ITP Filtrate Benzene Removal Alternatives

Transcription

1 / WSRC-R P TP Filtrate Benzene Removal Alternatives by L. 0. Dworjanyn Westinghouse Savannah River Company Savannah River Site Aiken, South Carolina DOE Contract No. DE-AC09-89SRl8035 This paper was prepared in connection with work done under the above contract number with the U. S. Department of Energy. By acceptance of this paper, the publisher andor recipient acknowledges the U. S. Government's right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper, along with the right to reproduce and to authorize others to reproduce all or part of the copyrighted paper.

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4 Westinghouse Savannah River Company Savannah River Technology C e n t e r WSRC-gP Keywords: Benzene s t r i p p i n g, G a s Sparging, GOOD ~ L E Znsa-Process, R e t e n t i o n Period: Permanent cc T.P. Gaughan, H G.A. Taylor, H G. K. Georgeton, 3-H D.B. Burns, 676-T J. F. McGlynn, 773-A E.W. H o l t z s c h e i t e r, 773-A L. F. Landon, -T M.D. B o e r s m a, -T M.A. B a i c h, -T W.E. Stevens, 773-A S. D. Fink, 773-A D. D. Walker, 773-A SRTC Records ( ) May 21, Authorized Derivative h a s s i f ier TO: W.L. Tamosaitis, 773-A FROM: SUMMARY E x i s t i n g T P f i l t r a t e hold t a n k s smafs@'r8vkde s u f f i c i e n t c a p a c i t y and r e s i d e n c e t i m e t o s t r f i p d i s s o l v e d benzene from t h e incoming f i l t r a t e u s i n g n i t r o g e n s p a r g i n g i n t h e bottom of t h e hold t a n k s. T h i s is based on e q u i l i b r i u m s u p p o r t e d by Late Wash t e s t data u s i n g aged washed s l u r r y, Theoretical c o n s i d e r a t i o n s i n d i c a t e t h a t benzene s t r i p p i n g w i l l be more d i f f i c u l t f r o m t h e T P unwashed h i g h s a l t f i l t r a t e s due t o reduced m a s s t r a n s f e r. T h e r e f o r e e!acpj3'if&3~-%~~~kn~*.'aata is needed t o q u a n t i f y t h e t h e o r e t i c a l effects. &ioam%%g l i m i t s which d i c t a t e allowable s p a r g i n g rate wm*-&%o:-zhaveto be-established. Sparging i n t h e

5 w:l. WSRC-RP Tamosaitis hold tanks will require installation of sintered metal spargers, and possibly stirrers and foam monitoring/disengagement equipment. The most critical sparging needs are at thg start of the precipitation/concentration cycle, when the filtrate flux rate is the highest, and at the end of the wash cycle where Henry's equilibrium constant falls off, requiring more gas to sparge the dissolved benzene. With adequate recycle (for proper p#k$#mg distribution\)-or sparging in the hold tanks, the630 inch column could be used for the complete TP process. A cosrser packing would reduce back pressure while enabling benzene stripping. The Late Wash tests indicate adequate benzene stripping even at reduced gas flow. This will require experimental verification under TP conditions. Using the 30 in. column vs 18 in. during the wash cycle will enhance stripping without need for additional sparging provided the minimum flow requirements are met. RECOMMENDATONS / ' Provide experimental data to validate sparging in hold tanks under TP conditions. Use only the larger 30 in. stripper column for the whole TP cycle. a Evaluate other (covrser) packings in the 30 inch column. BACKGROUND Unexpectedly high pressure drops were observed in an TP benzene stripping column during cold chemicals testing of cross-flow filters. The 30 in. column pressure drop reached the 0 in. water operating limit at half the design liquid flow and one quarter design nitrogen stripping gas flow. Subsequent vendor tests using simulated filtrates showed that the pressure drop is due to foaming and can be significantly reduced with tributyl phosphate. The dissolved benzene also had a small anti-foaming effect. Also different batches of simulant showed different levels of foaming and resulted in differing pressure drops. The pressure drop was not predicted during the initial stripper column design and real tank waste may behave differently, even in the presence of tributyl phosphate. Benzene removal from T P filtrate is essential to meet Class A saltstone quality. Existing T??equipment has been examined to provide back-up benzene removal alternatives. DSCUSSON P Late Wash sparg' g tests'(ref 1) show that benzene is removed to near equilibrium as determined by Henry's Law Constant. The stripping kinetics remain very fast down to about 2 ppm g-e&b benzene concentration (Fig 2). The tests were conducted on washed P g e d k (irratiated) slurry in a well stirred 30 gal. tank

6 w:'l. Tamosaitis -3- WSRC-RP (1/200 t h scale STRAT) u s i n g s p a r g e rates of 0. 3 t o 0.6 SCFM/ft2. The sparge rate d e t e r m i n e s t h e benzene s t r i p p i n g rate and i s ( u l t i m a t e l y l i m i t e d by foaming. The vendorzrecommends 0.62 SCFM/ft2 of sparge s u r f a c e a t 0.5 micron p o r o s i t y. The Baich data p r o v i d e s o n l y a p r o d u c t of Henry's Constant and s p a r g i n g e f f i c i e n c y s i n c e t h e a c t u a l Henry's Constant f o r L a t e Wash i r r a d i a t e d s o l u t i o n i s n o t known. For TP s t r i p p i n g w e assumed a 50% s p a r g i n g e f f i c i e n c y. T h i s may improve i n s i d e t a l l e r t a n k s as t h e c o n t a c t t i m e i n c r e a s e s, o r decrease as mixing becomes less effective i n larger t a n k s. Based on Gus Georgeton's c a l c u l a t i o n s ( R e f 5 ), t h e mass t r a n s f e r c o e f f i c i e n t w i l l decrease w i t h i n c r e a s i n g s a l t c o n c e n t r a t i o n a n d t h e assumed 50% e f f i c i e n c y may ~B5"rea%e-w3ud.-ntj T h i s w i l l r e q u i r e experimental t h e qaacg?fitratimhcycle. - s r m i n a t i o n under selected TP c o n d i t i o n s. Benzene s t r i p p i n g f o r batch o p e r a t i o n can be r e p r e s e n t e d by ( R e f 1): C ( t ) = Co exp (- SHEtjV) where C ( t ) i s benzene c o n c e n t r a t i o n a t t i m e t (min) Co is benzene c o n c e n t r a t i o n a t t h e s t a r t S i s sparge gas f l o w (SCFM) H i s Henry's Constant E i s the s t r i p p i n g e f f i c i e n c y V i s t h e t a n k l i q u i d working volume ( f t 3 ) Henry's C o n s t a n t s f o r "average" s a l t s o l u t i o n have been provided by Walker ( R e f 2 ) as a f u n c t i o n of t e m p e r a t u r e and ( N a + ] concentration. The v a l u e s f o r "average" s a l t c a n be a d j u s t e d f o r h i g h and l o w OH' as shown i n Table 1 t o better r e p r e s e n t t h e T P p r e c i p i t a t i o n / c o n c e n t r a t i o n / washing c y c l e ( R e f 3 ). The c a l c u l a t e d t i m e f o r s p a r g i n g 1 0, gal of h i g h OH f i l t r a t e d u r i n g t h e c o n c e n t r a t i o n c y c l e i s shown i n F i g u r e 3. The s p a r g i n g t i m e t o r e d u c e benzene c o n c e n t r a t i o n f r o m 120 p p m t o 2.5 ppm f a l l s off r a p i d l y as the s a l t c o n c e n t r a t i o n i n c r e a s e s, a s s u m i n g - t h e e f f i c i e n c y remains a t 50%. However, w e would e x p e c t t h e e f f i c i e n c y t o decrease w i t h i n c r e a s e d s a l t c o n c e n t r a t i o n and e x p e r i m e n t a l data must be p r o v i d e d t o v a l i d a t e t h e assumptions. During t h e w a s h c y c l e benzene c o n c e n t r a t i o n w i l l be higher, and t h e s t r i p p i n g t i m e w i l l i n c r e a s e as t h e s a l t i s washed o u t f r o m SM t o say 1M sodium, F i g u r e. A t 500 p p m s t a r t i n g c o n c e n t r a t i o n it m a y t a k e 10 h r s. t o s t r i p t h e benzene t o 2. 5 ppm. The estimated t i m e t o f i l l t h e h o l d t a n k s and t o s t r i p t h e benzene o v e r t h e whole T P c y c l e i s shown i n F i g u r e 1, based on T P cold chemicals f i l t r a t e f l u x r a t e F i g u r e 5 ( R e f ). During t h e c o n c e n t r a t i o n c y c l e benzene s t r i p p i n g w i l l be r e l a t i v e l y c o n s t a n t and q u i c k s i n c e t h e h i g h l y c o n c e n t r a t e d s a l t s o l u t i o n holds l i t t l e benzene. As t h e s o l i d s are c o n c e n t r a t e d t h e f i l t e r f l u x rate will f a l l off a l l o w i n g more t i m e f o r s t r i p p i n g.

7 W.'L. Tamosaitis -- WSRC-RP During the wash cycle the filter flux will remain relatively constant and low, however, the stripping time will increase as the salt is washed out and Henry's constant decreases by a factor of 10. The flowrates, benzene concentration, %md Henry's constant a* used are highlighted in Table 1. REFERENCES (1) M.A. Baich, "Qualification of Data 1/200 th Scale STRAT Late Wash Permeate Sparging", WSRC-RP , 1/20/93. (2) D.D. Walker to G.T. Wright, "Vapor Pressure of Benzene, Methanol and soprapanol over Salt Solutions", DPST , 3/28/89. (3) D.D. Walker, "Material Balance for the TP Process with Late Washing", SRT-LWP-92-07, 7/1/92. () L.O. Dworjanyn to W.L. Tamosaitis, "TP Filter Particulate Decontamination", WSRC-RP , 5/21/ 93. (5) 'G.K. Georgeton, "DevelopGent and Application of a Mathematical Model for the Benzene Stripping Columns in the TP Process", WSRC-RP-89-12, 12/28/89.

8 WSRC-W HoldTank Stripping 10,OOO gal Tank FiU 115.u hr Avh O S S operation: x* Plot Fill Tim,hr Gallons to filter Hours Needed Walker Hours #3 TP Filter Flux KTPB, W/' Hw, W F Flowfilter, gpm Two Filters, Qpm o d hr m Contemn ' adegc HiOH hr ' hr Avh adegc HiOH 113 ; A' 3.73 ' Washing hr Hi Hi Hi Hi Hi Mid Hi Total Total oooO 52 16oooO OOM 173OOO 173OOO oM O ist

9 w. WSEC-RP FGURE 1 L. Tamosaitis TP Cycle Sparging Time Time to Reduce Benzene to 2.5 ppm Starting Conc ppm Benzene '10' Concentration (wt% Solids) NOTE: 5( During CONCENTRATON filtrate flux rate decreases, increasing the available time for sparging. Assumed temperature = 35OC ' Washing [Na+] During PRECPTATE WASHNG salt concentration decreases, decreasing Henry's constant and increasing sparging time.

10 * W.L.. FGURE 2 Tamosaitis WSRC-e Late Wash Sparging Mark Baich, Test Run #3 v bo 150 Sparge time (min) Data 200 Theory Fit 250

11 W.L WSRC-RP FGURE 3 Tamesaitis : Hold Tank Sparging 120 ppm Benzene to 2.5 ppm Salt Concentration [Na+] Deg C -t 35 D e c j C - 5 Deg C

12 W:L. WSRC-RP FGURE Tamesaitis Hold Tank Sparging 500 ppm Benzene to 2.5 ppm L a, c a, E E 2 F 0 ' Salt Concentration[Na+] 6

13 .. W.L. Tamosaitis * WSRC-RP FGURE 5 * TP Filtrate Flux Unit #3 STRlP.WQ1 LOG-FT 0.01! Slurry Solids Concentration (wt%)