PROCESS FOR THE PURIFICATION OF PHENOL

Abstract: A stream of raw phenol, outcoming from the process of production of phenol and acetone from cumene, via cumene hydroperoxide, is purified by means of a combination of treatments of distillation and on silico-aluminates treated with acid.

Description: The present invention relates to a process for the purification of phenol obtained from the acidic splitting of cumene hydroperoxide. Processes are known in the art for the production of phenol by means of the oxidation of cumene into cumene hydroperoxide, followed by the splitting of so-obtained hydroperoxide, in the presence of acidic catalysts. The product of acidic splitting contains phenol and acetone, as main products, besides minor amount of various by-products. The fractional distillation of the splitting product allows producing phenol with purity satisfactory for various uses. However, in the production of such compounds as bisphenol, polycarbonate, nylon, and so forth, a phenol with very high purity characteristics, difficultly obtainable by means of distillation and rectification treatments, is required. The purity of phenol is generally determined by means of a test of colour development following nitration, chlorination or sulphonation, in that these tests are very sensible to the presence of chromogen impurities. A high purity phenol is that which does not practically develop a colour in these tests. Thus, the normal purification of phenol is carried out through sequential steps of fractional distillation, whilst the removal of colour-developing impurities is usually achieved by means of particular treatments. To that purpose, treatments have been proposed with acidic condensing agents (German Patent Appl. 2,211,616), acidic silico-aluminates used at high temperature, normally higher than 200°C (U.S. 3,454,653), alkalies and ferric chloride (U.K. 920,905), polyamines (U.S. 3,965,187), and hydrogen peroxide in alkaline medium (U.S. 2,971,893). All these treatments show one or more drawbacks, such as high cost, problems concerning the regeneration of the agent used in purification treatment, and problems connected with the introduction of foreign chemicals into the substance to be purified. Moreover, these treatment difficultly allow a complete or substantially complete removal of impurities from phenol. Also phenol purification processes based on distillation treatments have been proposed, such as disclosed, e.g., in U.K. Patent 1,021,759 and in European Patent Application publ. N. 4,168. Such distillation products however involve operating difficulties, in that they require a strict control of operating conditions; in any case, even if they allow a satisfactory removal to be achieved of particular carbonylic compounds such as hydroxyacetone and mesityl oxide, are not suitable to separate in a complete, or substantially complete way, all colour-producing substances. It has been discovered now that the drawbacks of the known art can be overcome, and that obtaining phenol free, or substantially free from chromogen substances is possible, by means of a simple and convenient process, which is essentially based on a combination of steps of distillation and of treatments with solid catalysts which display their activity within a range of relatively low temperatures. Accordingly, according to the process of the present invention: - a stream of raw phenol containing alpha-methyl-styrene, water, cumene, mesityl oxide, hydroxyacetone and other minor impurities, is fed to an intermediate point between the head and the bottom of a first distillation column, provided with reboiler, condenser and reflux, inside which a bottom stream, essentially consisting of phenol and alpha-methyl-styrene, and a head stream, essentially consisting of alpha-methyl-styrene, water, cumene, phenol and hydroxyacetone, is separated, said head stream being condensed with the formation of an aqueous phase and of an organic phase, the organic phase being partly recycled as reflux to the head of the column; - the bottom stream from the first column is fed into the nearby of the head of a second distillation column provided with reboiler, and free from condenser and from reflux, inside which a bottom stream essentially consisting of phenol, and a head stream essentially consisting of alpha-methyl-styrene and phenol is separated, said head stream being recycled directly in the nearby of the bottom of the first column; - the bottom stream from the second column is placed in contact with a natural silico-aluminate having a silica/alumina weight ratio equal to or greater than 3/1, previously treated with mineral acid; - the stream treated over natural silico-aluminate is fed into an intermediate point between the head and the bottom of a third distillation column, in which a stream of phenol with light impurities as the head product, a stream of phenol with heavy impurities as the bottom fraction, and a stream of purified phenol as side flow from same column is separated. Said purified phenol shows a total content of carbonylic compounds lower than 50 ppm, and is practically colourless at sulphonation test. According to the present invention the product of acidic splitting of cumene hydroperoxide, after the neutralization of catalyst, is submitted to distillation treatments to separate the light products, in particular acetone and a portion of hydrocarbons, such as cumene and alpha-methyl-styrene and the heavy products, such as acetophenone, alpha-methyl-styrene dimers and pitches. The stream is thus obtained of raw phenol, which is the feed to the process of the present invention, and which typically contains cumene within the range of from 0.1 to 1% by weight, alpha-methy-styrene within the range of from 2 to 10% by weight, water within the range of from 2 to 4% by weight, mesityl oxide within the range of 100 - 400 ppm, hydroxyacetone within the range of 100 - 2000 ppm and many other impurities among which phenylbutenes, aliphatic ketones with 9 carbon atoms in their molecule, and acetic acid. According to the present invention, such a stream of raw phenol is fed into an intermediate point between the head and the bottom of the first distillation column, provided with reboiler, condenser and reflux, and operating under a pressure lower than atmospheric pressure Typically, this first distillation column contains 40 trays, and the feed occurs between the 20th and the 30th tray. Moreover, it is typically run under a pressure measured at the head of the order of 120 mmHg, with a head temperature of the order of 100°C, with a bottom temperature of the order of 130°C, and with a reflux ratio of 25-30. In particular, the organic phase is refluxed, as it shall be illustrated hereinunder. By operating under these conditions, at the head of the column a vapour stream, essentially constituted by alpha-methyl-styrene (40-90% by weight), water (0,5-5% by weight), cumene, the most of hydroxycetone present in the feed, and variable amounts of other impurities and of phenol, in proper amounts due to the azeotropes with alpha-methyl-styrene and cumene, is separated. This vapour stream is condensed and separated into an aqueous phase (containing the most of hydroxyacetone, a little of phenol and other impurities) and into an organic phase (which is essentially constituted by alpha-methyl-styrene, cumene, phenol and traces of hydroxyacetone). The aqueous phase is removed from the system, and a portion of the organic phase constitutes the reflux to the distillation column, the balance is recovered. It should be noted that the presence of water has a beneficial function, in that hydroxyacetone and the other impurities are separated in the aqueous phase, thus avoiding on one hand the presence of hydroxyacetone in the reflux, and on the other hand, the presence of hydroxyacetone in the organic phase separated at the head, and intended for recycle. Thus, the stream of raw phenol fed to the first distillation column must contain an amount of water not lower than about 2% by weight, to achieve the above-described effects. Conveniently, the maximum concentration of water inside said stream of raw phenol does not exceed about 4% by weight, in that beyond such a value no appreciable advantages are obtained. This water can be present in the stream of raw phenol outcoming from previous distillation treatments, or it must be added to the same stream, before that this may be submitted to the process according to the present invention. By operating as previously described, at the bottom of the first distillation column a liquid stream is separated, essentially constituted by phenol and by alpha-methyl-styrene, with a content of alpha-methyl-styrene of from 30 to 50% by weight, and preferably at a value close to the high limit of said range, in that it allows a better separation of the impurities. The bottom stream from the first distillation column is fed into the nearby of the head of a second distillation column, provided with reboiler, but free from condenser and reflux. Typically, this second distillation column contains 90 trays, and the feeding takes place in correspondence of the 90th tray. This column is moreover typically run under a pressure measured at the head of the order of 340 mmHg, with a head temperature of the order of 137°C, and with a bottom temperature of the order of 175°C. By operating under these conditions, at the head of the second column a vapour stream of alpha-methyl-styrene and phenol, containing small amounts of impurities, is separated. This stream is directly recycled to the bottom of the first distillation column. Moreover, at the bottom of the second column a liquid stream essentially constituted by phenol (purity higher than about 99.5% by weight) and with a content of carbonylic compounds generally ranging from 200 to 400 ppm is separated. In this distillation, reducing the amount of carbonylic compounds, such as hydroxyacetone, to very low values, e.g., to values lower than about 50 - 70 ppm, is not necessary. The reduction of these carbonylic compounds under such levels can be indeed carried out, according to the present invention, in the subsequent treatments, whose function is of transforming and eliminating in a substantially complete way both the impurities of carbonylic character and non-carbonylic impurities. More particularly, according to the present invention, the phenol liquid stream separated at the bottom of the second distillation column is placed in contact with a natural silico-aluminate, having a silica/alumina weight ratio equal to or greater than 3/1, previously treated with mineral acid. Natural silico-aluminates suitable to that purpose are bentonites, in particular those showing a high content of montmorillonite. It is known that montmorillonite is a silico-aluminate of sodium, calcium and magnesium, whose general formula is as follows: [Al 1.67 Mg 0.33 Na 0.33 ]Si₄O₁₀ (OH)₂ The concentration of montmorillonite inside bentonites is generally ranging from 60 to 80% by weight, the balance being formed by such materials as silica, calcite and illite. By treating these silico-aluminates with mineral acids, such as sulphuric acid, hydrofluoric acid and hydrochloric acid, the alkaline metals are totally or partly removed and replaced by hydrogen, and the so-called "activated acidic earths" are obtained. To the purposes of the present invention, those natural silico-aluminates are useful, which show a silica/alumina weight ratio equal to or greater than 3/1, and which have been pre-treated with a mineral acid, up to remove the alkaline metals, or at least up to reduce them to a content lower than about 0.1% by weight. A product of this type is commercially available, under the trade name Filtrol G-24, from the firm Filtrol Co., of Los Angeles (USA). Thus, according to the present invention, the contact between the liquid stream of phenol outcoming from the second distillation column and the silico-aluminate having the characteristics as above described, is carried out at a temperature of from 120 to 200°C, from a time of from 10 minutes to 4 hours and preferably of from 0.5 to 2 hours, under a pressure of from atmospheric pressure to about 5 kg/cm². Conveniently, the silico-aluminate with granulometry of from 10 to 100 mesh, is given the form of a fixed bed in a reactor, at whose bottom the liquid stream of phenol to be processed is fed. The silico-aluminate having the characteristics as previously described, shows the advantage, compared to other silico-aluminates of the known art, of being active within a range of quite low temperatures. Relatively to the sulphonic resins, used to the same purpose, the silico-aluminates according to the present invention yield a series of advantages, such as the possibility of operating within such a temperature range as not to require the cooling of the phenol liquid stream outcoming from the second distillation column, the lower cost, the absence of any types of activation and regeneration steps, and the absence of acidic residued inside treated phenol. The treatment with silico-aluminate according to the present invention allows converting mesityl oxide, hydroxyacetone, and any other carbonylic of olephinic impurities present into lighter or heavier compounds, which can be easily separated from phenol in the subsequent rectification step. Hence, according to the present invention, the liquid stream of phenol treated over silico-aluminate is fed into a point intermediate between the head and the bottom of a distillation column provided with reboiler, condenser and reflux, typically containing 50 trays, the feeding taking place in correspondence of the 18th tray. This column is typically run under a pressure measured at the head of the order of 100 mmHg, with a head temperature of the order of 120°C, with a bottom temperature of the order of 159°C, and with a reflux ratio of the order of about 1. Under these conditions, at the head of the column a stream of phenol vapour with light impurities, and at the bottom a liquid stream of phenol with heavy impurities is separated. These two phenol streams are recycled. Purified phenol is drawn from the column as side stream (preferably in correspondence of the 45th tray). At the chemical analysis, this purified phenol shows a content of carbonylic compounds lower than 50 ppm, Moreover, this purified phenol is submitted to the following sulphonation test. To 20 ml of purified phenol, maintained under vacuum, at 45°C and under stirring for 10 minutes, 20 ml of high-purity concentrated (96%) sulphuric acid are added. The mixture is left standing for 4 minutes, the stirring being so adjusted as to prevent the formation of air bubbles. The vacuum is then withdrawn, and the mass is cooled for 5 minutes at 20°C. The measurement of transmittance at 532 mµ is immediately carried out, inside 10-mm cells, using demineralized water, whose transmittance is posed equal to 100, as the reference. Phenol purified according to the process according to the present invention shows transmittance values, at sulphonation test, similar to those of water. In the following experimental example, the parts and percentages are to be intended as being by weight, if not otherwise specified. Example Referring the figure of attached drawing table, to the distillation column 1, through the line 5, a liquid stream is fed of raw phenol (1570 parts/hour) heated at the temperature of about 70°C, and having the following composition: Column 1 is a 40-tray distillation column, provided with reboiler, condenser and reflux. The feeding (line 5) takes place in correspondence of the 26h tray of column 1, and the column is run under the following conditions: head pressure 120 mmHg, head temperature 100°C, bottom temperature 130°C and reflux ratio 29. Under these conditions, at the head of column 1 (line 6) 114 parts/hour are drawn of a distillate which is cooled in exchanger 7 and is separated, in decanter 8, in an organic phase containing cumene, alpha-methyl-styrene and phenol (71 parts) and in an aqueous phase (43 parts) containing 6.2% of hydroxyacetone, in addition to small amounts of phenol. The aqueous phase is recovered (line 9) and the organic phase, drawn through the line 10, is partly sent to column 1 as reflux (line 11), and the balance is recovered (line 12). By operating under the above indicated conditions, from the column a bottom product is recovered, having the following composition: This bottom product is sent (line 13) to the head tray of a distillation column 2, of 90 trays, provided with reboiler and operating under the following conditions: head pressure 340 mmHg, head temperature 137°C bottom temperature 175°C. Under these conditions, at the head of column 2 a distillate is drawn, which is directly sent (line 14) to the bottom of column 1, whilst at the bottom of column 2, a liquid stream (1456 parts/hour) is drawn (line 15), having the following compositions: This liquid stream is fed (line 15) to the bottom of reactor 3, packed with a silico-aluminate having the following composition: in the form of pellets of from 30 to 60 mesh and known under the trade name FILTROL G-24, available from Filtrol Co. of Los Angeles. Inside reactor 3, the process is carried out at 170°C, under a pressure of 4 kg/cm², and with a permanence time of about 1 hour, and a liquid stream is recovered (line 16), which is fed, in correspondence of the 18th tray, into the distillation column 4, equipped with 50 trays, provided with reboiler, condenser and reflux. Column 4 is run under a head pressure of 100 mmHg, with a head temperature of 120°C. with a bottom temperature of 159°C, and with a reflux ratio around 1. Under these conditions, at the head of the column (line 17) a phenol stream containing light impurities is separated, and at the bottom of the column (line 18) a stream of phenol containing heavy impurities is separated. Sideways to column 4, in correspondence of the 45th tray, a stream of purified phenol (960 parts/hour) is drawn (line 19). The streams in (17) and (18) are recycled to the process, the stream (19) is recovered and submitted to the analysis. The chemical analysis shows a content of carbonylic compounds lower than 50 ppm. The sulphonation test shows a value of transmittance at 532 mµ of 96.5. When operating in the way as previously indicated, but omitting the treatment over silico-aluminate in reactor 3, a stream is obtained of purified phenol (line 19) with a content of carbonylic compounds of about 300 ppm which, at the sulphonation test, shows a value of transmittance at 532 mµ of 87.5.

Claims: 1. Process for the purification of a stream of raw phenol outcoming from the process of production of phenol and acetone from cumene, via cumene hydroperoxide, wherein: - said stream of raw phenol containing alpha-methyl-styrene, water from 2% to 4% by weight, cumene, mesityl oxide, hydroxyacetone, other minor impurities, is fed to an intermediate point between the head and the bottom of a first distillation column provided with reboiler, condenser and reflux inside which a bottom stream, essentially consisting of phenol and alpha-methyl-styrene, and a head stream, essentially consisting of alpha-methyl-styrene, water, cumene, and hydroxyacetone, is separated, said head stream being condensed with the formation of an aqueous phase and of an organic phase, the organic phase being partly recycled as reflux to the head of the column; - the bottom stream from the first column is fed into the nearby of the head of a second distillation column provided with reboiler, and free from condenser and from reflux, in which a bottom stream essentially consisting of phenol, and a head stream essentially consisting of alpha-methyl-styrene and phenol is separated, said head stream being recycled directly into the nearby of the bottom of the first column; - the bottom stream from the second column is placed in contact with a natural silico-aluminate having a silica/alumina weight ratio equal to or greater than 3/1, previously treated with mineral acid, up to eliminate the content of alkaline metals, or to reduce said content to values lower than 0.1% by weight; - the stream treated with natural silico-aluminate is fed to an intermediate point between the head and the bottom of a third distillation column, in which a stream of phenol with light impurities as the head product, a stream of phenol with heavy impurities as the bottom fraction, and a stream of purified phenol as side flow from same column is separated. 2. Process according to claim 1, wherein raw phenol fed to the first column contains cumene within the range of from 0.1 to 1% by weight, alpha-methyl-styrene within the range of from 2 to 10% by weight, water within the range of from 2 to 4% by weight, mesityl oxide within the range of from 100 to 400 ppm, hydroxyacetone within the range of from 100 to 2000 ppm and other minor impurities. 3. Process according to claim 1, wherein the first distillation column contains 40 trays, the feeding of phenol occurs between the 20th and the 30th tray, and said column is run under a pressure measured at the head of the order of 120 mmHg, with a head temperature of the order of 100°C, with a bottom temperature of the order of 130°C, and with a reflux ratio of 25 - 30, and as bottom product a stream essentially constituted by phenol and alpha-methylstyrene, with a content of alpha-methyl-styrene of from 30 to 50% by weight is separated. 4. Process according to claim 1, wherein the second distillation column is run under a pressure measured at the head of the order of 340 mmHg, with a head temperature of the order of 137°C and with a bottom temperature of 175°C. 5. Process according to claim 1, wherein the treatment with natural silico-aluminate is run at a temperature comprised within the range of from 120 to 200°C, for a time of from 10 minutes to 4 hours, and under a pressure comprised within the range of from atmospheric pressure up to about 5 kg/cm². 6. Process according to claim 1, wherein the third column is run under a pressure measured at the head of the order of 100 mmHg, with a head temperature of the order of 120°C, with a bottom temperature of the order of 159°C, and with a reflux ratio of the order of about 1.

Inventors: BRUZZI, VITTORIO, LORENZONI, LORENO, MESSINA, GIUSEPPE, VOLTOLINI, MAURIZIO
Applicants: ENICHEM ANIC S.P.A.
Application year: 1986
Main classification: C07C37
IPC codes: C07C37, C07C39