Diluent-slurry process

Figure 93. Flow diagram of the diluent-slurry process...

Diluent slurry processes for polypropylene are expensive to build and to run because of the number of pieces of equipment involved. They have largely been replaced by the more efficient bulk and gas-phase processes. Most of the remaining diluent slurry plants in the world now focus on producing speciality polymers, as diluent slurry processes do offer some advantages over other bulk and gas-phase processes. An example is the production of high-crystallinity polypropylene (HCPP), where most of the atactic polymer is dissolved in... 

In this type of process, polymerization takes place in liquid propylene without the use of an inert diluent. This is a significant simplification over the traditional diluent slurry process, as propylene can be separated from the polymer by flashing, and there is no need for the extensive diluent recovery system. 

The share of competing technologies in production of polypropylenes in the three industrialized regions is undergoing dramatic change at the present time. Many of the existing heavy diluent slurry process plants are being revamped to bulk slurry processes, and many more new bulk slurry plants are presently under construction. 

Some slurry processes use continuous stirred tank reactors and relatively heavy solvents (57) these ate employed by such companies as Hoechst, Montedison, Mitsubishi, Dow, and Nissan. In the Hoechst process (Eig. 4), hexane is used as the diluent. Reactors usually operate at 80—90°C and a total pressure of 1—3 MPa (10—30 psi). The solvent, ethylene, catalyst components, and hydrogen are all continuously fed into the reactor. The residence time of catalyst particles in the reactor is two to three hours. The polymer slurry may be transferred into a smaller reactor for post-polymerization. In most cases, molecular weight of polymer is controlled by the addition of hydrogen to both reactors. After the slurry exits the second reactor, the total charge is separated by a centrifuge into a Hquid stream and soHd polymer. The solvent is then steam-stripped from wet polymer, purified, and returned to the main reactor the wet polymer is dried and pelletized. Variations of this process are widely used throughout the world. 

Montedison and Mitsui Petrochemical iatroduced MgCl2-supported high yield catalysts ia 1975 (7). These third-generation catalyst systems reduced the level of corrosive catalyst residues to the extent that neutralization or removal from the polymer was not required. Stereospecificity, however, was iasufficient to eliminate the requirement for removal of the atactic polymer fraction. These catalysts are used ia the Montedison high yield slurry process (Fig. 9), which demonstrates the process simplification achieved when the sections for polymer de-ashing and separation and purification of the hydrocarbon diluent and alcohol are eliminated (121). These catalysts have also been used ia retrofitted RexaH (El Paso) Hquid monomer processes, eliminating the de-ashing sections of the plant (Fig. 10) (129). 

Most commercial processes produce polypropylene by a Hquid-phase slurry process. Hexane or heptane are the most commonly used diluents. However, there are a few examples in which Hquid propylene is used as the diluent. The leading companies involved in propylene processes are Amoco Chemicals (Standard OH, Indiana), El Paso (formerly Dart Industries), Exxon Chemical, Hercules, Hoechst, ICl, Mitsubishi Chemical Industries, Mitsubishi Petrochemical, Mitsui Petrochemical, Mitsui Toatsu, Montedison, Phillips Petroleum, SheU, Solvay, and Sumimoto Chemical. Eastman Kodak has developed and commercialized a Hquid-phase solution process. BASE has developed and commercialized a gas-phase process, and Amoco has developed a vapor-phase polymerization process that has been in commercial operation since early 1980. 

In the slurry process, propylene monomer is dissolved in a hydrocarbon diluent in which the polymerization process occurs. The polymerization products are either soluble (the highly atactic components) or insoluble. Both the insoluble and soluble components are collected and form separate product streams. The insoluble species form a slurry in the solvent, from which they are removed by centrifugation. The soluble, atactic component is removed with the solvent as another product stream. To separate the atactic polymer from the solvent, the solution is heated allowing the solvent to flash off, leaving the atactic polymer behind. Any un reacted monomer is degassed from the solution and recycled to the start of the polymerization process. 

In the bulk process, liquid propylene (polymer grade propylene here too) replaces the hydrocarbon diluent used in the slurry phase process. The PP is continuously withdrawn from the solution and any unreacted monomer is flashed off and recycled. The back end of the process, atactic PP removal and catalyst deactivation and removal, is the same as the slurry process. 

One of the most economical routes to most commercial grades of olefin polymers is the loop slurry process with a paraffin diluent. This process was introduced by Chevron Phillips in 1960 (7). There, a mixture of catalyst particles, growing polymer particles, comonomers, and diluent is pumped in a loop. The polymer particles are harvested by guiding a side stream of the slurry to settling chambers, where the polymer particles settle toward the bottom. 

The polymerization process can be carried out at a temperature low enough that the resulting polymer is largely insoluble in the diluent. The pressures in the loop slurry process are in the range of 2.5-4 MPa. 

Three processes are used commercially to make linear polyethylene-solution, slurry, and gas phase. All are called low-pressure processes (< 50 atm) to distinguish them from the free radical or high-pressure process that makes highly branched polyethylene. In the solution mode a hydrocarbon solvent at 125-170°C dissolves the polymer as it forms. The reaction usually slows as the solution becomes viscous because it becomes difficult to stir ethylene into the liquid phase. In contrast, The slurry process uses a poor solvent and low temperature (60-110°C) to prevent dissolving or even swelling of the polymer. Each catalyst particle creates a polymer particle several thousand times larger than itself. There is no viscosity limitation in the slurry method the diluent serves to transfer heat and to keep the catalyst in contact with ethylene and other reactants. Finally, the gas-phase process is much like the slurry method in that polymer particles are formed at similar temperatures. A bed of catalyst/polymer is fluidized by circulating ethylene, which also serves as a coolant. 

Whatever the merits of each process in a continuous commercial operation, the slurry process is very convenient for batch polymerization studies in the laboratory. The diluent permits precise control of the temperature and serves to dissolve ethylene and other reactants that must contact the catalyst during polymerization. Most of the work reported here was done in a slurry reactor. 

Current Processes. The development of superactive third-generation supported catalysts enabled the introduction of simplified processes, without sections for catalyst deactivation or removal of atactic polymer. By eliminating the waste streams associated with the neutralization of catalyst residues and purification of the recycled diluent and alcohol, these processes minimize any potential environmental impact. Investment costs arc reduced by approximately one-third over slurry process plants. Energy consumption is minimized by elimination of the distillation of recycled diluent and alcohol. The total plant cost for the production of polymer is less than 130% of the monomer price, when a modem process is used, compared to 175% for a slurry process. 

Slurry processes in which dissolved ethylene is polymerised to form solid polymer particles suspended in a hydrocarbon diluent... 

The slurry process is the oldest and still widely used method for manufacturing polymers of ethylene, propylene and higher a-olefins. In this process, the monomer dissolves in the polymerisation medium (hydrocarbon diluent) and forms a solid polymer as a suspension containing ca 40 wt-% of the polymer the polymerisation occurs below the melting point of the polymer. In slurry polymerisation, the temperature ranges from 70 to 90 °C, with the ethylene pressure varying between 7 and 30 atm. The polymerisation time is 1-4 h and the polymer yield is 95-98 %. The polymer is obtained in the form of fine particles in the diluent and can be separated by filtration. Removal of the catalyst residues from the polymer can be achieved by the addition of alcohol (isopropanol, methanol), followed by recovery and extraction of the catalyst residues. The polymer is freed from diluent by centrifuging and then dried. In the case of polypropylene manufacture, the atactic fraction remains in the diluent [28,37]. 

The development of supported catalysts has permitted the elimination of the expensive catalyst removal stage. Therefore, in slurry processes taking advantage of highly active catalysts, the diluent is recovered after centrifugation and recycled without purification (Figure 3.53) [51]. 

In the slurry process, the reaction is carried out in a liquid dispersant (paraffinic in nature), in which catalyst and polymer remain in suspension. Reaction temperature is held below 110°C. to prevent dissolution of the polymer. Catalyst does not necessarily remain in the middle of a polymer particle but spalls and is scattered throughout the polymer. The slurry of polymer and hydrocarbon is withdrawn from the reactor and flashed to remove diluent and unreacted olefin for recycle. Because of the high productivities obtained in this process, it is unnecessary to remove catalyst for many polymer applications. 

Synthesis. The early PP plants used a slurry process adopted from polyethylene technology. An inert liquid hydrocarbon diluent, such as hexane, was stirred in an autoclave at temperatures and pressures sufficient to keep 10-20 percent of the propylene monomer concentrated in the liquid phase. The traditional catalyst system was the crystalline, violet form ofTiCl3 and A1C1(C2H5)2. Isotactic polymer particles that were formed remained in suspension and were removed as a 20-40 percent solid slurry while the atactic portion remained as a solution in the liquid hydrocarbon. The catalyst was deactivated and solubilized by adding HC1 and alcohol. The iPP was removed by centrifuging, filtration, or aqueous extraction, and the atactic portion was recovered by evaporation of the solvent. The first plants were inefficient because of low catalyst productivity and low crystalline yields. With some modifications to the catalyst system, basically the same process is in use today. 

Diluents must be inert toward the catalyst system and are usually saturated hydrocarbons such as propane, isobutane and hexane. Slurry processes typically operate at temperatures from about 80 to 110 °C and pressures of 200-500 psig. Polyethylene precipitates as formed resulting in a suspension of polymer in diluent. The catalysts most commonly used in slurry processes are chromium-on-silica or supported Ziegler-Natta catalysts. 

Features - particles of growing polymer form as suspension in hydrocarbon diluent - catalyst residence time 1 hour for Phillips loop slurry process - morphology and psd of catalyst are important - wide range of comonomers may be used... 

Slurry processes in hydrocarbon diluent are used in the production of HDPE, including bimodal polymers produced in the cascade process in which different hydrogen concentrations are applied in two or more reactors in series. Liquid loop reactors are generally used with a light hydrocarbon diluent such as isobutane, whereas heavier hydrocarbon diluents are typically used in continuous stirred tank reactors. 

In PP manufacture, modern bulk (liquid monomer) and gas-phase processes have largely replaced the earlier slurry processes in which polymerization was carried out in hydrocarbon diluent. The most widely adopted process for PP is Basell s Spheripol process.317 Homopolymer production involves a pre-polymerization step at relatively low temperature, followed by polymerization in a loop reactor using liquid propylene random co-polymers are produced by introducing small quantities of ethylene into the feed. The pre-polymerization step gives a pre-polymer particle with the capacity to withstand the reaction peak, which occurs on entering the main loop reactor. The addition of one or two gas-phase reactors for EP co-polymerization makes it possible to produce heterophasic co-polymers containing up to 40% of E/P rubber within the homopolymer matrix. 

The slurry process was developed shortly afterward, offering better economic efficiency. The slurry process uses a poor solvent, called a diluent, and a low reaction temperature (70-110 °C) to prevent dissolution or even swelling of the polymer [20,21,711,717]. Each catalyst particle creates a polymer particle several thousand times more massive than itself, provided that it is not broken by the forces of the slurry circulating in the reactor. The polymer product is therefore formed as a powder. In practice, however, most polymer particles are broken several times, and low-MW homopolymers (which have weak particles) are broken more easily than high-MW copolymers (which have greater toughness). 

A higher concentration of slurry, which can contain up to 50% polymer by weight and 80% by settled volume, contributes to the improved economic efficiency of the loop-slurry process. The diluent serves to transfer heat and to keep the catalyst in contact with ethylene and other reactants. Isobutane is now used almost entirely as the diluent. This diluent allows the polymer to be separated by a flash separation, which is generally rather simple in comparison with the separation in the solution processes. 

The "gas-phase" (fluidized-bed) process is much like the slurry process in that polymer particles are formed at similar temperatures, but a liquid hydrocarbon diluent is not used. A bed of catalyst/polymer is stirred either by mechanical means, or, more often, by fluidization, while ethylene, N2, and other hydrocarbons that act as a coolant are circulated [718-722], Although the "gas phase" process offers many advantages, including the lack of a diluent that can cause polymer swelling, its weak point is poor heat removal from the polymer particles, because of the low heat capacity of a gas. Thus, reactor fouling still occurs,... 

TABLE 75 Maximum Operating Temperatures When Various Hydrocarbons were Used as Diluents in the Slurry Process for Homopolymer Production with Cr/Silica... 

Alternative slurry and gas phase processes for the production of EPDM are an improvement with respect to the solvent recovery step [2]. However, in gas phase processes the possibility of incorporating large amounts of heavier monomers is hmited because of the low vapor pressure of these monomers. Furthermore, conventional slurry processes use aliphatic diluents such as iso-butane or, in some processes, supercritical propane, which are highly flammable. 

The Mitsui CX process is typical of a modem slurry process for HDPE production (Figure 2.37). It consists of two CSTRs (hexane is used as diluent), a centrifuge to separate the diluent from the polymer, a dryer to remove the residual diluent, and a diluent recovery system to separate the low molecular weight polymer or wax that is dissolved in the diluent. The two polymerization reactors can be operated in series or in parallel. When run in... 

Like the slurry process for polyethylene manufacture, this is also the first commercial process for the production of polypropylene. The basic process includes a series of CSTRs and polymerization takes places in an inert diluent. Many different variations of the slurry process were developed in the early 1970s. The design of the process was dictated by the available catalyst at the time, which typically had low activity and produced polymer with low isotacticity. Since catalyst activity was low, a series of reactors were needed to push the reaction to completion. It is not imusual to see processes with five reactors in series, and some with even seven reactors in series, designed in those earlier days. Deashing was required to remove the high level of catalyst residue from the product an atactic polypropylene removal step was also necessary. Different variations of the slurry process included the use of diluents ranging from Ce to C12 hydrocarbons. 

The only important commercial elastomer prepared by a cationic polymerization is butyl rubber, i.e., a copolymer of isobutene and isoprene. The latter monomer is incorporated in relatively small proportions (-1.5 mole % [76]) in order to introduce sufficient unsaturation for sulfur vulcanization. The slurry process with aluminum chloride at -98 to -90°C in methyl chloride diluent can be described by the accompanying flow sheet [115,116,124]. In this process the polymerization is almost instantaneous and extensive cooling by liquid ethylene is required to control the reaction. 

In continuous slurry processes, the temperatures are kept between 90 and 100°C and pressures between 400 and 450 Ib/in.. The catalyst concentrations range between 0.004 and 0.03% and typical diluents are w-pentane and w-hexane. Individual catalyst particles become imbedded in polymer granules as the reaction proceeds. The granules are removed as slurry containing 20 0% solids. 

Most cellulose ethers including HMHEC polymers are produced by heterogeneous slurry processes consisting of separate organic and aqueous/cellulose ether phases. Yokota of Daicel" studied the propoxylation of cellulose using an aqueous toluene//-butyl alcohol diluent, and he reported a diluent phase separation in which essentially all the caustic partitioned into the lower aqueous phase during the heterogeneous reaction. After the addition of aqueous caustic, two phases form (tlvee if one counts the solid cellulose as a... 

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