Document 0qx315DYyaOv4GOpbk0RBZRjM

KaS'jjaiiiijjJfeil il-w. J U I!: - Suspension-PVC in large reactors Using large (200 m3) reactors, special equipment and techniques, the Huls process is economical and reliable and complies with low VCM standards B. Terwiesch, Chemische Werke Hills AG, Marl, West Germany As one of the most widely used thermoplastics, PVC has a world-wide capacity of about 13 million metric tons/ sear. Its properties can be substantially determined by different production methods, and today, PVC is manu factured by suspension, emulsion, bulk and solution poly merization processes. The largest output is by the suspen sion process and about 80 percent of the U.S. production is by the suspension process. Suspension polymerization is a batch process with up to 90 percent conversion. Attempts toward an economical continuous suspension polymerization process have so far failed. The installation of a very large number of small units, with all the necessary accessories for each reactor, is very expensive and also involves environmental prob lems. During the last few yean, the trend has been towards large reactors. Huls has successfully scaled up the reactor to 200 m3. SPECIAL FEATURES During design and development of large polymerization reactors, quite a number of technical problems required solution.12 Moreover, safety and pollution control have become increasingly important when handling large amounts of highly combustible and environmentally harm ful vinyl chloride monomer (VCM). The reactor shell. Vinyl chloride (VC) suspension poly merization occurs at approximately 40-70 C, correspond ing to a VC saturated vapor pressure of 6-12 bar. A design pressure of 18 bar provides an operating pressure safety Fig. 1--Transporting a 200 in1 reactor from the Hiils workshop to the PVC plant. margin. The reactor dimensions are 5.5 m diameter and 10 m high. To avoid stress relief after welding and to keep within transport we:ght limits, attempts were made to use as small a wall thickness as possible. By using a special high strength steel, a transport weight of less than 100 tons was achieved. Of course, special welding methods should be used. Fig. I shows the transportation of a 200 m3 reactor Irom the Hiils workshop to the PVC plant. Stirring. In the late 1960s when Hiils began development of large reactors, polymerization reactors were normalK about 6 to 40 m3 volume. Such a reactor is shown in Fig. 2. Its characteristic features are the agitator driven from above through a long shaft, the finger-shaped baffle and heat removal via a cooling jacket. In planning larger reactors, the first consideration should be vvhether adequate intermixing of the contents is pos sible, e.g., under what conditions can the same stirring and mixing effect as in the smaller reactors be achieved. Theoretical considerations and our experience in scaling up the S-PVC process from 6 m3 to 40 m3 reactors pre sented no basic problems. Thcte are, however, construction Hydrocarbon Processing November 1976 117 COLORITE 006044 SUSPENSION-PVC Fig. 2--Conventional polymerization reactor. TABLE 1--Heat removable from a 200 m} reactor by wall cooling at different temperature differences and Heat transfer coefficients Cooling Atm of Reactor Jacket (ma) 145 At CO 10 50 Over-all Heat Removable Heat-Transfer Through Coefficient Reactor Wall (W/m* C) (W) Conversion Rate (t PVC/h) 130 0.19 X 10* 260 0.3& X 10* 0.4 0.8 130 0.94 X 10* 2 260 1.9 X 10* 4 Attempts toward an economical continuous PVC polymerization process have failed and batch suspension with 90 percent conversion is still used problems in stirring systems for very large reactors. These stirrers cannot be top driven by the usual agitator shaft entering through the reactor hood. The agitator shaft for a 200 m3 reactor would be too long, thick and expensive. The solution to this problem is shown in Fig. 3. Here, a three-vane impeller inside the reactor and near the reactor bottom is driven through a short and relatively thin shaft from a drive unit located below the reactor bottom. Two baffles are installed in the upper part of the reactor. Agitator shaft sealing is by a double mechanical seal. To prevent ingress of monomer and suspension between the slide rings, the annular clearance on the reactor shaft is constantly flushed with water.3 Cooling and temperature control. In large polymeriza tion reactors, polymerization heat removal presents special problems, because of the decreasing surface to volume ratio with increasing reactor volume. Vinyl chloride polymerization heat is approximately 0.47 kWh/kg. During VC polymerization, up to 16 metric tons per hour can be reacted in a 200 m3 reactor with the evolution of approximately 7.4 MW of heat.4 Table 1 shows the heat rates removable through the wall of a completely jacketed 200 m3 reactor for two different AT's and heat transfer coefficients. It can be seen that in the case of mere wall cooling the maximum conversion rate is only 4 metric tons per hour which is not economically sufficient. Therefore, additional cooling equipment is required. A very effective method of removing polymeriza tion heat is the installation of a reflux condenser on top of the reactor. Thus, space-time yields comparable to those obtained in conventional reactors can be achieved in 200 m3 reactors. Another important point is temperature control. In principle, large reactor temperature control operates the same way as in small reactors. However, if reactor mixing is poor, e.g., if the overturn is too slow, serious interference is caused by delays in boiling. However, mea surements have shown that the temperature differences inside the reactor are negligibly low. Cleaning. In many polymerization processes, reactor cleaning is an important feature, as it is in VC suspension polymerization. In the past PVC reactors were usually cleaned out by hand using scraping tools. Also, the reactor required clean ing after a few batches, in some cases after each batch. During the last few years various companies switched to cleaning with pressurized water or organic solvents. These methods are more economical and ensure more reliable and safer operation. For the 200 ms reactors a cleaning system was de veloped using pressurized water in a highly automated way. The great advantage of this cleaning system is that it is possible to keep the reactor closed, thereby saving time and reducing environmental pollution. In addition, Hiils has made great efforts to develop methods to avoid polymer deposits inside the reactor. Different methods have been developed. One is a special antifouling agent which can be used without any change of existing equipment. It has performed successfully in the two Hiils suspension PVC plants for more than two years for all suspension PVC grades produced by HUls. Periodic reactor inspections are necessary to check safety valves function and remove, if necessary, smaller I 18 November 1976 Hydrocarbon Processing COLORITE 006045 deposits. In these cases, the VCM is almost completely removed by purging the reactor with steam. Safety precauti ns. The large amounts of monomer which are handled in a 200 m3 reactor require special safety precautions for the protection of plant and per sonnel. The first condition for safe operation of a large reactor is the proper sizing of the cooling and stirring systems. However, account must be taken that extraordinary cir cumstances might arise that result in failures of or damage to the cooling or stirring systems or both. The computer controlled Htils S-PVC process provides for complete emergency protection. In the event of a total power failure, the system automatically manages a controlled shutdown. Two safety measures can be taken--emergency degassing and emergency shortstop. Emergency degassing is mainly used if the cooling system fails or the heat produced cannot be completely removed. Hiils has developed a reliable collection system5 for emer gency degassing as shown in Fig. 4. In case of emergency, a safety valve in the reactor cover opens and allows gas to flow out. Heat of vaporization is removed in this way. In order to protect the environment and because of the danger of explosions, the released gas is condensed and collected for re-use. In case of a total power failure a substance is added to the batch which shortstops the reaction. A system which is independent of the normal energy supply is available for introducing the ``stopper-' into the reactor. C mputer system. Since early 1967, Hiils has pioneered computer technology for the control of PVC production.* With hardware and software, the on-line closed loop operating computer is designed to supervise and control all checking, metering, polymerization and discharge op erations automatically, to initiate corrective measures auto matically on operational deviations, to actuate specific alarms and to print out essential parameters. It also serves to control reactor agitators and recycle pumps. Due to built-in check features, faulty commands cannot be trans mitted to the plant prior to a computer outage. The computer controls the essential reactor control loops by DDC. It is furnished with a hand control backup which is automatically actuated on computer outage, in which case all initiated polymerization operations can be com pleted manually, and polymerization vessel temperatures and pressures can always be observed on analog recorders. Computer advantages include: High plant capacity utilization Reduced manpower requirements More uniform products Safeguard against malfunction Reduced reject rate. PROCESS DESCRIPTION The Hiils S-PVC process consists of six main sections as shown in Fig. 5. Fa d preparation. In the feed preparation section, sus pension agents are dissolved in demineralized water. The solutions and other liquid chemicals are fed via the Fig. 3--Stirring system of a large reactor with bottom drive impeller and baffles at vortex level. REACTOR . CONDENSER VCM ' . COOLING WATER ACCUMULATOR STORAGE Fig. A--Emergency degassing system. TABLE 2--Characteristics of a S-PVC polymerization unit using 200 m' reactors compared with a plant using 20 mJ reactors based an 160,000 metric tons/year production Capital investment (including emergency system, Gulf Coast ba$i<) Space requirement.............................................................................. Utility consumption......................................... Manpower.......................................................................... WT SO'Tc metering station to the reactor. Charging of the reactor is computer controlled according to the stored recipes. Polymerization. The polymerization section of the Hiils large reactor plant located in Marl consists of two 200 mJ Hydrocarbon Processing November 1976 119 COLORITE 006046 SUSPENSION-PVC FEED ROLTMERIZATIOW WEPADinOH i , DEGASSING DEMWERAUZID WATER DEGASSING VACWM SYSTEM GAS HOLDER (ORTIONAL) absorption unit VCM RECOVERY TO EMERGENCY SLOWDOWN POLrMERIZER DEGASSWO CEGASSINS INTERMEDIATE VESSEL COLUMN SUSPENSION STORAGE Fig. 5--Flow diagram of the Hills S-PVC process. DWYER SYSTEM WATER SCRUBBER VCM TANIC - reactors (Fig. 6) and their associated equipment but in the next few years the number of reactors will be in creased. One reactor is polymerizing while the other is charged, emptied and cleaned to use the associated equip ment in a most economical way. The total batch cycle is similar to that of smaller reactors. Degassing. The degassing section separates the unreacted monomer from the water/polymer slurry in two stages. The batch is discharged from the reactor into the pressure degassing vessel and from there the slurry is pumped con tinuously to a vacuum degassing column. In the degassing column residual monomer is removed by a vacuum system via a knockout drum and condenser into the recovery section The residual monomer content in the degassed slum can be controlled by temperature, pressure and resi dence tune. All water effluents which are contaminated with YCM are stripped in a water degassing column before stoing to the sewage svstem. Product drying. From the intermediate suspension stor age the slurry is pumped to a centrifuge. The wet cake is fed to a hot air dryer system. We use a two-stage system of a flash and a rotary dryer. Other systems can be applied such as a single or two-stage flash dryer or a combination flash and fluid bed dryer or a single fluid bed or a single rotary dryer. The dried product is pneumatically trans ported to a screening device to separate oversize. Filters or cyclones are used to separate the product from drying and transport air. From the vent stream PVC dust is removed by a water scrubber. VCM recovery. The VCM recovery section compresses the unreacted monomer from the degassing section, knocks out any water and returns the monomer to storage for reactor feed. Inerts loaded with VCM are scrubbed in our plant in an absorber/desorber unit to avoid VCM emission. The solvent is stripped, recovered and recycled. The inerts leaving the scrubber are incinerated. TABLE 3--Specifications of Hiils 5-PCV grades produced in the 200 mJ suspension polymerization r actors Prnpvrde* Vi'a.osjiy mile* (1SO/R 174) K.v.iIiip Apparent bulW density (ISO/R t>0). n/l Sieve amly-us (ISO/R 1<24) >M .atm. >2.`ill .aMI, ' J < Sulfate.) .uti .ISO/R 1270), < Volatile nutter 'ISO/R IL'ijO) L"T ........................ , ' Hull Grades For Proceuinfi With Plasticizers S 6554 S 7054 S 7554 S 8054 105 125 145 169 65 70 75 80 500 475 460 460 90 0.5 001 0.2 90 0.5 0.01 0.3 95 0.5 0.01 0.3 95 0.5 0.01 0.3 Huls Grades For Proceatflnft Without Plasticizers S 5858 S 6058 S 6558 S 6858 82 88 105 115 58 60 65 68 580 580 5S0 580 98 2 0.01 0.3 98 2 0.01 0.3 98 2 0.01 0.3 98 2 0.01 0.3 120 November 1976 Hydrocarbon Processing COLORITE 006047 In addition to :'.? {iax ibis end rigid go.y.'inyic'dorida graces, disperse ? PVC resins sre a'so produc ?-d in if, s i. 'is P 'go reactor poiyvinyisi'bQnds giant Emergency blowdown. The emergency blowdown sec tion provides a means of cooling and deactivating the large monomer release from the reactor section in the event of cooling water and/or power failures as described in safety precautions. LARGE REACTOR ADVANTAGES The use of large reactors for the polymerization of vinyl chloride provides economic and technical advantages. Even in regions where temperatures below zero occur, the large kettles can very easily be installed in an open air plant as the pipe connections are large and short length. It is obvious that an open air plant makes control of atmospheric VCM concentration easier than in a closed building. Table 2 compares the characteristics of a S-PVC poly merization unit using four 200 m3 reactors with a plant using 40 20 m3 reactors, based on 160,000 metric tons/ year. According to our recent calculations, the capital investment for the 200 m3 reactor polymerization plant is only about 75 percent of that required for a plant of the same capacity using 20 m3 reactors, and the large reactors require only 65 percent of the space needed for the smaller ones. Savings in utilities occur mainly in the lower electrical power consumption for stirring. Considerable savings are achieved in manpower. A plant with only four reactors needs less operating personnel than a plant with 40 re actors. The cost of maintenance and repair are reduced by the same percentage as that shown for capital investment. Installing large reactors minimizes potential leakages-- nozzles, flanges and mechanical seals. With fewer but larger nozzles in a large reactor plant, it is necessary and economical to have remote controlled valves in all pipe connections. Thus, a fully automated and perfectly con- About the author B. Terwiesch is production manager for PVC with Chemische Werke Hills, Marl, Germany. Dr. Terwiesch received his Chsmie-Ing. from Ingenieurschule Essen and his Diplom-Chemiker and Dr. rer. nat. from Teehnische TJniversitat Karlsruhe. trolled interlock system in combination with the process computer is obtained. S.PVC GRADES The S-PVC grades which can be manufactured in the 200 m3 reactor are shown in Table 3. The designations refer to the names of PVC grades manufactured by Hiils, i.e., S 7054. The first two figures refer to the K-value and the last two to the grade classification. There are two groups: the 54 group for flexible applications and the 58 group for rigid applications. Both groups contain products with different K-values. For flexible grades a high granular porosity is primarily desired, which ensures good plasticizer adsorption and free flowing characteristics for the plasticized mixture for dry powder processing. The rigid grades necessarily show a lower apparent bulk density but the tendency to form "fish eyes" is very low. In the case of rigid grades a high apparent bulk density and good free flowing properties are primarily demanded, which in the processing of powder favorably influence the intake and the pressure build-up in the extruder. In addition to these S-PVC grades, dispersion PVC resins are also produced in the Hiils large reactor PVC plant. Adapted from a presentation before the I&EC Division Symposium on New Processes and Technology of the Overseas Chemical Industries, New York, April 1976. literature cited 1 Beckmann. G., Chemttch, May 1973. p- 304. * Albright, L. F.. Ch. G. Bild. Chcm. Eng. (1975), p. 121. ^ 1 DT Patent Application P 2019017. Chemische Werke Huh AG, 4 Beckman, G- Chemie-Ttchnik 5 (1975), p. 133. % DBP.2428 705, Chemiwhe Werke H&ls AG. * DT-AS 1349 397. Chemwche Werke Hnh AG. -- Hydrocarbon Processing November 1976 121 COLORITE 006048 engineering Getting out the last traces of VCM Tenneco process uses activated carbon to remove vinyl chloride monomer from PVC plant gas streams. It's one of several methods producers are licensing More than a year's experience with its carbon adsorption method of controlling vinyl chloride emissions from polyvinyl chloride plants has convinced Tenneco Chemicals (Saddle Brook, NJ.) that it has found the best route. So it is setting out to license the method to others. When the cancer-causing properties of vinyl chloride monomer became known, explains Robert S. Miller, manager of polymers research, the firm had to de velop its own technology to control emis sions. Now it is marketing the know-how- in the U.S. and in European countries that are following the U.S. lead in setting stringent emission standards. The firm is just one of a number of U.S. producers that have developed li censing packages in an effort to recoup part of the millions they have spent to de velop the new technology. B.F. Goodrich is licensing its steamstripping process, which removes VCM from PVC suspension resins. Diamond Shamrock has developed a proprietary technique for removing residual VCM from PVC dispersion resins. Monsanto has been licensing its PVC reactor clean ing know-how for more than a year. Lonza (Basle. Switzerland) is offering a procedure for preventing suspension resin polymer buildup on reactor walls. And several others have developed in strumentation to monitor the amount of VCM in the air and in PVC. Thirsty Carbon: Tenneco's develop ment is aimed at recovering the residual VCM in vent gas streams (CM'', Sept. 18, 1974, p. 66). A unit built by Chemical De sign (Lockport, N.Y.) was installed at Tenneco's PVC plant in Pasadeasa Tex., in early 1975. A year's experience with the equip ment, says Roy T. Gottesman, associate director of research and development, shows that less than 4 ppm. of vinyl chlo ride monomer is escaping in the adsorber effluent stream. During that time, the ac tivated carbon in the unit has undergone more than 1,300 adsorption and regener ation cycles. That makes it good enough to meet the proposed Environmental Pro tection Agency standard of 10 ppm. Moreover, after 10 months of contin uous service, samples of carbon with drawn from the adsorber showed that it retained 90% of its original adsorptive ca pacity. Further, there was no evidence of vinyl chloride polymerization in the ad sorption unit or within the carbon. Trouble Spots: Aside from leaks, which manufacturers try to keep to a minimum, there are several points where VCM can escape to the atmosphere: at the reactor, the slurry tanks, and equip ment purge streams. PVC is made primarily by either sus pension or emulsion polymerization tech niques. Generally, in these processes VCM and the polymerization initiator are dispersed in water by the use of sus pending or emulsifying agents. The poly merization temperature is selected to ob tain the desired molecular weight. But not all of the vinyl chloride is converted into PVC. Usually 17-20% of monomer that does not react remains at the end of the polymerization cycle. Much of that can be recovered and recycled to the front end of the process. But as much as TENNECO'S GOTTESMAN: New process cuts PVC plants' VCM loss to below 4 ppm. 4% VCM can remain in the PVC slurry from the reactor. That can be removed by specialized stripping techniques, such as the ones Goodrich, Diamond Shamrock and Tenneco now offer. Residual VCM from that cleanup phase is one of the potential sources of feed for the carbon adsorption step. Another source is the, slurry tanks, where operators flush air or nitrogen through the vapor space of the tank to keep VCM concentrations down and pre vent a flammable mixture from forming Depending on the level of stripping of the slurry downstream, VCM concentra tions in the nitrogen purged may reach 15 mol percent. Various equipment in the process is also purged with inert gar to reduce the explosion hazard and to remove the last traces of VCM. These off-gas purge streams generally contain 10-30 mol per cent vinyl chloride. New Twist: Vinyl chloride has been around a long time and so has activated carbon. But up to now, researchers have worked with pure monomer. Tenneco's contribution has been to make the pro cess work with recovered vinyl chloride, says Gottesman. He explains that this material can contain residual initiators, catalyst fragments, and suspending agents, so that it is prone to polymerize and foul the carbon system. Making the process work is a matter of selecting the right operating conditions and the right type of activated carbon. Another method of scrubbing VCM from stack gas, called oxvphoiolysis. has been investigated by Robiniech of Fort Worth, Tex. (CIV Technology Newslener. July 9, 1975). The firm says it ts still eval uating the economics of installing the unit, which so far has not been used com mercially. Although carbon adsorption is the pro cess that Gottesman believes will attract the widest interest, the company is arm ing itself with a variety of other tech nology. "We have filed 13 patent appli cations on widely differing subjects." says Gottesman. "They include a spray appa ratus for cleaning PVC reactors, processes for removing VCM from suspension and emulsion processes, the destruction of VCM by ozonolysis and the removal of monomer from dispersion latex by spar ging." If others' experience is any indication, there is a big demand for the know-how. B.F. Goodrich Chemical says it has signed agreements with six corporations tn Sweden, Italy, Japan and the U.S. for its suspension-resin stripping technology. Diamond Shamrock became a licensor in March, offering its proprietary tech nique for removing residual VCM front August 11, 1976 CHEMICAL WEEK 35 COLORITE 006049 Aminopanidine Bicarbonate: You can have plenty; get it fast! Aminate reacts with acids to generate carbon, dioxide and pro duce solutions of the correspond ing salt. It's used in the manufacture oftetrazene, an ingredient for small arms primers. Further applications: in photon graphic emulsions, azo dyes, depilation of animal skins, textile softening agents, flameproofing agents. Also as a blowing agent in manufacture of foam as a vinyl sta bilizer, and in the preparation of aminoplast resins. Let your chemical imagination roam. Find all the uses you want, because we have all the Aminate you could want. Available as a fine, white, odorless crystalline powder. Delivery is fast. The price is right. For full details, write today to the IMC Chemical Group, Inc., Sobin Park, Boston, Mass. 02210. imces rucmirQi esc industrial GfiOUPlnc CHE",CaLS0,VIS," A subsidiary of imsrnstional Minrsti & Crwticat Corporation 36 CHEMICAL WEEK August 11, 1976 engineering dispersion resins. Ditlerent techniques are required for the two svstems because of differences in particle size. The company explains that in the sus pension process, monomer droplets sus pended in water typically produce PVC particles ranging in size from 90 to 130 microns. In the dispersion or emulsion process, a colloidal dispersion of mono mer droplets creates PVC particles aver aging 0,5 micron in size. These smaller particles are more sensitive to heat and mechanical disturbance than suspension resins, thus different technology is re quired. Says Diamond Shamrock. "We are us ing it ourselves. We have a number of people seriously interested, including some from outside the U.S We do not at the moment have any licensees. But it is a relatively new offer." It seems doubtful that anyone will reap a financial bonanza from the new tech nology. Rather, licensors see their effort as one of trying to recover part of their development cost. Diamond Shamrock adds' "Licensing this kind of technology is not going to be a giant money-maker for anybody, as far as we can see, once you put the income generated up against what it cost to de velop it in the first place. We hope we will be able to defray our developmental costs--that is, spread them out through the industry." Insight into energy A new 900-page United Nations study of energy use has turned up extensive data on the use and distribution of com mercial energy since 1950 Among the findings: The U.S., with 5% of the world's population, used 30% of the world's com mercial energy in 1974, compared with 45% in 1950. Its per-capita gasoline con sumption was nine times the world aver age. The U.S.S.R., the world's secondlargest user, increased its share from 11.5% in 1950 to 16.6% in 1974. China and Japan are the third- and fourth-1 argest consumers, respectively. China still relies mainly on solid fuel. Ja pan is a leading importer of all types of energy. The report updates earlier studies on commercial energy production, trade and consumption and includes preliminary data on the production of primary fossil fuels for 1975. It is available from the UN Sales Sec tion (New York) for S30. COLORITE 006050