Polyacrylonitrile reactor

Solution Polymerization These processes may retain the polymer in solution or precipitate it. Polyethylene is made in a tubular flow reactor at supercritical conditions so the polymer stays in solution. In the Phillips process, however, after about 22 percent conversion when the desirable properties have been attained, the polymer is recovered and the monomer is flashed off and recyled (Fig. 23-23 ). In another process, a solution of ethylene in a saturated hydrocarbon is passed over a chromia-alumina catalyst, then the solvent is separated and recyled. Another example of precipitation polymerization is the copolymerization of styrene and acrylonitrile in methanol. Also, an aqueous solution of acrylonitrile makes a precipitate of polyacrylonitrile on heating to 80°C (176°F). 

Two different polyacrylonitrile precursor carbon fibers, an A fiber of low tensile modulus and an HM fiber of intermediate tensile modulus were characterized both as to their surface chemical and morphological composition as well as to their behavior in an epoxy matrix under interfacial shear loading conditions. The fiber surfaces were in two conditions. Untreated fibers were used as they were obtained from the reactors and surface treated fibers had a surface oxidative treatment applied to them. Quantitative differences in surface chemistry as well as interfacial shear strength were measur-ed. 

Bead Polymerization Bulk reaction proceeds in independent droplets of 10 to 1,000 pm diameter suspended in water or other medium and insulated from each other by some colloid. A typical suspending agent is polyvinyl alcohol dissolved in water. The polymerization can be done to high conversion. Temperature control is easy because of the moderating thermal effect of the water and its low viscosity. The suspensions sometimes are unstable and agitation may be critical. Only batch reactors appear to be in industrial use polyvinyl acetate in methanol, copolymers of acrylates and methacrylates, polyacrylonitrile in aqueous ZnCb solution, and others. Bead polymerization of styrene takes 8 to 12 h. 

The enantioselective hydrolysis is carried out in an organic-aqueous two-phase reactor (toluene/water), where the phase contact is established by a hydrophilic hollow-fiber membrane (polyacrylonitrile). The lipase is immobilized onto a spongy layer by pressurized adsorption. The productivity is about 40 kg trans-(2R,3S)-(4-methoxyphenyl) glycidic acid methyl ester m-2 a-1. This process has been operated since 1993. 

An enzymatic production process for Diltiazem (54), a coronary vasodilator and calcium channel blocker, was started in 1993 by Tanabe Seiyaku, Japan [7, 77]. The epoxide (2i, 3S)-52 is a key intermediate in this synthesis (Scheme 17) and can be produced via asymmetric hydrolysis of rac-52 catalyzed by Serratia marescens lipase immobilized on spongy layers. The whole process takes place in a polyacrylonitrile hollow fiber membrane reactor and produces (2i, 3S)-52 in yields of 40-45%. The hydrolyzed product (2S,3i )-53 is not stable under the prevailing reaction conditions and decarboxylates to aldehyde 55, a strong enzyme deactivator. The aldehyde needs therefore to be removed, which is achieved by continuous filtration of its bisulfite adduct 56. Using this enzymatic process it was possible to bring down the number of required steps en route to 54 from nine to five. This process is also carried out by other companies (e.g., DSM) with a worldwide annual production of 1001. 

Polyacrylonitrile, like PVC, is insoluble in its own monomer. Consequently, the polymer precipitates from the system during bulk polymerization. Aerylonitrile ean be polymerized in solution in water or dimethyl formamide (DMF) with ammonium persulfate as the initiator (redox initiation). The polyacrylonitrile homopolymer ean be dry spun from DMF direetly from the polymerization reactor or wet spun... 

The bank of 60 m commercial-scale membrane reactor modules in diltiazem production facility employing hydrophilic polyaCTylonitrile hollow fibers was demonstrated. This example describes enzymatic resolution of diltiazem precursor using hydrophilic polyacrylonitrile hollow fibers and an aqueous-organic interface on the outside surface of these fibers (Eigure 4.16) [1,3,116]. 

From the overview presented above it can be found that there are numerous methods of preparation of polymeric photocatalytic membranes. However, it should be stressed that in case of polymer membranes there is always a danger of destruction of the membrane structure by UV light or hydroxyl radicals. This risk is associated with the reactor configuration. Application of a photocatalytic membrane requires irradiation of the membrane itself in order to perform the photodecomposition of pollutants. The lowest UV resistance is exhibited by membranes prepared from polyether-sulfone (PES) and polysulfone (PSU) (Chin et al, 2006 Molinari et al, 2000). This can be attributed to the fact that PES and PSU contain sulfone groups which are highly sensitive to UV light. Other membranes exhibiting low UV resistance are those made of polypropylene (PP), polyacrylonitrile... 

Continuous Bulk Process. Polyacrylonitrile is not soluble in its monomer and precipitates from the medium. The polymerization exhibits autocatalytic behavior, and as polymerization proceeds, it becomes increasingly difficult to remove the heat of polymerization as viscosity increases. Consequently, in a batch process, the polymerization can run out of control. Therefore, continuous operation is used to overcome the difficulties (85-87). As an example, the following streams are continuously charged into a 2.5-L reactor at 40°C, equipped with an agitator and filled initially with acrylonitrile to one-half of its volume ... 

A special variation of solution polymerization is precipitation polymerization. The only difference here is that the polymer becomes more and more Insoluble in the solvent as its molecular weight increases. The polymer precipitates from the solution and can be isolated by filtration. One interesting feature of precipitation polymerization is that the viscosity of the solution in the reactor remains almost constant. The production of polyacrylonitrile in water is an important example of this polymerization process. 

Polyacrylonitrile can be produced by continuous bulk, continuous slurry, and emulsion polymerization processes. In a continuous slurry process, small monomer droplets are suspended in an aqueous medium. In this process, heat removal is more efficient than in the bulk process. In one example [58], a 0.3% H2SO4 aqueous solution, a catalyst solution (15% NaaSOs and 4.22% NaClOs in water), and a monomer solution are continuously supplied into an agitated reactor. At 35°C, 90% monomer conversion is achieved at 1.7 hr of residence time. A portion of the polymerizing mixture... 

The dimerization of isobutene carried out in a forced-flow polymeric catalytic membrane reactor was reported by D. Fritsch and co-workers. The authors prepared composite porous membranes consisting of a catalytic layer made of solid add catalysts, such as siUca supported Naflon , Nafion NR50, Amberlyst 15 and silica supported tungstophosphoric add dispersed in polymeric binders such as Teflon AF, Hyflon AD, polytrim-ethylsilylpropyne, or polydimethylsiloxane (PDMS), cast on microporous support membranes made of polyacrylonitrile (PAN) or Torlon . The membranes were assembled in the membrane reactor into which isobutene was fed in the retentate side with a build-up pressure of 4 bar. The liquid product was collected on the permeate side. 

---