Propane-propylene separation relative volatility

Propane is separated from propylene by distillation. The compounds have close boiling points and the relative volatility will be low. For a feed composition of 10 per cent w/w propane, 90 per cent w/w propylene, estimate the number of theoretical plates needed to produce propylene overhead with a minimum purity of 99.5 mol per cent. The column will operate with a reflux ratio of 20. The feed will be at its boiling point. Take the relative volatility as constant at 1.1. 

The distillation column used in this example separated a binary mixture of propylene and propane. Because of the low relative volatility and large number of trays, the dominant time constant is very large (500 minutes). Despite this large time constant, a sampling period of 9.6 minutes gave poor results. The period had to be reduced to 1,8 minutes to get good identification, both dynamic and steadystate gain. 

Concurrently with the work on carbon dioxide and hydrogen sulfide at General Electric, Steigelmann and Hughes [27] and others at Standard Oil were developing facilitated transport membranes for olefin separations. The principal target was the separation of ethylene/ethane and propylene/propane mixtures. Both separations are performed on a massive scale by distillation, but the relative volatilities of the olefins and paraffins are so small that large columns with up to 200 trays are required. In the facilitated transport process, concentrated aqueous silver salt solutions, held in microporous cellulose acetate flat sheets or hollow fibers, were used as the carrier. 

Most volatile mixtures have a relative volatility that varies inversely with column pressure. Therefore, their separation requires less energy at lower pressure, and savings in the range of 20 to 40 percent have been achieved. Column pressure can be minimized by floating on the condenser, i.e., by operating the condenser with minimal or no restrictions. In some columns, such as the propylene-propane splitter, pressure can be left uncontrolled. Where it cannot, the set point of the... 

A zeotropic and extractive distillations have been used through the years in the chemical industry to separate mixtures where the relative volatility of the key components is very close, or equal, to unity. Applications from the classical dehydration of alcohol with benzene (1) to more recent ones such as the propylene-propane separation (2) and aromatics recovery from hydrocarbon mixtures with N-methylpyrrolidone (3), indicate a continuous interest through the years in this area. 

Tphis work explores the important variables which must be considered - to design an extractive distillation process. The discussion identifies the economic effects of these variables and their possible interactions. Some of the design variables may have synergistic effects in terms of separation cost while others may not. As a result, the optimum design for an economic extractive distillation process must be a compromise set of values for the different process variables. These compromises are discussed and are illustrated for a particular case—i.e., separation of propane-propylene mixtures. For this commercially important separation fractional distillation is most often used, regardless of the low relative volatility (about 1.13-1.19 at 200 psia). 

Solvent Loading. The solvent circulation rate is a function of the reflux ratio in the primary tower and the liquid-phase concentration of the solvent. For a given solvent selectivity, as the solvent concentration rises, the propane-propylene relative volatility increases and hence the required reflux rate falls. The increased relative volatility results in a decreased number of equilibrium stages required for the desired separation. Figure 4 shows the effect of solvent concentration on the number... 

Selectivity and Activity Coefficients. The solvent selectivity determines the relative volatility of the propane-propylene system, with a higher selectivity yielding a higher relative volatility of propane to propylene. An increasing selectivity therefore results in a smaller reflux ratio and fewer equilibrium stages required for the separation. The lower reflux rate corresponds to a lower vapor-flow rate in the tower and hence to a thinner tower. The lower reflux rate also results in a smaller solvent-flow rate with a consequent decrease in the solvent-cooler size and duty and in the reboiler duties. 

What is the minimum number of theoretical stages required for a Cj splitter to separate propylene from propane if the geometric mean relative volatility between the top and bottom is 1.12, and the desired separation is 99.5% propylene in the distillate and 1% propylene in the bottoms ... 

Cryogenic distillation has been nsed for over 70 years for the recovery of ethylene and propylene from olefin plants, refinery gas streams, and other sources (Keller et ah, 1992). These separations are difficnlt to accomplish because of the close relative volatilities. The ethane/ethylene distillation is performed at about -25 °C and 320 psig in a colnmn containing over 100 trays. Propane/propylene distillation is performed at abont —30 °C and 30 psig. These are the most energy-intensive distillations in the chemical and petrochemical industry (Safarik and Eldridge, 1998). 

The propylene/propane gas separation is a well-known energy consuming process, especially when the traditional distillation is used. It requires more than one hiuidred contact steps since the relative volatility is near to one at temperatures between 244 K and 327 K and total pressures between 1.7 to 22 bar... 

We consider a binary mixture with constant relative volatility a = 1.12 to be separated in a distillation column with 110 theoretical stages and the feed entering at stage 39 (counted from the bottom with the reboiler as stage 1). Nominally, the feed contains 65 mole% of light component (zp = 0.65) and is saturated liquid (qp = 1.0). This represents a propylene-propane splitter where propylene (light component) is taken overhead as a final product with at least 99.5% purity (xp > 0.995), whereas unreacted propane (heavy component) is recycled to the... 

For a fixed separation, tray and reflux requirements increase as the relative volatility decreases, i.e., the separation becomes more difficult. For example, the propane and n-butane separation is easier than propane-propylene but more difficult than propane and n-pentane. 

Extractive distillation can be generally used to separate close boiling liquids or azeotropes, which cannot be separated through conventional distillation process. A solvent is introduced into the distillation column to alter the relative volatility of the feed components, and to avoid the formation of azeotropes. The extracted less volatile components leave from the bottom, whereas more volatile components come out as top products in pure form. Extractive distillation can replace conventional distillation or extraction processes resulting in improved separations, reduced capital investment and energy consumption. Industrially, extractive distillation can be implemented for binary separations resolving the close boiling mixtures, namely m-xylene/ o-xylene, methyl-cyclohexane/toluene, propylene/propane, 1-butane/1,3-butadiene, and azeotropic mixtures such as iso propylether/acetone, ethyl acetate/ethanol/water, MTBE/ethanol, etc. 

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