Prelab for Ch 22.2 Organic Chemistry LabFull description
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Geometry
Notes on solid state
Solid Desiccant Dehydration
Several solid desiccant processes are available to dry liquid hydrocarbons. Liquid velocity is usually 3-5 ft/minute through solid desiccant beds with a minimum travel of at least 5 feet to ensure good distribution. Direction of ow can be up-ow or down-ow in the adsorption cycle. Special care must be ta!en in designing the bed supports in the liquid dehydrator vessels to prevent desiccant loss" desiccant damage" and to ensure proper distribution. Layers of ceramic balls are installed in decreasing si#e from the support screen. $he support ball si#es may vary with the type and si#e of solid desiccant used but the layers of support balls should never be graduated in si#e more than twice the diameter of the balls being supported. $he regeneration of solid desiccant beds is very similar to gas dehydrators with the following e%ceptions& Liquid draining and 'lling time must be allowed. • (ressuring and de-pressuring must be done carefully to avoid bed movement. • )dequate bed cooling is required before liquid re-entry to minimi#e ashing. • *t is important to prevent movement of the bed particles to prevent attrition that would require premature replacement. )lso" desiccant dust particles can cause downstream pluggi plugging" ng" equipm equipment ent damage damage"" and e%ces e%cessiv sive e 'lter 'lter mainte maintenan nance. ce. Liqui Liquid d and vapor vapor velocities must be controlled carefully and ashing of liquids or accelerated blow-down rates that would +lift, or +oat, all or portions of the bed should be avoided. Desiccant bed life can be e%tended by doing several or all of the following activities& (revent (revent the desiccant particles from moving. • • ee eep p co cont ntam amin inan ants ts out out of the the dehy dehydra drati ting ng port portio ion n of the the bed bed by upst upstre ream am conditioning or by providing a sacri'cial layer of less e%pensive desiccant to act as a catcher of any compounds. (reve revent nt over overhe heati ating ng the the bed bed to redu reduce ce the the forma formati tion on of ca carb rbon on duri during ng the the • regeneration cycle. • )naly#e the heating/cooling regeneration temperature cycles to minimi#e the time the bed is at elevated temperatures. temperatures. $his will also minimi#e energy requirements. requirements. ) typical heating/cooling regeneration temperature cycle plot is shown in ig. 0-12" 0-12" with a description of the stage activities.
$here are typically typically four 4 distinct stages stages in a normal cycle& Stage 67irst bed-heating stage Stage 7Desorption stage
Stage 37Second bed-heating stage Stage 478ed-cooling stage or a period of time after the heat source is introduced into a desiccant bed being dehydrated" the bed must be heated to a temperature where the water will start to be desorbed Stage 6. )s the water is desorbed Stage " the bed temperature will usually rise only a few degrees because the regeneration gas heat is utili#ed to provide the heat of vapori#ation of the water being removed. $he completion of the water desorption stage is characteri#ed by a rapid increase in bed temperature measured as the outlet temperature. )t this point the heating may be discontinued while bed heating will continue from residual heat in the heating cycle Stage 3. )s the unheated regeneration gas stream continues to pass through the bed" the bed will be cooled Stage 4. )t near ambient pressures" regeneration of silica gel and alumina can be accomplished at 3009. :olecular sieve requires 500-5509 to maintain the low dew point potential" and the higher temperatures may increase desiccant life by providing more complete removal of adsorbed hydrocarbons. ;apacity and performance data for new solid desiccants are usually presented based on a static test.
:olecular sieve is not normally used for liquid dehydration because the required level of water removal is usually moderate and the cost of molecular sieve is considerably more than other types of suitable desiccants" such as activated alumina. >owever" in e%treme cases where the moisture content of the liquid must be !ept at an unusually low concentration" molecular sieve should be considered. :olecular sieve may be used for removing other undesirable compounds" such as >S" ;?S" mercaptans" etc." from liquid streams. Dehydration may be a secondary bene't of using this type of treating method. ADSORPTION PROCESSES
$he two types of adsorption are physical adsorption and chemisorption. *n physical adsorption" the bonding between the adsorbed species and the solid phase is called van der @aals forces" the attractive and repulsive forces that hold liquids and solids together and give them their structure. *n chemisorption" a much stronger chemical bonding occurs between the surface and the adsorbed molecules. $his chapter considers only physical adsorption" and all references to adsorption mean physical adsorption. (hysical adsorption is an equilibrium process li!e vapor−liquid equilibria and equations analogous to Aquation B.6 apply. $hus" for a given vapor-phase concentration partial pressure and temperature" an equilibrium concentration e%ists on the adsorbent surface that is the ma%imum concentration of the condensed component adsorbate on the surface. igure B.B shows the equilibrium conditions for water on a commercial molecular sieve. Such curves are called isotherms. $he 'gure is based upon a water−air mi%ture but is applicable to natural gas systems. $he important parameter is the partial pressure of waterC total pressure has only a minor e=ect on the adsorption equilibrium. 8ecause adsorbate concentrations are usually low" generally only a few layers of molecules will
build up on the surface. $hus" adsorption processes use solids with e%tremely high surface-to-volume ratios. ;ommercially used synthetic #eolites i.e" molecular sieves have surface-to-volume ratios in the range of 250 cm /cm3" with most of the surface for adsorption inside of the adsorbent. *n the case of molecular sieves" the adsorbent consists of e%tremely 'ne #eolite particles held together by a binder. $herefore" adsorbing species travel through the macropores of the binder into the micropores of the #eolite. )dsorbents such as silica gel and alumina are formed in larger particles and require no binder. (ore openings that lead to the inside of commercial adsorbents are of molecular si#eC they normally range from appro%imately 4 6 = 60−1 cm to 600 . :olecular sieves have an e%tremely narrow pore distribution" whereas silica gel and alumina have wide distributions. >owever" a molecular sieve binder" which is usually about 0E of the weight of the total adsorbent" has large pores capable of adsorbing heavier components. $wo steps are involved in adsorbing a trace gas component. $he 'rst step is to have the component contact the surface and the second step is to have it travel through the pathways inside the adsorbent. 8ecause this process is a two-step process and the second step is relatively slow" solid adsorbents ta!e longer to come to equilibrium with the gas phase than in absorption processes. *n addition to concentration i.e." partial pressure for gases" two properties of the adsorbate dictate its concentration on the absorbent surface& polarity and si#e. ow si#e a=ects adsorption depends upon the pore si#e of the adsorbent. )n adsorbate too large to 't into the pores adsorbs only on the outer surface of adsorbent" which is a trivial amount of surface area compared with the pore area. *f the pores are suFciently large to hold di=erent adsorbates" the less volatile" which usually correlates with si#e" adsorbates will displace the more volatile ones. $herefore" ethane is displaced by propane. *n commercial practice" adsorption is carried out in a vertical" '%ed bed of adsorbent" with the feed gas owing down through the bed. )s noted above" the process is not instantaneous" which leads to the formation of a mass transfer #one :$G in the bed. igure B.2 shows the three #ones in an adsorbent bed& 6. $he equilibrium #one" where the adsorbate on the adsorbent is in equilibrium with the adsorbate in the inlet gas phase and no additional adsorption occurs . $he mass transfer #one :$G" the volume where mass transfer and adsorption ta!e place 3. $he active #one" where no adsorption has yet ta!en place *n the mass transfer #one :$G" the concentration drops from the inlet value" y in" to the outlet value" y out" in a smooth S-shaped curve. *f the mass transfer rate were in'nite" the :$G would have #ero thic!ness. $he :$G is usually assumed to form quic!ly in the adsorption bed and to have a constant length as it moves through the bed" unless particle si#e or shape is changed. $he value of y in is dictated by upstream processesC the y out value is determined by the regeneration gas adsorbate content. $he length of the :$G is usually 0.5 to B ft 0. to 6.1 m" and the gas is in the #one for 0.5 to seconds $rent" 004. $o ma%imi#e bed capacity" the :$G needs to be as small as possible because the #one nominally holds only 50E of the adsorbate held by a comparable length of adsorbent at equilibrium. 8oth tall" slender beds" which reduce the percentage of the bed in the :$G" and smaller particles ma!e more of the bed e=ective. >owever" smaller particle si#e" deeper beds" and increased gas velocity will increase pressure drop. or a point in the :$G" the gas phase adsorbate content increases in time from y in to y out in an S-shaped curve that mirrors the curve shown in igure B.2.
*n principle" beds can be run until the 'rst sign of brea!through. $his practice ma%imi#es cycle time" which e%tends bed life because temperature cycling is a maHor source of bed degeneration" and minimi#es regeneration costs. >owever" most plants operate on a set time cycle to ensure no adsorbate brea!through. $rent 004 presents data that show a change in the L/D from 0.1 to .2 in the bed increases the useful adsorption capacity from 1.2 to 60.0 wtE in useful water capacity for an equal amount of gas dried. >owever" the pressure drop increases from 0.4 to 4.3 psi 0.00 to 0.0 !(a. @hen used as a puri'cation process" adsorption has two maHor disadvantages& I *t is a '%ed-bed process that requires two or more adsorption beds for continuous operation. I *t has limited capacity and is usually impractical for removing large amounts of impurity. >owever" adsorption is very e=ective in the dehydration of natural gas because water is much more strongly adsorbed than any of the al!anes" carbon dio%ide" or hydrogen sul'de. Jenerally" a higher degree of dehydration can be achieved with adsorbents than with absorption processes. )lthough this discussion uses molecular sieve as the e%ample of an adsorbent to remove water" with the e%ception of regeneration temperatures" the basic process is the same for all gas adsorption processes. igure B.1 shows a schematic of a two-bed adsorber system. ?ne bed" adsorber K6 in igure B.1" dries gas while the other bed" adsorber K" goes through a regeneration cycle. $he wet feed goes through an inlet separator that will catch any entrained liquids before the gas enters the top of the active bed. low is top-down to avoid bed uidi#ation. $he dried gas then goes through a dust 'lter that will catch 'nes before the gas e%its the unit. $his 'lter must be !ept wor!ing properly" especially if the gas goes on to a cryogenic section with plate-'n heat e%changers" as dust can collect in the e%changers and reduce heat transfer and dramatically increase pressure drop.
igure B.1 shows a slip stream of dry gas returning to the bed that is being regenerated. Sales gas is sometimes used instead of a slip stream. $he sales gas stream has the advantage of being free of heavier hydrocarbons that can cause co!ing. $his gas is usually about 5 to 60E of gas throughput. egeneration involves heating the bed" removing the water" and cooling. or the 'rst two steps" the regeneration gas is heated to about B00° 365°; to both heat the bed and remove adsorbed water from the adsorbent. *f ;?S formation is a problem" it can be mitigated by lowering regeneration temperatures to 400 to 450 ° 00 to 30 °; or lower" provided suFcient time for
regeneration is available" or by switching to 3). egeneration gas enters at the bottom of the bed countercurrent to ow during adsorption to ensure that the lower part of the bed is the driest and that any contaminants trapped in the upper section of the bed stay out of the lower section. $he high temperature required ma!es this step energy intensive and in addition to furnaces" other heat sources e.g." waste heat from gas turbines that drive compressors are used when possible. $he hot" wet regeneration gas then goes through a cooler and inlet separator to remove the water before being recompressed and mi%ed with incoming wet feed. $o complete the regeneration" unheated regeneration gas passes through the bed to cool before it is placed in drying service. Jas ow during this step can be concurrent or countercurrent. $he Angineering Data 8oo! 004b recommends that the bed pressure not be changed more than 50 psi/min B !(a /s. $herefore" if the adsorption process operates at high pressure" regeneration should ta!e place at as high a pressure as possible to reduce the time needed for changing the pressure. >owever" as :alino 004 points out" higher pressures increase the amount of water and hydrocarbons that condense at the top of the bed and fall bac! onto the adsorption bed. $his unavoidable reu%ing is a maHor cause of bed aging" as it leads to adsorbent brea!down and subsequent 'nes agglomeration ichman" 005. $he ca!ing leads to higher pressure drop. ;ondensation at the bed walls can also occur" which can cause bed channeling.
