LPG splitter

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. 

Assuming a typical operating pressure of 12 barg (or an absolute pressure of 13.01325 baia or 12.84308 atm) for a LPG splitter, and the Antoine coefficients given in Table 12.2, we obtain the values for dT/dP shown in Table 12.6. 

On trains of distillation columns there may be other possible methods of reducing sample delay. Consider the arrangement in Figure 12.123 in which our case study LPG splitter is the last column. 

It is common to have specifications on the C2 and C4 content of propane. We have considered schemes that might control the latter by manipulation of the reflux on the LPG splitter. But to control the C2 content we manipulate the reboiler duty on the preceding de-ethaniser. A composition controller based on a C2 analyser on the propane product will respond very slowly to changes in its MV. A better approach would be to locate an analyser on the de-ethaniser bottoms. The C2/C3 ratio will be close to the C2 content of propane but provides a much earlier indication and hence much faster disturbance rejection. 

Similarly it is common to have C3 and C5 specifications for butane. Control of C3 would likely be by manipulation of the LPG splitter reboiler duty. However control of C5 would be manipulation of the reflux on the debutaniser- two columns upstream Measuring the C5/C4 ratio of debutaniser distillate would provide a huge dynamic advantage. 

In the same way that pressure can be manipulated to exploit the difference in product prices, so can target compositions. We saw in the first section of this chapter how cut and separation can be adjusted to maximise revenue. The feasible operating region developed for these variables can be converted to one based on the actual MVs. For our case study LPG splitter. 

Figure 13.3 shows an example of the calculation that might be performed on a very high purity LPG splitter. The distillate specification is 0.05 mol% C4 in propane. The material sold from storage has a concentration very close to this specification and so there would appear little benefit to be obtained from better controlling it. However the rundown... 

A key part of this work in the development of fractionation sections for the reformate and A6 splitter. We compare the model predictions of the temperature profiles of the LPG column DA301 and reformate separator DA302. We note good agreement with plant measurements. 

---