Q.7 For the process described below draw a process flow diagram, assuming that [8] the process is Unit 200. Specify the number of all the equipment appropriately. In a process to separate and purify propane from a mixture of propane and heavier straight-chain saturated hydrocarbons (e.g., n-butane, n-pentane, etc.),

the feed stream is fed to the 18th tray of a 24-tray distillation column. The overhead vapor stream from the column is totally condensed in a water cooled heat exchanger prior to being fed to an overhead reflux drum. The liquid product from the drum is sent to the reflux pump (which has a spare), and the discharge from the pump is split into two streams. One of these streams is the overhead reflux to the column and is fed back to the column on Tray 1. The second liquid stream from the pump discharge is the overhead product and is sent to storage.

The bottom of the distillation column is used to store the liquid leaving the bottom plate. From the bottom of the column a liquid stream leaves and is immediately split into two. One stream is the bottom product, which is sent for further processing in Unit 400. The other stream is sent to a thermosyphon reboiler where a portion of the stream is vaporized by condensing low pressure steam on the other side of the exchanger. The partially vaporized stream from the reboiler is returned to the column just below the 24th tray. The two-phase mixture separates, with the vapor portion passing upward through the bottom plate to provide the vapor flow in the column. The liquid portion returns to the liquid accumulated at the bottom of the column.

On the process flow diagram developed add the control systems to give the [4] desired control action for the overhead portion and bottom product of the column described below:

i) The flow of overhead product going to storage is controlled by a signal from the liquid level indicator on the reflux drum, which is used to control the position of a pneumatic control valve in the product line (pipe).

ii) The flow of reflux back to the column is also regulated by a pneumatic control valve, which uses the signal (electrical) from a flow indicator on the overhead product line to adjust the valve such that the flow of reflux is always in a certain proportion to the product flow.

iii) The flow of bottom product sent to Unit 400 is controlled by a pneumatic valve that receives a signal from a liquid level indicator that senses the level of accumulated liquid in the bottom of the column.