Low- pressure recycling

A simplified flow sheet of the industrial process is shown in Fig. 5.1-1. In the first section fresh ethylene is mixed with the low-pressure recycle at 5 MPa and is compressed to 15 -35 MPa by means of a five-stage piston compressor. Fresh ethylene should have a high purity of above 99.9 vol.%. Further specifications of polymerization-grade ethylene are given in Table 5.1-1. 

Figure 4.43. Plant layout for reactive processing. 1 - tanks for reagents 2 - heat exchangers 3 - low-pressure pumps 4 - high pressure metering pumps 5 - low-pressure recycling line 6 - high-pressure recycling line 7 - chamber for shock mixing 8 - mold.

Fig. 13.40 Schematic representation of a typical RIM machine. The machine can be divided into three basic parts (1) low-pressure recirculation or conditioning (bounded by the dotted lines) (2) high-pressure metering and (3) the impingement mixhead. The mold is usually considered separately. The figure shows the machine in low-pressure recycle mode. [Reprinted by permission from C. W. Macosko, RIM Fundamentals of Reaction Injection Molding, Hanser, Munich, 1989.]...

Refer to Figure 4.1. Fresh ethylene and a chain transfer agent (modifier) are mixed with the low-pressure recycle stream previously compressed in a booster compressor, from almost atmospheric conditions up to 20-50 bar (depending on fresh ethylene conditions). The combined stream is then compressed to an intermediate pressure of 200-300 bar approximately (supercritical conditions) in a multistage primary compressor. Optionally, comonomers such as vinyl acetate, acrylic add, and methyl acrylate can be used for the production of ethylene copolymers, and in this case are typically fed at the discharge of the primary compressor. 

The stripper off-gas going to the high pressure carbamate condensers also contains the carbamate recovered in the medium and low pressure recirculation sections. Both of these systems ate similar to those shown in the total-recycle process. 

The unit has virtually the same flow sheet (see Fig. 2) as that of methanol carbonylation to acetic acid (qv). Any water present in the methyl acetate feed is destroyed by recycle anhydride. Water impairs the catalyst. Carbonylation occurs in a sparged reactor, fitted with baffles to diminish entrainment of the catalyst-rich Hquid. Carbon monoxide is introduced at about 15—18 MPa from centrifugal, multistage compressors. Gaseous dimethyl ether from the reactor is recycled with the CO and occasional injections of methyl iodide and methyl acetate may be introduced. Near the end of the life of a catalyst charge, additional rhodium chloride, with or without a ligand, can be put into the system to increase anhydride production based on net noble metal introduced. The reaction is exothermic, thus no heat need be added and surplus heat can be recovered as low pressure steam. 

The reaction is initiated with nickel carbonyl. The feeds are adjusted to give the bulk of the carbonyl from carbon monoxide. The reaction takes place continuously in an agitated reactor with a Hquid recirculation loop. The reaction is mn at about atmospheric pressure and at about 40°C with an acetylene carbon monoxide mole ratio of 1.1 1 in the presence of 20% excess alcohol. The reactor effluent is washed with nickel chloride brine to remove excess alcohol and nickel salts and the brine—alcohol mixture is stripped to recover alcohol for recycle. The stripped brine is again used as extractant, but with a bleed stream returned to the nickel carbonyl conversion unit. The neutralized cmde monomer is purified by a series of continuous, low pressure distillations. 

Ethylene Stripping. The acetylene absorber bottom product is routed to the ethylene stripper, which operates at low pressure. In the bottom part of this tower the loaded solvent is stripped by heat input according to the purity specifications of the acetylene product. A lean DMF fraction is routed to the top of the upper part for selective absorption of acetylene. This feature reduces the acetylene content in the recycle gas to its minimum (typically 1%). The overhead gas fraction is recycled to the cracked gas compression of the olefin plant for the recovery of the ethylene. 

Quench Converter. The quench converter (Fig. 7a) was the basis for the initial ICl low pressure methanol flow sheet. A portion of the mixed synthesis and recycle gas bypasses the loop interchanger, which provides the quench fractions for the iatermediate catalyst beds. The remaining feed gas is heated to the inlet temperature of the first bed. Because the beds are adiabatic, the feed gas temperature increases as the exothermic synthesis reactions proceed. The injection of quench gas between the beds serves to cool the reacting mixture and add more reactants prior to entering the next catalyst bed. Quench converters typically contain three to six catalyst beds with a gas distributor in between each bed for injecting the quench gas. A variety of gas mixing and distribution devices are employed which characterize the proprietary converter designs. 

A significant concern in all nitration plants using mixed acids centers on the disposal method or use for the waste acids. They are sometimes employed for production of superphosphate ferti1i2ers. Processes have also been developed to reconcentrate and recycle the acid. The waste acid is frequently first stripped with steam to remove unreacted HNO and NO. Water is then removed by low pressure evapori2ation or vacuum distillation. 

Nuclear utiUties have sharply reduced the volume of low level radioactive waste over the years. In addition to treating wastes, utiUties avoid contamination of bulk material by limiting the contact with radioactive materials. Decontamination of used equipment and materials is also carried out. For example, lead used for shielding can be successfully decontaminated and recycled using an abrasive mixture of low pressure air, water, and alumina. 

Recycle and Polymer Collection. Due to the incomplete conversion of monomer to polymer, it is necessary to incorporate a system for the recovery and recycling of the unreacted monomer. Both tubular and autoclave reactors have similar recycle systems (Fig. 1). The high pressure separator partitions most of the polymers from the unreacted monomer. The separator overhead stream, composed of monomer and a trace of low molecular weight polymer, enters a series of coolers and separators where both the reaction heat and waxy polymers are removed. Subsequendy, this stream is combined with fresh as well as recycled monomers from the low pressure separator together they supply feed to the secondary compressor. 

A Hquid-phase variation of the direct hydration was developed by Tokuyama Soda (78). The disadvantages of the gas-phase processes are largely avoided by employing a weakly acidic aqueous catalyst solution of a siHcotungstate (82). Preheated propylene, water, and recycled aqueous catalyst solution are pressurized and fed into a reaction chamber where they react in the Hquid state at 270°C and 20.3 MPa (200 atm) and form aqueous isopropyl alcohol. Propylene conversions of 60—70% per pass are obtained, and selectivity to isopropyl alcohol is 98—99 mol % of converted propylene. The catalyst is recycled and requites Htde replenishment compared to other processes. Corrosion and environmental problems are also minimized because the catalyst is a weak acid and because the system is completely closed. On account of the low gas recycle ratio, regular commercial propylene of 95% purity can be used as feedstock. 

At conditions of high temperature and low pressure, for sufficient catalyst activity and acceptable reaction rates, equiUbrium conversions maybe as low as 5%, necessitating recycle of large amounts of unreacted propylene (101). 

C, 0.356—1.069 m H2/L (2000—6000 fU/bbl) of Hquid feed, and a space velocity (wt feed per wt catalyst) of 1—5 h. Operation of reformers at low pressure, high temperature, and low hydrogen recycle rates favors the kinetics and the thermodynamics for aromatics production and reduces operating costs. However, all three of these factors, which tend to increase coking, increase the deactivation rate of the catalyst therefore, operating conditions are a compromise. More detailed treatment of the catalysis and chemistry of catalytic reforming is available (33—35). Typical reformate compositions are shown in Table 6. 

The carbon monoxide product is removed from the top of the column and warmed against recycled high pressure product. The warm low pressure stream is compressed, and the bulk of it is recycled to the system for process use as a reboder medium and as the reflux to the carbon monoxide column the balance is removed as product. The main impurity in the stream is nitrogen from the feed gas. Carbon monoxide purities of 99.8% are commonly obtained from nitrogen-free feedstocks. 

The processiag costs associated with separation and corrosion are stiU significant ia the low pressure process for the process to be economical, the efficiency of recovery and recycle of the rhodium must be very high. Consequently, researchers have continued to seek new ways to faciUtate the separation and confine the corrosion. Extensive research was done with rhodium phosphine complexes bonded to soHd supports, but the resulting catalysts were not sufficiently stable, as rhodium was leached iato the product solution (27,28). A mote successful solution to the engineering problem resulted from the apphcation of a two-phase Hquid-Hquid process (29). The catalyst is synthesized with polar -SO Na groups on the phenyl rings of the triphenylphosphine. 

Ethylene Oxide Recovery. An economic recovery scheme for a gas stream that contains less than 3 mol % ethylene oxide (EO) must be designed. It is necessary to achieve nearly complete removal siace any ethylene oxide recycled to the reactor would be combusted or poison the carbon dioxide removal solution. Commercial designs use a water absorber foUowed by vacuum or low pressure stripping of EO to minimize oxide hydrolysis. Several patents have proposed improvements to the basic recovery scheme (176—189). Other references describe how to improve the scmbbiag efficiency of water or propose alternative solvents (180,181). 

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