Propane-butane splitters

Figure 1.9 illustrates the operation of a simple propane-butane splitter. The tower controls are such that both the pressure and bottoms temperature are held constant. This means that the percent of propane in the butane bottoms product is held constant. If the operator increases the top reflux flow, here is what will happen if... 

In brief, an increase in heavy nonkeys concentration will "fool a bottom section temperature controller into letting more light keys out of the bottom, but will have little impact on a top section temperature controller. Conversely, an increase in light nonkeys in the feed will fool a top section temperature controller into letting more heavy non-keys out of the top, but will barely affect a bottom section temperature controller. In two different troublesome cases (239, 378), the top section temperature controller of an isobutane-normal butane splitter was frequently fooled into letting normal butane into the top product each time propane concentration in the column feed suddenly rose. In one of these (378), the problem was cured by using an analyzer/temperature control (see Sec. 18.3). The author is familiar with several similar experiences of temperature controller fooling. 

Now suppose the refinery crude unit that contributes feed to the butane splitter suddenly increases the propane content of its butane product. Assume this change raises the propane content in the splitter s feed by 20%. If the tower top temperature is maintained at 140°F, the isobutane product composition would be 13% propane, 66% isobutane, and 21% normal butane. 

Most distillation processes are multicomponent. A column common to many industries is the LPG splitter. The feed composition is shown in Table 12.1. It is primarily a propane/ butane mixture which is required to be separated to specified purity targets. While at first glance the feed appears to comprise four components, butane has two isomers (isobutane and n-butane) and pentane has three. Further it is likely that small amounts of unsaturated material will be present, such as CsHg, C4Hg, C5H10 and their isomers. Thus an apparently binary distillation actually involves a substantial number of components. 

The C3+-fraction of the C2 /C3+ splitter enters the C3/C4+ splitter that separates propane, propene, propadiene, and propyne from all heavier products. The C3 stream undergoes a selective hydrogenation step in a fixed bed reactor that converts propyne and propadiene mainly into propene. Propene and propane are separated in a very similar way as ethane/ethene. Again, distillation columns with more than 100 trays are applied, making these separation units very costly in investment and energy consumption. The bottom fraction of the C3/C4+ splitter is transferred to C4/C5+ splitter. The C4 fraction leaving this column at the top contains mainly butadiene, isobutene, 1-butene, 2-butene, and butane. The further use of this crack-C4 mixture is described in detail in Section 5.3. 

Refer again to Fig. 50.4, the reflux drum of the CjC splitter with 10 mol% Butane and 90 mol% Propane in the vapor, and the reflux drum temperature at 100°F. 

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