Propylene-propane distillation

Third, on a new design, the addition of the membrane separation unit can lead to a reduction of the required number of theoretical stages, lowering thus the size of the distillation column. This trend was already observed in a former paper which demonstrated that the addition of relatively small area of a silver-based facilitated transport membrane (several dozen square meters) to a small capacity propylene/propane distillation column was leading to a reduction of the required number of trays from 135 to less than 105 (feed flow = 2.78 mol s propylene feed content = 0.44 mol mol propylene distillate content = 0.99). 

The design of distillation columns is commonly phrased in terms of vapor and liquid flows. For example, we will speak of vapor flow of 600 mol/sec of 99.7% propylene coming out of the top of a propylene propane distillation tower. We will consider the effect of changing the product purity by changing the reflux ratio, and hence the ratio of Hquid to vapor flows in the column. We will evaluate the effect of partially vaporizing some of our feed, and hence reducing the liquid flow in the lower, stripping section of our column. 

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. 

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. 

On the other hand, a pervaporation membrane can be coupled with a conventional distillation column, resulting in a hybrid membrane/distillation process (228,229). Some of the investigated applications of such hybrid pervaporation membrane/distillation systems are shown in Table 9. In hybrid pervaporation/ distillation systems, the membrane units can be installed on the overhead vapor of the distillation column, as shown in Figure 13a for the case of propylene/ propane splitting (234), or they can be installed on the feed to the distillation column,... 

Figure 13 Combined distillation/pervaporation systems for (a) propylene/ propane splitting and (b) aromatic/aliphatic hydrocarbon separation. (Part a from Ref. 234 part b from Ref. 235.)...

The above applies to both binary and multicomponent distillation. In multicomponent distillation, once the above are specified, other components will distribute according to the equilibrium relationship. Frequently, a product spec sets the maximum concentration of impurities that can be tolerated in the product. Product specs are less than" specifications. The one impurity which is dependent on the column separation and is most difficult to achieve sets the composition specification in the column. This is illustrated in Table 3.1 for a propylene-propane separation (Ca splitter). Since the light nonkeys (hydrogen, methane, ethylene, ethane, and oxygen) end up in the distillate, their concentration in the distillate is independent of the column. Of the others, the most difficult purity to achieve sets the composition specification, Similarly, the heavy nonkeys (MAPD, C4 and... 

As in the acetaldehyde process, this can be carried out commercially in either a single-step or a two-step process. The latter is economically more favourable because a propylene/propane mixture (made by petroleum cracking) can be directly used as the feedstock. Propane behaves like an inert gas and does not participate in the reaction. Acetone is separated from lower and higher boiling compounds in a two-step distillation. 

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. 

Process Design Considerations for Extractive Distillation Separation of Propylene-Propane... 

Extractive distillation is evaluated as an alternative to ordinary distillation for the separation of propylene-propane mixtures. Particular attention is given to the necessary compromises between different design factors solvent concentration within the primary column, solvent selectivity, solvent loss, etc. A major expense is associated with the sensible heat requirements of the circulating solvent process modifications so as to minimize this expense are discussed. The process analysis explores combinations of solvent selectivity and other solvent properties which might make extractive distillation attractive. It appears that in almost all cases extractive distillation offers no advantage compared with ordinary distillation. Only in special cases may circumstances warrant extractive distillation. External factors favoring the use of extractive distillation are identified. 

The basis and various parameters for the economic analysis are given in Table II. The overall column efficiency used was obtained from a plot of efficiency vs. the product of relative volatility and liquid viscosity (9), corrected to match predicted (10) data for the propane-propylene system. The value from the plot (9) was increased by a factor required to make the efficiency of the propane-propylene binary distillation equal to 100%. Costs were calculated by the Venture Analysis method (II), because this method yields the appropriate weighting factors for the fixed and operating costs in order to calculate the total costs. Results are expressed as annual costs, before taxes. The important process variables are discussed below. 

Propylene is also recovered as a by-product of other refinery operations, principally from the fluid catalytic cracking (FCC) of gas oils and to a lesser extent from the volatile products of coking, when coking is used. All refinery streams containing recoverable fractions of propylene will be combined into a mixed C3 stream for propylene separation. Distillation of this combined stream then gives propylene (b.p. —47.7°C) as the overhead product and propane (b.p. —42.1°C) plus traces of other higher boiling point products as the bottom fraction. 

It is required to design a distillation column to separate a propylene-propane stream according to the following specifications ... 

The crude acetone produced in the first step of the reaction is first rid of gaseous components (unconverted propylene, propane etc.). The small amounts entrained are absorbed by scrubbing with water at 50°C and atmospheric pressure. The acetone is then distilled to separate the light ends, particularly propionaldehyde, with heavy ends separation to remove the heavy products (water, acetic add, chlorinated compounds such as mono- and dichloroacetones, mono- and dichloropropionaldehydes, etc.). Treatment with caustic soda, carried out in the same final column, facilitates the destruction of the chlorinated compounds and improves the final yield of the operation. 

Rather than setting the pressure of a distillation column at a level sufficiently high to permit the use of cooling water in the overhead condenser, one may specify a lower pressure and use a refrigerant in the condenser. For example, the separation of propylene-propane, as specified in Fig. 17.2, can be conducted by low-temperature distillation at a 100-psia column overhead pressure, as shown in Fig. 17.8, if a feed system such as shown in Fig. 17.9 is provided. There, the feed mixture is compressed in two stages with an intercooler. A refrigerant-cooled condenser, which follows a water-cooled aftercooler, prepares a saturated liquid feed for the distillation operation. In Fig. 17.8, refrigerant must be supplied to the partial condenser to condense the overhead vapor to obtain reflux at 43 F. At... 

Figure 17.9, Feed system for separation of propylene-propane system by distillation at lOOpsia.

Figure 17.5. Separation of propylene-propane by high-pressure distillation.

Figure 17.7. Double-effect distillation process for propylene-propane separation.

Due to the above limitations, an energy balance control scheme such as scheme 16.7 is not recommended, but some situations exist where this scheme can offer better product composition control than many other alternatives. Superfractionators with a reflux to distillate ratio of 10 to 1 or more are one example. Here, distillate flow may be too small to satisfactorily control either accumulator level or column temperature. The author has experienced a satisfactory operation of scheme 16.7 in a propylene-propane splitter, with intermittent operator intervention to ac ust the material balance. The cycles in reflux and reboil (see above) could be tolerated, as the column was not operating close to its limits. In this column, scheme 16.7 gave tighter composition control than scheme 16.4d. 

In the separation train, the gas stream is partially liquefied before entering the demethanizer at 320 bar. The overhead vapor, containing methane and hydrogen, is sent to a membrane separator in which these products are separated. The pressure of the bottoms product is reduced to 270 bar and fed to the deethanizer. In this column, the ethylene and ethane are removed in the distillate, whose pressure is reduced to 160 bar before the species are separated in the C-2 splitter. The bottoms product from the deethanizer, containing propylene, propane, and the heavier species, is throttled to 190 bar, mixed with the bottoms product from the condensate splitter, and fed to the depropanizer. The overhead product of the depropanizer is a mixture of propane and propylene and the bottoms product is throttled to 50 bar and sent to the debutanizer. In this column, the butylenes and butadiene are separated from the SCN. 

EXAMPLE 9.4 Separation of a Propylene-Propane Mixture by Distillation... 

Figure 9.22 Distillation system for propylene-propane separation. Table 9.4 Properties for Propylene-Propane Separation...

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