Control scheme of typical chemical unit

▲ Pressure control system

1) Gas pressure

Gas pressure is similar to liquid level in that it is a measure of the imbalance in and out of the material in the system, so the pressure control of the gas changes either the inflow or the outflow.

2) Liquid pressure

Because of the incompressibility of the liquid, the pressure control of the liquid is very similar to the flow control

3) Steam pressure

Common boiler drum pressure control, rectification tower, evaporator pressure control, which is essentially the control of heat transfer, the steam pressure of the system characterizes the state of heat balance. So in this kind of control system its characteristics are in some ways similar to temperature control.

(4) Liquid level control system

The liquid level of a equipment or storage tank represents the accumulation of the difference between its inflow and outflow. In chemical production, due to the continuity of production, liquid level control is to serve the material balance, and liquid level control should be completed.

Keep the retention in the equipment or storage tank within the specified high and low limits, so that they have a certain buffer capacity;

At each retention rate, the balance between the inlet flow and the outlet flow is maintained most of the time;

Through the buffer volume to maintain the balance of the load before and after the process, when the flow needs to change, it is hoped that the flow can be gradually and smoothly adjusted.

(5) Component control system

In the production site, the most problematic is often the component control system. The object of the component control system is also multi-capacity, and the time constant is large, and the pure lag time is large. The reasons for the poor work of the component control system, as well as the structure of the analyzer itself is relatively complex, the sampling system and the sample pretreatment part are not working well, and the pure lag is too large. In general, when a suitable component analyzer is not selected, indirect controlled variables such as temperature and temperature difference can also be used instead.

1.Requirements for selecting control instruments

Accurate and reliable, high sensitivity, rapid response, small lag, easy to use and maintain, cheap price.

2.Control scheme of typical chemical unit

▲ Reactor

Requirements for reaction control:

1) Make the reaction reach the specified conversion rate or the product reach the specified concentration.

2) The processing capacity is relatively stable. For example, in a reaction system with material circulation, in order to maintain material balance, necessary auxiliary adjustment systems should be configured, such as automatic empouting or elimination of inert materials.

3) In order to prevent the process parameters from entering the dangerous area or abnormal working conditions, some alarm, interlock or automatic selective adjustment system should be configured to send a signal when the process parameters are outside the normal range, and some valves should be opened, cut off or kept in the restricted position when approaching the dangerous area.

Reaction control scheme:

▲ rectification

Distillation is a common liquid-liquid separation method, and the main purpose of distillation control is to meet the specified separation requirements. The main variables are feed amount, composition, temperature, operating pressure, cooling and heating medium, pressure and temperature transformation.

3.Energy Consumption Calculation

At present, the common task facing mankind is to protect resources, reduce environmental pollution, maintain ecological balance and achieve sustainable development. The heat transfer process in chemical production is a frequent occurrence, so the rational use of energy and saving energy consumption are the responsibilities of each chemical process designer. The common methods of process energy analysis are: pinch point analysis and three-link energy analysis. Common energy saving measures are as follows:

Select the appropriate reaction pressure or temperature.

Improve the reaction conversion rate and reduce the occurrence of side reactions: Choose advanced reactors or reaction processes, and can also use catalysts with strong selectivity and high conversion rate.

Select advanced process routes and raw material routes.

Select the appropriate equipment, optimize the operating conditions, and reduce the energy consumption of the separation process.

The selection of efficient equipment and new insulation materials, such as the selection of efficient pumps.

Pay attention to waste heat recovery and energy recovery.

In the whole process, the step-by-step use of heat energy should be considered, and cold and fever should be avoided.

4.Security Settings

In the PID design phase, you must work with the automation professional to design the necessary safety protection facilities. The safety control of hazardous materials in chemical equipment and storage and transportation is one of the most effective measures for fire and explosion prevention.

4.1 Layout of the plant area and each chemical plant area

Fire protection design, fire alarm system should be implemented in accordance with the "Design Code for Fire Protection of Petrochemical Enterprises" (GB50160-92), as revised in 1999;