Fract Clmn Composition Control

1. Purpose

This procedure describes the background and current method for material balance control systems used to maintain product quality of the desired process stream.

2. Traditional Control

The traditional control configuration for the top of a column (Figure 1) is temperature (composition) control adjusting the reflux, with the receiver level control adjusting the net overhead product. This configuration is used when the receiver is used as a surge drum (to minimize flow fluctuations of the overhead product) for downstream processing units.

Figure 1

The traditional control configuration for the top of a column does not, however, respond well to external disturbances, such as heat input variations and rapid changes in ambient conditions (i.e., a rainstorm) that will change the external reflux temperature or cause substantial heat loss from the column. These external disturbances will affect the internal reflux by the relationships shown in Figure 2.

Figure 2

To compensate for an ambient temperature upset with the traditional configuration, additional instrumentation is required to measure the external reflux and overhead vapor temperatures. As shown in Figure 2, the internal reflux is calculated from a heat and material balance around the top tray (if the column has a sidecut stream, the sidecut is also used in the calculation). The calculated internal reflux is then used as the measured variable, and the external reflux flow is adjusted to maintain the internal reflux at the set point. With the internal reflux held constant, the control tray temperature is not affected by changes in ambient temperature.

In general, reliable internal reflux control was not obtained due to the difficulty in establishing the correct physical properties of the external reflux and overhead vapor stream, because the physical properties would change with changes in feed composition. For many applications the direct control of internal reflux has been replaced by the material balance control configuration.

3. Material Balance Control (Original)

In 1975, Inflection Point Engineering began using temperature (composition) control of the net product, with the receiver level controlling the reflux. This configuration (Figure 3) is preferred because it is more direct and provides better response to changes in heat balance and feed composition.

Figure 3

The amount of overhead product that can be withdrawn is determined by the feed composition. If the feed contains 10 moles of light component “A” and 90 moles of heavy component “B”, the maximum amount of overhead product that can be withdrawn without affecting the purity is 10 moles. If the feed composition changes to 10.1 moles of “A” and 89.9 moles of “B”, the additional light component will travel up the column to the control tray and the temperature will decrease. The composition controller will increase the net overhead product rate, which causes the receiver level to decrease and decrease the reflux flow to maintain the composition on the control tray. Since the overhead product rate is generally much smaller than the reflux rate, the resulting change in reflux will be more in proportion to the change in overhead composition than if the reflux was directly controlled by the composition controller.

When the column is in balance and an upset occurs in the reboiler heating medium to increase the heat input, the vapor rate up the column will increase and cause an increase in the receiver level. If the column was recovering the maximum amount of overhead product before the upset, the increase in overhead vapor must include more of the heavier component being taken overhead as an impurity. The increase in receiver level will immediately increase the reflux with the material balance control configuration. When the composition controller detects heavier material on the control tray, it will decrease the flow of net product, which will additionally increase the level in the receiver and directionally increase the reflux more. If the upset is severe, the column may go on total reflux until the composition gradient on the trays is reestablished and the product purity is back to normal. This helps protect the overhead product specification. (The traditional control configuration would send the contaminated product out to storage until the composition controller can react and increase the reflux.)

As discussed earlier, changes in external reflux temperature require additional instrumentation with the traditional configuration to maintain the internal reflux constant. However, the material balance configuration compensates automatically for changes in reflux temperature. If the overhead system is designed with an air condenser and a sudden rainstorm occurs decreasing the temperature of the reflux, the colder external reflux will condense additional vapors resulting in an increase in the internal reflux. The vapor rate to the overhead receiver will decrease causing the receiver level to decrease. When the receiver level controls the external reflux, the external reflux will also decrease. The reaction is the same as the traditional configuration with internal reflux control, but no extra instrumentation is required, and there are no computations to introduce errors into the loop. The material balance configuration is self-correcting.

If the column has a high reflux to distillate ratio (above 10:1), changes in product rate have a relatively small effect on the mass of material in the column, and take considerable time for that effect to appear. If the column has a low reflux to distillate ratio (below 1:1), changes in product rate have a relatively large effect on the mass of material in the column, which could upset the column. In these cases, automatic composition control of the overhead product rate becomes difficult, and the traditional control configuration is used.

With the original material balance configuration (Figure 3) a slow response of the receiver level controller to changes in overhead product rate exists. The receiver inventory is large in comparison to the product flow rate, and small changes in product flow would not quickly change the receiver level. A very small proportional band (large gain) setting was required for the receiver level controller to minimize measurement offset and increase controller sensitivity to a level change caused by a change in product rate. This procedure was not highly successful and often lead to receiver level control instability where small proportional bands (between 2 and 10%) were required to detect small level changes.

4. Material Balance Control (Modified)

In 1985, a modification (Figure 4) to the original material balance configuration was implemented to improve the response of the reflux to small changes in product flow.

Figure 4

The external reflux orifice plate was relocated to the total overhead liquid line from the receiver. This modification reduced the need for a high gain on the receiver level controller, since the reflux responds immediately to changes in net product flowrate via the total overhead flow controller.

In 2002, the above was modified to put the orifice plate back in the reflux line, and remove the total overhead liquid orifice plate (Figure 4a). The net overhead and reflux flow are then summed together and used as the measured variable for the flow controller for the reflux valve. This gives the same control as Figure 4, but now the field hardware is in place to allow for reconfiguration to traditional control if required.

Figure 4a