The importance of control system

A control system is critically important to maintain equipment performance within the bounds of safe operation. It is also used as protection to process equipment and environment. In addition, the control systems are becoming more critical to the profitable operation of process plant because of rising energy costs, limited availability of raw material, and tighter safety and environmental regulations. Objectives of control systems and instrumentation are:

(a) Safety of plant operation

Maintain the operating temperature at 260^oC to avoid overheating or overcooling while the operating pressure at 200 kPa to avoid pressure build up and cause explosion hazard.

(b) Production rate

Achieve the desired product output which is 98% conversion of acrolein in reactor.

(c) Product quality

Produce acrylic acid with standard requirement and ensure the better product quality control

Since the reaction was carried out in the packed bed reactor at pressure of 200 kPa and temperature of 260°C, it is required of installation the temperature indicator as well as pressure indicator to measured the reactor condition for convenient the process monitoring. Beside, the high level alarm and low level alarm of temperature, pressure and flow is installed to alert operators in case of runaway in process condition. The gate valves are utilized as isolation valve and check valve to prevent the back flow.

7.3.4.1 Types of Controller 7.3.4.1.1 Temperature Control

The primary control variable in most oxidation reaction is temperature. For a closer control, a cascade system is preferred for temperature control where cooling water is used to remove the exothermic heat of reaction. The control variable, reactor temperature which responds slowly to the change in heat transfer medium flow (cooling water as manipulated variable) is allowed to adjust the set point of temperature secondary loop which respond rapidly to the flow changes.

The purpose of secondary loop is to correct the outside disturbances (i.e. temperature changes in heat transfer medium) without allowing them to affect the reaction temperature. If temperature exists more or less than 10% of set point, high or low temperature alarm will be activated. PID is chosen as it gives faster response, avoid lag time to system and eliminate the offset of the set point. ^[11]

7.3.4.1.2 Pressure Control

When the pressure rises, that is the indication of excess gaseous reactant accumulation in the reactor. Thus, the pressure controller will reduce the gas flow. As the result, the gas consumption balance will re-establish. If the pressure exists more or less than 10% of the set point, high or low pressure alarm will be activated. PID controller is used to achieve effective control systems. ^[11]

7.3.4.1.3 Level Control

Level is important in reactor and it should be maintained to achieve the desired conversion. If the inflow to the reactor varies, the easiest way to maintain the level is by manipulating reactor outflow. The ratio of volume to outflow is controlled by manipulating on the flow rate of the outlet stream from the reactor. Cascade control is used and PI controller is chosen because of its efficiency in controlling the level of reactor.

7.3.4.1.4 Feed Flow Control

The objective of flow control is to maintain a constant production rate and make sure the amount of catalyst is sufficient enough for the occurrence of reaction. Flow transmitter (FT) is installed to detect the flow of the inlet stream to the reactor. Signal is then send to Flow Controller (FC) that will be controlling the limitation of feed flow rate at set point value by adjusting the final control element (control valve). For flow control system, feed forward control configuration is applied. If flow rate exist more or less than 10% of the set point, high or low flow alarm will be activated. PI controller is used since it is sufficient for controlling the feed flow. ^[11]

7.3.4.1.5 Feed Composition Control

The molar ratio of reactant affects the conversion of the reaction and the composition of the product. Hence, to maintain these two output variable at desired set point, chemical composition

analyzer is installed such as infrared analyzer, conductometric analysis ultraviolet or visible radiation analyzer to detect the concentration of the components. PI controller is selected because the integral action can eliminate the offset thereby establish precise ratio of streams. ^[11]

7.3.4.2 Control System Loop

7.3.4.2.1 Feed Flow Control (Control System Loop 1)

This control system is used to control the reactor feed stream in a constant flow rate. The flow transmitter is utilized to detect the changes of the feed flow. A signal will be generated by the flow transmitter and send to the flow controller. The flow controller will convert the received signal and send to the flow control valve, which adjust the flow of the feed stream to the desired flow rate.

7.3.4.2.2 Reactor Pressure Control (Control System Loop 2)

This control system is used to control the reactor pressure in a constant value. The pressure transmitter is utilized to measure the pressure of the reactor. When the reactor pressure is high than the set point value, the pressure transmitter send a signal to the pressure controller and the high pressure alarm will sounded to instruct the operators. The pressure controller and then will adjust the flow to maintain set point value.

7.3.4.2.3 Cooling Stream and Reactor Temperature Control (Control System Loop 3)

This control system is used to control the cooling system to maintain reactor temperature. Heat transfer is necessary in the exothermic reaction to maintain the reactor in isothermal condition.

Normally, heat transfer in the reactor is a slow process. Therefore, the reactor temperature can easily undergo a large deviation from the set point before control is again established. In order to have a better response to the temperature fluctuation, Cascade control is utilized rather than other control because can provide faster response of reactor temperature control to a disturbance in cooling water temperature. In this system, the output of primary controller becomes the set point of secondary controller which is used to control the flow rate of cooling water into the reactor.

Under these conditions, the primary controller adjusts indirectly the flow rate of cooling water.

The control and instrumentations of packed bed reactor is show as below:

Figure 7.4: Control System for Packed Bed Reactor

FC  Flow controller  TC  Temperature controller  PC  Pressure controller  FI  Flow indicator  TI  Temperature indicator  PI  Pressure indicator  FT  Flow transmitter  TT  Temperature transmitter  PT  Pressure transmitter  FAH  High flow alarm  TAH  High Temperature alarm  PAH  High Pressure alarm  FAL  Low flow alarm  TAL  Low Temperature alarm  PAL  Low Pressure alarm 

Table 7.25: Control Variables for Packed Bed Reactor

Cascade/ PI Control product

Acrolein feed Feed flow rate from oxidation reactor

Cascade/ PI Control feed flow

rate

Acrolein:O2:N2:H2O is 1:1.6:6.4:1