Jul-2014

Selective control for a total reflux column

Operating problems with a de-ethaniser column led to the development of a new strategy for an advanced process control system

NIYAZI BOZKURT
Tüpraş Kirikkale Refinery

Article Summary

A selective approach for a total reflux de-ethaniser column is introduced in this article. The applied algorithm proposed here is based on advanced process control (APC) techniques and implemented in the amine treating unit of a de-ethaniser column at Turkish Petroleum Refineries’ Kirikkale refinery. The objective of the method is to establish appropriate priorities for controlled variables.

Before implementation, there was no operational relationship between reflux flow and top temperature. Following successful implementation of the application, the reflux flow controller is cascaded to the top temperature if the reflux drum level is within the secure operating range. Because this application has a successful outcome, an algorithm such as this can be applied to any total reflux column.

The main objective of the de-ethaniser column is to achieve the best quality in LPG production. The de-ethaniser column at Kirikkale refinery has a total of 30 trays, 10 in the rectification region and 20 in the exhaust region. The most important disturbance that occurs in the column is the feed flow which is taken from the 11th tray. Another problem for the column is the reboiler steam pressure which was changing too much and too often. Because of the varying reboiler steam pressure, the bottom temperature could not be stabilised easily. The bottom temperature affects all operations inside the column. Another important feature of the de-ethaniser column at Kirikkale is that it is a total reflux type.

In this work, the control strategy of the column is improved by considering in particular the total reflux properties. Selective control is applied to the top of the column. In addition, column operation has become more stable as the bottom temperature has become stable for normal operations.

Operation and control of a de-ethaniser column
The de-ethaniser separates the overhead stream from the stabiliser into a C2 stream, usually used for fuel gas, and an LPG C3/C4 stream which is sent to the de-propaniser. The control scheme is shown in Figure 1.
 
Bottom of the column
In the bottom of the column, there is a flow controller valve for adjusting the amount of reboiler steam. This flow controller is cascaded to the column’s bottom temperature controller. Furthermore, the bottom product temperature is monitored with the help of a temperature indicator.

Feed to the column
The feed to the column is from the debutaniser and other units. The feed is taken into the column from the 11th tray by means of a flow controller. The most important source of disturbance for the column is the feed.

Top of the column
Reflux drum pressure is controlled with a pressure controller. This pressure value is so important that the column’s operation is overly affected even by small pressure changes. This is a total reflux column. If the amount of feed is not high enough, there could be almost no reflux inside the column at certain times. The amount of reflux is controlled with a flow controller valve which is cascaded to the reflux drum level controller (LC). The top temperature of the column is not controlled with a control valve; there is just a temperature indicator. In short, for our de-ethaniser column, the top temperature is free and reflux flow is controlled with a reflux drum level controller. The main work in this study is focused on this fact for the purposes of developing the selective control algorithm.

Separation Index
Reboiler duty or steam flow sets the separation index in the column by affecting the vapour and 
liquid rate. These features set the column traffic and sharpness of separation.

Effective cut point
This feature is most affected by the mass balance of the column. The main role of the effective cut point is concerned with reflux drum pressure. As the drum pressure decreases, the gas product contains more heavy ends and gas flow increases.

Proposed approach
In this section, the progress of the application is explained in detail. Furthermore, alternative and recommended approaches which could be implemented in similar processes are introduced.

Base layer enhancement studies
Here, the control elements of the column are revised to fix the main operating problems. The problems are solved and appropriate values for the control parameters (Proportional: P, Integral: I, Derivative: D) are decided. In addition, the control loops are reviewed, taking into account the higher resulting efficiency of the column.

Data collection
The data for this application was collected almost one month before its implementation.
The control element data were collected carefully; these control elements include the bottom temperature, reflux flow, top temperature and reflux drum pressure. Quality values were taken from laboratory results. These quality results are C2 at the top and N-C5 at the bottom of the column.

Control structure parameters
Manipulated variables
Bottom temperature controller
This control valve affects almost every operation inside the column and is required to be stable at all times. The bottom temperature controller is in cascade with the reboiler duty which affects the separation index in the column.

Reflux drum pressure controller
This control valve is particularly important for the effective cut point of the column. The product quality of the column is directly related to the pressure controller on the reflux drum.

Reflux flow controller
The de-ethaniser column is of the total reflux type, so the amount of reflux is crucial to the column’s operation. The reflux flow controller is cascaded to the reflux drum level controller in the original configuration.

Controlled variables
Pressure compensated bottom temperature
Bottom product quality (C5 in LPG) can be estimated and controlled by the bottom temperature. The compensating pressure, the only pressure value in the column, is the reflux drum pressure. The formula used for this value is the following:

3000 / (3000/ ([Bottom Temperature] + 273.15) - ln(24.5 / ([Reflux Drum Pressure]))) - 273.15                                (1)