In the automatic control industry there are two main types: open or close control (discrete control) and process control (process control).
Basic principles of process control
The collapsible control is an automatic control system commonly used for assembly plants. In the chemical industry in general as well as in the oil refining and gas processing industry in particular, open and close control is not common, but also indispensable and plays an important role, especially in applications. start up, shutdown, factory safety.
The inputs and outputs of this control type are only in one of two open or on (on or off) state. The control method of this type is logic, with OR, AND, NAND gates etc. 40 years ago the controller of this type was a system of relays and time relays placed in a panel cabinet. With the development of the electronics industry, a controller capable of programmable PLC (Programmable Logic Control) was born, making the relay system obsolete.
In oil refineries, petrochemicals, and gas processing plants, this type of controller is mainly used. The manufacturing process is continuous, control parameters include temperature, pressure, liquid level, flow rate, pH, concentration etc.
The input device is usually from transducers that output standard analogue signals such as 4-20 mA or 3-15 psig. The usual output devices are control valves. Control method is usually the control algorithm (Proportional), Integral (Integral) and Differential (Differential) abbreviated as PID. 
DCS dispersion control system in a modern oil refinery
Due to the fact that many factories are scattered in a large area and there are many inputs and outputs for each workshop, most of the refineries today use DCS distributed control systems ( Distributed Control System). The system is composed of many smaller systems scattered in each workshop, each of these small systems is responsible for ensuring the control process at the workshop it is in charge of, it is under the management of the systems. host above, can receive or provide signals to host systems. These dispersing systems themselves will directly manage field devices such as valves, sensors, motors, etc.
Gather all the data from the distributed systems in each plant to be sent to higher level systems, which are usually centralized in the central control room of the plant, where the engineers Operators and managers directly make decisions about the operating mode of the plant.
Established in the mid-70s, the DCS distributed control system has brought a real revolution to the central control rooms of the refineries by digitizing control rings and information representation of process to the monitor screen.
|DCS control system model|
The advantages that DCS brings can be mentioned as:
- Ensuring high safety during operation.
- Storing information during the operation for statistics, research, strategic planning.
- Provides an overview of the plant's operations.
- The computational module allows the implementation of control strategies aimed at optimizing technological efficiency and economic efficiency.
- User-friendly interface with language and graphics… 
Some control techniques commonly used in factories
Feed back control feedback
This is the traditional method of controlling a process. The content of the method is to measure the process variable PV (Process Variable) and compare it with the set value SP (Setpoint), based on the deviation e = SP - PV, the controller will give the signal. corresponding control changes the valve opening.
|Feed back control scheme|
Most industrial simple control loops use this control ring.
- Advantages: Simple design, low investment cost.
- The biggest disadvantage of this control loop is that the controller only intervenes in the process after there has been an interference effect on the process.
- The quality of the control process is not high.
Cascade control serial
Serial control is a control technique that uses two controllers inside a control ring. One controller is nested in another controller, the output of the first controller is the set point SP (Set point) of the second controller. This means that the two controllers are not independent but are linked together in order to control the process variable PV (Process Variable) to reach the desired SP value.
Serial control can improve the responsiveness and achieve process control, especially for those with a significant delay, or the response time of the first controller. very big. The picture below is an example of a conditioning system with TRC on sensitive plates and a product quality control system.
Cascade control example
Cases for using Cascade control:
- Improve system responsiveness and increase ease of control.
- The deadtime of the first control loop must be large. If this is not met, the cascade control will be ineffective.
- At the same time, the overall dynamics of the second ring must be faster than the first.
- Application of cascade control in the chemical industry can be mentioned as follows
- LC liquid level control via flow control via FC valve.
- Temperature control at a plate in the tower (usually a sensitive plate). or TC tower top temperature through FC reflux control.
- Temperature control at a plate in the bottom of the tower (usually sensitive plate) TC through control of the heat supply FC FC.
- Control the temperature in the TC reactor through the control of the heating or cooling flow ...
Feed forward control early:
Feed forward control is also called early control technique, with this technique, the control device will measure the disturbance affecting the process and react accordingly. This means that the quality of the control process will be significantly improved as the noise variable will not have an impact on the system, it has been pre-measured and controlled by the controller.
Use case is Feed forward
Although there are significant advantages of eliminating the effects of noise affecting the process, not all processes are able to implement this Feed forward control technique.
Since the nature of this control is to react against noise before it affects the process, it is important to know information about the noise, which is:
- The magnitude of the disturbance, that is, the disturbance in this case must be measured, the most common disturbance variables are temperature disturbances and flow disturbances.
- The way in which the noise affects the process means that we have to know its mathematical model, which will help the controller to set up the control algorithm to eliminate this noise.
Without these two information, the Feed forward control technique will not work.
The example below is a feed forward control system to regulate the operation of a 2-product distillation tower using TRC on the sensitive plate, affecting the capacity of the reheating device when the material changes.
|Feed forward control example|
Combination of Feed back and Feed forward
This is the method most commonly used in industrial control loops. To explain the benefits of this combination, we study the following control scheme for the purpose of controlling output concentration.
|Control model combines Feed back and Feed forward|
Comparing with the diagram in Figure 1.4 without the feedback stage, it is found that when there are disturbances such as the temperature or the concentration of the material flow, the control quality will not be high. The incorporation of the Feed back control ring improves the quality of the control significantly, even in the case of disturbing effects of temperature or material flow concentration.
Technology controlled by computer
This is a modern control technique thanks to the advancement of computational techniques.
With a computer, a simulation of the entire control process can be performed. This model can compute outputs for control valves and other actuators.
Control technique selective Override control:
Selective control is a control technique in which the tuning variable can be controlled by one of many different process variables.
The illustration below shows the main control variable, which is the flow rate of heat through the Reboiler that can be controlled via the liquid level or the bottom of the tower. Here we will analyze this carefully.
In practice there are many cases where the control loop takes in data from many process variables.
The most common application of this control technique is in startup or factory shutdown.
Currently, the modern control technology used to control the process is Advanced Processec Control (APC), but in the dynamic simulation software Dynsim is not used, so we do not study in depth. , 
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. Dr. Nguyen Dinh Lam. Curriculum of auxiliary processes in oil refining technology.
. Pham Van Hoanh. Graduation topic. Class 01H5