Click Here to View the PVC Process Flow Diagram
This facility produces polyvinyl chloride polymer resin (PVC) by polymerizing vinyl chloride monomer (VCM) in water/PVC slurry. After the polymerization step is completed, residual vinyl chloride is stripped from the polymer slurry (two strippers). Filters convert the slurry to a wet cake. Pneumatic conveying systems transfer the dried PVC polymer resin to product silos before it is shipped to the customers.
Product goes through six Bird centrifuges and then to six natural gas fired rotary kilns to dry the wet cake. Pneumatic conveying systems transfer the dried PVC polymer resin to product silos before being shipped to the customers.
The raw material VCM arrives by rail and is unloaded into the 5 MM lb capacity storage sphere. There are also two large bullet tanks for additional VCM storage capacity. There is no VCM gas holder in this process.
Although it may appear to be six individual lines, the product comes together at two main places in the process so that the resultant end product is completely homogenous. These common locations are the reactor dump tank and the stripper feed tank. There are also only two stripper columns; therefore it would be more accurate to describe the plant as having two trains rather than six.
The raw materials (VCM, water, suspension agents, and catalyst) are added to the reactor for each batch. The reactors typically run at about 135°F and 105 psig with a batch cycle time of about 5 hours, depending on which steps are included. The 18,000 gallon reactors are 1” thick stainless steel (304L) and are rated for 200 psig at 200°F. They are jacketed for heating or cooling (800 gpm cooling water flow). They are agitated with one set of paddles coming up through the bottom of the reactor with 200 hp drives. Organic peroxide catalysts (front-end and back-end catalysts) are used to initial the polymerization of VCM. About 80% of the VCM is converted to PVC on each batch in the reactor. The 20% unreacted VCM is recovered and recycled back to the reaction unit. The process uses direct steam into the reactor for the initial heat up. This is more efficient and has also led to much more reliable reactor jacket operation. The reactors are also cooled by a unique process. The 2,612 ft 2 condensers are mounted directly on top of the reactors in the vertical position and actually cool the reactor while condensing overhead VCM vapors. They have 1” diameter by 10’ long tubes made of 304L stainless steel with cooling water on the jacket running at 2,500 gpm per condenser. AMS (alpha-methyl-styrene) is used as a “kill agent” for stopping any runaway reaction. The entire process is continuous after the polymerization reactors.
The stripping process was modified when EVC technology was purchased. The total project cost $30 MM. Prior to this, the unreacted VCM was stripped in the reactors, which greatly reduced overall production. Two spiral exchangers made of Duplex 2205 stainless steel (1,313 ft2 each) are used for the stripping condensers. The agitated dump drum holds 40,000 gallons and is made of 304L stainless steel. The agitated stripper feed vessel holds 10,750 gallons and is also made of 304L stainless steel.
The stripper columns (2) are 64” diameter by 35’ tall with 34 dual-flow trays. The columns are made of Duplex 2205 stainless steel. These columns can accommodate 230 gpm of process flow in each stripper.
The VCM recovery system includes two 60 hp Sulzer-Bingham model 4G vacuum pumps (780 acfm each) and four 150 hp Nash model 1256 compressors (400 acfm each). All of this equipment is “liquid ring” technology and comes with the liquid ring pumps and condensers.
PVC Slurry Conversion
The PVC slurry proceeds to six Bird 24” X 36” centrifuges. These units spin at 1,775 rpm and can handle 50,000 lb/hr of slurry each. They are constructed mainly of 304L stainless steel and better alloys and have 75 hp drive motors.
The 13,000 lb/hr Bartlett-Snow rotary kilns are direct fired so that the combustion gases are contacting the wet product. This efficient process removes almost all of the 30% water content from the centrifuge outlet product. The kilns operate at 400°F inlet and 200°F outlet. They are approximately 8’ diameter by 38’ long and rotate at about 7 rpm. Five of them are made of solid 304L stainless steel and one kiln is stainless steel lined. It is also larger at 9’ diameter by 40’ long. Filtered air is fed to the burner and dust collectors are used to remove PVC particles from the discharge gases.
The product is classified in vibrating sieves (6) and then a dilute phase pneumatic transfer system is used to transfer the finished PVC powder. The system uses stainless steel transfer lines.
Storage and Loading
There are four large carbon steel storage silos for the finished PVC. They each hold 470,000 lbs (density of PVC powder is 50 lb/ft3). Two of the large storage silos are mounted on weigh cells. There are six 304L stainless steel railcar loading silos positioned above the railcar loading area so that railcar loading is gravity fed.