Control of Pipestill

A petroleum fractionator, such as a pipestill or a cat fractionator, is almost overwhelmingly complex. In addition to the main column, there are strippers that have vapor and liquid streams going back to and coming from the main column. There are a very large number of control loops to set up. Let us enumerate the loops that we will set up, considering both the preflash column and the pipestill. 

Temperature loops Temperatures of both furnaces (two loops). 

Flow loops Two cmde feeds, steam to two column bases, steam to three strippers, and Stage 19 liquid in the pipestill (eight loops) 

Pressure loops Condenser in both columns and three strippers (five loops) 

Level loops Base level of two columns, water level in two reflux drums, organic level in two reflux drums, and base levels in three stripper bases (nine loops) 

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Figure 8-1 indicates the location of the various control instruments that may be used in a topping plant. Among the most important are (1) top temperature controller (2) recording pyrometer at vaporizer and at inlet and outlet of pipestill (3) pressure at inlet and outlet of pipestill, at charge pump, and in tower (4) rate of flow of crude oil and (5) level controls at bottom of tower and the side-draw plates. Control instruments are extensively used in natural gasoline plants (Fig. 8-2). Finally, Figs. 12-6 and 12-7 on dewaxing, and Fig. 20-11 on alkylation, show instrumentation.

Individual circulation systems are often maintained for each pipestill. With such an arrangement the rate of firing may be controlled by regulating the pressure in the fuel line without visiting the burner. The pressure may be conveniently regulated at a central control board. Such a system is not satisfactory unless clean, fluid fuels are avaUable and unless minor adjustments of the firing rate are all that are necessary. 

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