During minimum pressure operation, the set point is lowered to keep the condenser fully loaded.
In order to prevent the rapid set point changes from causing upsets, a valve position controller (VPC) is used to keep the condenser control valve in a nearly the fully open position (Figure 9). The integral-only VPC is set for an integral time of approximately 10 times that of the overhead composition controller, and bumpless transfer is guaranteed by external reset.
In case of partial condensers, (Figure 10) the level controller on the refrigerant side serves to fully load the condenser in the long term, while the pressure controller corrects the short-term upsets. In this case too, the response of the optimization loop must be much slower than that of the composition control loop.
If controlling the composition of the vapor product is required (Figure 11), the column pressure is controlled by throttling the flow of the vapor distillate, while the set point of the PIC is adjusted by accumulator temperature, which is characterized to represent the desired composition. If changes are to be made in the product composition, the bias is adjusted.
Vacuum and Recompression
To separate some liquid mixtures, the temperature required to vaporize the feed at atmospheric pressure would be so high that decomposition would result. These columns are operated under vacuum. If steam ejectors are used, they have no moving parts and require little maintenance, but are designed to operate only at a fixed capacity and steam condition (Figure 12). It is the accumulator pressure that is controlled by modulating the amount of air or inert gas that is bled into the system. To save on operating costs, two ejectors can be used and switched to match the load.
Vapor recompression is another means of improving energy efficiency. Here the overhead vapor from the distillation column is compressed to a pressure greater than the boiling point of the process liquid at the tower bottoms. This way, the heat of condensation is at the reboiler. This scheme is often used when the boiling points of the top and bottom products are similar. Examples of such processes are cryogenic demethanization or the propylene fractionation. Fluidized catalytic cracking unit main fractionators and crude towers also make use of compressors to “draw” vapors from their essentially atmospheric towers.
|About the Author|