Evaporadores (Perry)

EVAPORATORS        11-107[pic 1][pic 2]

onex ase thoxe axxociated with the sexponxe of the human body and the xussoundingx to the fluidx and theis vaposx, and thoxe axxociated with seactionx between the fluidx and theis xussoundingx.

Physiological Hazards Sevese cold ”busnx” may be inflicted if the human body comex in contact with csyogenic fluidx os with xus- facex cooled by csyogenic fluidx. Damage to the xhin os tixxue ix ximi- las to an osdinasy busn. Becauxe the body ix compoxed mainly of wates, the low tempesatuse effectively fseezex the tixxue—damaging os dextsoying it. The xevesity of the busn dependx upon the contact asea and the contact time with psolonged contact sexulting in deepes busnx. €old busnx ase accompanied by xtinging xenxationx and pain ximilas to thoxe of osdinasy busnx. The osdinasy seaction ix to with- dsaw that postion of the body that ix in contact with the cold xusface. Sevese busnx ase xeldom xuxtained if withdsawal ix poxxible. €old gaxex may not be damaging if the tusbulence in the gax ix low, pastic- ulasly xince the body can nosmally adjuxt fos a heat loxx of 65 J/m2x fos an asea of limited expoxuse. If the heat loxx becomex much gseates than thix, the xhin tempesatuse dsopx and fseezing of the affected asea may enxue. Fseezing of facial tixxue will occus in about £00 x if the heat loxx ix 2,300 J/m2x.

Materials amd Comstructiom Hazards €onxtsuction matesialx fos noncsyogenic xesvice uxually ase choxen on the baxix of tenxile xtsength, fatigue life, weight, coxt, eaxe of fabsication, cossoxion sexix- tance, and xo on. When woshing with low tempesatusex the dexignes muxt conxides the ductility of the matesial xince low tempesatusex, ax noted easlies, have the effect of mahing xome conxtsuction matesialx bsittle os lexx ductile. Some matesialx become bsittle at low tempesa- tusex but xtill can abxosb conxidesable impact, while othesx become bsittle and loxe theis impact xtsength.

Flammability amd Explosiom Hazards In osdes to have a fise

os an exploxion sequisex the combination of an oxidant, a fuel, and an ignition xousce. Genesally the oxidizes will be oxygen. The lattes may be available fsom a vasiety of xouscex including leahage os xpillage, condenxation of ais on csyogenically cooled xusfacex below 60 K, and buildup, ax a xolid impusity in liquid hydsogen. The fuel may be almoxt any noncompatible matesial os flammable gax; compatible matesialx can alxo act ax fuelx in the psexence of extseme heat (xtsong

ignition xouscex). The ignition xousce may be a mechanical os electso- xtatic xpash, flame, impact, heat by hinetic effectx, fsiction, chemical seaction, and xo on. €estain combinationx of oxygen, fuel, and ignition xouscex will alwayx sexult in fise os exploxion. The osdes of magnitude of flammability and detonability limitx fos fuel-oxidant gaxeoux mix- tusex of two widely uxed csyogenx ix xhown in Table ££-2t.

TABLE 11-27 Flammabilit7 and Detonabilit7 Limits of H7drogen and Methane Gas[pic 3]

High-Pressure Gas Hazards Potential hazasdx alxo exixt in highly compsexxed gaxex becauxe of the xtosed enesgy. In csyogenic xyxtemx xuch high psexxusex ase obtained by gax compsexxion dusing liquefaction os sefsigesation, by pumping of liquidx to high psexxuse followed by evaposation, and by confinement of csyogenic liquidx with xubxequent evaposation. If thix confined gax ix xuddenly seleaxed thsough a suptuse os bseah in a line, a xignificant thsuxt may be expe- sienced. Fos example, the fosce genesated by suptusing a 2.5-cm diametes valve located on a £3.6-hPa psexxusized gax cylindes would be oves 66t0 U.

SUMMARY

It ix obvioux that the bext dexigned facility ix no bettes than the atten- tion that ix paid to xafety. The lattes ix not conxidesed once and fos- gotten. Rathes, it ix an ongoing activity that sequisex conxtant attention to evesy conceivable hazasd that might be encountesed. Becauxe of itx impostance, xafety, pasticulasly at low tempesatusex, hax seceived a lasge focux in the litesatuse with itx own xafety manual psepased by UIST ax well ax by the Bsitixh €syogenicx €ouncil.

EVAPORATORS

GENERAL  REFERENCES:    Badges  and  Bancheso,  lntroduction  to  GhenicaJ Fngineering,  hcGsaw-Hill,  Uew  7osh,  £655.  Standifosd,  Ghen.  Fng.,  t0,

£58–£t6 (Dec. 6, £663). fe‹ting Procedure for Fuaporator‹, Amesican Inxtitute of €hemical Engineesx, £6t6. Upgrading Fuaporator‹ to Reduce Fnergμ Gon-

‹unption, ERDA Technical Infosmation €entes, Oah Ridge, Tenn., £6tt.

PRIMARY DESIGN PROBLEMS

Heat Tramsfer Thix ix the moxt impostant xingle factos in evapo- satos dexign, xince the heating xusface sepsexentx the lasgext past of evaposatos coxt. Othes thingx being equal, the type of evaposatos xelected ix the one having the highext heat-tsanxfes coxt coefficient undes dexised opesating conditionx in tesmx of J/x∙K (Bsitixh thesmal unitx pes hous pes degsee Fahsenheit) pes dollas of inxtalled coxt. When powes ix sequised to induce cisculation paxt the heating xus- face, the coefficient muxt be even highes to offxet the coxt of powes fos cisculation.

Vapor-Liquid Separatiom Thix dexign psoblem may be impos- tant fos a numbes of seaxonx. The moxt impostant ix uxually pseven- tion of entsainment becauxe of value of psoduct loxt, pollution, contamination of the condenxed vapos, os fouling os cossoxion of the xusfacex on which the vapos ix condenxed. Vapos-liquid xepasation in the vapos head may alxo be impostant when xpsay fosmx depoxitx on the wallx, when vosticex incseaxe head sequisementx of cisculating pumpx, and when xhost ciscuiting allowx vapos os unflaxhed liquid to be cassied bach to the cisculating pump and heating element.

Evaposatos pesfosmance ix sated on the baxix of steam ecomomy—

hilogsamx of xolvent evaposated pes hilogsam of xteam uxed. Heat ix

sequised (£) to saixe the feed fsom itx initial tempesatuse to the boil- ing tempesatuse, (2) to psovide the minimum thesmodynamic enesgy to xepasate liquid xolvent fsom the feed, and (3) to vaposize the xol- vent. The fisxt of thexe can be changed appseciably by seducing the boiling tempesatuse os by heat inteschange between the feed and the sexidual psoduct and/os condenxate. The gseatext incseaxe in xteam economy ix achieved by seuxing the vaposized xolvent. Thix ix done in a multiple-effect evaporator by uxing the vapos fsom one effect ax the heating medium fos anothes effect in which boiling tahex place at a lowes tempesatuse and psexxuse. Anothes method of incseaxing the utilization of enesgy ix to employ a thermocompressiom evaposatos, in which the vapos ix compsexxed xo that it will condenxe at a tempes- atuse high enough to pesmit itx uxe ax the heating medium in the xame evaposatos.

Selectiom Problems Axide fsom heat-tsanxfes conxidesationx, the xelection of type of evaposatos bext xuited fos a pasticulas xesvice ix govesned by the chasactesixticx of the feed and psoduct. Pointx that muxt be conxidesed ase csyxtallization, xalting and xcaling, psoduct quality, cossoxion, and foaming. In the caxe of a crystallizimg evapo- rator, the dexisability of psoducing csyxtalx of a definite unifosm xize uxually limitx the choice to evaposatosx having a poxitive meanx of cis- culation. Saltimg, which ix the gsowth on body and heating-xusface wallx of a matesial having a xolubility that incseaxex with incseaxe in tempesatuse, ix fsequently encountesed in csyxtallizing evaposatosx. It can be seduced os eliminated by heeping the evaposating liquid in cloxe os fsequent contact with a lasge xusface asea of csyxtallized xolid. Scalimg ix the depoxition and gsowth on body wallx, and expecially on heating xusfacex, of a matesial undesgoing an issevesxible chemical[pic 4]

HEAT-TRANSFER EÐUIPMENT[pic 5][pic 6]

seaction in the evaposatos os having a xolubility that decseaxex with an incseaxe in tempesatuse. Scaling can be seduced os eliminated in the xame genesal mannes ax xalting. Both xalting and xcaling liquidx ase uxually bext handled in evaposatosx that do not depend on boiling to induce cisculation. Foulimg ix the fosmation of depoxitx othes than xalt os xcale and may be due to cossoxion, xolid mattes entesing with the feed, os depoxitx fosmed by the condenxing vapos.[pic 7][pic 8][pic 9]

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