Dynamic Design of a Cryogenic Air Separation Unit - · PDF fileDynamic Design of a Cryogenic...

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Dynamic Design of a Cryogenic Air Separation Unit

Samantha Schmidt and Russell Clayton

Lehigh University

Spring 2013

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Abstract

We present the dynamic design of an air separation unit (ASU), which is used to separate oxygen,

nitrogen, and argon products from air at very low temperatures. This unit can be used as an isolated

plant to produce the products to be sold or as a unit joined to a larger plant, also called a “piggy back”

plant, to produce products to be used in an additional process [1]. These gases are commonly used in

refineries and oil recovery efforts. This process is convenient because the raw material is available at no

cost and in limitless amounts. The primary cost in the process is compression. In order to minimize

costs, heat integration is used throughout the process. This particular ASU will produce 1500 metric tons

of 99.5% oxygen, 5000 metric tons of 99.5% nitrogen, and 58 metric tons of 95% crude argon each day

to a customer. Heat integration will eliminate all costs from heating and cooling in the process. The

compressor capital cost will be $16.5 MM1. The venture guidance appraisal for this process is $118.5

MM. When running at full capacity, the plant will sell $113.9 MM worth of products. With a total

annual cost for equipment and utilities of $39.0 MM, this plant will yield a yearly profit of $73.4MM.

An integrated control structure has been designed to maintain product purity and reject atmospheric

(temperature and pressure) disturbances, as well as flowrate disturbances. An overview of both a feed

control and on demand scheme is included in the following pages, along with disturbance testing for

both schemes.

1. Million dollars

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Table of Contents

Process Overview 3

Design Problem 3

Control Objectives 4

Argon Column Control 5

Compression Control 7

Integrated Column Control 8

Plant Wide Control 10

On Demand Control Structure 11

Process Adjustments 12

Total Annual Cost 13

Safety 13

Lessons Learned 16

References 18

Appendix 20

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Process Overview

In this process, air is fed into a molecular sieve column, in which carbon-containing molecules

and water are removed by adsorption. The air then enters a series of compressors in which the air is

compressed to high pressure. The air stream enters a high pressure column from which two streams are

separated. The liquid distillate is rich in nitrogen, and the liquid bottoms product is rich in oxygen.

These streams enter a low pressure column that separates the nitrogen and oxygen products. The reboiler

of the low pressure column acts as the condenser of the high pressure column, eliminating energy costs

for both columns. A vapor side stream is taken from this low pressure column, and feeds into a single

crude argon tower. The distillate from this column is the crude argon product. The bottoms product of

this column is recycled to the low pressure column. The process flow diagram illustrating the process is

shown in Appendix 1. The property package used in our simulation was Peng-Robinson because this

model was in good agreement with experimental data found in literature [4].

Design Problem

This particular plant is required to produce 1500 metric tons of 99.5% oxygen, 5000 metric tons

of 99.5% nitrogen, and 58 metric tons of 95% crude argon each day [2]. A mass balance showed that

9480 kmol/hr of air must be fed into the process. For the plant design, our constraints are the purity

specifications and the necessary product flowrates. Because the composition of air cannot be changed,

our smallest product relative to the amount of air entering is nitrogen. Thus, we will have some excess

argon and oxygen produced when running at design conditions.

This plant will be operated in Bethlehem, Pennsylvania. The feed air will contain contaminant

amounts found in Bethlehem air, 275 ppm hydrocarbons, and will be at 1 atm and ambient air

temperature, 315 K. The amount of water in the air will be dependent on the humidity in the air on a

given day. We over estimated an amount of water equivalent to the amount of hydrocarbons in the air, in

order to guarantee our purification process can rid the feed air of water

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In this plant simulation, one column is shown.

Industrially, this system would be built as one large column

with a high pressure section on the bottom and a low pressure

section on top. Figure 1 shows a schematic of the one column

design. In this design, the condenser/reboiler is a heat

exchanger in between the high pressure and low pressure

sections of the column. In some cases the high pressure and

low pressure sections are built as two separate towers. In this

case, the condenser-reboiler would be a large heat exchanger

outside of the columns. A pump would be required to pump

the liquid from a vessel into the high pressure column. We are

assuming our simulation is for a system in a single column.

This process is operated at cryogenic temperatures. At very low temperatures, the chemicals can

be in the liquid state. In order to achieve the necessary separation, it is required that the chemicals are

partially liquid. To obtain these low temperatures, the feed gas is compressed, and the compressed

stream is cooled by the nitrogen and oxygen products to reduce the temperature. The temperatures in the

process are also mandated by the heat duty specifications of the condenser in the high pressure column

and the reboiler in the low pressure column. The heat duties of these two columns must be equal. The

cryogenic temperatures pose intricacies in the design because of the materials required for the

equipment, as well as additional potential safety problems the low temperatures can cause.

Control Objectives

Our primary control objective is to maintain the purity of our products. The purity is the most

important product specification. These objectives are explained in greater detail in the following pages.

The disturbances that must be rejected are primarily pressure and temperature changes. These

Figure 1. Single ASU column

separated in high pressure and low

pressure sections [2]

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disturbances are important because the feed is the ambient air which is subject to temperature and

pressure change based on the weather. Because the feed into the process is air from the atmosphere, we

do not expect a significant composition change. There is possible hydrocarbon impurity fluctuation for

the feed air but, this has been compensated for by designing a molecular sieve purification system that is

twice as large then would be necessary to account for all fluctuations of hydrocarbon and water

impurities in the feed. The system was designed this way because Aspen software is incapable of

simulating a molecular sieve and other methods of purification would not sufficiently remove the

hydrocarbon and water impurities in the system. Otherwise, the composition of the three key

components: nitrogen, oxygen, and argon are not expected to change so testing was not done on

composition changes of these components. We do, however, test that our argon column can reject a

change in the composition of the side stream from the integrated columns, in case the composition in

that column would change due to a pressure or temperature change in the process.

For this type of plant, both feed control and on demand control structures can be used. In this

report, we explore both options and weight the benefits of using each.

Argon Column Control

For the crude argon column, the primary control objective was to control the purity of the crude

argon product. Due to the lack of a re-boiler in this column, the crude argon column only had five

degrees of freedom as opposed to the normal six degrees of freedom in a normal column. The

parameters that must be controlled are the reflux drum and sump liquid levels, the feed flow rate, the

pressure into the column, and the composition of the crude argon product. The reflux drum level is

controlled by the reflux flow rate into the column and the sump level is controlled by the flow rate of the

oxygen rich bottoms recycle. A simple flow controller was used to control the feed into the crude argon

column from the side stream coming off the low pressure column. The pressure is controlled by the heat

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duty of the condenser. This leaves only one degree of freedom to handle the control objective once all

of the inventory controls are set.

The composition in the crude argon product could be controlled with one of two methods:

temperature control or composition control. The temperature control scheme holds the temperature on

stage 21 of the crude argon column by manipulating the distillate product flow rate. The composition

control scheme measures and controls the concentration of the oxygen impurity in the distillate leaving

the reflux drum by manipulating the distillate product flow rate.

The appropriate disturbance for the column is a feed composition disturbance. When disturbance

testing was performed, both control structures could reject with a five percent change in argon

composition of the feed. However, the temperature controlled scheme could not hold the product within

its specified purity of 95% argon, whereas the composition control scheme could reject the disturbances

almost entirely. Therefore, the composition control scheme was chosen to be used based on its ability to

meet the control objectives. The composition response is further improved when the gain for the reflux

drum level control is increased to 20. Figure 2 shows the control scheme chosen for the crude argon

column. The disturbance analysis for this column is shown in Appendix VIII.

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Recycle Valve

Argon Product1.1 atm88.2 K

68.0 kmol/hr.955 Argon

.045 Oxygen

T-103Crude Argon Tower

1.1 atm88.2 K

Packing Height: 5.85 m

80

1.5 atm90.7 K

3432 kmol/hr.96 Oxygen.04 Argon

CondenserCooled using Liquid Nitrogen

6.6 MW

1.5 atm93.9 K

3500 kmol/hr.058 Argon

.942 Oxygen

Side Valve

Argon Valve

PC

LC

LC

FC1.1 atm93.9 K

3500 kmol/hr.058 Argon

.942 Oxygen

CC

Figure 2. Composition control structure for crude argon column

Compression Control

The control objectives for control of the compressor train were to maintain a steady feed pressure

and temperature. These objectives were accomplished by using a temperature controller to maintain the

inter stage temperature by manipulating the appropriate cooler’s heat duty and using a single pressure

controller to control the exit pressure by controlling the power supplied to each compressor. This

control scheme rejected 50K temperature disturbances and 20% pressure disturbances quickly and

efficiently. Figure 3 shows the control scheme used for the compression train control. The disturbance

analysis for this system is shown in Appendix IX.

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C-1014.3 MW

Feed Air300 K1 atm

9482 kmol/hr.7812 Nitrogen.2095 Oxygen.0093 Argon

275 ppm HydrocarbonsTrace Water

TC

PC

E-1054.4 MW2580 m²

CW

E-1034.3 MW2570 m²

CW

E-1044.3 MW2570 m²

CW

E-1013.7 MW2550 m²

CW

E-1024.3 MW2560 m²

CW

C-1044.3 MW

C-1054.3 MW

C-1024.3 MW

C-1034.3 MW

TC

TC

TC

TC

TC

Figure 3. Compression train control structure

Integrated Column Control

For the heat integrated columns, the key variables we are trying to control are the nitrogen and

oxygen product stream purities and the composition of the vapor side stream that goes into the argon

column. The integrated condenser/reboiler has constraints of its own. Because of this, there are fewer

degrees of freedom in the design. Typically, a single column has six degrees of freedom. This two

column integrated system only has six degrees of freedom. Thus, the design to control several

parameters with fewer control variables must be integrated.

In order to ensure heat integration in these two columns, flowsheeting equations first had to be

used to make the reboiler and condenser heat duties equivalent. Figure 4 shows the flowsheeting

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equations used. The duty of the reboiler on the low pressure column is first defined using the heat

transfer area and the change in temperature across the heat exchanger. The heat duty of the condenser is

then set to be the negative value of the reboiler heat duty. This will specify the heat integration in these

columns.

Figure 4. Flowsheeting equations used to equate the heat duties of the integrated condenser/reboiler

As stated above, there are fewer degrees of freedom in this integrated column system. The

parameters that must be controlled are the pressure in one column, the sump levels of both the high and

low pressure columns, the composition of the nitrogen and oxygen products and the flowrate of the feed

stream into the column. There are various ways to control these parameters. The heat duty of the

condenser reboiler is set by the above flowsheeting equations. The flowrate of the nitrogen rich and

oxygen streams coming from the high pressure column, the flowrates of the product streams, and the

flowrate of the reflux stream from the condenser/reboiler, as well as the flowrate of the feed stream can

be used to control the necessary parameters. A temperature controller is used to maintain the purity of

the nitrogen product and a composition controller is cascaded to the compressor train to maintain the

oxygen product purity. The temperature profile for the low pressure column is shown in Figure 5. The

control design chosen for this system is shown in the process flow diagram Appendix II.

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Block T -101: Temperature Prof ile

Stage

Tem

pera

ture

K

1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0

94.5

95.0

95.5

96.0

96.5

97.0

97.5

98.0

98.5

99.0

99.5

100.0

Temperature K

Figure 5. Temperature profile of low pressure column. A temperature controller is used on stage 7 to

maintain the purity of the nitrogen product.

The appropriate disturbances for this system are temperature and pressure changes. We test this

system in conjunction with the compression train to assess the effects of temperature and pressure on

this system. A 20% pressure change and 50 degree temperature change is made. The key disturbance

graphs are shown in Appendix X.

Plant Wide Control

Once each component of the system was designed and tested, the integrated column system was

combined with the argon column and the front end compression train. The primary plant wide control

concern is the product purities. The main concern in bringing together the crude argon column and the

integrated column system was the oxygen-rich recycle stream from the crude argon column. However,

when connected and tested, this stream did not cause many issues.

The main issues originated from the vapor side stream that is taken from the integrated columns

to the crude argon column. When pressure and temperature disturbances are made in the feed stream,

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there is a short period when there are also pressure and temperature disturbances that occur downstream

in the process. Because of the temperature and pressure disturbances, there is variation in the

composition on each stage of the low pressure column. This variation in the composition on stage 35

leads to a composition change in the vapor side stream. Depending on the type of disturbance made, this

leads to either an excess or deficiency of argon in this stream. These changes in argon flowrates in the

side stream also lead to changes in the flowrate of the crude argon product. Ratio controls were used in

various manors to try to properly adjust the side stream flowrate depending on the change in argon in

this stream. A composition controller was also implemented to test its effectiveness in manipulating the

side stream flowrate depending on the amount of argon. However, these more complicated, more

expensive schemes did not work any better in helping to better control the product flowrate. Thus, we

decided to go with a simple flow controller to maintain the flowrate of this stream. This is only an issue

when a large flowrate disturbance is made. The system can easily and readily reject any pressure or

temperature disturbances that it may experience. Appendix XI shows the disturbance graphs for the

plant wide control structure.

On Demand Control Structure

In order to truly understand the process, an on demand structure was also designed. This

structure differs because the throughput manipulator is the flowrate of the nitrogen product. Although

the control structures for the crude argon column and the compression train do not change, the controls

on the integrated column system and for the plantwide control must be considered and altered. The

flowrate of the nitrogen product is controlled by a valve on the nitrogen product stream. This valve

previously controlled the pressure on the low pressure column. This pressure will now be allowed to

vary. The pressure in the high pressure column, which previously was allowed to “float”, is now

regulated by a valve in the feed line to this column. All other controls remain the same. The control

structure for the integrated columns is shown below in the process and instrumentation diagram in

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Appendix II. The on demand structure would be used in a “piggy-back” plant because the flowrate of the

nitrogen product would be specified by the amount necessary in the larger plant that utilizes the

separated elements.

The on demand structure also implements a ratio control to regulate the flowrate of the vapor

side stream from the low pressure column. As described earlier, temperature and pressure differences in

this column lead to composition differences in the column and in the side stream. By maintaining the

design ratio between the feed stream and the vapor side stream, larger flowrate disturbances can be

made to the nitrogen product stream. This system can withstand larger flowrate disturbances than the

feed control system. This system can also withstand and pressure and temperature disturbances in the

feed. The disturbance tests are shown in Appendix XI. A 20% pressure change and 50 degree

temperature change yield the same results as those from the feed control the system.

Process Adjustments

A few adjustments to the process needed to be made in order to meet the control objectives. All

three distillation columns were redesigned to contain five additional stages each to allow for more

flexibility for the control system as per feedback on the steady state design. Pumps and valves were

added to the system to ensure that the process was entirely pressure driven and could have a control

system added to it. The crude argon column was redesigned to have a total of 80 stages, up from the

original 40. This increase in stages was made during the design of the crude argon control structure due

to the 40 stage column’s inability to handle composition disturbances well enough to keep the product

on specification. The larger distillation column could both reject disturbances with the control structures

tested on it and keep the product within purity and flow rate specifications. Finally, a calculation of heat

duty proved that the nitrogen product from column two was not sufficient enough to cool condense the

argon distillate, and so it was decided that liquid nitrogen would be used as an external cooling utility

instead.

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Total Annual Cost

Detailed information on the total annual cost for each piece of equipment is given in Appendix

III. It is assumed that composition controls will cost about 0.1MM$ each and other controls will have

costs that are ultimately negligible compared to the cost of the process. The venture guidance appraisal

for this plant is 118.5 MM$. The total annual cost for this plant is 40.6 MM$. The yearly income from

product sales for this plant will be 113.9 MM$, with our products selling for $30/ton, $30/ton, and

$2000/ton for nitrogen, oxygen, and argon, respectively. The cash flow for this plant for the next

eighteen years is shown in Figure 5. The startup year will be 2016.

Figure 6. Cash Flow Diagram for the next eighteen years.

Safety

The conditions required for the separations and the products themselves, nitrogen, argon, and

oxygen, are what make safety considerations necessary. The specific safety considerations include

control of pressurized gases, exposure to cryogenic temperatures, risk of asphyxiation from oxygen

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displacement by argon, and explosion risks in the distillation columns due to oxygen reactivity,

combustion, and pressurization.

In cryogenic air separation the air is compressed to pressures in the range of 8-10 atmospheres

through a series of compressors and heat exchangers. While the gas in the process does not stay at these

high pressures for very long, any high pressure gas can be dangerous in the event of a leak or pipe

rupture. However, the risk to personnel is relatively low because cryogenic air separation needs to

maintain a tight set of conditions and generally does not have maintenance access ways. As a precaution

pressure gauges can be placed to monitor the pressure across pipes at regular intervals as a check against

leaks in the pipes and pressure relief valves can be used to safely vent over pressurized gases which may

have resulted from equipment malfunction.

Asphyxiation due to argon and nitrogen, and the cryogenic temperatures are hazards that occur in

the insulated ‘cold boxes’ that house the parts of the process that work at cryogenic temperatures. Leaks

in this part of the process would allow exposure to the extremely cold fluids. This can also be a problem

in the condenser of the crude argon column, which is cooled using 70K liquid nitrogen. Additionally, a

leak in the argon and nitrogen rich pipes would cause them to displace the air near the ground, which in

the confined spaces of the cold box would displace the breathable air. Again, limited access ways and

the limited number of personnel during active process limits these risks and pressure valves can be

installed to monitor for leaks. As a further precaution oxygen sensors can be installed in the cold boxes

to monitor for low oxygen content and maintenance crews can be equipped with portable breathing

apparatus, which will be available at cold box entry ways for work in the cold box

Explosion risk is the primary concern related to the process. There are factors to cause reactions

with oxygen, which in turn would lead to combustion and an explosion inside the low pressure

distillation column. Most distillation column explosions are caused by problems with the reboiler. This

process contains only one reboiler at the bottom of the low pressure column. This is the primary area of

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concern regarding the explosion hazard. When the air is fed into the process, the air contains trace

amounts of hydrocarbons. These hydrocarbons would freeze under the operating conditions and collect

at the bottom of the low pressure column and in the reboiler. Over time, enough hydrocarbons would

build up to react with the oxygen, causing combustion which could lead to flash vaporization in the

column and, subsequently, an explosion. The damage from these explosions can vary from leaks in the

reboiler to destruction of the entire process and product pipelines [10]. To prevent this, hydrocarbons

must not be allowed to enter the cold section of the process. To this end an adsorption unit must be used

to capture these flammable impurities. The unit must be designed to adsorb more than the expected

amount of impurities in the ambient air. Also, it has been found that the carbon dioxide concentration

left in the air is related to the amount of hydrocarbons in the air [10]. Thus, if carbon dioxide

concentrations leaving the adsorption unit are monitored, the hydrocarbons are also monitored. An

alarm condition needs to be set at a concentration of carbon dioxide of 1ppm of air leaving the adsorber,

at which point the operator should switch adsorber beds to a properly regenerated bed and then monitor

sump concentrations of hydrocarbons [10]. Should these exceed safe values, then the plant must be shut

down and either warm air must be sent through the system to remove the built up hydrocarbons, or

maintenance must be done on the process. The removed hydrocarbons would be disposed of as waste

gas. Should this precaution fail, a way to limit the explosion is to ensure that the packing used in the

low pressure column does not combust more readily in the presence of reaction oxygen. Using copper

or brass packing instead of aluminum is a safe alternative to limit the explosion which will in turn cut

down on repair costs and lower the risks of injury. Additionally, a composition controlled purge stream

is used to vent trace amounts of hydrocarbons that are caught in the sump of the low pressure column.

A HAZOP summary of the failure of the low pressure column is included in Appendix XIII.

A simulation was run in which a compressor failed. The results of this simulation showed that

the process came back to steady state after approximately six hours. The crude argon product stream

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increased significantly, but this was due to the changes in composition in the low pressure column

caused by pressure changes in the system. The changes in each of the other streams was negligible.

Since the increase or decrease in pressure is a concern in our process, it must be able to withstand

changes like this. In the control scheme, there is enough redundancy that most safety concerns are

rejected. Figure 6 shows the disturbance graphs from this simulation.

Figure 7. Disturbance analysis of process when compressor stage fails

Lessons Learned

We learned that Aspen Plus and Aspen Dynamics contain many nuances that enable process

design to be easier. We would suggest utilizing online resources to research Aspen implementation and

learning about the many ways Aspen can be made easier. We learned that plant wide control can have a

very large effect on the disturbance rejection. We would suggest considering plant wide control

obstacles that may arise throughout the design of the process and working on plant wide control sooner.

We learned that safety is very important, even if you are working with a raw material as seemingly

0 1 2 3 4 5 6 7 80.9

0.95

1

1.05

Time, hr

Nitro

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n P

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Controlled variables

0 1 2 3 4 5 6 7 8180

185

190

195

200

Time, hr

T-1

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Manipulated variables

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1

1.05

Time, hr

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0 1 2 3 4 5 6 7 83000

3500

4000

4500

5000

Time, hr

Co

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kW


0 1 2 3 4 5 6 7 80.9

0.95

1

1.05

Time, hr

Cru

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Arg

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0 1 2 3 4 5 6 7 840

50

60

70

80

90

100

Time, hr

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Pro

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harmless as air. We suggest thoroughly researching safety hazards and understanding the entirety of the

process so that dangerous areas can be identified and precautions can be taken. We learned that although

integrated columns cut down on costs greatly, they are very difficult to design. Both in steady state and

dynamics, these columns necessitate a thorough understanding of the process. Particularly in dynamics,

the integration takes away degrees of freedom and requires an integrated control scheme. We would

suggest that the engineer researches and understands the process entirely before undertaking the task of

converging the heat duties of the condenser/reboiler or designing an integrated plant wide control

scheme.

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References

1. "Basic Air Separation Unit Description." Ranch Cryogenics Inc. RSS. N.p., n.d. Web. 25 Oct.

2012. <http://www.ranchcryogenics.com/about/basic-air-separation-unit-description/>.

2. "Air Separation into Oxygen, Nitrogen, Argon." West Virginia University ChE. West Virginia

University, n.d. Web. 10 Sept. 2012. <http://www.che.cemr.wvu.edu>.

3. ".Argon Ar Properties, Uses, Applications - Gas and Liquid." Universal Industrial Gases, Inc.

N.p., n.d. Web. 2 Nov. 2012. <http://www.uigi.com/argon.html>.

4. Bernstein, Joseph T. "Cryogenic Argon Production." Proc. of Modern Air Separation Plant

Technology Conference, Chengdu, People's Republic of China. N.p.: Cryogenic Consulting

Service, n.d. N. pag. Print.

5. Luyben, William L. Distillation Design and Control Using Aspen Simulation. Hoboken, NJ:

Wiley-Interscience, 2006. Print.

6. Turton, Richard. Analysis, Synthesis, and Design of Chemical Processes. 3rd ed. Upper Saddle

River, NJ: Prentice Hall PTR, 2009. Print.

7. "Air Separation Tutorial." Air Separation Tutorial. Carnegie Mellon University, n.d. Web. 12

Nov. 2012. <http://www.cheme.cmu.edu/course/06302/airsep2/Part2.html>.

8. "Air Separation Plants." Linde Engineering. The Linde Group, n.d. Web. 12 Nov. 2012.

<http://www.linde-engineering.com/en/process_plants/air_separation_plants/index.html>.

9. "Air Separation Technology— Structured Packing." Cryogenic Air Separation. Air Products,

n.d. Web. 10 Oct. 2012.

10. Argent, Roger et al. “Safe Operation of Reboilers/Condensers in Air Separation Units.” Asia

Industrial Gases Association. AIGA 035/06. Web. 25 Nov. 2012

11. "Molecular Sieve Zeolite." 13X Zhengzhou Gold Mountain Science and Technique Co. Ltd. N.p.,

n.d. Web. 27 Nov. 2012.

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12. "Activated Alumina Dessicant." ECompressedAir. N.p., n.d. Web. 27 Nov. 2012.

13. “Airtek Air Dryer.” Full Systems Engineering. Web. 6 Dec. 2012.

14. Luyben, William L. "Design and Control of a Fully Heat-Integrated Pressure-Swing

Azeotropic Distillation System." Industrial & Engineering Chemistry Research 47.8 (2008):

2681-695. Print.

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Appendix

I. Process Flow Diagram

See attached

II. Piping and Instrumentation Diagram

See attached

III. Total Annual Cost

Table II. Total Annual Cost of Plant Equipment

Equipment Capital Cost ($) Utility Costs ($/year)

C-101 Compressor in Train 8683000 2950000





E-101 Compressor Cooler 1050000 39000





E-106 Feed Cooler 1 2690000 0

E-107 Feed Cooler 2 7550000 0

E-108 Precompressor Hx 1050000 46000

T-101 HP Tower 499000 0

T-102 LP Tower 1250000 0

T-103 Crude Argon Tower 904000 2136000

Condenser/Reboiler 1041000 0

Crude argon Condenser 20100 0

P-101 Oxygen Recycle Pump 57000 6880

P-102 Oxygen Product Pump 47000 3620

P-103 Argon Product Pump 36000 460

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V-101 Molecular Sieve Vessel 3670000 0

Cold Box (Cryogenic Container) 1140 0

4 Composition controllers 400000 0

Total ($MM): 68.35 17.4

TAC 3 year pay back ($MM) 40.6

IV. Equipment List

Table III. Design parameters for all equipment in plant

Equipment Summary for Proposed Cryogenic Air Separation Plant

Heat Exchangers E-101 E-102 E-103

Type Fixed or U-Tube S/T Fixed or U-Tube S/T Fixed or U-Tube S/T

Area (m2) 1280 1280 1280

Duty (kW) 3690 4330 4330

U (kW/m2*°C) 0.06 0.06 0.06

Shell

Temp In (°C) 27 27 27

Press (psi) 14.7 14.7 14.7

Phase Liquid Liquid Liquid

MOC 316 Stainless Steel 316 Stainless Steel 316 Stainless Steel

Tube

Temp In (°C) 97 105 105

Press (psi) 22.2 33.8 51.3

Phase Vapor Vapor Vapor


Heat Exchangers E-104 E-105 E-106

Type Fixed or U-Tube S/T Fixed or U-Tube S/T Fixed or U-Tube S/T

Area (m2) 2570 2580 3780

Duty (kW) 4340 4360 7260

U (kW/m2*°C) 0.06 0.06 0.06

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Shell

Temp In (°C) 27 27 -179

Press (psi) 14.7 14.7 22.1

Phase Liquid Liquid Liquid


Tube

Temp In (°C) 105 105 49

Press (psi) 77.9 118 118

Phase Vapor Vapor Vapor


Heat Exchangers E-107 E-108

Type Fixed or U-Tube S/T Fixed or U-Tube S/T

Area (m2) 10700 1280

Duty (kW) 8200 4330

U (kW/m

2*°C) 0.03 0.06

Shell

Temp In (°C) -192 27

Press (psi) 22.1 14.7

Phase Vapor Liquid

MOC 316 Stainless Steel 316 Stainless Steel

Tube

Temp In (°C) -44 105

Press (psi) 118 33.8

Phase Vapor Vapor


Towers T-101 T-102 T-103

Operational Temp (°C) -178 -192 -185

Press (psi) 73.5 22 16.2


of 41

Size

Height (m) 1.6 8.9 14.4

Diameter (m) 4.2 4.2 2.8

Internal 1.35 m Copper Mesh

Packing

7.35 m Copper Mesh

Packing

5.85 m Copper Mesh

Packing

HETP (m) 0.15 0.15 0.15

Pumps/Compressers P-101 P-102 P-103

Flow (kmol/hr) 3450 1987 9480

Shaft Power (kW) 9.57 5.02 0.64

Type Centrifugal Centrifugal Centrifugal

Efficiency 0.7 0.64 0.30


Temp In (°C) -182 -179 49

Press In (psi) 16.0 22.0 16.2

Press Out (psi) 52.2 52.9 66.2

Compressors C-101 C-102 C-103

Flow (kmol/hr) 9480 9480 9480

Shaft Power (kW) 4230 4320 4320

Type Centrifugal Centrifugal Centrifugal

Efficiency 0.72 0.72 0.72


Temp In (°C) 42 49 49

Press In (psi) 14.7 22.2 33.8

Press Out (psi) 22.2 33.8 51.3

Compressors C-104 C-105

Flow (kmol/hr) 9480 9480

Shaft Power (kW) 4320 4320

Type Centrifugal Centrifugal

Efficiency 0.72 0.72


of 41

Temp In (°C) 49 49

Press In (psi) 51.3 77.9

Press Out (psi) 77.9 118

Molecular Sieve V-101

Temp (°C) 49

Press (psi) 118

MOC 316 Stainless Steel

Size

Height (m) 19

Diameter (m) 2.5

Internal

11 m3 Alumina

adsorbant, 18.8 13X

Zeolite adsorbant

V. Compression Analysis

Analysis was performed to optimize number of compressors

Figure 63. Total utility requirement for compressor trains with 1-7 stages

0

5000

10000

15000

20000

25000

30000

0 2 4 6 8

Uti

lity

Re

qu

ire

d (

kW)

Number of Stages

Utility Requirement for Compression

of 41

Figure 14. Total annual cost for compression utility (electricity) for compressor trains with 1-7 stages

Figure 15. Capital cost of compressor-drive system for compressor trains with 1-7 stages

0

5

10

15

20

0 2 4 6 8

Co

st (

MM

$)

Number of Stages

Annual Utility Cost for Compression

0

0.5

1

1.5

2

2.5

3

3.5

4

0 2 4 6 8

Co

st (

M$

)

Number of Stages

Compressor Capital Cost

of 41

Figure 16. Total capital cost for heat exchangers in compressor trains. Each train contains the same

number of heat air cooler heat exchangers as compressors

Figure 17. Annual utility cost for shell and tube heat exchangers using cooling water

0

0.05

0.1

0.15

0.2

0.25

0.3

0 2 4 6 8

Co

st (

MM

$)

Number of Stages

Heat Exchanger Capital Cost

0

0.05

0.1

0.15

0.2

0.25

0.3

0 2 4 6 8

Co

st (

MM

$)

Number of Stages

Heat Exchanger Utility Cost

of 41

Figure 18. Total annual cost of compression, including capitals costs of compressors, drives, and heat

exchangers, and utility for drives and heat exchangers

VI. Stream Table

Table IV. Properties of each stream in plant simulation

Stream 1 2 3 4 5 6

Temperature (K) 315 370 322 378 322 378

Pressure (atm) 1.00 1.52 1.52 2.30 2.30 3.49

Flow Rate (kmol/hr) 9480 9480 9480 9480 9480 9480

Nitrogen 0.7812 0.7812 0.7812 0.7812 0.7812 0.7812

Argon 0.0093 0.0093 0.0093 0.0093 0.0093 0.0093

Oxygen 0.2095 0.2095 0.2095 0.2095 0.2095 0.2095

Vapor Fraction 1.000 1.000 1.000 1.000 1.000 1.000

Stream 7 8 9 10 11 12

Temperature (K) 322 378 322 378 322 229

Pressure (atm) 3.49 5.29 5.29 8.00 8.00 8.00

Flow Rate (kmol/hr) 9480 9480 9480 9480 9480 9480

Nitrogen 0.7812 0.7812 0.7812 0.7812 0.7812 0.7812

Argon 0.0093 0.0093 0.0093 0.0093 0.0093 0.0093

Oxygen 0.2095 0.2095 0.2095 0.2095 0.2095 0.2095

Vapor Fraction 1.000 1.000 1.000 1.000 1.000 1.000

0

5

10

15

20

25

0 2 4 6 8

Co

st (

MM

$)

Number of Stages

Compression Total Annual Cost

of 41

Stream 13 14 15 16 17 18

Temperature (K) 127.6 124.1 94.4 81.2 98.3 84.7

Pressure (atm) 8.00 5.00 5.00 1.50 5.00 1.50

Flow Rate (kmol/hr) 9480 9480 4412.1 4412.1 5067.9 5067.9

Nitrogen 0.7812 0.7812 0.99 0.99 0.6 0.6

Argon 0.0093 0.0093 0.002 0.002 0.016 0.016

Oxygen 0.2095 0.2095 0.008 0.008 0.384 0.384

Vapor Fraction 1.000 1.000 0.000 0.143 0.000 0.138

Stream 19 20 21 22 23 24

Temperature (K) 81.2 215.2 93.9 94 319.4 90.7

Pressure (atm) 1.50 1.50 1.50 1.50 1.50 1.10

Flow Rate (kmol/hr) 7443 7443 3500 1969 1969 3432

Nitrogen 0.995 0.995 -- -- -- --

Argon 0.002 0.002 0.06 0.003 0.003 0.04

Oxygen 0.003 0.003 0.94 0.997 0.997 0.96

Vapor Fraction 1.000 1.000 1.000 0.000 1.000 0.000

Stream 25 26

Temperature (K) 90.7 88.2

Pressure (atm) 1.50 1.10

Flow Rate (kmol/hr) 3432 68

Nitrogen -- --

Argon 0.04 0.955

Oxygen 0.96 0.045

Vapor Fraction 0.000 0.000

VII. Sample Calculations

a. Assumed or given values:

From Turton, Analysis, Synthesis, and Design of Chemical Processes [6]:

of 41

From Air Separation Tutorial [7]:

Molecular Sieve Densities:

[11]

[12]

b. Molecular Sieve Calculations (zeolite used as an example):

1. Mass of adsorbant (calculated using 1.333 more adsorbant than minimum

needed as per regulation) [7]:

2. Volume of adsorbant:

3. Height of Vessel (assuming same diameter as column T-101, 4.2 m)

(

)

of 41

( )

c. Column Calculations (crude argon tower used as example):

( ( ))

( ( ))

( ( )) ( (

)

)

( ( ))

( (

)

)

d. Heat exchanger/condenser/reboiler area:

e. Compression ratio:

( )

(

)

of 41

VIII. Crude Argon Disturbance analysis

Composition Controller, 5% Feed Composition Disturbance

Temperature Controller, 5% Feed Composition Disturbance

of 41

Composition Controller, Drum Level Controller Gain=20, 5% Feed Composition Disturbance

Control Scheme Comparison: -5% Feed Composition Disturbance

of 41

IX. Compressor Disturbance Analysis

Compressor Control: 20% Pressure Disturbance

Compressor Control 50K Temperature Disturbance

of 41

X. Integrated Column Disturbance Analysis

Integrated Column Control 20% Pressure Disturbance

Integrated Column Monitored Variables 20% Pressure Disturbance

of 41

Integrated Column Control 50K Temperature Disturbance

Integrated Column Monitored Variables 50K Temperature Disturbance

of 41

XI. Plant Wide Control Disturbance Analysis

Plant Wide Feed Control Feed Flow Disturbances

Plant Wide On Demand Nitrogen Product Flow Disturbances

of 41

Plant Wide Control Both Models 20% Pressure Disturbance

Plant Wide Control Both Models 50K Temperature Disturbance

of 41

T-101 Pressure with 20% Pressure Disturbance

of 41

XII. HAZOP of Low Pressure Column

Gu

ide

wo

rdC

ause

Co

nse

qu

en

ceSa

fegu

ard

Re

com

me

nd

atio

n

Hig

h T

em

pe

ratu

reLo

ss o

f fe

ed

Sep

arat

ion

wo

uld

no

t o

ccu

r, p

ress

ure

incr

eas

e

Tem

pe

ratu

re

ind

icat

or

on

colu

mn

Hyd

roca

rbo

n B

uil

d

Up

Ad

sorp

tio

n s

yste

m f

ault

Bu

ild

up

of

oxy

gen

an

d c

ou

ld c

ause

exp

losi

on

Pu

rge

str

eam

an

d

hyd

roca

rbo

n

mo

nit

or

Low

leve

l ala

rm

Hig

h P

ress

ure

Flo

w c

on

tro

lle

r o

n

Nit

roge

n p

rod

uct

fau

lt

Sep

arat

ion

wo

uld

ch

ange

, aff

ect

ing

con

seq

ue

nt

pro

du

ct p

uri

tie

s

Pre

ssu

re in

dic

ato

r

on

co

lum

n

Un

it: T

-102

(Lo

w P

ress

ure

Co

lum

n)

Hig

h L

eve

lLe

vel c

on

tro

lle

r fa

ult

Flo

od

ing

of

colu

mn

an

d

reb

oil

er/

con

de

nse

r st

op

s w

ork

ing

Hig

h le

vel a

larm

ind

ep

en

de

nt

of

leve

l co

ntr

oll

er

Low

Le

vel

Leve

l co

ntr

oll

er

mal

fun

ctio

n o

r lo

w f

low

fro

m h

igh

pre

ssu

re

colu

mn

Co

nd

en

ser/

Re

bo

ile

r st

op

s w

ork

ing

of 41

XIII. Controller Specifications

Unit Control Variable Gain Integral

Time Direction

HX-1 Temperature 4.01244 20 Reverse

HX-2 Temperature 0.995011 8.5 Direct




C1-C5 Pressure 20 12 Reverse

E-104 Temperature 0.510111 8.5 Reverse

T-101 Temperature (Nitrogen Composition) 10.491378 29.03999 Direct

T-103 Composition 8.81741 71.28 Reverse

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