R&D on New Polyphenylene Sulfide Manufacturing Methods Using Hydrogen Sulfide as Feedstock

Transcription

1 1999D R&D on New Polyphenylene Sulfide Manufacturing Methods Using Hydrogen Sulfide as Feedstock 1. Contents of R&D In petroleum refining, the byproduct hydrogen sulfide (H2S) is recovered as sulfur by means of a sulfur recovery unit. The production volumes of this substance are increasing because facilities such as light oil desulfurization units or heavy oil desulfurization units are being newly installed or augmented as part of environmental policy. In addressing environmental problems, demand has arisen for a reduction of aromatic components in gasoline, especially benzene content, and in oil refining in the future, the quantities of benzene yielded as the byproduct will trend upward. Accordingly, development of new applications for sulfur and benzene has become a vital issue. Polyphenylene sulfide (PPS), a form of engineering plastic, is chemically comprised of about 30 percent sulfur and about 70 percent benzene by weight. It has captured wide attention as a material for automobile and electronic parts but its manufacturing costs are high and it has not yet been spread adequately. In the conventional PPS manufacturing process, sodium sulfide (Na2S) and sodium hydrogen sulfide (NaSH) are used in S source; these are made to react with paradichlorobenzene, produced by chlorination of benzene in solvent (N-Methylpyrrolidone), and polymerized PPS. In this process, because NaCl insoluble in the solvent is produced in the polymer, continuous manufacturing is difficult and a water washing process is required; there is also the problem that solvent recovery and wastewater treatment are heavy burdens. It was conjectured, therefore, that the cost of continuously manufacturing PPS could be drastically reduced by taking the hydrogen sulfide byproduct of the oil refining process as the feedstock, and applying a new reaction system that circulates and uses byproduct salt, which does not become deposited during polymerization; it was also proposed that expanding demand for sulfur and benzene could be met by expanding its applications through quality improvements. Our study was undertaken in order to develop a new process of continuous PPS manufacture, taking hydrogen sulfide and paradichlorobenzene, a benzene derivative, as feedstock, and in order to verify its effectiveness from the following standpoints. (1) Hydrogen sulfide coming as a byproduct of oil refinement can be used, and continuous operation is possible. (2) Washing by polymerization solvent is possible, and the PPS yield after washing is the same as, or greater than, the yield from the conventional process. (3) The various characteristics of PPS obtained by the new process are the same as, or better than, those of the conventional PPS. The details of our research are presented below. 1.1 Acquisition of chemical engineering data (1) Investigation of method of PPS feedstock continuous synthesis Hydrogen sulfide gas was fed into the solvent and made to react with alkali metal compound; the synthetic reaction conditions of metal sulfide, the PPS feedstock, were examined. 1

2 (2) Investigation of method of continuous PPS polymerization Metal sulfide and paradichlorobenzene, as feedstock monomer, were subjected to polymerization under prescribed conditions of reaction, and the rate of polymerization reaction was then calculated. Next, reaction conditions were changed and PPS polymerization was implemented, the granular configuration of the PPS thus produced was examined, and the conditions for controlling the scattering of polymer droplets within the polymerization reaction vessel were investigated. Using hydrogen sulfide yielded as a byproduct of oil refining, polymerization feedstock was synthesized; the molecular weight of PPS produced under prescribed conditions of polymerization was measured, and polymerization performance was evaluated. (3) Investigation of melt washing method After polymerization, the PPS was washed using a prescribed quantity of solvent; the metal residue was analyzed and changes in molecular weight and in molecular weight distribution were evaluated. 1.2 Verification by bench plant and acquisition of scale-up data (1) Examination of continuous polymerization Hydrogen sulfide was fed into solvent containing alkali metal compounds, paradichlorobenzene was then added to make PPS prepolymer. Next, PPS prepolymer was fed continuously into a polymerization reaction vessel; polymerization took place at prescribed conditions and the molecular weight of the PPS thus produced was measured. (2) Examination of continuous melt washing PPS polymer fluid produced in the polymerization process was mixed with solvent of prescribed volume and washed. It was then fed continuously into a liquid- liquid standing vessel; supernatant and polymer fluid were separated, and washing performance was evaluated. (3) Examination of PPS thermal degradation behavior after polymerization Thermal degradation of PPS polymer fluid was examined under prescribed conditions, and then molecular weight, molecular weight distribution, and the impacts on product properties were investigated. (4) Examination of continuous solvent removal PPS polymer fluid was fed continuously into a decompressed degassing equipment; solvent was removed from the top, and solvent removal performance was evaluated. (5) Evaluation of the properties of continuously manufactured PPS product Additives were added to PPS obtained through continuous manufacturing, and product properties were evaluated. 1.3 Investigation of PPS quality improvements Such things as the molecular weight, the molecular weight distribution and molecular structure of PPS produced under various conditions of polymerization were measured; the impacts of polymerization conditions and control methods were investigated; and thermal stability, toughness and melt characteristics were evaluated. 2

3 1.4 Evaluation of the effectiveness of the continuous process Based on bench plant operation for verification and scaled up data, the possibilities for reducing production costs were investigated, and the new continuous manufacturing process was evaluated for effectiveness. 2. Empirical research results and analysis thereof The continuous manufacturing process and reaction formula for PPS production are shown below. Continuous manufacturing processes Hydrogen sulfide Paradichlorobenzene Hydrogen sulfide absorption process Polymerization process Washing / Drying process Recovery process PPS manufacturing reaction type Alkali metal Hydrogen sulfide Paradichlorobenzene Byproduct 2.1 Acquisition of chemical engineering data (1) Investigation of method for continuous synthesis of PPS feedstock Our study was done on the method of continuous synthesis of metal sulfide, the PPS feedstock. The reaction formula is shown below. (1) (2) 3

4 For reaction (1), the feedstock is metal hydroxide, and by feeding hydrogen sulfide of the same mole or greater, a fixed quantitative reaction was proceeded. Temporal changes in the concentration of reactant under typical reaction conditions are depicted in Figure It was confirmed, however, that at the reaction time taken as the standard, MSH can be obtained in virtually fixed quantities. The impact of H2O concentration on hydrogen sulfide absorption speed is represented in Figure As the concentration of liquid phase H2O is increased, H2S partial pressure drops because of water vapor production, and it was found that H2S absorption rate drops. Metal sulfide concentration (mmol/g) Time (min.) H2S absorption rate (mol/sec) Liquid phase H2O concentration (wt%) Figure Temporal change in metal sulfide production volume Figure Impact of water on H2S absorption rate Next, an investigation was made of continuous synthesis of MOH, the synthesis feedstock of metal sulfides. In the recovery phase of the continuous manufacturing process, MOH crystal can be recovered continuously by adding alkali liquid to salt byproduct during the polymerization reaction. The reaction formula is as follows. Salt byproduct + Alkali liquid MOH + Salt byproduct Shown in Figure is the impact of alkali liquid volume used on alkali metal recovery rate. When the mole ratio of alkali liquid vs. salt byproduct was at the standard or above, the theoretical volume of MOH could be recovered, and the deficient component was only saturated soluble complement. Next, the optimum conditions for reaction (2) were investigated. The impact of average retention time on M2S production is shown in Figure For M/S ratio, the theoretical volume is approached as the feed speed is lowered, that is, as the average retention time is prolonged, and at the standard average retention time or greater, it was confirmed that M2S can be synthesized at nearly fixed quantities. 4

5 Metal recovery rate (%) Recovery Concentration Alkali liquid/salt byproduct M 2 S production Concentration of mother liquid metal M/S ratio (mol/gl) (mol/mol) M/S ratio H 2 O concentration Average retention time H 2 O concentration (wt%) Figure Alkali metal recovery rate Figure Impact of average retention time on M2S production (2) Investigation of PPS continuous polymerization method Polymerization rate in continuous polymerization was investigated with respect to metal sulfide (M2S) and paradichlorobenzene (PDCB), the monomer feedstock. The temporal changes in monomer conversion rate and in reaction yield are represented in Figure Based on these results, it was found that the polymerization rate (monomer consumption rate: -ra) could be processed with a second-order reaction formula for both monomer concentrations as depicted in Figure Here, k is the reaction rate constant, (PDCB) is that paradichlorobenzene concentration, and (M2S) is the metal sulfide concentration. Monomer conversion rate (%) Figure Conversion rate Yield inh Polymerization time (hr) Temporal change in monomer conversion rate and in yield Molecular weight (dl/g) Figure PDCB 2 S Polymerization rate 5

6 Next, stirring conditions for favorable control of polymer droplet scattering inside a polymerization reaction vessel were investigated. As indicated in Figure 2.1-7, the impact of stirring force and of stirring blade shape were found to be great, and the optimum stirring force per unit volume and the optimum stirring blade were determined. Under conditions thus determined, the status of polymer droplet scattering in polymerization and the molecular weight of polymer thus produced were investigated. As shown in Figure 2.1-8, the status of droplet scattering could be maintained favorably and molecular weight could be regulated within the target range. Following an investigation of the impact of reactant conditions on yield during polymerization reaction, it was discovered that a PPS polymer yield equivalent to, or greater than, that from the conventional process could be maintained even if the feedstock ratio was varied over a broad range. Average grain diameter (mm) Figure Stirring blade A Stirring force (Pv) Control of scattering in polymerization reaction vessel Molecular Stirring blade B weight (dl/g) Grade of polymer grain shape (dispersibility) Dimensionless cavity time () Figure Scattering status by continuous polymerization and molecular weight behavior Next, hydrogen sulfide generated as a byproduct in oil refining was directly used as H2S feedstock for synthesis of metal sulfides; polymerization of PPS was continued, and comparison was made with cases in which a purchased cylinder of high purity H2S is used. The results are shown in Table The purity of H2S byproduct of oil refining is low, but the PPS manufactured by using this H2S assumed an adequately high molecular weight, and in the performance evaluation of a single slew compared here, roughly the equivalent results were obtained as in the case of using purchased cylinder product. It was also found that there are 20 or more different types of microscopic contaminants in H2S byproduct of oil refining. Studies must be conducted on the effect of contaminants accumulating during continuous manufacturing, and on methods of preventing and removing such contaminants. 6

7 Table Comparisons of polymerization results vs. hydrogen sulfide purity Purchased cylinder product Purity Polymerization results (PPS molecular weight (VOL%) VOL% ( dl/g) dl/g) Oil refining byproduct (3) Investigation of melt washing method A study was done on the changes in material balance and in polymer properties during melt washing. Presented in Table are the changes in residual metal content, in PPS polymer yield, in PPS molecular weight and molecular weight distribution. The metal ingredients arising from salt byproduct during polymerization could be removed efficiently by melt washing, and the residual metal content in PPS after polymerization could be gradually reduced through repeated melt washing. On the other hand, polymer yield after melt washing could be maintained at the target level. It was also discovered that low molecular weight polymer (PPS oligomer) resulting from melt washing shifts to the solvent side and molecular weight distribution becomes narrow. Table Changes in PPS phase by melt washing Cleaning frequency Residual metal weight (ppm) PPS yield rate (%) Molecular weight () Molecular weight distribution (Mw/Mn) Next, in order to investigate the conditions for recovery of PPS oligomer, a byproduct of polymerization, by means of the melt washing process, liquid-liquid equilibrium data on the PPS oligomer/solvent NMP/byproduct salt system was corrected, and the results are shown in Figure In comparison to PPS polymer, phase separation of PPS oligomer is difficult unless large volumes of lean solvent are also present. It was found, however, that the byproduct salt in PPS oligomer shifts largely to the solvent phase side as a result of adding more lean solvent additive. On the other hand, in the polymerization reaction fluid, numerous types of byproduct low molecular weight ingredients or degradation products are mixed in as impurities, and studies must be done on the effects of such accumulations in the continuous manufacturing process and on methods for prevention and removal of impurities. 7

8 M/solvent (wtppm) PPS oligomer PPS polymer Figure Metal ion concentration of each phase during melt washing 2.2 Verification by bench plant and acquisition of scaled up data (1) Investigation of the use of hydrogen sulfide by bench plant When using hydrogen sulfide (purchased cylinder product) containing almost no contaminants as the feedstock, it was discovered that, as batch reactions using a large bench plant reactor, the basic reactions (feedstock synthesis, PPS prepolymer synthesis, PPS synthesis) advance with no problems and can be used. Measurements of the material balance during PPS prepolymer synthesis indicated that the difference in the computed of prepolymer weight and the actual measure was about 1%. (2) Feed test of raw material liquid slurry A feed test was conducted on metal sulfide liquid slurry, the PPS feedstock, and it was found that stable feeding can be achieved by selecting the optimum pump. The results are shown in Figure Flow volume (kg/hr) Elapsed time (hr) Figure Stability of raw material liquid slurry flow volume 8

9 (3) Continuous polymerization Conditions of polymerization were selected based on the results of laboratory-scale studies, and the results of an evaluation of continuous operational performance are presented in Figure The feasibility of the polymerization rate presented earlier was confirmed; continuous operation was implemented until the feedstock was depleted, polymerization progressed stably, and PPS of high molecular weight could be obtained. Molecular weight (dl/g) Monomer conversion rate (%) Figure Time period of continuous operation (hr) Polymerization by continuous bench plant operation (4) Continuous melt washing Following continuous polymerization, the polymer washing performance of continuous melt washing was evaluated. As indicated in Figure 2.2-3, the residual metal content in polymer was reduced by about 1/25 for single washing stage, and it was found that it could be reduced to the target concentration. Metal weight PPS in feed (ppm) Mixer Solvent phase Liquid- liquid separation standing vessel PPS phase Cleaning solution Figure Residual metal weight in PPS PPS 0.04 (ppm). Performance evaluation of continuous melt washing 9

10 (5) Thermal degradation of PPS after polymerization It was discovered that when the PPS polymer obtained by continuous polymerization is stored for a long time at high-temperature, the monomers remaining after polymerization are gradually consumed; once fully consumed, degradation of the PPS progresses rapidly. This degradation activity, however, can be reduced by lowering the storage temperature, and it can be curbed by having the feedstock ratio at the time of polymerization kept at or above a constant. Also investigated were the temporal changes in the molecular weight distribution and in the properties of PPS stored for a fixed time period at the melt washing temperature. The results, as shown in Table 2.2-1, indicated that with the passage of time the distribution of molecular weight tends to expand, but within the assumed time period for melt washing, no adverse effects on product properties could be noted. Table Change in product properties due to high-temperature storage High-temperature storage time (hr) Molecular weight (, dl/g) Molecular weight distribution (Mw/Mn) Fluidity (mm) Toughness (kj/m 2 ) time (6) Continuous solvent removal In relation to the performance of continuous solvent removal by means of a solvent removal equipment, comparisons were made between test samples in which the molecular weight of PPS polymer differs and the results are shown in Figure With respect to PPS of high molecular weight, it was found that the volume of residual solvent could be adequately reduced, as in the case of low-molecular-weight PPS, and there were no other problems. Figure Laboratory Bench (: standard ) Amount of Bench (: -0.09) solvent remaining in PPS (ppm) Pressure (Torr) Performance evaluation of continuous solvent removal 10

11 (7) Product properties of continuously produced PPS Additives were added to PPS produced by the continuous process and product properties were evaluated. Comparisons were made with the performance of a typical grade of PPS produced by the conventional process and the results are presented in Figure PPS roughly equivalent to the conventional grade could be obtained through optimization of such things as the conditions of modification reaction during polymerization, the volume of product additive used, and molding conditions. Conventional process Continuous manufacturing process Figure Fluidity Measurability Toughness Product properties of PPS by continuous manufacturing method 2.3 PPS quality improvement Control factors were investigated in order to improve the thermal stability of PPS. It was discovered that the temporal change in the molecular weight of PPS, when it has been stored at high-temperature in the solution state, diminishes as the feedstock percentage during polymerization reaction enlarges, and that it is outstanding in thermal stability. An investigation of factors for improving PPS toughness revealed that the impact of the compositional percentage of feedstock ingredient during PPS polymerization is great and that with a gain in M/S ratio, strength and toughness are increased. In studying melt characteristics, it was found that the impact of modification reaction conditions due to additive is great and that improvements can be made by adjusting the reaction time period during modification reaction. 2.4 Evaluation of the effectiveness of the continuous manufacturing process Based on the research results presented above, the costs of manufacturing by the continuous manufacturing process were calculated, and estimated s for commercial scale production were compared with those of the conventional process. It was discovered that manufacturing costs could be reduced 20 to 30 percent depending on production scale. 3. Results of empirical research (1) It was confirmed that hydrogen sulfide obtained as a byproduct of oil refining can be used as feedstock, and that basic reactions such as monomer synthesis or polymerization can be advanced with no problems. 11

12 (2) It was found that, by continuously feeding feedstock into a polymerization vessel, continuous stable operation can be maintained until the feedstock is depleted and PPS of high molecular weight can be produced. (3) Polymer yield during PPS polymerization reaction is the same as with the conventional process even if the conditions of polymerization are changed somewhat; the yield after washing by polymerization solvent can also be kept at target s. (4) It was confirmed that a product of the same quality or better than PPS by the conventional method can be produced through optimization of polymerization conditions and/or washing using the polymerization solvent. (5) Through conversion of feedstock to hydrogen sulfide, conversion to the continuous manufacturing process, and/or conversion to the melt polymer washing process, the cost of manufacturing conventional PPS can be drastically reduced, and expanded applications can be expected through quality improvements. The effectiveness of the continuous manufacturing process was verified. 4. Summary In light of the research results presented above, the following subjects are scheduled to be investigated. (1) The effects of unknown contaminants accumulated in the process The effects of contaminants in hydrogen sulfide byproduct from oil refining and of other unknown substances accumulated in the process, including degradation products and byproducts, will be studied. (2) Prevention methods and processes for removal of accumulated contaminants Methods of preventing contaminants from accumulating and processes for removing such contaminants will be established. Copyright 1999 Petroleum Energy Center all rights reserved. 12