Comparison of solution rubber finishing processes - part II.

The manufacturing process for most synthetic rubber is done by either solution or emulsion polymerization. Solution polymerization is done in a solvent at low rubber solids levels followed by steam stripping and a water slurry finishing step. The solution process offers several key advantages, such as a lower level of residuals in the final product, more flexibility and superior product properties.

The primary disadvantage of the solution process with water slurry finishing is the comparatively higher manufacturing cost. This is due to the need to purify and dry the monomer and recycle solvent before feeding them to the reactor.

Solution rubber can also be recovered from the solvent by another finishing process, direct devolatilization of the solvent from the rubber solution using a twin screw extruder. This process eliminates the steam stripping step and greatly reduces the high recycle solvent purification costs.

By comparing the manufacturing costs and economics of the two rubber finishing methods, the water slurry process and the direct devolatilizing extruder process, the economic advantage of eliminating the steam stripping step can be determined. This can influence the decision for the type of finishing process specified for new or modernized solution rubber facilities.

Part I of this article appeared in the March 2000 issue of Rubber World, and described solution rubber process steps, solvent and monomer purification, solution rubber polymerization, water slurry finishing and direct devolatilizing extruder finishing.

Devolatilizing extruder operation

Refer to figure 3 for the devolatilizing extruder operation. The hot rubber solution feed is flashed across a valve into the second barrel of the devolatilizing extruder. Solvent vapor is generated and removed out the rear vent at atmospheric pressure. About half of the solvent can be flashed out the rear vent in the first barrel. The solution in the extruder becomes more viscous and is pumped forward and heated by shearing elements in the extruder.

[Figure 3 ILLUSTRATION OMITTED]

A second vent removes another portion of the solvent. Some vacuum is pulled on this vent. The solvent from both the rear and second vent is collected and directly recycled back to the reactor without further purification. This is a major cost saving step.

The remaining solvent, about 15% of the total, is taken off in two more vents under higher vacuum. A stripping agent can be added to help reduce residuals. The recycled solvent may have some impurities in it, so a small purification step is needed for this before going back to the reactor.

The rubber is discharged out of the extruder. If a porous pellet is required, a 1% water steam is injected into the extruder and mixed into the rubber. This material goes through an air injected cutter to form the porous pellets and convey them to the next operation. If a clear pellet is desired, water is not injected and a pelletizer is used to form the pellets and dust them with a non-stick talc.

In the rear vent, an atmospheric pressure or slightly higher is used to preven entrainment of any powder if the rubber is cooled below the Tg. Some rubbers have a tendency to melt fracture and form fines in the last extruder vacuum vent. This is a concern in rubber devolatilization. Local high shear rates in the vent must be minimized, and special vent designs are made to handle the fines generated.

The direct devolatilizing extruder finishing process will give significantly less environmental emissions than the water slurry process with its expeller, expander and dryer. The devolatilizing extruder is a closed environment system and all vent effluents are captured and condensed.

Economics comparison basis

The solution rubber polymerization reactor usually runs in the range of 7% to 30% rubber solids. A 20% rubber solids in the solvent from the reactor is taken as the basis for this finishing process comparison study. Solvent makes up the remaining part, since small amounts of monomer are flashed off and recycled directly back to the reactor.

A 100 million pounds per year world scale plant is studied in this comparison of finishing process alternates. Two lines are required for each process alternate. This is equivalent to each line running at about 7,000 pph product

rate. Unit costs for utilities, raw materials and labor are based on a U.S. Gulf Coast plant installation.

Finishing process economics comparison

Estimates were made for the capital cost for each finishing process. Details cannot be given here, but the finishing area capital is about $20 million for the two line operation. Within the estimate accuracy, capital cost is about the same for the two finishing alternates.

Consumption rates of utilities for each finishing process are given in table 1. A credit is taken for 30% of the steam usage. Also given in the table are the labor requirements and raw material usage. The consumption rates and labor needs were taken from actual situations, as described in references 13-14. The Phillips cis-polybutadiene process information was used for the water slurry process, and the Stamicarbon "compact" polyethylene solution process was used for the devolatilizing extruder process.

Table 1 - consumption rates for finishing process alternative

Basis - 100 million pounds         Wet slurry process
per year

Raw materials                  Usage                Yield
                               Lb./100 lbs.         %

Rubber solution - 20% solids   100                  97.0%
Solvent added                  5                    99.0%

Utilities                      Use
                               Per 14,000 lbs.

Elect - extruders              2,088.8 KWH
Elect - other total            671.4 KWH
Process water                  180 KGAL
Steam - 200 psig               130 KLBSTM
Steam credit                   39 KLBSTM

Labor                          Use

Shiftwork
  Supervise                    1/shift
  Operating                    9/shift
  Laboratory                   1/shift
Daytime
  Supervise                    1

Basis - 100 million pounds           Devolatilizing
per year                            extruder process

Raw materials                  Usage              Yield
                               Lb./100 lbs.       %

Rubber solution - 20% solids   100                98.0%
Solvent added                  3                  99.0%

Utilities                      Use
                               Per 14,000 lbs.

Elect - extruders              2,088.8 KWH
Elect - other total            447.6 KWH
Process water                  90 KGAL
Steam - 200 psig               30 KLBSTM
Steam credit                   9 KLBSTM

Labor                          Use

Shiftwork
  Supervise                    1/shift
  Operating                    4/shift
  Laboratory                   1/shift
Daytime
  Supervise                    1

Raw material usage for the devolatilizing process is lower due to fewer purification steps needed and fewer losses in the finishing step. Labor is less in the devolatilizing process, since there is less equipment and fewer steps in the process for operators to watch.

The manufacturing cost of each alternate is calculated according to standard plant practices. The manufacturing costs for the water slurry finishing process and the direct devolatilizing finishing process are given in tables 2 and 3.

Table 2 - water slurry solution rubber process manufacturing cost estimate

Basis - 100 million pounds per year
        14,000 pph total
         7, 000 pph each line - two extruder lines

                                      Usage             Cost
Raw material                   lb./100 lbs.            $/lb.

Rubber solution                         100             0.24
Solvent added                             5             0.16
Raw materials total

Utilities                          Use Unit/100lbs.   $/Unit

Elect expel/dryer #1, HP              1,400   14.92     0.05
Elect expel/dryer #2, HP              1,400   14.92     0.05
Elect - other, HP                       900    4.80     0.05
Process water, gpm                    3,000    1.29     0.10
Steam - 200 psig, pph               130,000    0.93        5
Steam credit, pph                    38,000    0.28        5
Utilities total

Manufacturing costs

Capacity lb./yr.
Variable costs
  Raw materials
  Utilities
Fixed costs
  Labor/supervise
  Maint./material 5% capital
  Control lab
Allocated costs
  Admin./R&D/taxes
Total Mfg. cost

Basis - 100 million pounds per year
        14,000 pph total
         7,000 pph each line - two extruder lines

Raw materials                        Yield         Cost
                                         %   $/100 lbs.

Rubber solution                      97.0%        24.74
Solvent added                        99.0%         0.81
Raw materials total                               25.55

Utilities                             Unit         Cost
                                             $/100 lbs.

Elect expel/dryer #1, HP              KWHR         0.75
Elect expel/dryer #2, HP              KWHR         0.75
Elect - other, HP                     KWHR         0.24
Process water, gpm                    KGAL         0.13
Steam - 200 psig, pph               KLBSTM         4.64
Steam credit, pph                   KLBSTM        -1.39
Utilities total                                    5.11

Manufacturing costs             $/100 lbs.   $/100 lbs.
                               100 million   50 million

Capacity lb./yr.
Variable costs
  Raw materials                      25.55        26.66
  Utilities                           5.11         5.11
Fixed costs
  Labor/supervise                     1.77         3.53
  Maint./material 5% capital          1.09         2.18
  Control lab                         0.50         1.19
Allocated costs
  Admin./R&D/taxes                    1.98         3.70
Total Mfg. cost                      36.10        41.27

Table 3 - direct devolatilizing solution rubber process manufacturing cost estimate

Basis - 100 million pounds per year
14,000 pph total
7, 000 pph each line - two extruder lines

                                      Usage             Cost
Raw materials                  lb./100 lbs.            $/lb.

Rubber solution                         100             0.24
Solvent added                             3             0.16
Raw materials total

Utilities                         Use Unit/100 lbs.   $/Unit

Elect - extruder #1, HP               1,400   14.92     0.05
Elect - extruder #2, HP               1,400   14.92     0.06
Elect - other, HP                       500    3.20     0.05
Process water, gpm                    1,500    0.64     0.10
Steam - 200 psig, pph                30,000    0.21        5
Steam credit, pph                     9,000    0.06        5
Utilities total

Manufacturing cost

Capacity lb./yr.
Variable costs
  Raw materials
  Utilities
Fixed costs
  Labor/supervise
  Maint./material 5% capital
  Control lab
Allocated costs
  Admin./R&D/taxes
Total Mfg. costs

Basis - 100 million pounds per year
14,000 pph total
7, 000 pph each line - two extruder lines

Raw materials                        Yield         Cost
                                         %   $/100 lbs.

Rubber solution                      98.0%        24.49
Solvent added                        99.0%         0.48
Raw materials total                               24.97

Utilities                             Unit         Cost
                                             $/100 lbs.

Elect - extruder #1, HP               KWHR         0.75
Elect - extruder #2, HP               KWHR         0.75
Elect - other, HP                     KWHR         0.16
Process water, gpm                    KGAL         0.06
Steam - 200 psig, pph               KLBSTM         1.07
Steam credit, pph                   KLBSTM        -0.32
Utilities total                                    2.47

Manufacturing cost              $/100 lbs.   $/100 lbs.
                               100 million   50 million

Capacity lb./yr.
Variable costs
  Raw materials                      24.97        24.97
  Utilities                           2.47         2.47
Fixed costs
  Labor/supervise                     1.07         2.14
  Maint./material 5% capital          1.09         2.18
  Control lab                         0.60         1.19
Allocated costs
  Admin./R&D/taxes                    1.98         3.70
Total Mfg. costs                     32.18        36.66

Overhead items are the same for each alternate, since capital costs are approximately the same. The overhead costs are estimates for a typical Gulf Coast plant in the U.S. Depreciation is the same for both alternates, and is not added into the manufacturing cost total.

The key cost difference is in the utilities area. Because of the need for purification of all the recycled solvent and an extra steam stripping step, the water slurry process has higher utilities costs by about 2.6 c/lb. Other factors are the labor and raw materials costs. These costs are about 1.3 c/lb. higher for the water slurry process.

Overall, the direct devolatilizing finishing process has a manufacturing cost advantage of about 4 c/lb. If a steam credit is not taken, this advantage can be about 5 c/lb.

Conclusions

The direct devolatilizing extruder process can give a significant reduction in manufacturing costs for solution rubber finishing. Detailed estimates of the manufacturing costs for water slurry finishing and devolatilizing extruder finishing gives a cost savings of about 4 c/lb. for the devolatilizing extruder process. It would increase to 5 c/lb. if a steam usage credit were not used.

This is a 10% reduction of the water slurry manufacturing cost. Solution rubber costs using devolatilizing extruder finishing can be almost the same as emulsion rubber cost.

Other advantages of the devolatilizing extruder finishing are less plant space required, less monomer and solvent waste, fewer emissions and it is ecologically beneficial. Both clear pellets and porous pellets can be made. The main concern is generation of some fines in the vacuum vent for some rubber types.

The devolatilizing extruder finishing process has not been widely adopted yet by the rubber industry. However, it is being used for many thermoplastic rubber products like SB, SIS, SBS and a few synthetic rubbers. It can be applied on EPM, EPDM, SSBR, BR and butyl. It is just a matter of time and effort as to when the rubber industry will generally adopt the technology. Some research study verification is warranted by the results presented in this article.

References

(1.) Halasa, A.F., Austin, L., Hsu, W., Gross, B.B., U.S. Patent 5,679,751 to Goodyear Tire and Rubber Co. (October 1997).

(2.) Smith, D.R., Zelinski, R.P., U.S. Patent 3,178,402 to Phillips Petroleum Co., (April 1965).

(3.) Van Den Berg, C., U.S. Patent 4,319,010 to Stamicarbon B.V. (March 1982).

(4.) Simon, R.H.M., Falling strand devolatilization," Polymer Devolatilization, Marcel Dekker, New York, 1996, pp. 261-290.

(5.) Hagberg, C.G., U.S. Patent 3,966,538 to Monsanto Co. (June 1976).

(6.) Fink, P., et al, U.S. Patent 4,153,501 to BASF (May 1979).

(7.) Albalak, R.J., ed., Polymer Devolatilization, Marcel Dekker, New York, 1996.

(8.) Nichols, R.J., Lindt, J.T., "Devolatilization in counter-rotating, nonintermeshing twin screw extruders," Polymer Devolatilization, Marcel Dekker, New York, 1996.

(9.) Hagberg, C.G., Bash, T.F., "Extruder devolatilization of low solids solutions,' Seminar Series, NPE 1997.

(10.) Skidmore, R.H., U.S. Patent 3,963,558 to W Bar E Inc., (June 1976).

(11.) Brasz, A.J.H., et al, U.S. Patent 4,452,977 to Stamicarbon B. V. (June 1984).

(12.) Salmon, E.J., U.S. Patent 4,186,047 to Phillips Petroleum Co. (January 1980).

(13.) Cooper, E.D., "Phillips cis-polybutadiene process," Handbook of Chemicals Production Processes, McGraw-Hill Book Co., New York, 1986, p. 2.1-3.

(14.) Staff, "Stamicarbon compact polyethylene solution process," Handbook of Chemicals Production Processes, McGraw-Hill Book Co., New York, 1986, p. 2.5-1.