US2376425A - Diolefin production - Google Patents

Description

May 22j? w45 F. E. FREY DIOLEFIN PRODUCTION Filed June 2, 1941 2 Sheets-Sheet l Filed June 2, 1941 2 Sheets-Sheet 2 NOLLBOSGV "IVODHVHD Nolidaosov "woQdvI-m aolvuvdas BUTADIENE INVENTOR FREDERICK E.F`REY I BY www @A wml W NEY Patented May 22, 1945 DIOLE-FIN PRODUCTION Frederick E. Frey, Bartlesville, Okla., assigner to Phillips Petroleum Company, a corporation of Delaware Application June 2, 1941, Serial No. 396,344

(Cl. 26o-680) 8 Claims.

This invention relates to the production of dioleiln hydrocarbons from low-boiling hydrocarl bons heavier than methane. It relates more particularly to the production of diolens such as butadiene, pentadiene, isoprene, cyclopentadiene, and the like by treating hydrocarbons of two to ve 'carbon atoms per molecule at elevated temperatures and relatively low pressures An appreciable number of diolens such as those just mentioned have been known to the art for a number of years. These have been produced in a number of ways which have included cracking of heavier oils, the inter-reaction of acetylene and ethylene to form butadiene, catalytic or thermal conversion of alcohols, both of the same number of carbon atoms per molecule as the desired diolen and of ya fewer number of carbon atoms per molecule, as well as the dehydrogenation of the corresponding olens which in turn may have beenproduced by the dehydrogenation of lthe corresponding parains. Diolens have also been reported as being present in the pyrolysis products ofllight hydrocarbons, including even methane. There are enormous quantities of such lighter hydrocarbons which are potentially available for the production of such q diolens, but apparently most of the commercial "installations and processes to date have been one of the more involved types previously mentioned rather than 'the direct conversionof cheap and abundant light hydrocarbons to diolens.

I have now found that I can successfully produce substantial yields of low-boiling diolens of the type first mentioned from lighter gaseous hydrocarbons by a suitable combination of hightemperature conversion steps, to be more fully disclosed hereinafter, yand also that I can'operate such a process quite economically to produce high yields of diolens when the high-temperature conversion is further combined with certain simple but ecacious separative steps, also to be more fully disclosed hereinafter. may be converted in a first high temperature conversion step to produce heavier, but low-boiling, hydrocarbons which can be further treated in additional high-temperature conversion steps to produce the desired dioleflns, I generally do not desire to have large amounts of methane in the initial hydrocarbon material to be converted. I prefer to treat primarily hydrocarbons of two to four or five carbon atoms per molecule, with hydrocarbons of two and/or three carbon atoms per molecule being the best adapted to my process. Methane may be present in the charge stock, sometimes in relatively high proportions, but will While methane generally not enter extensively into the reactions which take place under the most desirable conditions.

In one modication of my invention an ethylene-rich gas is converted at an elevated temperature and low pressure for a time suilicient to produce a discernible but relatively small amount of light oils, a C4 and heavier hydrocarbon fraction which contains the desired dloleilns is removed from the eilluents, a substantial portion of the C3 and lighter .fraction is recycled directly to the conversion step, and methane and other light gases are separated from an additional portion of theetlluent with recycle of the C2 and C:

hydrocarbons.

In another modification of my invention an ethane-propane fraction is converted at an elevated temperature to produce an optimum yield of unsaturates of low molecular weight, the effluent is passed to a first separating means wherein heavier hydrocarbons and lighter gases are removed, the resultant unsaturate-containing Cz-Ca fraction is passed to a second conversion step yat an elevated temperature and low pressure to form an optimum yield of dioleflns, dioleflns are separated from the effluent, a portion of the remainder is recycled to the second conversion step, and a further portion is introduced into said first separating means to eiect a removal of light gases and retention in the system of desired C2 Iand C3 hydrocarbons.

It is an object of my invention to convert lowboiling hydrocarbons into low-boiling diolen hydrocarbons.

Another object of my invention is to convert low-boiling paran hydrocarbons to diolens by a lunique combination of two or more high temperature conversion steps.

Still another object of my invention is to effect a simple and unique combination of thermal conversion and separating steps whereby a suitable recycle stock for the thermal conversion step is readily separated from desired products and undesired by-products.

Further objects and advantages oflthis invention will become apparent from the accompanying disclosure and discussion. I

'The features of my invention will now be disclosed in connection with the accompanying drawings, which illustrate diagrammatically by way of ilow sheets the manner of practicing my invention, together with auxiliary steps which may be Vincorporated with various modifications of it.

Figure 1 shows a complete arrangement of apparatus in which my invention may be practiced, together with modifications.

Figure 2 shows another arrangement of apparatus for the recovery of butadiene, particularly useful in connection with my invention.

Referring now to Figure 1, low-boiling predominantly saturated hydrocarbons of two to five carbon atoms per molecule are introduced to the process through pipe I and valve to the conversion or cracking zone I2, represented as a coil in a. furnace |3. In the conversion zone l2 the hydrocarbons are subjected to elevated temperatures while under low pressure and low-boiling unsaturated hydrocarbons are formed in substantial amounts. The eiiiuent of the conversion is passed through pipe i4 and valve i5 to separating means i6. Hydrocarbon material containing hydrocarbons of more than i'lve carbon atoms per molecule may be removed from the system through pipe i1 and valve |3. The remaining lower boiling material, which will also include methane and hydrogen formed in the conversion, is passed from the separating means l0 through pipe Iil), and at least a substantial portion thereof is passed through valve 2| to separating means 22. In one modification of operation a portion of this material may be passed from pipe 20 through pipe 23 and valve 20 to pipe i0 for retreatment in conversion zone i2. In separating means 22 light gases such as methane and hydrogen are separated from low-boiling unsaturated hydrocarbons and are discharged through pipe 25 and valve 26.

In a preferred form of my invention low-boiling unsaturated hydrocarbons pass through pipe 3@ to the heating and/or conversion zone 3i, represented by-a tube coil in a suitable heating unit 32. With the initial step just described as a part of the process, such low-boiling unsaturated hydrocarbons are passed from separating means 22 through pipe and valves 29 and 39. The material charged to the conversion zone 3| may be augmented by similar low-boiling unsaturated hydrocarbons introduced from any suitable outside source through pipe 21 and valve 28, which may at times constitute the sole charge to the process. In one modification the zone 3| is primarily a heating zone operated in conjunction with a reaction chamber 33 to which it is joined by a transfer line 34. When using such a reaction chamber, the material passing through pipe 30 is rapidly heated in zone 3| to a temperature only suicient to initiate an uncatalyzed reaction, which takes place in reaction chamber 33. This chamber is preferably insulated to preclude substantial heat transfer through the walls thereof, and the inlet pipe 34 and the outlet pipe 36 are preferably so placed that a direct passage ofthe contents from the linlet to the outlet is not favored but rather so that a dilution and mixing of the chamber contents with the incoming mixture is favored. A clean-out device or manhole illustrated by conduit 59 equipped with a valve .59' is provided for the reaction chamber, by means of which any heavy oils or tar or hard carbonaceous material which may tend to accumulate therein may be removed as desired or necessary. The

. reaction effluents pass to a compressor 43, after being suitably cooled by means such as cooler |03, and are passed through pipe 44 to suitable separating means such as absorber 45. When the reaction chamber 33 is not used and the desired extent of reaction is effected in zone 3|, the effluclosing valve 35 in pipe 34 and valve 40 in pipe I8. When reaction chamber 33 is used the pipe Il is not included in the system, as may be effected by closing valve 42.

The conversion effected in the zone 3| and/or 33 is such as to produce from the low-boiling unsaturated hydrocarbons charged through pipe 30 an optimum yield of low-boilii'g diolens of the nature of butadiene, pentadiene, and cyclopentadiene. I have found that such optimum conditions exist when the reaction is carried out in a temperature range suitable for the production of low-boiling aromatic hydrocarbons for a period of time such that only a minor portion of such aromatics are produced. More specically I prefer to operate within a range of about 1300 to 1700" preferably between about 1375 and 1550 F. for a period of time such that the eiiiuent contains not more than about 10% by weight of C4 and heavier hydrocarbons which are a result of synthesis reactions. However, of course, an appreciable amount of C4, and heavier hydrocarbons should be produced by synthesis reactions, and I have found that the diolenn content does not approach a desirable value until at least about 3% by weight of the eiiiuent consists of such ,C4 and heavier hydrocarbons. When the material charged through pipe 30 is substantially free of C4 and heavier hydrocarbons, the extent of conversion is readily determined by a simple analysis of the effluent. When such hydrocarbons are present in the charge stock, a material balance made in conjunction with an analysis of the charge and eiiuent will readily indicate to one skilled in the art the proportionate amount of C4 and heavier hydrocarbons which have been produced from lighter hydrocarbons by synthesis reactions. The overall synthesis reactions are exothermic with highly unsaturated charge stocks, and endothermic when the charge is relatively saturated, and suitable-means for removing or adding heat may bevprovided to maintain a fairly uniform reaction temperature. When a tube coil reactor is used, such as is illustrated by the heating and conversion zone 3|, the rst portion of the tube coil should be used to heat the charge rapidly to the reaction temperature, and reaction will take place in a continuation of the tube coil, which may be in heat exchange relationship with a suitable heat exchange medium such as iiue gases, adjusted to maintain a more or less even reaction temperature. In some instances, however, it will often be desirable to allow the temperature in the tube coil reactor to increase slightly as the reaction proceeds and especially when exothermic reactions are occurring in the tube coil, with little or no removal of heatA of reaction. In many cases it may be desirable to operate with the insulated reaction chamber 33, which should be quite large in cross section in comparison to the cross section of the tube coil 3|, so that advantageously the contents can circulate repeatedly past the inlet. With such a reactor and a highly unsaturated stream, the

tions that any temperature increase of the eilluent over the charge represents the heat developed by the reaction, and sothat the temperature increase brings the charge to a desired reaction temperature. In most modifications the pressure should be low throughout the reaction, generally not in excess of about 50 pounds per square inch absolute. and may be subatmospheric although pressures lower than about 1.5 pounds per square inch absolute are generally not practical. With some relatively more unsaturated charge stocks. the formation of butadiene, and the line. will result to a substantial extent from polymerizationy type reactions, and pressures as high as 100 pounds per square inch may be employed. However, such pressures also tend to promote formation of other and less desired products of higher molecular weight. With such pressures the throughput per unit of time is alsov increased. With relatively less unsaturated charge stocks, lower pressures are more desirable. Low partial pressures with the total pressure somewhat above atmospheric. may be readily employed by using diluent gases, which may be gases such as nitrogen, methane or other suitable gases which are substantially unreactive under the reaction conditions. or which may be in part or entirely gases such as steam or carbon dioxide, which are not strictly reactants but whose presence appears to `suppress or compensate for undesired side reactions such as the formation of tar and heavy carbonaceous material. Carbon monoxide also tends to take up hydrogen under the reaction conditions. which also has the effect of favoring diolefin formation. When such diluents are used, the extent of conversion discussed above should be calculated on a basis free of such diluent, or of compounds formed from it.

Reaction efliuents are passed to 'separation means for recovery of dioleflns produced in the reaction. In one preferred manner of practicing my invention, a substantial portion of the Ca and lighter material is separated and returned to the conversion zone just discussed, in admixture with fresh charge being passed through pipe 30. The relative volume of such recycle C3 and lighter material to the volume of fresh charge entering zone 3| is preferably between about 1:1 and 5:1. and advantageously is of the order of about 3:1.

Separation of C3 and lighter material from C4 and heavier maybe accomplished in any one of a number of operations, but will be described in one preferred modification with reference to Figure 1 to the use of an oil absorb-er with simple ash stripping of the rich absorption oil. which may be used with particular advantage in conjunction with the foregoing conversion step as a modification of the present invention. In this modification. eiiiuent of the conversion zone is passed to a low point of absorber 45. In absorber 45 a separation is effected between C4 and heavier hydrocarbons, and C3 and lighter material, which will generally include some free hydrogen. `The absorption pressure need not be excessively high although it should be appreciably above atmospheric. The conditions in any particular case will be dependent on the composition of the conversion effluent and upon the characteristics of the absorption oil used, which is introduced near the top of absorber 45 through pipe 46. The absorber 45 is so operated that the gaseous effluent through pipe 41 and valve 48 is substantially free of C4 and heavier hydrocarbons. The rich absorption. oil is passed from a low point of absorber 45 through pipe 50 and valve 5I to a ilrst flash stripper 52. Stripper 52 is operated under such conditions of pressure and temperature that substantially all C3 and lighter hydrocarbons in this rich oil are vaporized. The resulting vapors are passed from the top of stripper 52 through pipe 53 and valve 54, and are com- Dressed by compressor 55 to a pressure suitable to permit their injection at a low point of absorber 45. Under conditions of operation such that the gases passing through pipe 41 are substantially free of C4 hydrocarbons considerable Ca and lighter' material will be included in the oil passed through pipe 50. In order for the flashed oil passing from stripper 52 through pipe 56 to be substantially free of Ca and lighter material, an appreciable amount of vdesired .C4 hydrocarbons will be included in the vapors passing through pipe 53. When operating under steadystate conditions the return of these vapors to absorber 45 offsets these disadvantages of simple absorption and ash stripping so that there is little or no net loss of either of these materials andthe amounts of C; and lighter passed through pipe 41 and of C4 and heavier passed through pipe 56 and subsequently recovered are substantially equivalent to the amounts of these materials passed to absorber 45 through pipe 44.

The liquid from stripper 52 is passed therefrom through pipe 56 and valve 51 to a second iiash stripper 58. which is operated at a lower pressure and/or higher temperature than stripper 52, and under conditions such that substantially all C4 and C5 hydrocarbons are vaporized and removed through pipe 60. These vapors are passed through valve 6I and are passed by a vapor pump, or compressor, 62 through pipe 63 for further use as may be desired. They may be discharged through valve 69 for separation and purication of various individual hydrocarbons, or may be sent entirely or in part from pipe 63 through pipe 92 and valve 93 to the latter part of pipe 13 and separating means I6. so that heavier products may be recovered along with similar products produced in cracking zone l 2.

The absorption oil used in absorber 45 should have a suiiiciently low volatility that no appreciable part of it is vaporized in any part of the system but should not have such a high molecular weight that C4 hydrocarbons, especially butadiene and the like, are not effectively absorbed. After having the low-boiling products ilashed from it in stripper 58 the resultant lean absorption oil is removed through pipe 64, and may be returned, at least in part, through pipe 66 and valve 61 to pipe 46 and the absorber 45. This its entirety, or in any continuous portion, as may best fit the particular conditions involved, as may be readily ascertained by one skilled in the art.

In connection with this modification of my invention a high eilcincy of operation is achieved if a substantial amount of the C3 and lighter gases present in the effluent of the conversion zone are returned to the conversion zone. This is accomplished in the present instance by passing unabsorbed gases from absorber 45 through(` pipe 41 and valve 46. andpassing a substantial .The contaminated oil may be removed.

portion thereof through valve 1| in pipe 10 to pipe 30. The remainder continues through pipe 41 and may be discharged from the system through valve 12 therein. Unsaturated hydrocarbons may be separated from such discharged material and returned to the process through pipe Such a recovery of light unsaturated hydrocarbons from accompanying lighter gases may be accomplished within the system, by passing material from pipe 41 through pipe 13 and valves 16 and 15 to pipe |4 and separating means d6 and 22, or through pipe 15 and valve i1 to pipe 20 and separating means 22 alone, with suitable control of valve 15.

The content of low-boiling unsaturates in the charge passed through pipe 30 to the zone 8| and produced from saturated hydrocarbons entering `the system through pipe l may be increased over that value readily obtainable by a once-through conversion in zone i2, in any one of several ways. A portion of the low-boiling effluent may be returned through pipe 28 and valve 2d, as previously mentioned. However, this also entails return oi other products such as methane and hydrogen, which is not always desirable'. To preclude this, such material may be removed in separating means 22, and a portion of the efiluent of means 22 removed through pipe 30 is passed thrugh pipe se and valve si to be mixed with the charge being passed through pipe i0. When the charge to the system is a more or less saturated, heavier low-boiling material such as propane or butane, or 'a predominantly vsaturated, C: or Cs fraction, a. further modification of my invention may be practiced. Such a heavier, low-boiling material may be introduced in liquid phase through pipe :a controlled by a valve 3'@ to the top of demethanize'r 22 as an absorption liquid, with suitable temperature and pressure conditions imposed on the operation of demethanizer i2. The rich absorption liquid passes from demethanizer 22 through pipe 30, and through pipe Si@ controlled by valve 38 to fractionator 89, valves E9 and 89 in pipe 30 being closed. In fractionator 89 a suitable separation is made between light absorbed gases, including ethylene and propylene, and heavier hydrocarbons including ethane, some propylene, and the absorption liquid. v'I'he light fraction passes from fractionator 89 through pipe 18, controlled by valve 19, to pipe and the reaction zone 3|. The heavier hydrocarbon material is removed from the bottom of fractionator 89 through pipe 90, controlled by valve 9|, and passed through pipe 80 for treatment as discussed herein.

In other instances a separate conversion zone may be'advantageously used, such as is represented by coil 82 in the heater or furnace 83. A suitable charge stock is removed from pipe 80 through pipe 84 and valve 85, and the converted material is passed through pipe 86 and valve 81 to pipe 13 and subsequent separating means. When the conversion zone 82 is operated to produce light unsaturated hydrocarbons with a negligible amount of heavier hydrocarbons, the effluent may be passed directly to separating means 22. Under many conditions of operation, however, an appreciable amount of heavier material includingvaluable diolens is so produced, and in such cases the eiiiuent is passed through valve 15 to separating means I6. AIn some cases the conversion zone 82 may operate under similar conditions, or may be a substitute for the conversion zone 3| and/or 33. In such a case the operating conditions will be similar to those described for these zones and the dioleiln product recovered from pipe |1. will be a principal product o! the process. When desirable a part or all of the heavier hydrocarbon material removed from the bottom of fractionator 89 may be removed from the system for further treatment through conduits 90, 80, 84 and |00 and valves 9| and 0|.

The separation step for the eiiluent of the reaction zones 3| and/or 33 has been discussed in connection with the use of a relatively nonvolatile and nonselective absorption oil. With such an operation the dioleiin material recovered through pipe 63 will have associated with it appreciable amounts of other hydrocarbon material within the same general volatility range. In some cases it may be found more desirable to employ an absorption medium or solvent which is more or less selective in its action for highly unsaturated hydrocarbons such as butadiene, pentadiene, cyclopentadiene and the like. Such solvents known to the art include water, furfural, ethers and halogenated ethers such as sym.-dichloroethyl ether, various glycols and their organic esters such as the glycol acetates, glyceryl ethers, lactic acid esters, alkyl amines, and the like, and especially' aqueous solutions of such materials. A suitable but somewhat different class of solvents is the S02oleiinresin oils disclosed by Snow in his Patent 2,163,588. In another modification of the separation step, a material may be used which reacts selectively with vdioleiins to form readily decomposible chemical compounds. Such materials known to the art include aqueous solutions or compositions of a salt of a. heavy metal of groups I and Il of the periodic system, especially of such a metal in monovalent form, as a cuprous, mercurous or silver salt, and especially an aqueous alkali-halide-containing solution, or suspension, of a cuprous halide such as cuprous c hloride. When such selective materials are used, the hydrocarbon material from which dioleiins are removed my be either in gaseous or liquid phase.

With any of the foregoing liquid materials, the absorber 45 will be equipped internally with suitable baiiles, bubble-plates, packing, or the like to ensure efficient contact between a descending liquid phase and an ascending phase. The iiash strippers may be as described in connection with the use of an absorption oil, or may be also equipped for countercurrent iiow of liquid and vapor with intimate contact of the two to provide more efcient separation, although of course equipment investment will be greater.

Solid absorbents, or adsorbents, may be used as well as absorption oil. Charcoal is one solid adsorbent which may be used with advantage, and

various adsorptive oxide gels, such as silica gel may also be used. Such an adsorbent may be employed in a manner similar to that discussed in connection with the use of an absorption oil. Figure 2 illustrates a modification of the separation step which employs solid adsorbents for the separation of C4 hydrocarbons from material lighter and heavier than C4 hydrocarbons. Like numerals in Figures 1 and 2 designate parts performing similar functions. In a preferred man-y ner of operating the separation equipment as illustrated in Figure 2, hydrocarbon material, having a composition similar to that material which is passed from a conversion zone through pipe 44 to a low point of absorber 45 in Figure 1, is passed through pipe 44 and pipe |01 controlled by valve |08 to chamber ||0 when valve |04 is closed. Chamber ||0 contains solid adsorbent material such as silica gel, which removes C5 and higher boiling hydrocarbon material from C4 and lighter material. Chamber is so operated that the gaseous eflluent through pipe ||4, valve ||1 and pipe IIB is substantially free of Cs and heavier hydrocarbons. The conditions in any particular case will be dependent on the composition of the charge to and on the adsorptive quality of the adsorbent employed in said chamber. When the adsorbent material in chamber ||0 is substantially no longer effective in removing Cs and heavier hydrocarbons and such hydrocarbons tend to appear in the material passing through pipes 4 and IIE, the adsorbent material is said to be spent and charge material `through pipe 44 is passed through valve |04 and pipe |05 controlled by valve |06 to chamber and valve |08 is closed. Chamber contains solid adsorbent material similar in character to fresh adsorbent material originally contained in chamber I |0, and is operated in a manner similar to that described herein for chamber ||0. While chamber is on stream for the treatment of material charged through pipe 44, C4 and lighter material pass from chamber I through pipe ||2, valve I3 and pipe ||6 when valve |2| is closed and C5 hydrocarbons Il! is open and valves ||1 and |2| are closed.

When the pressure on the adsorbent material is so reduced, Ct and heavier hydrocarbon material pass through pipe ||4 valve I |5 and pipe |20. Such hydrocarbonmaterial may be further treated as will appear most desirable. The initial desorbed material as long as it is substantially free of C5, may be added to the eiliuent of chamber |I|.

Removal of Cs and heavier hydrocarbons from the spent adsorbent material revivies said material thereby allowing it to be re-used for adsorption of additional hydrocarbons charged through pipe 44. The spent adsorbent material in chamber I I0 may be reviviiled by treatment with steam 'which is introduced through pipe |24 and valve y introducing air diluted with ilue gas.

When the adsorbent material in chamber III has become spent, chamber ||0 is put back on stream by appropriate manipulation of valves and chamber III is subjected to the desorption step similarin character to that described in connection with chamber I0. C5 and heavier hydrocarbons are removed from chamber III through pipes ||2 and |20 land valve |2| when valve I|3 is closed. When the spent adsorbent in chamber I II contains silica gel it may be reviviiied by passage of steam therethrough which is charged through pipes |24 and |26 and Valve |21 when valve |25 is closed. In such a manner chambers ||0 and are operated so that while the adsorbent in one is employed for the adsorption of Cs and heavier'hydrocarbons the spent adsorbent in the other is undergoing reviviiication and/or regeneration by desorption of material on the adsorbent, thereby aiording a continuous -process for the `separation of material in conduit 44. Al-

though only two chambers for the removal of C5 and heavier hydrocarbons are illustrated in the drawings, it is within the scope of this .invention that any number of chambers may be employed and operated intermittently as is desirable for any particular charge.

The material passing through pipe ||8 and valve |32 contains substantially C4 and lighter material. This material is passed to a charcoal adsorption system either through pipe |35 or pipe |36. When the hydrocarbon material in pipe 44 contains substantially no hydrocarbons boiling higher than C4 hydrocarbons, chambers I|0 and II| may be omitted and said material may be passed directly through pipe 44 controlled by valves |04 and |3| to pipe ||6 when valves |08 and I 06 are closed.

In a preferred manner of operating, C4 and lighter material in pipe IIS ispassed through pipe controlled by valve |34 to chamber |40 which contains activated, adsorbent charcoal. Chamber is so operated that the charcoal therein removes substantially all of the C4 hydrocarbons as well as some of the C3 and C'z hydrocarbons, methane and small amounts of hydrogen from the C4 and lighter material passed therethrough. Unadsorbed' lighter material which Will contain C3 and C2 hydrocarbons as well as methane and hydrogen is passed through pipe |44, valve |41, pipe 41, and valve 48 after which it is treated in a manner similar to the treatment of the material in pipe 41 as described in connection with Figure 1 and the oil absorption method of separation. When the charcoal in chamber |40 is no longer effective in removing substantially all the C4 hydrocarbons and C4 hydrocarbons appear in pipe |44, charge material in pipe ||6 is passed through valve |31 and pipe |36 to chamber |4| and valve |34 is closed. Chamber |4| also contains charcoal and is operated in a manner similar to chamber |40 so that substantially all of the C4 hydrocarbons :as well as some of the lighter hydrocarbons are adsorbed by the charcoal from the material passed therethrough. Unadsorbed material from chamber |4| passes through pipes |42 and |50 controlled by valve |5| to pipe 41 and through valve 48, when valve |43 is closed, after which it is treated as previously discussed. While chamber |4I is on stream and C4v hydrocarbons are removed therein from the material charged through pipe I6, material adsorbedon the charcoal .in chamber |40 is desorbed. This is done in a preferred manner by reducing the pressure in chamber |40 by means of pump |53 when valves |41, |34 and |65 are closed. The pressure is so reduced at first that the gas passing through pipes |44 and |52 and valve |45 to pump |53 is substantially free from C4 hydrocarbons. This gas is passed by pump |53l through pipes |54 and 41 and valve |51 into the gas stream leaving chamber |4|. After an initial period of applying lower pressure to the charcoal in chamber |40 additional gases lighter than C4 hydrocarbons together with some C4 hydrocarbons are desorbed, and these are passed by pump |53 from pipes |44 and |52 through valve |45, and through pipe |54, valve |55, pipe |66 and valve |61 to pipe IIS to be combined with the incoming charge to the charcoal chamber that is in use, which in this partieular instance is chamber |4I. C4 hydrocarbons contained in the material passing through' pipe |66 are thus removed by the charcoal chamber on stream. After this, the desorption of the bulk of C4 hydrocarbons from the charcoal in chamber |40 can be accomplished by conventional steaming methods, or the like. For example, steam may be passed through pipe |6| controlled by valve |65 and through pipe |36 to chamber |40 whereupon substantially all of the C4 hydrocarbons are removed and passed by means of pump |53 through pipe lid and valves |55 and lli to cooler |13 and thence through pipe liti to separating means H5.

After the charcoal in chamber |4| is no longer effective for the removal of Ci hydrocarbons chamber |40 is put back on stream by the proper manipulation of valves, after the charcoal within this latter chamber has been rendered suitable for re-use such as by removing substantially all material previously adsorbed. The charcoal in chamber |4| is then subjected to a desorption operation similar in character to the desorption operation discussed in connection with the charcoal in chamber Md. Steam for desorption of C4 hydrocarbons can be admitted to chamber |4| through pipe |64, valve |63, pipe |62 and pipe |36. Eilluent from chamber |4| containing desorbed material is removed by pump |53 through pipe |42 controlledby valve |43 when valve |5| is closed. It is to be appreciated that additional charcoal adsorption chambers may be employed in the separation equipment and as many as will appear desirable for continuous operation of this separation process.

When steam is used to desorb the charcoal in chambers |40 or |4| the stream from cooler |13 will contain hydrocarbon material and water. Water is preferably removed from this stream by any conventional means, such as by dehydrating agents, before passing the stream to separating means |15. Separating means |15 is then so operated that a stream comprising substantially C4 hydrocarbons is removed through pipe 6U controlled by valve 6| and a stream comprising substantially C: and some lighter hydrocarbons is removed through valve |.1i and pipe 41 for further treatment as herein described. The hydrocarbon material removed through pipe 68 will contain butadiene and may be treated in-a manner as herein described for similar material removed through conduit 60 when obtained from an oil absorption separation process.

When a means, such as a dehydrating agent, for removing water from the stream in pipe llt is not employed, separating means |16 can be so operated thatwater is separated through pipe |80 controlled by valve iti.

Chambers HU, |40, and |4| can be charged with appropriate adsorbent material through means represented by conduits H, |28, |36 and |48, respectively, which in turn are controlled by valves H9, |29, |39, and |49, respectively. Simi.. larly, when desirable, material may be removed from chambers llt, Iii, |40, and lil through suitable means represented by conduits |58, ltd, |18 and |12, respectively, which in turn are controlled by valves |59, |69, |19, and |16, respectively.

It is the primary function of the first adsorp-I tion step, carried out in chambers |||l or lil, to remove Cs and heavier material,A and to allow substantially all C4 and lighter material to pass through for subsequent treatment. Some of this heavier material is diiilcult to remove from the adsorbent, and it may be necessary from time to time to supplement a desorption process by a burning out process. For these reasons, it is desirable to use an adsorbent which will not take up lighter hydrocarbons to any great extent and which will also withstand a burning mit opera... tion, and ordinary silica gel, alumina, some bauxites, and the like can be used to advantage. With this preliminary removal of heavy material, full advantage may be taken of the great adsorptive powers of activated charcoal to remove and recover butadiene in an adsorption step carried out in chambers |40 or |4I. Material dimcult to remove from the charcoal will not be present to be taken up by it, and eiiicient and economical operation can be readily realized in the procedure disclosed, with continued repeated use of the charcoal without treatment other than that described. Other adsorbent material having adsorptive properties comparable with those of activated charcoal may, of course, be used in its place.

Although the adsorption of C4 and lighter material has been described with respect to charcoal as an adsorbent, as other modications it is within the scope of operation and the invention to employ materials such as granular or sheet rubber in a pure or compounded form, natural or synthetic, instead ofcharcoal. Such material will be more selective in its action for the adsorption of the diolens than charcoal and absorbing oils previously discussed. Completely hydrogenated or other saturated rubbers, however, will generally not be found satisfactory in such a step. Solid adsorbents impregnated with selective agents, such as bauxite impregnated with cuprous chloride, may also be used.

Although such a modification is accompanied by considerable refrigeration and heating expense, which of course may be reduced by elcient heat-exchange arrangements, a fractional condensation-absorption process may be used to recover the C4 and heavier hydrocarbons from the reaction effluent. Such a. modification will be most effectively used when the charge to the zone 3 is substantially free of C4 and heavier hydrocarbons, so that substantially all such material present in the eiliuent will be present as a result of synthesis reactions and will not be present in excessively large amounts, as discussed in connection with this conversion step. With such a modiiication the absorber 45 will have associated with it cooling and condensing-equipment suitable for cooling the overhead gases and vapors, and 'condensing a substantial portion of the C3 hydrocarbons. This condensate is allowed to pass downwardly through the absorber, as an absorption liquid, and contact countercurrently the ascending gases. In this manner the gases which contact the cooling and condensing equipment will contain only small portions of C4 and heavier hydrocarbons and the liquid which leaves the bottom of the absorber will comprise substantially all of these hydrocarbons which are introduced to the absorber. The liquid from the bottom of the absorber 45 is preferably separated in fractional distillation steps, as by operating stripper 52 and/or 58 as fractionating columns, with return of C3 and lighter material more or less as discussed.

The conversion carried out in zone I2 will in most cases be one which is so operated as to produce an optimum yield of low-boiling unsaturates, especially those of two and three carbon atoms per molecule, from paraiiin hydrocarbons. Efficient conversion is readily realized at low expense by conducting the process in the absence of catalysts, especially when the charge introduced through pipe I0 comprises predominantly ethane and/or propane. Dehydrogenation catalysts may, of course, be used under conventional dehydrogenation conditions, and their use Will result in a lower production of methane. More especially when this step is' operated noncatalytically, a

' tion. and the butadiene is recovered in a substansubstantial production of the desired unsaturated hydrocarbons will be accompanied by a production of some C4 and heavier hydrocarbons whichv may be realized by recovering butadiene throughpipe Il, removal of methane andv hydrogen by means 22 from substantially all of the light efuent passing through pipe 20, and a. return of an intermediate fraction, which will be rich in ethylene back to the cracking z'one I2 through pipe 80. At times, when propane is the charge, it may be worthwhile to provide for a higher concentration of ethylene in this recycle fraction by also removing ethane in means 22.

The various separating means are to be understood as diagrammatic, and the actual separa-` tions discussed may be carried out in a manner which seems most desirable and economical in conjunction with the conditions of any particular case. Since the drawings are diagrammatic, the

application of my invention on a commercial scale will require the use of much conventional equipment, such as pumps, heaters, coolers, separators, accumulators, and the like, not shown in detail, but which may be readily applied and adapted for any particular installation by one skilled in the art. The general process, operating conditions, and material flows have been disclosed and discussed sufficient to serve as eflicient guides for such purposes.

` EXAMPLEI As one example of the operation of one modiiication of my invention, an unsaturated Cz-Ca mixture is converted to butadiene in the single conversion coil 3|. Such a charge stock, with a composition as shown in column l of Table I, is introduced to the system through pipe 21, and is joined by about six times as much, by weight, of a recycle stock having a composition as shown in column 2. This recycle is formed by mixing about two parts by weight of the stream passing through pipes 41 and 10, having a composition as shown in column 5, and one part by weight of a substantially methane-free material passed from demethanizer 22 through pipe 30 and valve 29. The combined charge to the cracking coil 3I has a composition Aas shown in column 3. This combined charge is reacted in coil 3| at about atmospheric pressure and an average temperature of about 1470 F. for a reaction time of one second. From the eilluent of the coil 3|, which has a composition as shown in column 4, the C4 and heavier material is separated from C: and lighter material. About two-thirds of the C3 and lighter fraction, with a composition as shown in column 5, is returned directly to the reaction zone, and the remainder is passed to a demethanizer. 'I'he methane-free product of the demethanizer is also passed to the reaction zone,.as discussed. The C4 and heavier stock, which contains a-large portion of butadiene, is subjected to further'separatially` pure state. This product also contains other valuable, low-boiling hydrocarbons, including piperylene, isoprene, cyclopentadiene, benzene, and toluene, among others.

Table I Per cent by weight 1 v 2 3 4- 5 Component y l 'l tal T t l C kin Gflseouts o o a rac g e uen Charge recycle charge etlluent from absorber El". 1. 35 1.15 1.75 1.90 CH4.. 10. 9 11. 00 14.10 16. 35 C2H2. 2. 08 1. 78 1. 80 1.97

01H4. 62. 9 54. 6 54. 5 59. 3 02H1. 16. 03 17. 7 14. 05 15. 1.4 CaHa. 5. 98 6. 97 5. 15 5. B2 CsHl. 0. 76 1. 80 0. 65 0. 72 CIEL. 2. 36 CAHL 0. 83 04H10 0.36 Heavier 4. 46

EXAMPLE )I As-an example of the operation of another 1 lmodiiication of my invention, liquid propane is passed to demethanizer 22 through pipe I9 as an absorption liquid. Methane and hydrogen are removed through pipe 25, and a rich absorption liquid is removed through pipe 30 and passed to fractlonator 89 through pipe 39 and valve 33.

From the bottom of fractionator 851 a Ca fraci tion, having a composition as shown in column l of Table II and being in amount about 1.6 times by weight of the propane charged through pipe I9, is passed through pipes 90 and 80 to the cracking coil I2. This material is reacted at a temperature of about 1470 F. for a period of about 0.7 second under about atmospheric pressure, the eiliuent having a composition as shown in column 2. This eilluent is passed through pipe I4 to separating means I6, wherein C4 and heavier hydrocarbons are separated and recovered through pipe Il, for further recovery and purification of butadiene and other hydrocarbons. From separating means I6 the C3 and lighter material is charged through pipe 20 to the demethanizer 22. No material is charged through pipe I0 or recycled through pipe 23.

Table II Per cent by weight Cracking coil Conversion c'oil Compouent Elu- Total Elu- Charge ent Charge Recycle charge em 0. 5l l 7 28 23.' it? 2s. 4.' 21 11. 0. 56 l. 2. 97 2. 1. 10 0. c3 c. 43 Heavier. 5. 5; 64

The light material from fractionator 89,` hav- `ing a composition as shown in column 3 of Table II, is joined by a recycle stream from pipe l0 having a composition as shown in column 4. The ratio of recycle from pipe 10 to charge through pipe 18 is about 3:2 by weight, and the total charge has a composition as shown in column 5.

as shown in column 5 is returned directly to the reaction zone through pipes 41 and 10, as mentioned, and the remainder is passed to demethanizer 22 from which a fraction containing about This total charge is passed to conversion coil 3l, 6 3.6 per cent hydrogen and 96.4 per cent methane wherein it is reacted at a temperature of about is removed as a gaseous effluent through pipe 25. 1470 F. for a period of about one second at about A heavier fraction from the demethanizer is atmospheric pressure. The conversion effluent passed to another separating means as representhas a composition as shown in column 6, and is ed by numeral 89 in Figure 1, from which a gasepassed to separating means 45. C3 and lighter l" ous eflluent is removed and passed to the reaction material is separated from C4 and heavier, and is zone through pipe 18, as discussed.

Table III Per cent by weight Component Gaseous ial Material Material Total Total Cracking mater Char e in con in conin cong recycle charge eillueut duits 47 duits() mm3 v and 70 passed from separating means through pipe 41. About two-thirds of this material is directly recycled through pipe 10, as discussed, and the rest is passed from pipe 41 through pipes 13 and 16 to pipe 20 and demethanizer 22. The C4 and heavier material is passed through pipes 60, 63, and 92 to the'latter part of pipe 13 and to separating means I6, wherein a separation and recovery of butadiene and other desired heavier products produced both in cracking coil I2 and conversion coil 3l is effected.

EXAMPLE III As an example of the operation of another modiication of my invention, an unsaturated charge stock comprising predominantly ethylene is converted to butadiene in the single conversion coil 3l. Such a charge stock, with a `composition as shown in column 1 of Table III, is introduced` to o the system through pipe 21, and is joined by about six times as much, by weight Vof a recycle stock having a composition as shown in columnv 2. This recycle is formed by mixing about 5.5 parts by weight of the stream passing through pipes 41 and 10, having a composition as shown in column 5, and one part by Weight of a stream suby shown in column 4 is passed to a first separator for the separation of C4 and heavier material from C3 and lighter material, as in absorber 45 and associated equipment. About 80 per cent of acetylene.

In a further modification, the heavy fraction from 89 may be passed through pipes 80, 84 and 85 to heating and/or conversion coil 82 for additional production of butadiene.

' EXAMPLE IV As an example of the operation of still another modification of my invention, essentially pure ethylene is charged to the conversion zone 3l through pipes 21 and 30 to the substantial exclusion of higher and lower boiling hydrocarbons. This material is reacted at a temperature of about 1570 F. for a period of 0.16 second under a pressure, just sufhciently above atmospheric to provide a positive pressure and iiow through the system, the effluent having a composition as shown in column 1 of Table IV. This effluent is passed to silica gel-charcoal adsorption equipment as illustrated in Figure 2 for the separation of various fractions. The C4 and heavier material, which comprises about,9.5 per cent by Weight of the eiiiuent,.has a composition shown in column 2 vof Table IV. The material separated and passing means I6, with valves 1I, 12 and 11 closed. Matethe c3 and lighter fraction, with a composition 75 rial in pipe 41 is joined in pipe 13 by material from conversion zone 82. From separating means I6, C3 and lighter material is passed to demethanizer 22 from which methane and hydrogen are removed through pipe 25 controlledby valve 26. C; and Cz hydrocarbons are removed through pipe 30 and passed through valve 29 through pipes and 84 and valve 85 to conversion zone 82, with valves 38, 49, Il, and IDI closed. In conversion zone 82 substantial amounts of ethylene in combination with other hydrocarbons are con- When a mixture of 73.7 mol per cent of steaml and 26.3 mol per cent of essentially pure ethylene is charged to the conversion zone 3|, as through pipes 21 and 30, and valve 28, and is reacted therein at a temperature of about 1570 F. i'or a period of 0.2 second, under about atmospheric pressure, the eilluent has a composition as shown in column 3 of Table IV. Subsequent separation and further treatment of this eilluent is similar to that described above in connection with the treatment of the ellluent from the conversion of relatively pure ethylene in the absence of steam, with production of a C4 and heavier fraction as shown in column 4 of Table IV, which comprises about 7.3 per cent by weight of the hydrocarbon ellluent.

It is seen from Table IV that when steam is present in the cracking of ethylene to produce butadiene, the effect is apparently more than one oi' simple dilution. While on a once-through basis the yield of butadiene is only slightly greater under comparable time-temperature conditions, the yield of heavier material is quitesubstantially lessened, and the acetylene produced is very markedly increased. A recycle of C2 hydrocar- Table IV Per cent by weight Conversion of ethylene Conversion of ethylene alone in presence of steam Component 1 2 3 4 Conversion Hydrocoil eiluent C and Conversion carbon (free of oxyheavier coil e'luent material in gen-containfraction pipa 60 ing compounds) 0.38 1. 43 l. 97 2.55 4. 49 20. 20 78. 32 62. 30 2. 50 4. 93 2. 55 1. 29 0.33 3. 95 4l. 7 3. 28 1. 33 14. 1 l. 25 0. 07 0. 7 0. 3l 4. il 43. 5 2. 46

Reaction conditions:

Temperature F l, 570 1 570 Time sec, 0.16 0.16 Pressure Atm. Atm. Ethylene reactcdmpercent 21.6 36. I claim:

1: A process for producing dioleiin hydrocar-l bons having four or more carbon atoms per molehydrocarbons in a flrst reaction zone at an elevated conversion temperature and low pressure for a period of time suicient to produce an optimum yield of low-boiling unsaturated hydrocarbons, passing the eilluent of said first reactionzone to a iirst separating means, separating therefrom a Cz-Cs hydrocarbon fraction, subjecting said hydrocarbon fraction in a second reaction zone to an elevated conversion temperature at a low pressure for a period oi time sutilcient to produce between about .3 and 10 per cent by weight of synthesized C4 and heavier hydrocarbons in the effluent, passing the eiiluent to a second separating means and separating from the ellluent and 'removing from the process a C4 and heavier hydrocarbon material containing diolefins so produced, separating also from said eflluent C3 and lighter material, returning a portion of said Ca and lighter material to said second conversion zone, and passing a further portion of said C3 and lighter material to said iirst separating means.

2. A process for producing diolen hydrocarbons having at least four carbon atoms per molecule from lower boiling saturated hydrocarbons, which comprises subjecting such lower boiling hydrocarbons in a iirst reaction zone to :an elevated conversion'temperature and a low pressure for a period of time sufcient to produce an optimum yield of low-boiling unsaturated hydrocarbons, passing the eiluent of said iirst reaction zone to a separating means, separating therefrom a Ca-Ca hydrocarbon fraction, subjecting said hydrocarbon fraction in a second reaction zone to an elevated conversion temperature within the range of 1300-1700" F. at a low pressure within the range of 1.5 to 50 pounds per square inch absolute for a period of time sufficient to produce between about 3 to 10 weight per cent of hydrocarbons of four to live carbon atoms per molecule in the eiiiuent-of said second conversion zone comprising low-boiling dioleiin hydrocarbons of at least four carbon atoms per molecule, separating from the eiiluent and removing from the process a C4 and heavier hydrocarbon material, returning a portion of the remaining Ca and lighter material of said eflluentto said second reaction zone in an amount at least equal in amount to the said Cz-Cs fraction charged, and passing a further portion of said remaining C: and lighter material to said rst separating means.

3. A porcess'for producing low-boiling dioleiins of four and iive carbon atoms per molecule, which comprises subjecting an ethane-propane mixture to dehydrogenation at a low pressure and elevated temperature to produce an optimum yield of unsaturated hydrocarbons of two and three carbon atoms per molecule, passing the dehydrogenation eiiluent to a rst separating means, separating a Ca-C; hydrocarbon fraction in said first separating means, subjecting said Cae-C3 fraction in admixture with between about one and five parts by weight of a recycle fraction for each part of said Cz-Cs fraction to conversion at an elevated temperature within the range of about 1300-1700 Rand a low pressure not greater than about 50 pounds per square inch for a period of time sufficient to produce between about 3 and 10 per cent by weight of synthesized C4 and hea-vier hydrocarbons in the eilluent, in a second separating means separating from the effluent and removing from the process a C4 and heavier fraction containing dioleiins of four and turning a portion of the remaining C: .and lighter material to said conversion as said recycle iraction, and passing a further portion of the said remaining Cs and lighter material to said iirst separating means. Y

4. A process for producing diolefin hydrocar bons of four to live carbon atoms per molecule from low-boiling hydrocarbons of at least two carbon atoms per molecule, which comprises converting an ethylene-rich gas at a low pressure and elevated temperature to produce butadiene, cooling the resultant gas to a near-atmospheric temperature, passing said gas through a body of a rst adsorbent to remove Cs and heavier hy-v drocarbons, passing unabsorbed gases .through a rst body of a second adsorbent to remove Ct hydrocarbons, recovering ethylenel from at least a portion of unabsorbed gases and converting same as a part of said ethylene-rich gas, removing said rst body of second adsorbent from servI ice to recover adsorbed C4 hydrocarbons and substituting therefor a second body oi' said second adsorbent, subjecting said rst body of said second adsorbent to an initial desorption to remove Cs and lighter gases substantially free of C4 hy drocarbons and adding a gas so removed to unabsorbed gases eiiluent from a second body of said second adsorbent, subsequently removing a de- `sorbed gas containing minor amounts of C4. hy-

drocarbons and adding a gas so removed to a gas passing to a second body of said second adsorbent, and finally recovering a desorbed gas rich in C4 hydrocarbons and containing butadiene as a product of the process.

5. A process for recovering C4 hydrocarbons from a. normally gaseous mixture containing between r3 and 10 per cent of C4 and heavier hy drocarbons, which comprises passing said normally gaseous mixture through a body of a first 1 adsorbent to remove C5 and heavier hydrocarbons, passing unabsorbed gases through a first body of a second adsorbent to remove C4 hydrocarbons, removing said first body of second adsorbent from service to recover adsorbed C4 hydrocarbons and substituting therefor a second body of said second adsorbent, subjecting said irst body of said second adsorbent to an initial desorption to remove C: -and lighter gases substantially free oi' C4 hydrocarbons and adding a gas so removed to unabsorbed gases eilluent from a second body of said second adsorbent, subsequently removing a desorbed gas containing minor amounts of C4 hydrocarbons and adding a gas so removed to a gas passing to a second body 'of said second adsorbent, and nally recovering a desorbed gas rich in C4 hydrocarbons as a product of the process.

6. lA process for producing diolefinl hydrocarbons from low-boiling hydrocarbons of at least two carbon atoms per molecule, which comprises subjecting an ethylene-rich gas to an uncatalyzed conversion at a temperature between about 1300 and 1700 F. under a low pressure for a time suffi cient to produce between about 3 and 10 per cent by weight of synthesized C4 and heavier hydrocarbons in the eiiiuent, cooling the resultant gas to a near-atmospheric temperature, passing said gas through a body of a first adsorbent to remove Cu and heavier hydrocarbons, passing unabsorbed gases through a rst body of a second adsorbent to remove C4 hydrocarbons, recovering ethylene from at least a portion of unabsorbed gases and converting same as a part -of said ethylene-rich gas, removing said -flrst body of second adsorbent from service to recover adsorbed C4 hydrocarbons and substituting therefor a second body of said second adsorbent. sub- Jecting said first body of said second adsorbent to aninitial desorption to remove Cs and lighter gases substantially free of C4 hydrocarbons and adding a gas so removed to unabsorbed gases efiiuent from a second body of said second adsorbent, subsequently removing a desorbed gas containing minor amounts of C4 hydrocarbons and adding a gas so removed to a gas passing to a second body of said second adsorbent, and finally recovering a desorbed gas rich in C4 hydrocarbons and containing butadiene as a product of the process.

7. A process for producing diolefln hydrocar bons having atleast four carbon atoms per molecule from a feed composed principally of saturated hydrocarbons having two to three carbon atoms per molecule which comprises subjecting said feed in a rst reaction zone to an elevated conversion temperature and low pressure for a period of time sufcient to produce an optimum yield of low boiling unsaturated hydrocarbons, passing the eiliuent of the iirst reaction zone to a separating means, separating therefrom a hydrocarbon fraction consisting of hydrocarbons having two to three carbon atoms per molecule, subjecting said hydrocarbon fraction in a second reaction zone in the absence of a catalyst and in the abesnce of an oxidizing substance to an elevated conversion temperature within the range of 1300 to 1700 F. at a low pressure within the range of 1.5 to pounds per square inch-absolute for a period of time sufficient to produce at least 3 but not more than 10 per cent by weight of synthesized hydrocarbons of four to five carbon atoms per molecule in the eilluent thereof, passing the efiluent of said second reaction zone to a second separating *means and separating from. the eiiluent C4 and heavier vhydrocarbons 4o containing dioleiins, separating also from said eiiluent Cs and lighter hydrocarbons, passing at least part of said C3 and lighter hydrocarbons to said rst separating means, and recycling another portion of said Cs and lighter hydrocarbons to said second reaction zone.

8. A process for producing diolen hydrocarbons having four and five carbon atoms per mole.. cule from ethane and propane which comprises subjecting a hydrocarbon mixture composed principally of ethane and propane to an elevated temperature and low pressure in a iirst reaction zone eiiecting conversion of a portion of said mixture to ethylene, separating from the eiiiuent of said zone a hydrocarbon fraction consisting of hydrocarbons of two to three carbon atoms per molecule including ethylene, subjecting said hydrocarbons in a second reaction zone in the absence of a catalyst and in the absence of an oxidizing substancev to an elevated conversion temperature within the range of 1300 to 1700 F. and a low pressure within the range of 1.5 to 50 pounds per square inch absolute for a period of time suicient to produce at least 3 but not more .than 10 weight per cent of synthesized hydrocarbons of four to five carbon atoms per molecule in the eiiiuent of said second reaction zone, and separating said synthesized hydrocarbons from the eiiluent of the second reaction zone, separating lfrom said efiluent C3 and lighter hydrocarbons,

passing at least part of said Ca and lighter hydrocarbons to said nist-named separating step, and recycling another portion of said 'C3 Vand lighter hydrocarbons to said second reaction zone.

FREDERICK E. FREY.

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