US2372668A - Process for selective absorption - Google Patents

Description

April 3, 1945- I K. H. HACHMUTH 2,372,668

PROCESS FOR SELECTIVE ABSORPTION Filed Oct. 5, 1942 2 Sheets-Sheet 1 POLYMER FOOMATION IN FURFURAL FURFURAL' MAKE-UP ADDED HERE o 0 FIRST RUN x SECOND RUN o 2 4 e a lo I2 WEIGHT PERCENT POLYMER IN FURFURAL INVENTOR KARL- H. HACHMUTH BY'M WW FIG. "W 7 ATTORNEYS Ap 3, 1945. K, H. HACHMUTH- ,5

PROCESS I FOR SELECTIVE ABSORPTION Filed Oct. 5, 1942 2 Sheets-Shee't 2 ATTORNEYS Patented Apr. 3, 1945 PROCESS ron SELECTIVE nnsoar'riou ,Karl H. Hachmuth, Bartlesville, Okla, assignor to Phillips Petroleum Company, a corporation of Delaware Application October 5,1942, Serial No. 460,874

. vent extractionwith furiural. In the preferred 9 Claims.

This invention relates to a process for the separation of oleilns 'or diolefins from hydrocarbon mixtures containing the same by means of furfural as a selective solvent. Still more particularly' it relates to an improved method for the utilization of furfural as an absorbent or selective solvent for the segregation of low-boiling aliphatic olefins or aliphatic conjugated dioleflns, especially those containing four carbon atoms to the molecule, namely, normal butenes or butadiene. f

This application is a continuation-in-part of my prior and copending application Serial No. 430,307, filed February 10, 1942.

Various hydrocarbon conversion processes,

'such as dehydrogenation of parafiins or oleiins or mixtures thereof, high temperature-low pressure cracking of low-boiling parafiin hydrocarbons, heavy oils,'etc., produce complex mixtures of hydrocarbons containing valuable olefins or practice, what is now known as extractive distillation is employed. that is the extraction is combined m'th fractional distillation or'rectification in a, fractionating column, the furfural being supplied continuously to the top of the column and descending therein, and the hydrocarbon teed being vaporized upwardly countercurrently to the furfural, a reboiling arrangement being provided at the bottom of the column. The separation of butadiene in this manner is described dioleflns or both. Such mixtures are valuable sources of these valuable olefins and diolefins when significant amounts of the materials can be economically separated in a suitably pure form. This is especially true of aliphatic coniugated diolefins, such as butadiene, which have important uses in the preparation of high molecular weight polymers, such as synthetic rubber.

Efllcient fractional distillation of such complex hydrocarbon mixtures ordinarily results in separation into portions consisting essentially of hy-.

drocarbon atoms and exhibiting boiling ranges of say 10 to'- 30 F. Furtherv precise fractionation may eiiect appreciable segregation of the individual components, but ultimate separation by this method is expensive and generally impracftical because of the closeness of the boiling points in my copending applications, Serial No. 454,312, filed August 10, 1942, and Serial No. 438,844, filed April 13, 1942, and in the copending application 01 F. D. Mayfield, Serial No. 363,821, filed March 17, 1941. The separation of butenes and butadienes from their respective streams in this'manner is described in the copending application of Arnold, Serial No. 450,725, filed July 13, 1942.

The temperature at the bottom of the extraction-rectification column is the boiling point oi the liquid at that point under the operating pres- 7 sure of the column. The operating pressure is sufiici'ently high to permit condensation of a portion of the overhead vapors for refiux. The liquid in the bottom of the absorption column is composed of-furiural having dissolved therein the a selectively absorbed hydrocarbons. The iurfural dissolves diolefins in preference to the more satuand theformation of constant boiling hydrocar bon mixtures or azeotropes. V

For example, the clean separation'of butadiene from admixture with isobutylene or butene-l or both by fractional distillation is practically impossible because of their extremely close boiling points: Again, the separation by fractional distillation of butadiene from normal butane is impossible becausethese two components form an azeotrope which contains approximately 80% buitadiene and 20% normal butane and boils be-' tween butene-l and butadiene so that it is praci v-tically inseparable from'butene-l and isobutyl- The practice has now been developed in the art of separating 'butenes, especially normalbutenes,

and of butadiene from C4 hydrocarbon streams containing the same by means of selective sol-=' rated olefins or parafiins and dissolves olefins in preference to parafilns. The bottoms liquid is removed and introduced into another column near the top. In this second column the dissolved hydrocarbons are stripped out of the top while the lean furfural bottom-liquid is returned to the absorption column. Again, the temperature at the bottom of the stripping column is the boiling point of the liquid in the column at the operating pressure.

. Thus the furfural used in a process of the type outlined above is continuously being subjected to repeated vaporization at relatively high'temperatnres, the highest occurring inthe kettle of the stripping column where the kettle product is furfural practically'free 'from absorbed hydrocarboos.

.Furfural employed in the process described above in the presence of butadiene, butenes and related unsaturated hydrocarbons and under the influence of the heat and pressure employed deteriorates. This deterioration primarily involves the formation of heavy polymeric material causedby polymerization of iurfural. The formation of the po ymer is also accompanied by an increase in acidity of the furfural which is objectionable minor part of the acids formed are may be obtained in the extractive distillation volatile and the balance non-volatile. The

mechanism. of the polymerization of furfural under' the foregoing conditions is not understood at present and the invention is not limited by any theory with respect thereto. The formation ;of polymer, gum, and other heavy or high-boiling materials may be due to polymerization or condensation of the furfural or of the dioleflns and/or other unsaturates or to co-polymerizastantially reduces the polymer formation in addition to improving the selectivity of the solvent for the light unsaturated hydrocarbons, it does not by any means completely eliminate the polymerization. The process or the present invention i equally applicable whether anhydrous or water-containing turfural is used. In this specification, unless the context requires otherwise, the term furfural is to be taken as comprehending either anhydrous furfural or a solution of water in furfural.

Among the disadvantages of the polymer formation are the yield of desired hydrocarbon prod ucts is reduced, appreciable quantities of furfural are destroyed involving the continual expense of providing make-up iurfural, and the efiiciency of the iurfural as a solvent is diminished due to the presence of dissolved and suspended heavy material. Furthermore, the equipment becomes clogged and corroded by the polymer and attendant acid, reducing the capacity and efiiciency of the equipment and requiring frequent shutdowns for cleaning.

The principal object of. this invention is to provide an improved process of separating olefins or diolefin from hydrocarbon mixtures containin the same by means of furfural. Another object is.

to provide an improved method of extracting butadiene by means of furfural. Another object is to provide an improved process for the extractive distillation of a butadiene-containing 'Ci stream with f urfural. Another object is to provide a process of the foregoing type for the segregation of butene. Another object is to provide a vented. Numerous other objects will hereinafter appear.

In the accompa ying drawings: Fig. 1 gives two curves showing the variation in polymer concentration with use in extraction of butadiene from a C4 stream by means f furfural, and illustrates how the polymer formation is greatly accentuated after its concentration reaches the 5% figure. Fig. 2 is a schematic diagram of one arrangement of equipment found suitable for carrying out the process of the present invention.

I have discovered that greatly improved results of iural and is a non-solvent for the polymer.

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low-boiling aliphatic olefins and diolefins from hydrocarbon streams by means of furfural as the solvent if the furfural be removed from the system either periodically or continuously, and treated to free it. from polymer before its re-introduction to the system.

For example the stream of stripped polymercontaining iurfural withdrawn from the stripping column may be, in part or in toto, passed to a polymer removal unit wherein polymer-free fur-, fural is separated. This polymer-free furfural is recycled to the extraction-rectification column.

The removal of polymer may be accomplished in a number of ways, for example, by distillation of the furfural away from the polymer preferably with steam, for example by the method described in the copending application of Buell and Cooper, Serial No. 460,004, filed September 28, 1942, wherein the f'urfural is recovered from admixture with polymer by passing the mixture through a.

series of successive fractionating zones maintained. at successively decreasing temperatures, steam being injected directly into the final zone only.

Instead of the. preferred distillation of polymer-free furfural away from polymer, I may use other methods of separating furfural from polymer. Thus I may extract the fur-fural from the polymer-furfural solution or mixture by means of a liquid which preferentially dissolves the fur- Water is the preferred solvent for this purpose. For example, I may admix the polymer-furfural mixture with a relatively large amount of Water to effect precipitation of the Water-insoluble polymer dissolved in the furfural by dissolving the furfural in the water, following which I may separate the aqueous furfural solution from the polymer as by filtration, centrifuging, decanting, etc., and treat same to recover concentrated furfural for re-use in the extraction-distillation column. Thus the water-furfural phase may be distilled to remove overhead an azeotrope of furfural and water which may be condensed to from two layers, one layer of water containing furfural in solution and which may be recycled to the polymer precipitation step, and the other of furfural containing in solution a small amount of water and which may be recycled to the extraction column. i

The solution of the furfural in water may be expedited by the use of a temperature sufficiently high to efiect miscibility, so that a substantially smaller amount of water may be employed. Thus the polymer-furfural mixture may be admixed with the limited amount of water whereupon the mixture is heated to say 275 F. under a pressureof 40 pounds gauge to effect miscibility between the water and furfural and precipitation of the polymer which may then be settled out. The hot furfural-water phase is then separated and cooled to say 80 F. at which temperature water is no longer miscible with furfural so that two layers are formed, one of water containing limited amounts of furfural in solution and which may be recycled to the precipitation mina, etc., whereby the high molecular" weight posits may be converted to coke.

other methods of polymer removal now known to the art or hereafter discoveredmay be utilized in carrying out the invention in its broad aspects, although I. prefer to distill the furfural The separation of furfural from its polymer and other impurities which have accumulated therein. has, in accordance with the presentin vention, been accomplished by distillation of the furfural from the solution of furfural in polymer, thereby leaving the polymer and other impurities as bottom product. The temperature of distillation for furfural removal from furfural-polymer solutions increases quite rapidly as the percentage of polymer, gum and other impurities increases. Since the formation of polymer, gum and coke in the solutions increases quite markedly with rise in temperature, it desirable to carry out purification steps involving heating of furfural or lurfural solutions at as low a temperature as possible. As pointed out hereinafter, I have still further found that the rate of polymer formation is greatly accelerated after a certain polymer concentration, usually about 5%; is attained and therefore I prefer to carry out the butene or butadiene extraction-rectification, the stripping,

and the furfural purification steps at polymer concentrations below this point and as low as possible inorder to avaid this undesirable effect. Other disadvantagesattend concentrations of polymer above the point referred to. For example, where the purification of furfural is carasmcos polymeric material is selectively adsorbed. Still from the polymer under conditions preventing minimum loss of furfural in forming undesirable products. A j Even greater advantages are attained when the furfural distillation is conducted in the preferred manner, namely, by means of steam. In such a furfural recovery process, the overhead consists of i'uriural and water and upon condensation yields two layers, one of furiural saturated with water and sdaptedfor =re-use in the extraction process and the other of watersaturated with furfural. When using steam distillation to seping polymer initially somewhat in excess of 5%, 40

under the usual conditions of operation, the heating of the solution is accompanied by further I polymer formation and considerable coke formation. This polymer and/or coke deposits on the heating coils and acts-as a'heat insulator reducing the rate of and eventually preventing the distillation of the furfural. The polymer de- Furthermore the local overheating due to the initial coke formation together with an apparently catalytic eflect of the initial coke formed greatly accelerv I 4 ates the rate of carbonization of the furfural. Another disadvantage of this type of operation is the corrosive nature of the decomposition products of turfural which. together with localized overheating results in excessive corrosion. This disadvantage coupled. with the auto-catalytic action of-the gum or polymer when the polymer concentration exceeds 5% makes operation at polymer concentrations above 5% exceedingly unfavorable.

I have found that if the f'urfural solution being purified contains initially, less than 5% by weight of the polymer, the purification even by arate the furfural from polymer, the temperature is substantially below theboilins point of pure furfu-rsl, and may vary from 200 to 300 F.

depending upon .the pressure maintained and mer percentage in the iurfural is plotted as ab'scissae against the cumulative hydrocarbon feed as ordinates. The break in the curve representing the first run is due to the addition of makeup furfural which has, the effect of diluting the polymer and thereby delaying somewhat the ill eilects of excessive polymer.

' The following tabulation of calculations made from the curves of Fig. 1 shows the rate of polymer formation in relation to the percentage of polymer already in, the .furfural. The rate .of

polymer formationmay be considered to be' the rate of change of slope of the curve. of polymer concentration plotted against time or length of use or cumulative feed treated; Y

Rate of polymer-formation, percehtiormed per thou- Wt. per cent polymer in iurfurel sand throughput I First run Second nm' Promthistabulatiomitwillbeseenthatthe rate oi! gum or polymer formation increases rapidly as the polymer content of the mum in? creases 'above about,5% by weight. Thus, the

maintenance or a relatively low polymer concentration by means of remnning a suitable amount of the furtural minimises furfural losses by polymerization.

Referring to me. 2 of the drawlnss, thebutadime-containing C4 hydrocarbon feed enters the via line I to extraction-distillotion column I which is supplied atits top with furlural introduced via line I. The tower is equipped with rebelling coil 4, and with means (not shown) for refluxing a suitable portionof The stripped butadie'ne-' the overhead vapors. free'vapors leave via line 5. The butadiene-rich i'uri'uralpassesvialineitostrippingcolumnl where itvis stripped oi butadiene in known lull;-

refluxing a suitable portion or the overhead is usually provided.

The stripped furfural leaves tower ll via line Ml. This furiural contains polymer in the same proportion relative to furfural as is present in columns 2 and 71. Part or all of this polymer-containing furfural, depending upon conditions and the wishes of the operator, is passed via line i l to polymer removal steps presently to be described. Any balance is recycled to tower 2 via lines l2 and 3.

Polymer removal is shown as effected by steam distillation in tower or still 83 to the base of which the steam is admitted via line it. The unit it may comprise a series of evaporating units or zones as described in the Buell and Cooper application above-identified. Overhead via line it pass the pure furfural and water free from polymer. The polymer, together with small amounts of Water and with or without small amounts of residual furfural leaves as bottoms via line iii.

Since at atmospheric temperature the polymer is dense and so viscous that it does not flow readily, it is desirable to retain a small amount of furiural in the polymer to keep it sufficiently fluid to permit easy removal from the still and to prevent local overheating and consequent coke formation on the heating elements of still iii.

The overhead vapors of pure furfural and water flow via line l5 to condenser [ll where they are condensed to liquid which flows via line it; to

separation unit it in which layer separation, as by gravity is affected. The upper layer of water leaves via line while the lower layer of furfural leaves via line 26 and is recycled to iurfural storage. 22 or via line 23 to line 3. Makeup furfural may be introduced via line 26 directly to storage tank 22 or via line 26 to line Sufficient of the polymer is removed from the furfural in accordance with the present invention to maintain the polymer concentration in the furfural in the system, i. e., in at least the extrac tion and stripping units, any furfural storage tank and the associated piping and other equipment at a figure not exceeding that at which the ratio of polymer formation is greatlyaccelerated, or in other words, the point at which the rate of change of slope of the polymer concentration curve de n'atesfrom the linear in thedirectlon of disproportionately increased polymer formation. In the practical applications of the invention, both in commercial plant operations and in experimental work, this upper limit of operation has been at-the 5% polymer-in-furfural figure While it is conceivable that lower boiling point of furfural containing less than 5% of polymer by weight, thus allowing more emclent and lower temperature absorption and stripping. Another advantage is that the furfural low in polymer has higher selectivity for the unsaturated hydrocarbons being extracted.

In a continuous operation in which the furfural-polymer mixture is continuously passed through the furfural still, a further and marked advantage is that the length of time during which the polymer is-exposed to the distilling tempera.- ture is ataminimmn.

A very important additional advantage or" my invention is that most of the acids which are formed in the furfural'in the extraction and stripping columns, apparently concomitantly' as acetic acid, was reduced from 0.292% to 0.040%. Since the larger proportion of the acidsv is concentrated with the polymer in the polymer removal by distillation, the action of the acids in accelerating corrosion of equipment, polymer for- Parts by weight .Propane 8 Butadiene s 63 Butene-i 13 Isobutylene 5 Butene-Z v l3 Butane 3 Vinyl acetylene Trace mation, and other undesirable efiects is largely overcome by my invention.

It will be obvious from the foregoing that all or a portion of the furfural-polymer mixture may be removed either continuously or batchwise the furfural distilled therefrom either batchwise Example Using the apparatus portrayed in Fig. 2, an essentially C4 liquid feed consisting of was fed via line i to absorber ii to which lean furfuralwas introduced via line 3.

Rich furfural containing allof the butadiene and some of the butene-2 (both high and low boiling) in the relative proportions by weight of 4 parts butadiene and 0.3 part butene-2 was withdrawn from the bottom of absorber 2 via line 6 to stripper l. The absorber 2 was operatedat a pressure of 65 pounds per square inch absolute,

and passed to a fractionation step (not shown)- wherein 98.5%. pure butadiene was separated.

asvaeos, i f 5 steps with the result that under the conditions 7 of stripper I via line It. This analyzed approximately as follows:

I Per cent by weight 'Furiural 7 94.8 Polymer .,-l.4 Water .3.8- v

Eighty per cent of this lean furfural was recyeach zone after 'the first is passed to the preceding zone. The furfural and water vapors leav-- ing the first zone are free from polymerand are passed via line ii to condenser I I where they are condensed and thence via 'line t8 t separator l9 in which layer separation is accomplished. The lower layer is continuously recycled via lines 2i and lines 23 to line 3 for re-u'se in tower 2. Makeup furfural is added to line 3 25 as needed to compensate for system losses.

From the' final zone in the furfural still is there is removed via line It a mixture of poly. r

mer, water and 3% of furiural by weight."

.By proceeding in the manner of this example the polymer concentration in the extraction and stripping zones is maintained substantially at the. figure of 1.4% by weight of the furfural-waterpolymer mixture.

As used herein, the reference to per cent of polymer in furfural means the per cent by-w'ei-ght of the polymer based enthe weight of turfural and polymer and any water present in the furiural. H While the above disclosure sets forth as lower.

Iclaim: g

1. In a process for the separation of an unsat mated aliphatic hydrocarbon by the steps comprising extracting said hydrocarbon from a stream containing the same by means of furfural inan prevailing therein turiural is converted to polymer, the improvement which comprises removing polymer from the 'iurfural in the system to 5 such an extent that the polymer concentration in said furfural in the system does'not exceed 5% by weight based on the weight 01' furfural,

- polymer and any water dissolvedin the furfural. 3; In a process for the separationiof an unsaturated aliphatic hydrocarbon by the steps. comprising extracting said hydrocarbon from a stream containing the same by means 01' 'furiural in an absorption zone, introducing the furiuralrich in said hydrocarbon to astripping zone and there '16 stripping said hydrocarbon from said rich furfural, withdrawing leanfurfur'al from said stripping zone and recycling it to said extracting step,

and wherein elevatedtemperatures and pressures are employed in said extracting and stripping steps with the result thatunder the conditions prevailing therein iurfural is converted to polymer,-the improvement which comprises efiecting removalot said polymer by treating'suflicient of said withdrawn =le'an furfural to remove sufi'i-.

'- concentration in furtural in said extracting and stripping steps at a level below that at which the rate ofpolymer formation deviates from linear no in the direction of increased polymer formation,

and recycling the resulting polymer-free furiural to said extracting step..

i 4. In a process for the separation of an unsat- Q urated aliphatic hydrocarbon by the steps com prisin extracting said hydrocarbon from a stream containing the same by means of furfural in an absorption zone, introducing the furfural rich in said hydrocarbon to a stripping zone and there stripping said hydrocarbon from said rich w furfural, withdrawing lean furfurai from said the upper limit oi polymer concentration for stripping zone and recycling it to said extracting step, and wherein elevated I temperatures and pressures are employed in said extracting and stripping steps with the result that under the 5 conditions prevailing therein furfural is converted. to polymer, the improvement which comprises effecting removal of said polymer by treating sufiicient of said withdrawn lean furfural to remove sufiicient p lymer therefrom prior to said o p n Z n ntr ucin the furfural rich -recycling to maintain the concentration of poly-' in said hydrocarbon to a stripping zone and there. Y

stripping said hydrocarbon from saidrich furfural, withdrawing lean furi'ural from said stripand wherein elevated temperatures and pressures steps with the result that under the conditions prevailing therein furfural iseonverted topolymet, the improvement which comprises removing polymer from the furfural' in the system-to such an extent that the polymer-concentration in said'iuriural inthe system does no exceed that point at which increase in polymer concentration takes place in excess of linear.-

'2. 'Ina process for the separation of an unsaturated aliphatic hydrocarbon by the steps comprising extracting said hydrocarbon from a stream containing the same by means of furfural in an absorption zone, introducing the iurtural rich in said hydrocarbon to a stripping zofne and there s rippins said hydrocarbon from said rich fursteps at a figure not exceeding 5% by weight based on the weight of furfural, polymer and any I ping zone and recycling it to said extracting' step,

water dissolved in the furfu'ral, and recycling the resulting polymer-free furfural to said ex-' are employed in said extracting and stripping tracting step.

.. 5. Ina process .ior theseparation of an unseturated aliphatic hydrocarbon .by the steps comprising extracting said hydrocarbon from a 60, stream containing the same by means of iurfural .in. an absorption zone, introducing the iurfural rich in said hydrocarbon to a stripping zone and there stripping said hydrocarbon from said rich furfural, withdrawing lean furfural from said 5 stripping zone and recycling it 'tosaid extracting step, and wherein elevated temperatures and pressures, are employed in said extracting and stripping stepswith the result that under the conditions prevailin therein furfural is converted topolymer, the improvement which comprises efiecting'removal of said polymer by steam dis- 'tilling a-suflicfe furmral prior to recycling thereof to removesuificient polymer to maintain the concentration of .76 polymer in furfuraljin the extracting and stripcient polymer therefrom prior to said recycling to said extracting step to maintain the polymer mer in furfural in said extracting and stripping nt portion of said withdrawn lean said rich iuriural, withdrawing lean furfural from said stripping zone and recycling it to said extracting step, and wherein elevated temperatures and pressures are employed in said extracting and stripping steps with the result that under the conditions prevailing therein iurfural is converted to polymer, the improvement which comprises effecting removal of said polymer by steam distilling a suficient portion of said with drawn lean furiural prior to recycling thereof to remove sufdcient polymer to maintain the concentration of polymer in furfin'al in the extracting and stripping steps at a figtn'e not exceeding 2% by weight based on the weight oiiurfural, polymer and any Wamr present in the iurfural, and recycling the resulting polymer-free furfural to said extracting step.

'7. In a process for the separation of an unsaturated aliphatic hydrocarbon by the steps comprisin extracting said hydrocarbon from a stream containing the same by means of iurfural verted to polymer, the improvement which comasraees recycling said furfural phase to said extracting step.

8. In a process for the separation of unsaturated aliphatic hydrocarbons from hydrocarbon mixtures containing the same wherein the hydrocarbon stream is introduced at an intermediate point to a combined extraction and rectiflcatio'n column, said column is maintained under superatmospheric pressure sumciently high to allow condensation of overhead vapors, furfural is introduced adjacent the top of said column, heat is introduced into the bottom of said column to cause boiling of the bottom product under the pressure maintained therein, an overhead product comprising inore saturated hydrocarbons ls removed from the top of said column, a pditionof said overhead is condensed and returned to said column as reflux therefor, a bot- 7 tom product comprising said unsaturated hyprises effecting removal of said polymer by steam drocarbons dissolved in said iuriural is removed i low condensation of overhead vapors, heat is in trcduced into the bottom of said stripper column to cause boiling of the bottom product therein under the pressure maintained therein, an overhead product comprising said unsatm'ated hydrocarbons is removed from the stripper columns a portion of said last-named overhead is condensed and returned to said stripper column as reflux therefor, and a bottom product comprising furiural andv polymer is removed from the stripper column, the improvement which cornprises steam distilling at least a portion of said removed stripper bottom product to remove over-- head vapor comprising water and furfural free from polymer, condensing said vapor, separating the furfural phase of the condensate from the water phase, and recyclin said furi'ural phase to the top of said combined column, the amount of polymer so removed in said steam distilling step being such as to maintain the concentration of polymer in furfural in said combined column and said stripper column at not exceeding 5% by weight based on the Weight of furfural', polymer and any water dissolved in the furiural.

9. A process as set forth in claim 8 Where the unsaturated hydrocarbon is butadiene and the hydrocarbon mixture is a C4 stream containing the same.

KARL H. FACHMUi'i-i.

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