Propylene propane, separation

In this chapter, we examine the thermodynamic efficiency of the propane-propylene separation process by distillation. The tools necessary for this analysis are developed using the first and second laws of thermodynamics. Sources of thermodynamic inefficiency are pinpointed and, finally, some options are discussed to improve the efficiency of the separation. 

In general, a C3 stream is obtained that contains propane, propylene, pro-padiene, and propyne, and these are separated in a C3 distillation column, also referred to as the C3 splitter. Propane-propylene separation and, as a rule, olefin-paraffin separation, are energy-intensive, and some estimates are that 1.27 x 1017 J are used for olefin-paraffin separation on an annual basis [4] while roughly 3% is used by paraffin-olefin distillation columns [5]. This provides an incentive to examine the propane-propene separation, which is an example of paraffin-olefin separation. 

In case of the propane-propylene separation, the y = x line and the i/x-equilibrium line are very close, as shown in Figure 10.9. This means that a large number of trays is necessary, which is the case in practice. Another consequence of the close proximity of the y = x line and i/x-eqtiilibrium line is that the slope of the top operating line, L/V, will be close to unity. This... 

Separation, CO2/C2 hydrocarbons Separation of mixtures Separation, membrane Separation, membrane, asymmetric Separation, propane/propylene Separation, xylenes SFe-Xe diffusion in BOG, modelling Shape selectivity 11-P-24 25-P-07 25-... 

Benali, M. and Aydin, B. (2010) Ethane/ethylene and propane/propylene separation in hybrid membrane distillation systems Optimization and economic analysis. Separation and Purification Technology, 73 (3), 377-390. 

Figure 13.11 Two-step propane dehydrogenation to propylene with minimum CO2 emissions (1) Combustion chamber (2) post-combustion chamber (3) convective furnace (4) and (5) membrane modules for H2 separation (6) boiler (7) propane/propylene separator (8) H2 compressor.

Two types of sorbents have been examined for ethane/ethylene and propane/propylene separations zeolites/molecular sieves and r-complexation sorbents. The... 

Liquefied Petroleum Gas The term liquefied petroleum gas (LPG) is applied to certain specific hydrocarbons which can be liquefied under moderate pressure at normal temperatures but are gaseous under normal atmospheric conditions. The chief constituents of LPG are propane, propylene, butane, butylene, and isobutane. LPG produced in the separation of heavier hydrocarbons from natural gas is mainly of the paraffinic (saturated) series. LPG derived from oil-refinery gas may contain varying low amounts of olefinic (unsaturated) hydrocamons. 

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 reactor is followed by a gas-liquid separator operating at 30 bar from which the liquid phase is heated with steam to decompose the catalyst for recovery of cobalt by filtration. A second gas-liquid separator operating at atmospheric pressure subsequently yields a liquid phase of aldehydes, alcohols, heavy ends and water, which is free from propane, propylene, carbon monoxide and hydrogen. 

The volumetric expansion parameter S may thus be taken as 0.9675. The product distribution will vary somewhat with temperature, but the stoichiometry indicated above is sufficient for preliminary design purposes. (We should also indicate that if one s primary goal is the production of ethylene, the obvious thing to do is to recycle the propylene and ethane and any unreacted propane after separation from the lighter components. In such cases the reactor feed would consist of a mixture of propane, propylene, and ethane, and the design analysis that we will present would have to be modified. For our purposes, however, the use of a mixed feed would involve significantly more computation without serving sufficient educational purpose.)... 

In the second scheme, the alkane is transformed to the olefin by oxidehydro-genation, and the outlet stream is sent to the second oxidation reactor without any intermediate separation." Isobutane and isobutene are recycled, together with oxygen, nitrogen, and carbon oxides. Finally, the third scheme differs from the first one in that hydrogen is separated from propane/propylene after the dehydrogenation step, and oxygen is preferably used instead of air in the oxidation reactor." ... 

One possible arrangement for a hydrofluoric acid alkylation unit is shown schematically in Fig. 1. Feedstocks are pretreated, mainly to remove sulfur compounds. The hydrocarbons and acid are intimately contacted in the reactor to form an emulsion, within which the reaction occurs. The reaction is exothermic and temperature must be controlled by cooling water. After reaction, the emulsion is allowed to separate in a settler, the hydrocarbon phase rising to the top. The acid phase is recycled. Hydrocarbons from the settler pass to a fractionator which produces an overhead stream rich in isobutane. The isobutane is recycled to the reactor. The alkylate is the bottom product of tile fraetionater (isostripper). If the olefin teed contains propylene and propane, some of the isoshipper overhead goes to a depropanizer where propane is separated as an overhead... 

Hayashi, J., Mizuta, H., Yamamoto, M., Kusakabe, K. and Morooka, S. (1996) Separation of ethane/ethylene and propane/propylene systems with a carbonizad BPDA-pp ODA polyimide membrana. Industrial el Engineering Chemistry Research, 35, 4176. 

Table I shows the products from a well-designed gas-recovery unit in a typical refinery having a catalytic-cracking unit and a thermal-cracking unit. Where only the propane propylene is charged to the polymerization unit a depropanizer is added to separate the Cs and lighter from the C and heavier, shown in the last column of the table.

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. 

It is evident therefore that the thermodynamic characteristics (S00, y°°C3H8, S, ycsHgj etc.) of the propane-propylene-solvent system are most important in determining the economics of extractive distillation for this separation. Accurate prediction (or experimental determination) of these factors is essential for economic analysis of an extractive distillation process. 

Hafslund (6) reports that extractive distillation has been used commercially in one particular case to separate propane-propylene. The feed to the separation process was the off-gas from a reactor in a new process for making acrylonitrile and consisted of 40 wt % inerts, 39 wt % C3H6, 8 wt % C3H8, 7 wt % acrylonitrile, 5 wt % water, and 1 wt % by-product impurities. The propylene was to be recovered for recycle to the reactor, and the propane and inerts were to be purged from the system. Acrylonitrile was chosen as the solvent for extractive distillation. The topmost section of the primary column was replaced by a water scrubber to recover the acrylonitrile in the vent gas. The process reported by Hafslund (6) is therefore somewhat different from the processes illustrated in Figures 1, 8, and 10. 

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