Furfural Extractive Distillation

manufacturing processes in connection with the government synthetic rubber program ... built for the recovery of butadiene from a cracked gas stream a...
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Furfural Extractive Distillation FOR SEPARATION AND PURIFICATION OF C, HYDROCARBONS C. K. Boell a n d R. G. Boatright PHILLIPS PETROLEUM COMPANY, PHILLIPS, TEXAS

HE furfural extractive column. I-Butene, isobutylThe furfural extractive distillation process for the separation distillation process for ene, the butanes, and undisand purification of C4 hydrocarbons is used extensively in butadiene the separation and purisolved 2-butenes are n-ithmanufacturing processes in connection with the government synthetic fication of C4 hydrocarbons drawn overhead. The rich rubber program. Furfural, as a selective solvent, alters the normal is used extensively in butafurfural from. the bottom of relative volatilities and makes possible the separation of several of diene manufacturing procthe extractive d is t i l l a t i o 11 the C, hydrocarbons which would be difficult, if not impossible, to column is charged to a fifteenesses. The purpose of this separate b y simple fractional distillation. This paper presents paper is to present infornixp l a t e s t r i p p i n g column. information relative to the development and commercial operation tion relative to the developLean furfural is withdrawn of this process. Solvent characteristics are discussed, a commercial ment and commercial from the bottom of this process flow arrangement is described, performance data are preoperation of this process. column, cooled, and recycled sented, and the effects of operating variables with respect to selecConstruction of a pilot to the extractive distillation tion of process control methods and degree of separations obtained plant IWS started in 1938 for column. The butadiene and are discussed. Factors pertaining to selection of materials of conrecovery and concentration 2 - b u t e n e . ; Tieparated from struction and equipment items and their performance characteristics of small amounts of butathe furfural are fed to a 150during more than three years of plant operations are reviewed. plate fractionating column. diene from cracked gases. I n 1940 a commercial unit was In this last. tower butadiene built for the recovery of butadiene from a cracked gas stream a t is separated from 2-butenes. The purity of the butadiene ohPhillips, Texas. The first method used a t this plant for the retnined averages '39.6$. covery of butadiene from the cracked gases \vas straight fractionaEarly in 1912, under agreement,< lvith Defen5e PIxit Corporation. The feed to thir unit contained approximately 0.40 mole pc tion and Rubbe.; Reserve Company (now Office of Rubber Itebutadiene. The gas was first passed through n aerie3 of separaserve), Phillip Petroleum Compaiiy supervised the design and tion steps to remove propane and lighter components. The resultconstruction and undertook the operation of the Plains plant, ing C, and heavier fraction was fed into a 200-plate fractionating near Borgei,, Texas, which n-as designed to manufacture butadiene column in which the 1-butene and lighter C, hydrocarbons tvere hy the tn.o-stape catalytic dehydrogenation of n-butane in annual distilled overhead, and the butadiene and heavier compounds 45,000-ton amoiints. Operations were started in 1943. formed the kettle product. This kettle product n-a,- charged t o a The C4 concentrate from the first-stage (n-butane) catalytic 100-plate fractionator in which butadiene was distilled overhead. de1iydrogcn;itor consists essentially of a mixture of n-butylenes Thie plant successfully produced butadiene, but because of an and n-butane. The C, concentrate from the second-stage (nazeotrope which is formed by n-butane and butadiene, it n-as butylene) catalytic dehydrogeriator consists principnlly of hutanecessary either for the product stream to contain n-butane, or diene and n-butylenes. The furfural extractive distillation unit for some of the butadiene to be lost in order to remove the na t Plains plant for recovery of butadiene from thp second-stage butane as the azeotrope. Since contamination n-ith n-butane in dehydrogenation unit effluent stream was designed .on the h i s small amounts does not prevent the use of butadiene for rubber of Phillips' first commercial butadiene recovery unit and is pracmanufacture, the former alternative was chosen and the butadiene tically identical with that unit except in size. The furfural exproduct obtained was 95% pure. tractive distillation unit for recovery and purification of Ti-butylT o meet a demand for butadiene of highest purity, this plant enes in the effluent from the firsbstage dehydrogenation unit was \vas modified in 1941 t o use furfural extractive distillation for not exactly duplicated by any pre-existing installation. Howpurification of the butadiene instead of straight distillation. Furever, t'he various portions of this unit do not differ in principle fural, as a selective solvent (liquid against vapor), alters the norfrom the butadiene separation unit. A detailed description of mal relative volatilities of the C4 hydrocarbons. This makesposthese separation processes follows later in this paper. sible the separation of several of the Ci's which are difficult, or I n addition t o the installations mentioned, the furfural extrncimpossible, to separate by simple fractional distillation. tive distillation process is being used in the butadiene plants operThe first portion of the plant, consisting of the equipment used ated by the Seches Butane Products Company at Port iieches, for separating the Ca and lighter from the Cd and heavier hydrocarTexas (d), and Sinclair Rubber, Inc., a t Houston, Texas. bons, is not modified from the original design. The C, and heavier SELECTIVE SEPARATION OF C4 HYDROCARBONS hydrocarbons are passed into a fifty-plate debutanizer in which a separation is made between butadiene and compounds heavier Prior to the commercial development of the furfural extractive than C,. The 2-butenes are divided in this column, part going distillation process, extensive studies were conducted on the overhead and part out of the kettle. The butadiene and other Ci properties of mixtures of hydrocarbons of interest in butylene and hydrocarbons from the top of the debutanizer pass into a 100butadiene separations. Table I gives the normal boiling points of plnte extractive distillation column. The hydrocarbon feed those hydrocarbons likely t o be present in a roughly stabilized C4 mixture resulting from previous operations in which the reaction enters near the center of this column. The solvent, furfural, enters near the top. The furfural breaks the azeotrope betxeen products from the dehydrogenation steps are freed from the n-butane and butadiene. Butadiene and some of the 2-butenes lighter hydrocarbons. These hydrocarbons are listed in order of are dissolved in the rich solvent Ivithdrawn from the base of the descending volatility.

T

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TABLE I. NORMAL BOILING POIXTS CShydrocarbons Propylene Propane XIethylacetylene C4 hydrocarbons Isobutane Isobutylene 1-Butene 1.3-Butadiene

Boiling Point,

F.

- 53.86

(1) -43.73 ( I ) 9.76 ( 1 )

-

10.89 ( 1 ) 19.58 ( 1 ) 20.73 ( 1 ) 24.06 ( 1 ) 31.10 ( 1 )

;4:E1 . 2

i.___.i._-._

Butadiene (diacetylene) Dimethylacetylene

(6) 47.7 (1) 49.6 (6) 80.58 ( 1 )

The relative ease of separation of two hydrocarbons may be determined from the so-called vaporization equilibrium constants of a mixture of the two. This constant, I e t o Butadiene ( a t 65 L b / S q I n Abs. a n d 130' F.) 2.6 (appros.) 2,020 1.718 1,666 1,190 1,065 1.000

I n general, the diolefins are the most soluble in polar selective solvents, the mono-olefins the next most soluble, and the paraffins the least soluble. An efficient selective solvent, therefore, tends t o dissolve butadiene readily and a t the same time to reject the paraffins and, t o a lesser extent, the mono-olefins. The relative volatilities are a n indication (inverse) of the solubilities of the

0

20 40 60 80 MOLE PER CENT WATER

100

Figure 1. Temperature-Composition Diagram for Water-Furfural System at 64.2 Pounds per Square Inch Absolute

compouiid>i n the solvelit. That is, the lower the volatility ratio, the more soluble the compound. For example, a t 150" F. the concentrations of n-butane, 1-butene, and 1,3-butadiene in the solvent furfural plus 4 weight % water required to give a vapor pressure of 64.2 pounds per square inch absolute are 5.9,9.2,and 18.9 mole yc,respectively. Presentation of extensive liquid-vapor phase equilibrium and solubility data for the hydrocarbon-furfural-water systems under discussion is outside the scope of this paper. I t must be recognized t h a t the relative volatilities listed in Table I11 will be affected by temperature, pressure, and composition of the hydrocarbons present in the system. I n addition, in hydrocarbonsolvent systems the relative volatilities are also a function of the amount of hydrocarbon dissolved in the solvent. Some data O I I the phase behavior of isobutane, n-butane, 1-butene, and 1,3butadiene in hydrocarbon-furfural-xater systems are available i n the literature ( 5 ) . The C, SEGRCGATIOS AXD PL-RIFICATIOX OF TL-BTTYLEXES. concentrate from the catalytic dehydrogenation of n-butane consists essentially of a mixt'ure of 1-butene, n-butane, and 2-butenes (Ion. and high boiling isomers). Separation by simple fractionation ~i-ouldrequire two steps: ( a ) to remove l-butene as an overhead product and ( b ) to remove n-butane as an overhead product from the mixture remaining from the first fractionating step. Experience, as well as knowledge of t,he relative volatilities of the several compound$, has shown t h a t separation by simple fractional distillation b e k e e n 1-butene and n-butane is difficult but practical (such separations require about one hundred trays and 25 to 1 reflux ratio). Hon-ever, separation betITeen n-butane and 2butenes, particularly the l&v boiling isomer, by fractional distillat,ion is not practical (see Table 11). - i t the same time separation between n-butane and 2-butenes by solvent extractive distillation is relatively easy while separation b e h e e n n-butane and 1-butene is comparatively difficult (see Table 111). Therefore, separation of 1-butene from n-butane is accomplished by straight fractionation and separation of n-butane and 2-butenes is made by extractive distillation. SEGREGATIOS ASD PURIFICATIOX OF BUTADIEXE. The C4 concentrate from the catalytic dehydrogenation of n-butylenes comprises 1-butene, 1,3-butadiene1 2-butenes1 and some nbutane, isohutane, and isobutylene. Complete separation 1)y

I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

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simple fractional distillation is not possible in this case since nbutane forms a n azeotrope, of the minimum boiling point type, with 1,3-butadiene’. The separation of n-butane is possible, however, by furfural selective solvent distillation since the relative volatility of n-butane to butadiene in the solvent is about 2.0 (Table 111). Separation of 2-butenes from butadiene by straight distillation is easier than by estractive distillation, whereas the separation of 1-butene from butadiene is easier by extractive distillation than by straight distillation. Therefore, briefly, 1-butene and nbutane are sepnrated from hiitadiene tp- extractive distillation, and butadiene and 2-butenes are separated by straight fractionation. Separation of isobutylene from 1-butene is n o t practical b)either straight fractionation or extractive distillation with furfurnl. Tile i d x t y l e n e may he removed by selective polymerization. R ~ i b b e sptliesis r requires a butadiene product not only of high purity, \)ut also ab free as possible of traces of materials deleterious t o the synthesis operations or to the quality of ruhber produced. I n processes for the manufacture of butadiene by the catalytic dehydrogenation of n-butylenes, small quantities of acetylenes appear i n the product7 of reaction. The quantities 1 T h e equilibrium constants of mixtures of butadiene with the other C4 hydrocarbons having volatilities nearest to t h a t of butadiene (1-butene, n-butane, a n d %butene, low boiling isomer) have been determined experimentally. T h e results of these studies indicate t h a t in such systems, including n-butane a n d butadiene, the equilibrium constants of these two compounds become equal a t some concentration, which shotis the existence of a n azeotrope. I n t h e two component system n-butane-butadiene, the composition of t h e azeotrope a t 115 pounds per square inch absolute is 21 mole % ’ n-butane and 79 mole ’?Lobutadiene.

BUTANE

present are ordinarily of such lo^ magnitude as to b e termed “trace impurities.” However, the volatility of methylacetylene and the C1 acetylenes is such that they may increase to a n undesirable concentration in the refined butadiene product unless suitable steps are taken for their removal. The butadiene separation and purification process described here is designed so that the sequence of separation steps used t o make the major hydrocarbon separations also effects the removal of acetylenes, as will he esplained later in this paper. S O L V E N T CHARACTERISTICS

CHOICEOF SOLVEKT.Prior to the selection of furfural, qu:intitative measurements were made in the laboratory of the effects of a large number of solvents on the relative equilibrium constants of C4 hydrocarbons a t various temperatures. Of the solvents tested, only furfural, acetone, phenol, and wat.er met the requircments of high selectivity for butadiene and commercial availability. Consequently, more detailed determinations of equili1)rium constants were largely limited to mistures of C, hydrocarbons in t,hese four. K a t e r has the best selectivity, but the C, hydrocarbon> have negligible solubility in it. Consequently its use is impractical because of the large volumes of water that xvould have to be heated, cooled, and circulated in treating a smallvolume of hydrocarbons. The nest most selective solvent is furfural plus water. Phenol plus water offers about the same degree of selectivity as furfural plus water, but the solubility of hydrocarbons in the pheiiolwater solvent’ is small. The physiological properties of phenol are also objectionable. The selectivity of acetone for butadiene is appreciably lower than that of furfural.

I

Figure

2.

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n-Butylene Purification System

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Although solvent decomposition and lor polymerization is not detectable on a once-through basis, there is a gradual build-up of polymeric bodies in the recirculated solvent. To maintain the selectivity of the solvent and to prevent fouling of heating surfaces, it is necessary t o keep the polymer concentration within tmunds. Consequently, a small side stream of lean furfural is constantly processed in a furfural repurification unit for polymer removal. In addition to losses of furfural through polymer formation, t h e are some physical l o s e s through pump and valve leakage :tiid h?-drncarboii product streams. Total solvent consumption is in the r m g e of 0.01 to 0.02% of the circulation rate, depending principally on residence time, temperature, and diene concentration in the solvent. Plant balances indicate that polymer formation :tccountr for about 60% of the total furfural consumption; thr rem:iining 40?, results from physical losses. BUTYLENE PURIFICATION

Figure 3.

Concentration Gradient for 2-Butene Extractive Distillation Column

In addition to the desirable properties of good selectivity and reasonably high miscibility with C, hydrocarbons, furfural is nontoxic (except to a few persons who are allergic to aldehydes). It is relatively nonvolatile, hut volatile enough to permit good stripping of the Ca hydrocarbons from the rich solvent. I t is available in large quantity and a t reasonably low cost. Finally, under conditions of use, furfurnl has a relatively high degree of stability. .ill of these economic and engineering factors entered into the choice of furfural. It is not the intent of this paper to compare the relative merits of t,he furfural-water solvent with other iolvents being used commercially for similar separations. PHYSICAL PROPERTIES. Commercial furfural is an nml~ercolored liquid having an odor similar to that of oil of bitter :11monds. The physical constants are listed in Table IV (8). In practice a water concentration of about 4 to 6 weight r;c (18 to 25 mole 7 )of t,he solvent is maintained to lower the boiling point of the furfural, as shown in Figure 1. It will be noted that a n azeotrope is formed a t 292.0" F. having a composition 91.5 :md 8.5 mole yc water and furfural, respectively. At this tempera; ture, water and furfural are miscible in all proportions. At atmospheric pressure and a t temperatures below the critical solution temperature, 253.0' F. (Q), furfural and water are only partially miscible. A i t 100" F. the solubilities of ivater in furfural and furfural in water are 6.3 and 8.9 Tveight rc, reapectively ( 7 ) . STABILITY OF SOLVEST.Furfural is commonly regarded iis n thermally stable material. Dunlop and Peters (3) report the decomposition rate of refined furfural to be so slo\v that from a n industrial viewpoint it is thermally stable. However, in t h r presence of water, butadiene, and related hydrocarbons, arid under the influence of heat arid pressure, furfural tends to polymerize gradually. It is known that a reaction product of 1)nt:idiene and furfural also constitutes a part of the polymers formed.

The purpose of the butylene separation and purification step is to prepare a concentrated n-butylene feed stock for subsequent dehydrogenation to butadiene, and t o return essentially- pure nbutane as recycle feed to the butane dehydrogenation process. Figure 2 illustrates the n-butylene purification system. Effluent from the butane dehydrogenation unit is compressed and roughly stabilized by conventional means. .it this stage the principal constituents are n-butane and the three isomeric n-butylenes. Butadiene is present, in lox concentration, but in sufficient qu:intity to n.arrant recovery by recycle means, as will he discupsed suhsequentlp. Concentrations of isohutane and irobutylene are so small thiit they are insignificant in the butylene retem. Small quantities of constituents heavic,r and 1 C:, iirr a l w present. Feed to the purification step is t:iken into a hundred-tray coliiniii which make:: a precise separation of 1-butene from n-butane and 2-butenes t)y Gimple fractionation. Separation of butadiene a t this stage i. f:icilitated by formation of the minimum-boiling azeotrope ivith n-hutane, nhich enables substantially all the butadiene to he removed overhead n.ith 1-butene. Cy and lighter, iwhutaiie, and isobutylene also are present in small quantities in the l-tutenr stream. The overhead product from the 1-hut,ene column ip depropanized and t,aken to the butadiene purification tem, ivhere hutadiene is removed, and 1-butene is returned in thr niixed recycle streams to the hutylene dehydrogenation unit. Tile l-'tnitenr column kettle product is depentanized in a twentytrxy coliiniii and taken to an extractive distillation column for separ:rtion of 2-hutenes from n-t)ut:ine. Furfural containing :ihoiit 6 w i g h t 5; Lvater is used a s :I wlectivePolvent for thisoperation. The separation is carried o u t in a hundred-tray column. The top section of the column acts : n absorber for 2-butene t the n-butane which The hottom section serves to strip absorhed to some extent in t,he top section. The n-butane overhead product is recycled to the hutane dehydrogenation unit. The 2-hutene> in the rich solvent removed from the kettle of the extractive disti1l:rtion column are separated from the bolvent in a tTventy-tray i.tripping column and taken to the n-butylene dehydrogenation unit. The lean or denuded solvent is removed from the kettle of the stripper, cooled, and taken to surge tanks for rccirculation to the top section of the extractive distillation column. =i modification of this process is described by Happel et al. ( 4 ) . In this case isohutane and isobutylene are present in appreciahle concentrations in the raw feed to the n-butylene purification step. Thus, the 1-butene rich stream, containing substantially all the isobutylene and isobut,ane, is taken to a cold-acid selective polymerization unit for removal of isobutylene and then to an extractive distillation system for separation of 1-butene from isohut m e . Table V gives test data illustrative of normal operating conditions and hydrocarbon concentration gradients throughout the

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hundred-tray extractive distilTABLE Iv. PHYSIC.4L CONSTANTS O F FURFURAL lation column separating. n323 Other Constants (at 60' to 450' F . ) butane from 2-butenes. Boiling point, F . Freezing point F. - 34 Vapor Latent heat Figure 3 is a plot of vapor Flash point (Cieveland open cup), a F. 131-5 pressure. at vapor 1.5261 Tynp.. lb./sq. in. Sp. heat, pressure, Refractive index, ';n phase composition of the major F. Density abs. B.t.u./lb.B.t.u./lb. Thermal conductivity a t 100' F . , components (n-butane and low B.t.u./(hr.)(sq. i t . ) ( " F . per f t . ) 0,1525 60 1,164 0.035 0.39 ... and high boiling 2-butenes) Viscosity, centipoises 100 1.140 0.130 0 41 ... At 100' F . 1.35 150 1.110 0.540 0.42 ... against tray number. At the At 130' F . 1.09 175 1.095 0.950 0.43 At 210' F . 0 . 6 8 200 1 . 0 8 0 1 . 6 5 0 0 . 4 3 5 :07.5 time these test d a t a were obHeat of vaporization. cal./g. 107 51 250 1.049 4.40 0,475 -00.5 Dielectric constant at 77' F . tained, the hydrocarbon feed 38 300 1.019 11.50 0,565 194 5 2 1 Explosire limit at 257' F . . % 350 . . . 2 2 . 5 0 . . . 187,O was introduced a t the fiftieth Jurfare tension, dynes/cni. 49 400 ... 43 5 , . . 177.5 450 ... 77.0 165.0 tray and the lean solvent a t the ninety-sixth tray, numbered from the reboiler. The external hydrocarbon reflux was and roughly stabilized by conventional means. Since appreciintroduced on the hundredth tray. Although the solvent is relaable concentrations of C3hydrocarbons are present in the stabilized tively nonvolatile, four trays are used t o "knock back" the small C, mixture, the first step in the purification process is to separate amount of solvent t h a t would otherwise pass overhead in the C1 from C;. This is accomplished in a forty-tray depropanizing column and be removed in the product stream. I n t h e operation of an extractive distillation column with a given quantity and quality of feed, i t is customary t o set t'he column pressure, solvent rate, reflux rate, and solvent temperature a t constant values. The heat input t o t h e reboiler is then adjusted as required t o maintain the desired separation. The bromine Orsat analysis offers a simple and rapid control test for the separation of 2-butenes from n-butane. Samples of vapor from the thirtieth tray of the extractive distillation column are withdraxn a t 2-hour intervals and analyzed for total unsaturates by bromine absorption. -4djustments in steam flow to the reboiler are made t o maintain the total unsaturate concentration a t this point in t h e range 30 to 3 5 5 . Under normal opernting conditions, if the olefin concentration is held within this range, the percentage of olefin in the n-butane overhead product n-ill be about 3 - 4 5 , n-hile the purity of the olefin stream removed overhead in the solvent stripper n-ill be about 95-985. Bihourly bromine Orsat analyses are also made on the overhead product, t o serve as a check analysis on the purity of the n-butane overhead ptream und to assure that the lean solvent is completelystripped of nlworbed olefins. If the solvent is not thoroughly stripped, the percentage of olefins passing oi-erliend i n the extractive distillation column \Till increnze markedly. .IsTable V shows, at the time these te>tdata were obtained, the volume ratio of lean mloeiit to hydroc~lrbonfeed \vas 11 t o 1,the solvent temperattire 135" F., : i d the ratio of external hydrocarbon reflux to feed :illout 0.4 to 1. The purity of tlie n-butane overhead product x x s 9GC;, and tlie purity of the n-butylenes The effect of operating dripped from the rich solvent IWS 97$. variables on column pei.formancc )vi11 be discussed later in this paper. Operation of the solvent stripper is more or less conventional. IIowever, t o prevent loss of solvent in the 2-butenes overliead product and loss of 2-hutenes in the solvent removed from the kettle, this column must lie carefully controlled. For high separnt'ion efficiency, a hyc1rocart)on reflux rate of ahout 0.2 times the t,ich solvent feed rate is used. Heat input is controlled by column temperature, the control point being located on an intermediate tray of the column. Periodic checks are made on the hydrocarbon concentration in the solvent removed from the bottom of the stripper. This concentration is maintained a t less thnn 0.1 liquid volume yohydrocarbon. BUTADIENE PURIFICATION

The purpose of the butadiene separation and purification system is t o separate butadiene produced in the dehydrogenation units into a final product steam of 98.0% minimum purity, and to recover unconverted n-butylenes for return as recycle feed to the butylene dehydrogenation unit. Figure 4 is a flow diagram of the butadiene purification system. Effluent from the butylene dehydrogenation unit is compressed

Furfural Columns at the Plains Butadiene Plant

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is charged t o a hundred-tray extractive diqtillntion column. Pressure, The upper section of this tower .knnlyais of C1 Fraction, Alole % Te;rip.a, Lb./Gq. Flow Rate, acts as an ab.snrl)er for hut,aTect Point F. I n . Gage Gal./Hr. n-CaHio 2-CdHsb 2-CaH8C 1-CaHs Other Cd's diene. The bottom section acts 5,000 2.4 0.i 0 9 Reflux 90 95 8 0 2 2 3 0.7 0 7 95.9 0 4 100th tray 1 d p o r 112 44 as a stripper t o remove 10 8 2.5 1.0 0 4 95.3 96th tray vapor 139 44 1 0 4.1 1.1 0 3 93.5 90th tray 5apor 141 45 butene, which is almr1,ed to ' 2 0 3 7 0 1 6 88.9 80th tray vapor 143 46 some extent by tlie furfural iii 4.2 1 4 84.0 0.9 9.9 t 0th tray vapor 144 48 12.4 6.6 0 1 1 5 78.8 60th tray vapor 144 the upper section of the tower. 8.9 0 7 12.6 1 0 76.8 5 l s t tray Lapor 145 50 The separation is cnrried out 12,000 75.A 12 9 10 6 164 1.1 0.3 Feed (50th tray) 12 5 52 . .. 75 8.2 3.2 0.4 145 40th tray vapor to remove i s o b u t s n e , i s o .... 67 i 18 0 9.1 0.4 4.8 146 30th tray vapor butylene, and n-butane over37 5 16.1 0.5 36 6 54 9.3 148 20th tray vapor 55 4 26.9 0 4 8 4 8.9 10th tray vapor 154 head with the I-but'ene while 57 40.1 2 0 50 3 5.8 1 8 295 Reboiler vapor retaining butadiene in the rich Stripper overhead product ... .. .... 0.6 48.3 4i.7 1.4 2 0 solvrtit removed from the tower Lean solvent 135 .. 131,600 ... ... ... ... ... bottorl:. The 2-hutenes are a Temperature of liquid on de~ignatedtray distribut,edin tlie overhead and Low bailing isomer. bottoms product. The overC High boiling isomer. head product from the extractive distillation column is returned to the butylene dehydrogenation unit. Table VI presents typicaloperating conditions column, designed to remove essentially all of the C, in the feed. of the butadiene extractive distillation column. Pressure, temSubstantially complete removal of CBassures that a very high perature, and butadiene concentration gradients in this tower are percentage of methylacetylene will also he removed from the C , shoil-n in Figure 5 . hydrocarbons. Although the normal boiling point of methylProcess variables are controlled in the butadiene extractive disacetylene is higher t h a n t h a t of propane (Table I), this compound tillation tower in much the same manner as in the 2-butene exforms a minimum boiling azeotrope with propane, which facilitractive distillation tower. All conditions are maintained as tates its removal from the C4 hydrocarbons. (At 322 pounds per nearly constant as possible, except heat input n-hich is adjusted square inch absolute the composition of this azeotrope i F 8-1 mole to maintain the desired separation. Control limits of 40 t o 50% 5 propane and 16% methylacetylene.) butadiene in the twent,y-sixth tray vapor have heen found satisTlie depropanizer was operated init'ially to separate an overhead product containing substantially no C1. However, it was factory for optimum separation over the range of feed composition normally encountered. Bihourly ultraviolet spectrometer found t'hat polymer deposition in the column was accelerated by hutadiene analyses are made 011 the vapor from the twenty-sixth the internal temperature level necessary for operation with water tray above the reboiler. Also, the overhead product is nnalyzed as the condensing medium. To reduce column fouling, the presfor butadiene every 2 hours as a check on butadiene concentrasure was decreased and the overhead vapor product returned t o tion in the overhead product. Sormally, the concentration of the vapor recovery unit for recovery of C4 components. butadiene in the overhead product is ahout 1 . 0 5 . A continuThe second step in the sequence of purifications is to separate ously recording ultraviolet spectrometer is a valuable aid in mainpiirtially the 2-but,enes and C4 acetyIenes from butadiene and taining high separation efficiency. An ultraviolet instrument utilighter components. T h e bottoms product from the depropanlizing an air bellows arrangement to control the steam input has izer, v,-hich consists principally of n-butylenes and butadiene, but proved successful, but such precision control is unnecessary since also contains substantial amounts of isobutane, isobutylene, nonly infrequent manual adjustments of steam flow are required. butane, Caacetylenes, and C6and heavier compounds, is taken to Solvent rates required for efficient separation vary between intermediate surge and thence t o a hundred-tray 2-butene rather n-ide limits, depending upon average feed purity. At the column. I n this column the butadiene fraction is concentrated Plains plant, where the feed purity is in the order of 25% butathrough removal, as a reboiler product, of most of the high boiling diene, a solvent to feed ratio of about 12 to 1 is generally used; 2-butene, as well as part of the low boiling 2-butene and C4 acetyhonever, a t Phillips' private plant where the feed purity is above lenes. The n-butane splits in this column, part going over5 0 5 , a solvent to feed ratio of 6 to 1 is adequate for good separahead with butadiene and the more volatile hydrocarbons, and part going out with the. kettle product. .$ portion of this tion. Because of the greatel, soluhility of the hydrocarbons in t h r kettle product stream may be recycled t o the deoiler in the butylene purification system (Figure 2) t o prevent an excessive accumulation of n-butane in the feed stream to the butylene dehyTABLEVI. CONCESTR.4TIOs GRADIESTFOR BUTADIENE drogenation unit. EXTRACTIVE DISTILLATIOS CoLmrs This second step is not necessary from the standpoint of acetyButadiene Pressure b , Concn. in C r lene removal since the C4 acetylenes can be removed with the Teomp.", Lb./Sq. In. Flow Rnte, Fraction, 2-hutene reboiler product in the final butadiene separation. Tlie I-. Gage Gal./Hr. Mule % Test Point pr,imary purpose of this operation is t o reduce the volume of feed 100th tray vapor 100 52 ... 0.6 116 96th tray vapor .... 0.9 to the subsequent furfural extractive distillation step and also to 80th tray vapor 128 53 . .. 3.1 129 72nd tray vapor 54 .... 7.0 make the separat,ion easier as a result of the enriched feed. This 60th tray vapor 131 55 12.6 ... separation also accomplishes the removal of light oils or polymers 52nd tray vapor ... 17.7 Feed (to 50th tray) i 2 0 56 4;500 2 2.4 that are present in the stabilized C4 mixture. If not removed, 38th tray vapor 139 .... 58.5 31.3 18th tray \.apor 57.5 144 60 .. . these materials will accumulate in the solvent and, thereby, de4th tray \.apor 154 .... 61 79.7 crease solvent selectivity and foul heating surfaces. Reboiler 63 ... ... 297 Stripper overliead The third step in the purification sequence is the separation of product Lean furfural 132 1-butene from butadiene. The butadiene concentrate removed Hydrocarbon reflux 78 overhead in the 2-butene column contains principally 1-butene a Value from smoothed curie of obberved tray liquid temperature. and butadiene, along with lesser amounts of isobutane, isobutylb \-due from smoothed curve of observed data. ene, n-butane, 2-butenes, and C4 acetylenes. This concentrate FOR 2-RUTEXE EXTR.4CTIVE DISTILLATIOS COLGIfS T.IBI,Ev. COXCESTR.4TIOS GR-~DIEKT

1

70 1

I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

June 1947

!-BUTENE

/-

EXTRACTIVE DISTILLATION COLUMN

COLUMN

C 5 8 HEAVIER

Figure 4.

2- BUT EN E$

Butadiene Purification System

solvent, the butadiene extractive distillation tower is generally operated with higher internal hydrocarbon reflux rates than are used in the butylene purification process. The rich solvent leaving the bottom of the extractive distillation tower contains butadiene, 2-butenes, and traces of other C4 hydrocarbons. The rich solvent is pumped to a stripping column where the previously extracted hydrocarbons are stripped from the solvent. The concentration of butadiene in the stripper overhead product is normally in the order of 85%. Table VI1 gives typical analyses of the extractive distillation column feed and overhead product and the furfural stripper overhead product. Substantially all of the C4 acetylenes t h a t pass overhead in the 2-butene column are absorbed in the furfural solvent. This, of course, would be predicted since these compounds are less volatile than the other C4's (Table I) and a t the same time are more highly unsaturated and, hence, more soluble in the solvent. RIost of the methylacetylene present in the feed to the extractive distillation column will also be absorbed in the solvent, since its volatility in furfural is only slightly higher than t h a t of butadiene. The fourth step in the purification sequence is a rigorous separation between butadiene and the 2-butenes. The overhead product from the solvent stripping tower is passed to a 120-tray butadiene column, in which butadiene and traces of lighter materials (isobutane, isobutylene, l-butene, n-butane, and methylacetylene) are removed as a n overhead product while the 2butenes and C4 acetylenes are withdrawn from the reboiler. Traces of furfural solvent or polymers t h a t are carried over from the solvent unit are also eliminated as a reboiler product. Bottoms from the butadiene column are combined with the bottoms from the 2-butene column and taken to a twenty-tray deoiler for

removal of Ca and heavier before return to the butylene dehydrogenation unit. Any methylacetylene not removed in the depropanizing operation, which will ultimately appear in the finished butadiene overhead product, may be further reduced by taking a small vapor bleed stream from the butadiene column reflux accumulator. The butadiene in the methylacetylene-rich vapor bleed stream may be recovered by recycling the vapors to the compression system. Under this mode of operation (as illustrated in Figure 4), the finished butadiene is withdrawn from tray 101 of the 120tray column. Table VI11 shows analyses of samples from the reflux and tray-101 liquid during a time when the final product was being withdrawn from the tray 101. (These analyses were made by removing the butadiene in the original sample to concentrate impurities, analyzing the impurities, and then scaling the results to the percentage butadiene in the original sample.)

TaBLE

1'11. TYPICAL ANALYSESO F BUTADIEKEEXTRACTWE DISTILLaTION SYSTEM

Component Isobutane Isobutylene 1-Butene Butadiene n-Butane 2-Butene (low boiling isomer) 2-Butene (high boiling isomer) Total

(STOLE PEIl

Extractive Distn. Colu Overhead% Feed product 5.8 8.3 6.8 9.8 29.8 42.8 26.2 1.2 15.1 21.6

CEXT)

I

Furfural Strip er Overhead P r o A r t

... ...

...

83.8

...

13.0

14.6

9.2

3.3

1 7

7.0 100.0

loo.0

ioo.0

,

702

I N D U S T R I A L A N D EN G I N E E R I N G C H E M I S T R Y

Vol. 39, No. 6

apparent volatility with respect to the low boiling 2-butene and, therefore, acts as a light key component and increases rapidly in concentration. I n the upper portion of the tower, whew the butadiene is present in high concentration, the C, acetylene has a lo\v volatility with respect to butadiene and, therefore, acts iis a heavy key component and decreases in concentration. This behavior is typical of a component of intermediate volatility and immediately indicates the presence of an azeotrope in the tower. If the C; acetylene did not form an azeotrope, it would derrcase more or less constantly as it progressed up the tower since it liar a lorver normal volatility than the other C4hydrocarbons in the misturc. Since immcdi:itely ahove the feed tray the C4 acetylrxrie exhi1)its hehavior similar to a heavy key component with rrrpect t o hut:idit.ne, its concentration should continue t o decrearr progressively in the upper portion of the tower. However, instead of decreasing, the acetylene concentration on tray 120 i:, about tcri times tliat i'orind on tray 100. This indicates that prartically ail of the acetylene in the top part of the tower i i methylacetylenc, as the methylacetyleilr silollld tiehaw :IS a compotient lighter than the light Icry component, hutadicne, :ind increase in concentration in tlir top few trays of the column. SOLVENT R E P U R I F I C A T I O N U N I T

Figure 5.

TABLE

Concentration Gradient for Butadiene Extractive Distillation Column

1-111.

Sample Reflux l O l P t tray liquid

-4A.4I.YSES O F s A J I P L E S FRC).\I BVT.IUII:SE ( A I O L E P E R ('EST)

?-Butene n ( H . B . ) I3utnne

I3uta&ene

'OLt-ltx

1.01- 2-Butene butyl- 1.t~- .icetyButene (I,.B.) ene hur:inr leiie

0.02

0 06

95 70

1 30

0.14

0 47

0 21

2.10

0.11

0.06

98.70

0.12

0.87

0 07

0 00

0.0i

Butadiene column concentration gradient dnta indicnte tllnt an azeotrope is formed between the 2-butenes and onr or more of the C, acetylenes. Since ethylacetylene constitutes the greatest percentage of the C4 acetylenes, the azeotrope is therefore probably formed betn-een ethylacetylene and the 2-hutriies. This azeotrope is of the minimum boiling type, eshit)itiiig :I vnlatility someviliat higher than tlie lo\v boiling 2-l>uten~. Tlierefot,r, depending upon the C, acetylene concentration in the feed, more reflux may have t o be returned to this column than the quantity calculated as necessary to separate butadiene from tlir lou- tmiling 2-butene. OtherIvise the C4 acetylene concentrution in tlic finished product may exceed the specification limit. Figure 6 illustrates the concentration gmdient of tlie C,