CPVC

30,000 t/a Chlorinated Polyvinyl Chloride (CPVC) Process Scheme Using Aqueous Acid-Phase Chlorination Method

1. Process Block Diagram

2. Material Balance Table

3. Raw Material and Utilities Consumption

Table 3-1 Consumption Quotas and Quantities of Raw Materials and Utilities

Table 3-2 Chemical Consumption Quotas and Quantities

Table 3-3 Auxiliary Material Consumption Quotas and Quantities

4. Product Standards

5. Raw Material Specifications (PVC)

6. Plant Layout
A. Batching, chlorination, neutralization, and storage tank area: 80 m × 30 m.
B. Drying area: 98.5 m × 32 m.
C. Control room area: 30 m × 22 m.
D. Warehouse and packaging area: 185 m × 30 m.
E. Raw material storage area: 70 m × 38 m.

7. Brief Process Description
Basic Principle of Aqueous Acid-Phase CPVC Production
Polyvinyl chloride (PVC) is a linear polymer and an amorphous thermoplastic resin produced by free-radical polymerization. Its chlorination proceeds by a substitution reaction. As the degree of chlorination increases, the irregularity of the molecular chain increases, making crystallization less likely. Meanwhile, the polarity of the molecular chain increases, resulting in stronger intermolecular forces. The aqueous-phase suspension chlorination process uses water or hydrochloric acid as the suspension medium to facilitate mass and heat transfer. The CPVC produced is a heterogeneous chlorinated product with a block-like molecular structure, characterized by low solubility and high thermal stability. The process is simple with a short production flow and relatively low production cost. With a chlorine content of up to 68%, the product exhibits excellent heat resistance and mechanical properties, making it suitable for processing into heat-resistant rigid materials.
In terms of unit operations, the process can be divided into batching, chlorination, post-treatment, centrifugation, and drying units. The plant is also equipped with supporting systems including a hot water circulation system, a tail gas absorption and treatment system, instrumentation and control systems, and storage tank areas for raw materials, auxiliary materials, and waste collection (solid, liquid, and gaseous wastes).
（1）Batching Unit
Based on the different media, the batching unit can be further divided into four sub-sections: acid preparation, chlorination suspension preparation, soda ash preparation, and caustic soda preparation.
Acid Preparation:
Filtration and blending of the 25% concentrated acid and 5% dilute acid obtained from acid filtration.
The 25% concentrated acid and 5% dilute acid obtained from acid filtration first enter the concentrated acid settling tank and the dilute acid settling tank, respectively, for material recovery. They then enter the 25% hydrochloric acid intermediate tank and the 5% hydrochloric acid intermediate tank, and are subsequently pumped by the 25% hydrochloric acid filter pump and the 5% hydrochloric acid filter pump into the 25% hydrochloric acid filter and the 5% hydrochloric acid filter, respectively. The entire filtration process is automatically controlled.The filtrate is sent to the 25% hydrochloric acid product tank and the 5% hydrochloric acid product tank, respectively, while the filter residue is sent to one of the soaking kettles.The 25% hydrochloric acid product tank and the 5% hydrochloric acid product tank are each equipped with a hydrochloric acid pump, namely the 25% hydrochloric acid product pump and the 5% hydrochloric acid product pump.The flow rates are regulated in a certain proportion through the respective flow meters and control valves at the pump outlets. Based on the actual acid concentration, 16% hydrochloric acid can be obtained.
The liquid level of the 16% hydrochloric acid tank is interlocked with the 25% hydrochloric acid product pump and the 5% hydrochloric acid product pump, enabling automatic acid feeding and automatic control. The blended acid is then ready for use.
Start the agitator of the batching kettle and start the 16% hydrochloric acid pump to feed the required amount of acid into the batching kettle. The entire process is fully automatic. Then close the acid feed valve and stop the 16% hydrochloric acid pump. Add the additives to the batching kettle and allow to stand for 10 minutes. Then open the vent valve of the batching kettle and open the bottom valve of the raw material metering hopper, and feed the pre-weighed raw materials into the kettle through the screw conveyor. After mixing and emulsification for approximately 0.5 hours, the batch becomes homogeneous. Based on the temperature of the material in the batching kettle, the heating system is automatically activated by the PLC program throughout the process, eventually raising the temperature to 70℃ for standby. The discharge valve of the batching kettle is remotely opened by the DCS system, and the transfer pump is started to deliver the slurry from the batching kettle to the corresponding chlorination reactor. This unit consists of two batching kettles, each serving five chlorination reactors.
Sodium Carbonate Preparation:
After adding a specified amount of pure water, switch to the circulation mode and start the soda ash pump. Manually add the required quantity of solid soda ash through the feed port at the top of the soda ash tank. Once the concentration in the tank has stabilized, stop the soda ash pump and switch back to normal mode. This process is carried out manually.
Caustic Soda Preparation:
After the caustic soda circulation tank has been emptied, add a specified amount of water, then start the caustic soda liquid pump to charge the required quantity of 32% caustic soda solution into the circulation tank. This process is carried out manually.
(2) Chlorination Unit
Liquid chlorine from the pipeline passes through the shut-off valve into the liquid chlorine distribution header, and is then fed into each chlorination reactor under flow control. Open the discharge valve of the batching kettle and the feed valve of the chlorination reactor, and open the vent valve from the chlorination reactor to the chlorination tail gas buffer tank. Start the agitator of the reactor and transfer the material from the batching kettle to the chlorination reactor by the transfer pump. After the batching kettle has been emptied, close the feed valve of the chlorination reactor. Keep the vent valve of the chlorination reactor open, as dissolved oxygen in the reactor needs to be gradually released as the temperature rises during the later stage of chlorination.
The jacket steam heating system of the chlorination reactor is started with one-click operation, and an automatic control system for hot water and cooling water is also provided. The jacket temperature is automatically controlled by the control valves on the steam, hot water, and cooling water lines to raise the reactor temperature to the initial reaction temperature, typically 70℃. The vent valve from the chlorination reactor to the chlorination tail gas system is then automatically closed. The chlorine gas flow control valve is opened to introduce liquid chlorine into the reactor, and the instantaneous chlorine flow rate is automatically regulated to allow a gradual increase in reactor pressure and temperature until chlorine feeding is completed. The entire reaction process is carried out in accordance with the specified process parameters, as defined in the process instruction sheets issued by the technical department, with full automatic control based on the set parameters throughout the process. After chlorine feeding is completed, the jacket cooling water inlet and outlet valves are automatically opened to cool the reactor contents until the material temperature drops to 80℃. The DCS remotely opens the compressed air valve of the chlorination reactor and the discharge valve of the chlorination reactor, and opens the feed valve of the receiving reactor to transfer the material by air pressure to the receiving reactor. If necessary, the manhole cover of the chlorination reactor shall be opened for slag removal, and the filter on the receiving reactor shall be cleaned periodically.
(3) Post-Treatment Unit
The post-treatment unit includes concentrated acid filtration, dilute acid filtration, neutralization, and slurry preparation.
Concentrated Acid Filtration:
There are four receiving and cooling reactors in one production line. Start the concentrated acid filter, open the bottom discharge valve of the receiving reactor, and start the transfer pump to deliver the material from the receiving reactor to the concentrated acid vacuum filter. (The filter is fully enclosed and equipped with a vacuum system. The hydrogen chloride gas extracted is scrubbed with water and then sent to the workshop fugitive emission absorption system for discharge in compliance with emission standards.) The separated 25% hydrochloric acid goes to the 25% hydrochloric acid settling tank, while the filter cake, together with process water for filter cloth rinsing and 5% dilute acid, enters the soaking kettle.
Soaking and Dilute Acid Filtration:
After the soaking kettle is charged, the material is thoroughly soaked and agitated to allow complete release of acid from the particles. The two soaking kettles are operated alternately, with one kettle in soaking mode and the other in discharge mode. Process water is added to each kettle, and the operating cycle is 2 hours. After soaking, start the deacidification centrifuge and open the feed control valve of the deacidification centrifuge. Open the control valve of the recovery pump for the finished centrifuge mother liquor settling tank, and control the flow rate to transfer the mother liquor from the settling tank to the intermediate transfer kettle. Finally, open the bottom discharge valve of the soaking kettle and start the transfer pump to feed the deacidification centrifuge, with the feed rate controlled by the control valve. The filter cake from the centrifuge goes directly to the intermediate transfer kettle. The filtrate is sent to the 5% hydrochloric acid settling tank and then to the intermediate tank after settling. After centrifugation, start the transfer pump to deliver the material to the corresponding neutralization reactor. The volatile matter content of the filter cake from the deacidification centrifuge is approximately 15%.
Caustic Neutralization:
Before the neutralization kettle is started, the alkali neutralization program is initiated. The system will automatically adjust the motor frequency according to the feed liquid level, and automatically add alkali to regulate the pH value, ensuring that the pH value is always maintained within the set range. When the liquid level reaches the set value, the system will automatically start steam heating to the set temperature. The pH value is controlled at 6.5–7, and the temperature is maintained constant for 3 hours. The neutralization process achieves full-process automatic control.
De-alkalization and Desalting:
Start the de-alkalization filter, open the discharge valve of the neutralization reactor, and start the feed transfer pump to evenly deliver the neutralized material to the belt filter for removal of excess alkali and salt. The filter cake, together with process cold water, is sent to the finished product mixing kettle, while the weakly alkaline filtrate is discharged after treatment. The finished product intermediate transfer kettle homogenizes the material through batch-to-batch blending to minimize product variability, ensuring more stable product quality.
（4）Separation and Drying Unit
The CPVC slurry from the mixing tank is fed by the slurry pump into the CPVC centrifugal separator. After dewatering by the centrifuge (the centrifugal water is collected in the mother liquor tank and then sent to the wastewater treatment station for combined treatment), the CPVC with a moisture content of ≤30% is mixed with a certain amount of anti-caking agent through a twin-screw conveyor and then fed into the fluidized bed dryer. The CPVC is deeply dried by hot air displacement and gradually moves toward the discharge outlet. Once the product reaches the qualified moisture content (≤0.4%), it enters the cooling section at the end of the fluidized bed. When the material temperature reaches 45℃，it is discharged from the fluidized bed dryer. (The tail gas is purified by a primary cyclone separator and a secondary bag filter. After meeting the emission standards, the tail gas is pressurized by the induced draft fan and vented to atmosphere. The collected material is returned to the fluidized bed for re-drying.) The CPVC product from the fluidized bed is screened by a vibrating screen. Oversized particles are manually collected and sold as off-spec product, while the qualified product is conveyed through a positive-pressure pneumatic conveying system to the packaging silo and then packaged by an automatic packaging system for warehousing. (The tail gas is purified by a cyclone separator and a bag filter to meet emission standards.)
（5）Tail Gas Absorption and Treatment
The waste chlorine gas from the top of the chlorination reactors passes through the tail gas settling kettle and then enters the tail gas pre-treatment kettle. In the pre-treatment kettle, the waste chlorine gas, together with emergency waste chlorine gas from the rupture discs of each chlorination reactor and from the safety valve discharge of the liquid chlorine main header, is absorbed by 32% caustic soda solution. The tail gas leaving the pre-treatment kettle enters the primary tail gas absorption tower. Other waste gases that may also enter the primary tail gas absorption tower include those from the batching kettle, receiving and cooling reactors, 25% hydrochloric acid intermediate tank, tank farm, emergency chlorine absorption pipe, dilute acid deacidification centrifuges, and de-alkalization filter. In the primary tail gas absorption tower, the waste chlorine gas is neutralized by 10–15% caustic soda solution. The tail gas from the primary tower then enters the secondary tail gas absorption tower for further neutralization with 15% caustic soda / sodium hypochlorite solution. The treated tail gas exiting the secondary tower (mainly inert gases such as nitrogen and water vapor) meets the emission standards and is discharged to atmosphere by the tail gas fan. The emergency chlorine absorption ports are located at five positions among the ten chlorination reactors. When the chlorine gas detection and alarm system reaches the interlock set point, the suction valves at the emergency chlorine absorption ports are opened by interlock to draw the emergency chlorine gas from the vicinity of the chlorination reactors to the primary tail gas absorption tower. The two-stage caustic absorption system treats waste gases containing Cl₂ and HCl, with the primary tail gas absorption tower and the waste chlorine treatment tower serving as the main treatment towers, and the secondary tail gas absorption tower serving as the protective tower. The circulating absorption system comprises the relevant tail gas absorption towers, two sets of sodium hypochlorite circulation tanks, sodium hypochlorite circulation pumps, and sodium hypochlorite coolers. Each circulation system can independently circulate and absorb gases containing Cl₂ and HCl. In the event of a failure in one circulation system, the other system can still function, ensuring that the waste gas treatment facilities do not cease operation.
（6）Instrumentation and Control
The plant is controlled by a DCS system. Batching is carried out by pneumatic conveying with automatic metering, and product packaging is fully automated. Only minor additives (chemical auxiliaries) require manual weighing.
The chlorination, drying, and packaging units are controlled by the DCS system, with all critical parameters displayed and recorded centrally in the DCS system located in the control room. Under normal operating conditions, operators can safely operate the plant continuously from the control room. Electrical equipment in the control room is powered by a dedicated UPS supplied by the instrumentation discipline.In consideration of the hazardous nature of the chlorination process, appropriate alarm, regulation, and interlock control loops are provided for parameters that may pose risks or are critical to production. A SIS safety interlock system is implemented with a two-out-of-three voting scheme for reactor pressure, temperature, and agitator motor to trip the liquid chlorine feed valve. An independent toxic gas detection and alarm GDS system is also provided. Toxic gas detectors for chlorine and hydrogen chloride are installed in the chlorination building, using fixed-point electrochemical and photoionization sensors.

8.Brief Equipment List

9. Waste Discharge Standards
A. Waste Acid Discharge: 25% hydrochloric acid, 12.26 t/h
Option 1: Deep desorption — hydrogen chloride is used for VCM production, while the remaining 1% hydrochloric acid is sent to the chlorination reaction batching system.
Note: 1.23 tons of steam is consumed per ton of 25% hydrochloric acid.
Option 2: Concentration to 31% hydrochloric acid for sale as a product.
B. Centrifuge Mother Liquor Discharge: 430 t/d
Option 1: Sent to secondary wastewater treatment after neutralization.
Option 2: Sent to the caustic soda primary brine dissolving system after organic removal.
C. Tail Gas Absorption Tower Discharge: Q = 50 Nm³/h, chlorine content in tail gas: 1 ppm
D. Drying Tail Gas Discharge: Q = 20,000 Nm³/h, dust content in tail gas: 20 ppm