Name: Automatic Control of Ion-Exchange Membrane Caustic Soda Production
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I. Introduction

In the ion-exchange membrane industry, the SUPCON WebField series DCS has been successfully applied in over 50 sets. As of October 2006, the maximum application scale reached 250,000 tons per year. In the production process of ion-exchange membrane caustic soda, the SUPCON DCS is responsible for the fully automatic control of all processes, ranging from primary brine, secondary brine purification, electrolysis, dechlorination, Cl2 treatment, H2 treatment, Cl2 liquefaction, to caustic soda evaporation, achieving successful first-time commissioning. The SUPCON DCS has successfully established communication with the PLC of the brine Kelmembrane filter and fully assumed the sequential control of resin towers (2-5 towers) without requiring a dedicated PLC. This optimizes the control of all production processes in the electrolysis unit. In particular, SUPCON has accumulated extensive integrated experience in monitoring unit cell voltages in electrolyzers, successfully implementing safety interlock protection under various electrolysis process conditions.

Ion-Exchange Membrane Caustic Soda Production

II. Brief Introduction to the Process Flow

Currently, caustic soda production mainly relies on the ion-exchange membrane process. The process flow is sequentially divided into primary brine, secondary brine purification, electrolysis, dilute brine dechlorination, Cl2 treatment, and H2 treatment. The core processes are secondary brine purification and electrolysis.

The primary purpose of primary brine purification is to control the content of suspended solids (SS) and various impurity ions within the required range, preparing for secondary brine purification. The most critical part of secondary brine purification is the chelating resin tower, which removes divalent cations from the crude brine. In some processes, carbon tubes or other types of filters are installed before the brine enters the chelating resin tower during secondary purification to further reduce the suspended solids content.

The electrolysis section is the key process in caustic soda preparation. When purified brine that meets electrolysis specifications flows through the electrolyzer, ions migrate through the ion-exchange membrane under the action of direct current. Ultimately, caustic soda is formed in the cathode liquid phase, dilute brine is produced in the anode liquid phase, H2 is generated in the cathode gas phase, and Cl2 is generated in the anode gas phase.

Ion-Exchange Membrane Caustic Soda Production

III. Control Strategy

3.1 Primary Brine Purification

From a control perspective, the control loops for primary brine purification are relatively simple and mainly consist of single-loop controls. The most complex equipment control lies in the filtration process. Domestically, Gore filters are generally used, mostly controlled by the equipment's built-in PLC. The SUPCON WebField series DCS supports dedicated network gateways to share data with third-party PLCs via serial communication, enabling the monitoring of the filters.

3.2 Secondary Brine Purification

The sequential control of the chelating resin tower is the core of secondary brine control and is also one of the more complex parts of ion-exchange membrane control. It demands high reliability from executing components such as control valves, and its control functions are now mostly implemented precisely by the DCS.

There are two modes for resin towers: three-tower series and two-tower series. Additionally, the operation of resin towers varies slightly depending on the specific process. The following outlines SUPCON's control strategy based on the Asahi Kasei three-tower mode.

Key controls required for the Asahi Kasei three-tower mode include:
◆ Sequential control for normal filtration and regeneration.
◆ Switching and control for normal regeneration, double acid washing, and triple acid washing.
◆ Sequential control for filtration and regeneration during faults.
◆ Differentiated handling for long-term shutdowns and temporary shutdowns.
◆ Gap handling for switching to ensure the safety of field equipment.

Distinctive control strategies:
◆ Since the regeneration tower serves as the main line while the other two towers filter in series, program switching is quite simple. Furthermore, SUPCON provides a step-by-step operation mode for the regeneration program, and the operating time for each step can be set in the parameter setting screen. Therefore, if the primary brine quality fails to meet standards and contaminates the resin tower, the resin can be easily cleaned and regenerated.
◆ During the execution of the sequential control program, many steps require judging the feedback signals of on-off valves to determine the next action. To prevent the feedback sensors of on-off valves from failing after long-term operation and thereby affecting the sequential control program, the system provides a selectable switch to bypass valve feedback signals. When a valve fault signal disrupts the sequential control program, if the operator confirms that the valve is functioning normally and the feedback sensor is faulty, they can choose to bypass the corresponding valve feedback signal, and the program will no longer evaluate it.
◆ During production, resin towers may undergo temporary shutdowns or long-term shutdowns for maintenance. Different shutdown methods have varying requirements for process fluid and valve control. SUPCON provides operation panels in the monitoring screens, allowing operators to easily select the appropriate shutdown mode.

3.3 Electrolysis

The electrolysis section mainly involves directing purified brine through the anode and cathode of the electrolyzer cells while maintaining a specific pressure difference. Normal ion transfer occurs under the action of direct current. It is essential to strictly control the pH value and flow rate of the brine entering the electrolyzer, the differential pressure between the anode and cathode inside the electrolyzer, the Cl2-H2 differential pressure at the electrolyzer outlet, and the DC voltage of the electrolyzer units. To ensure the safe and normal operation of the ion-exchange membranes in the electrolyzers, an extremely reliable interlock protection system must be established.

Loop Control
◆ Cl2-H2 Differential Pressure Control
A double closed-loop ratio control system. By adjusting the pressure in the Cl2 header and the H2 header, the hydrogen pressure is controlled to follow the chlorine pressure changes according to a specific variable ratio relationship.
◆ Control of Anolyte Recirculation Flow Rate
◆ Control of Pure Water Addition Flow Rate to Recirculated Caustic Soda
★ Forms a cascade control with caustic soda concentration.
★ Forms a ratio control with the total current of the Asahi Kasei electrolyzer; a ratio switch is used to toggle between them.
◆ Control of Purified Brine Flow Rate to the Electrolyzer
The purified brine flow rate to the electrolyzer forms a ratio control with the electrolyzer current, but the flow rate must not fall below a certain setpoint.
◆ Control of Acid Addition Flow Rate to the Electrolyzer Anode
The acid addition flow rate to the electrolyzer anode is adjusted based on a ratio control tied to the electrolyzer current.
◆ Detection of Anode/Cathode Differential Pressure Inside the Electrolyzer
The purpose of this detection is to protect the ion-exchange membrane and the normal operation of the unit cell. When installing transmitters, factors such as corrosion resistance, explosion-proofing, and insulation must be considered.
Electrolyzer Interlock Protection

Electrolyzer interlocks consist of two parts: common interlocks and single-cell interlocks. Common interlocks include total plant power failure, complete shutdown of chlorine compressors, excessively high or low Cl2-H2 differential pressure, and instrument air failure. If any of these common interlocks are triggered, all electrolyzers will shut down via interlock. Single-cell interlock conditions include excessively high electrolyzer voltage, low purified brine flow rate into the cell, or low caustic soda recirculation flow rate. When a single-cell interlock condition is met, it only affects that specific cell.

Ion-Exchange Membrane Caustic Soda Production

IV. Typical Applications

Sitara Chemical Industrial Ltd. (SCIL) in Pakistan is the largest chemical group in the country, primarily engaged in the production of caustic soda and chemical fibers. In this 60,000 tons/year ion-exchange membrane caustic soda project—the largest of its kind in Pakistan—the electrolyzers use UHDENORA products, and the rectifiers use Frima products. The control system utilizes SUPCON's DCS products. The control scope includes primary brine, secondary brine, and electrolysis, fulfilling all control requirements, including electrolyzer interlocks, rectifier load ramp-up/ramp-down control, automatic resin tower control, and complex loop control. The control performance has met and exceeded the process specification requirements, receiving full recognition from SCIL, UHDENORA, Frima, and other companies.

The successful implementation of this project marks a breakthrough step for domestic control systems in the caustic soda industry as they expand into the international market. It also signifies that SUPCON's comprehensive optimization control solutions in this industry have been widely recognized, placing it at the forefront of the domestic automatic control sector.