Recycle phenol hydrogenation

Figure 5.9 presents the flowsheet prior to heat integration. Fresh and recycled phenol is evaporated and mixed with hydrogen in the evaporator (Ev-1) at about 2 bar. The gas mixture enters the catalytic hydrogenation reactor (R-l). The inlet temperature should be kept strictly constant, in this case at 150°C, to avoid the... 

The impurities can be grouped into two categories lights (water, cyclohexene, cyclohexadiene) and heavies (phenol, dicyclohexyl-ether, cyclohexenyl- cyclohexanone). To limit their amount, the conversion is kept around 80% with a selectivity of about 98%. The hot reactor effluent is cooled in countercurrent with the feed in FEHE, and finally for phase separation in the heat exchanger (E-2) at 33 °C. The simple flash (S-2) can ensure a sharp split between hydrogen, recycled to hydrogenation reactor, and a liquid phase sent to separation. 

The unreacted cyclohexane is distilled off and recycled. The ca. 1 1 mixture of cyclohexanol and cyclohexanone is then subjected to dehydrogenation over a palladium catalyst (the same catalyst as is used in phenol hydrogenation) to give pure cyclohexanone. 

The yield of acetone from the cumene/phenol process is beUeved to average 94%. By-products include significant amounts of a-methylstyrene [98-83-9] and acetophenone [98-86-2] as well as small amounts of hydroxyacetone [116-09-6] and mesityl oxide [141-79-7]. By-product yields vary with the producer. The a-methylstyrene may be hydrogenated to cumene for recycle or recovered for monomer use. Yields of phenol and acetone decline by 3.5—5.5% when the a-methylstyrene is not recycled (21). 

The recovery area of the plant employs fractionation to recover and purify the phenol and acetone products. Also in this section the alpha-methylstyrene is recovered and may be hydrogenated back to cumene or recovered as AMS product. The hydrogenated AMS is recycled as feedstock to the reaction area. The overall yield for the cumene process is 96 mol %. Figure 1 is a simplified process diagram. 

Production of a-methylstyrene (AMS) from cumene by dehydrogenation was practiced commercially by Dow until 1977. It is now produced as a by-product in the production of phenol and acetone from cumene. Cumene is manufactured by alkylation of benzene with propylene. In the phenol—acetone process, cumene is oxidized in the Hquid phase thermally to cumene hydroperoxide. The hydroperoxide is spHt into phenol and acetone by a cleavage reaction catalyzed by sulfur dioxide. Up to 2% of the cumene is converted to a-methylstyrene. Phenol and acetone are large-volume chemicals and the supply of the by-product a-methylstyrene is weU in excess of its demand. Producers are forced to hydrogenate it back to cumene for recycle to the phenol—acetone plant. Estimated plant capacities of the U.S. producers of a-methylstyrene are Hsted in Table 13 (80). 

The oxidation of cyclohexane to a mixture of cyclohexanol and cyclohexanone, known as KA-od (ketone—alcohol, cyclohexanone—cyclohexanol cmde mixture), is used for most production (1). The earlier technology that used an oxidation catalyst such as cobalt naphthenate at 180—250°C at low conversions (2) has been improved. Cyclohexanol can be obtained through a boric acid-catalyzed cyclohexane oxidation at 140—180°C with up to 10% conversion (3). Unreacted cyclohexane is recycled and the product mixture is separated by vacuum distillation. The hydrogenation of phenol to a mixture of cyclohexanol and cyclohexanone is usually carried out at elevated temperatures and pressure ia either the Hquid (4) or ia the vapor phase (5) catalyzed by nickel. 

It has been proposed that the a-tocopheroxyl radical can be recycled back to tocopherol by ascorbate producing the ascorbyl radical (Packer etal., 1979 Scarpa et al., 1984). The location of a-tocopherol, with its phytyl tail in the membrane parallel to the fatty acyl chains of the phospholipids and its phenolic hydroxyl group at the memisrane-water interface near the polar headgroups of the phospholipid bilayer, enables ascorbate to donate hydrogen atoms to the tocopheroxyl radical. The suitability for ascorbate and tocopherol as chain-breaking antioxidants is exemplified (Buettner,... 

Microautoclave data was also obtained with Wilsonville Batch I solvent utilizing Indiana V coal. Batch I solvent was obtained from Wilsonville in mid-1977. Other batches of recycle solvent were received later. Batch I solvent had inspections most like the Allied 24CA Creosote Oil used for start-up at the Wilsonville Pilot Plant. Succeeding batches of solvent received by CCDC showed substantial differences, presumably due to equilibration at various operating conditions. As the Wilsonville solvent aged and became more coal derived, the solvent aromaticity decreased with an increase in such compounds as indan and related homologs. The decrease in aromaticity has also been verified by NMR. A later solvent (Batch III) also showed an increase in phenolic and a decrease in phenanthrene (anthracene) and hydrogenated phenanthrene (anthracene) type compounds. 

The alpha-methyl styrene can be recovered as a product or catalytically treated with hydrogen and converted back to cumene for recycling. The acetophenone has some commercial use in pharmaceuticals and at one time was used to make ethylbenzene. A high purity phenol is sometimes made by a crystallization step, since phenol freezes at about 109°F. With alpha-methyl styrene recycled, the ultimate yield is about 97%. 

A substantial amount of a-methylstyrene is produced during the cumene oxidation step in the production of phenol and acetone. Slurry processes applying Raney nickel and a fixed-bed operation with palladium developed by Engelhard326,341 are used to hydrogenate and recycle a-methylstyrene to produce more phenol and acetone. 

Vacuum distillation recovers the unreacted cumene and yields a-methylstyrene, which can be hydrogenated back to cumene and recycled. Further distillation separates phenol, boiling point 181°C, and acetophenone, boiling point 202°C. 

In the structure CS2 (Figure 5.24) the phenol fresh feed is raised to 165 kmol/h, while the hydrogen flow to reactor is kept constant at the value from the nominal steady state at 388kmol/h. This control structure does not work. Phenol accumulates in recycle because the hydrogen is insufficient for reaction, leading to plant upset. 

Figu re 5.24 CS2 Production rate by phenol fresh feed and fixed hydrogen recycle. At t = 2h, fresh phenol increases from 149.5 to 165kmol/h hydrogen on makeup. 

In CS3 (Figure 5.25), the hydrogen fresh feed is increased by about 9% from 348.5 to 380, while the recycle flow of phenol remains fixed to 220kmol/h. This control structure works well. Both the production of cyclohexanone and cyclohexa-nol is increased by about 4%, while phenol makeup increases with 8%. The purity of both products remains above 98%. A somewhat shorter transition time is obtained. The fact that hydrogen pushes the plant better than phenol is quite surprising, but it can be explained by the fact that there is no snowball effect on the gas-recycle side. 

Figure 5.25 CS3 Production rate by hydrogen fresh feed and fixed phenol recycle. At t = 2h, fresh hydrogen increases from 348.5 to 383kmol/h phenol on makeup.

The discovery of TS 1 led rapidly to the development of a process for phenol hydroxylation (25). This process has numerous advantages over the previous processes using peracid or Co2+, Fe2+ as catalysts higher conversion of phenol (30% instead of 5-9%) requiring less phenol separation/recycle steps, comparable or higher yields relative to both hydrogen peroxide and phenol, wider range of catechol/hydroquinone ratio (0.5-1.3 instead of 1.2-1.5 or 2.0-2.3) (24, 26). 

Phenol is recovered from the acetone finishing column bottoms (12) by extraction with caustic. AMS in the raffinate is then concentrated (13), hydrogenated (14) and recovered as cumene for recycle to oxidation. Refined AMS production is optional. 

The world production of phenol, of ca. 8.4 Mt/a, is mostly dependent on the cumene process. The yields and selectivities of the process are almost quantitative. However, the per-pass yield is relatively low (< 8.5%) and ca. 0.6 tonne of acetone is co-produced per 1 tonne of phenol. The hydrogenation-dehydration of acetone and its recycle has been considered but is not practised commercially. 

On completion, water is added to the mixture after which it is fractionated. Cyclohexane (b.p. 81°C) containing some benzene is collected from the top of the column, and after hydrogenation of the benzene, is recycled. The cyclo-hexanol-cyclohexanone mixture consists of approximately equal volumes of cyclohexanol (b.p. 161°C), cyclohexanone (b.p. 156°C), plus a mixture of several esters and ethers. It is collected from the bottom with 80+% yields on cyclohexane. An alternative route to cyclohexanol used by some plants is to catalytically hydrogenate phenol. 

This process is a variant of the PO/SM process that uses cumene instead of ethylbenzene and recycles the coproduct cumyl alcohol via dehydration to a-methyl-styrene and hydrogenation back to cumene (the latter two steps can be combined into a single hydrogenolysis step). Cumene hydroperoxidation technology is well-known for its use in phenol and acetone production. 

Acetone and phenol can be recovered after the neutralization of the acidic mixture from the cleavage reactor with sodium hydroxide or phenolate solution. The neutralized mixture is then subjected to a series of distillations. Acetone is first distilled, then cumene is recovered, together with a-methylstyrene, which is either purified and marketed or hydrogenated back to cumene and recycled to the oxidation. Phenol is finally distilled with a purity up to 99.99%, suitable for the production of polycarbonate grade bisphenol A and other chemicals and polymers. 

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