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Temperature Control

How to Compensate Pressure Changes in Temperature Control

02/12/2013 | Autor / Editor: YUVRAJ LAWATE & RUSHIKESH JANAI / Dominik Stephan

The values of the coefficients are derived from the simulated data in the absence of actual plant operating data. Thus these coefficients need to be fine-tuned during operation.

Vapour pressure vs. Temperature data from PRO II and from equation 3 with coefficients from table 3 are plotted in figure 2. From the plot it is evident that the vapour pressure data calculated from equation 3 is in close agreement with PRO II simulation.

Vapour Pressure vs. Temperature

2. Debutanizer: The Debutanizer separates normal butane (n-C4) from C6 plus heavier hydrocarbons. Normal butane is produced as an overhead product and is sent to tankage. The large difference in boiling points makes this a relatively easy separation. Debutanizer has an air-cooled total condenser and thermosyphon reboiler. Refer figure 3 for schematic of Debutanizer column. Operating conditions of Debutanizer are mentioned in table 4.

Table 4: Debutanizer column operating conditions
Table 4: Debutanizer column operating conditions (Source: Aker Solutions)

Debutanizer control schemes: Control scheme of debutanizer is similar to that of Deisobutanizer column.

Temperature control: Tower heat balance is maintained by controlling the pressure compensated temperature of the downcomer liquid from the sixth tray from bottom. This controller resets the flow of medium pressure steam to the reboiler via direct acting flow controller. This controller in turn achieves proper reboiler duty by adjusting the set point of condensate pot level controller, which exposes proper number of tubes and thus the proper amount of heat transfer surface for condensation in the reboiler.

Debutanizer column- PCT Control: Problem statement – Inferential temperature control is employed in Debutanizer column which is separating butane from the mixture of heavier hydrocarbons. Temperature transmitter is located at the sixth tray from the bottom in the column. Pressure compensation is provided to eliminate the effects of pressure variations in the column. Pressure transmitter used for pressure compensation is located at the bottom most tray. Pressure – temperature correlation is to be developed and values of coefficients in the equation are to be determined.

Analysis: In this scenario, tray composition where the temperature transmitter is located (i.e. sixth tray from bottom) would be different than bottom product composition. Thus Debutanizer column is simulated using PRO II to get the composition at sixth tray. For this composition, required bubble point - pressure data is generated using Pro II and is mentioned in table 5.

Effect of changes in Debutanizer pressure on temperature is studied. It is observed from the table 5 that column temperature may vary by about 80C i.e. from 102 to 1100C even if pressure varies by (+/-) 0.5 Kg/cm2 from operating pressure. Thus pressure compensation is essential to nullify the effect of pressure change.

Table 5: Bubble point – Pressure data for Debutanizer column from Pro II
Table 5: Bubble point – Pressure data for Debutanizer column from Pro II (Source: Aker Solutions)

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