US 2881862 A
Description (OCR text may contain errors)
R. N.- FtEcK TAL l ADsoRPTIvE FRACHONATION PRocEss 2 sheets-sheet 1 F1 1ed hay s. 195'/ il-f l v April 14, 1959 R. N. FLEcK ErAL 2583,1,862
-v y ADsoRPTIvE FRACTIONATION yPROCESS Filed May. 1957 y 2 Sheets-Sheet 2 United States Patent ADSORPTIV E FRACTIONATION PROCESS Raymond N. Fleck, Whittier, and Carlyle G. Wight, Fullerton, Calif., assignors to Union Oil Company of California, Los Angeles, Calif., a corporation of California Application May 3, 1957, Serial No. 656,824
18 Claims. (Cl. 18S-114.2)
'This invention relates to the fractionation of uid rmixtures, whether gaseous or liquid, and relates in particular to the fractionation of hydrocarbon mixtures, such as gasoline for example, to separate components or impurities therefrom.
Gasoline improvement to raise the antiknock rating iis required to provide an adequate fuel for modern, high compression, internal combustion, reciprocating piston engines. Gasoline in general comprises many individual components, including hydrocarbons having between about 4 and about 12 carbon atoms per molecule. In this range are found many straight and branched chain parain hydrocarbons, straight and branched chain olefin hydrocarbons yin cracked gasolines, as well as many naphthene and aromatic hydrocarbons. The complexity of gasoline stocks is appreciated by the fact that there are some 660 possible paraffin isomers and over 3800 possible olen hydrocarbon isomers in this range of from 4 to 12 carbon atoms per molecule. In addition, there are a great many naphthene and aromatic hydrocarbons. Each component has its own tendency to knock in internal combustion engines and its molecular character strongly governs this tendency. In general, the straight chain or normal parafin hydrocarbons have the greatest tendency to knock and therefore have the lowest antiknock rating. The naphthene hydrocarbons have a slightly higher antiknock rating, followed by the normal olefins, the iso-olefins, the iso-parains, and the aromatic hydrocarbons have the highest antiknock rating.
The gasoline is a mixture therefore that has anantiknock rating which is a function of its composition. Straight-run gasoline generally has a low rating due to its relatively high normal paraflin content and relatively low iso-paraflin and aromatic content. Cracked gasolines on the other hand have higher antiknock ratings due to the presence of normal and iso-olefin hydrocarbons. Distillation alone is not especially effective in removing only the low antiknock rating components since the rating is essentially determined by molecular structure rather than boiling point.
In the past, effective antiknock rating increase has been accomplished by the addition of materials to the gasoline such as antiknock agents like tetraethyllead, the addition of hydrocarbon components of very high knock ratings such as the alkylate of iso-butane and iso-butylene, or aromatic hydrocarbons, and the like. Antiknock rating increase is also obtainable by high temperature treatment such as thermal or catalytic cracking, rcatalytic or thermal dehydroaromatization or reforming, and the like. Such treatment effectively converts some of the low antiknock rating components into components ,having higher knock ratings.
2,881,862 Patented Apr. 14, 1959 ice The addition of antiknock agents or of hydrocarbons of usually high antiknock rating is expensive since usually they must be purchased or synthesized for such use. Even the high temperature treatments to raise the knock rating are only partly effective since the product from these processes still contains appreciable quantities of low antiknock rating materials. For example, a desulfurized naphtha having a knock rating of 51 (F-l clear) is reformed to effect dehydrogenation of naphthenes and dehydrocyclization of normal parains to form aromatic hydrocarbons over a platinum catlyst to produce prod uct having a knock rating of 96 (F-l, clear). Although this knock rating is satisfactory in some cases, the product still contains about 13% by volume of normal parailins whose antiknock ratings are of the order of 20 or less.
Certain aromatic hydrocarbon mixtures such as coal tar fractions and the efuent produced by catalytic dehydrogenation or aromatization of certain light petroleum fractions contain rather high percentages of aromatic hydrocarbons. These streams are desirable in the blending of high antiknock rating gasolines as indicated above, in the production of aromatic solvents, in the preparation of pure aromatic hydrocarbons for chemical synthesis, and in other uses. The presence of nonaromatic hydrocarbons such as parains and the like in gasoline blending stocks is undesirable. The separation of such hydrocarbons is conventionally performed by superfractionation, low temperature crystallization, and the like. These processes are expensive because of their unusual and frequently extreme operating conditions. `The present invention is especially applicable to these separations, as well as to others.
Therefore the present invention is directed to an improved process adapted to the fractionation of complex uid mixtures in the presence of a solid adsorbent of a particular type, and in which the process is improved by the sequential contact of the rich adsorbent with an exchange component in two stages including a subsequent contact of the lean adsorbent with the eiluent produced in the first exchange component contact stage.
It is accordingly the primary object of this invention to provide an improved adsorptive fractionation process for the separation of complex mixtures to produce at least one fraction consisting essentially of molecules of a particular species.
It is a further object of this invention to increase substantially the contacting efficiency and product purity in such adsorptive fractionation processes by employing plural stage displacement exchange of the adsorbed materials with a recirculated displacement medium under essentially isothermal conditions to produce an intercut for recirculation in the process.
It is a more specific object of this invention to reduce to essentially zero the contamination of product fluids in an adsorptive fractionation process in which the adsorbed fraction is desorbed under isothermal conditions.
It is also an object of this invention to provide an improved process step for contacting adsorbent solids with a fluid mixture whereby progressively increasing uid velocities are maintained throughout the adsorbent contacting zone. g
Other objects and advantages of the present invention will become apparent to those skilled in the art as the description and illustration thereof proceed.
Briefly the present invention comprisesan improved adsoptive fractionation process in which a complex feed mixture is contacted in a first stage with the adsorbent whereby the more readily adsorbable constituents are adsorbed forming a rich adsorbent leaving the less readily adsorbable materials as a substantially unadsorbed lean eiuent. This lean efuent consists of the unadsorbable constituents of the feed stream together with substantial quantities of a displacement exchange fluid displaced from the adsorbent when the adsorbable feed fraction was adsorbed. It is distilled or. otherwise fra :.y
tionated to. separate these components, the displacement exchange stream being recirculated in the process and the nonadsorbable portion of the feed being produced as a process product.
The recovery of the adsorbed fraction from the rich adsorbent according to this invention is characterized by the fact that itis substantially isothermal relative to the feed contacting step and that elevated temperatures are not required in order' to strip or desorb the adsorbed phase from the adsorbent. There are no adsorbent heatf ing or cooling steps as in the conventional adsorptive. fractionation processes. Rather, the contacting tempera-l tures are all about equal, differing slightly and only incidentally from each other, due to some internal thermal effects and to differences in inlet temperatures ofthe adsorbent and the fluid entering the contacting zones. The rich .adsorbent is next contacted with a relatively minor fraction of the total displacement exchange stream in a second stage of the process to produce an intercutfconsisting of a mixture of the displacement exchange fluid and the initially displaced fraction ofuids present' on the adsorbent. These uids include a major proportion of components of the feed which normally arenonadsorbable and a minor proportion of materials which are adsorbable. This intercut is separated and employed in a subsequent step to contact the adsorbent. The thus treated adsorbent is next contacted in the third stage with a major proportion of the displacement exchange stream under substantially isothermal conditions 'to displace the adsorbed materials from the ad sorbent. The stream thusv produced consists of the adsorbable fraction of vthe yfeed together with the displacement exchange lluid. This mixture is separately distilled or otherwise fractionated to recover the displacement exchange stream for recirculation in the process leaving. the adsorbed feed fraction as a process product. Y
The. adsorbent lat this lstage is now saturated -with the displacement exchange stream. It is then contacted in the fourth stage of the process with the intercut produced in the rst displacement exchange stage. The relatively small quantity of adsorbable materials in the intercut is retained by the adsorbent displacing some of the exchange component therefrom. The displaced exchange component together with the nonadsorbable feed conf stituents in the intercut Ypass through substantially unaffected. The composition of this stream is generally very similar to vthat `produced during the vfirstvstage -contact and therefore these two streams are combined. In other words, the eluent from the intercut or fourth stage contact is combined with the effluent from the feed or rst stage contact and is subsequently treated `with itvin the `manuel' referred `to above.
'Ihe `condition of the adsorbent following the fourth stage Vis one in which .the adsorbent is essentially saturated with displacement lexchange fluid but also `'contains a. small proportion of adsorbable constituents derived from the feed and which have accumulated on the adsorbent during `the lfourth or intercut contacting stage. The process is thenrepeated starting with the first or feed contact stage, and ycontinued indefinitely.
Because of the utilization of two-stage contacting with the displacement exchange stream and the separate treatment -of the intercut thus produced, the quality of the products produced is improved substantially, the degree of 'contaminationof cach of` theproduct streams lby corn- 4 ponents desired in the other stream is decreased substantially to zero.
In one specific application of the present invention, petroleum or other naphthas boiling in the gasoline boiling range are very effectively treated to remove normal parain hydrocarbons of low antiknock rating selectively from the other types of hydrocarbons to produce a nonadsorbed high antiknock rating gasoline stream containing the naphthene, iso-paraffin, and aromatic hydrocarbons.
In another application of this invention, highly aromatic hydrocarbon mixtures, such as those produced in catalytic reforming of naphthenic gasoline streams or those produced in the distillation of coal, may be. fraction-4 ated to remove selectively the aromatic hydrocarbons from any vnormal paraflins, naphthenes, and iso-paraflins.
The present type of adsorption on the particular adsorbents referred to has been found to progress at a rate which is of rst order of magnitude. In other words, the rate of adsorption of the adsorbable constituents is proportional to the concentration of the adsorbable constituents in the fluid being contacted. Mathematically, this rate of adsorption is expressed as wherein c is the concentration of the adsorbable material, t is time, andK is a proportionality constant depending upon the system. In order to achieve extensive degrees o f'adsorptive fractionation in the process, the concentration of theadsorbable componentinthe iluid must ap-y proach zero. According to the above relationship the rate -offadsorption therefore also approaches zero so that the time necessary to achieve the desired concentration may become quite long. Accordingly, in one modification 0f the process of this invention, progressively increasing tluid velocities are employed in the various contacting zones whereby the mass transfer coetiicients are substantially increased.
The process and apparatus of thepresent invention will be more readily understood by reference to the accompanying vdrawings in which:
y in the static bed and moving bed modications respectively.
Referring now more .particularly ito Figure l, the essential'` apparatus of the .invention includes first, second, third, and fourth contacting stages or zones 10, 12, 14,
p and 16, and rich eluent .and lean etluent distillation columns or zones A18 and 20:in -whichthe displacement exchange recycle stream is vrecovered from the adsorbable and the nonadsonbable feed constituents respectively.' Thefuidow through each of the .contacting zones is cyclic as controlled by. ithe four inlet control valves 22, 24, 26, and 28 and the four. .outlet control valvesz, .32, 34, and .36 respectively. Cycle .timer operator v38 controlsthe vpositions :of Veach of .the .eight .inlet and .outlet control `valves so that theuid flow through each contacting .zoneprogresses through the series of four contacting periods-in I.the correct staggered sequence outlined above.
In :Figure l fthe first, second, third, and fourth con.- tacting-Qzonesfl, 12, 14,and 16 are. shown .inttheiirsty second, tthird, V`and fourth. .contacting I'stages `referred yto previously. In contactingzone `10 theradsorbent isvbeing contacted Y=with ythe feedzmixture -and :is producing the exchange stream to produce the intercut, in zone 14 it is being contacted with the larger proportion of the displacement exchange stream to produce the adsorbable components as the rich euent, and in zone 16 it is being contacted with the intercut.
For purposes of illustration it is assumed that the feed stream in the present case comprises a catalytically reformed aromatic gasoline having the following propertes:
Table 1 Boiling range C6 400 F. Gravity, API 49.8 Aromatic hydrocarbons, volume percent 44.4 Normal parain hydrocarbons, volume percent 12.9 Knock rating, F-l 3 ml. TEL 97.0
The granular solid adsorbent comprises a specific zeolitic metallo alumino silicate adsorbent having pore diameters of approximately 5 A. and having a chemical composition discussed in more detail below. This particular adsorbent exhibits preferential adsorptive forces for the normal paraffin hydrocarbons while exhibiting substantially no preferential adsorption for the other species of hydrocarbons.
The feed gasoline is introduced in the vapor phase at a temperature of about 440 F. through line 40 at a rate controlled by valve 42 into feed manifold 44. This material is directed through line 46 by inlet valve 22 into rst adsorption zone in which the normal paraffin hydrocarbons of the feed are preferentially adsorbed thereby displacing the adsorbed normal pentane, which in this case is lighter than any feed component and is the displacement exchange stream recycled in the process. The non-normal parafn hydrocarbons and the displaced normal pentane ow through outlet 48 and outlet valve 30 into raffinate manifold 50. Raifinate continues through line 52 into lean effluent distillation column 20. Column 20 is provided with reboiler 54 and overhead condenser 56 and herein the normal pentane is distilled substantially completely from the higher boiling non-normal paratlin hydrocarbons in the lean euent. The normal pentane vapor is passed through line 58 into displacement exchange recycle manifold 60. A portion of the normal pentane is returned as shown as reflux to the column. Line 62 is provided with Valve 64 through which net products of pentanes may be taken including any traces of iso and normal pentane which may be present in the feed stream at a ow rate equivalent to that of the entering feed. This prevents isopentane from building up in the displacement exchange recycle stream to an equilibrium value which would be suiciently high to inhibit effective displacement exchange of normal paratfns of the feed stream by the normal pentane displacement exchange stream.
The normal pentane-free non-normal parafns of the feed stream are produced through line 66 at a rate controlled by valve 68 from the bottom of lean eiuent column 20. This material is cooled in aftercooler 70 and sent to production or further processing facilities not shown through line 72.
In second stage 12 the adsorbent, saturated with normal paran hydrocarbons of the feed stream and containing some residual non-normal parafn hydrocarbons of the feed, is being contacted with a minor portion of the displacement exchange stream to produce the intercut. This material flows from displacement exchange recycle manifold 60 through line 74 and linlet Valve 24 through second adsorption zone 12. The produced intercut, containing the residual non-normal hydrocarbons displaced from the adsorbent and some exchange pentane, is discharged through line 76 and outlet valve 32 through line 78 into intercut recycle manifold 80.
The intercut so produced is returned by means of blower 82 at a rate controlled by valve 84 through intercut heater 86 if necessary tomaintain the intercut in 6 the vapor phase. As indicated previously, the intercut constitutes the feed stream to the fourth adsorption stage in this process.
In third contacting zone 14 the adsorbent is being contacted with the major proportion of the displacement exchange stream. This material ows from manifold 60 through line 88 and inlet valve 26 into and through contacting zone 14. Here substantially all the adsorbed normal paraffin hydrocarbons adsorbed from the feed in stage one are preferentially displaced from the adsorbent in exchange for the normal pentane displacement exchange stream. This produces a rich euent stream comprising the displaced normal paratlins together with some normal pentane. This material flows through outlet valve 34 into extract manifold 90. Manifold 90 opens into rich eluent distillation column 18 provided with overhead condenser 92 and reboiler 94. Here the normal pentane is separated as overhead product from the heavier normal parans derived from the feed stream. The overhead pentane vapor is discharged into displacement exchange recycle manifold 60 through which it is recirculated into the appropriate contacting zone by means of blower 96 at a rate controlled by valve 98. Reheater 100 is provided to raise the pentane vapor displacement stream from its distillation temperature to the contacting temperature of about 440 F. and also to overcome heat losses in the system.
The adsorbed normal paratlns, now free of normal pentane displacement exchange material, are removed from the bottom of distillation column 18 at a rate controlled by valve 102 and passed through aftercooler 104 through line 106 to further processing or storage facilities not shown.
In fourth contacting zone 16, the lean adsorbent is contacted with the intercut stream flowing from manifold and line 81 through inlet valve 28. In zone 16 the small quantities of adsorbable normal paraflin constituents present are adsorbed displacing an equivalent amount of normal pentane into admixture with the unadsorbable non-normal parain hydrocarbons. This eluent contains the same constituents as the lean etlluent from the rst adsorption stage 10. It is therefore passed from outlet valve 36 via lines 83 and 52 into the lean effluent column 20 where the normal pentane exchange component is recovered from both streams.
In this process the composition and physical characteristics of the non-normal paraflin lean product sheam and of the normal paran richproduct stream are given below in Tables 2 and 3 respectively:
1Blending number in iso-octane, 50/50 blend.
It should be understood that each of the four contacting zones shown in Figure 1 and described in the foregoing description is operating in a staggered sequence in which each goes through the four contacting stages described, but out of phase with the others so that the feed stream may be processed continuously and the lean and rich product streams may also be produced continuously.
Referring now more particularly to Figure 2, the process as above described is illustrated in a schematic flow diagram form utilizing a recirculating stream of the granular absorbent passed downwardly throughcontacting.- column 1510-. 'The contacting zoneis provided at successively lower levels. with. sealing, zone 112, lean effluent disengaging zone 114, adsorption zone 116,.. feed engaging; zonev 1'1-8, second sealing-,zone 120,. intercut disengaging zone 122, intercut-:displacement zone. 124, first-.displacement exchange. engaging zone 1'26, third. seal.` ingr zone 128, ,richeiuent dis/engaging zone 130, second` displacement exchange zone. 132, second displacement fluid` engaging zone` 13d-fourth sealing zone 136, secondi leanf eluent disengaging. zone 138,. intercut adsorption: zone,14.0-, intercut engaging zone. 142, and fth sealing zone, 14.4;
- The. granular, solid adsorbentV passesy downwardly in succession through these. contacting zones inthe, form of a. compact movingbed. The adsorbent is removed from thebottom` of contacting column 110 and; recirculated through conveyor 146 to the top of the column. for re-V passage therethrough. In generalthe fluid. pressure existingu in: contacting. column 110.: increases at successively lower; levels. in the. column.. For purposes of close control@ it is preferred thereforeto employ a conveyor of the typewhich has. a pressure dropfrom bottomto. top and: therefore theY pneumatic gaslift type solids conveyors inwhich the solids move asa dilute. dispersion in a iluid, or the densephaseconveyors in whichthe granular solids moveupwardly as a dense packed mass at substantially the same density as the downwardly moving bed maybe used; In either case thesolids conveying forceis generated by a. yiiuid how.. Both of these typesv of conveyors are now-well. known in theartof solids conveyance.
In order to prevent fluid contamination between the topzand.y the bottom of the columnby. reason of the factl that Huid ow through the conveyor can occur, lines 148;.. and- 150 are provided; controlledY respectively by valves 152- and 153v at thetop; and bottom ofthe column for the introduction of.' a seal` tluid. These same lines may be employed if desired for the removal of a sealiluid; comprising a mixture of the conveyance fluid intro` duced into the conveyor through line 154v controlled by valve 156. and removed at the top oftheconveyor throughV line.158. i
`I-or purposes of illustration the moving bed processes of- Figure Zwill be described in connectionwith the selective adsorption of aromatic hydrocarbons contained in a catalytically reformed C7 to. C9 hydrocarbon gasoline streamt. Thejzeolitic metallo alumino silicate adsorbent onev having pores-of about' 1-3 A. in diameter and which, becausey of its. preferential adsorption characteristics, preferentially adsorbsthe aromaticfhydrocarbons leavingthe paraflinic, naphthenic, andiso-parathnic hydrocarbons substantially unadsorbed. The composition of the adsorbent is detailed below.
The feed stream is introduced in the vapor phase at a..
temperature of about 440 'F. through line 160 at a rate. controlled-by valve 162-. This-material passesupwardly countercurrent to the descending adsorbent through adsorption zone 116. The `displacement exchange stream comprises benzene and accordingly the adsorbenty in zone 116 is saturated therewith. The C7 through C9 aromatic hydrocarbons are adsorbed on the adsorbent and the non-aromatic C7 through C9 hydrocarbons remain unadsorbed and' become admixed with the displacement benzene forming thev lean effluent stream. This effluent is removed from disengaging zone 114 through line 164 and is introduced together with the product taken from intercut adsorption zone 140- into lean eluent column 166. This column is provided with overhead condenser 168 and bottoms reboiler 170. The lower-boiling benzene displacement exchange material is removed overhead' and recirculated through displacement exchange. recycle manifold .172. The nonaromatic hydrocarbons boilingabove'benzene areremoved through line. 174 at alratecontrolled by valve 17.6, as the llean or non-aromatic hydrocarbon product..
tained. The acid solubility@ for example, of the extract;
` 'lhe ricl'r adsorbent ows. from the bottom of adsorption zone.K 116 downwardlyinto first displacementyexchange. zone 124. A. relativelyminor` proportion ofthe displacement exchange.. stream .is .passed by means of blower 178 at a rate controlled by; valve 180,v through heater. 182: to raise the benzene. Vapor from:y its. distillation temperature to-the: contacting temperature. The vapor flows,- upwardly countercurrent tothe adsorbenty thereby displacing residual feed components from the adsorbent:`
' f together with a minor proportion of the adsorbed aromatic hydrocarbons. This intercut efuent together withY recycle benzene is removed from zone 124 and passed through line 184 by meansl of recycle blower 18.6. into intercut adsorption zone 140 subsequently described.` e
The thus treated adsorbent then continues downwardly into secondV displacement exchange zone 132. The remaining and: major proportion of the recycle benzene is passed' from; manifold: 17-2. by meansof blower 188` at a rate. controlled: by valve 190- through heater 192. which .f raises the vapor from the distillation temperature to the contacting temperature. The; vapor continues upwardlycountercurrent to the rich` adsorbent in zone 132. Here benzene vapor is;adsorbed on theadsorbent inV exchangefor the adsorbed aromatic hydrocarbons which are dis?. placedproducing an aromatic-hydrocarbon or rich efuent containing part of the recycled-benzene. This eiduenthows through line. 194.l into rich eiuent column 196 provided-with overhead condenser` 198. and bottoms reboiler 200;. The displacementexchange benzene is distilled-over v' head from the heavier aromatic hydrocarbons and intro` duced into recycle. manifoldy 1-72 through line 202 at al rate controlled by valve 204, The aromatic hydrocarbons tree of benzenev are produced as the rich process.4 product through. line 206. controlled by valve 208,.from
- the bottom'of; extract column 196.
As previously indicated, the. intercut isv introduced:v by.v means of blower 186 through a trimmer heater 187 to make upv heatV losses into adsorption zone 140 in which itf is., treated. countercurrently to. the benzene-saturated* adsorbent` discharged from second displacement exchangezone 132-. l-Here the intercut is treated to adsorby the,` traces. of residual heavier aromatic hydrocarbons in ex change. for the adsorbed benzene. The residual non-l adsorbablefeed components of the intercut and thedis-Y. placed'. benzene, constituting` theY intercut adsorption zoneefiluent, flow together through line 210 at a rate con-- trolled by-valve- 212 into-admixture with the lean effluent from adsorption zoney 116. This. material isy as statedf fractionated in leaneihuent still' 1-66 for separation of the-r displacement exchange benzene and the nonaromaticfeed hydrocarbons.
Line 167 controlled by valve 1692 is-.provided to; bleed out any nonadsorbablehydrocarbons-l boiling below the C, hydrocarbons which may. appearl in the feed in trace amounts.
Inorder to-,avoidintermix-ingof'the-uids in the 'rst, -second third,. and fourth contacting zones 116, 124-, 142, and 1.40 respectively, a diiferential pressure controllerl instrument is provided. connected between each of. zones 118-122, 126-4305. andi. 1-34--1-38y and respectively.` actuating a valve in the line opening from the disengaging.. zoneassociatedwith eachof the three pairs of adjacent zones. A zero pressure differential is thus maintained1 by. thesercontrollersacross. each pair of zone-s to mini-. mize-,interflow A typical connection of this type is illustrated at zones; 118-122 and involves controller 123 and valve 1-25 in the outlet. line. 184..
The composition of the lean nonaromatic and rich aromatic products produced indicates that a substantially complete separation of the aromatic hydrocarbons from the nonaromatic hydrocarbons present in the feed is obis 100% whereas the acid solubility ofthe raffinate is cus.- tornarily lessithan 05% indicating that. a substantially complete separation at near 100% eiciency ofthe aro-` matic hydrocarbonsresults. y
The relative quantities or liow rates of the displacement exchange lluid in the second or intercut displacement zone and the third or rich efuent displacement zone vary somewhat depending upon the nature of the adsorbed materials, the nature of the exchange uid, and the quantity of feed iiuid held up in the adsorbent after feed contact. In general however between about 1% and about by volume of the total flow of exchange uid is used in displacement of the intercut and the remaining 90% to 99% is used in the subsequent displacement of the rich efliuent. In most cases however these relative ows are between about 2.5% and about 7.5% and between about 97.5% and about 92.5% respectively.
In the static adsorbent bed modification, this means that after the minor portion of exchange iuid has been introduced into the adsorber and the intercut collected, the outlet is shifted to collect the rich etiiuent which follows as the major portion flows in.
In the moving adsorbent modication, the relative flow rates of exchange fluid into the second and third contact stages or zones are controlled to maintain these same proportions.
Referring now more particularly to Figure 3, a simpliiied elevation View of a modification of the contacting zones shown in Figure 1 is illustrated in which a static vbed of granular adsorbent is employed. Adsorption vessel 220 is provided at its upper end with fluid inlet 222 and at its lower end with outlet 224. A static bed of adsorbent 226 is indicated schematically within the vessel. As illustrated, the `diameter of the vessel and accordingly its cross sectional area open to flow decreases in the flow direction. The extent of this decrease may vary depending upon the circumstances, but preferably it is sucient to impart to the unadsorbed phase passing through the adsorption zone an increase in velocity of at least 50% and preferably between about 150% and 500% of the velocity. In other words, in this process where exchange displacement takes place, the cross sectional `area open to fluid flow through the adsorption vessel shall decrease at least about 33% and preferably between about 60% and about 83% from the open area at the iiuid inlet in order to effect this `desired increase in fluid velocity. In this Way it has been found that the usual adverse eect upon adsorption rate of decreasing concentration of the adsorbable constituents in the uid contacting the adsorbent may be largely overcome and substantial decreases in the size of the adsorber and in the quantity of adsorbent necessary in each adsorption zone are realized.
It is intended that the apparatus shown in Figure 3 and described above be substituted for any or all of the various adsorption zones 10, 12, 14, and 16 illustrated in Figure l.
Referring now more particularly to Figure 4, a fragmentary view of a modification of the moving bed adsorption column 110 of Figure 2 is shown in which the cross sectional area open to uid tiow increases with distance from the solids inlet in any given contacting zone. This is provided in adsorption vessel 228 having upper solids inlet 230, lower solids outlet 232, a lower fluid inlet 234 and an upper iiuid outlet 236 to permit countercurrent solids fluid contact. If desired, the tapered portion 2.38 of vessel 228 may be provided with an upper cylindrical portion 240 in which relatively high velocities are maintained for a period after the velocity increase. The ow rate through the vessel in relation to the transverse area open to iiow may not be increased above a certain maximum value or else the downward movement of solids is impeded by the pressure gradient developed by the ow, but the relative velocity changes described above may still be effected.
The foregoing descriptions illustrate the application of the present invention to the processing of two industrially representative streams of complex Huid mixtures.v
It is to be understood that this is not intended to be re- Table 4 Boiling range, F 120-218 n-Parans, vol. percent 26.6 Gravity, API 71.2 Knock rating, F-1-|3 ml. TEL 87.4
This material was treated in the vapor phase at atmospheric temperature with a bed of the 5 A. zeolitic metallo alumino silicate adsorbent discussed above at a temperature of 300 F., the exchange component was normal pentane vapor at the same temperature, no intercut displacement or readsorption was used, and the lean nonnormal parain hydrocarbon efliuent and the rich normal paratrn hydrocarbon effluent had the properties given respectively in Tables 5 and 6.
Table 5 Boiling range, F. 1Z0-218 n-Paratiins, vol. percent 4.0 Gravity, API 67.8 Knock rating, F-l-i-S m1. TEL 94.3
Table 6 Boiling range, F. 120-218 n-Paraffins, vol. percent 80.0 Other hydrocarbons, vol. percent 20.0 Knock rating, F-1 clear 12.5.0
1 Blending number in iso-octane, 50/ 50 blend.
Under the same conditions however, as above given, but in which an intercut was displaced from the rich adsorbent and was readsorbed by the lean adsorbent according to this invention, the corresponding data for the lean and rich eiiiuents respectively are given below in Tables 7 and 8.
Table 7 Boiling range, F. 1Z0-218 n-Paraiins, vol. percent 0.5-1.0 Gravity, API 68.4 Knock rating, F-1i3 m1. TEL v94.9
Table 8 Boiling range, F. 120-218 `n-Paratins, vol percent 97.0 Other hydrocarbons, vol. percent v 3.0 Knock rating, F-l clear 115.0
1Blending number in iso-octane, 50/50 blend.
The substantial improvement in product purity is apparent from these data, and specifically the normal parafiin contamination is lowered from 4% to below 1.0% in the lean non-normal paraiiin efuent while the concentration of normal parans in the rich normal paratin eiuent has been raised from to 97% by volume.
The adsorbent employed in the process of this invention is a solid granular material having a' mesh size range between about 2 and 200 mesh. It is used in the form of a dense compact bed of material, preferably between about 4 and about 30 mesh, through which the feed and displacement exchange recycle streams pass, either in the vapor phase or in the liquid phase. The process may employ the adsorbent in the formof a single static bed 0f material inwhich case the process is only semicontinnous. Preferably two or more static beds of adsorbent are employed with appropriate remotely operable. valving sothat thefeedi stream is passed through one or more of theadsorbers'in. a set while the exchange displacement stream passes through one or more of the other adsorbers in the set. In this case, the feed andproduct iiowsv are continuous, in either the vapor or liquid phase, and either up. o1: down through the adsorbent.. The more physically ruggedf types of granular adsorbent permit the use of the moving solids bed modification in which -ow of feedv is maintained continuously through an adsorption zone, the flow` of displacement exchange fluid is maintained continuously through a desorption zone, and the granular adsorbent is recirculated successively through these two zones.v With the smaller sized mesh ranges of adsorbent', such as from about 30 to 300 mesh, the material may beluidize'd in and by the iiuid streams contacting it, although the compact bed modifications are preferred since a lgreater number of theoretical and actual contact stages are morel readily obtained in smaller and simpler equipment.
Although the present invention may be carried out with mostall ofy the commonly available solid granular ad sorbents, particular adsorbents lendl themselves to processing particular feedstocks depending upon the specific separation which is to bemade. For example, in the sepa-ration of aromatic hydrocarbons from coal oil or petroleum hydrocarbon fractions, an adsorbent which preferentially adsorbs the aromatic hydrocarbons should be used. Adsorbents such ask silica gel', certain ofthe preferred adsorbents, and other polar adsorbents are effective in this service. In the `treatment of gasoline streams to remove straight chain, low antiknock rating components, an adsorbent which is highly eflicient and preferred in this application of the present invention are the; natural or synthetic crystalline partially dehydrated.
metallo alumino. silicates. The composition of one typical synthetic zeolite having a pore size of about 4 A. is
[QNa2O-A1ZO3.-(SO2),2]. It may be prepared by heatingstoichiometric quantities of alumina and silica and excess caustic under pressure. The excess is washed outy and the desired metal ion may then be introduced by ion exchange. Part of the sodium in this material can be ion exchanged with concentrated salt solutions at superatmospheric pressure and4 temperatures of 15C-300 C. to `introduce other metal ions such as calcium. to produce having a pore size of about 5 A. Certain naturally occurring minerals such as chabazite, analcite, gmelinite, and. the like, can be heated to dehydrate the molecule and obtain an activated zeolitic adsorbent similar in adsorption properties to the manufactured materials.
The foregoing adsorbentsl are typical of those having pore diameters of 7 A. and less and these are especially welladapted to the adsorption of paran hydrocarbons.
from complex hydrocarbon mixtures.
Another form of zeolitic metallo alumino silicate has a composition substantially corresponding to slargo-.m1203-15sio2 and this has, uniform pores ofl approximately 13 A. in diameter. Another similar material has a, composition correspondingy substantially to 5CaO6Al2O315SiO2 and it, has pore diameters of about 1'0 A. These and other zeolitic. adsorbents having pore'diameters of greater than aboutl7 A. are not, particularly effective in parain hydrocarbonadsorption, but they are also extremely ecient, in` the adsorptive separation of aromatic hydrocarbonsI and sulfur compounds from complex hydrocarbon streams` containingl these materials.Y
These` natural andA syntheticy materials are all zeolite's.v and their sodium: andy calcium derivatives are; very stable adsorbents which apparently 4have pores availabley for adscrgticuwhich are quite uniform in size., Other de 12 rivatives have different sized pores These are the preferred adsorbents for use in the process of this inven tion. The molecules which are the more readily ad'- sorbable and for which the adsorbent, having pores, of lessthan 7 A., exert preferential adsorptive forces are those having straight chain molecules whose minimum dimensions are equal yto or slightly less than these pore dimensions. Thus the normal paraflins and normal olefins with cross chain dimensions of under 5 A. are very strongly and Very readilyadsorbed by these below 7 A. materials. However the branched chain parans or ole-V iins, and the naphthene and aromatic hydrocarbons, allhaving molecular dimensions in the shortest direction in excess of 5 A. are substantially nonadsorbable. These adsorbents are thus selective for normal paraflins, and normal olefins if present, and will not adsorb any appreciable quantity of other hydrocarbons. Other derivatives of these particular inorganic adsorbent materials have uniformly sized pores greater than about 7 A. and ranging as high as from 12 to 13 A. These will adsorb molecules having a dimension less than about 12 A. such asaromatic hydrocarbons and will exclude a material whose.
minimum molecular dimension is above l2 A.
Other adsorbents such as activated charcoal, activatedk alumina, and other well known materials are applicable herein to particular feed mixtures. However, their specilic affinites for particular compounds must be consideredV in connection with the specific feed mixture in order to determine whether a particular component will be present in the extract phase produced from the adsorbentv by the displacement exchange or will be presentr in the raiinate phase from the adsorber.
Some of these adsorbents, particularly silica gel and activated alumina tend to adsorb rather strongly polarr materials to varying degrees. The metallo alumino silicates adsorb polar molecules to a considerably lesser extent. In gasoline treating this interferes with the fractionation of gasoline hydrocarbons as a function of molecular shape. Accordingly it is contemplated in this invention to contact the feed stream first` with a material which exhibits very strong adsorptive forces for these polar materials and remove them from the stream to be treated. This pre-adsorption or pretreating of the feed may be accomplished by contacting the feed stream with an inorganic halide such as copper chloride, calcium chloride, magnesium chloride, and the like. In this way large and highly polar materials such as ethers, thioethers,
water alcohols, mercaptans, and amines are readily removed from the feed. Also removable in this way are the highly polar nitrogen and sulfur compounds which commonly occur in small amounts in gasolines. These specifically include such materials as thiophene and the alkylated thiophenes, pyridine and alkylated pyridines. Thus this pretreatment removes these polar materials and prevents them from interfering with the subsequent fractionation in which the feed is separated into streams containing components of a specic molecular size or structure.
Although the pre-adsorption step prevents rapid deactivation of the main adsorbent beds, some deactivation may eventually occur. It is within the contemplation of this invention to regenerate the adsorbent periodically by high temperature stripping with steam etc. to desorb impurities with hot flue gas, or to burn off the impurities as in catalyst regeneration, or both.
A particular embodiment of the present invention has been hereinabove described in considerable detail by way of illustration. It should be understood that various other modilcations and adaptations thereof may be made by those skilled in this particular art Without departing from the spirit and scope of this invention as set forth in the appended claims.
1. A process for separating u'id mixtures which comprises iirst contacting the mixture With a solid adsorbent saturated with a displacement exchange component thereby adsorbing the more readily adsorbable constituents of said mixture and displacing said exchange component into admixture with the less readily adsorbable constituents forming a rst lean efHuent, separating said exchange component from said lean effluent leaving said less readily adsorbable constituents, next contacting the adsorbent with a minor portion of recirculated exchange component to displace an intercut efuent, then contacting the adsorbent with the remaining portion of said exchange component thereby resaturating said adsorbent with part of said exchange component and displacing said more readily adsorbable constituents from the adsorbent into admixture with the remainder of said exchange component to form a rich eilluent, separating said exchange component from said rich effluent leaving said more readily adsorbable constituents, contacting said adsorbent with said intercut eluent to adsorb the more readily adsorbable constituents present therein and displacing into admixture with the less readily adsorbable constituents thereof some of the exchange component as a second lean eiuent, combining said second lean eluent with said rst lean eliluent for separation of said exchange component, and recirculating the exchange component recovered from the combined lean effluents and from said rich euent in the process.
2. A process according to claim l wherein said displacement exchange component has a boiling point different from that of most of the constituents of said said fluid mixture, and wherein it is separated from said lean and rich eiiluents by distillation.
3. A process according to claim 1 in combination with the steps of controlling the llow of exchange component so that said minor and major portions thereof are between about 1% and about 10%, and between about 99% and about 90%, by volume, respectively.
4. A process according to claim 1 wherein the adsorbent is contacted with said feed mixture, the minor and major portions of exchange component, and said intercut eiluent in sequence at substantially the same temperatures.
5. A process according to claim l wherein a plurality of static beds of said adsorbent is contacted in a staggered sequence with said liuid mixture, said exchange stream, and said intercut to produce substantially continuous streams of the less readily adsorbable and more readily adsorbable constituents as process products.
6. A process according to claim l wherein said absorbent is recirculated in a stream successively through a feed mixture contacting zone, an intercut displacement zone, a displacement exchange zone, and an intercut adsorption zone in a contacting column.
7. A process according to claim 1 in combination with the step of increasing the velocity of fluid flow in the ow direction during contact of at least one of said uids with said solid adsorbent.
8. A process according to claim 1 wherein said uid mixture comprises a hydrocarbon mixture.
9. A process for the separation of fluid hydrocarbon mixtures which comprises contacting the feed mixture with a lean adsorbent saturated with an exchange hydrocarbon boiling apart from said mixture thereby adsorbing the more readily adsorbable hydrocarbons and displacing said exchange hydrocarbon into admixture with the less readily adsorbable hydrocarbons of said mixture as a first lean eifluent, then contacting the adsorbent with a minor portion of recycled exchange hydrocarbon to displace an intercut effluent therefrom, next contacting the adsorbent with a major portion of recycled exchange hydrocarbon thereby resaturating said adsorbent with one part and displacing said more readily adsorbable hydrocarbons into admixture with the other part of said major portion to form a rich efliuent, finally contacting the adsorbent with said intercut efuent to adsorb traces of said more readily adsorbable hydrocarbons to form a second lean eluent, combining said rst and second lean effluents, distilling said exchange hydrocarbon therefrom, distilling said exchange hydrocarbon from said rich eluent, recirculating the thus recovered exchange hydrocarbon in the process, controlling the relative volumes of said minor and major portions of said exchange hydrocarbon to between 1% and 10% and between 99% and 90% by volume respectively and thereby effecting eihcient adsorptive separation of said less readily and more readily adsorbable hydrocarbons from one another without the usual cyclic heating and cooling of said adsorbent.
10. A process according to claim 9 wherein said adsorbent comprises a zeolitic metallo alumino silicate activated by partial dehydration and having substantially uniform diameter pores of 7 A. and less, and said more readily adsorbable hydrocarbons comprise normal paraiiins.
11. A process according to claim 10 wherein said zeolitic metallo alumino silicate has a composition corresponding substantially to [CaO-A12O3(Si02)2]0.7 [Na2OAl2O3-(SiO2)2]0,3 and has substantially uniform diameter pores of about 5 A.
12. A process according to claim 9 wherein said adsorbent comprises a zeolitic metallo alumino silicate activated by partial dehydration and having substantially uniform diameter pores in excess of 7 A. and said more readily adsorbable hydrocarbons comprise aromatic hydrocarbons.
13. A process according to claim 12 wherein said zeolitic metallo alumino silicate has a composition corresponding substantially to 5Na2O-6Al203'15SiO2 and has substantially uniform pore diameters of about 13 A.
14. A process according to claim 9 in combination with the steps of maintaining an adsorption zone, an intercut displacement zone, a displacement exchange zone, and an intercut adsorption zone, passing said adsorbent as a moving bed downwardly by gravity through said zones in succession, and recirculating said adsorbent from the lowermost to the uppermost zone for repassage therethrough.
15. A process according to claim 9 in combination with the steps of maintaining an adsorption zone, a rst sealing section, an intercut displacement zone, a second sealing section, a displacement exchange zone, a third sealing section and an intercut adsorption zone; passing said adsorbent in a moving bed downwardly by gravity through each of said zones in succession; and controlling the flow of iiuid from one zone to another so as to maintain a pressure differential substantially equal to zero across each of said sealing sections.
16. A process according to claim 9 in combination with the step of adjusting the temperature of said exchange hydrocarbon distilled from said lean and rich effluents to a value substantially equal to that at which said feed mixture contacts said adsorbent whereby said more readily adsorbable hydrocarbons are recovered through displacement exchange from said adsorbent at substantially the same temperature at which they were adsorbed thereby from said feed mixture.
17. In a process for the adsorptive fractionation of a Huid mixture of constituents of different degrees of adsorbability which comprises contacting a solid adsorbent with the fluid mixture to adsorb part thereof leaving an unadsorbed lean efHuent and then desorbing the adsorbed part by contact with a desorption uid to form a rich effluent and leaves lean adsorbent, the improvement which comprises collecting an intercut efliuent produced during the ow of not more than the iirst 10% by volume of the desorption uid separately from the rich efuent produced during the flow of at least the subsequent of said fluid, then contacting said adsorbent with said intercut effluent to produce a second lean efluent, and combining said second lean euent with said unadsorbed lean effluent whereby intercontamination between the lean and rich eiuents is substantiallyA reduced. 18. A process according to claim 17 wherein the fluid mixture and said' desorption fluid contact said adsorbent separately at substantially the same temperature, in cornbination with the steps of separating said desorptionuid from said'v lean and rich effluents and4 recirculating it as a displacement exchange fluid into contact with said adtherewith;
References Cited in the le of this patent UNITED STATES PATENTS Weierman Ian. 24, 1950 Berg Mayv 19, 1953