Question

1. Two liquids (water, 50% sodium hydroxide) must be continuously pumped into a tank and mixed together to ensure a constant (and consistent) product flow. As an engineer you must design this system.

1. Use a diagram to describe how your design
2. Describe the control structure/s within your circuit (1 mark)
3. Describe the safety features of your circuit (1 mark)

Answer 1

The mixing is carried out in an open tank, a continous stirred tank, where mixing is done by means of an Agitator. The ratio of two liquids (L1 – Water and L2 – NaOH solution) are kept constant by controlling the flow of both L1 and L2 individually. The mixing tank is large enough to ensure perfect mixing throughout the tank. A level transmitter is provided for the tank which works on the feedback from the level indicating sensors from the tank, thereby ensuring a constant flow of product output. Here there is nothing to stop the tank from running dry or overflowing. For ensuring correct mass balance, there must either be an implicit mechanism in the process, or an inventory controller must be supplied. The valve for the holdup control loop goes on the remaining stream. In a liquid system the inventory or holdup is measured by level.

PLC based automated liquid mixing system ensures safety and the output of the product in desirable quantities and qualities.