Application of PLC in a Mixing Process

INTRODUCTION TO MIXING PROCESS:

Mixing is a unit operation in which a uniform mixture is obtained from two or more components,
by dispersing one within the other(s). Mixing is used to reduce inhomogeneity in order to achieve the desired process result. The inhomogeneity can be in concentration, phase or temperature. The secondary effects of mixing are mass transfer, reaction, and product properties.

Mixing plays a vital role in a wide range of industries, like

• Fine chemicals and pharmaceuticals
• Petroleum and Petrochemical industries
• Biotechnology
• Oil and gas refineries
• Polymer processing 
• Paints and Automotive finishes
• Cosmetics
• Food
• Paper and pulp
• Drinking water and wastewater treatment
• Mineral processing

A SIMPLE MIXING PROCESS

Simple Mixing Process 

The simple mixing process, shown in the figure, illustrates the mixing of two liquids. The two liquids are pumped into the mixing vessel through separate pipes. The quantity of each liquid entering the vessel can be controlled manually or automatically. In the manual mode, the pumps have to be switched on to start the supply of liquid and switched off to stop the liquid supply. In the automatic mode, there are two level switches such as high-level switch and low-level switch. Here, the system automatically senses the level of the tank and the pump will start or stop according to the liquid level. When the liquid level is above or below the low-level switch, it closes a contact and turns on the pump to supply liquid to the vessel. When the liquid level reaches the high-level, the pump is turned off to stop the liquid supply. 

The mixing vessel is provided with a stirrer. which is connected to a motor. The stirrer rotates and facilitates the mixing of two liquids. The mixing time is fixed depending on the characteristics of the liquids. Once the liquids are mixed, it is pumped out of the mixing vessel through the drain valve. After the draining process, the drain valve will close and the tank is filled by liquids and mixing process is continuously carried out.

Application of PLC in a Mixing Process

PLC stands for Programmable Logic Controller. A PLC is a special form of microprocessor based controller designed to operate in noisy industrial environment. It has a programmable memory to store control instructions. The term logic is used because the programming is based on logic, switching, sequencing, timing, counting and arithmetic operations. The programming language used in PLCs are simple. Example: Ladder Diagram

Inputs and Outputs in a Mixing Process

Inputs

For manual mode:

Start Switch of Pump 1 - I0.0

Start Switch of Pump 2 - I0.1

Stop Switch of Pump 1 - I0.2

Stop Switch of Pump 2 - I0.3

For Automatic mode:

High-Level Limit Switch - I0.4

Low-Level Limit Switch - I0.5

Outputs

Pump 1 - Q0.0

Pump 2 - Q0.1

Stirrer motor - Q0.2

Drain valve - Q0.3

Drain Pump - Q0.4

Ladder diagram of Mixing Process

Ladder Diagram for Mixing Process

Rung 1: When the start switch of pump 1 is turned on, the normally open contact is closed. This turns on the pump 1.
Rung 2: When the start switch of pump 2 is turned on, the normally open contact is closed. This turns on the pump 2.

Rung 3: When the mixing vessel is full of liquids, the two level switches closes the contact. This turns off pump 1, 2 and turns on the stirrer motor.
Rung 4: Once the stirrer motor starts its operation, the timer is activated. 
Rung 5: Once the timer timing is over, the stirrer motor is turned off. The draining process has to be stated so the drain valve and drain pump is turned on.
Rung 6: When the mixing vessel is empty, the two level switches opens the contact. This turns off the drain valve and drain pump and turns on the pump 1&2.
Rung 7: When the stop switch of pump 1 is turned on, the normally open contact is closed. This turns off the pump 1.
Rung 2: When the stop switch of pump 2 is turned on, the normally open contact is closed. This turns off the pump 2.

Also read: Application of PLC in Bottle Filling Process