13 Process Evaluation of Improved Solvents for Butadiene Recovery 1
G. D. DAVIS, E. C. MAKIN, JR., and C. H. MIDDLEBROOKS
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Monsanto Co., 800 North Lindbergh Blvd., St. Louis, Mo. 63166 Four solvents were evaluated for the recovery of 1,3-butadiene from crude C fractions by extractive distillation. Furfural [5 wt % water], methyl Cellosolve [10% water], acetonitrile [10% water] and β-methoxypropionitrile [5% water] solvents were studied at comparable operating con ditions in a 2 inch diameter, 140 tray column. Furfural and methyl Cellosolve solvents performed the desired separation between 1,3-butadiene and trans-2-butene only at high solvent-to-C feed ratios. Acetonitrile and β-methoxypro pionitrile solvents were far superior to furfural and methyl Cellosolve at equivalent solvent-to-C feed ratios. The supe rior solvents could perform the desired separation at half the solvent ratio. When related to a plant scale operation, β-methoxypropionitrile solvent could double the capacity of an existing butadiene plant using furfural. 4
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Butadiene is one of the most valuable chemicals produced in the modern chemical refining complex. The pure monomer is recovered from crude C streams by liquid-liquid extraction or extractive distillation with a selective solvent. A simplified diagram of an extractive distillation process is shown in Figure 1. The C feed is separated into two product streams in the extractive distillation column—a butane-butene stream which goes overhead from the column and a butadiene concentrate which is carried out the bottom of the column dissolved in the solvent. The butadiene concentrate contains cis- and irans-2-butene, and after being stripped from the solvent, it is subjected to normal distillation. Butadiene is distilled overhead at 99+% purity. 4
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Present address: Monsanto Co., Box 711, Alvin, Tex. 77511.
215 Spillane and Leftin; Refining Petroleum for Chemicals Advances in Chemistry; American Chemical Society: Washington, DC, 1970.
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REFINING P E T R O L E U M FOR CHEMICALS
In extractive distillation systems, good performance in economy and efficiency of operation can be achieved only with solvents having high selectivity for the desired component. To extract one component having a higher volatility from other components having lower volatilities requires a high selectivity because the relative volatilities must be reversed. For a given solvent, selectivity determines the number of stages required to achieve the desired separation. For a given number of stages and non-solvent reflux ratio, selectivity establishes the solvent-to-feed ratios which are permissible with that particular solvent. I n an existing purification plant, solvents with higher selectivities allow efficient operation at lower solvent to feed ratios [e.g., 6 or 8 to 1]. Plant operation at low solvent ratios is obviously preferred to attain higher production rates at lower energy requirements. BUTANE, BUTENE
BUTADIENE
SOLVENT FEED
C FEED 4
EXTRACTIVE DISTILLATION COLUMN
STRIPPING COLUMN
\—>2-BUTENE$ DISTILLATION COLUMN
Figure I . Schematic of a butadiene extractive distilfotion and purification system Other important solvent properties which must be considered are solute carrying capacity, ease of solute-solvent separation, thermal and chemical stability, and corrosivity. Properties of secondary importance include cost, density, viscosity, specific heat, and heat of vaporization. A number of solvents were studied as candidates for replacing furfural i n existing butadiene extraction plants. T h e significant data from that study have been reported (3), and the most promising solvents are compared in Table I.
Spillane and Leftin; Refining Petroleum for Chemicals Advances in Chemistry; American Chemical Society: Washington, DC, 1970.
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Table I.
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Butadiene Recovery
DAVIS E T A L .
Comparison of Butadiene Solvents at Optimum Water Concentration Relative Volatility of trsLns-2-Butene at 70 psig At Equivalent" Solvent Load
At Equivalent Temperature
1.292 1.302 1.349 1.442 —
1.269 1.303 1.350 1.408 0.848
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Solvent Furfural (5% water) M e t h y l Cellosolve (10% water) Acetonitrile (10% water) β-Methoxypropionitrile (5% water) Normal fractionation, no solvent
° This represents 20 mole % C^s in the solvent on a water-free basis. Temperature of solvent systems equivalent at 140°F.
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The solvent selectivities for butadiene over £ran$-2-butene (key pair) are compared at optimum water concentration and at equivalent tem perature and solvent loading. Solvent Selectivity is expressed i n terms of the relative volatility (
