US2348931A - Process for the treatment of hydrocarbons - Google Patents

Description

May 16, 1944.

W. A. SCHULZE PROCESS FOR THE TREATMENT 0F HYDRoCARBoNs Filed Dec. 1o, 1940 E z L `U H x kw R ,yN mA www5 mm n 5555 A w Y B n VM Patentes Mey 1s, 1944 lcE PROCESS'FOR THE TREATMENT 0F HYDBOCARBONS Walter A. Schulze, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Application December l0. 1940, Serial No. 369,491 3 claims. (ci. 2er-asis) This invention relates to and has for itsprlncipal object, the provision of a process for separating hydrocarbon mixtures containing saturated and unsaturated hydrocarbons into their constituents, and vmore particularly, a process whereby butadiene may be separated 'from butanebutene mixtures containing same.

Heretofore it has been ditllcult to separate substantially pure butadiene from hydrocarbon mixtures, particularly those containing mono-olefinic C4 hydrocarbons and other compounds whose boiling points lie close to that of butadiene. Normally, butadiene occurs or is produced in mix'- tures with considerable quantities of butenes and some butadiene, perhaps as much as one per cent v under certain conditions, but always contain considerable butenes and butanes. The same is true when cracking heavier petroleum fractions or treating butane by cracking and/or dehydrogenation processes to produce butadiene since a very considerable quantity of other C4 hydrocarbons is always present with the butadiene.

A C4 hydrocarbon fraction of the type described vmay be readily isolated from higher and lower boiling fractions in substantially pure form by fractional distillation. Further separation of this complex fraction by distillation, however, has been dimcult since the boiling points of all the components of this fraction are distributed over arelatively narrow range, as indicated below.

Boiling int Compound F. (160p3im: pressure) Isobutane l0 Bune-l 3) lsobutene 2l With emcient fractionating equipment it is pos- 'i For this reason recourse has been had to processes for the separation of butadiene based on hydrocarbon type rather than on boiling range, usually subsequent to preliminaryv concentration of C4 fractions by fractionation. By these processes I mean chemical separations such as absorptions in sulfuric acid o r reaction with cuprous .solutions or sulfur dioxide, as well as physical types of separation such as solvent extraction and the like, whereby the classes of mono-oleilns, diolens, and paraiilns may be separated from each other. These separations, however, have suffered from some disadvantages. They have to a certain extent lacked specificity and it has therefore not been possible to effect a relatively complete concentration of the butadiene. While separation of unsaturates from paraflins has been carried out fairly satisfactorily, the further separation of the mono-olefins from the diolefins has been much more difficult because of the similarity of the reactive group in each.

More recently there has been described in copending application Serial No. 369,490, :dled December 10, 1940, of which I am co-inventor, a process for the separation of butadiene from C4 hydrocarbon mixtures by the steps of (1) deisobutanizing, (2) converting butene-l to butene2 by catalytic isomerization, (3) removing isobutene selectively by acid absorption, and (4) distilling the resultingmixture to produce a butadiene fraction containing unconverted butene-l and a bottoms fraction containing n-butane, butene-2 and some butadiene.

The equilibrium concentrations ofbutene-l and butene-2 produed by the isomerization treat-v ment of step (2) vary according to the temperatures of said treatment in a manner illustrated by the following table:

. v Concentration in mol percent Temperature, F.

' Butene-l Butenei-l m 2. 9 97. l m0 7. 7 92. 8 440 18. 7 M. 3 oso 30.3 e0. 1

Thus. even with emcient isomeriaation at temperatures of 15o-600 F., 5-15 per cent of the butenes may remain as butene-l.

The last named step (4) produces a butadiene concentrate whose purity depends on the quantity of butene-i remaining therein. and for further purification.AV a second isomeriaation treatment may be required. Further, when conducting the fractionation to prevent any traces ofn-butane and/or trans-butene-Z from going into the overhead fraction it is usually impossible to strip butadiene completely from the bottoms fraction. Thus, there is a relatively small but constant loss in'yield of butadiene in the process.

I have now discovered an improved method of separating butadiene from C4 hydrocarbon mixtures and producing butadiene of higher purity and in greater yield by means of the steps hereinafter described.

I have found that when isobutane and isobutene have been removed from a C4 hydrocarbon mixture and the major proportion of n-butenes has been converted to butene-2, the resulting mixture may be fractionated more efllciently when mixed with additional n-butane which as an intermediate boiling component is distributed between the overhead and the'bottoms fraction. The function of this n-butane is to increase the ratio of n-butane to trans-butene-2 in the mixture. Thus, when n butadiene is completely stripped from the bottoms fraction by taking overhead a portion of higher boiling material, the higher-boiling material is n-butane to the exclusion of trans-butene-2 which is higher boiling than n-butane. The addition of n-butane prior to fractionation is necessary in processing mixtures in which the n-butane content is low as is ordinarily the case in butadiene-containing mixtures.

The fractionation in the presence of added nbutane results in an overhead product containing butadiene, butene-l. and n-butane, while the bottoms fraction contains n-butane and butene-2. I have found that butadiene is most satisfactorily recovered in substantially pure form by a second distillation of said overhead fraction in the presence of sufllcient sulfur dioxide to form azeotropic mixtures with the contaminating hydrocarbons. The sulfur dioxide azeotropes of the C4 hydrocarbons have the following approximate boiling points:

Azeotrope, B P 9F Hydrocarbon (760 mm.)

Afactorily separated from butadiene.

A further advantage of the second distillation with sulfur dioxide is that if isobutane is present in such small quantities that an initial de-isobutanizing step is not justified, this isobutane is removed along With the n-butane as an azeotrope and thus is eliminated from the butadiene concentrate.

The process according to my invention may be more readily understood by referring to the accompanying drawing. This represents schematically one type of apparatus in which my process can be used.

This drawing shows a C4 hydrocarbon fraction, such as that derived from cracking still vapors, dehydrogenation of butenes, or similar processes entering the system through line I into heater 2, which may be either a heat exchanger v f or furnace type heater.

The heated vapor's pass by line 4 into catalyst cases 5, in which equilibrium isomerization of butene-l to butene-2 is carried out. If the charge is already at the proper temperature it may pass by line 3 direct to the catalyst. The treated vapors pass by line 6 through condenser 1, and the liquid from 1 passes through line 8 to acid treater 9 for the removal of isobutene. Fresh acid enters treater 9 through line I0, while spent acid and acid-soluble compounds are removed through line I I. The hydrocarbon liquid from this acid treater is then delivered through line I2 to surge tank I3'where n-butane is added by line I4 from storage I5.

The mixture then passes by line I6 to the iracline I9 to fractionator 20. Sulfur dioxide is added from storage 2l through line 22 either into the feed line or directly into the `column. The distillation carried out'in column 20 produces butadiene as the bottoms product while sulfur dioxide azeotropes of n-butane and butene-l are taken overhead. The butadiene fraction leaving by line 23 is treated for the removal of any traces of sulfur dioxide in washer 24 which may utilize an alkaline solution. From washer 24 the butadiene is taken to storage through line 25. The overhead products from fractionator 20 are taken through line 26 to condenser 21, and the condensate is introduced into accumulator 28 where it is cooled until two immiscible phases separate,

one predominantly hydrocarbon, and the other predominantly sulfur-dioxide. The sulfur dioxide may be recovered by distillation of the sulfur dioxide phase in rectification unit 29 and returned to storage 2|. The hydrocarbon phase may be rectified in unit 3U and hydrocarbons thus recovered removed by line 3|. If n-butane is recovered in a satisfactory state of purity it may be recycled by line 32 to vessel I5.

In one specific embodiment of the invention, the C4 fraction containing butenes, butadiene, and butanes is de-sobutanized, and passed in the vapor form over a catalyst of an acidic nature, such as dilute sulfuric acid supported on an inert carrier such as silica gel, at a temperature in the range 200 to 600 F. and at flow rates equivaof catalyst to isomerize butene-l to butene-2. The vapors are cooled and condensed and washed with sulfuric acid of 60 to '70 per cent concentration, thereby absorbing the isobutene present. The acid-treated stream is then admixed with n-butane and subjected to fractional distillation in which an overhead fraction of butadiene, n-butane and butene-l and a bottoms fraction comprising butaneand butene-2 isomers is obtained. The overhead fraction is then mixed with sufiicient sulfur dioxide to form azeotropes with the butene-l and n-butane and the mixture is fractionated with azeotropes of n-butane and butene-l being taken overhead while the butadiene is the bottoms fraction substantially free of other hydrocarbons. f s

ization ot isobutene.

In the isomerization step I have found that low temperatures favor the butene-2 isomers as against butene-l, and' that in processes for carrying out the isomerization oi' butene-1 temperatures should be kept as low as practicable and a catalyst should be employed to obtain rapid In the isomerization step, splitting' ofthe oleiins to form octenes or .the like may occur. Polymerization is favored' by low temperatures also, but since it results in considerable removal of isobutene as the most readily polymerized component itis not usually considered as detrimental to my process unless appreciable amounts of butadiene -are also consumed. y

As catalyst for my isomerization step I prefer to employ diluted strong mineral acids and/or acid salt materials either alone or in solution as mixtures or adsorbed on carriers. I prefer to use catalysts which are highly active so that lower isomerization temperatures may be employed.

` Acidic salts such as aluminum phosphate and the like may be used. Evenmore active, however, are solutions of sulfuric or phosphoric acids adsorbed on suitable carriers whereby the acid strength is maintained at values below the range which would cause polymerization of the butadiene.

It is not to be understood from the foregoing description of one specific method of carrying out the isomerization of butene-1 to butene-2 .that the application ofthe invention is limited thereto. This invention contemplates the use of catalytic isomerization as the first step in a process of concentrating butadiene. The means whereby this isomerization of butene-1 to butene-2 is carried out may be selected at will from a variety of methods, using one of the many different catalyst known tothe art.

The removal of isobutene from` the hydrocarbon mixture may be readily carried out by absorption in sulfuric acid solution. Usually acid strengths of 60 to 75 per cent are employed for rapid absorption of isobutene with only' slight absorption of normal butenes. When dealing with a hydrocarbon mixture containing buta' diene, however, solutions of about '70 per cent or more acid are-less satisfactory'because of consid.. erable adsorption of butadiene, which is rather more reactive than the normal butenes. To achieve a relatively clean cut separation of isobutene without loss of butadiene, solutions of less than '10 per cent and preferably 65 to 'i0 per cent ac id are used with treating temperatures maintained below about 100 F. By treatment with these acid solutions. the isobutene content of the hydrocarbon mixture may be substantially rebutadiene. I prefer to carry out this acid-absorption step subsequent to the isomerization step but it may be ycarried out prior thereto; only rarely will it be donesubsequent to the fractional distillation. With mixtures containing low concentrations of butadiene the isomerization and acid-absorption steps may even be combined using a single acid'solution catalyst to promote the isomerization of butene-1 and the polymer- Such a'combination, however. may result in larger losses of butadiene through polymerization at elevated temperatures in the presence of strong acids.

Other methods of removing the isobutene from the gas mixture may be employed, for instance selective polymerization of the highly reactive isobutene over suitable catalysts to form di- .isobutene orA co-polymers of iso and normal butenes.

Following the steps of isomerization of' butene-1 and absorption of the isobutene, the C4 mixture comprises essentially butadiene, a small amount of n-butane and the isomers of butene-2.

To this fraction, n-butane is added to serve as an intermediate in the separation between butadiene and trans-butene-Z.

The quantity of n-butane added will depend and may vary widely with different mixtures.

Normally the content of :saturates in cracked or dehydrogenated C4 fractions is quite, low, but in some cases appreciable percentages may be present. In practicing the invention I find it desir, able to add sumcient n-butane so that vthis component constitutes a `large proportion of -the mixture boiling higher than butadiene, and usually a major proportion with respect to the next higher-boiling component trans-butene1-2.' The minimum quantity to be added may be determined rom the analysis oi. the mixture, while the maximum quantity will be limited by economic considerations in the subsequent distillation step.

The requisite amount of ln-butane may be added to the C4 mixture prior to treatment by my process, if desired. However, it is usually more economical to add the ni-fbutane just before the rst fractionation step in order to avoid the expense of handling the added volumeof inert material in the isomerization and isobutene-removal steps. Obviouslynvif the C4 mixture undergoing treatment contains suiiicient n-butane, none need carrying out the first fractionaentire butadiene content of the mixture is recovered in a fraction from which it can later be separated in substantially pure form without further losses. The ultimate yield of butadiene is thus increased by an amount corresponding to as much as 5-10 per cent or more of the amount produced, and the purity of the final product is also. greatly improved by the more complete removal of the contaminating C4 compounds.

When the first distillation is operated in such a manner that butadiene is substantially removed from the bottoms fraction, the overhead fraction lis free of butene-2 which would complicatethe second distillation. step. The quantity or the percentage of the n-butane which must be taken overhead to accomplish this separation will depend on the fractionating column eiiiciency and the operating conditions. In general, when nbutane is added according to my process, the separation may be more easily and quickly eected with columns having somewhat fewer trays and using smaller reflux ratios so that operation and equipment are less expensive. The bottoms fraction may he utilized in any suitable process where a butane-butene mixture is required such as further dehydrogenation for the production ot butadiene.

In the second fractionation step wherein the distillation is carried out in the presence oi sulfur dioxide, the requisite amount of sulfur dioxide may be added to the hydrocarbon feed ahead of the fractionator, or it may be introduced directly into the tower at some tray'below the fed tray. In the presence of sulfur dioxide, butene-l and n-butane are obtained as minimum boiling azeotropes both lower boiling than the butadiene azeotrope so that separation is substantially effected in a single step. The removal of n-butane is complete with moderately efiicient fractionation, and the completeness of removal of butene-1 will depend on the closeness of the cut between the butene-l azeotropeboiling at 3 F. and the butadiene azeotrope boiling at 8 F. However,

butadiene concentrates of over 95 per cent purity' and often over 98 per cent are obtained without further processing aside from removal of traces of sulfur dioxide by suitable means.

The n-butane and butene-l, after removal of sulfur dioxide, may be further processed as desired. The n-butane, for example may be separated from butene-l and returned to the system at the point of injection ahead of the first fractionation. Or, if butene-l is present in negligible quantities such that no pyramiding will occur, the total hydrocarbon fraction recovered from the overhead azeotropic mixtures may be recycled in this manner.

The separation of sulfur dioxide from the azeotropic mixtures with butene-l and n-butane may be accomplished by any suitable means. It is usually satisfactory to cool the overhead condensate to produce two liquid phases and to re-distill said liquid phases to recover therefrom substantially pure Asulfur dioxide and hydrocarbons respectively. In this manner a continuous recovery and re-use of sulfur dioxide may be established.

My process may very conveniently be applied to the process of catalyticaly dehydrogenating butane or butenes to butadiene. be integrated into such a process as a most convenient and economical step requiring only a bare minimum of equipment. Since the dehydrogenation step is ordinarily carried out at high temperatures of 1000 to 1300 F., and the efiluents are then immediately cooled and subjected to fractionation, catalyst chambers for the isomeri ization step operating in the range of 200 to 600 F. may be inserted by merely dividing the aforementioned cooling into two stages.

Many modications of my process are possible,

due to the low concentration of this component and the mixture was treated at approximately v 250 F. over a catalyst to convert butene-l. to

Indeed, it may butene-2. This catalyst was prepared by spraying 6-14 mesh silica gel with 85 per cent HsPOi and subsequently drying at 250 F. The gas was passed over this catalyst at a ow rate of about 0.5 liquid volume of charge per hour per volume of catalyst,`in which operation butene-l was about 92 per cent isomerized to butene-2. The gas was cooled, condensed, and washed with a solution of 65 per cent sulfuric acid to extract isobutene. To the acid-washed product approximately 33 moles oi normal butane was added per 100 moles of material and the mixture had the composition listed as the charge to the first fractionation in the second column of the following table. This charge was then fractionated, producing an overhead fraction boiling below about 30 F. and a bottoms fraction boiling above this temperature. The overhead fraction comprised about 50 per cent of the total and had the composition indicated in the third column of the table. Fifty per cent of bottoms containing no butadiene was recovered. The overhead stream was fed to a second column, and a volume of sulfur dioxide sufficient to form azeotropes with the butene-l and n-butane was supplied at a lower plate. The overhead product'comprising sulfur dioxide azeotropes was condensed andtreated for the recovery of sulfur dioxide and the hydrocarbons. The bottoms product was withdrawn and caustic washed. The iinal bottoms product had the composition indicated in the fourth column of the table, and comprised about 45 per cent of the original gas mixture.

When fractionated after'removal of isobutane and isobutene, but without the added butane, the same charge gave overhead and bottoms products whose composition is shown in the last two columns of the table under the heading Prior process." In this case about six per cent oi! the butadiene in the charge was lost in the bottoms fraction and the purity of the butadiene concentrate was only about 90 per cent.

Present process. volume per cent prllflgmnzcl' Hydrocarbon( o im l Ohrggto Ovrgtead Bottoxs l na l'S 800011 charge iractioniractionfraction- Overhed Bottoms ation ation ation isobutme g o. s 1. e i110 "ift "ifi "f 1112112111 39.9 31.8 .64.0 Dao 90.3 5.o 3.2 21.4 21.0 ai Z-butenes- 33. 3 36. 4 88. 9

depending on the particular hydrocarbon mix- I claim: ture undergoing treatment and the mosteiilcient 70 1. A process for the separation of butadiene combination or sequence of the steps of said process. These modiiicationsof my process will be obvious to those skilled in the art and thus are within the scope of my invention. l

from a C4 hydrocarbon mixture containing isobutane,butene1, isobutene and butadiene, which comprises fractionally distilling said mixture to separate isobutene, contacting the de-isobutan- The following example will still further illus- 7l ized mixture with an isomerization catalyst to convert butene-l to butenes-2, selectively removing isobutene from the isomerizedmixture, adding to the substantially isobutene-free mixture ysuicient n-butane to providea concentration of n-butane substantially in excess of the transbutene-Z content, fractionally distilling the resulting mixture to produce an overhead fraction free from butene-Z and comprising butadiene, n-butane and unconverted butene-l and a bottoms fraction free from butadiene and comprising n-butane and butenes-2, and fractionally di'stilling said overhead fraction admixed with sulfur dioxide to produce an overhead fraction comprising the sulfur dioxide azeotropes of n-butane and and butene-l, and a bottoms fraction comprising butadiene.

2. A 'process for the separation of butadiene.

from a C4 hydrocarbon mixture containing isobutane, butene-l, isobutene and butadiene, which comprises contacting said mixture with an isomerizationVv catalyst to convert butene-l to`bu tenes-2, selectively removing isobutene from the eiiiuent of the isomerization treatment by sulfuric acid absorption, adding"to the substantially isobutene-free mixture suiiicient n-butane to provide an n-butane content substantially in excess of the trans-butene-Z content, fractionally distilling the resulting mixture to produce an overhead fraction free .from butene\2 an'd comprising isobutane, n-butane, unconverted butene-l` and butadiene and a substantially butadiene-free bottoms fraction comprising n-butaneand butene-2, and fractionally distilling said overhead fraction admixed with sulfur dioxide to produce an overhead fraction comprising. sulfur dioxide azeotropes of isobutane, n-butane and butene-i and a bottoms fractions comprising substantially pure butadiene.y y

3. A process for the separationof 'butadiene from a C4 hydrocarbon mixture containing same along with n-butane and n-butenes, which comprises contacting said mixture with anisomerization catalyst to convert butene-l to butenes-2, adding to the eilluent of the isomerization treatment sufficient n-butane to provide an n-butane content substantially in excess of the trans-butene-2 content, fractionally distilling the resulting mixture to produce an overhead fraction free from butene-2 and comprising butadiene, n-butent, fractionally distilling the resulting mixture to produce an overhead fraction free from butene-2 and comprising butadiene, n-butane,v and unconverted butene-l and a bottoms fraction free from butadiene and comprising n-butaneand butenes-2, and fractionally distilling said overhead fraction admixed with sulfur dioxide to produce an overhead fraction comprising sulfur dioxide azeotropes of n-butane and butane-1 and a bottoms fraction comprising butadiene.

5. A process for facilitating the separation of butadiene from a hydrocarbon mixture comprising butadiene and butenes-2, which comprises providing said mixture with an n-butane content substantially in excess of the trans-butene-2 content and fractionally distilling the resulting mixture to produce an overhead fraction free from butene-2 and-comprising butadiene and n-butane and a bottoms fraction free from butadiene and comprising n-butane and butenes-Z.

6. A process for the separation of butene-l and butadiene from a C4 hydrocarbon mixture containing same along with cisand trans-butenes-2 and n-butane in amount less than the amount of trans-butene-Z present which comprises adding sufficient normal butane to said mixture prior to fractionation to provide an excess of normal butane over trans-butene-2, and fractionally distilling the resulting mixture to produce an over head fraction free from trhns-butene-Z and comprising butane-1, butadiene and n-butane and a bottoms fraction Asubstantially free of butadiene and comprising n-butane and butenes-2.

WALTER A. SCHULZE.4

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