Unitech

Division of Ecodyne 2720 U.S. Highway 22 Union, New Jersey, 07083 (201) 964-2600

Technical Reprint

u101

Concentration of Wastewater with High Calcium Scaling Tendencies By Evaporation

SETH LEVINE ROBERTO MANAS UNITECH DIVISION OF ECODYNE Union, New Jersey

Presented at THIRTEENTH MID ATLANTIC INDUSTRIAL WASTE CONFERENCE

June 28-30,1981

INDUSTRIAL WASTE Proceedings of the Thirteenth Mid-Atlantic Conference

C. P. Huang, Editor

ANN ARBOR SCIENCE PUBLISHERSINC /THE BUTTERWORTH CROUP

. CONCENTRATION OF WASTEWATER VITH HIGH CALCIUM SCALING TENDENCIES BY EVAPORATION

Seth Levine, Unitech D i v i s i o n of Ecodyne, Union, New J e r s e y Roberto Manas, Unitech D i v i s i o n of Ecodyne, Union, New J e r s e y

Copyright 0 1981 by the University of Delaware Newark, Delaware 1971 1 Published 198 1 by AM Arbor Science Publishers, Inc. P.O. Box 1425,230 Collingwood, Ann Arbor, Michigan 48106 Library of Congress Catalog Card Number 8 1-65971 ISBN 0-250-40473-7 Manufactured in the United States of America All Rights Reserved

Water recovery and w a s t e volume r e d u c t i o n have become i n c r e a s i n g l y important i n p r o c e s s p l a n t d e s i g n i n view of governmental d i s c h a r g e requirements and s c a r c i t y of water i n some areas. The removal of i n o r g a n i c s from p r o c e s s streams, o r desalting, w i l l play an ever increasing r o l e i n the years ahead. The a p p l i c a t i o n s i n t h e U . S . w i l l i n v o l v e r e s p o n s e s t o w a t e r s h o r t a g e s , r e t u r n waterflow cleanup, r e u s e water cleanup, and d r i n k i n g w a t e r cleanup. A s a r e s u l t of t h e F e d e r a l Water P o l l u t i o n C o n t r o l A c t s , many p r o c e s s p l a n t s a r e being pushed towards z e r o d i s c h a r g e of p o l l u t a n t s . For example, many s t e a m e l e c t r i c p l a n t s are b e i n g b u i l t w i t h c o o l i n g water r e c y c l e r a t h e r t h a n once through condenser cooling. A s a result, cooling water loops r e q u i r e reuse water t r e a t m e n t . I n most cases r e c y c l i n g is l i m i t e d by s c a l i n g upon c o n c e n t r a t i o n . [ l ] In many cases t h e water w i t h i n a p l a n t can b e r e u s e d s e v e r a l t i m e s . However, i n t h e above case where z e r o d i s c h a r g e i s a n t i c i p a t e d , u l t i m a t e d i s p o s a l of t h e c o o l i n g tower blowdown w i l l b e r e q u i r e d . If ponds are n o t s a t i s f a c t o r y , t h e n f u r t h e r w a s t e w a t e r t r e a t m e n t w i l l be r e q u i r e d . Evaporation t o accomplish waste volume r e d u c t i o n i s one such approach. A c o o l i n g tower blowdown e v a p o r a t o r could r e d u c e w a s t e volumes c o n s i d e r a b l y w h i l e r e c o v e r i n g 90% o r b e t t e r i n most c a s e s of high p u r i t y water f o r r e c y c l e . T h i s t y p e of e v a p o r a t o r w i l l be d i s c u s s e d later.

26

C H ~ I S " ~

I n discussing d e s a l t i n g evaporators, let us f i r s t look a t t h e c h e m i s t r y of t h e major water t r e a t m e n t problems. Chemical compositions o f t y p i c a l c o o l i n g tower blowdowns, as w e l l as b r a c k i s h w e l l waters, w i l l inlclude calcium, magnesium, sodium, c h l o r i d e , s u l f a t e , c a r b o n a t e and s i l i c a amongst o t h e r s . I n many s i t u a t i o n s t h e p r o c e s s stream is s a t u r a t e d o r n e a r s a t u r a t i o n i n some of t h e s e c o n s t i t u e n t s . T h e r e f o r e , as e v a p o r a t i o n o c c u r s t h e s c a l i n , g t h r e s h o l d s of some of t h e s e I n some cases t h e i o n s w i l l c o n s t i t u e n t s may b e reached. tend t o form compounds which have s o l u b i l i t i e s t h a t w i l l decrease w i t h temperature. When p r e c i p i t a t i o n o c c u r s , compounds may form hard scales. I n h e a t exchangers, c h i s w i l l o f t e n d e c r e a s e h e a t t r a n s f e r t o a p o i n t where chemical and mechanic a l c l e a n i n g is r e q u i r e d . I n t h e e v a p o r a t o r several w a t e r treatment s t e p s are i n c o r p o r a t e d t o p r o t e c t t h e h e a t t r a n s f e r s u r f a c e from e x c e s s i v e s c a l i n g . Calcium c a r b o n a t e and magnesium hydroxide are two such scales which a r e s t r o n g l y a f f e c t e d by pH as w e l l as temperature. A t high enough temperatures b i c a r b o n a t e w i l l d i s i n t e g r a t e t o c a r b o n a t e and form calcium c a r b o n a t e scale when f a v o r a b l e c o n d i t i o n s e x i s t . F u r t h e r d i s i n t e g r a t i o n of c a r b o n a t e w i l l form hydroxyl i o n s which can form magnesium hydroxide scales a t f a v o r a b l e c o n d i t i o n s of t e m p e r a t u r e and pH. These may b e c o n t r o l l e d though by a c i d t r e a t m e n t o r chemical a d d i t i v e s a s w i l l b e d i s c u s s e d l a t e r . Calcium s u l f a t e and i t s h y d r a t e s on t h e o t h e r hand w i l l form r e l a t i v e l y independent of pH. Due t o i t s i n v e r s e s o l u b i l i t y , a t high enough temperatures p r e c i p i t a t i o n w i l l occur as c o n c e n t r a t i o n proceeds. Conventional t r e a t m e n t i n v o l v e s keeping s o l u t i o n s below t h e s c a l i n g t h r e s h o l d s of calcium s u l f a t e . [ 2 ] A s w i l l b e d i s c u s s e d l a t e r , t h e s c a l i n g t h r e s h o l d can b e determined from a s o l u b i l i t y product c u r v e f o r t h e b r a c k i s h water of concern. However, i t becomes d i f f i c u l t t o develop an a c c u r a t e c u r v e s i n c e v a r y i n g amounts of o t h e r i o n s w i l l a f f e c t t h e s o l u b i l i t y . Another t r e a t m e n t i s by a c o p r e c i p i t a t i o n s e e d i n g t e c h n i q u e which w i l l be d i s c u s s e d l a t e r . Using t h i s method w i l l e n a b l e o p e r a t i o n o f t h e e v a p o r a t o r i n t h e p r e c i p i t a t i o n regime w i t h o u t s c a l i n g heat transfer surfaces. Lastly, silica w i l l also present s c a l i n g problems a t high enough c o n c e n t r a t i o n s . There h a s been s u c c e s s i n handling s i l i c a a t l i m i t e d c o n c e n t r a t i o n s u s i n g t h e c o p r e c i p i t a t i o n s e e d i n g technique. DESCRIPTION OF THE MVR With e v e r i n c r e a s i n g energy c o s t s , t h e NVR is f i n d i n g wider u s e i n e v a p o r a t o r s f o r water recovery and d e s a l i n a t i o n . The mechanical vapor recompression e v a p o r a t o r , o r MVR, works on t h e p r i n c i p l e o f r e u s i n g t h e v a p o r s d r i v e n o f f d u r i n g 27

e v a p o r a t i o n . These v a p o r s c o n t a i n n e a r l y as &ch energy as b o i l e r steam. The d i f f e r e n c e between vapor and steam i s , t h e r e f o r e , n o t t h e energy c o n t e n t b u t t h e q u a l i t y of energy. I n o r d e r t o t r a n s f e r h e a t , a temperature d i f f e r e n c e i s r e q u i r e d . I n two-phase systems, ( l i q u i d - v a p o r ) , t h i s temperat u r e d i f f e r e n c e must b e accomplished by a p r e s s u r e d i f f e r e n c e , t h u s t h e need t o i n c r e a s e t h e p r e s s u r e of t h e vapors. I n MVR's t h i s p r e s s u r e i n c r e a s e i s achieved w i t h e l e c t r i c a l o r steam d r i v e n c e n t r i f u g a l compressors. The v a p o r s g i v e n o f f i n t h e e v a p o r a t o r , which c o n t a i n approximately 1000 BTU/lb , are piped t o t h e s u c t i o n of a compressor where t h e vapor i s compressed a t a n e t e x p e n d i t u r e of about 40 BTU/lb, depending on t h e compression r a t i o . The compressor t h e n d i s c h a r g e s t h e s e v a p o r s t o t h e steam c h e s t of t h e e v a p o r a t o r t o d r i v e o f f more v a p o r s t o t h e compressor s u c t i o n . C e n t r i f u g a l compressors are l i m i t e d t o low compression r a t i o s . While some a p p l i c a t i o n s f i n d compression r a t i o s of 2.0, t h e v a s t m a j o r i t y of i n s t a l l a t i o n s o p e r a t e between 1 . 3 and 1 . 5 Because t h e h e a t i n g s u r f a c e i n MVR's a l s o f u n c t i o n s as a condenser, l i t t l e o r no c o o l i n g w a t e r i s needed. This can b e of g r e a t advantage i n zany a p p l i c a t i o n s . P r o c e s s D e s c r i p t i o n of t h e C r y s t a l l i z i n g lWR I n w a t e r recovery a p p l i c a t i o n s , t h e c r y s t a l l i z i n g MVR is used a s a means t o c o n t r o l calcium s c a l i n g . F i g u r e 1 shows t h e t y p i c a l c o n f i g u r a t i o n f o r t h i s t y p e of e v a p o r a t o r . A s a c a s e s t u d y a b r a c k i s h w a t e r e v a p o r a t o r h a s been s e l e c t e d f o r t h i s d i s c u s s i o n . F i g u r e 4 i s a photograph of t h e p a r t i a l l y complete i n s t a l l a t i o n . A b r i e f p r o c e s s d e s c r i p t i o n of t h i s i n s t a l l a t i o n follows. A s t h e f e e d w a t e r e n t e r s t h e system, t h e pH i s a d j u s t e d w i t h s u l f u r i c a c i d , and pumped t o a t a n k f o r decarbonation and s t o r a g e . A s i t i s pumped t o t h e p r e h e a t e r , t h e f e e d i s dosed w i t h an a n t i s c a l a n t . The purpose of t h e a n t i s c a l a n t i s t o p r o t e c t t h e p r e h e a t e r from s c a l i n g , n o t t h e h e a t i n g s u r f a c e i n t h e main e v a p o r a t o r .

I n t h e p r e h e a t e r t h e temperatvre of t h e f e e d i s r a i s e d t o n e a r i t s b o i l i n g p o i n t by t r a n s f e r of t h e s e n s i b l e h e a t c o n t a i n e d i n t h e h o t d i s t i l l a t e . The preheated f e e d t h e n e n t e r s a c o u n t e r f l o w d e a e r a t o r , where carbon d i o x i d e g a s i s removed and vented t o atmosphere. Feed a c i d i t y i s almost t o t a l l y n e u t r a l i z e d by removal of t h e carbon d i o x i d e . A i r i s a l s o removed t o a l e v e l which p r e c l u d e s c o r r o s i o n . C a u s t i c soda may b e i n j e c t e d if f u r t h e r n e u t r a l i z a t i o n i s r e q u i r e d . 28

0 U

5

0

mal 34J o m

zg I

The evaporator vents provide the required stripping steam for the deaerator. The deaerated feed then mixes with the brine which is continually circulated through the evaporator. The brine enters the top liquor box of the heating element and is distributed to the inside of each tube as a thin film. As the brine falls down the tubes, water evaporates. The vapor passes through a wire mesh entrainment separator and enters the compressor where its temperature and pressure are upgraded. As the vapor condenses on the shell side of the tubes, its heat of condensation is transferred to brine inside the tubes, resulting in more water evaporating. The brine inside the evaporator consists of a calcium sulfate slurry. Calcium sulfate crystals in the slurry provide an ideal site for crystal growth, preventing scaling in the evaporator through preferential crystallization on the existing crystals. The evaporator must be continually blown down in order to purge the soluble salts which accumulate during evaporation. The need to maintain high concentrations of calcium sulfate in the slurry precludes blowing down the evaporator directly to waste. Although crystallization does take place, direct blowdown would result in almost total loss of crystals. In order to retain these crystals, a hydrocyclone is used t o separate the slurry into two streams. The overflow is nearly free of crystals, and it is this stream which provides the major portion of the blowdown. The underflow contains nearly all of the crystals which are recycled back to ihe evaporator, except for a small portion which is purged to compensate for nev crystal growth. Case Study

5N H

To understand the chemical treatment approaches further, we will look at the system described earlier which is an existing system operating on brackish well water. A typical feed analysis for this system in shown in Table 1. As can be seen, carbonate scale prevention would be required. Acid treatment in this case was used as is shown in Figure 1. Sulfuric acid was added to the feedwater to lower the pH to 4.5-5.0. At this point, carbonates could be removed from the system as carbon dioxide by degasification.

30

A

*

+

i i

+

Table 1.

T y p i c a l Feedwater A n a l y s i s

60001 PH Chloride ( a s C l ) "P" A l k a l i n i t y ( a s CaC03) "?I" A l k a l i n i t y ( a s CaC03) T o t a l Hardness ( a s CaC03) Calcium ( a s c a ) Magnesium ( a s Mg) S u l f a t e ( a s SO4) S i l i c a ( a s S102) Suspended S o l i d s (ppm) T o t a l Dissolved S o l i d s (%) Organic and V o l a t i l e

x operatina point

7.2 5,250 ppm 50001

0 83 3,340 786 334 3,200 39 5 1.3

ppm ppm ppm ppm PPm PPm

---

A s p r e v i o u s l y mentioned, scale formation i n t h e h e a t r a n s f e r s u r f a c e of t h e e v a p o r a t o r i s prevented by c o n t r o l l e d rystallization. P r i o r t o s t a r t i n g t h e e v a p o r a t o r , a calcium u l f a t e s l u r r y i s prepared by mixing food grade calcium u l f a t e w i t h w a t e r i n a n a g i t a t e d t a n k , which i s t h e n t r a n s erred t o t h e evaporator. I n order f o r c r y s t a l l i z a t i o n t o ake p l a c e , some d e g r e e of s u p e r s a t u r a t i o n i s r e q u i r e d . o r t u n a t e l y , i n o r d e r f o r spontaneous c r y s t a l l i z a t i o n t o o c c u r , h i g h e r d e g r e e of s u p e r s a t u r a t i o n l e v e l t h a n f o r growth on x i s t i n g c r y s t a l s i t e s i s r e q u i r e d . T h i s i s t h e p r i n c i p l e on h i c h p r e f e r e n t i a l c r y s t a l l i z a t i o n works. By e n s u r i n g u f f i c i e n t c r y s t a l s , and t h u s s u f f f c i e n t s u r f a c e area f o r a p i d growth, s u p e r s a t u r a t i o n i s k e p t under s a f e l i m i t s . The .pper range t o c r y s t a l c o n c e n t r a t i o n i s l i m i t e d by t h e quipment. The c o n c e n t r a t i o n of calcium s u l f a t e c r y s t a l s i n h e s l u r r y is maintained w i t h i n i t s s p e c i f i e d r a n g e by a d j u s t ng t h e blowdown r a t e from t h e c y c l o n e underflow.

F i g u r e 2.

S o l u b i l i t y Product Curves f o r Calcium S u l f a t e i n Brackish Water (Data from OSTJ R&D P r o g r e s s Report N o . 529)

I n h i b i t o r Pump

N

r:

U \

A t a c o n c e n t r a t i o n r a t i o of 9 t o 1 under which t h i s v a p o r a t o r o p e r a t e s , s i l i c a s c a l i n g would become a problem f o r tandard e v a p o r a t o r s . However, because s i l i c a c o p r e c i p i t a t e s p t o c e r t a i n limits w i t h calcium s u l f a t e , t h e c r y s t a l l i z i n g VR can extend t h e o p e r a t i n g l i m i t s normally imposed by formai o n of s i l i c a scale.

Calcium s u l f a t e though p r e s e n t e d f u r t h e r problems p r i o r o t h e e v a p o r a t i o n loop. During p r e h e a t i n g , h y d r a t e s of alcium s u l f a t e began p r e c i p i t a t i n g i n t h e p l a t e p r e h e a t e r . s c a n be s e e n from t h e s o l u b i l i t y product curve of F i g u r e 2, n i t i a l feedwater c o n c e n t r a t i o n s of calcium and s u l f a t e i o n s ere a t n e a r s a t u r a t i o n . T h e r e f o r e , as t h e t e m p e r a t u r e n c r e a s e d , s o l u b i l i t y l i m i t s w e r e exceeded. The s o l u t i o n t o he problem involved t h e a d d i t i o n o f a chemical t o t h e f e e d

31

c 7 bd

a 0 h

n

5

10

Elapsed T i m e Hours Figure 3.

P r e h e a t e r P r e s s u r e Drop

32

15

4

.

.up%tream of t h e p r e h e a t e r . A d d i t i v e s a s mentioned e a r l i e r have been used i n p r e v e n t i o n of calcium c a r b o n a t e scale such a s polyphosphates r a t h e r than by a c i d p r e t r e a t m e n t . However, i n t h e c o n t r o l of calcium s u l f a t e p r e c i p i t a t i o n s e v e r a l problems e x i s t e d . The a d d i t i v e had t o remain s t a b l e a t tempe r a t u r e s above 212°F. The a d d i t i v e had t o b e a c c e p t a b l e f o r p o t a b l e water systems. L a s t l y , t h e a d d i t i v e had t o b e s t r o n g enough t o prevent calcium s u l f a t e s c a l i n g i n t h e p r e h e a t e r b u t y e t weak enough s o a s n o t t o a f f e c t t h e c o p r e c i p i t a t i o n seeding technique i n a d e l e t e r i o u s manner. One might a s k why n o t u s e such a n a d d i t i v e i n s t e a d of c o p r e c i p i t a t i o n ? The answer i s t h a t none are a v a i l a b l e which are s a t i s f a c t o r y a t t h e c o n c e n t r a t i o n s maintained i n t h e e v a p o r a t o r loop. The a d d i t i v e chosen w a s EL2438, a chemical manufactured by t h e Calgon Corp. The chemical c o n t a i n e d phosphonate which a c t s a s a t h r e s h o l d a g e n t . Atoms o f t h e phosphonate s t r u c t u r e i n t e r j e c t i n t o t h e c a l c i t e l a t t i c e and prevent c r y s t a l growth. When t h e f e e d w a s dosed w i t h a d d i t i v e , t h e r e w a s no f u r t h e r s c a l i n g i n t h e p r e h e a t e r . Operation of t h e evaporator proceeded normally a t c a p a c i t y . The s u c c e s s of t h e pretreatment is f u r t h e r exemplified by F i g u r e 3. The graph of p r e s s u r e drop v e r s u s t i m e shows t h a t d u r i n g a f a i l u r e of t h e a d d i t i v e i n j e c t i o n system, s c a l i n g occurred i n t h e p r e h e a t e r as shown by i n c r e a s e d p r e s s u r e drop a c r o s s t h e p r e h e a t e r . Once i n j e c t i o n continued, t h e p r e s s u r e drop r e t u r n e d t o normal. T h i s i n d i c a t e d t h a t e x i s t i n g s c a l e w a s f l u s h e d o u t and no f u r t h e r s c a l i n g occurred.

F i g u r e 4.

Mechanical Vapor Recompression I n s t a l l a t i o n Unitech D i v i s i o n of Ecodyne

SUMMARY Evaporator systems p r e s e n t themselves as a n e x c e l l e n t means f o r c o n c e n t r a t i n g w a s t e w a t e r s from a v a r i e t y of indust r i a l a p p l i c a t i o n s . S p e c i f i c a l l y , c r y s t a l l i z i n g mechanical vapor recompression systems have a t t a i n e d v e r y h i g h w a t e r recovery rates from wastewaters and b r a c k i s h waters c o n t a i n i n g calcium scale forming compounds. A l k a l i n e scales have been c o n t r o l l e d w e l l by c o n v e n t i o n a l means such as a c i d treatment o r chemical a d d i t i v e s . I n water systems where wastewaters are c o n c e n t r a t e d beyond calcium s u l f a t e s o l u b i l i t y l i m i t s , a seeding c o p r e c i p i t a t i o n technique c o n t r o l s calcium s u l f a t e scale. I n systems where t h e feed e n t e r s s a t u r a t e d i n calcium s u l f a t e , chemical a d d i t i v e s have been used t o p r o t e c t evaporat o r p r e h e a t s e c t i o n s without a f f e c t i n g t h e seeding c o p r e c i p i t a t i o n technique. Two such systems which u t i l i z e t h e s e treatment schemes t o produce p o t a b l e w a t e r have been b u i l t f o r t h e Saudi N a t i o n a l E l e c t r i c Company.

33

Bibliography [l]

F l u o r Engineers and C o n s t r u c t o r s , "Desalting P l a n s and Progress", O f f i c e of Water Research and Technology 14-34-0O01-77O7, (1978)

[2]

Howe, E.D., Water D e s a l i n a t i o n , Marcel Dekker, I n c . , pp. 35-37, 1974.

34