Propane-propylene separation reflux ratio

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. 

Martin and coworkers described an application of optimization to an existing tower separating propane and propylene. The lighter component (propylene) is more valuable than propane. For example, propylene and propane in the overhead product were both valued at 0.20/lb (a small amount of propane was allowable in the overhead), but propane in the bottoms was worth 0.12/lb and propylene 0.09/lb. The overhead stream had to be at least 95 percent propylene. Based on the data in Table E12.4A, we will determine the optimum reflux ratio for this column using derivations provided by McAvoy (personal communication, 1985). He employed correlations for column performance (operating equations) developed by Eduljee (1975). 

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. 

Use the Underwood equation to estimate the minimum external reflux ratio for the separation by distillation of 30 mole% propane in propylene to obtain 99 mole% propylene and 98mole% propane, if the feed condition at a column operating pressure of 300psia is ... 

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