In the second semester of my senior year, I have to apply my process control knowledge onto my senior design project. Last semester, my group and I finished the design for a Ethylene Oxide production plant. This semester, we’re going to put some controllers onto the system to make sure the plant operates under control without explosions.
One of the most important unit of my project is the distillation column. Distillation column is a big deal in general. Roughly speaking 80,000 distillation column are in operation world-wide. In addition, 50% of them are in the United States. Also, the distillation column is very essential to process industries, contributing towards $6 trillion dollars each year.
The first step to controlling a distillation column is to determine its control objectives. The control objectives comes in many different flavors. For example, it can be plain vanilla process safety. It can also be for other control objectives such as meeting regulatory requirements, satisfying process constraints and optimization reasons. Any control design must be able reject process disturbances within a reasonable time.
Here’s a example of one of the fundamental control schemes:
This distillation column has 16 total stages. FC is the flow controller that is controlling the feed stream flow rate into the column by adjusting the valve opening. It is typically reverse acting because as the flow increases the valve needs to be close down. The two LCs are the level controllers each controlling the liquid level in the sump and the condenser drum, by increasing and decreasing the valve level on the exit stream. The sump level controller is direct acting because as the liquid level accumulates the valve on the bottoms stream needs to be opened more to let it out. The condenser drum level is also direct acting for the same reason: The liquid distillate needs to open more in case there’s too much accumulation in the condenser drum. The column stage temperature is controlled by the reboiler duty. If the temperature gets too hot, the reboiler duty needs to be lowered so less heat is generated (reverse acting). Another flow controller is used to control the reflux rate back into the column. Lastly, the column pressure is controlled by the cooling water flow rate into the condenser.