Process and apparatus for producing dihydric phenolic compound

A dihydric phenolic compound is produced, with a high selectivity thereof, by a plurality of oxidation reactors connected to each other in series, by (1) feeding a monohydric phenolic compound with a temperature of 30 to 100.degree. C., a peroxide compound, a catalyst and optionally a ketone compound into a first reactor to oxidize the monohydric phenolic compound, and delivering a resultant reaction mixture containing the produced dihydric phenolic compound and non-reacted monohydric phenolic compound from the first reactor; (2) passing the reaction mixture through one or more reactors succeeding to the first reactor, to further oxidize the monohydric phenolic compound, while, in the steps (1) and (2), a portion of the peroxide compound is fed into the first reactor and the remaining portion of the peroxide compound is fed into at least one succeeding reactor; and (3) delivering a first reaction mixture produced in a rearend reactor and comprising the produced dihydric phenolic compound, the non-reacted monohydric phenolic compound and peroxide compound and the catalyst from the rearend reactor.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a process and apparatus for producing a
 dihydric phenolic compound. More particularly, the present invention
 relates to a process and apparatus for producing a dihydric phenolic
 compound at a high stability and safety, at a high selectivity and with a
 high efficiency.
 Still more particularly, the present invention relates to a process and
 apparatus for producing a dihydric phenolic compound, for example,
 catechol or hydroquinone, from a monohydric phenolic compound at an
 excellent selectivity at a high stability, while containing the reaction
 liquid temperature of the monohydric phenolic compound in an appropriate
 range, which process and apparatus are suitable for industrial practice.
 2. Description of the Related Art
 Processes for producing a dihydric phenolic compound, for example, catechol
 together with hydroquinone by an oxidation reaction of monohydric phenolic
 compounds with peroxide compounds, for example, hydrogen peroxide and
 ketone peroxide, in the presence of specific catalysts are well known
 from, for example, Japanese Examined Patent Publications No. 52-38,546
 (corresponding to U.S. Pat. No. 4,078,006), No. 52-38,547 (corresponding
 to U.S. Pat. No. 4,072,722), No. 52-38,548 and No. 52-38,549.
 In the conventional processes for producing dihydric phenolic compounds,
 however, an industrial means for industrially and easily controlling the
 rapid temperature rise in the reaction liquid for the oxidation reaction
 of the monohydric phenolic compound due to the heat generated by reaction
 has not been concretely known. Therefore, the conventional processes are
 disadvantageous in that not only an explosion of ketoneperoxide contained
 in the reaction liquid cannot be surely prevented, but also the
 selectivity of the target dihydric phenolic compound significantly
 decreases due to the rapid rise of the reaction liquid temperature, and
 the yield of the dihydric phenolic compound decreases. Accordingly, when a
 high safety of the reaction procedure and a high selectivity of the target
 dihydric phenolic compound are required above all, the reaction procedure
 must be carried out at a low conversion of the monohydric phenolic
 compound.
 SUMMARY OF THE INVENTION
 An object of the present invention is to provide a process and apparatus
 for providing dihydric phenolic compounds by a continuous oxidation
 reaction of a monohydric phenolic compound, at enhanced selectivity and
 yield of the target dihydric phenolic compound, while preventing a rapid
 rise of the reaction liquid temperature due to a generation of heat of
 reaction and maintaining a conversion of the monohydric phenolic compound
 at a satisfactory level, which process and apparatus are suitable for
 industrial continuous production of the dihydric phenolic compounds.
 The above-mentioned object can be attained by the process and apparatus of
 the present invention.
 The process of the present invention for producing dihydric phenolic
 compound comprises oxidizing a monohydric phenolic compound in the
 presence of a catalyst by using a continuous multi-stage oxidation
 apparatus comprising a plurality of oxidation reactors connected to each
 other in series in such a manner that (1) in a first reactor of the
 oxidation apparatus, a monohydric phenolic compound having a temperature
 of 30 to 100.degree. C., a peroxide compound and a catalyst are fed
 thereinto, to oxidize the monohydric phenolic compound into a dihydric
 phenolic compound, and the resultant reaction mixture containing the
 produced dihydric phenolic compounds, the non-reacted monohydric compound,
 the non-reacted peroxide compound and the catalyst is delivered from the
 first reactor; (2) the reaction mixture delivered from the first reactor
 is passed through one or more reactors succeeding to the first reactor, to
 further oxidize the non-reacted monohydric phenolic compound, while, in
 the steps (1) and (2), a portion of the peroxide compound is fed into the
 first reactor and the remaining portion of the peroxide compound is fed
 into at least one succeeding reactor; and (3) a final reaction mixture
 produced in a rearend reactor and comprising the produced dihydric
 phenolic compound, the non-reacted monohydric phenolic compound, the
 non-reacted peroxide compound and the catalyst is delivered from the
 oxidation apparatus.
 In the dihydric phenolic compound-producing process of the present
 invention, optionally, at least one ketone compound is fed together with
 the monohydric phenolic compound, the peroxide compound and the catalyst
 into the first reactor.
 In the dihydric phenolic compound-producing process of the present
 invention optionally, before the remaining portion of the peroxide
 compound is fed into at least one reactor succeeding to the first reactor,
 the reaction mixture delivered from a next reactor located in front of the
 succeeding reactor to which the remaining portion of the peroxide compound
 is fed is cooled to a temperature of 40 to 100.degree. C. by a cooling
 means, and then is fed, together with the remaining portion of the
 peroxide compound, into the succeeding reactor, and the temperature of the
 reaction mixture passing through the oxidation apparatus is controlled to
 a level of 40 to 120.degree. C.
 The continuous multi-stage oxidation apparatus of the present invention for
 producing dihydric phenolic compounds by the process of the present
 invention as defined above, comprises a first reactor and one or more
 reactors succeeding the first reactor which reactors are connected to each
 other in series, and are each suitable for catalytically oxidizing a
 monohydric phenolic compound into dihydric phenolic compounds, wherein the
 first reactor is connected to a feed source of a monohydric phenolic
 compound, a feed source of a peroxide compound and a feed source of a
 catalyst, at least one succeeding reactor is connected to a feed source of
 the peroxide compound, and a rearend reactor has an outlet for delivering
 a final reaction mixture produced therein.
 In the dihydric phenolic compound-producing apparatus of the present
 invention, optionally, the first reactor is further connected to a feed
 source of a ketone compound.
 The dihydric phenolic compound-producing apparatus of the present invention
 optionally further comprises a cooling means arranged between the
 succeeding reactor connected to the peroxide compound-feed source and a
 next reactor located in front of the succeeding reactor connected to the
 peroxide compound-feed source.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In the process of the present invention for producing dihydric phenolic
 compounds by oxidizing a monohydric phenolic compound in the presence of a
 catalyst, a continuous multi-stage oxidation apparatus comprising a
 plurality of oxidation reactors connected to each other in series is used.
 The continuous multi-stage oxidation apparatus preferably comprises 2 to 10
 reactors, more preferably 2 to 6 reactors, still more preferably 3 to 5
 reactors, connected to each other in series.
 In the process of the present invention,
 (1) in a first reactor of the oxidation apparatus, a monohydric phenolic
 compound having a temperature of 30 to 100.degree. C., a peroxide compound
 and a catalyst are fed thereinto, to oxidize the monohydric phenolic
 compound into dihydric phenolic compounds, and the resultant reaction
 mixture containing the produced dihydric phenolic compounds, the
 non-reacted monohydric compound, the non-reacted peroxide compound and the
 catalyst is delivered from the first reactor;
 (2) the reaction mixture delivered from the first reactor is passed through
 one or more reactors succeeding to the first reactor, to further oxidize
 the non-reacted monohydric phenolic compound, while in the steps (1) and
 (2), a portion of the peroxide compound being fed into the first reactor
 and the remaining portion of the peroxide compound being fed into at least
 one succeeding reactor;
 (3) a final reaction mixture produced in a rearend reactor and comprising
 the produced dihydric phenolic compounds, the non-reacted monohydric
 phenolic compound, the non-reacted peroxide compound and the catalyst is
 delivered from the oxidation apparatus.
 The process and apparatus of the present invention will be explained in
 detail below, while referring to the attached drawings.
 Referring to FIG. 1, three oxidation reactors 1, 10 and 20 are successively
 connected to each other in series. Into a lower portion of the first
 reactor 1, a monohydric phenolic compound, for example, phenol, having a
 temperature of 30 to 100.degree. C., preferably 40 to 95.degree. C. is fed
 from a supply source thereof (not shown) through a feed line 2, a peroxide
 compound, for example, a portion of hydrogen peroxide, is fed from a
 supply source thereof (not shown) through a feed line 3, a catalyst is fed
 from a supply source thereof (not shown) through a feed line 4, and
 optionally, a ketone compound is fed from a supply source thereof (not
 shown) through a feed line 5. In the first reactor 1, the monohydric
 phenolic compound is catalytically oxidized with the peroxide compound to
 produce dihydric phenolic compounds as target products. The feed lines 2,
 3, 4 and 5 are connected, at locations close to each other, to the lower
 portion of the first reactor 1. The resultant reaction mixture produced in
 the first reactor 1 and containing the produced dihydric phenolic
 compounds, the non-reacted monohydric phenolic compound, the non-reacted
 peroxide compound, the catalyst and optionally the non-reacted ketone
 compound is delivered from the top portion of the first reactor 1 through
 a delivery line 7, is cooled by a cooler 6, for example, a heat-exchanger
 type cooler, to a temperature of 40 to 100.degree. C., and then the cooled
 reaction mixture is fed into a lower portion of a second reactor 10
 through a feed line 12. Into the lower portion of the second reactor 10,
 another portion of the peroxide compound is fed from a supply source
 thereof (not shown) through a feed line 13 and optionally an additional
 amount of the catalyst is fed from a supply source thereof (not shown)
 through a feed line 14, to further oxidize the monohydric phenolic
 compound into the dihydric phenolic compounds.
 The feed lines 12, 13 and 14 are connected at locations close to each
 other, to the lower portion of the second reactor 10.
 The resultant reaction mixture produced in the second reactor 10 and
 containing the produced dihydric phenolic compound the non-reacted
 monohydric phenolic compound, the non-reacted peroxide compound, the
 catalyst and optionally the non-reacted ketone compound is delivered from
 the top portion of the second reactor 10 through a delivery line 17, is
 cooled by a cooler 16 to a temperature of 40 to 100.degree. C., and then
 the cooled reaction mixture is fed into a lower portion of a third reactor
 20 through a feed line 22. Into the lower portion of the third reactor 20,
 the remaining portion of the peroxide compound is fed from a supply source
 thereof (not shown) through a feed line 23 and optionally a further
 additional amount of the catalyst is fed from a supply source thereof (not
 shown) through a feed line 24, to further oxidize the monohydric phenolic
 compound into the dihydric phenolic compounds.
 The feed lines 22, 23 and 24 are connected at locations close to each
 other, to the lower portion of the third reactor 20.
 The reaction mixture produced in the third reactor 20 and containing the
 target dihydric phenolic compound, the catalyst and the non-reacted
 monohydric phenolic compound, peroxide compound and optionally ketone
 compound (which may not be contained) is delivered from the top portion of
 the third reactor 20 through a delivery line 27. The delivered reaction
 mixture is cooled or heated to a desired temperature by a heat-exchanger
 25 and then withdrawn from the continuous oxidation apparatus through a
 withdrawal line 28.
 When the peroxide compound is fed into at least one of the reactors
 succeeding to the first reactor, the reaction mixture delivered from a
 next reactor located in front of the succeeding reactor into which the
 peroxide compound is supplemented is preferably cooled by a cooler to a
 temperature of 40 to 100.degree. C., more preferably 50 to 100.degree. C.
 and then fed into the succeeding reactor. By feeding the peroxide compound
 into two or more reactors, the monohydric phenolic compound can be
 converted to dihydric phenolic compounds with a high selectivity of the
 dihydric phenolic compounds.
 In each of the oxidation reactors, the reaction mixture flows upward and
 the temperature of the reaction mixture is preferably controlled to 40 to
 120.degree. C., more preferably 55 to 115.degree. C.
 In the process and apparatus of the present invention, the oxidation
 reaction apparatus preferably includes 2 to 10 reactors, more preferably 2
 to 6 reactors, still more preferably 3 to 4 reactors, connected to each
 other in series.
 In this oxidation reaction apparatus, the peroxide compound, for example,
 hydrogen peroxide is dividedly fed into the first reactor and to at least
 one of the succeeding reactors, preferably 1 to 8, more preferably 1 to 5,
 still more preferably 2 to 3 of the succeeding reactors.
 In the process of the present invention, the catalyst may be fed in a whole
 amount into the first reactor. Alternatively a portion of the catalyst may
 be fed into the first reactor and the remaining portion of the catalyst
 may be fed into one or more of the succeeding reactors. Preferably, the
 catalyst is dividedly fed into the first reactor and to at least one of
 the succeeding reactors into which a portion of the peroxide compound is
 fed. For example, in the multi-stage oxidation apparatus as shown in FIG.
 1, a portion of the total amount of the peroxide compound is fed into the
 first reactor 1 through the feed line 3, and the remaining portion of the
 peroxide compound is fed dividually into the second and third reactors
 through the feed lines 13 and 23, respectively.
 Also, a portion of the total amount of the catalyst is fed into the first
 reactor 1 through the feed line 4, and the remaining portion of the
 catalyst is fed dividedly fed into the second and third reactors 10 and 20
 through the feed lines 14 and 24, respectively.
 In the process of the present invention, the production reaction for the
 dihydric phenolic compounds, namely the oxidation reaction of the
 monohydric phenolic compound, in the oxidation reaction apparatus is
 preferably carried out to such an extent that the monohydric phenolic
 compound is reacted at a low conversion of 0.1 to 10 molar %, more
 preferably 0.3 to 6 molar %, and the peroxide compound, particularly
 hydrogen peroxide, is reacted at a high conversion of 80 to 100 molar %,
 more preferably 95 to 100 molar %. For this purpose, the total amount of
 the monohydric phenolic compound and the total amount of the peroxide
 compound fed into the multi-stage oxidation apparatus are preferably in a
 molar ratio of 10:1 to 100:1, more preferably 17:1 to 33:1. This specific
 range of the molar ratio of the monohydric phenolic compound to the
 peroxide compound contributes to obtaining the dihydric phenolic compounds
 at a high selectivity thereof.
 In the process of the present invention, the production of the dihydric
 phenolic compound in each reactor of the multi-stage oxidation apparatus
 is preferably carried out to such an extent that the monohydric phenolic
 compound is reacted at a very low conversion of 0.1 to 3 molar %, more
 preferably 0.5 to 2 molar %, and the peroxide compound is reacted at a
 high conversion of 70 to 100 molar %, more preferably 75 to 100%. The very
 low conversion of the monohydric phenolic compound and the high conversion
 of the peroxide compound contribute to preventing or restricting an
 irregular rapid rise in the temperature of the reaction mixture.
 In this case, the amount of the monohydric phenolic compound fed into the
 first reactor and the amount of the peroxide compound fed into each of the
 reactors to which the peroxide compound are preferably in a molar ratio of
 33:1 to 1000:1, more preferably 50:1 to 200:1, to decrease the conversion
 of the monohydric phenolic compound in each of the reactors and to
 restrict the exothermic reaction of the monohydric phenolic compound.
 In the process of the present invention, the temperature of the reaction
 mixture delivered from a flow end portion of the each reaction, namely,
 when the material or the reaction mixture is fed into a bottom portion of
 the reactor, a top portion of the reactor is the flow end portion of the
 reactor and is preferably maintained at a level of 120.degree. C. or less,
 more preferably 115.degree. C. or less, to prevent side reactions and to
 increase the selectivity of the dihydric phenolic compound.
 Also, in the process of the present invention, the increase in temperature
 of the reaction mixture between the feed inlet portion and the delivery
 outlet portion of each reactor is preferably controlled to 3 to 30.degree.
 C., more preferably 5 to 20.degree. C., to maintain the conversion of the
 monohydric phenolic compound due to the oxidation reaction thereof at a
 low level.
 The monohydric phenolic compound usable for the process of the present
 invention includes phenolic compounds having a hydroxyl group attached to
 a benzene ring, for example, phenol, o-, m- and p-cresols, o-, m- and
 p-ethylphenols, o-, m- and p-(n-propyl)phenols, o-, m- and
 p-isopropylphenols, o-, m- and p-(n-butyl)phenols, o-, m- and p-isobutyl
 phenols, o-, m- and p-(tert-butyl)phenols, p-pentylphenol, p-hexylphenol
 2,3,6-trimethylphenol and methylsalicylate. The phenol is preferred for
 the process of the present invention.
 The dihydric phenolic compounds which can be produced by the process of the
 present invention are, phenolic compounds having two hydroxyl groups
 attached to a benzene ring, for example catechol, hydroquinone,
 3-methyl-catechol, 2-methylhydroquinone, 4-methyl catechol,
 3-ethylcatechol, 2-ethylhydroquinone, 4-ethylcatechol, 3-propylcatechols,
 2-propylhydroquinones, 4-propylcatechols, n-butylcatechols,
 2-butylhydroquinones, 4-butyl-catechols, 4-pentylcatechol,
 4-hexylcatechol, 2,3,5-trimethylhydroquinone and methyl
 2,5-dihydroxybenzoate.
 The phenol can be converted to catechol and hydroquinone, the o-cresol to
 3-methylcatechol and 2-methyl-hydroquinone, the m-cresol to 3-methyl
 catechol, 2-methylhydroquinone and 4-methylcatechol and p-cresol to
 4-methylcatechol.
 The peroxide compound usable for the process of the present invention, is
 selected from inorganic peroxide compounds, for example, hydrogen peroxide
 and perchloric acid and organic peroxide compounds, for example, ketone
 peroxides and aliphatic percarboxylic acids. In the process of the present
 invention, hydrogen peroxide, ketone peroxides and mixtures of hydrogen
 peroxide and a ketone compound are preferably employed. When the peroxide
 compound is dividedly fed into a plurality of reactors of the multi-stage
 oxidation apparatus of the present invention, hydrogen peroxide is more
 preferably employed as a peroxide compound.
 The ketone compound which is optionally employed in the process of the
 present invention, is preferably selected from aliphatic dialkyl ketones
 which more preferably have 3 to 20 carbon atoms, for example,
 dimethylketone, diethylketone, diisopropylketone, diisobutylketone,
 methylethylketone, methyl-n-propylketone, methylisopropylketone,
 methylisobutylketone, ethylisopropylketone, ethylisobutylketone and
 ethylhexylketone. In the process of the present invention, more
 preferably, the ketone compound is selected from aliphatic di-lower alkyl
 ketones of which each alkyl group has 1 to 9 carbon atoms, for example,
 dimethylketone, diethylketone, di-n-propylketone and diisopropylketone.
 In the process of the present invention, optionally, the ketone compound is
 preferably employed in a total amount of 0.5 to 10% by weight, more
 preferably 1 to 5% by weight, based on the amount of the monohydric
 phenolic compound fed into the first reactor, and in a total molar amount
 of 0.1 to 10 moles, more preferably 1 to 5 moles, per mole of the peroxide
 compound fed into the multi-stage oxidation apparatus.
 The ketone peroxides usable as a peroxide compound for the process of the
 present invention, are preferably selected from dialkylketone peroxides,
 for example, dimethylketone peroxide, diethylketone peroxide,
 diisopropylketone peroxide, diisobutyl peroxide, methylethylketone
 peroxide, methyl-n-propylketone peroxide, methyl-isopropylketone peroxide,
 methylisobutylketone peroxide, ethyl-isopropylketone peroxide, and
 ethylisobutylketone peroxide.
 The aliphatic percarboxylic acids usable for the process of the present
 invention are selected from, for example, peracetic acid and perpropionic
 acid.
 The oxidation catalyst usable for the process of the present invention may
 be selected from conventional catalysts usable for oxidation reactions.
 Preferably, the catalyst comprises at least one member selected from
 phosphoric acid, sulfuric acid, chloric acid and tungstic acid. Also, in
 the process of the present invention, the oxidation catalyst may be
 employed together with a complexing agent for metal ions, selected from,
 for example, phosphoric acid complexing agents such as phosphoric acid,
 alkyl phosphates and polyphosphoric acid.
 In the process of the present invention, the oxidation catalyst is
 preferably employed in a total amount of 1 to 1000 ppm, more preferably 30
 to 200 ppm, based on the amount in weight of the monohydric phenolic
 compound fed into the first reactor.
 The process of the present invention can be carried out by using the
 continuous multi-stage oxidation apparatus as shown in FIG. 2, which has
 four reactors 1, 10, 20 and 30 connected to each other in series, and in
 which a monohydric phenolic compound is catalytically oxidized, at a low
 conversion thereof, to produce dihydric phenolic compounds with a high
 selectivity thereof.
 Referring to FIG. 2, a monohydric phenolic compound is fed at a temperature
 of 30 to 100.degree. C., preferably 40 to 95.degree. C. into a lower
 portion of the first reactor 1 through a feed line 2, and a portion of a
 peroxide compound (particularly hydrogen peroxide), a portion of a
 catalyst and optionally a ketone compound are fed into the lower portion
 of the first reactor 1 respectively through feed lines 3, 4 and 5. In the
 first reactor, the monohydric phenolic compound is catalytically oxidized
 with the peroxide compound to produce corresponding dihydric phenolic
 compounds, while flowing upward from the lower portion to the top portion
 through the middle portion of the first reactor 1. The reaction mixture
 produced in the first reactor 1 is delivered from the top portion of the
 first reactor 1 through a delivery line 7, is cooled by a cooler
 (heat-exchanging cooler) 6 to a temperature of 40 to 100.degree. C. The
 cooled reaction mixture is fed into the lower portion of the second
 reactor 10 through a feed line 12, and a portion of the peroxide compound
 and optionally a portion of the catalyst are fed into the lower portion of
 the second reactor 10 through the feed lines 13 and 14, respectively. The
 reaction mixture is subjected to a catalytic oxidation reaction of the
 monohydric phenolic compound with the peroxide compound while passing
 upward through the reactor 10. The reaction mixture produced in the second
 reactor 10 is delivered from a top portion of the second reactor 10
 through a delivery line 17 and is directly fed into a lower portion of a
 third reactor 20 through a feed line 22 connected to the delivery line 17.
 A reaction mixture produced in the third reactor 20 is delivered from a
 top portion of the third reactor 20 through a delivery line 27, is cooled
 by a cooler 26 to a temperature of 40 to 100.degree. C., and then is fed
 into a lower portion of a fourth reactor 30 through a feed line 32. Also,
 a remaining portion of the peroxide compound is fed into the lower portion
 of the fourth reactor 30 through a feed conduit 33. Optionally a
 supplementary amount of the catalyst is fed into the lower portion of the
 fourth reactor 30 through a feed line 34. A reaction mixture produced in
 the fourth reactor 30 is delivered from a top portion of the fourth
 reactor 30 through a delivery line 37 and is heat-exchanged by a heat
 exchanger 35. The resultant reaction mixture having a desired temperature
 and containing the target dihydric phenolic compounds is withdrawn from
 the heat-exchanger through a withdrawal line 38.
 In the process of the present invention as shown in FIG. 2, a monohydric
 phenol compound for example, phenol is oxidized to produce dihydric
 phenolic compounds, for example, catechol and hydroquinone at a high
 selectivity thereof.
 In the process of the present invention using the continuous four stage
 oxidation apparatus as shown in FIG. 2, the reaction mixture delivered
 from the first reactor is preferably cooled by the cooler 6 to a
 temperature of 40 to 100.degree. C., more preferably 50 to 95.degree. C.,
 and the temperature of the reaction mixture passing through each reactor
 is preferably controlled to 40 to 120.degree. C., more preferably 55 to
 115.degree. C., to catalytically oxidize the mono-hydric phenolic compound
 under moderate conditions.
 In the process of the present invention, as shown in FIGS. 1 and 2, the
 catalyst can be fed into the continuous multi-step oxidation apparatus in
 such a manner that a portion of the total amount of the catalyst is fed
 into the first reactor, and the remaining portion of the catalyst is
 dividedly fed into one or more of the succeeding reactors, for example,
 the second and third reactors 10 and 20 of FIG. 1 or the second and fourth
 reactors 10 and 30 of FIG. 2.
 When the oxidation apparatus of FIG. 2 is employed, the reaction mixture
 delivered from the second reactor 10 through the delivery line 17 has a
 relatively low temperature of about 80 to 110.degree. C., and thus is fed
 into the third reactor 20 without feeding additional amounts of the
 peroxide compound and the catalyst. Therefore, the non-reacted monohydric
 phenolic compound, peroxide compound and optionally ketone compound
 contained in the reaction mixture fed from the second reactor 10 into the
 third reactor 20 are subjected to the oxidation reaction of a temperature
 of 120.degree. C. or less in the third reactor 20. Namely, the oxidation
 reaction of the monohydric phenolic compound is continuously carried out
 in the second and third reactors 10 and 20.
 In the process of the present invention, each of the catalyst and the
 ketone compound can be fed in an entire amount thereof into the first
 reactor, without feeding into the succeeding reactors. Alternatively, each
 of the catalyst of the ketone compound may be dividedly fed in one or more
 of the succeeding reactors, as long as the amount of the catalyst or the
 ketone fed into each proceeding reactor is similar to that of the peroxide
 compound fed into the proceeding reactor.
 In the process of the present invention, when the reaction mixture
 delivered from the rearend reactor (the third reactor 20 of FIG. 1 or the
 fourth reactor 30 of FIG. 2) through the delivery line 27 in FIG. 1 or the
 delivery line 37 in FIG. 2 contains non-reacted monohydric phenolic
 compound and peroxide compound, the temperature of the delivered reaction
 mixture is adjusted, for example, is heated or cooled to a desired level,
 for example, about 80 to 125.degree. C., by a heat-exchanger 25 in FIG. 1
 or a heat-exchanger 35 in FIG. 2, and the temperature-adjusted reaction
 mixture is preferably transported to and stored in a storage container
 (not shown in FIGS. 1 and 2) wherein the non-reacted monohydric phenolic
 compound is oxidized with the non-reacted peroxide compound, as an aging
 reaction, to completely consume the peroxide compound.
 As mentioned above, the reaction mixture withdrawn from the rearend reactor
 of the continuous multi-stage oxidation apparatus is optionally subjected
 to the aging reaction. The aged or non aged reaction mixture contains the
 target dihydric phenolic compounds, for example, catechol and
 hydroquinone, together with the non-reacted monohydric phenolic compound,
 the catalyst and optionally the non-reacted ketone compound. The reaction
 mixture is subjected to a refining procedure, for example, a distillation
 refining procedure, to separate the compounds from each other, to collect
 the target dihydric phenolic compounds and to separately recover the
 catalyst and the non-reacted ketone compound and monohydric phenolic
 compound.
 The continuous multi-stage oxidation apparatus of the present invention for
 producing a dihydric phenolic compound comprises a plurality of oxidation
 reactors connected to each other in series. In FIG. 1, the oxidation
 apparatus comprises a first reactor 1 and two reactors 10 and 20
 succeeding to the first reactor 1. The first reactor 1 is connected to a
 feed source (not shown) of a monohydric phenolic compound through a feed
 line 2, to a feed source (not shown) of a peroxide compound through a feed
 line 3, to a feed source (not shown) of a catalyst through a feed line 4
 and optionally to a feed source (not shown) of a ketone compound through a
 feed line 5. At least one of the succeeding reactors 10 and 20 is
 connected to a feed source (not shown) of the peroxide compound through a
 feed line 13 or 23 and optionally to a feed source (not shown) of the
 catalyst through a feed line 14 or 24. The rearend reactor (the third
 reactor) 20 has an outlet for delivery a final reaction mixture produced
 therein.
 The oxidation apparatus of the present invention optionally comprises at
 least one cooling means 6 or 16 each arranged between the succeeding
 reactor 10 or 20 connected to the peroxide compound-feed source through a
 feed line 13 or 23 and a next succeeding reactor 1 or 10 located in front
 of the succeeding reactor 10 or 20 connected to the peroxide compound-feed
 source through the feed line 13 or 23.
 In the oxidation apparatus as shown in FIG. 2 in accordance with the
 present invention, four oxidation reactors 1, 10, 20 and 30 in each of
 which a reaction mixture can be subjected to the catalytical oxidation
 reaction while passing therethrough, are connected to each other in
 series. In the first reactor 1, a feed line 2 for a monohydric phenolic
 compound, a feed line 3 for a peroxide compound, a feed line 4 for a
 catalyst and optionally a feed line 5 for a ketone compound are connected
 to a lower portion of the first reactor 1. Also, the second and fourth
 reactors 10 and 30 succeeding to the first reactor 1 have feed lines 12
 and 32 for the reaction mixtures each produced in a reactor located in
 front of the second or fourth reactors 10 and 30, feed lines 13 and 33 for
 the peroxide compound and optionally feed lines 14 and 34 for the
 catalyst.
 Further, a cooling means (cooler) 6 is arranged between the first reactor 1
 and the second reactor 10 to cool the reaction mixture delivered from the
 first reactor 1, and a cooling means (cooler) 26 is arranged between the
 third reactor 20 and the fourth reactor 30 to cool the reaction mixture
 delivered from the third reactor 20. The third reactor 20 does not have
 the feed line for the peroxide compound and the feed line for the
 catalyst. In the oxidation apparatus of the present invention, the rearend
 reactor (for example, the third reactor 20 in FIG. 1 and the fourth
 reactor 30 in FIG. 2) has a heat-exchanger (25 in FIG. 1 and 35 in FIG. 2)
 by which the temperature of the reaction mixture delivered from the
 rearend reactor is adjusted to a desired temperature. The
 temperature-adjusted reaction mixture is transported to and stored in a
 storage container (tank) (not shown) through a withdrawal line (28 in FIG.
 1 and 38 in FIG. 2). In the storage container, the peroxide compound
 remaining in the withdrawn reaction mixture may consumed by an oxidation
 reaction of the non-reacted monohydric phenolic compound with the
 remaining peroxide compound to produce dihydric phenolic compounds.
 On the oxidation apparatus of the present invention, there is no limitation
 to the type of the cooling means for cooling the reaction mixture
 delivered from a preceding reactor, as long as the reaction mixture in the
 state of a liquid can be cooled to a desired temperature at which a rapid
 oxidation of the monohydric phenolic compound with the peroxide compound
 can be controlled when an additional amount of the peroxide compound is
 added to the reaction mixture in a succeeding reactor. Preferably, a
 heat-exchanging cooler using a cooling medium consisting of cold water is
 used as a cooling means. In the oxidation apparatus of the present
 invention, the succeeding reactors connected to a feed line for the
 peroxide compound is preferably further connected to a feed line for the
 catalyst.
 In the oxidation apparatus of the present invention, each reactor is
 preferably of a cylindrical type such that no backward flow of the
 reaction mixture occurs in the reactor. If necessary, as shown in FIGS. 1
 and 2, the reactors are provided with partitioning plates 9, 19, 29 and
 39, for example, perforated plates or baffles. The type and scale of the
 reactors may be designed in consideration of the conversion (reaction
 degree) of the monohydric phenolic compound, the amounts of the peroxide
 compound, the catalyst and optionally the ketone compound used, and the
 increase in temperature of the reaction mixture. The reactors may be the
 same as or different from each other in type and scale (size) thereof.
 The reaction mixture withdrawn from the heat exchanger connected to the
 rearend reactor may be transported to and aged in the storage container,
 and then the aged reaction mixture may be subjected to a recovery
 procedure for the non-reacted monohydric phenolic compound. Alternatively,
 the reaction mixture delivered from the rearend reactor may be directly
 subjected to the recovery procedure for the non-reacted monohydric
 phenolic compound.
 In the non-reacted monohydric phenolic compound-recovery procedure, the
 reaction mixture is subjected to a distillation procedure by which the
 monohydric phenol compound is recovered as a distillation vapor fraction,
 and a non-vaporized liquid fraction containing the target dihydric
 phenolic compound in an increased concentration is obtained. The vapor
 fraction obtained by the distillation is refined to remove water, and to
 obtain a mixture containing the ketone compound and by-products, for
 example, aliphatic carboxylate esters. The mixture may be returned to the
 first reactor and reused for the oxidation reaction.
 The monohydric phenolic compound recovered by the above-mentioned
 distillation procedure is recycled and fed, together with fresh monohydric
 phenolic compound, into the first reactor, and is reused for the oxidation
 procedure, at a low conversion, of the monohydric phenolic compound. The
 recycling amount of the recovered monohydric phenolic compound is
 preferably 10 to 100 times, more preferably 17 to 33 times, the total
 amount of the monohydric phenolic compound consumed in the oxidation
 procedure or the amount of the fresh monohydric phenolic compound fed into
 the first reactor.
 The recovered mixture obtained by the distillation procedure for recovering
 the non-reacted monohydric phenolic compound and containing the target
 compounds in a high concentration of 80 to 100% by weight, particularly 85
 to 95% by weight, is subjected to further distillation procedures in which
 the individual compounds in the mixture are successively separated from
 each other. In this procedures, for example, catechol and hydroquinone are
 separated from each other and refined.
 EXAMPLES
 The present invention will be further explained by the following examples.
 Example 1
 A continuous multi-stage oxidation apparatus as shown in FIG. 2 was used
 for a continuous multi-stage oxidation procedure for phenol. This
 multi-stage oxidation procedure was continuously carried out at a low
 conversion of phenol for 40 days.
 In the oxidation apparatus as shown in FIG. 2, the inside volumes of the
 individual reactors are 1.1 m.sup.3 in the first reactor 1, 3.2 m.sup.3 in
 the second reactor 10, 3.2 m.sup.2 in the third reactor 20 and 11.3
 m.sup.3 in the fourth reactor 30. Also, the inner diameter of the
 individual reactors are 600 mm in the first reactor 1, 650 mm in the
 second reactor 10, 650 mm in the third reactor 20 and 1200 mm in the
 fourth reactor 30.
 In the first reactor 1, (a) a mixture of a recovered (recycling) phenol
 containing 0.36% by weight of catechol and having a temperature of about
 75.degree. C. at a feed rate of 16,054 kg/hr and fresh phenol having a
 temperature of about 70.degree. C. at a feed rate of 661 kg/hr; (b) a
 catalyst comprising sulfuric acid at a feed rate of 0.65 kg/hr and an
 additive consisting of phosphoric acid at a feed rate of 0.16 kg/hr; (c)
 an aqueous hydrogen peroxide containing 60% by weight of hydrogen peroxide
 at a feed rate of 146 kg/hr; and (d) a mixture of a recovered light
 fraction mixture comprising about 50% by weight of diethylketone and about
 50% by weight of ethyl propionate at a feed rate of 654 kg/hr with fresh
 diethylketone at a feed rate of 18 kg/hr, were fed altogether into a lower
 portion of the first reactor, to allow the fed reaction mixture to pass
 upward through the first reactor 1 for a residing time of 3.5 minutes,
 while catalytically oxidizing phenol with hydrogen peroxide. Then the
 resultant reaction mixture was delivered at a temperature of 89.degree. C.
 from the top portion of the first reactor 1, cooled to a temperature of
 80.degree. C. by a cooler 6 and then fed into the bottom portion of the
 second reactor 10.
 Into the bottom portion of the second reactor 10, sulfuric acid was fed as
 a catalyst at a feed rate of 0.65 kg/hr and an aqueous hydrogen peroxide
 solution containing 60% by weight of hydrogen peroxide was fed at a feed
 rate of 146 kg/hr, together with the reaction mixture delivered from the
 first reactor 1. The fed reaction mixture was allowed to pass upward the
 second reactor 10 for a residing time of 10.0 minutes, to catalytically
 oxidize phenol with hydrogen oxide. The resultant reaction mixture having
 an increased temperature of 98.degree. C. was delivered from a top portion
 of the second reactor 10 and fed into a bottom portion of the third
 reactor 20. The fed reaction mixture passed upward through the third
 reactor for a residing time of 10.0 minutes, while oxidizing phenol with
 hydrogen peroxide. The resultant reaction mixture having an increased
 temperature of 100.degree. C. was delivered from a top portion of the
 third reactor 20. The delivered reaction mixture was cooled to a
 temperature of 89.5.degree. C. by a heat-exchanger type cooler 26. The
 cooled reaction mixture was fed into a bottom portion of the fourth
 reactor 30.
 In the bottom portion of the fourth reactor 30, a catalyst consisting of
 sulfuric acid was fed at a feed rate of 0.65 kg/hr and an aqueous hydrogen
 peroxide solution containing 60% by weight of hydrogen peroxide was fed at
 a feed rate of 146 kg/hr, together with the reaction mixture delivered
 from the third reactor 20.
 In the fourth reactor 30, the fed reaction mixture passed upward
 therethrough, for a residing time of 36.0 minutes, while oxidizing phenol
 with hydrogen peroxide. Then the resultant reaction mixture having an
 increased temperature of 106.7.degree. C. was delivered from a top portion
 of the fourth reactor 30 at a delivery rate of 17,827 kg/hr, and was
 temperature-adjusted to a temperature of 115.degree. C. by a heat
 exchanger 35.
 The temperature-adjusted reaction mixture was aged in a storage tank (not
 shown in FIG. 2) to completely consume hydrogen peroxide contained in the
 reaction mixture.
 The resultant reaction mixture in the storage tank contained 89.8% by
 weight of the non-reacted phenol, 3.8% by weight of a mixture of
 diethylketone with ethyl propionate, 2.7% by weight of catechol, 1.7% by
 weight of hydroquinone, 1.6% by weight of water and a very small amount of
 the remaining catalyst.
 The reaction mixture was subjected to distillation procedures to recover
 the phenol. In the first distillation procedure, a light fraction
 comprising water and diethylketone is removed as a vapor fraction and
 then, the remaining liquid fraction is subjected to a second distillation
 procedure to collect the non-reacted phenol as a vapor fraction having a
 phenol content of 96.3% by weight. The collected phenol-containing vapor
 fraction is liquefied and recycled to the first reactor 1. Also, a liquid
 fraction containing 53.5% by weight of catechol, 37.4% by weight of
 hydroquinone, and 9.1% by weight of the others was recovered at a recovery
 rate of 783 kg/hr.
 The liquid fraction obtained by the non-reacted phenol-recovering
 distillation procedure is subjected to a third distillation procedure by
 which catechol having a degree of purity of 99% by weight or more was
 collected at a collecting rate of 387 kg/hr and hydroquinone was obtained
 at a collecting rate of 281 kg/hr.
 In the above-mentioned oxidation procedures for phenol by using the
 continuous multi-stage oxidation apparatus, the molar ratio of the amount
 of phenol fed into the first reactor 1 to the total amount of hydrogen
 peroxide fed into the oxidation apparatus was 23, and in each of the
 first, second and fourth reactors, the molar ratio of the amount of phenol
 fed into the first reactor 1 to the amount of hydrogen peroxide fed into
 each reactor was 69. Also, it was confirmed that the temperature of the
 reaction mixture passing through the oxidation apparatus including the
 four reactors did not exceed 110.degree. C.
 In the above-described example, the conversion of phenol (which refers to
 molar % of the amount of phenol consumed in the oxidation apparatus to the
 amount of phenol fed into the first reactor) was about 3.9%, the
 selectivity of catechol was 55.9 molar %, the selectivity of hydroquinone
 was 40.0 molar %. Also, in this example, the conversion of hydrogen
 peroxide was substantially 100%. Further, the selectivity of catechol and
 hydroquinone was 85.4% based on the molar amount of hydrogen peroxide.
 Comparative Example 1
 The same oxidation procedures for phenol with hydrogen peroxide as in
 Example 1 were carried out except that a single reactor was employed.
 Namely, phenol in the same amount as used in Example 1, hydrogen peroxide
 in the same amount as the total amount used in Example 1, the catalyst in
 the same amount as the total amount used in Example 1 and the ketone
 compound in the same amount as in Example 1 were fed into the single
 reactor, and the oxidation procedure was carried out until the conversion
 of phenol reached about 3.9 molar %. In the single reactor, a heat of
 reaction was generated and thus the temperature of the reaction mixture
 increased by about 52.degree. C.
 Namely, when phenol was oxidized by feeding phenol, hydrogen peroxide,
 sulfuric acid and diethylketone at a temperature of 75.degree. C. into the
 single reaction, the temperature of the reaction mixture rised to
 127.degree. C. which is higher than a threshold temperature of explosion
 of diethylketone peroxide, namely, 120 to 125.degree. C., and thus the
 resultant reaction system was very dangerous in industrial practice. In
 the oxidation procedure at the above-mentioned high temperature, the total
 yield of catechol and hydroquinone based on hydrogen peroxide was 75%, and
 the resultant reaction mixture was significantly colored and contained
 many by-products. Also, the selectivity of catechol and hydroquinone was
 low.
 The process and apparatus of the present invention are useful for producing
 dihydric phenolic compounds by continuously catalytically oxidizing a
 monohydric phenolic compound in industrial practice with a high
 selectivity of the dihydric phenolic compounds. Also, in the process of
 the present invention, the explosion of ketone peroxide produced during
 the oxidation procedure can be surely prevented.