PURIFICATION OF CUMENE

The present invention concerns a process and an apparatus for removing impurities from a cumene feed, for example for use in phenol production, hi particular, it concerns a process for the removal of impurities from a cumene feed which in part comprises cumene recycled from one or more stages of a phenol production process.

Phenol production is generally based on the oxidation of cumene to cumene hydroperoxide (CHP) and cumene is thus considered as the raw material in the production process. For achieving an efficient production process, the purity of the cumene raw material is essential. Typically, the cumene has the specification that it has to be over 99.9% pure.

The phenol production process contains process steps wherein phenol and acetone are produced through the oxidation of cumene to cumene hydroperoxide (CHP), the CHP is concentrated and cleaved into phenol, acetone and other cleavage products, the cleavage products are neutralized, washed and desalted and the desired products finally separated and purified. In this production process, some secondary products, by-products and impurities are formed, thus lowering the yields of the desired products, particularly if these impurities follow the portion of unreacted cumene being recycled to the oxidation feed. Impurities and by-products are mainly formed during the oxidation and the cleavage. The exact identities and amounts of these impurities depend on the conditions used in the process, but some possible impurities and by-products are listed below:

Formed during oxidation: Formaldehyde Formic acid

Methanol Acetophenone

Dimethyl phenylcarbinol (DMPC) Methylhydroperoxide Dicumylperoxide

Other dicymylcompounds Phenol

Ethylbenzenehydroperoxide Propylbenzenehydroperoxide

Cyclohexanol α-Methylstyrene (AMS) Benzoic acids

Formed during cleavage and the AMS hydrogenation: α-Methylstyrene (AMS) Phenyl cumyl ether Mesityl oxide Hydroxyacetone (HA) 2-Methylbenzofuran

Further derivatives of phenol

3 -Methyl cyclopentanone (3 -MCP)

Isopropylcyclohexane (IPCH)

The main impurities formed in the phenol production process are dimethyl phenyl carbinol (DMPC) and acetophenone. The DMPC is formed in large amounts, but it can be effectively converted into cumene and, thus, does not cause a significant problem. The acetophenone, on the other hand, forms an azeotrope with phenol and needs to be removed from the process, but it is unlikely to follow the recycled cumene to the oxidation feed.

Small quantities of impurities may get into the cumene oxidation phase with the cumene feed, particularly if the feed or a portion of the feed consists of recycled cumene from the other steps of the phenol production process. Unreacted cumene can be recovered from the oxidation reaction and/or following cleavage of CHP.

Some of these impurity components react with the oxygen in the air and thus increase the number of unwanted side reactions, thereby lowering the efficiency of the main reaction. The components that slow down cumene oxidation to CHP are for example phenol, α- methyl styrene (AMS) and salts.

Other components may cause undesired premature cleavage of the CHP, i.e., cleavage reactions occurring before the actual (i.e. desired) cleavage step or cleavage reactions resulting in other products than the desired ones. Among the mentioned components are

acidic components, such as the mentioned phenol and formic acid. The latter is formed by reaction of acetophenone into methanol which reacts further into formic acid.

A high concentration of formic acid and other acidic compounds in the cumene feed causes a decrease of the pH in oxidation and therefore early cleavage of CHP into phenol and acetone. Phenol, in addition to causing early cleavage, slows down or inhibits the cumene oxidation process even in low concentrations. If the phenol concentration in the oxidation reaction mixture is high, an increased temperature and an increased reaction volume are required for the CHP production. The increased temperature, on the other hand, causes a decrease in the selectivity of the reaction.

An extremely harmful impurity in the cumene oxidation is sulfur, which stops the formation of CHP. In the reaction mechanism the sulfur has two effects:

• sulfur reacts with free radicals, therefore preventing the formation of new radicals, and

• sulfur gets oxidized into sulfur oxide and from thereon into sulfuric acid, which causes early CHP decomposition into phenol and acetone.

Organic sulfur solutions, such as thiophene, have the same effect on CHP formation as sulfur.

As can be expected with recycled solutions, recycled cumene contains impurities, such as the ones mentioned above. Many of these impurities can be removed by a caustic wash, as described in US 5,120,902, US 5,220,103 and US 5,908,962. Amongst other things, this caustic wash has the benefit of neutralizing any acids, which may slow down the oxidation reaction or reduce the selectivity and the yield of the oxidation. However, caustic residues may also be harmful, particularly when the cumene is used in a dry oxidation procedure. For example, caustic components accelerate CHP decomposition into acetophenone and carbinol, further causing cumene losses. Carry-over of caustic components to the cleavage phase of the phenol production process may also neutralize the acid used to catalyze the cleavage reaction. Further, any remains of salts formed through the reaction of the alkali used in the caustic or alkaline wash with the acidic by-products and impurities of the cumene feed slowly contaminate the heat exchanger surfaces of the oxidation reactors, such that the reactors will require periodic purification.

Thus, it can clearly be seen that there is a need for an alternative method of decreasing the amount of impurities in the cumene oxidation feed, in particular in a cumene feed which comprises cumene recycled from one or more stages of a phenol production process, in particular from the oxidation of cumene. Such recycled cumene contains phenol in addition to other impurities.

The present invention provides a process for removing impurities from a cumene feed. Particularly, the present invention provides a process for removing by-products and impurities from a cumene feed, which slow down or inhibit the oxidation of cumene to CHP or accelerate the cleavage of the CHP.

The present invention concerns a process for removing impurities from cumene (e.g. for use in phenol production), which process comprises providing a cumene feed containing cumene as well as phenol and one or more water-soluble components, mixing the cumene feed with an aqueous alkaline solution which contains at least one alkaline compound to form a first aqueous cumene mixture, and separating the aqueous solution from the first aqueous cumene mixture to provide a first purified cumene stream. Following separation of the aqueous solution, the cumene-containing stream is further subjected to a water wash whereby to further reduce the amount of any remaining impurities and any residual alkaline compound(s). Separation of the washed cumene stream from the aqueous phase provides a cumene feed which is particularly suitable for use in a dry oxidation procedure.

The present invention also concerns an assembly adapted for use in carrying out the process herein described, e.g. for purifying a liquid stream of cumene.

Considerable advantages are obtained by means of the invention. Thus, the present invention provides an efficient process for removing impurities as well as excess alkaline washing solution and salts from a cumene feed. When used in the production of phenol, this will give the cumene process an improved selectivity and yield as well as cause the fouling to decrease in the cumene oxidation reactors and the subsequent phase of CHP concentration. The invention will also cause a decrease in the risk of alkaline carryover to the phase of CHP cleavage, thus decreasing the risk of neutralization of the cleavage catalyst. Further, the invention provides the possibility to use a more concentrated alkaline

solution in the first step of cumene purification, since excess solution may be effectively washed away.

Next, the invention will be described more closely with reference to the attached Figure 1 which is a schematic drawing of the purification assembly of an embodiment of the present invention.

The present invention concerns a process for removing impurities from cumene, which preferably is to be used in phenol production. The process comprises providing a cumene feed containing cumene as well as water-soluble components, such as alcohols and acidic components, i.e., components that in solution have a pH below 7, mixing the cumene feed with an aqueous alkaline solution, i.e. a solution containing at least one alkaline component and having a pH of 8-14, preferably 10-14 to form a first aqueous cumene mixture, and separating the aqueous solution from the first aqueous cumene mixture to provide a first purified cumene stream.

Particularly, the present invention concerns a cumene purification process comprising mixing the first purified cumene stream with water to form a second aqueous cumene mixture, separating the water with water-soluble components from the second aqueous cumene mixture to provide a second purified cumene stream, and recovering the second purified cumene stream.

The term "water-soluble component", as used in the context of the present invention, is intended to mean a component which substantially dissolves in excess water to form a solution, losing its particulate form and which becomes essentially molecularly dispersed throughout the aqueous solution. More particularly, a "water-soluble component" is one that forms a clear mixture when added to water in an amount of up to 1 g/100 ml, which mixture will not undergo phase separation upon standing for up to 10 h at room temperature.

The water-soluble components that may be present in the cumene feed used in the present invention include one or more components, such as phenol, aldehydes, organic acids and alcohols. Common water-soluble components of such feeds are formic acid, methanol, cyclohexanol and formaldehyde.

The process of the present invention preferably takes place in a purification assembly (Fig. 1), which consists of at least two washing units (I, II), which are in fluid connection with each other, comprising: 1 alkaline wash drum

2 water wash drum.

Preferably, the assembly further comprises:

3 cumene inlet 4 alkaline inlet

5 water inlet

6 alkaline recycle outlet

7 outlet for first purified cumene stream

8 inlet for first purified cumene stream 9 cumene outlet

10 water outlet

11 alkaline discharge

12 water recycle line 13, 14 static mixers 15 water discharge (optional)

Thus, the purification assembly of the present invention comprises a first washing unit (I) for washing the cumene stream with an alkaline solution, said washing unit (I) comprising a first wash drum 1 being equipped with an inlet 3 for the cumene stream, an inlet 4 for the alkaline solution, an outlet 7 for the cumene stream washed with the alkaline solution and an alkaline recycle outlet 6, and a second washing unit (II) for washing with water the cumene stream obtained from the first washing unit (I), said second washing unit (II) being arranged in a cascade with (i.e. downstream from) the first washing unit (I) and comprising a second wash drum 2 equipped with an inlet 8 for the cumene stream washed with the alkaline solution connected to the outlet 7 of the first wash drum 1, an inlet 5 for water, an outlet 9 for the second purified cumene stream and a water outlet 10, said inlet 3 of the first wash drum 1 being connected to a source of recycled cumene.

Preferably, a portion of the alkaline solution being conducted out of the first wash drum 1 through the alkaline outlet 6 is recycled back to the drum 1 through the alkaline inlet 4, whereas another portion of the alkaline solution is removed through an alkaline discharge line 11. Likewise, a portion of the aqueous solution conducted out of the second wash drum 2 through the water outlet 10 may be recycled back to the first drum 1 through a water recycle line 12. Optionally a portion of water recycle can be discharged through a water discharge line 15.

In order to facilitate mixing of the solutions, static mixers 13, 14 may be provided in the feedlines for the cumene stream and the first purified cumene stream.

According to a preferred embodiment of the present invention, the first wash drum 1 is an alkaline wash drum 1, and the second wash drum 2 is a water wash drum 2.

The process and the assembly of the present invention may be used to purify any organic compound or a mixture of compounds from undesired by-products. Preferably, the organic solution is a solution containing cumene and by-products, such as phenol and one or more other essentially water-soluble components, such as alcohols and acidic components. More preferably, the assembly of the invention is arranged in a cumene circulation line of a phenol production process (i.e. the cumene feed comprises cumene recycled from one or more stages of such a process). The phenol production process typically comprises process steps, wherein phenol and acetone are produced through the oxidation of cumene to cumene hydroperoxide (CHP) and the subsequent concentration of the CHP, cleavage of the CHP into phenol, acetone and other cleavage products, neutralizing, washing and desalting of the cleavage products and finally separation of the acetone from the phenol and purification of both desired products. The cumene feed which is treated in accordance with the invention may comprise unreacted cumene recovered from the oxidation phase or from the cleavage phase of the phenol production process. Preferably, this will comprise cumene recycled from the oxidation phase, e.g. that recovered following concentration of CHP.

According to the invention, at least a portion of the cumene feed to the oxidation is washed, preferably in two stages, to ensure a sufficient grade of purity. Preferably, the cumene feed is not subjected to any additional purification steps (e.g. treatment with acid)

prior to carrying out the process steps herein described. It is also preferred that no post- treatment steps (e.g. distillation) are carried out before the purified cumene feed is subjected to oxidation in a phenol production process.

Subsequently, the cumene may be subjected to oxidation, such as dry oxidation, using air or concentrated oxygen as the mentioned air feed. Where concentrated oxygen is used this is a gaseous mixture containing up to 100% oxygen, preferably about 22-80% oxygen, the rest being mostly inert gases.

Particularly preferably, the treated cumene is used in an oxidation process (e.g. a dry oxidation process) in which it forms a portion (preferably up to 90%, more preferably from 70 to 90%, e.g. from 75 to 85%) of the cumene feed. The remainder of the feed may comprise fresh cumene, preferably fresh cumene which has also been subjected to a purification process as herein described. By "dry oxidation" it is intended that all components used in the oxidation of cumene (i.e. cumene and air) should be substantially free from water droplets (i.e. free water) and that no additional process water is added during the oxidation procedure, i.e. no water-emulsion is produced. Where any water is present during a dry oxidation process, this will generally be at a level below saturation point. A particular feature of a dry oxidation process is the absence of any purification stage between cumene oxidation and concentration of CHP.

According to a preferred aspect of the present invention, the CHP formed during the oxidation is further processed, e.g. by increasing its concentration in a series of concentration steps, and subjecting it to a cleavage process. Recycled cumene from the concentration steps is preferably conducted to the cumene wash of the present invention.

According to this preferred aspect, the obtained cleavage product mixture, which contains, for example, phenol, acetone, water, cumene, AMS, hydroxyacetone, mesityl oxide, acetophenone, carbinol and heavy hydrocarbons, i.e., components that remain in liquid form during a distillation, is conducted further to a distillation section, wherein it is distilled.

In order to feed only cumene that has a sufficient quality to the oxidation phase, the oxidation inhibitors and other impurities mentioned above, such as phenol and water-

soluble components, such as alcohols and acidic components, which may develop in the phenol production process and be carried over with the recycled cumene to the cumene feed are removed with an alkaline wash and a water wash. Preferably, a cumene feed having a maximum concentration of phenol of less than lppm, is achieved. When utilizing the cumene feed in a dry oxidation procedure, the purification of the recycled cumene is even more important compared to utilizing a wet oxidation procedure, since the dry process lacks the further purification stage present in the wet process, namely a purification stage between the oxidation stage and the concentration stage, where the by-products and impurities that are harmful specifically for the concentration or the cleavage stages may be separated from the streams.

Thus, according to the present invention, phenol production process liquid flows that are returned to the cumene feed in order to be reused are first subjected to a purification process.

In a first step of the present process, the cumene feed is subjected to a washing step, i.e. an alkaline treatment, in a first washing unit (I) using an aqueous alkaline solution, i.e. a solution containing at least one alkaline component and having a pH of 8-14, preferably 10-14. By mixing the feed with the alkaline solution, a first aqueous cumene mixture is produced. Subsequently, after this alkaline treatment, an aqueous phase containing a majority, preferably about 50-90%, more preferably about 75-90%, of the acidic components, such as phenol, formic acid and sulfuric compounds, and of the alkaline compounds, such as components of the alkaline washing solution, is separated from the cumene mixture to produce a first purified cumene stream.

The first purified cumene stream still contains residues of water-soluble components, such as alcohols as well as remaining acidic components and alkaline compounds, and is, as a second step of the process, introduced into a second washing unit (II), where it is contacted with water in order to produce a second aqueous cumene mixture. The amount of water is sufficient essentially to dissolve all the mentioned remaining residues and the desired amount can readily be determined by those skilled in the art. Subsequently, an aqueous phase containing essentially all said residues is separated from the cumene mixture so as to give a second purified cumene stream, which is essentially free from acidic components and alkaline compounds. Typically, the amount of any acidic and alkaline components in

the second purified cumene stream will be below the limits of detection. Separation of the aqueous phase provides a cumene feed (second purified cumene stream) which is suitable for recycling purposes.

The water used in the second washing step should preferably be essentially free from acidic and alkaline components. Preferably, the water is essentially free from chlorides, particularly having a chloride concentration of less than 10 ppm, preferably less than 0.1 ppm. More preferably, the water is condensed water.

The cumene feed and the aqueous alkaline solution are contacted in a first washing unit (I) and the first purified cumene stream is recovered as an effluent of the first washing unit (I). Likewise, the first purified cumene stream and the water are contacted in a second washing unit (II) and the second purified cumene stream is recovered as an effluent of the second washing unit (II).

According to a preferred embodiment of the invention, the cumene feed and the aqueous alkaline solution are mixed in a feed line connected to the first wash drum 1 , whereby the cumene feed and the aqueous alkaline solution are contacted with each other both in the feed line and in the first wash drum 1. Likewise, according to the preferred embodiment, the first purified cumene stream and the water are mixed in a feed line connected to the second wash drum 2, whereby the first purified cumene stream and the water are contacted with each other both in the feed line and in the second wash drum 2.

The washed cumene is conducted to a feed tank, which typically has a large volume compared to the drums 1 , 2 of the washing units (I, II), from the purification assembly and it may possibly contain remains of water carried over from the second washing unit (II). In the feed tank the water phase may separate, but it can then be removed from the bottom of the feed tank.

According to an embodiment of the present invention, fresh cumene, i.e. cumene essentially not containing water-soluble or acidic or caustic components, can be fed straight to the cumene feed tank without further purification. The fresh cumene feed can alternatively be washed in the two-stage process of the present invention, if needed, to remove harmful impurities.

According to a preferred embodiment, the cumene feed at least partially comprises cumene, which is recycled from one or more of the stages of a phenol production process.

Particularly in startup situations, the fresh cumene is fed together with the recycled cumene through the purification assembly to the cumene feed tank. This procedure is carried out to remove acidic compounds, which may form during the transportation of cumene to the phenol production facilities. Thus, the cumene feed comprises a mixture of fresh cumene and recycled cumene from one or more of the stages of a phenol production process.

The aqueous alkaline solution used in the process of the invention is preferably a sodium hydroxide (NaOH) solution, which preferably has a concentration of 0.1-8%, more preferably 2-5%. Thus, it is ensured that most of the organic acids and the phenol are removed from the cumene feed, which is to be conducted to the oxidation reactors. The aqueous alkaline solution may also contain other alkaline components, e.g. sodium carbonate. Subsequently, the remains of the NaOH solution or any other alkalines and the other water-soluble components are removed from the feed in a washing step using water as the washing solution.

The temperatures of the streams conducted through the washing units (I, II) of the present invention are preferably maintained at a level of 20-90°C, preferably about 30-50°C. The pressure in the washing units (I, II) is typically maintained close to atmospheric pressure, alternatively at a slight over-pressure, i.e., a pressure slightly above atmospheric pressure, preferably about 105-30OkPa.

The following non-limiting Example is intended to further illustrate the invention:

Example

100 kg/h of unreacted cumene recycled from the CHP concentration and oxidation reactor off-gas condensation stage of a phenol production process are washed in a washing unit with 1.0 kg/h of 2.0% NaOH solution at a temperature of 5O ⁰ C. The pressure in the washing unit is maintained at atmospheric pressure.

The resulting solution is separated to remove the aqueous phase. The organic phase is subsequently washed with 0.04 kg/h of condensed water in a second washing unit to remove residual NaOH solution at a temperature of 50 ⁰ C.

Following separation of the aqueous phase a purified cumene stream is obtained. This is mixed with fresh cumene (i.e. cumene added from a source external to the system) in a ratio of 4:1 (recycled cumene: fresh cumene) and used in a dry oxidation procedure.