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PROPER DESIGN SAVES ENERGY FORMOLECULAR SIEVE DEHYDRATION SYSTEMS

John BarrowRay VeldmanCoastal Chemical Company

The molecular sieve system is a s igni f -icant energy user in the cryogenic gasplant . Designing and operating th esystem properly ca n save thousands ofdollars in fuel each year. A poorlydesigned energy saving system canresul t in poor plant operation, freezeups, and los t plant production.The molecular sieve system is a batchprocess. One tower dehydrates whilethe other tower i s being regenerated byheating. Energy can be conserved inseveral ways.

Molecular sieve is the desiccant usual ly chosen to remove water vapor aheadof the natural ga s processing cryogenicplant . The molecular sieves are neededto dry the gas to less than 0.1 ppmv.Concentrations of water higher than 0.1ppmv ca n cause ice to form and plugl ines in the cryogenic plant . Theseplants typical ly operate from -100 to-lBOF.The molecular sieve .system is a s igni f -icant energy user in the plant . Onebed is adsorbing while the other bed isheated then cooled. At the end of th ecycle the beds are switched. The be dwhich was adsorbing is switched intoheating to remove the water or regenerate th e bed. To effect ively removethe water from th e sieve requiresheating the be d higher than 450F (normally 550F) with a s l ip stream of gastaken from somewhere in th e plant . Asl ip stream of dr y effluent from th ebed in adsorption is one source of re-generation gas, while residue ga s i sanother choice. In e i ther case the

HEATPRODUCT

e>-S . . . . . . . . . . . .DIAMETER (I. D. ) ......8EO HEIGHT .5.S. HEIGHT _._ ...PRESSURE DROP ...........

CYCLES------':DSOR8 ......... .....

HEAr . ..................COOL. .

REGENERATION CASFLOW _ .PRESSURE .... _... _ _.HOT GAS INLET........ ....MAX IHUH OUTLET . ...........

HOLECUl.AR SIEVETyPE .SiZE _ _ .WE IGHT I DEHYDRATOR .

50 HHSCPD100'P615 psla90 Ib "zO/mmsc f

25. 5 f t-12.0 ft .14. a ft 3, 0 psi8 hr.5 hr.3 hr.

itA molecular s ieve1/8"13,ISO lb s

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ESL-IE-84-04-145

Proceedings from the Sixth Annual Industrial Energy Technology Conference Volume II, Houston, TX, April 15-18, 1984
http:///reader/full/DSOR8...%E5%AE%AE.%E5%AE%AE...%E5%AE%95%E5%95%95http:///reader/full/DSOR8...%E5%AE%AE.%E5%AE%AE...%E5%AE%95%E5%95%95http:///reader/full/INLET......%E5%AE%AE%E5%80%80http:///reader/full/INLET......%E5%AE%AE%E5%80%80http:///reader/full/DSOR8...%E5%AE%AE.%E5%AE%AE...%E5%AE%95%E5%95%95http:///reader/full/INLET......%E5%AE%AE%E5%80%80

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s l ips tream i s hea ted , then used to hea tth e s ieve bed. From the bed, the gasis cooled an d water i s condensed a t theregenerat ion gas scrubber . From theregenerat ion as scrubber the gas i sre turned to the molecular s ieve i n l e t( for dry ef f luen t regenerat ion) or toLhe sa les gas l ine ( for r es idue regenera t ion) . (Figs . 1 & 2)Regenerat ion r equi r es about 7,200 BTU'sp r pound of water removed. This expense can be el iminated or grea t ly re duced in seve ra l d i f f e r en t ways. Reducing the amount of water removed v iamolecular s ieve , u t i l i z i n g waste hea trecovery , an d operat ing the system inan energy e f f i c i en t manner are , a l lway t ha t l e ss energy can be used toopera te a mole u la r s ieve system.The compressors whichrepressure thegas a f t e r the cryogenic plan t a r e usual ly driven by gas tu rb ines or gasengines . Heat from the xhaus t can ber cov red a d used to heat the regenera t ion gas stream. There are many gasto gas exchangers which can be used inth i s service . Ut i l i z ing compressorwaste hea t es sen t i a l l y e l imina tes theenergy xpense. However, it can alsopresent th op r a to r with some d i f f i cu l t problems. Some plan t operatorshave been r e luc tan t to bui ld a gas toas exchanger because a leak wouldcer ta in ly m an a f i r e when the highpressur regener t ion gas would bem'xed with an exhaust gas containingox y en. One so lu t ion to th i s p ~ u b l e m i s 0 hea t a s a l t bath with compressor ex h us t then heat the regenerat iongas with the s a l t .Waste hea t recovery has r esu l ted inoperat ing probl ms for some plants whenthey run a t redu ed r a t e s . I f thewaste hea t uni t s were designed so tha ta l l compressor exhausts were needed tomainta in both th e volume and th e temperature of the required regenerat ions ream, and one compressor i s down dueto low throughput, the opera tor has therol lowing choices . He ca n accept aeg e r a t ion stream flowing a t the corr c t ra te an d lower temperature, o rhe can c oose r a te lower than designa t the car r e t tempera ure . Since mosts ie e sys t ms are designed a t minimumt mperature and minimum r egenera t iongas r a t e , e i t h e r choice r e su l t s in poorr gen r a t i ns and f reeze-ups . An auxi l l a ry burner in the gas to ga s exchanger could so lve t h i s problem.We t e s t ed the economics fo r the f o l lowing ideas using a 50 rnmscfd systemdescribed by Table I .

l ~ s t comp nies design fo r 8 hourcyc les . When waste hea t i s not u s e . 8hours may not be the bes t time chos n.Fig 3 shows he c o r r e c t cyc le t ime(propor t ion 1 to pounds required) a afunc t ion f opera t ing co t an d capi a linv s tment . Larger beds requ i re m i es ieve an d a higher cap ' t a l investment,but save energy through fewer regenJe ra t ions . simple quick look t tileprobl m, show' t h a t the s l i g h t inCrjaSein cap i t 1 inv stment (Larger beds) i sa good en rgy s ving ide and t ha t eoptimum i s 10-12 hour cyc les . The ngineer ing hous and p r o j e c t engineshould spend the t ime to decide on hemost en rgy e f f i c i e n t system to bui d.A glycol uni t i s an economical waydehydrate natura l gas bu t w i l l onlydehydrate to a lev 1 of 1 to 7 poun sof water per mmscf. A glycol u n i t sesl e ss energy because the he t o f absorp t ion of water in to glycol i s no ashigh as the heat of absorpt ion of , t e ronto molecular sieve . Put t ing a glycoluni t an a molecular siev system iser ies w i l l allow removal of wat r 0l e ss than 0.1 ppmv an d use l ess ene gy.Th e economics, how ver , a re disappoin t ing (Table I I ) as th ex t racap i t a l inves tment takes 5-7 year 0p a y o u t . H wever, a r e t r fit of a Icryogenic pL 1 t to an ex i s t ' ng olderplan t which a l r eady has a glycol un tmay u t i l i z e the old g lycol uni t economically.In t e rn a l l insu la t ing th e moleculars ieve b ds w i l l s ve 20- 0% of therequired energy. Th e economics sho a2 to 3 year pa y b ck p e r i a . f thr ef rac to ry l in ing i s cor rec t ly in s t a l l ed th i s idea w i l l w rk we l.fth e insu la t ion i s ins ta l l ed i ncor r ec t l y , wet gas w i l l channel down t ewalls an d f eeze ups w i l l be a consfantproblem.The reg nera t ion system which u es r -s idue gas regener t ion can be dr iv I bydropping the en t i re r es idue gas s t r ampressure across a cont ro l valvean d forcing a s l ip stream of gasthraug the rege e ra t ion loop. New rplan t s a re using a smal l com re ssor topump the l ips ream and save horesp?werin the re ompressors. This idea may besu i tab le fo r r e t r o f i t to ex is t ingplan t s .Another energy savings idea i s headdi t ion of a th i rd be This woulal low the regener t i on gas to be pre heated as the bed DaIs, saVing n rgy.A b r i e f review f the e o n o m i ~ s showsha t the energy sa ed pays fo r the in vestment i. 5 to 7 y e a r .

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IlOPTUIUl'1 CYCLE TUIEGRAPHICAL S O L ~ T I O N 1Z

PRESENTVALUEINCREMENTALCAPITALCOST

10

oob 0

'"

\0 \2. \(, 2.0 a

The s ing le l a r g e s t waste of energyr esu l t s froQ operat ing with cycles UTILIZE WASTE HEATt imes which are too shor t . We f r e- TABLE I Iquent ly f ind plan ts which are opera t ing USES COfIPRESSOR EXHAUST ASon 8 hour cycles (which i s des ign) but A HEAT SOURCEnow process s ig n i f i can t ly l e ss gas, orgas which has been dehydrated in the REQUIRES THE INSTALLATION OFf ie ld . Th e only way to operate a t the AN AUXILLARY BURNERWITHOUTmaximum poss ib le cycle time i s to run PROBLEMS OCCUR WITHf requent break through t e s t s . These TURNDOWN (MULTIPLEt e s t s are simply made, the opera to r s RECOMPRESSORS)put the bed swi tching mechanism on REGENERATION GAS VELOCITYmanual and switch the bed when the OR TEMPERATURE NUST DROPplan t I:1oisture monitor shows an in - WITH THE SHUT DOWNOF 1 ENGINEcrease in the wate r content of the gasl eaving the bed. Ins ta l l ing a newprobe before the t e s t assu res t h a tbreakthrough w i l l be eas i ly observedand t ha t the plan t w i l l not f reeze up.Sel le r s of molecu la r s i eves sometimesdo these t es t s for f r ee . I f a break-through t e s t shows t h a t 24 hours onadsorp t ion i s poss ib le and you oper-a te on only 8 hour cyc les , the excessenergy b i l l can be very high.This paper has presented a b r i e f over-view on how to save energy in a cryo-genic plan t . I f your next cryogenicp l a n t cannot use waste heat for theregenerat ion gas , the ideas presentedshould be evaluated . I f waste hea t i sused, be sure t ha t the design isf l ex ib le enough to a llow good oper-a t ions a t processing r a t e s wel l belowdes ign .

FIGURE JCYCLE TIME PRUPORTIONALTO SIEVE QUANTITY

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