Income $

AS Annual Sales $

ATE Annual Operating Expenses $

BEP Break Even Point Years

C1 Loss Control Credit Factor- Process Control

C2 Loss Control Credit Factor- Material Isolation

C3 Loss Control Credit Factor- Fire Protection

CBM Bare Module Cost $

Conc Concentration (Mass) kg/m3

Cp Purchase Cost $

Cp Heat Capacity kJ/kg-K

CPI Chemical Engineering Plant Cost Index

Da Impeller Diameter m

x

DCFRR Discounted Cash Flow Rate of Return %

Di , ID Inner Diameter of Tubes m

Do, OD Outer Diameter of Tubes m

f Fanning Friction Factor dimensionless

F1 General Process Hazards Factor

F2 Special Process Hazards Factor

F3 Unit Hazards Factor

FBM Bare Module Factor

Fm Material Factor

Fp Pressure Factor

fd Future Interest Rate for Year i

g Gravitational Constant m/s2

h Convective Heat Transfer Coefficient W/m2K

H Enthalpy kJ/mol, kJ/kg

Hproducts Enthalpy of Product kJ/mol

Hreactants Enthalpy of Reactant kJ/mol

Hrxn Enthalpy of Reaction kJ/mol h Height m

i Interest Rate %

k Thermal Conductivity W/mK

K K-value for Valves and Fittings

Ksp Solubility Product

LD50 Lethal Dose that Kills 50% of Population mg/kg Administered to

xi

LTCU Low Temperature Crystallization Unit

Ltubes Length of Tubes m

MW Molecular Weight g/mol

m& Mass Flowrate kg/hr

NPV Net Present Value $

n Year number Years

N Impeller Rotational Speed rad/s, RPM

NL Number of Tubes in Bank of Tubes

NP Impeller Power Number dimensionless

NQ Impeller Pitch Flow Number dimensionless

NuD Nusselt Number dimensionless

P Pressure Pa

P Power Consumption W

Patm Atmospheric Pressure Pa

Pi Pressure at Position i Pa

Pi Pressure Change over Portion i Pa PBP Payback Period Years

Pr Prandtl Number dimensionless

q Heat Transfer Rate kW

q& Volumetric Flowrate m3/s

Q Volumetric Flowrate m3/hr

ReD Reynolds Number dimensionless

Rf Fouling Factor m2K/W

xii

SD Tube Spacing Diagonal To Flow m

SL Tube Spacing Parallel To Flow m

ST Tube Spacing Normal To Flow m

T Temperature oC, K

TLM Log-mean Temperature K

TLV Threshold Limit Value mg/m3

u Velocity m/s

U Overall Heat Transfer Coefficient W/m2K

um Mean Velocity m/s

Vmax Maximum Fluid Velocity Between Tubes m/s

Ws Shaft Power W

Correlation Factor dimensionless

Greek Symbol Definition Units

I Intrinsic Efficiency D Driver Efficiency Viscosity Pa-s Delta, Change in

Density kg/m3

1

1. INTRODUCTION

1.1. Background

The client that submitted the project was ERCO Worldwide, a division of Superior Plus.

ERCO operates a closed system sodium chlorate plant north of Saskatoon, Saskatchewan,

Canada. The technology used by ERCOs sodium chlorate plant is an electrolysis cell

reactor. A generalized schematic of a simple chlorate cell is shown in Figure 1.1.

2

Figure 1. 1: Simple Chlorate Cell

3

The principle overall chemical reaction in a chlorate cell is given by:

236

2 33 HNaClOOHNaCle ++ + (1.1)

If all sodium chlorate is made by the above equation only, the cell efficiency is 100%.

However, this is an extremely simplistic expression of a series of desirable chemical

reactions that occur in a chlorate cell and ignores a number of undesirable reactions. The

desirable reactions are as follows:

+ eClCl 22 2 (1.2) ++ OHHeOH 222 22 (1.3)

+ +++ ClHHOClOHCl 22 (1.4) + + ClOHHOCl (1.5)

+ +++ HClClOClOHOCl 222 3 (1.6)

In addition to the desirable reactions above, there are a number of undesirable reactions

which result in a loss of current efficiency in a chlorate cell. Undesirable reactions

occurring at the anode include:

+ eOSSO 222 28224 (1.7)

2212

42282 22 OSOHOHOS +++ + (1.8)

2242

282 242 COSOHCOHOS ++++ + (1.9)

+ ++ eOHOH 442 22 (1.10) + +++ eCOHCOH 442 22 (1.11)

+ +++ ClHCOCHOCl 222 2 (1.12) + +++++ eOClHClOOHClO 646236 22132 (1.13)

4

+ +++ eHClOOHClO 22423 (1.14)

The discharge of sulphate ions, Equation (1.7), occurs in a chlorate cell whenever the

sulphate content in the feed brine reaches relatively high levels. If metal anodes are

employed, such as in ERCOs chlorate process, sulphate ion discharge results in the

evolution of oxygen as in Equation (1.8). The accumulation of sulphate ions adversely

affects electrolytic power consumption. Equations (1.9), (1.11), and (1.12) are of concern

only when the anodes are composed of graphite.

Equation (1.14) illustrates the discharge of the chlorate ion to form perchlorate. This

reaction will occur whenever there is a low level of chloride in a cell together with a high

concentration of chlorate. All of these undesirable reactions at the anode can be

minimized or eliminated by adequate brine treatment and appropriate cell design and

operating conditions.

The cathode also has its share of undesirable reactions resulting in a loss of current

efficiency; undesirable reactions occurring at the cathode include:

+++ OHCleOHClO 222 (1.15) +++ OHCleOHO 663 23 (1.16)

Fortunately Equations (1.15) and (1.16) are almost completely inhibited by the addition

of sodium dichromate to the cell solution. The sodium dichromate forms a hydrated

chromium oxide layer on the cathode. Dichromate also acts as an excellent buffer

maintaining the pH of the cell at an optimum value via the reaction:

+ ++ HCrOOHOCr 22 242272 (1.17)

5

Other benefits of sodium dichromate are that it greatly reduces corrosion on steel surfaces

and it also inhibits the electrochemical discharge of hypochlorite or other ions which

evolve oxygen.

The importance of controlling the undesirable reactions without increasing capital cost is

obvious, when one considers that the cost of power accounts for half the cost of

producing sodium chlorate. Of the balance almost one third is the cost of capital. Thus,

the control of Equations (1.7), (1.10), (1.11), (1.13), (1.14), (1.15) and (1.16) is of

paramount importance to the chlorate cell designer and operator. Of equal importance to

the cell designer and operator is the cell voltage, since power cost is a function of voltage

as well as current efficiency. For this reason, the removal of sulphate and perchlorate are

of importance for reducing the power costs.

1.2. Expansion Plans

ERCO is developing plans to increase chlorate production at their Saskatoon sodium

chlorate plant to take advantage of the high market price of chlorate. The problem with

this being the brine evaporator, which limits the amount of sulphate entering the process,

is operating at full capacity. This means that further brine requirements would bypass the

brine evaporator and greatly increase levels of sulphate in the chlorate cells. This is

obviously undesirable given that sulphate, at higher concentrations, will cause decreased

cell current efficiencies and drive up operating costs. For this reason, ERCO submitted a

project to the Department of Chemical Engineering at the University of Saskatchewan to

achieve the low temperature removal of sodium sulphate (Na2SO4) and sodium

6

perchlorate (NaClO4) from their closed system sodium chlorate plant. The project

description submitted by ERCO can be found in Appendix A.

1.3. Current Mitigation

Currently, ERCO maintains acceptable levels of sulphate (and perchlorate) by purging

chlorate liquor product monthly. The minimum purge rate is governed by the amount of

sulphate entering the process with the feed brine. The higher the sulphate level, the

greater the chlorate liquor purge rate. When sulphate levels increase to ~29 g/L in the

reactors, the purge (at ~ 0.5 m3/hr) is started and ran until the sulphate levels drop to ~26

g/L. Overall ERCO ends up purging about 60 - 80 m3 of mother liquor per month. This

purge rate seems to maintain perchlorate levels at ~10-12 g/L. The current purge stream

is not operated continuously. The purge is drawn from the level control line that goes

from the Chlorate II 76-J-101 Crystallizer to the 74-T-104 Mother Liquor Tank. The

level control line is found on P&ID 07-07100 in Appendix Q. It is a 1" line that should

be indicated just before the level control valve LV-103.

The least expensive and technically preferred method to control the plants sulphate level

is by having a sufficient chlorate liquor product rate. This, of course, requires customers

willing to take the necessary quantities of chlorate liquor product. Fortunately, ERCO is

able to transport this chlorate liquor product to a sister plant where facilities exist to

process the sulphate by addition of calcium chloride to make a calcium sulphate mud.

This method of sulphate concentration maintenance is costing ERCO approximately $191

000 per year, and may not be feasible when production increases are implemented due to

7

the high levels of sulphate ingress which will result. Therefore, ERCO considers this a

potential threat to the long term operation of the sodium chlorate plant.

1.4. Project Deliverables

ERCO had indicated that patents show that low temperature crystallization of sodium

sulphate is feasible at temperatures below 0oC for concentrations experienced at the

Saskatoon location. (These patents can be found in Appendix S). ERCO was unsure of

the precipitation of sodium perchlorate, but internal evaluations showed that dual

precipitation could be economical. The project was to develop a conceptual process

design of a sulphate and perchlorate removal system. The project involved:

Literature search to confirm current data on sulphate and perchlorate solubility; Bench work and process simulations to confirm and optimize the design; Development of a conceptual process design based on the knowledge gained from

the above work;

Hazard and operability review of the design; Estimation of installed cost of the major equipment; Estimation of the operating cost and potential savings from the proposed design; Project justification.

The project deliverables included:

1. Conceptual process design that included a preliminary P&ID and a mass and

energy balance for the project;

2. Equipment sizing and estimated cost of major equipment;

8

3. Hazard Analysis of preliminary project design;

4. Project justification.

9

2. DESIGNALTERNATIVES

2.1. Low Temperature Crystallization of Sulphate

United States Patents exist for the low temperature removal of sulphate from a sodium

chlorate plant and can be found in Appendix S. At subzero temperatures, sodium

sulphate will crystallize from the slurry more readily than during normal operation. The

solubility of sulphate at various temperatures is located in Appendix B.

By drawing a small slip stream from the existing process, the chlorate slurry could be

cooled and placed in a crystallizer to remove the sulphate. If the entire stream was

cooled, the refrigeration requirements would be extremely high and unreasonable.

Removing the sulphate from the slip stream would adequately maintain the sulphate

levels in the process liquor and remove ERCOs present necessity to purge one railcar of

process liquor per month to keep sulphate levels in check. In fact, if the low temperature

crystallization of sulphate was chosen, this would allow sulphate levels to be steady

rather than purging when the levels reach their intolerable level. For this reason, this

system would allow for improved efficiency in the electrolysis process as the sulphate

ions inhibit the electrolysis process.

The crystallized sodium sulphate (Na2SO4) would be stable, thus it could be combined

with the sales product with minimal impact on quality as there is already sulphate crystals

10

present in the product. Thus, this alternative would remove the sister plants necessity of

using calcium chloride (CaCl2) to produce calcium sulphate (CaSO4) which then proceeds

to mud disposal. Thus this alternative would reduce ERCOs environmental footprint by

removing 4.1 - 5.6 metric tons of mud which requires disposal.

An additional benefit of this design is that it does not require handling of hazardous

materials handling other than the dichromate which is already present in the process

liquor. However, this alternative would require a number of key pieces of equipment

resulting in a high capital cost. Fortunately, ERCO has equipment from an old part of the

plant that was shut down that may be retrofitted for use in this process to reduce the

capital cost. (If ERCOs used equipment is suitable in the design, testing would be

required to ensure the equipment still meets safety standards under the new operation).

As well, this unit would be high operating costs from the refrigeration costs.

Since this design alternative was discussed in patents, experiments were performed in

order to ensure that the patent concepts would be applicable to ERCOs sodium chlorate

slurry. The experiments are discussed in Section 3.

2.2. Low Temperature Crystallization of Sulphate and Perchlorate

with the Addition of Potassium Chloride

The objective of this project was to design a system to remove sodium sulphate; the

removal of sodium perchlorate was a secondary objective. Sodium perchlorate and

sodium sulphate are undesirable components that build up in ERCOs sodium chlorate

11

process over time. Many patents have been developed for the removal of sulphate from a

sodium chlorate plant using low temperature crystallization, but do not contain discuss on

the removal of perchlorate. While the preliminary experiment focused on the primary

objective of the removal of sulphate, the second experiment also focused on the removal

of perchlorate. If the experiments indicate that the removal of perchlorate can be done in

conjunction with the removal of sulphate, ERCO will have the opportunity to develop its

own patent and avoid patent fees.

The addition of potassium chloride may be beneficial, as it would increase the removal of

perchlorate from the system. The literary search completed showed that the solubility of

potassium perchlorate is lower than that of sodium perchlorate; this solubility data is

located in Appendix S. However, the solubility data could only be found for

temperatures greater than 0oC, thus experimental data had to be collected to ensure that

the addition of potassium chloride to the slurry would indeed result in enhanced

perchlorate removal. As well, the solubility data indicated that there was a possibility

that potassium chlorate crystals could form, which would result in a loss of product

(sodium chlorate) rather than a removal of perchlorate.

The pros and cons of this alternative are very similar to that of the discussed in Section

2.1. Since the potassium chloride would be an additional cost, the experiments had to

show successful removal of perchlorate. If experiments did indicate that potassium

chloride addition increased the removal of perchlorate, this alternative would be chosen

over the Low Temperature Crystallization of Sulphate alternative. Similar to the

12

crystallized sulphate, the perchlorate crystal could be added to the chlorate product

without affecting the product quality.

2.3. Precipitation of Barium Sulphate by the Addition of Barium

Chloride

If barium chloride (BaCl2) was added to the slurry, it would result in the precipitation of

barium sulphate (BaSO4). This alternative would also remove the necessity to purge one

railcar of process liquor per month to keep the sulphate levels in check. However, unlike

the low temperature crystallization alternatives, this alternative has the advantage of

removing nearly 100% of the sulphate resulting in a higher efficiency in the electrolysis

process. Unfortunately, it would not remove any of the perchlorate. While the barium

chloride addition would be an expensive operating cost, this alternative would not require

a refrigeration unit and would therefore require a lower equipment cost.

The removal of the sulphate would not be 100% because the barium cannot be added in

excess as barium is a very toxic substance. If barium was added in excess, the barium

which would remain in the chlorate liquor would contaminate ERCOs entire process and

the consequences of barium in the electrolysis process are unknown. As well, it should

be noted that at ERCOs sister plant which handles the purged chlorate liquor containing

sulphate, barium is not used. The sister plant uses calcium chloride (CaCl2) to produce

calcium sulphate (CaSO4). The calcium sulphate then proceeds to mud disposal. Since

barium is a toxic substance, the disposal of the barium sulphate would be more delicate

13

than the present mud disposal of calcium sulphate. Therefore, this alternative was not

considered further.

2.4. Sulphate Ion Exchange Resin

Another alternative that was considered for the removal of sulphate was a sulphate ion

exchange resin.

Ion-exchange resins are light and porous solids, usually prepared in the form of granules, beads, or sheets; they contain any wide variety of organic compounds synthetically polymerized and containing positively or negatively charged sites that can attract an ion of opposite charge from a surrounding solution. In chemical analysis, ion-exchange resins are used for the separation or concentration of ionic substances (2008 Encyclopedia Britannica, Inc.).

However, a literature search did not turn up any information about this technology being

used for sulphate removal in a saturated solution. Because the ion exchange effectiveness

is unknown due to the other compounds in the system, this alternative would require

expensive bench scale and pilot plant testing. As well, a sulphate ion exchange resin

would be expensive and would require a sulphate-free stream to reactivate the resin.

Similarly to the Precipitation of Barium Sulphate by the Addition of Barium Chloride

alternative discussed in Section 2.3, while this alternative would not require refrigeration

and there would be no perchlorate removal. Due to the high cost and the unknown

effectiveness of exchange resins, this alternative was not considered further.

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3. EXPERIMENTS

3.1. Bulk Density of Slurry Experiment

Sample mass and volumes were analyzed at ERCO to obtain a density trend based on

slurry volume percent. This analysis was conducted at 22.4oC in the lab. The data

collected can be found in Table 3.1. The data was used to determine the relationship

between the bulk density of the slurry and the slurry percent. Excel was used to

determine a line of best fit as shown in Figure 3.1.

( ) ( ) 3.1358%99.935%66.290 23 ++= SlurrySlurrymkgDensityBulkSlurry Table 3. 1: Data Collected for the Determination of the Relationship between Bulk Density of Slurry and % Slurry

Sample Description Tare

Mass of Cylinder

(g)

Total Mass of Sample & Cylinder

(g)

Sample Mass (g)

Total Sample Volume

(mL)

Slurry Volume

(mL) Slurry

%

Sample Bulk Density of

Slurry (kg/m3)

A Slurry 196.49 638.93 442.44 250.0 130 52% 1770 B Slurry 196.49 619.09 422.60 247.0 110 45% 1711 C Slurry 196.49 607.05 410.56 248.0 88 35% 1655 D Slurry 196.49 596.00 399.51 247.0 75 30% 1617 E Decant 152.93 289.40 136.47 100.5 0 0% 1358

15

Figure 3. 1 Relationship Between the Bulk Density Slurry and Percent Slurry

16

3.2. Preliminary Experiment

The first lab experiment was performed on Wednesday, October 17 at 1:30pm in the

Chemical Engineering undergraduate lab located in the Engineering building room 1D25.

The purpose of the preliminary experiment was to determine if low temperature

crystallization would effectively reduce the sulphate concentration in ERCOs chlorate

slurry.

Tim Barnstable, the design contact at ERCO, provided 12 - 1L samples of the chlorate

slurry from the 76-J-101 Primary Crystallizer for the Chlorate II system shown in P&ID

07-07100 in Appendix Q. He also provided the brine wash solution, hydrochloric acid

solution and potassium chloride salt.

Prior to the experiment, MSDS for hydrochloric acid, potassium chloride, sodium

dichromate anhydrous, potassium perchlorate, and sodium chlorate crystal were collected

and reviewed. These MSDS can be found in Appendix T.

The initial plan was to analyze the liquor samples for sulphate prior to being crystallized

and after a residence time of fifteen minutes in the cooler. Thus, to begin the experiment,

a blank sample supplied by ERCO (Sample 0) with no pH adjustment or cooling

completed was analyzed for sulphate using a Sulphate Test Kit Model SF-1. The

Sulphate Test Kit was provided by the Physical Chemistry Lab Coordinator, Bryan

Wilson, at the University of Saskatchewan. However, the kit did not give accurate results

17

for the feed sample which was expected to have sulphate levels of approximately 30 g/L.

Instead, the ERCO lab in Saskatoon completed sulphate analysis for the samples.

The first step of the procedure was to add 11% hydrochloric acid dropwise while stirring

continuously. The pH was analyzed using a pH meter. Initially, the pH of the sample

was at approximately 8 - 8.7, and was dropped down to approximately 4.5. If the pH

dropped below 4.0, there was a risk of releasing chlorine gas or chlorine dioxide gas due

to the composition of sodium chlorate. Thus, it was very important that each sample was

stirred continuously, and that each drop of hydrochloric acid was allowed to distribute

throughout the sample. The formation of chlorine dioxide gas is shown in equation (3.1)

and the formation of chlorine gas is shown in equation (3.2).

OHClClONaClHClNaClO 222123 2 ++++ (3.1)

OHNaClClHClNaClO 223 336 +++ (3.2)

In case the pH did drop below 4.0, despite the dropwise addition of hydrochloric acid,

this step was completed under the fume hood.

The pH adjusted sample was then placed in the glycol cooler which contained anti-freeze;

the anti-freeze temperature was set to a setpoint of 0oC, -5oC or -10oC depending on the

sample. A stirrer was placed in the beaker containing the slurry sample to maintain

fluidization of the sample. The slurry temperature was checked periodically; once the

sample reached the desired temperature, it remained in the glycol cooler for an additional

15 minutes. A residence time of 15 minutes was chosen based on time constraints, as

five samples were to be prepared that afternoon.

18

After the sample was removed from the glycol cooler, the volume increase in crystals was

noticeable to the naked eye indicating good crystallization. The crystals that formed had

distinct edges and good shape. As well, an additional thin crystal layer could be seen on

top of the initial chlorate crystals; the crystals in this layer were white in color and much

finer than the chlorate crystals. It was believed that this layer was the newly crystallized

sulphate crystals. A sample of these crystals was collected from the thin layer; once these

were placed in the crucible, it was noted that they were in a very high surface tension

fluid.

The sample was given time to settle and then the liquor was decanted. A portion of the

decant was collected in a plastic sample bottle and labeled by its sample number followed

by the letter A. A portion of the crystals (after mixing well) was placed in a funnel

containing a filter paper. An ejector was placed on the tap and used to vacuum filter the

sample to remove the remaining liquor from the crystals. These wet crystals were then

placed in a sample bottle and labeled with the letter B. Additional crystals were vacuum

filtered and placed in a small aluminum crucible; this crucible was then placed in the

oven at 100oC to dry overnight. The next day, the dry crystals were collected in sample

bags and labeled with the letter C.

For a couple of the samples, the vacuum filtered crystals were brine washed. However,

the brine wash was at room temperature and resulted in a substantial amount of

dissolution of the crystals. The brine wash did effectively remove the dichromate from

19

the crystals as after the wash the crystals were white. The brine washed crystals were

labeled with the letter D.

In addition to the sample labels, each sample was labeled as toxic and carcinogenic as a

precaution. All the samples were yellow from the dichromate in the solution and

dichromate is toxic and carcinogenic. Throughout the experiment, the proper personal

protection equipment was worn including safety glasses and gloves. All of the apparatus

and counter top area were cleaned and rinsed; all the waste (including the water used for

rinsing and all the soiled napkins and gloves) was collected in a hazardous waste

container and returned to ERCO for proper handling and disposal.

All five samples were prepared uniquely. A summary is shown in Table 3.2. Sample 0

was not cooled or pH adjusted, but used to compare the results of the samples which were

cooled and pH adjusted. Sample 1 was chilled to a temperature of 0oC but pH was not

adjusted. This was done to see if the pH adjustment was required.

Table 3. 2: Preliminary Experiment Sample Preparations

Sample ID Temperature pH Residence Time Desired KCl

Concentration Mass KCl

Added (oC) (min) (g/L) (g)

0A 35 8.7 1A 0 8.7 15 2A 0 4.6 15 3A -5 4.62 15 15 10 4A -5 4.65 15 5A -10 4.61 15

Sample 1 indicated that if the crystallization is done at a pH of 8.7, then sodium

dichromate also precipitates as expected. This is undesired as dichromate is carcinogenic

20

and toxic, and thus a stable precipitate cannot be washed away. The dichromate must

remain in solution so that it can be easily removed using a brine wash.

Potassium chloride was added to Sample 3 prior to it being chilled. When the sample

was removed from the chiller it was noted that it had poor settling qualities which made it

difficult to decant. This is a result of the potassium perchlorate forming fine solids that

settle very slowly.

All the samples were sent to the ERCO laboratory in Saskatoon. Perchlorate analysis was

not completed for this preliminary experiment as the laboratory in Saskatoon did not have

the necessary resources to analyze the samples for perchlorate. The request for the

analysis for the crystals can be found in Appendix C. The results for the decanted liquor

are shown in Table 3.3; the results of the wet and dry crystals can be found in Appendix

D.

Table 3. 3: Preliminary Experiment Results for Decanted Liquor of Processed Samples

Sample ID NaClO3 NaCl Na2SO4 Na2Cr2O7-2H2O

Na2SO4 Removal

Na2SO4 Removal

(g/L) (g/L) (g/L) (g/L) (%) (%) 0A 566.9 127.6 31.4 4.1 1A 405.1 138.4 27.1 4.5 13.7% 13.7% 2A 405.9 138.6 23.9 4.3 23.9% 23.9% 3A 378.8 146.6 20.9 4.2 33.4% 33.4% 4A 387.4 139.6 25.0 4.3 20.4% 20.4% 5A 371.9 139.9 33.9 4.4 -8.0% -8.0%

From these results it can be seen that sulphate does precipitate in the solution as a

significantly hydrated crystal product. As well, when sample 1 is compared to the other

samples it is obvious that the pH adjustment to approximately 4.5 does help to facilitate

21

the crystallization of sulphate. It should also be noted that as the samples are chilled, the

solubility of chlorate decreases from 567g/L at 35oC to approximately 382g/L at -5oC.

The maximum sulphate removal achieved was 33% and this sample had 15 g/L of

potassium chloride added. While it is unexplained, it will be reassessed during the

second experiment. In conclusion, the preliminary experiment confirmed that low

temperature crystallization is feasible.

3.3. Secondary Experiment

While the preliminary experiment focused on the primary objective of the removal of

sulphate, the second experiment also focused on the removal of perchlorate. Many

patents have been developed for the removal of sulphate from a sodium chlorate plant

using low temperature crystallization. If the removal of perchlorate in conjunction with

the sulphate is successful, ERCO will have the opportunity to develop its own patent and

avoid paying patent fees. By avoiding the existing sulphate removal patents,

implementation of the LTCU will be eased.

The second experiment was performed on Friday, January 4 at 9:30am in the Chemical

Engineering undergraduate lab located in the Engineering building room 1D25. The main

objectives of this experiment were as follows:

Confirm the composition of the liquor in the original slurry (Sample 0) of the preliminary experiment at 35oC;

Determine the liquor composition of the slurry chilled to -5oC and -10oC in order to compare the effective crystallization of sulphate and perchlorate;

22

Determine the effect of the addition of perchlorate would have on the liquor composition at -5oC;

Determine if the addition of potassium would increase the removal of perchlorate from the liquor at -5oC and -10oC.

Table 3.4 lists the samples that were prepared.

Table 3. 4: Second Experiment Sample Preparations

Sample ID Description Temp pH

Residence Time

Desired KCl Concentration

Mass KCl

AddedDesired ClO4- Concentration

Mass NaClO4-XH2O

Added (oC) (min) (g/L liquor) (g) (g/L liquor) (g)

0A Blank 35 9.8 1A -5 4.47 20 2A -5 4.47 30 3A -10 4.53 20 4A -5 4.65 20 5 2.9 5A -5 4.66 20 10 5.8 6A -5 4.25 20 15 8.6 7A -10 4.25 20 10 5.8

8/9A Blank 35 9.7 8A Spiked Blank 35 9.7 20 64 29 9A -5 4.54 20 64 29

The liquor samples were diluted 1:10 and the crystal samples were dissolved and

transported to Toronto. The analysis for the second experiment was completed in the

ERCO laboratory in Toronto as they had the analytical instrumentation needed for the

analyses of all the liquor components including sulphate, perchlorate and potassium.

The results for the decanted liquor are shown in Table 3.5.

23

Table 3. 5: Second Experiment Results for Decanted Liquor of Processed Samples

Sample ID NaClO3 NaCl Na2SO4

Na2Cr2O7-2H2O NaClO4 K

Na2SO4 Removal

NaClO4 Removal

(g/L) (g/L) (g/L) (g/L) (g/L) (g/L) (%) (%) 0A 548 125 26.7 3.3 10.71 1.6 1A 400 137 25.9 3.5 12.03 1.7 3.0% -12.3%2A 385 138 24.8 3.6 11.96 1.7 7.1% -11.6%3A 383 137 28.4 3.6 11.87 1.7 -6.4% -10.8%4A 387 140 22.3 3.6 11.89 3.4 16.5% -11.0%5A 384 143 18.3 3.6 12.07 3.3 31.5% -12.6%6A 386 141 22.3 3.6 12.03 3.1 16.5% -12.3%7A 361 142 19.7 3.7 11.98 2.8 26.2% -11.8%

8/9A 540 129 27.5 3.4 11.07 1.7 8A 497 128 27.5 3.4 46.90 1.7 9A 391 135 27.8 3.6 58.84 1.7 1.1% -25.5%

Sample 3A in this experiment as well as Sample 5A in the preliminary experiment had a

negative sulphate removal; both these samples were completed at -10oC suggesting that

dropping the temperature below -5oC will not increase the removal and may even inhibit

it.

At high doses of potassium (Sample 4 7) there is a strong possibility of precipitating

KClO3. Thus, the potassium concentration in the liquor is limited by solubility of KClO3.

This possibility of precipitating KClO3 is unfavorable because it would result in a loss of

sodium chlorate product. When the potassium chloride is added, a portion will

precipitate KClO4 and the remaining portion may precipitate KClO3.

From the results it appears that the highest KCl concentration in the liquor that can be

attained is around 2.8 - 3.4 g/L. The potassium chloride concentrations added were 5, 10

24

and 15 g/L; the potassium concentration results do not show an increasing trend. Thus, it

can be assumed that whatever additional KCl that is added and cannot remain in solution

will precipitate out as KClO3. The results indicate that 2.8 - 3.4 g/L is the maximum

achievable concentration of potassium in the liquor. For this reason, in order to

precipitate the potassium perchlorate, the concentration in the liquor must be greater than

16 g/L. This number was determined using the NaClO4 in the liquor as function of

steady state KCl concentration figure (Figure B-2 in Appendix B). Therefore, because

the current level of perchlorate in ERCOs liquor is so low, the removal of perchlorate is

not possible even at the maximum dosage of potassium.

However, the perchlorate levels are expected to increase as the expansion is brought

online, and therefore, a potassium chloride addition system was still put in place. For this

reason, sodium perchlorate was added to samples 8 and 9. An increase in perchlorate

concentration seemed to affect only the concentration of chlorate, the other components

did not tend to salt out. While these results do not look promising for perchlorate

removal, there appears to be errors as the level of perchlorate increased in the liquor after

chilling. In order to remove the required amount of perchlorate enough, potassium must

be added to first increase the background potassium concentration to the level that

corresponds to the expected level concentration of perchlorate in the treated stream and

secondly, form the necessary quantity of KClO4. Considering the samples that did not

have potassium chloride added had a concentration of 1.7 g/L potassium, only a small

amount of potassium chloride would have to be added to bring the background

concentration to 3.4 g/L.

25

After the decant was collected from Sample 1, the sample was returned to the cooler for

an additional 10 minutes resulting in a total residence time of 30 minutes. When the

decant results of Sample 1 and 2 are compared, it can be seen that the increased residence

time increased the amount of sulphate removal. The Low Temperature Crystallization

Unit (LTCU) will have a residence time of three hours in the crystallizer allowing for

longer precipitation times, so the sulphate removal is expected to increase. It is possible

that the 20 minute residence time in the glycol cooler was not sufficient time for reaching

complete equilibrium.

In addition to this, the results show that the amount of NaClO3 is decreasing, indicating

precipitation. As the chlorate drops out of the liquor, the solution volume decreases as it

contains fewer components in solution. As the volume decreases, the concentration of

the components not precipitating will increase. Therefore, if the sulphate concentration

in the liquor remains unchanged, this indicates that the sulphate has indeed precipitated as

well.

The maximum sulphate removal achieved in both experiments was 33% and these

samples which had the highest removal were those which had 10 - 15 g/L of potassium

chloride added. While it is unexplained, it seems to be the trend. A possible explanation

is the precipitation of K2SO4; however, the solubility data indicates that KClO4 and

KClO3 would precipitate first. For this reason, the potassium chloride will be added at 10

g/L despite the results indicating that only 2.8 3.4 g/L is required.

26

3.4. Additional Experimental Work Performed by ERCO

Yuri Omelchenko, an ERCO chemist, performed additional experimental work at the

ERCO laboratory in Toronto. The experiments he completed were done on a liquor

containing 379g/L NaClO3, 126 g/L NaCl, 62.8 g/L NaClO4 and 3.33 g/L Na2Cr2O7-

2H2O. This formulation is close to the liquor composition that was formed at subzero

temperatures during the preliminary and secondary experiments. Yuri Omelchenko drew

the following conclusions when he performed a perchlorate removal study on the liquor:

The experimentally solubility product, Ksp, value for KClO3 was found to be 0.0103 at -6oC which is somewhat higher than the calculated Ksp value of 0.0083.

The maximum achievable concentration of potassium in the liquor at 6oC was confirmed to be in the order of 3 g/L (5.7 g/L KCl).

The lowest concentration of NaClO4 that can be achieved through precipitation with KCl is 13 - 16 g/L. With the initial concentration of approximately 63 g/L

NaClO4, this shows that a 75% removal of perchlorate.

The precipitation performed using 22% KCl solution occurred quickly (within 10 minutes). If crystal KCl is added, the equilibration takes much longer (more than

an hour).

Experiments were also done at the laboratory in Toronto to confirm the removal of

sulphate. It was found that the concentration of Na2SO4 could be brought from 30 g/L

down to 9 g/L resulting in a 70% removal of sulphate at -6oC. It was noted that the

27

sulphate crystallization occurred after a notable delay; this explains why the results of the

preliminary and secondary experiment discussed in Section 3.2 and 3.3 only showed a

33% removal as the residence times were 15 and 20 minutes respectively.

28

4. DETAILEDQUALITATIVEPROCESSDESCRIPTION

4.1. Process Description

While this description states that the sulphate concentration entering the LTCU process

will be 20 g/L, initially the slurry will enter at approximately 30 g/L. Once this LTCU is

brought online, the target operating sulphate level will be 20 g/L. However, during the

first few months of operation, the LTCU will be required to lower the steady state

concentration of sulphate down to 20 g/L from the current level of 30 g/L.

A slipstream of the process slurry at the plant will be drawn at 2.5 m3/hr (4300 kg/hr).

The stream is drawn between the Chlorate II 76-J-101 Crystallizer and the 76-CF-101

Centrifuge as found on P&ID 07-07100 in Appendix Q. The Chlorate II plant was

chosen because it is the primary plant at ERCOs facility. The Chlorate I plant is the

secondary system and only runs part-time. Therefore, drawing from the Chlorate II

system would more effectively keep sulphate levels in check. The slurry entering the

system will have a sulphate level of 20 g/L and a perchlorate level of 36 g/L. The slurry

enters the F-100 Chlorate Mix Tank where it is combined with potassium chloride (KCl)

and hydrochloric acid (HCl) while an agitator keeps the slurry well-mixed. The 22 wt%

potassium chloride is pumped in at 0.096 m3/hr by a metering pump from a tote in order

to reach a mass concentration of 10 g/(L of liquor) of KCl in the mix tank. The KCl is

added to precipitate potassium perchlorate as it is less soluble than sodium perchlorate.

29

(Further solubility discussion can be found in Appendix B). The hydrochloric acid is

added dropwise into the mix tank until the pH reaches 4.5 from its original pH of 8.5.

The pH must be dropped to facilitate the precipitation of sodium sulphate and to prevent

the precipitation of sodium dichromate. The addition of HCl needed to be monitored and

controlled to prevent the pH from dropping to 4. If the pH drops to 4, chlorine and

chlorine dioxide gas may form; this must be prevented as both these gases are very

dangerous. In the event that gas forms in the mix tank there is a gooseneck vent to draw

air in from the surroundings, sweep the gases at the top of the tank and send them to the

off-gas fume scrubber. The line to the fume scrubber has a pressure indicator on it to

determine if there venting is drawing gas as designed. There is a recycle line on the mix

tank to ensure the slurry is well-mixed and to prevent blocking the K-110 Crystallizer

Feed Pump.

The slurry is then pumped by the K-110 Crystallizer Feed Pump to the D-120

Crystallizer. The slurry enters the top of the crystallizer and is forced down the draft tube

by the agitator. The slurry then comes back up through the tubes of the crystallizer where

it comes in contact with the cold calcium chloride brine (21 wt%). The calcium chloride

brine flows through the shell of the crystallizer and passes by five baffles to help with

heat transfer. The calcium chloride is cooled to -7oC by the CIMCO (50 ton)

Refrigeration unit and is pumped to the crystallizer by the K-123 Calcium Chloride Pump

at 130 m3/hr. The refrigerant used in the refrigeration skid is R-507. The calcium

chloride brine cools the process slurry to -5oC in the crystallizer. There is only a 2oC

temperature difference between the process slurry and the calcium chloride because of

30

scaling effects in the crystallizer. The temperature differential was provided by ERCO as

they have past experience with the crystallizer; however, it is recommended that when the

LTCU is brought online the temperature differential be increased gradually while scaling

does not occur. The process slurry then exits the bottom of the crystallizer where the pH

is analyzed again to verify previous readings in the process. The slurry passes by a

deionized (DI) water flush point to free any line blockages that may occur prior to or in

the K-130 Centrifuge Feed Pump.

The process slurry is then pumped to the H-140 Pusher Centrifuge by the K-130

Centrifuge Feed Pump. The slurry enters the center of the horizontal centrifuge where it

is spun and centrifugal forces force the liquid through the molecular sieves. The crystals

build up on the sieves and are pushed to the outlet of the centrifuge as shown in Figure

4.1.

31

Figure 4. 1: Diagram of a Pusher Centrifuge Sulphate free brine is added to the centrifuge at 0.15 m3/hr to wash the dichromate out of

the crystals and this is monitored by a flow indicator. Dichromate is added in another

portion of ERCOs plant to protect the plates in the electrolysis process and is

carcinogenic and toxic and must be removed from the crystal product. The brine is

cooled by the calcium chloride brine in the E-124 Brine Wash Heat Exchanger to prevent

the dissolution of crystals and the temperature of the brine is monitored and used to

control the flowrate of the calcium chloride brine.

The crystals are then transported to the existing Chlorate II 76DR101 Dryer, as found on

P&ID 07-07130 in Appendix R by the I-150 Dryer Feed Screw Conveyor. The mass

flowrate of crystals entering the dryer will be 2300 kg/hr. The solution from the

32

centrifuge will be gravity fed at 2000 kg/hr into the existing Chlorate Mother Liquor

Tank 74-T-104 with sulphate levels of 10 g/L and perchlorate levels of 16 g/L. This will

result in a removal of 50% of sulphate.

4.2. Control Scheme

All of the valves in the process are fail-closed valves in order to stop the flow in the event

that air pressure is lost. The flowrate of the HCl will be controlled with a control valve

and the pH of the recycle line on the mix tank is analyzed and controls a valve on the

inlet of the HCl line. There is also a sample point on the recycle line for operators to

manually check the pH. The level analyzer will monitor the level in the mix tank and

control a valve on the chlorate slurry feed into the mix tank to ensure there is no overflow

of the mix tank.

There is also a level controller on the crystallizer to control the inlet flowrate to the

crystallizer and to ensure that it is not overfilled. There is a flow indicator on the calcium

chloride line so the flowrate can be monitored. A cascade controller will be used for the

temperature controls between the refrigeration circuit and the crystallizer. With this

control, the inlet temperature of the CaCl2 coming from the refrigeration skid does not

have to be constant.

The cascade control system consists of a primary and secondary control loop. The

secondary control loop in this case is transmitting the temperature of the CaCl2 brine at

the outlet of the cooling jacket. The primary control loop is transmitting the temperature

33

of the chlorate slurry leaving the crystallizer. This control system is recommended

because the CaCl2 brine outlet temperature would indicate a disturbance more quickly

than the outlet slurry temperature. For example, an increase in the CaCl2 brine outlet

temperature would cause the VFD pump to speed up, whereas, it may take longer for the

slurry outlet temperature to show an increase in temperature and indicate that the slurry is

above -5oC. However, if the chlorate slurry outlet temperature transmitter did show a

disturbance from the desired -5oC temperature, then it would also cause the VFD pump

speed to change as it is the master controller and serves as the set point. The chlorate

outlet temperature will control the VFD pump. Considering the control system further, it

was decided to include a temperature transmitter on the CaCl2 brine line at the inlet of the

cooling jacket to ensure that there is only a 2 to 3oC difference between the CaCl2 brine

and the chlorate slurry outlet temperature.

There is a flow indicator on the inlet to the centrifuge to monitor the flowrate, as well as a

sample point to monitor product quality and calcium content to ensure the calcium

chloride brine is not leaking into the crystallizer.

4.3. Simulation

At the beginning of the project, a suitable simulation tool was sought after to aid with

calculations. HYSYS and ASPEN were determined to be not suitable due to the fact that

there are no crystallizer vessels in either software and the slurry is a saturated feed and

that would be difficult to model. After consultation with the Department of Chemistry

and the Department of Geology at the University of Saskatchewan, it was determined that

34

no suitable simulation software would be readily available to use in the design of this

project.

Microsoft Excel was used to aid in the calculation of mass and energy balances of the

process.

4.4. Plot Space Plan

The F-100 Chlorate Mix Tank will be placed on the third floor of ERCOs facility to

allow gravity to assist with the flow as can be seen in Figure 4.2. The D-120 will be

placed on the ground floor of ERCOs facility as there is room available as shown in

Figure 4.3. Because the plant is enclosed, it will be quite difficult to move these vessels

into position. This resulted in high installation costs for these pieces of equipment. The

refrigeration skid will be placed on the roof of one of the lower levels of the plant to ease

of installation and as a result of space limitations.

35

Figure 4. 2: Plot Space for F-100 Chlorate Mix Tank

36

Figure 4. 3: Plot Space for D-120 Crystallizer

37

5. EQUIPMENTSIZING

5.1. Mix Tank

The mix tank is an existing piece of equipment that ERCO has from previously shut

down plants. It is made with titanium to prevent erosion/corrosion from the chlorate

slurry and has an agitator. It has a height of approximately 4.3 m and a diameter of 1.1 m

with a total volume of 4 m3. This more than meets Ulrichs requirement for a feed tank

with a residence time of 1800 s. With the chlorate flowrate of 2.5 m3/hr, a feed tank with

a volume of 1.25 m3 would suffice but since ERCO already has this tank it will be

utilized. The tank will be equipped with a gooseneck vent to sweep any gas that may

form and send it to the off-gas fume scrubber. It will also have a recycle line to ensure

the slurry is well-mixed and to provide some flexibility in the flowrate to the crystallizer.

5.2. Pump Material Selection

Centrifugal pumps are used throughout the process because of the low slurry flows and

low pressures in the system. Fortunately, they are relatively inexpensive. The process

slurry pumps are stainless steel with wetted lined parts to prevent erosion/corrosion issues

because of the solid crystals in the slurry. Stainless steel with wetted lined parts was used

instead of titanium because it is significantly less expensive.

38

It was determined that a centrifugal stainless steel pump would also be sufficient for the

calcium chloride brine flow. A proposal for the K-123 Calcium Chloride pump was

provided by Goulds Pumps (shown in Appendix L) based on the conditions supplied by

SMK Integration Solutions.

5.3. Crystallizer Feed Pump

The K-110 Crystallizer Feed Pump was sized based on the pressure required to reach the

D-120 Crystallizer from the F-100 Chlorate Mix Tank. The top of the mix tank and the

top of the crystallizer were assumed to be at atmospheric pressure (101.3 kPa). The

suction pressure of the pump was based on the head of the mix tank and was calculated to

be 175 kPa. The pump will have to pump the slurry up 4.88 m for the recycle of the mix

tank, over the length of pipe from the pump to the crystallizer (6.1 m or 20 ft) and

through a globe valve with a K-value of 9.5 (1/2 open). Therefore, the discharge pressure

on the pump had to be 243 kPa. The pressure increase across the pump will be 68 kPa.

Chemical Engineering: Process Design and Economics; A Practical Guide (Ulrich,

2004) was used to determine the size of the pump needed from the pressure increase in

the pump using the following equation:

is

PqW = && (5.1)

The intrinsic efficiency of the pump, I, was determined to be 0.144 from the following equation:

)1)(12.01( 8.027.0 = qi & (5.2)

39

The viscosity of the slurry was supplied by ERCO to be 0.003 Pa*s. The shaft power,

sW& , was then determined to be 328 W.

The driver efficiency, D, was then determined from Figure 4.2 in Ulrich to be 0.82 and the power consumption, P, for the pump was found to be 400 W from the following

equation:

Di

PqP = & (5.3)

Further details of the calculations can be found in Appendix F.

5.4. Crystallizer

When operating a sub-zero temperature crystallizer, industry experience indicates that

heat transfer should have a temperature difference of no more than 2oC. This poses

restrictions on the operation of crystallizer and needs to be quantified. The crystallizer

vessel that will be used is a CAPO Reactor and is shown in Figures 5.1 and 5.2 below.

40

Figure 5. 1: Top of CAPO Reactor

41

Figure 5. 2: Bottom of CAPO Reactor

42

SMK Integrated Solutions performed a theoretical analysis of the CAPO Reactor vessel

based on a maximum temperature difference of 2oC. The analysis, based on first

principles and correlations presented in Fundamentals of Heat and Mass Transfer 5th

Ed., (Incropera & DeWitt, 2002) showed that the required rate of heat transfer could not

be achieved with the existing CAPO Reactor vessel and a 2oC temperature difference.

The analysis of the crystallizer as a heat exchanger was not a straight-forward calculation.

The difficulties are present in numerous aspects of the analysis; however, these

difficulties could be handled by making some assumptions, such as:

1. Exact measurements of the tube size and thickness were not known. Estimates of

tube sizes were made based on photographs of the end of the tube bundle. These

estimates were then verified against standard gauge tubing sizes. ERCO indicated

that the tubes were likely 18 gauge (referenced from a similar style titanium tube

sheet at their plant site), and were made of Grade 2 titanium.

2. Physical properties of sodium chlorate slurry relating to heat transfer were not

readily available:

a. The effective viscosity of the slurry was assumed to be 0.003 Pa-s. This

was based on a previous estimation made by ERCO when designing a

pump. This effective viscosity was estimated for slurry at process

temperature (18-35oC). The effect of cooling the slurry was not accounted

for in the use of this effective viscosity.

b. A Prandtl number could not be found in any literature for a saturated

solution of sodium chlorate, nor for the slurry. A Prandtl number was

43

found for the calcium chloride brine to be Pr(0oC, 30 wt% CaCl2) = 0.7.

This value of Prandtl number seemed reasonable and was assumed to be

the same for the sodium chlorate slurry for the purposes of the heat

transfer analysis.

3. The alignment of the tubes around the annulus of the agitator draft tube could not

be accurately described as either aligned or staggered arrangements, but a

combination of the two arrangements, as can be seen in Figure 5.3. Therefore,

best estimates of measurements of aligned and staggered arrangements were used

to calculate convective heat transfer coefficients and the average of the two results

was taken to be accurate.

44

Figure 5. 3: Interior of CAPO Reactor

45

4. The flow orientation is parallel flow as the slurry flows down the annulus and up

through the vertical tubes and the calcium chloride brine flows up the baffled shell

side of the heat exchanger as shown in Figure 5.4. Assuming that the exiting (hot)

calcium chloride brine can reach a temperature within 0.5oC of the exiting (cold)

slurry, the log-mean temperature difference (driving force for heat transfer) was

calculated to be 1.08oC. Due to the low low-mean temperature difference, a very

high flow of calcium chloride brine and chlorate slurry will be required to achieve

the required amount of heat transfer, and may lead to operability issues with

respect to pressure drops and pumping requirements for the refrigerant fluid.

5. When accounting for the heat transfer resistance of conduction through the

titanium tubes, the temperature difference of 2oC will not be sufficient to produce

the required heat flux.

46

Figure 5. 4: Dimensions of Heat Exchanger Jacket of CAPO Reactor (Crystallizer) Vessel, (a) Top View: Tubes Run Through Annulus; (b) Side View: 5 Baffles Equally Spaced

Slurry flow out (h,o)

Slurry flow in (h,i) 1360mm

300mm

1830

mm

267mm

(a) (b)

CaCl2 brine flow in (c,i)

CaCl2 brine flow out (c,o)

47

The calculated heat transfer rate when accounting for all resistances, with no fouling

resistance, under the conditions of 150 m3/hr CaCl2 brine flow in the shell side and 0.5-2

m/s flow velocity of slurry on the tube side, was only 2.2 kW. If the conduction

resistance is ignored from the calculation, there would be more than enough heat transfer,

but this would be technically incorrect.

After discussing this concern with ERCO and referencing design handbooks, it was

concluded that this result did not seem reasonable. Perrys Chemical Engineering

Handbook (1997) showed that a typical heat exchanger in chilling service would have a

typical U value (overall heat transfer coefficient) of 1135-1420 W/m2-K (200-250

Btu/oF.ft2.hr). Ulrichs Process Design and Economics: A Practical Guide (2004) showed

that a typical U value for a brine-brine service shell and tube heat exchanger was 800-

1000 W/m2-K. Using a U value of 1300 W/m2-K, the heat transfer capability was

determined to be 156 kW, slightly under the required rate of 190 kW. The client decided

this was accurate enough to assume the crystallizer will work for this application. SMK

Integrated Solutions recommends that ERCO verify the heat transfer capabilities of this

vessel before installing it for service in the proposed process.

Other parameters that were determined for successful operation of the existing CAPO

reactor vessel as the low temperature crystallizer include the impeller rotation speed and

the pressure drop across the shell side of the heat exchanger. To produce a slurry velocity

of 2 m/s in the tubes, the impeller would need to rotate at approximately 41 rotations per

minute (RPM). This parameter was calculated based on best estimates (from photographs

48

of equipment) and correlations from Perrys Chemical Engineering Handbook (1997).

The power requirement for the impeller operation was estimated to be 8.7 kW. The

pressure drop across the shell side of the heat exchanger jacket was estimated to be 500

kPa when pumping at a rate of 150 m3/hr. The pressure drop across the shell side of the

crystallizer was then calculated and it was determined to be only 64 kPa.

Detailed calculations of the heat transfer, pressure drop and impellar rate can be found in

Appendix H.

5.5. Centrifuge Feed Pump

The K-130 Centrifuge Feed Pump was sized based on the pressure required to reach the

H-140 Pusher Centrifuge from the D-120 Crystallizer. The top of the crystallizer and the

inlet of the centrifuge were assumed to be at atmospheric pressure (101.3 kPa). The

suction pressure of the pump was based on the head of the crystallizer and was calculated

to be 178 kPa. The pump had to be able to pump the slurry over 15.2 m (50 ft) and up

6.1 m (20 ft). Therefore, the discharge pressure on the pump needed to be 296 kPa. This

meant that there was a pressure increase in the pump of 118 kPa.

Ulrich (2002) was used to determine the size of the pump needed from the pressure

increase in the pump using the following equation:

is

PqW = && (5.4)

49

The intrinsic efficiency of the pump, I, was determined to be 0.144 from the following equation:

)1)(12.01( 8.027.0 = qi & (5.5) A slurry viscosity of 0.003 Pa-s was used for the calculations; this value was supplied by

ERCO. The shaft power, sW& , was then determined to be 569 W.

The driver efficiency, D, was then determined from Figure 4.2 in Ulrich (2007) to be 0.83 and the power consumption, P, for the pump was found to be 686 W from the

following equation:

Di

PqP = & (5.6)

Further calculations for the centrifuge feed pump can be found in Appendix I.

5.6. Centrifuge

The H-140 Pusher Centrifuge is an existing centrifuge currently at the ERCO facility but

in a shut down section of the plant and is shown in Figure 5.5. The centrifuge is titanium

to prevent erosion/corrosion issues and has a maximum flowrate of 3 m3/hr. Since the

flowrate in the process is 2.5 m3/hr, this centrifuge is acceptable. The solid crystals leave

the centrifuge and are transported to the Chlorate II Dryer by the I-150 Screw Conveyor

that is attached to the centrifuge. Using the existing 76-CF-101 Primary Centrifuge was

considered, however the 35oC chlorate slurry mixing with the chlorate slurry from the

LTCU would have resulted in dissolution of the sulphate crystals.

50

Figure 5. 5: Existing Pusher Centrifuge

51

5.7. Refrigeration Skid

The refrigeration skid was sized based on the amount of cooling needed to reduce the

crystal temperature from 35oC to -5oC in the crystallizer. The heat capacity of the slurry

was provided by ERCO and the flowrate was known; therefore, the amount of energy that

had to be removed from the slurry could be determined. There is also heat generated

from the crystallization of chlorate in the crystallizer and this was determined from the

amount of chlorate that will be crystallized and the heat of crystallization of chlorate.

Therefore, the total heat that will be necessary to remove is 190 kW. ERCO then

contacted CIMCO Refrigeration for a 50 ton of refrigerant (177 kW) refrigeration skid

proposal; the proposal is located in Appendix J. ERCO has also discussed using a 100

ton of refrigerant (354 kW) refrigeration skid that they have at their facility that is not

being used. After some discussion, ERCO was satisfied with the amount of cooling the

CIMCO refrigeration skid would provide, thus it was chosen to be implemented into the

process. The refrigeration skid includes the following major equipment:

(2) Copeland Semi Hermetic Screw Conveyors (1) Standard Refrigeration DX Chiller (1) Armstrong Pump (2) CIMCO Shell and Tube Condenser (2) CIMCO High Pressure Receiver (1) CIMCO Control Panel (standard relay-logic type c/w all motor

starters)

Associated valves, piping and fittings Armaflex insulation

52

Structural steel base Corrosive paint Compresser Oil initial charge Initial drier cores Initial refrigerant charge

Not included in the CIMCO refrigeration skid is any electrical wiring and/or electrical

components and any water connections.

5.8. Calcium Chloride Pump

The K-123 Calcium Chloride Pump was sized based on the pressure required to pump the

calcium chloride brine through the shell of the D-120 Crystallizer from CIMCO

Refrigeration unit. The suction pressure of the pump was assumed to be 203 kPa and the

pressure drop across the refrigeration unit was estimated to be 200 kPa. The pressure

drop through the crystallizer was estimated to be 500 kPa. The pump had to be able to

pump the slurry over the length of pipe from the pump to the crystallizer (46 m, 150 ft),

through the crystallizer, up 9.1 m (30 ft), over the length of pipe from crystallizer to the

refrigeration unit (46 m, 150 ft), and through the refrigeration unit back to the pump.

There is also a slipstream of calcium chloride going to the E-124 Brine Wash Heat

Exchanger but the flow in this line is so small that the pressure drop was assumed

negligible. Therefore, the discharge pressure on the pump needed to be 1153 kPa. This

meant that there was a pressure increase in the pump of 951 kPa. (The proposal can be

found in Appendix L).

53

Ulrich (2002) was used to determine the size of the pump needed from the pressure

increase in the pump using the following equation:

is

PqW = && (5.7)

The intrinsic efficiency of the pump, I, was determined to be 0.728 from the following equation:

)1)(12.01( 8.027.0 = qi & (5.8)

The viscosity of the calcium chloride brine used was 0.0045 Pa*s. The shaft power, sW& ,

was then determined to be 72.8 kW.

The driver efficiency, D, was then determined from Figure 4.2 in Ulrich to be 0.94 and the power consumption, P, for the pump was found to be 77.2 kW from the following

equation:

Di

PqP = & (5.9)

Details of the calculations discussed above can be found in Appendix K.

54

5.9. Brine Wash Heat Exchanger

The Brine Wash Heat Exchanger, E-124, was sized based on the brine wash flowrate

provided by ERCO. This brine wash flowrate was specified to be 2.5 L/min. Assuming

that the inlet temperature of the brine fluid was 20oC or 293 K, and the outlet temperature

was -5oC or 268 K, the heat transfer rate required was calculated to be 4.3 kW.

This heat exchanger was initially designed as a double-pipe (single-tube) heat exchanger

as these are typically the cheapest exchangers available. The heat transfer area required

for this exchanger was calculated to be 0.45 m2. The smallest commercially available

double-pipe heat exchanger is 0.6 m2, according to Ulrich (2002) Figure 5.36, so this size

was used for costing. After determining the dimensions of the double-pipe heat

exchanger (15 meters in length), it was decided that it would be too long for the space

that ERCO has available for this equipment. Therefore, a plate and frame type heat

exchanger would be used due to plot space restrictions.

The plate and frame heat exchanger was sized based on the same operating conditions

specified for the double-pipe exchanger. Assuming a coolant flow of 0.5m3/hr, a log-

mean temperature difference of 6.4oC was calculated. From Ulrich, a typical U value for