Epoxidation reactor

After epoxidation, propylene oxide, excess propylene, and propane are distilled overhead. Propane is purged from the process propylene is recycled to the epoxidation reactor. The bottoms Hquid is treated with a base, such as sodium hydroxide, to neutralize the acids. Acids in this stream cause dehydration of the 1-phenylethanol to styrene. The styrene readily polymerizes under these conditions (177—179). Neutralization, along with water washing, allows phase separation such that the salts and molybdenum catalyst remain in the aqueous phase (179). Dissolved organics in the aqueous phase ate further recovered by treatment with sulfuric acid and phase separation. The organic phase is then distilled to recover 1-phenylethanol overhead. The heavy bottoms are burned for fuel (180,181). 

Cmde propylene oxide separated from the epoxidation reactor effluent is further purified by a series of conventional and extractive distillations to reduce the content of aldehydes, ethylbenzene, water, and acetone (182,183). 

The coproduct 1-phenylethanol from the epoxidation reactor, along with acetophenone from the hydroperoxide reactor, is dehydrated to styrene in a vapor-phase reaction over a catalyst of siUca gel (184) or titanium dioxide (170,185) at 250—280°C and atmospheric pressure. This product is then distilled to recover purified styrene and to separate water and high boiling organics for disposal. Unreacted 1-phenylethanol is recycled to the dehydrator. 

A variant envisages the use of aqueous methanol for both the extraction of hydrogen peroxide and the epoxidation of propene as shown, in a simplified form, by Figure 18.3. Thus, no extra water is added to the epoxidation reactor, which decreases separation and waste-treatment costs. On the laboratory scale, the quantity of methanol dissolving into the working solution is small and apparently does not interfere with the AO cycle of reactions [154, 161]. 

In principle, process integration applies even more to direct synthesis of hydrogen peroxide, further improving the advantages over the anthraquinone route. Methanol can be used to replace water as the solvent and the dilute methanol solution obtained fed into the epoxidation reactor. Minimal purification may be required, for example for the removal of hydrogen bromide and other additives that may have been needed to increase the selectivity. 

Synthesis is carried out in two separate steps (Scheme 6.1). In the first reactor, propene reacts with CI2 to produce propene chlorohydrin via intermediate formation of the propene chloronium complex, then quenched by water. In the epoxidation reactor, the dehydrochlorination of propene chlorohydrin occurs using a base (usually calcium hydroxide). 

In epoxidation, the propene-to-CHP molar ratio is 10 1, the reaction temperature is 60 °C and the pressure is sufficient to maintain propene in the liquid phase. The feed to the epoxidation reactor must contain less than 1% water in order to limit the hydrolysis of PO to glycol. The reaction is catalyzed by a proprietary, silylated, titanium-containing silicon oxide catalyst. The conversion of CHP is greater than 95%. Selectivity for PO based on hydroperoxide is 95%, whereas selectivity based on propene is around 99%. By-products of the reaction are aldehydes, such as acetaldehyde and propionaldehyde, alcohols (methanol and propene glycol), ketones and esters (e.g., acetone and methyl formate). The catalyst fixed-bed is structured into multiple catalyst layers, with heat exchangers in between the layers. This prevents excessive increases in temperature due to the exothermal reaction that would cause both thermal decomposition of the hydroperoxide and consecutive reactions of PO. 

The dilute HP-alcohol solution (HP concentration less than 10%) is introduced in a fixed-bed epoxidation reactor. ATi silicalite (TS-1) catalyst is used also in this case, to produce PO from propene and HP. The reaction is carried out at 40 °C and 20 atm pressure. Process PO yield is estimated to be around 95 mol.% by-products are 1,2-propandiol and the ethers formed by the methanolysis of the oxirane ring (l-methoxy-2-propanol and 2-methoxy-l-propanol), which may further react with PO to yield dipropeneglycol monomethylethers. PO may also form propanol hydroperoxides (l-hydroperoxy-2-propanol and 2-hydroperoxy-l-propanol). Other side-reactions such as the decomposition of HP normally occur to a very low extent. 

The bottom product of the pre-evaporation stage (Figure 6.7) can eventually be sub] ected to hydrogenation in a trickle bed reactor, to purify the solvent recycle stream by eliminating impurities in the form of formaldehyde and acetaldehyde, reducing them to methanol and ethanol, and also to eliminate traces of unconverted HP. Moreover, traces ofhydroperoxypropanol and hydroxyacetone are converted into 1,2-propanediol. This allows a considerable decrease in catalyst deactivation in the epoxidation reactor and the improvement of product quality [20k]. 

An integrated process has also been reported, but not implemented at a commercial level, by ARCO (now Lyondell) [21]. In this case, the O-donor species is produced by the autoxidation of aryl-substituted secondary alcohols (a-methyl benzyl alcohol) to ketones, and the resulting solution is used to feed the epoxidation reactor. The alcohol is then recovered by hydrogenating the ketone. The main difference with respect to the EniChem process is the higher temperature of the autoxidation process (90-140 instead of 40 °C). 

In the second step, a dilute H P-methanol solution is introduced in a fixed-bed epoxidation reactor. Make-up propene, recycled propene and HP from the product purification stage are fed into the reactor. The reaction is catalyzed by titanium silicalite, and takes place at 40-50 °C and 300 psi. HP per-pass conversion is initially 96% but drops down to 63% after 400 hours. PO selectivity is 95 mol.% propene per-pass conversion is 39.8%. This technology gives capital savings compared to conventional hydroperoxidation technologies however, it is likely that the operating costs of such a plant are higher than that of the latter. 

The product from the epoxidation reactor is sent to the crude PO recovery unit. The unit contains a number of distillation columns in which this product is separated into unreacted propylene for recycle to the epoxidation section, crude PO, EB, and styrene precursors (mainly MPC and MPK). Light hydrocarbons going overhead in the various distillation columns can be sent to a hot oil furnace. 

Figure 1. Simulation of an industrial ethylene epoxidation reactor tube.

The bottoms product from the isobutane separation is a mixture of tertiary butyl alcohol and tertiary butyl hydroperoxide. This mixture enters the epoxidation reactor where it reacts with propylene to form propylene oxide. The catalyst is either molybdenum based as in the process developed by Halcon and practiced by ARCO or TiOj on silica in the Shell process. 

The epoxidation reactor effluent is sent to a propylene separation column where unreacted propylene is distilled from the propylene oxide product. The unreacted propylene is recycled to the epoxidation reactor and the propylene oxide is sent for further separation and recovery of propylene glycol by-products. 

The bottoms from the propylene oxide distillation column contains tertiary butyl alcohol, high molecular weight organic by-products, and catalyst. The catalyst is recovered and returned to the epoxidation reactor. The TEA is separated from the organic by-products, dehydrated to isobutylene, and distilled to separate the isobutylene from water. The isobutylene is sent to storage. 

Selectivity of TBHP to both propylene oxide and rer/-butyl alcohol in the epoxidation reactor is 83%. The balance of the TBHP decomposes to hydrocarbon by-products and oxygen. For simplification, it is assumed that the decomposition is entirely to butane and oxygen. 

Combinatorial chemistry can perhaps help discover new catalyst formulations for reactions presently of particular interest, such as oxidations or ammoxidation, and generally all reactions of alkanes. Reactions traditionally made in different kinds of processes are frequently shown to be also activated by heterogeneous catalysts (e.g., epoxidations). Reactors of unexpected design allow surprisingly selective reactions (e.g., monoliths for the oxidative dehydrogenation of light alkanes). However, the distance often remains long between these discoveries and the manufacture of active and selective catalysts adequately structured for particular use in an industrial reactor inserted in an industrial plant. 

Scheme I shows a simplified block diagram illustrating the four main stq>s of a new route to propylene oxide production. In die first step, an alkylanthrahydroquinone, propylene and air react through a series of reactors producing propylene oxide, a minor amount of solvolysis products and water. Propylene oxide is separated by distillation and recovered. In die next step, methanol, propylene glycol and its methyl ether derivatives are extracted widi water and purified. The remaining organic phase passes to the alkylanthraquinone purification/hydrogenation step and finally is fed widi methanol, back to the epoxidation reactors. The regeneration and purification of the working solution are not shown in Scheme I.

In the peroxidation reactor ethylbenzene is converted with air at 146 °C and 2 bar to form a 12-14 wt% solution of ethylbenzene hydroperoxide in ethylbenzene. The reaction takes place in the liquid phase and conversion is limited to 10% for safety reasons. The reactor is a bubble tray reactor with nine separate reaction zones. To avoid decomposition of the formed peroxide the temperature is reduced from 146 °C to 132 °C over the trays. In the epoxidation reactor the reaction solution is mixed with a homogeneous molybdenum naphthenate catalyst. Epoxidation of propylene in the liquid phase is carried out at 100-130 °C and 1-35 bar. The crude product stream (containing PO, unreacted propylene, a-phenylethanol, acetophenone, and other impurities) is sent to the recycle column to remove propylene. The catalyst can be removed by an aqueous alkali wash and phase separation. The crude PO, obtained as head stream in the crude PO column, is purified by distillations. The unconverted reactant ethylbenzene can be recycled in the second recycle column. The bottom stream containing a-phenylethanol is sent to the dehydration reactor. The vapor-phase dehydration of a-phenylethanol to styrene takes place over a titanium/alumina oxide catalyst at 200-280 °C and 0.35 bar (conversion 85%, selectivity 95%). 

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