Evaporative Recovery William J. McLay

Coming Glass Works Coming, NY

Reprinted from the materials submitted by Corning Glass Works for the Environmental Compliance & Control Course which was organized and presented by the

American Electroplaters’ Society. Copyright 1980.

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EVAPORATIVE RECOVERY Abstract

rative technology and the advantages and disadvantages various evaporator designs are discussed.

action and sizing, and on control of recycled impurities recovered baths.

Factors affecting orator design and operation are reviewed with particular asis on the impact of rinsing practice on evaporator

21.

LIST OF SLIDES

AES Slide Title Slide The Basic Recovery Loop Types of Evaporators Atmospheric Evaporator

aporator

Climbing Film Double Effect

Basic Heat Transfer Equation Heat Transfer in Liquids and Gases Basic Equation: Heating/Cooling of Liquids Basic Equation: Heat of Vaporization/Condensation Evaportation Energy vs. Vacuum Benefits of Vacuum Operation Counter Flow Cascade Rinsinq Pipeline Flow

25. Evaporator Sizing 26. Concentration in N 2 7 , Percent Capture Equatio 28. Fixed RR vs. No. Rinse Tanks 29. Fixed No. Rinses vs. Variable RR 30. Recoverable acid baths. 31. Recoverable Alkali 32. Contamina 33 (. Justifica 34, Energy Cos 35. Energy Costs/ 36. Plating Bath Value/Gallo 37. Potential Sa 38. Payback 39. Summary 40. Summary Continued All nghts m ~ " d P m t d m the Unitad Sutes 04 Amrltu. Thispubirot~nmaymrfbenpmdud. nordina".l u v r l m 01 tnrtMitud n whoh or In mrl. In any tom, w by a n y means. electronic. mechanical. photocop rscwdmp or 0 1 h . w l u . Vtthoui the prmr m k m r s * ~ ! of FES. rml Louarsna A m . Wintn Pu*. 32789

Copyright 1980, American Electr

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11.

INTRODUCTION

Economic incentive for more efficient use of water, chemicals and energy have made recovery and reuse of plating baths essential for profitable operation. Evaporative recovery technology has kept pace with this need. This course examines evaporation and the advantage and of various evaporator designs. The impact on evaporator size, recovery economics and trol are also discussed

disadvantages of rinse practice contaminant con- .

THE BASIC EVAPORATIVE RECOVERY SYSTEM

A typical evaporative recovery system is shown below:

I BASIC RECOVERY LOOP

,

SLIDE 3 .

Phrases like I f closed loop recovery, or "zero discharge" are frequently found in literature and advertisements. Despite the use of these phrases it is important to recognize that there is always tfsomethingff coming out of these systems. Note streams R, and P I . Stream R, will have a finite concentration which escapes from the system via drag-out. Where purification of recovered bath can be done, such as on chrome baths, removal of the contaminants from the purifier and disposal of these contaminants constitutes a break in the lfclosedlr loop.

In reality, absolute closed loop control of plating bath dragout is not practically achievable. Furthermore, it will be demonstrated that recovery and reuse of 90-99% of dragged-out bath can be quite easily and economically accomplished. Attemps to capture the remaining 1% will be very costly and therefore impractical.

And with some baths such as fluoborates, which cannot be easily purified, it may be desirable to allow a small percentage of the bath to escape to waste treatment in order to prevent impurities from buildina UD in the bath.

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I

111.

Recovery of a reasonable percentage of dragged-out bath is usually referred to as "partial recovery, I' versus the usually impractical but frequently referred to goal of total, closed loop recovery.

Before bath recovery is applied, bath contaminants are usually dragged out at a satisfactory rate. the bath also causes the contaminants to be retrieved with the bath so some form of bath purification or contaminant control is necessary.

Contaminant control is not the obstacle as it first ap- pears to be. Baths whose additive or brightener systems normally suffer electrolytic breakdown such as nickel or acid zinc and which must be periodically or continually carbon filtered should continue this treatment. Cationic contaminants are easily removed from chrome and chrome etch baths by use of a cation exchanger independently looped around the first rinse or feed tank to the evap- orator. (See Slide 3).

Contaminating cations in nickel baths are normally removed by periodic dummying, again, no change in standard pro- cedure. And in most cyanide baths, contaminating cations usually plate out with the base metal and are not a problem. Evaporators do not create carbonates but will recycle carbonates already formed by electrolytic breakdown of cyanide or absorption of CO, from the air, and previously lost through drag-out. So, treatment of cyanide baths for carbonate removal will continue as before, but perhaps more frequently.

Recovery of

EVAPORATION DEFINED

A plating bath is usually a water solution of inorganic and organic compounds. water in the rinse tanks. Separation of this rinse water (the solvent) and the original bath concentrate can be accomplished simply by boiling the diluted fluid under controlled conditions, usually under vacuum, to avoid destruction of any heat sensitive components.

The application of sufficient heat energy converts the water to a vapor. The water vapor is removed and con- densed back to the liquid state by contact with a cooled surface of a condenser.

Dragged-out bath is diluted with

Most technologists feel that separation of a solvent by phase change, i.e., changing a liquid to a vapor and back, is more demanding of energy, more energy intensive, other recovery techniques. costs vs. benefits are compared, evaporative recovery becomes very competitive and attractive.

than However, when total system

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IV.

When the simplicity, understandability and dependability of today's evaporators are recognized, it's clear why evaporation has achieved and maintained a dominant role against other recovery technologies.

EVAPORATIVE EQUIPMENT

A . TYPES OF EVAPORATORS

Four basic evaporator types including both atmospher- ic and vacuum designs are used by the metal finishing industry for recovering plating baths and rinse waters, and to reduce the volume of mixed liquid effluents for efficient and economical transporation to remote treat- ment and disposal sites.

The four basic types are shown in Slide 4.

SLIDE 4

There is a separate category of Specialized Evaporators Including Double Effect units which will be discussed.

B. HOW EVAPORATORS WORK

A n evaporator is a device which operates at either atmospheric pressure or more commonly under vacuum and uses heat energy to recover and reconcentrate dragged-out plating baths from plating rinse waters. A typical arrangement has already been shown in Slide 3 .

In some cases two or more of one particular evaporator design can be hydraulically coupled so the water vapor from the first evaporator becomes the heat source for the second evaporator which must operate at a lower absolute pressure or higher vacuum. Approximately . equal quantities of water are vaporized in each - evaporator (or each "effect"), thus a "double effect" unit evaporates twice the amount of water that a single evaporator, or single effect, does. This increased boil-off occurs at nearly the same cost in energy but at an increased equipment com- plexity and higher capital investment and maintenance costs.

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1. THE ATMOSPHERIC EVAPORATOR

Of the four bas i c designs t h i s i s the only one, as t he name implies , which opera tes a t atmos- pher ic pressure. genera l ly a cor ros ion- res i s tan t c y l i n d r i c a l tower, a s shown below.

An atmospheric evaporator is

ATMOSPHERIC EVAPORATOR

The tower is usua l ly constructed of f ibe rg la s s re- inforced p l a s t i c and conta ins a bed of loose ly dumped ceramic o r p l a s t i c packing mater ia l over which heated p l a t i n g r i n s e water is sprayed. Some water i s evap- ora ted from t h i s packed bed by a stream of a i r which i s drawn up through the wetted packing by a fan.

As t h e a i r i s drawn through t h e packing it is heated by the hot l i q u i d which it contac ts and which p a r t i a l l y vaporizes t h e l i q u i d and humidifies t h e a i r stream. The a i r stream is now humidified o r s a tu ra t ed .wi th water vapor a s a r e s u l t , and i s exhausted t o t h e atmosphere by t h e fan.

The "dewateredll o r l lconcentratedll r i n s e accumulates i n a r e s e r v o i r a t the bottom of t h e tower where it is mixed with incoming f r e s h r i n s e water and pumped a t a r a t e of 50 t o 200 ga l lons pe r minute o r s o , depending on t h e tower diameter, through a steam heated exchanger t o t h e top of t h e tower where it i s sprayed again over the tower packing. T h e steam heated exchanger usua l ly maintains t h e c i r c u l a t i n g f l u i d a t a temperature of 130-150OF. (55-65°C). By vaporizing some of t he water t h e exhaust a i r stream removes t h e hea t suppl ied by t h e hea t exchanger and t h i s i n tu rn lowers t h e f l u i d temperature t o roughly llO°F. (43OC). as w e l l a s t h e r i n s e water which i s vaporized.

T h i s hea t of vapor iza t ion is of course l o s t

2 - FLASH EVAPORATORS

Flash evaporators, as shown in Slide 6, operate under vacuum and consist of a boiler section, liquid circula- ting pump, a large flash or expansion chamber, a con- denser and vacuum source such as a vacuum pump or water eductor. This type of unit can be defined as a forced circulation, or forced thermosyphon, unit. Flash evaporators in the metal finishing industry usually operate at a high vacuum and consequently are rather large in size.

SLIDE 6

FLASH EVAPORATOR

The concept of flash evaporation requires the dilute feed liquid to be pre-heated prior to entry into the evaporator or to be heated by the evaporator reboiler to a temperature slightly above the boiling point corresponding to the vacuum which is maintained in

. the flash chamber. This overheated liquid enters the large expansion or flash chamber at an appreciable velocity, where some of it instantly vaporizes from the superheat. This vaporization takes heat from the circulating solution thereby reducing its temperature to that corresponding to the vacuum maintained by the vacuum source. The vacuum system also removes air and other non-condensable gases from the condenser. The resulting vapor passes through an entrainment knockdown device to the condenser where it is con- densed and normally pumped back to the rinse tanks. The recovered concentrate is either recirculated or pumped out of the unit back to the plating bath. special circumstances, such as hard chrome plating, when the amount of electrical energy used in the plating bath requires constant bath cooling or re- frigeration to maintain its proper operating tempera- ture, the bath can be fed directly to a high vacuum flash evaporator and does not require pre-heating. Using some of this excess bath heat can reduce the energy demand of a flash evaporator by roughly 20% of the heat required for a similar concentration rate in other evaporator designs.

In

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3. SUBMERGED TUBE EVAPORATORS

The most common submerged tube evaporator is a simple, integrated, low cost single vessel design which contains both the heating and condenser tubes or coils.

i

SLIDE 7

SUBMERGED TUBE EVAPORATOR

A water eductor operated by the cooling water stream from the condenser creates and maintains the vacuum. Plating rinse water is delivered to the bottom of the evaporator where it is evaporated by the steam or hot water heated submerged tubes. The resulting water vapor which is generated is condensed by the water cooled condenser coils at the top of the vessel and drips into an accumulator trough where a level con- troled pump returns condensate to the rinse tanks. Concentrate pumps also regulated by level controls return the recovered bath to the plating system.

4. CLIMBING FILM EVAPORATORS d

The climbing film design, sometimes called a rising film, as used by the metal finishing industry is actually a modification of the pure climbing film concept and is more accurately described as a vertical tube, natural thermosyphon evaporator. shown in Slide 8 , utilizes a steam or hot water heated vertical tube boiler, a liquid-vapor separator

vertical shell ,and tube water cooled condenser and a liquid ring vacuum pump.

This design,

. which also holds accumulated bath concentrate, a

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5

SLIDE 8

CLIMBING FILM EVAPORATOR

Rinse water is drawn by vacuum into the steam jacketed evaporator tubes where, because of the small volume of fluid involved, some water quickly evaporates and a high velocity mixture of water vapor and concen- trate enter the separator chamber, which allows the vapor to pass through a mesh entrainment knock-down pad to the condenser. Condensed water vapor is pumped back to the rinse tanks by the liquid ring vacuum pump which uses this recovered water to create and maintain the vacuum. Concentrate which accum- ulates in the separator is allowed to cycle back through the boiler to mix with incoming feed to be further concentrated. Chemicals contained in the incoming feed remian in the thermosyphon loop. When the concentration of this circulating fluid reaches a value preset in the control panel, the unit vents to atmosphkre, and discharges the concentrate to storage or to the plating tank. In this fashion a climbing film design functions as a continuous unit in terms of recycling recovered rinse to the rinse system, and as a batch unit in terms of periodic shutdown and discharge of recovered concentrate to the plating system.

DOUBLE EFFECT AND SPECIALIZED EVAPORATORS

Witn the exception of the atmospheric evaporator, the three vacuum designs can be provided as multiple effect units. A double effect evaporator is actually two evaporators of any one design connected in tandem. The first evaporator has no condenser but uses the boiler of the second evaporator as its condenser as shown in Slide 9.

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D O U L E - W C C T EVAPORITOR

SLIDE 9

TYPICAL DOUBLE EFFECT EVAPORATOR

r

The second evaporator must have its own vacuum system and must operate at a higher vacuum in order to achieve adequate boiling action. In this way the heat energy in the steam supply to the initial evap- orator is used twice, disregarding losses, which results in lower energy operating costs. However, the initial capital investment is a minimum of 60% higher and can be nearly twice the cost of a single effect evaporator. The complexity of a double effect unit demands more operator attention and the associated control and maintenance requirements are usually higher than for a single effect unit. You are actually opera- ting two evaporators which are hydraulically inter- connected and interdependent, one on the other, and the plating lines which the double effect system serves are consequently interdependent as well. trouble w i t h any segment, the entire system is affected.

If there is

For this reason most evaporators used in the plating in- dustry are single effect units. This is especially true where relatively untrained personnel will oversee the recovery system or low initial capital outlay is desired. Single effect evaporators are the simplest in design and therefore the easiest to understand, operate and maintain.

!

In very special circumstances with low corrosion baths and where bath and rinse water drag-out are high enough to justify the much higher capital cost, or where little or no stem or cooling water are available, a mechanical vapor recompression evaporator as shown in Slide 10 can be used.

SLIDE 10

MECHANICAL VAPOR RECOMPRESSION EVAPORATOR

In this design the water vapor created under vacuum is mechanically recompressed or pressurized by an electrically driven compressor. pressure vapor or steam is fed back to the boiler. While the mechanical recompression unit has the lowest operating cost of equivalent capacity single and double effect units the higher initial capital investment cost and its dependance on a complex mechanical compressor is a major disadvantage.

The resulting higher

Vapor recompressors are not readily available in small enough capacities to make them practical for typical plating rinse evaporators. Because of re- stricted materials of construction, they are .suited only for alkaline or neutral vapors. over of entrained droplets into the vapor stream to the compressor would cause severe corrosive damage in acid systems.

Even minor carry-

The last type of evaporator which should be mentioned is the wiped film evaporator. Wiped film units are used to concentrate viscous materials or to evaporate some substances to dryness. These units use a mechan- ical wiper or screw to spread and wipe the fluid to be concentrated o'n and off the walls of a heated cylinder. It is reported several metal finishing plants are using this type of unit to evaporate sludge to dryness.

QESIGN REQUIREMENTS

All evaporator designs, including the atmospheric type require heat energy to concentrate plating rinse waters. In the atmospheric evaporator the recirculating rinse water is maintained at an appropriate temperature (130-150OF or 55-65OC) by an external steam heated exchanger. Since evaporation requires heat, the heated rinse waters supply this energy.

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This heat helps vaporize water into the air stream. The vaporizing action in turn cools the rinse water sprayed over the packed bed. In this design the evaporated water and the heat energy required for the evaporation are both exhausted by a fan from the tower and are lost to the environment.

All other evaporators used for plating bath recovery are of the vacuum type and are designed to operate with either low pressure steam, common in most shops, or high pressure hot water.

As you have seen from the earlier description of the various vacuum units, each of these units has four common components:

SLIDE 11 2. Vapor Liquid Separator and

Concentrate Accumulator I

3. Vapor Condenser

4. Vacuum Source

3

Commonly, automatic controls are either all electric/electronic or a combination of electric/ pneumatic, depending on the manufacturer. In some units, separate reservoirs and pumps are required to remove the recovered bath and the rinse water for return to the plating system.

Regardless of which type of evaporator is selected, it must be designed for the bath it's intended to handle. Because of differences in bath chemistry, pH, and operating temperatures it is not possible to use a universal unit. Each evaporator must be designed for the material it will recover. Careful attention is given by manufacturers to materials of construc- tion, operating conditions (vacuum and temperature) and sizing of all components.

In most cases units are designed to operate with low pressure steam (3-15 psig) or high pressure (60-100 psig) hot water, and the quality of the condenser cooling water, whether it is municipal, recirculated well water or cooling tower water, must be considered when sizing the vapor condenser.

Today, most evaporators are p re fab r i ca t ed , come com- p l e t e with con t ro l s i n s t a l l e d , and a r e capable of a f u l l y automatic, f a i l s a f e operat ion.

D. SERVICESflTILITIES REQUIRED

SLIDE 12

Heat Enerqy: Steam is the most common source of h e a t energy used i n i n d u s t r i a l evaporators . Since most metal f i n i s h i n g shops operate with low pressure steam b o i l e r s (15 p s i g ) , most p l a t i n g r i n s e evaporators a r e designed t o opera te a t 3-15 p s i g steam pressure .

Very high vacuum evaporators may n o t r equ i r e steam pres- su re beyond 3-5 ps ig . Since t h e b o i l i n g p o i n t a t high vacuum i s q u i t e low, i n t h e order of llO-lOO°F, (43-38OC) f o r a 27-28" Hg vacuum, the hea t ing sur face shouldn ' t be more than 50-60°F (22-28OC) h o t t e r t o avoid unnecessary chemical breakdown o r s c a l e build-up.

I f your typ ica l1 sidered

shop u t i l i z e s a high pressure steam b o i l e r , -y 70-100 p s i g , (anything over 15 p s i g i s con- high pressure and r equ i r e s a l i censed s t a t i o n a r y

fireman f o r opera t ion) t h e steam pressure must be re- duced t o 15 ps igor lower by means of a pressure reducer before in t roduct ion i n t o the evaporator b o i l e r .

Pressure reduct ion of high pressure steam usua l ly pro- duces superheated low pressure steam. This i s low pres- su re steam a t a temperature higher than i t s condensing temperature. The energy re leased i n reducing t h e pressure inc reases t h e temperature of t h e lower pressure steam. This superheat , o r excess h e a t , i f n o t removed, w i l l overheat t h e evaporator b o i l e r sur faces . De-superheaters are used f o r t h i s purpose and a r e nothing more than water spray nozzles mounted i n t h e reduced pressure steam l i n e . The w a t e r spray vaporizes a s it absorbs the excess hea t from the superheated steam thus lowering t h e o v e r a l l steam temperature and a c t u a l l y c r e a t i n g more low pressure steam.

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i Proper steam pip ing p r a c t i c e should be observed t o main- t a i n c lean dry s a t u r a t e d steam a t the evaporator b o i l e r steam i n l e t .

High pressure ho t w a t e r can be u s e d i n p lace of steam b u t t h i s i s usua l ly done only on smaller i n s t a l l a t i o n s . pound of ho t water does n o t conta in the quan t i ty of hea t which a pound of steam does. condenses it r e l e a s e s approximately 1000 BTU's, which i n t u r n can evaporate approximately 1 pound o f r i n s e water. For example, one pound of ho t water a t 180°F ( 8 2 O C ) cooled t o 80°F ( 2 7 O C ) would release only 100 BTU's. Thus t e n times as many pounds of h o t water would be needed and must be pumped through the evaporator as pounds of steam, although t h e a c t u a l r i n s e water evaporating energy required, 1000 BTU's, would be t h e same. Hot water is t y p i c a l l y ava i l ab le up t o 100 p s i g and a t 160-180OF ( 7 1 - 8 2 O C ) although water a t 100 p s i can be obtained i n some i n s t a l l a t i o n s as high a s 210°F (99OC) o r higher .

A

When one pound of steam

Condenser Coolinq Water: from municipal water supp l i e s , w e l l s o r cool ing tower r e c i r c u l a t i n g systems. In any case, depending on the opera t ing vacuum and t h e allowable cool ing water t e m - pera ture rise, apF*$x t e l y 20 t o 50 ga l lons of cool ing w a e r a r e r equ i r e i n s i n g l e effect evaporator t o con- dense and c W C n e - ' " g a l l o n of r i n s e water. w r t e r i- ki--tke process and can be used elsewhere t o conserve energy.

Process water q u a l i t y should be monitored t o minimize o r prevent hard water depos i t s o r a lgae from impairing the performance of t h e condenser.

Electric Power: Power is requi red t o d r ive vacuum pumps o r pumped eductors , and feed o r product pumps and f o r t h e automatic c o n t r o l system. Voltages are t y p i c a l l y 110/220/ 440, t h r e e phase, 60 h e r t z and power consumption while usu- a l l y a small f r a c t i o n of recovery c o s t s , w i l l vary depending on t h e type of evaporator used and i t s operat ing parameters. For example, evaporators opera t ing a t higher vacuums w i l l consume more energy t o achieve and maintain t h a t vacuum than evaporators opera t ing a t lower vacuums. A common misconception is lower opera t ing temperatures (higher vacuum evaporat ion) saves energy.

Cooling water may be obtained

This Cooling

A i r : - A i r is gene ra l ly used only i n those u n i t s whose con- t r o l systems employ pneumatic sensors and valve ac tua to r s . Generally, a f e w cubic feet pe r hour of s tandard, c lean f i l t e r e d o i l - f r e e a i r a t 30 p s i g is a l l t h a t i s required.

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V. "DAMENTALS OF HEAT TRANSFER

MAJOR HEAT TRANSFER COMPONENTS

SLIDE 13

Boiler Section

Condenser Sectlon

The two major evaporator components which u s e o r t r a n s f e r h e a t energy, a r e t h e b o i l e r and the condenser. Both a r e usually o f s h e l l and tube design although the submerged tube evaporator simply uses steam tubing submerged i n a pool of l i q u i d i n the bottom of the evaporator.

Since an evaporator is a device which u t i l i z e s hea t energy i t ' s important t o understand the manner i n which h e a t energy i s con t ro l l ed and used.

The job of t he b o i l e r i s t o t r a n s f e r hea t from steam o r hot water t o t h e r i n s e so lu t ion causing it t o b o i l . The water, which is t h u s vaporized leaving behind a more concentrated l i q u i d , i s t r a n s f e r r e d t o another s h e l l and tube hea t ex- changer, ( a water cooled condenser). H e r e t h e vapor is condensed giving up i t s hea t t o t h e cooling water.

In each case, t h e hea t energy flows from the hot f l u i d t o t h e co lder f l u i d . Under n a t u r a l condi t ions hea t only runs downhill. That is , it always moves from a ho t body t o a co lder one. The d r iv ing force is t h e temperature d i f f e r - ence between two bodies of mat ter . The g r e a t e r t h e d i f - ference, o r AT, t h e f a s t e r t h e r a t e of hea t t r a n s f e r . N o h e a t energy i s t r a n s f e r r e d between two b o d i e s ' a t t h e same temperature.

There a r e r e s i s t a n c e s t o t h i s t r a n s f e r of hea t . Some ma te r i a l s conduct hea t bet ter than o thers . a r e so poor i n t h e i r a b i l i t y t o t r a n s f e r h e a t they a r e used as hea t i n s u l a t o r s . The - fac to r s governing t h e mechanism and e f f i c i e n c y of h e a t t r a n s f e r a r e f a r beyond the scope of t h i s course. The manufacturer must s e l e c t t he proper ma te r i a l s and design them i n such a way t o minimize t h e r e s i s t ances t o h e a t t r a n s f e r . force , o r AT. The AT must be g r e a t enough t o overcome any r e s i s t ances t o h e a t t r a n s f e r , and s ince i n our case the re i s a sur face ( a r e s i s t a n c e ) separa t ing the hot f l u i d s from the cold f l u i d s , t h e sur face a rea must be l a rge enough t o permit t h e amount of h e a t f l u x adequate t o do the evapora- t i o n job required.

Some ma te r i a l s

There must be a dr iv ing

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Transferred hea t is measured i n terms of BTU's p e r hour, o r Q .

The b a s i c equation is: 1 I I

BASIC HEAT TRANSFER EQUATION

SLIDE 14

9 = UAAT

where:

Q = heat transferred In ETU/hr U = 8 specfailzed computed factor depending on the

flulda belng handled, the reslstances Involved and the design of the heat t rade r devlce.

A = the surface area across which heat energy Is flowing.

AT = the temperature dlfference In O F (the thermal drlvlng force).

A. Heat Enerqy

SLIDE 15

HEAT ENERGY TRANSFER INVOLVES:

1. Heatlng & Coollng of Flulds

2 Phase Change of Flulds Vapor to Liquld Liquld to Vapor

Heating o r Cooling of Liquids: water one Farenhei t degree requi res one BTU of h e a t

To h e a t one pound of

energy. Conversely, t o cool one pound of water one Farenhei t degree, a BTU of hea t energy must be removed.

The amount of h e a t energy added o r removed t o hea t o r cool a f l u i d t h i s one Farenhei t degree i s the hea t capac i ty value of t h e f l u i d . For water, t h i s value ( C p ) i s 1 . 0 BTU/lb/OF. For o ther f l u i d s the value w i l l be higher o r lower than 1 .0 depending on the f l u i d . Therefore, t o c a l c u l a t e t h e thermal load Q, f o r hea t ing o r cool ing one need only know i t s hea t capac i ty va lue , t h e number of degrees of hea t ing o r cool ing requi red and t h e mass, o r pounds of f l u i d being so t r e a t e d .

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The formula is:

BASIC EQUATION HEATING-COOLING LlOUlDS

Q = m C , U Z - T ~ ) SLIDE 16

= total heat load In BTU m = pounds of fluid C, = heat capaclty of Nuld

TI = lower temperature, O F < T2 = hlgher temperature, "F

\

The t o t a l h e a t load f o r heat ing o r cool ing a f l u i d i s known a s the "sens ib le heat" load. A temperature change always occurs during sens ib l e h e a t t r a n s f e r .

Heat of Vaporization: Compared t o t h e sens ib l e hea t load f o r heat ing o r cool ing f l u i d s , t h e hea t energy required t o vaporize o r condense t h a t f l u i d , i s s u b s t a n t i a l l y g rea t e r .

Vaporizing a l i q u i d , o r condensing a vapor involves a phase change of mat ter . In t h i s case w e a r e e i t h e r converting a l i q u i d t o a gas, o r a gas t o a l i q u i d . The amount of energy required t o do t h i s , t o convert one pound of l i q u i d water t o water vapor a t atmospheric pressure, i s 970 BTU. This is known a s the hea t of vaporizat ion. This va lue d i f f e r s f o r d i f f e r e n t f l u i d s . Likewise, if a pound of water i s condensed a t atmos- phe r i c pressure , it would r e l e a s e these 970 BTU's of h e a t energy.

BASIC EQUATION VAPORIZATION-CONDENSATION

i

i

SLIDE 17 : Q = K = H r

Q = Heat Load In BTU H, = Heat of Vapodzatlon- H, = Condensutlon In BTU

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This i s what happens when steam condenses, o r when evap- o ra t ed water condenses i n the condenser. I n an evaporator t h e hea t of condensation given up by a pound of steam i n the evaporator b o i l e r becomes the hea t of vaporizat ion of roughly one pound of water from t h e p l a t i n g r i n s e . T h i s pound of water vapor then gives up i t s hea t of condensa- t i o n t o the condenser cooling water which is used i n suf- f i c i e n t quan t i ty t o avoid vaporizat ion b u t undergoes a temperature increase , a s ens ib l e hea t rise. Thus, two phase changes requi r ing and evolving hea t energy i s the mechanism by which an evaporator can separa te water from d i l u t e p l a t i n g r i n s e s .

I n most cases t h e r e i s nothing o the r than water which i s heat v o l a t i l e i n p l a t i n g r i n s e s so a c lean separa t ion can occur.

.

B. Role of Vacuum

Why i s vacuum used f o r evaporative recovery of p l a t i n g baths? I t does no t , a s some manufacturers seem t o imply, save energy. operat ion. t o achieve and maintain a higher vacuum. When considering the t o t a l h e a t load of both heat ing r i n s e water from room temperature t o a b o i l i n g po in t a t a s p e c i f i c vacuum, and then vaporizing t h a t water a t t h a t vacuum, t h e combined sens ib l e hea t and h e a t of vaporizat ion do decrease s l i g h t - l y a s vacuum is increased. However, when t h e addi t iona l energy required t o c r e a t e and maintain the higher vacuum is added, t h e o v e r a l l energy consumption increases s l i g h t - l y a s vacuum i n increased.

There i s a p r i c e t o pay f o r higher vacuum I t takes approximately 10-15% more energy

For a given evaporation r a t e , a high vacuum evaporator must be phys ica l ly l a r g e r t o accommodate t h e l a r g e r volume of lower dens i ty vapor t h a t i s generated. T h i s phenomen is shown i n S l ide 18, which l is ts the temper- a t u r e , h e a t of vapor iza t ion and s p e c i f i c volume re l a t ion - sh ips of water vapor a t increas ing vacuums. a l s o shows t h a t it requ i r e s more hea t energy t o vaporize water a t high vacuum than a t atmospheric pressure, about 10% more a t a vacuum of 0.5 ps ig .

S l ide 18

SLIDE 18

I

I -17-

So why operate at higher vacuum? Increasing the vacuum, or decreasing the absolute pressure in an evaporator, does several positive things:

SLIDE 19

Recovers more Heat-Senritlve Components Avoids Need for Feed Pump

,

High vacuum recovery is thermally gentle to heat sen- sitive, low temperature baths and very well justified when bath or disposal costs, or both, are high - in the order of $2.00 - $2.50 per gallon or more.

VI. EVAPORATOR SIZING AND RINSE PRACTICE

Although rinse theory and rinse technique is not the subject of this paper, a proper understanding of rinse theory and practice is essential for intelligent sizing and application of evaporative recovery systems. timized operation of existing rinse systems or the design of new rinse systems following a plating bath has a signi- ficant impact on evaporator capacity when the bath is to be evaporatively recovered.

Op-

The number and efficiency of rinse tanks determines the size of evaporator required. A good multi tank, air agitated, countercurrent rinse system requires a smaller evaporator for a given dragout rate than a less efficient high volume single tank system. It's that simple.

A. Rinse Theory: The objective of rinsing plated parts is to achieve and maintain clean, chemical-free active surfaces. This is easy to accomplish with high volumes of rinse water.

-18-

-

However, w i t h recovery a l l r i n s e waters must be pso- cessed through the evaporator, and t h e more water t h a t i s used, t h e l a r g e r w i l l be t h e evaporator and i ts assoc ia ted operat ing c o s t s . Therefore, r i n s e water conservation, cons i s t en t w i t h adequate c lean r i n s i n g is the f i rs t s t e p toward p r a c t i c a l and econ- omical evaporative recovery. T h i s i n t u r n requires an understanding of the advantages of multi- tank counter c u r r e n t a i r ag i t a t ed r in s ing . There is some semantic confusion concerning the phrases used t o describe r i n s e systems, so l e t ' s def ine our t e r m s :

SLIDE 20

1. Counter Flow: Simply means r i n s e water i s flowing i n a d i r e c t i o n genera l ly opposi te t o the flow of dragged i n chemicals. One cannot assume from this phrase alone t h a t t h e flow of ba th chemicals and r i n s e water are hydrau l i ca l ly op- posed o r t h a t r i n s i n g i s occurr ing o r being accomplished with a minimum quan t i ty of water.

2 . Cascade Rinsing: Cascade r i n s i n g is best descr ibed by Slide 2 1 .

SLIDE 21

-19-

This approach uses two or more rinse tanks hydrauli- cally interconnected but allows fresh water to skim across the tops of each rinse tank and overflow into the preceding tank without assurance of good mixing.

3. Countercurrent Flow: A method of hydraulically con- necting two or more rinse tanks generally referred to as pipeline flow, such that the water flow is hydraulically opposed to the flow of dragged in chemicals. possible at hydraulically opposed pipeline flow, good mixing, and, by repeated multi-stage use, a reduction in the quantity of water required for ef- fective rinsing.

This arrangement aims as nearly as

SLIDE 22

PIPELINE FLOW

The proper arrangement of a three-tank air agitated countercurrent rinse system is shown below.

SLIDE 23

THREE STAGE AIR AGITATED COUNTER- . CURRENT RINSE SYSTEM

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This tank arrangement, while conserving water and achieving quality rinsing, also pre-concentrates dragged-out bath in the first rinse tank by effec- -

tively I1pushing1l dragged-out bath chemicals back into this rinse tank. This pre-concentration, coupled with the reduced rinse water requirement, decreases the necessary evaporating capacity of an evaporative recovery system which of course will minimize its initial capital cost as well as its operating expense.

The acceptance and implementation of countercurrent rinsing techniques has greatly improved the economics of bath recovery, and while very effective by itself, can occasionally be enhanced by employing auxilliary techniques to reduce bath drag-out, or improve rinsing efficiency, such as an air knife or fog rinse directly over the plating bath or spray or fog rinsing directly over the rinse tanks. Remember, the objective is to reduce bath drag-out and the volume of rinse water required.

The quantity of rinse water flowing through a proper- ly designed and operated rinse system in gallons per hour, divided by the bath drag-out in gallons per hour is called the Rinse Ratio:

Rinse water Rate, GPH

Bath Drag-Out, GPH RINSE RATIO = SLIDE 24

Since all the water passed through a countercurrent rinse system must be processed through the evaporator, this then sets the stage for a discussion of evaporator sizing.

B. Evaporator Sizing

Once the effect of rinse water conservation is under- stood and accepted, sizing an evaporator for a specific plating system becomes a simple matter. Evaporator size, or boil-off capacity is the product of bath drag-out in GPH times a rinse ratio which will result in an acceptable or specified minimum concentration of the last countercurrent rinse tank for good, clean rinsing.

I I -21 -

The following data should be gathered:

1. Bath Drag-out in Gallons Per Hour

A reasonable estimate of drag-out can be made by using a still tank filled with fresh D.I. water and analyzing the concentration increase over several hours time.

Drag-out can be computed in a number of ways. For example, in decorative chrome plating its accepted that drag-out is approximately 80-90% of bath make-up chemical consumption. So, if bath addition records are available, drag-out can be estimated knowing the number of operating hours a year.

The same holds for nickel baths which usually only lose boric acid through drag-out. Since boric acid is carried in the bath at about 6 oz/gal., bath additions and yearly operating hours will again yield an approximation of nickel bath drag-out.

2 . Number of Rinse Tanks

Next, determine the number of air agitated rinse tanks following the bath which are available for countercurrent flow. Generally, a reasonable rinse ratio through a minimum of three or more countercurrent rinse tanks is 15:l.

The estimated evaporator boil-off capacity is then determined by multiplying the rinse ratio x bath drag-out in GPH.

SLIDE 25

Assuming 100% mixing in each rinse tank, the concentration of the last rinse tank, or any tank, in a multitank countercurrent flow rinse system can be estimated with the following simplified equation:

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SLIDE 26

I

!

I

1 I

t

An estimate of percent capture o r recovery can l i k e - w i s e be made using the following s impl i f i ed equation:

SLIDE 27

I t is clear t h a t t h e g r e a t e r t h e number of counter- c u r r e n t flow r i n s e tanks ava i l ab le which can be g r a v i t y flowed o r pumped, t h e less r i n s e water i s \ requi red and t h e smaller would be the evaporator.

However, from a p r a c t i c a l viewpoint, e s p e c i a l l y i n e x i s t i n g i n s t a l l a t i o n s , l a r g e numbers of counter- c u r r e n t , a i r a g i t a t e d r i n s e tanks are usua l ly not ava i l ab le , and t h e c o s t of both floor space and equipment modif icat ions t o i n s t a l l them f a r outweighs the opera t ing c o s t of a l a r g e r evaporat ive recovery system.

So t h e t rade-off i s evaporator capac i ty vs. quant i ty of r i n s e tanks. This trade-off i s demonstrated by t h e examples shown i n S l ides 28 and 29 . '

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,'-

OWEN.

FMCD RFNSE RATIO IEVAFORATOR SIZE) vs WUMsm os aiww TANKS

9 DECORATIVE CHRDME BATH m o z m ~ cm, lSO21GAL Cr+'

ORAGOUT 1 GPH 0 W TO FOUR CC RINSES AVAILABLE 0 RINSE RATIO 15 1

THEORETICAL RECOVERY

FIXED NUMBER OF RINSE TANKS rr WNSf RATIO IEVAPORATOR SIZE)

GIVEN 9 DECORATIVE CHROME BATH MOZIGAL Cm, 15 OZIGAL C P

*DRAGIN 1OPH .TWO CC RINSE TANKS 0 RINSE RATIO VARIABLE

REWIRED EVAPORATOR CAPACITY

16 GPH

120 GPH

SLIDE 28

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SLIDE 29

The p re fe r r ed arrangement f o r maximum capture and m i n i m u m c a p i t a l investment i s a t l e a s t a t h r e e s t a g e countercur ren t r i n s e system which y ie lds :

Rinse Rat io N o . 3 Rinse (Evap. Boi l -of f )

Theo.conc. ppm C r + 6 GPH

33.0 4 .2

I

15 30

O f course with higher r i n s e r a t i o s t h e number t h r e e r i n s e tank concentrat ion can be f u r t h e r depressed.

C . Draq-out vs . Draq-in: Hot vs . Cold Baths

Natural evaporation of water from a p l a t i n g ba th which normally opera tes warm o r hot , such as chrome, n icke l o r cyanide copper, c r e a t e s room i n the p l a t i n g tank t o allow the r e t u r n of concent ra te recovered by the evaporator from the r i n s e system.

But many cool o r room temperature ba ths such as cadmium cyanide o r ac id z inc ch lo r ide , c r e a t e no such room i n t h e p l a t i n g tank from na tu ra l evaporat ion t o allow r e t u r n of recovered bath. In f a c t , t he s i t u a - t i o n i s f requent ly worsened by the lower su r face tens ion of t h e ba th compared t o t h a t of t h e preceding r i n s e water. A g r e a t e r volume of r i n s e water is dragged i n t o t h e p l a t i n g tank than t h e volume o f ba th dragged o u t .

This dragged i n water, as well as adequate addi t iona l water, can be removed by evaporating some of t h e bath i t s e l f along with t h e r i n s e waters. In t h i s way ba th volume is con t ro l l ed and there is room t o r e t u r n recovered concentrate .

VII. RECOVERABL ,E BATHS

SLIDE 30

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SLIDE 31

There are few limitations on the applicability of evapora- tion. Consequently, its popularity and use as a recovery technique is increasing.

Most common plating baths are being routinely recovered for reuse by evaporation. of commonly hauled-away mixed liquid waste waters is sharply reducing transporation charges which currently range from $0.35 to $1.00 or more per gallon.

And evaporative consolidation

Where "single nickel" plating is done, i.e.; either Watts (dull), Woods nickel semi-bright or bright nickel, each individual bath can readily be recovered by evaporation. However, when "dual nickel" systems are used sequentially, 1.e.; either dull or semi-bright and bright, recovery by evaporation cannot readily be done unless the two baths are separated by a rinse system, which is usually not the case, to allow separate recovery of the individual baths. The semi-bright bath is usually dragged directly into the bright bath with no rinsing between. The recovered bath from a bright nickel system cannot be returned to a dull or semi-bright nickel bath. Dragged-out additives from a dull or semi-bright nickel bath won't harm a bright nickel bath. But bright nickel contains sulfur bearing additives which, if recovered and returned to the semi-bright or Watts bath, would be harmful. And because of the drag-in, drag-out hydraulics of dual nickel systems usually the only place to put recovered bright nickel bath is back into the semi-bright or dull nickel tank.

The reason is chemical.

Some baths aren't evaporatively recoverable because they either decompose chemically or aren't economically worth recovering. For example, ammoniated alkali zinc, electro- less copper and copper etchants used in printed circuit work, evolve ammonia gas when heated even slightly. The evolved ammonia, besides upsetting the bath composition, can create a potentially severe atmospheric pollution problem. recoverable as bath since there is usually little active nickel value in the rinse waters and the bath itself becomes spent rather quickly.

And electroless nickel baths aren't economically

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Certain baths such as low pH cyanides will lose a small percentage of cyanide. chloride, some silvers, or citrate gold may lose a portion of some of the bath additives during evaporation. These additive fractions are usually steam volatile and distill over with the water to the condensor and are condensed in the evaporator. It is a simple matter to adjust the re- covered concentrate accordingly before returning it to the plating tank.

Other baths such as acid zinc

VIII.TREATMENT OF RECYCLED BATH IMPURITIES

SLIDE 32 1

While covered in greater detail elsewhere in this lecture series the key methods of treatment and regulating impuri- ties recycled by evaporative recovery merit a brief review.

Because cation impurities transferred to most plating baths via dropped work or racks and by drag-in are also dragged out, they don't usually accumulate in the bath to a harmful level.

When cation contamination becomes a problem there are a number of existing "bath housekeeping'' routines such as dummying, chemical precipitation, or in some cases treat- ment with a cation exchanger, which are employed by platers to either remove or control these impurities at a tolerable concentration.

Other types of impurities, such as organics from additive or brightener breakdown which is caused by electrolytic action in the bath, or the accumulation of dirt or solid particles, can be routinely treated by standard carbon or particle filtration methods.

When a recovery system is installed, impurities previously lost through drag-out are returned to the plating tank along with the recovered bath. It is for this reason that some people feel evaporators Ilmake" impurities.

-27-

For example, i n cyanide ba ths , carbonates a r e formed by absorption o f CO, from the a i r by the highly a l k a l i n e baths and by e l e c t r o l y t i c breakdown of t h e cyanide r a d i c a l during p l a t i n g and these carbonates a r e dragged o u t along with the bath. An evaporator w i l l r e t u r n t h e dragged-out carbonates along with the ba th s o t h e rou t ine t reatment of chemical o r thermal ( c h i l l i n g ) p r e c i p i t a t i o n may have t o be done more f requent ly . The important p o i n t t o recognize here i s t h a t evaporators d o n ' t make carbonates , they simply recyc le carbonates.

O t h e r myths p e r s i s t . One involves n icke l ba ths . The myth is t h a t vacuum evaporation breaks down organic br ighteners t o harmful contaminating organic by products. Again, t h i s i s not t h e case. The e l e c t r o l y t i c ac t ion of the ba th causes the decomposition of the add i t ives .

An evaporator r e t u r n s these mater ia l s along with recovered ba th and recovered undamaged add i t ives t o t h e bath so more frequent carbon f i l t r a t i o n i s required. In many i n s t a l l a - t i o n s , however, carbon f i l t r a t i o n of n i c k e l baths i s con- t inuous, and seve ra l u s e r s of n icke l recovery evaporators have reported br ightener consumption is a c t u a l l y decreased by 10% o r more s i n c e normal drag-out a l s o includes add i t ives which a r e no t a f f ec t ed by t h e e l e c t r o l y t i c ac t ion of t h e bath. These a d d i t i v e s a re recovered.

A d e t a i l e d review of both advantages and disadvantages of evaporation and of t h e ind iv idua l evaporators covered by t h i s l e c t u r e are included i n t h e Appendix A-G.

Note: In t h e d iscuss ion given i n t h e next s e r i e s of pages on eva lua t ing opt ions the re a r e no s l i d e s provided.

I X . EVALUATING OPTIONS

Assuming you have an adequate r i n s e system and ba th drag- ou t t o j u s t i f y evaporative recovery, as opposed t o waste t reatment , ana lys i s and s e l e c t i o n of evaporative recovery systems, o r of an e n t i r e recovery/waste t reatment system, should begin w i t h w e l l established w r i t t e n OBJECTIVE which is based on condi t ions i n your p a r t i c u l a r p l a n t and the e x t e n t t o which you a r e , o r a r e no t , cu r ren t ly i n compli- ance. Your general ob jec t ive should be t o achieve com- p l i ance without s a c r i f i c i n g p l a t i n g con t ro l and q u a l i t y a t t he l e a s t o v e r a l l lfcostl l t o your company. (Cost and p r i c e are n o t t h e same th ing . More on t h i s l a t e r ) .

The re -a re hundreds of quest ions which can be proposed a t t h i s s t a g e , b u t a few of t h e more important ones you should cons ider a re :

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*

Is a new or existing plating line, machine or plant involved? Fitting a recovery system to an established process or plating machine with its typical limited rinsing cap'acity and space is one thing. Planning a completely new system allows inclusion of ample space and counter-current rinses.

What impurities are likely to occur. in the recovered bath and how will they be controlled or removed?

What is the overall space (plan area and height) requirement for the evaporator and any auxilliary equipment?

Can the existing steam boiler and cooling water supply handle the added load?

What percentage recovery is theoretically achievable in your system and will the remainder fall within compliance limits or must it be waste treated?

What is the prospective evaporator supplier's track record on baths similar to yours?

What is the projected installed cost of the evaporator and associated equipment?

What are the projected operating and maintenance costs?

What tion

is your prospective manufacturer/supplier reputa- for:

delivery product quality performance and mechanical guarantee operator and maintenance training and support field service spare parts availability

As you assess these and other data pertinent to your analysis keep in mind the difference between price and cost. Every decision has a trade-off, a cost. A low priced system or a highly energy efficient system may not necessarily be the lowest cost system in the long run once all factors are considered. The lowest operating cost per gallon of bath recovered may not be a worthy or practical objective if its achieved at a higher initial capital cost, or at the expense of constant operator and mainte- nance attention.

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X. RECOVERY ECONOMICS . -

JUSTIFICATION FOR EVAPORATIVE RECOVERY

SLIDE 33 Bath Costs/Gal. Waste Treatment COSVG8l. Sludge Disposal - Cost/Gal.

The j u s t i f i c a t i o n f o r evaporative recovery depends on t h e bath drag-out r a t e i n a manner s imi l a r t o which t h e s i z i n g of t h e evaporator (a l ready discussed) i s keyed t o ba th drag-out. The p l a t e r can determine t r u e ba th drag-out c o s t a s shown i n S l ide 33.

The t o t a l of t hese c o s t s represents t h e constant cash d ra in f o r every ga l lon of bath dragged ou t and no t re- covered.

The EPA has published a summary r epor t on Evaporative Re- covery containing a very d e t a i l e d sec t ion on recovery economics, including methods f o r evaluat ing R O I , d i s - counted cash flow r a t e s of r e tu rn , e tc . While some of t h e p r i c ing and operat ion c o s t da ta may requi re updating t h e reader i s urged t o consul t t h i s r e p o r t f o r an explanation of t h e methodology used f o r t h i s type of ana lys i s should it become necessary.

For t h e purpose of t h i s discussion a s impl i f ied approach f o r t h e j u s t i f i c a t i o n of recovery over waste treatment w i l l be presented.

Dragged ou t ba th represents a constant operat ing c o s t - a cash d ra in . per operat ing year , t h e c o s t of t h e recovered ba th p l u s t h e assoc ia ted reduct ion i n waste t reatment , sludge de- watering and d isposa l charges represent a before t a x savings which can t y p i c a l l y o f f s e t t h e i n s t a l l e d system and year ly operat ing c o s t of an evaporative recovery system wi th in two years o r less on average.

S l ide 34 represents the t y p i c a l c o s t i n t h e North Eas t United S t a t e s i n Apr i l , 1980 t o evaporate one ga l lon of water i n a s i n g l e e f f e c t evaporator.

Depending on t h e amount of ba th dragged o u t

SLIDE 34

And S l i d e 35 shows these r e s u l t s i n terms of u t i l i t y c o s t s a t a r i n s e r a t i o n of 15:l (evaporator bo i l -of f of 15 GPH w a t e r ) t o y i e l d the hourly u t i l i t y c o s t s per gal lon of BATH recovered!

SLIDE 35

S l i d e 36 t o t a l s t y p i c a l b a t h values and waste t reatment c o s t s f o r some common ba ths .

SLIDE 36

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' - With a proper rinse ratio a two or three stage counter-current rinse system will yield a theoretical capture percentage of 98-99%, so for all practical purposes one can assume 100% capture. Subsequently, Slide 37 relates the net dollar return and net percent return per gallon of dragged out bath based on a conservative average recovery cost of $0.73 per gallon of bath as neither sludge dewatering nor disposal charges are included.

SLIDE 37

For a detailed return-on-investment (ROI) computation these values would be considered "before tax" returns , multiply ratios by the estimated yearly drag-out will result in a total estimated yearly dollar savings. the projected combined installed and yearly operating cost will at least help you determine whether or not your actual or predicted bath drag out rate is sufficient to justify such a system.

This value when compared to

Typically the break even point for a bath with an average over- all cost of $5.00, (bath plus treatment and disposal Costs), which is quite conservative, for a single shift operation (the worst case) is approximately one gallon of drag o u t per hour. This will result in approximately a 2.5 to 3.0 year payback as shown below.

SLIDE 38

XI. SUMMARY

SLIDE 39

Evaporative equipment and technology have g r e a t l y improved s ince the f i rs t t r i a l by t h e metal f i n i sh ing indus t ry ap- proximately 30 years ago. Evaporation has become a f i rmly e s t ab l i shed and r e l i a b l e i n d u s t r i a l procedure with very broad appl ica t ion f o r recovering p l a t i n g baths and r i n s e waters f o r reuse.

The chemistry of most baths w i l l e a s i l y accommodate evapora- t i v e recovery. Successful app l i ca t ion and economic j u s t i - f i c a t i o n f o r specif ic baths depends on f a c t o r s shown i n S l ide 4 0 .

SLIDE 40

These th ree f a c t o r s determine t h e s i z e evaporator required. These f a c t o r s i n t u r n a r e f u r t h e r a f f ec t ed t o some e x t e n t by whether an e x i s t i n g o r a new p l a t i n g machine, l i n e , o r p l a n t i s being considered.

Hundreds of evaporat ive recovery systems a r e i n operat ion, no t only recovering baths and r i n s e waters b u t providing t h e added b e n e f i t s of lowering ove ra l l p l a t i n g cos t s and keducing the amount of p o l l u t a n t s t o be t r e a t e d .

L

Vacuum evaporators also recover rinse water for reuse which would otherwise have to be processed through waste treatment.

Evaporators reduce waste treatment costs by substan- tially reducing the quantity of sludge which is genera- ted and which must ultimately be disposed of in legal and secure landfill.

Most baths accommodate evaporative recovery without serious detriment to the bath chemistry. Accumulated impurities can be controlled by commonly practiced treatments and reasonable attention to standard bath maintenance practices.

By proper vacuum selection degradation of any heat sensititive bath component or scale formation is mini- mized or prevented.

Evaporators can also concentrate mixed process effluent, such as ion exchange regenerant solutions and washes, to reduce disposal or haul-away volumes and associated transport and/or storage costs.

Numerous items can be tabulated and debated when the merits of various evaportor designs are considered. Following is a summary of the generally acknowledged and accepted charac- teristics, both positive and negat- of the major evap- orator designs used in the metal finishing industry.

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o w

ADVANTAGES

Atmospheric Evaporator

Doesn't use vacuum. Runs a t atmospheric pressure.

U s e s no cool ing water o r condenser.

Low c o s t . simple concept.

Useful on baths l i k e hard chrome where l a r g e amounts of ba th hea t i s generated and must be removed. A c t s as a b a t h c h i l l e r i n such a case.

While most u se fu l with hard chrome ba ths , can recover ba th vapors and d rop le t s drawn i n t o vent ing duc ts , from any ba th by combining a i r scrub- b e r and evaporator i n one u n i t .

Typical opera t ing range is 130-150°F (55-65OC) although c e r t a i n feed so lu t ion may r equ i r e hea t ing t o a s high a s 170°F (77OC). Can be used f o r s o l u t i o n s which decompose a t e leva ted temperatures and which do no t r e l e a s e t o x i c o r v o l a t i l e components t o t h e a i r stream.

The low humidity i n a r i d regions makes t h i s design a t t r a c t i v e e s p e c i a l l y where heated ba ths on r i n s e water can supply a l l t he hea t of vapor iza t ion d i r e c t l y .

DISADVANTAGES

Units a r e l a r g e , may be over 6 feet i n diameter and over 20 feet t a l l .

Must be assembled on s i te . N o t prepackaged.

Evaporated r i n s e water l o s t t o t he atmosphere.

In some app l i ca t ions may r equ i r e 20% more energy than equiva len t capac i ty vacuum u n i t because ex- haus t a i r removes some of the h e a t supplied - b u t i n o the r appl ica t ions u n i t can be p a r t i a l l y heated w i t h waste energy as i n case of hard chrome recovery.

Concentration of re- covered ba th can vary.

Requires r i n s e water r e c i r c u l a t i n g tank; although bottom of tower usua l ly serves t h i s funct ion.

Packed bed i r r i g a t i o n r a t e s 'are s e n s i t i v e and r equ i r e ca re fu l con t ro l t o avoid e i t h e r channeling and dry packing from too l i t t l e water, o r poss ib le loading and flooding from too much water.

Cannot be used with ba ths which may re- l e a s e t o x i c components t o t h e atmosphere.

-C-

..

ADVANTAGES

Operate a t high vacuum. Gentle t o heat s e n s i t i v e ba ths .

Higher vacuum operat ion allows some energy economy from hea t conten t of ho t ba ths which can be d i r e c t l y f lashed - savings, roughly 20%.

Can be provided prepackaged and with con t ro l s .

E a r l i e s t evaporator used f o r ba th reconcentrat ion. Probably 15-20 year experience curve.

Eas i ly adapted t o double e f f e c t mode.

DISADVANTAGES

Doesn't handle wetters o r foaming agents w e l l .

Units a r e usua l ly q u i t e l a r g e and t a l l : 15-20 feet .

Generally requi res more cooling water o r l a r g e r condenser because of lower temperature d i f f e r e n t i a l between vapor and coolant .

Usually uses e x t r a pumps, t o withdraw concentrate and con- densate from evacuated u n i t .

Usually higher c o s t by v i r t u e of l a r g e r s i z e components.

Slow s ta r t -up .

-D-

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ADVANTAGES

submerged Tube Evaporators

Lowest c a p i t a l c o s t (20-25% less) of vacuum evaporators because of i n t eg ra t ed , s i n g l e chamber design.

Very compact. Shor tes t of vacuum u n i t s . Neat appearance.

Available prepackaged and w i t h con t ro l s .

Has opt iona l f ea tu re s ( a t add i t iona l c o s t ) .

Operates a t high vacuum. Lower temperature can be g e n t l e t o thermally s e n s i t i v e baths .

Double e f f e c t models ava i l ab le .

Steam o r ho t water i n s i d e submerged heat ing tubes. Bath ou t s ide making s c a l e o r s o l i d s e a s i e r t o remove when necessary.

DISADVANTAGES

Slow s t a r t -up because of mass of f l u i d t o be heated- Takes 15-30 minutes t o g e t good recyc le r i n s e r a t e e s t ab l i shed

Eas i ly c r e a t e s foam and becomes foam bound i f so lu t ion contains wetter.

D i s t i l l e d water very "di.r ty" a t s t a r t -up .

Because of lower temper- a t u r e d i f f e r e n t i a l uses more cool ing water than comparable climbing f i l m u n i t s i n acid service.

U s e s water eductor t o c r e a t e and maintain vacuum. R i s k s con- tamination of eductor w a t e r precluding reuse elsewhere.

Requires condensate and d i s t i l l a t e pumps.

-E-

ADVANTAGES

Climbinq F i l m Evaporators

F i r s t design t a i l o r e d s p e c i f i c a l l y t o p l a t i n g b a t h recovery.

Broad v a r i e t y of designs a v a i l a b l e f o r both high and intermediate vacuums t a i l o r e d t o ind iv idua l baths .

F a s t e s t s t a r t -up , 5 minutes o r less f o r c lean water d e l i v e r y t o r i n s e system.

Prepackaged and automated t o recover any p r a c t i c a l ba th concentrat ion. Can recover high concentration ba ths such as chrome-etch.

Vacuum s e t t i n g balanced a g a i n s t ba th c h a r a c t e r i s t i c s t o minimize u n i t s i z e and maximize performance.

DISADVANTAGES

Recovered bath i s in s ide steam heated v e r t i c a l tubes. Risk sca l ing o r plugging ii bath allowed t o over- concentrate o r i f feed contains s o l i d s .

Approximately 15-25% higher c a p i t a l c o s t than submerged tube design, e spec ia l ly f o r a lka l ine u n i t s .

Liquid r ing vacuum pump vulnerable t o corrosion from mal- operat ion, bu t can be hydraul ica l ly i s o l a t e d a t a s l i g h t l y (2-3%) higher cos t .

U s e s only one pump, a l i q u i d r i n g vacuum pump with ad jus tab le a i r b leed t o cont ro l vacuum.

-F-

6

ADVANTAGES DISADVANTAGES

Double Effect Evaporators (Submerged Tube, Flash, Climbing Film)

Use 50% of energy demand of Capital cost sig- single effect design - nificantly increased reduces overall operating (35-50%) over single cost by approximately 36%. effect design.

Units and controls are more complex - larger-require more floor area.

Individual effects mutually interdependent. Trouble with one upsets the other.

Requires longer start- up and more operator and maintenance attention.

Vapor Recompression Evaporators

Lowest overall operating cost - approximately 50% that for a single effect design of equivalent capacity.

Highest capital cost design - 50 to 65% greater than equivalent capacity single effect unit.

Depends on a complex and expensive mechanical compressor.

Because of compressor construction applicable only to alkaline baths.

-G-