Multitube reactor, vapor phase catalytic oxidation method using the multitube reactor, and start up method applied to the multitube reactor Number:7,144,557 from the United States Patent and Trademark Office (PTO) owispatent A multitube reactor, wherein tubes having smaller tolerance between a nominal size and actual sizes are used as reaction tubes to stably perform a high yield reaction for a long period, a catalyst is filled into the reaction tubes so that the catalyst layer peak temperature portions of the reaction tubes are not overlapped with the connection sites thereof with baffles to effectively prevent hot spots from occurring and stably perform a reaction for a long period without the clogging of the reaction tubes, a heat medium and raw material gas are allowed to flow in the direction of a countercurrent and a specified type of catalyst is filled into the reaction tubes so that activity is increased from the inlet of the raw material gas to the outlet thereof to prevent the autooxidation of products so as to prevent equipment from being damaged due to the reaction, and, at the time of starting, gas with a temperature of 100 to 400.degree. C. is led to the outside of the reaction tubes to increase the temperature of the reaction tubes and, a heat medium, which is solid at the room temperature, is heated to circulate to the outside of the reaction tubes to efficiently start up the reactor without affecting the activity of the catalyst.<H multitube, oxidation, Multitube, react, 7144557.html, Patent, pto, 7144557

Title: Multitube reactor, vapor phase catalytic oxidation method using the multitube reactor, and start up method applied to the multitube reactor

Abstract: A multitube reactor, wherein tubes having smaller tolerance between a nominal size and actual sizes are used as reaction tubes to stably perform a high yield reaction for a long period, a catalyst is filled into the reaction tubes so that the catalyst layer peak temperature portions of the reaction tubes are not overlapped with the connection sites thereof with baffles to effectively prevent hot spots from occurring and stably perform a reaction for a long period without the clogging of the reaction tubes, a heat medium and raw material gas are allowed to flow in the direction of a countercurrent and a specified type of catalyst is filled into the reaction tubes so that activity is increased from the inlet of the raw material gas to the outlet thereof to prevent the autooxidation of products so as to prevent equipment from being damaged due to the reaction, and, at the time of starting, gas with a temperature of 100 to 400.degree. C. is led to the outside of the reaction tubes to increase the temperature of the reaction tubes and, a heat medium, which is solid at the room temperature, is heated to circulate to the outside of the reaction tubes to efficiently start up the reactor without affecting the activity of the catalyst.

Patent Number: 7,144,557 Issued on 12/05/2006 to Yada, et al.

Inventors: Yada; Shuhei (Yokkaichi, JP), Hosaka; Hirochika (Yokkaichi, JP), Goriki; Masayasu (Yokkaichi, JP), Jinno; Kimikatsu (Yokkaichi, JP), Ogawa; Yasushi (Yokkaichi, JP), Suzuki; Yoshiro (Yokkaichi, JP)
Assignee: Mitsubishi Chemical Corporation (Tokyo, JP)

Appl. No.: 10/887,868

Filed: July 12, 2004

Foreign Application Priority Data


Jan 11, 2002 [JP]			2002-004636
Feb 14, 2002 [JP]			2002-036460
Mar 14, 2002 [JP]			2002-069820
Apr 09, 2002 [JP]			2002-105924

Current U.S. Class: 422/196 ; 422/198; 422/200; 422/201; 562/532; 568/476

Current International Class: B01J 10/00 (20060101); C07C 45/00 (20060101); C07C 51/16 (20060101); F28D 7/00 (20060101)

References Cited [Referenced By]

U.S. Patent Documents


5006131	April 1991	Karafian et al.
5149884	September 1992	Brenner et al.
5677261	October 1997	Tenten et al.
5772870	June 1998	Basse
6057481	May 2000	Brockwell et al.
6582667	June 2003	Ogata et al.
6808689	October 2004	Matsumoto et al.
6946573	September 2005	Matsumoto et al.
2003/0065194	April 2003	Weiguny et al.

Foreign Patent Documents


1289634	Apr., 2001	CN
1323779	Nov., 2001	CN
2140125	Feb., 1972	DE
62-201646	Sep., 1987	JP
63-054941	Mar., 1988	JP
6-13096	Feb., 1994	JP
6-38918	May., 1994	JP
8-92147	Apr., 1996	JP
2000-93784	Apr., 2000	JP
2001-310123	Nov., 2001	JP
01/68626	Sep., 2001	WO

Primary Examiner: Witherspoon; Sikarl A.
Attorney, Agent or Firm: Wenderoth, Lind & Ponack, L.L.P.

Parent Case Text

This is a continuation of International Application No. PCT/JP03/00160 filed Jan. 10, 2003.

Claims

The invention claimed is:

1. A multitube reactor comprising a plurality of reaction tubes having a catalyst packed therein, and a shell equipped with the reaction tubes inside and into which a heat medium flowing outside the reaction tubes may be introduced, wherein at least 95% of the reaction tubes are selected from tubes having same nominal outside diameter and same nominal wall-thickness, an outside diameter tolerance of .+-.0.62%, and a wall-thickness tolerance of +19% to -0%.

2. A multitube reactor comprising a plurality of reaction tubes having a catalyst packed therein, and a shell equipped with the reaction tubes inside and into which a heat medium flowing outside the reaction tubes may be introduced, wherein at least 95% of the reaction tubes are selected from tubes having same nominal outside diameter and same nominal wall-thickness, an outside diameter tolerance of .+-.0.56%, and a wall-thickness tolerance of +17% to -0%.

3. The multitube reactor according to claim 1, wherein the catalyst can oxidize gaseous propylene, propane, isobutylene, isobutanol, or t-butanol to (meth)acrolein and/or (meth)acrylic acid with a molecular oxygen-containing gas.

4. The multitube reactor according to claim 2, wherein the catalyst can oxidize gaseous propylene, propane, isobutylene, isobutanol, or t-butanol to (meth)acrolein and/or (meth)acrylic acid with a molecular oxygen-containing gas.

5. A method for starting up a shell-tube type reactor having a system for circulating a heat medium which is solid at normal temperature, the shell-tube type reactor having reaction tubes, and an introducing port and a discharging port for the heat medium, wherein the heat medium circulates outside the reaction tubes for removing heat generated inside the reaction tubes, wherein the method comprises: heating the reaction tubes through introduction of a gas having temperature of 100 to 400.degree. C. outside the reaction tubes; and then circulating a heated heat medium through the outside of the reaction tubes.

6. The method according to claim 5, wherein the heat medium which is solid at normal temperature has a solidifying point of 50 to 250.degree. C.

7. The method according to claim 5, wherein the heating through introduction of the gas is performed until the temperature of the reactor is equal to or higher than the solidifying point of the heat medium.

8. The method according to claim 5, wherein the circulation of the heated heat medium is carried out while heating the heated heat medium.

Description

TECHNICAL FIELD

The present invention relates to a multitube reactor applied to a method of producing (meth)acrolein and/or (meth)acrylic acid by oxidizing propylene, propane, isobutylene, isobutanol, or t-butanol with a molecular oxygen-containing gas, a vapor phase catalytic oxidation method using the multitube reactor, and to a start up method applied to the multitube reactor.

BACKGROUND ART

A conventional multitube reactor is equipped with a plurality of reaction tubes having a catalyst packed therein and a plurality of baffles inside a shell for feeding and circulating inside the shell a fluid for heat removal (hereinafter, referred to as "heat medium") introduced into the shell. A raw material gas fed inside the reaction tubes reacts in the presence of the catalyst inside the reaction tubes, to thereby generate heat of reaction. The heat of reaction is removed by a heat medium circulating inside the shell.

When differences of inner volumes among the plurality of reaction tubes equipped inside the shell is large, amounts of the catalyst packed inside the reaction tubes are irregular and a scatter arises. As a result, a flow rate of the raw material gas fed or a retention time differs among the reaction tubes, thereby becoming a factor causing yield reduction of a target product and reduced catalyst life. Further, a localized abnormal high-temperature site (hot spot) may form in the reaction tubes provoking a reaction out of control, thereby causing a problem of inhibiting a continuous operation.

Further, in a multitube reactor provided with the baffles, the heat medium does not flow at all in a portion where the baffles and the reaction tubes are fixed to each other when the baffles and the reaction tubes are fixed through welding, flanges, or the like. A reactor in which outer walls of the reaction tubes and the baffle are not fixed also exists, but the amount of the heat medium flowing through this clearance is limited. The following problems arise in a vapor phase catalytic oxidation method using a fixed bed multitube heat-exchanger type reactor as described above.

There is a state of poor heat removal in the reaction tubes in a portion where flow of the heat medium is insufficient inside the shell. A localized abnormal high-temperature zone (hot spot) may form in the reaction tubes which are in a state of poor heat removal, possibly resulting in a reaction out of control. Further, a reaction may not become out of control, but problems arise including ease of clogging the reaction tubes, yield reduction of the reaction product gas, deterioration of the catalyst life, and inhibition of a stable operation for a long period of time.

Many methods of suppressing hot spot formation have been proposed for the multitube reactor used in a vapor phase catalytic oxidation reaction. For example, JP 08-092147 A discloses a method of providing uniform heat medium temperature by: setting a flow direction of a reactant gas guided to a reactor and that of the heat medium inside a shell in a countercurrent; allowing the heat medium to flow further upward in a meandering way using baffles; and adjusting temperature differences of the heat medium from an inlet of the reactor to an outlet thereof within 2 to 10.degree. C. or less.

The multitube reactor generally consists of a plurality of tubes (bundle) arranged vertically, and thus a process fluid flow can be upflow or downflow by allowing a process fluid to flow from an upper portion or lower portion of the reactor. The heat medium can also be fed to the shell from the upper portion or lower portion thereof.

Therefore, the multitube reactor is classified into two types similar to a general shell and tube heat exchanger: a concurrent type allowing the process fluid and the heat medium to flow in the same direction; and a countercurrent type allowing the process fluid and the heat medium to flow in opposite directions.

Further, the multitube reactor may be classified into the following types considering the directions of the fluids: 1) a concurrent type of downflow process fluid/downflow heat medium; 2) a concurrent type of upflow process fluid/upflow heat medium; 3) a countercurrent type of upflow process fluid/downflow heat medium; and 4) a countercurrent type of downflow process fluid/upflow heat medium.

Proposed in JP 2000-093784 A is a method of suppressing hot spot formation by: allowing a raw material gas and a heat medium to flow in downward concurrent; and preventing a gas reservoir free of the heat medium. Further, the method allows an exchange of a catalyst in a vicinity of a catalyst layer inlet alone where most easily deteriorates by: feeding the raw material gas from an upper portion of a reactor; and allowing the raw material gas to flow downward inside the catalyst layer of reaction tubes.

However, the heat medium and the process fluid move in a concurrent according to the method, and gas temperature in an outlet portion of the reactor increases. Thus, the method has a fault that high concentration of a product (meth)acrolein easily causes an autooxidation reaction (autolysis reaction).

Further, with respect to the upflow, in a method of allowing the process fluid and the heat medium to flow in a concurrent, that is, in the same direction, heat medium temperature increases with heat of reaction. Thus, high temperature at a process outlet causes autooxidation at the reactor outlet easily. The autooxidation reaction results in problems of a combustion reaction of the product, equipment breakdown due to temperature increase, and yield reduction.

Proposed is a method of preventing autooxidation for a purpose of preventing temperature increase, by providing a cooling zone or heat exchanger in a downstream of a reaction portion for decreasing gas temperature. However, in a concurrent, heat medium temperature in the vicinity of the reactor outlet and process gas temperature in an outlet portion are high. Thus, an amount of heat removal becomes large and a cooling portion (cooling zone and heat exchanger) enlarges, thereby becoming disadvantageous in point of cost.

Further, even if a significant autooxidation reaction is not caused, an autooxidation reaction is caused by a part of a product, which arises a problem of yield reduction of a target product as a whole.

Further, in a shell-tube type reactor circulating a heat medium which is solid at normal temperature, there is a necessary to maintain the heat medium at temperature of the solidifying point or above to ensure fluidity thereof for circulating the heat medium inside the reactor.

JP 2001-310123 A discloses a reactor start up method for a multitube reactor having reaction tubes, an introducing port of a fluid flowing outside reaction tubes, and a discharging port thereof for removing heat generated inside the reaction tubes, the method being characterized by including: heating reaction tubes by introducing a gas having temperature of 100 to 400.degree. C. in the reaction tubes; and circulating a heated heat medium through the outside of the reaction tubes. Further, a gas not providing an effect when being mixed with a catalyst packed in the reaction tubes or with a raw material gas (such as air) is selected as the gas introduced to the reaction tubes.

However, a large volume of a high temperature gas is introduced to the reaction tubes according to the above-mentioned method, thereby changing an oxidation state of the catalyst. Therefore, catalytic activity and selectivity may be affected, possibly resulting in yield reduction or reduced catalyst life.

DISCLOSURE OF THE INVENTION

A first object of the present invention is to provide a multitube reactor for improving life of a catalyst packed inside the reaction tubes and for preventing yield reduction of a target product.

Further, a second object of the present invention is to provide a vapor phase catalytic oxidation method comprising: using the above-mentioned multitube reactor; circulating a heat medium through the outside of the reaction tubes; and feeding a reaction raw material gas inside the reaction tubes packed with a catalyst to obtain a reaction product gas, in which hot spot formation can be effectively prevented, the reaction tubes are not clogged, an yield of a reaction product gas is high, a catalyst life is long, and a stable operation can be performed over a long period of time.

Further, a third object of the present invention is to reduce process gas temperature at a product discharging port of the reactor in the vapor phase catalytic oxidation method using the multitube reactor described above.

Further, a fourth object of the present invention is to provide a method which makes a reactor start up effectively without adversely affecting the catalytic activity in a shell-tube type reactor of circulating a heat medium which is solid at normal temperature, the method being applied to a multituibe eactor such as the above-mentioned multitube reactor.

The present invention provides a multitube reactor represented by the following items (1) to (3) (hereinafter, may also be referred to as "multitube reactor of the present invention") as a means for solving at least the first object of the present invention.

(1) A multitube reactor comprising a plurality of reaction tubes having a catalyst packed therein, and a shell equipped with the reaction tubes inside and into which a heat medium flowing outside the reaction tubes is introduced, wherein the reaction tubes are selected from tubes having same nominal outside diameter and same nominal wall-thickness, an outside diameter tolerance of .+-.0.62%, and a wall-thickness tolerance of +19% to -0%.

(2) The multitube reactor comprising a plurality of reaction tubes having a catalyst packed therein, and a shell equipped with the reaction tubes inside and into which a heat medium flowing outside the reaction tubes is introduced, wherein the reaction tubes are selected from tubes having same nominal outside diameter and same nominal wall-thickness, an outside diameter tolerance of .+-.0.56%, and a wall-thickness tolerance of +17% to -0%.

(3) The multitube reactor according to the above item (1) or (2), which is used for production of (meth) acrolein and/or (meth) acrylic acid by oxidizing propylene, propane, isobutylene, isobutanol, or t-butanol with a molecular oxygen-containing gas.

Further, the present invention provides a vapor phase catalytic oxidation method represented by the following items (4) to (6) using the multitube reactor of the present invention for solving at least one of the second, third, and fourth objects of the present invention.

(4) A vapor phase catalytic oxidation method comprising: using the multitube reactor according to the above items (1) or (2), which further comprises baffles connected to the reaction tubes through connecting sites for changing a flow path of a heat medium introduced into the shell; circulating the heat medium through the outside of the reaction tubes; and feeding a reaction raw material gas inside the reaction tubes packed with a catalyst to obtain a reaction product gas; wherein the method comprises setting catalyst packing specifications in the reaction tubes so that catalyst layer peak temperature sites of the reaction tubes are not located at the connecting sites between the baffles and the reaction tubes.

(5) The vapor phase catalytic oxidation method according to the above item (4), wherein the method comprises: packing the reaction tubes with a Mo--Bi catalyst and/or Sb--Mo catalyst so that an activity increases from a process gas inlet to a process gas outlet of the reaction tubes; allowing the heat medium and the process gas to flow in a countercurrent; and oxidizing propylene, propane, or isobutylene, and/or (meth)acrolein through vapor phase catalytic oxidation with a molecular oxygen-containing gas.

(6) The vapor phase catalytic oxidation method according to the above item (4) or (5), wherein the method comprises: heating the reaction tubes through introduction of a gas having temperature of 100 to 400.degree. C. outside the reaction tubes; and circulating the heat medium which is solid at normal temperature outside the heated reaction tubes to start up the multitube reactor.

Further, the present invention provides a vapor phase catalytic oxidation method (hereinafter, may also be referred to as "first vapor phase catalytic oxidation method") represented by the following items (7) to (10) for solving at least the second object of the present invention.

(7) A vapor phase catalytic oxidation method comprising: using a fixed bed multitube heat-exchanger type reactor having a plurality of reaction tubes and baffles connected to the reaction tubes through connecting sites for changing a flow path of a heat medium flowing outside the reaction tubes; circulating the heat medium through the outside of the reaction tubes; feeding a reaction raw material gas inside the reaction tubes packed with a catalyst to obtain a reaction product gas, wherein the method comprises setting catalyst packing specifications in the reaction tubes so that catalyst layer peak temperature sites of the reaction tubes are not located at the connecting sites between the baffles and the reaction tubes.

(8) The vapor phase catalytic oxidation method according to the above item (7), wherein layers having different catalyst packing specifications are provided with two or more catalyst in one reaction tube.

(9) The vapor phase catalytic oxidation method according to the above item (7) or (8), wherein items for setting the catalyst packing specifications comprise a type of catalyst, an amount of catalyst, a form of catalyst, a method for diluting the catalyst, and lengths of reaction zones.

(10) The vapor phase catalytic oxidation method according to any one of the above items (7) to (9), wherein the method comprises oxidizing propane, propylene, and/or isobutylene with molecular oxygen through the vapor phase catalytic oxidation method to produce (meth) acrylic acid.

Further, the present invention provides a vapor phase catalytic oxidation method (hereinafter, may also be referred to as "second vapor phase catalytic oxidation method") represented by the following items (11) and (12) for solving at least the third object of the present invention.

(11) A vapor phase catalytic oxidation method comprises: using a multitube reactor which comprises: a cylindrical shell having a raw material feed port and a product discharging port; a plurality of ring-shaped tubes arranged on an outer periphery of the cylindrical shell for introducing or discharging a heat medium into or from the cylindrical shell; a circulating device connecting the plurality of the ring-shaped tubes one another; a plurality of reaction tubes restrained by a plurality of tube plates of the reactor and comprising a catalyst; and a plurality of baffles provided in a longitudinal direction of the reactor and for changing a direction of the heat medium introduced into the cylindrical shell; oxidizing propylene, propane, or isobutylene, and/or (meth)acrolein through vapor phase catalytic oxidation with a molecular oxygen-containing gas to obtain (meth)acrolein and/or (meth)acrylic acid; wherein the method comprises,

packing a Mo--Bi catalyst and/or Sb--Mo catalyst in the reaction tubes so that an activity increases from a process gas inlet to a process gas outlet of the reaction tubes; and

allowing the heat medium and the process gas to flow in a countercurrent.

(12) The vapor phase catalytic oxidation method according to the above item (11), wherein the Mo--Bi catalyst is represented by the following general formula (I) and the Sb--Mo catalyst is represented by the following general formula (II): Mo.sub.aW.sub.bBi.sub.cFe.sub.dA.sub.eB.sub.fC.sub.gD.sub.hE.sub.iO.sub.j (I) (wherein, Mo represents molybdenum; W represents tungsten; Bi represents bismuth; Fe represents iron; A represents at least one type of element chosen from nickel and cobalt; B represents at least one type of element selected from the group consisting of sodium, potassium, rubidium, cesium, and thallium; C represents at least one type of element selected from alkaline earth metals; D represents at least one type of element selected from the group consisting of phosphorus, tellurium, antimony, tin, cerium, lead, niobium, manganese, arsenic, boron, and zinc; E represents at least one type of element selected from the group consisting of silicon, aluminum, titanium, and zirconium; O represents oxygen; a, b, c, d, e, f, g, h, i, and j represent atomic ratios of Mo, W, Bi, Fe, A, B, C, D, E, and O respectively; and if a=12, 0.ltoreq.b.ltoreq.10, 0<c.ltoreq.10, 0<d.ltoreq.10, 2.ltoreq.e.ltoreq.15, 0<f.ltoreq.10, 0.ltoreq.g.ltoreq.10, 0.ltoreq.h.ltoreq.4, and 0.ltoreq.i.ltoreq.30; and j is a value determined from oxidation states of the respective elements); and Sb.sub.kMo.sub.l(V/Nb).sub.mX.sub.nY.sub.pSi.sub.qO.sub.r (II) (wherein, Sb represents antimony; Mo represents molybdenum; V represents vanadium; Nb represents niobium; X represents at least one type of element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), and bismuth (Bi); Y represents at least one type of element chosen from copper (Cu) and tungsten (W); Si represents silicon; O represents oxygen; (V/Nb) represents V and/or Nb; k, l, m, n, p, q, and r represent atomic ratios of Sb, Mo, (V/Nb), X, Y, Si, and 0 respectively; and 1.ltoreq.k.ltoreq.100, 1.ltoreq.1.ltoreq.100, 0.1.ltoreq.m.ltoreq.50, 1.ltoreq.n.ltoreq.100, 0.1.ltoreq.p.ltoreq.50, 1.ltoreq.q.ltoreq.100; and r is a value determined from oxidation states of the respective elements).

Further, the present invention provides a method for starting up (hereinafter, may also referred to as "start up method of the present invention") represented by the following items (13) and (14) for solving at least the fourth object of the present invention.

(13) A start up method for a shell-tube type reactor having a system of circulating a heat medium which is solid at normal temperature, the shell-tube type reactor having reaction tubes, and an introducing port and discharging port of a fluid flowing outside the reaction tubes for removing heat generated inside the reaction tubes, wherein the method comprises:

heating the reaction tubes through introduction of a gas having temperature of 100 to 400.degree. C. outside the reaction tubes; and circulating the heated heat medium through the outside of the reaction tubes.

(14) The start up method according to the above item (13), wherein the heat medium which is solid at normal temperature has a solidifying point of 50 to 250.degree. C.

Hereinafter, the present invention will be described in more detail.

<Multitube reactor of the present invention>

A multitube reactor of the present invention is a multitube reactor equipped with a plurality of reaction tubes inside a shell of the multitube reactor, the plurality of reaction tubes being selected from tubes having the same nominal outside diameter and the same nominal wall-thickness, an outside diameter tolerance of .+-.0.62% and a wall-thickness tolerance of +19% to -0%, particularly preferably an outside diameter tolerance of .+-.0.56% and a wall-thickness tolerance of +17% to -0%. The multitube reactor is suitably used in oxidizing propylene, propane, isobutylene, isobutanol, or t-butanol with a molecular oxygen-containing gas.

In the multitube reactor of the present invention, the phrase "the same nominal outside diameter and the same nominal wall-thickness" means that "in a reaction tube, a nominal outside diameter and an actual outside diameter are substantially the same and a nominal wall-thickness and an actual wall-thickness are substantially the same". In addition, the above-mentioned range of the tolerance is defined in the present invention as the range which represents "substantially the same" The actual dimensions of a reaction tube can be measured by means of a conventionally known method. The dimensions may adopt the measured value at a given position or the average value of a plurality of measured values.

An outline of the multitube reactor of the present invention will be described with reference to FIG. 1.

Reference numeral 2 represents a shell of the multitube reactor, and the shell 2 comprises reaction tubes 1a, 1b, and 1c each packed with a catalyst, the reaction tubes being fixed by both a lower tube plate 5b and an upper tube plate 5a.

The reaction tubes 1a, 1b, and 1c each have an inner diameter of about 20 to 40 mm.PHI., a wall-thickness of 1 to 2 mm, and a length of 3,000 to 6,000 mm. A carbon steel tube or stainless steel tube is used as a material for each of the reaction tubes 1a, 1b, and 1c.

The total number of the reaction tubes 1a, 1b, and 1c equipped inside the shell 2 varies depending on the amount of production of a target product but is typically 1,000 to 30,000. The arrangement of the tubes varies depending on outside diameter sizes of the reaction tubes, but the reaction tubes equipped inside the shell at intervals of 5 to 50 mm to establish a square arrangement or an equilateral triangle arrangement.

The above equilateral triangle arrangement is often used because the arrangement can increase the number of the reaction tubes 1a, 1b, and 1c equipped inside the reactor per unit area.

The outside diameter tolerance and wall-thickness tolerance of each of the reaction tubes used in the present invention are far more rigorous than the tolerance of JIS or of ASTM, and such tubes that satisfy the rigorous tolerances are used.

That is, each of the reaction tubes is desirably selected from tube products having the same nominal outside diameter and the same nominal wall-thickness, an outside diameter tolerance of .+-.0.62% and a wall-thickness tolerance of +19% to -0%, particularly preferably an outside diameter tolerance of .+-.0.56% and a wall-thickness tolerance of +17% to -0%. In the multitube reactor of the present invention, all the reaction tubes equipped inside the multitube reactor preferably satisfy the above conditions. However, it is sufficient that at least 95%, more preferably at least 99% of the tubes satisfy the above conditions.

The shell 2 has, at its top and bottom ends, inlet and outlet portions 4a and 4b for a raw material gas Rg for a reaction, and the raw material gas Rg flows through the reaction tubes 1a, 1b, and 1c in an upward or downward flow direction via the inlet and outlet portions 4a and 4b for the raw material gas arranged on the top and bottom ends of the reactor. The flow direction of the raw material gas is not particularly limited, but is more preferably a downflow.

In addition, a ring-shaped tube 3a for introducing a heat medium Hm is arranged on the outer periphery of the shell 2. The heat medium Hm pressurized by a circulating pump 7 is introduced into the shell 2 through the ring-shaped tube 3a.

The heat medium Hm introduced into the shell 2 flows upward while changing its flow direction as indicated by arrows due to baffles 6a, 6b, and 6a equipped inside in the shell 2. During this period, the heat medium Hm contacts the outer surfaces of the reaction tubes 1a, 1b, and 1c to remove heat of reaction, and then returns to the circulating pump 7 via the ring-shaped tube 3a arranged on the outer periphery of the shell 2.

Part of the heat medium Hm absorbing the heat of reaction flows toward a discharge tube 8b arranged on an upper portion of the circulating pump 7 to be cooled by a heat exchanger (not shown). Then, the heat medium is sucked again in the circulating pump 7 through a heat medium feed tube 8a to be introduced into the shell 2.

The temperature of the heat medium Hm introduced into the shell 2 is controlled by adjusting the temperature or quantity of flow of the heat medium flowing from the heat medium feed tube 8a. In addition, the temperature of the heat medium Hm is measured with a thermometer 14 inserted into the side of the inlet of the ring-shaped tube 3a.

An inner body plate of each of the ring-shaped tube 3a and a ring-shaped tube 3b is equipped with a flow-rectifying plate (not shown) in order to minimize the flow rate distribution of the heat medium in a circumferential direction. A porous plate or a plate with slits is used for the flow-rectifying plate. The flow-rectifying plate is arranged in such a manner that the same quantity of flow of the heat medium Hm is introduced into the shell 2 at the same flow rate from the entire circumference by changing an opening area of the porous plate or by changing slit intervals.

In addition, the temperature inside the ring-shaped tube 3a, preferably the temperature inside each of the ring-shaped tubes 3a and 3b, can be monitored with a plurality of thermometers 15 arranged at even interval along the circumference as shown in FIG. 4.

1 to 5 baffles are typically equipped inside the shell 2. In FIG. 1, 3 baffles (6a, 6b, and 6a) are equipped inside the shell 2. The presence of those baffles causes the flow of the heat medium Hm in the shell 2 to center on the central portion from the outer peripheral portion of the shell 2, to flow upward through an opening of the baffle 6a toward the outer peripheral portion while changing its direction, and then to reach the inner wall of the shell 2.

Then, the heat medium Hm changes its direction again to converge to the central portion while flowing upward through a gap between the inner wall of the shell 2 and the outer periphery of the baffle 6b. Finally, the heat medium Hm flows upward through an opening of the baffle 6a toward the outer periphery along the bottom face of the upper tube plate 5a in the shell 2 to be introduced into the ring-shaped tube 3b. After that, the heat medium Hm is sucked in the circulating pump 7 to be circulated in the shell 2 again.

The specific structure of a baffle used in the present invention may be any one of a segment-type noncircular baffle shown in FIG. 2 and a disc-type baffle shown in FIG. 3.

Both types of baffles have the same relationship between the flow direction of a heat medium and the axis of a reaction tube.

The baffle 6a has its outer periphery on the inner wall of the shell 2 and has an opening around its center. In addition, the outer periphery of the baffle 6b is smaller than the circumference of the inner wall of the shell 2 so that a gap is formed between the outer periphery of the baffle 6b and the inner wall of the shell 2.

The heat medium changes its direction at each opening and gap while moving upward, and the flow rate is changed.

Thermometers 11 are equipped inside the reaction tubes 1a, 1b, and 1c, which are equipped inside in the shell 2. Signals are transmitted from thermometers to the outside of the shell 2 to measure the temperature distribution of a catalyst layer packed inside a reaction tube in an axial direction of the reaction tube.

A plurality of thermometers 11 are inserted into the reaction tubes 1a, 1b, and 1c to measure temperatures at 2 to 20 points in an axial direction.

The reaction tubes 1a, 1b, and 1c equipped inside the shell 2 are divided by the 3 baffles 6a, 6b, and 6c, and are classified into 3 types depending on the relationship with the flow direction of the heat medium Hm.

That is, the reaction tube 1a is connected to the baffle 6b. Therefore, the flow direction of the heat medium Hm is restrained only by the baffle 6b. In addition, the flow direction is not restrained by the other two baffles 6a because the reaction tube 1a penetrates through opening portions of the two baffles 6a.

The direction of the heat medium Hm introduced into the shell 2 through the ring-shaped tube 3a is changed as indicated by an arrow shown in FIG. 1 at the central portion of the shell 2. Furthermore, the reaction tube 1a is located at the position where the direction is changed, and thus the heat medium Hm that flows along the outer periphery of the reaction tube 1a mainly flows in parallel with the axis of the reaction tube 1a.

The reaction tube 1b is connected to the 3 baffles 6a, 6b, and 6a so that the flow direction of the heat medium Hm is restrained by each of the baffles. In addition, the heat medium Hm flowing along the outer periphery of the reaction tube 1b flows at a right angle with the axis of the reaction tube 1b at nearly all positions of the reaction tube 1b. It should be noted that most of the reaction tubes equipped inside the shell 2 are located at the position of the reaction tube 1b.

In addition, the reaction tube 1c is not connected to the baffle 6b and penetrates through a gap between the outer periphery of the baffle 6b and the inner wall of the shell 2. Therefore, the flow of the heat medium Hm is not restrained by the baffle 6b at this position and is parallel with the axis of the reaction tube 1c.

FIG. 4 shows the positional relationship among the reaction tubes 1a, 1b, and 1c and the baffles 6a, 6b, and 6a, and the correlation of the flow of the heat medium Hm.

When an opening portion (the most inner circle indicated by broken lines) of the baffle 6a is located at a converging position of the heat medium Hm, that is, the center of the shell 2, the flow of the heat medium Hm is parallel with the reaction tube 1a. Moreover, nearly no heat medium Hm flows particularly at the center of the opening portion of the baffle 6a, and the flow rate is close to zero. In other words, heat transfer efficiency is extremely poor. Therefore, the reaction tube 1a is not provided at this position in some cases.

FIG. 5 shows another example of the present invention in which the shell 2 of the reactor is divided by an intermediate tube plate 9.

Different heating media Hm1 and Hm2 are circulated in spaces obtained by dividing the shell 2 and the temperatures of the media are separately controlled.

A raw material gas Rg is introduced through a raw material gas inlet 4a of the shell and is successively reacted to yield a product.

Heat media having different temperatures are present in the shell, and hence how each of the reaction tubes 1a, 1b, and 1c is packed with a catalyst is as follows. In a case (i), each reaction tube is entirely packed with the same catalyst and the temperature of the catalyst is changed at the inlet and outlet of the shell to allow a reaction. In a case (ii), a catalyst is packed at the inlet portion. For rapidly cooling a reaction product, no catalyst is packed at the outlet portion, that is, the outlet portion serves as a cavity or is packed with an inert substance without reaction activity. In a case (iii), different catalysts are packed at the inlet and outlet portions. For rapidly cooling a reaction product, no catalyst is packed at an intermediate portion or is packed with an inert substance without reaction activity.

For example, when propylene or isobutylene is introduced as a mixed gas with a molecular oxygen-containing gas, propylene or isobutylene is converted into (meth)acrolein at an upper portion and is oxidized to (meth)acrylic acid at a lower portion.

Different catalysts are packed in upper and lower portions in each of the reaction tubes 1a, 1b, and 1c, and the temperatures of the catalysts are controlled to respective optimum temperatures, to thereby carry out a reaction. An inert substance layer that is not involved in the reaction may be present as a partition between the upper portion and the lower portion. In the case, the inert substance layer is provided in a portion corresponding to the position at which the outer periphery of each of the reaction tubes 1a, 1b, and 1c is connected to the intermediate tube plate 9.

In FIG. 6, reference numeral 9 represents an intermediate tube plate, and 3 thermal shields 10 are fixed at the bottom face of the intermediate tube plate 9 by spacer rods 13.

As shown in the figure, 2 to 3 thermal shields 10 are provided within 100 mm below or above the intermediate tube plate 9, whereby preferably forming a flowless stagnant space 12 filled with the heat medium Hm1 or Hm2, to provide a heat insulation effect.

Thermal shields 10 are attached to the intermediate tube plate 9 for the following reason. That is, in FIG. 5, when the difference in controlled temperature between the heat medium Hm1 introduced into the lower portion of the shell 2 and the heat medium Hm2 introduced into the upper portion of the shell 2 exceeds 100.degree. C., heat transfer from the high-temperature heat medium to the low-temperature heat medium cannot be neglected. Thus, the precision in controlling the reaction temperature of a catalyst may degrade at lower temperatures. In such a case, heat insulation is necessary to prevent heat transfer above and/or below the intermediate tube plate 9.

Here, the types and ratios of raw material gas components and the importance of uniform packing of a catalyst will be described.

Introduced into a multitube reactor for use in vapor phase catalytic oxidation is a mixed gas of propylene or isobutylene and/or (meth)acrolein with a molecular oxygen-containing gas or with water vapor as a raw material gas Rg for a reaction.

The concentration of propylene or isobutylene ranges from 3 to 10 vol %. A molar ratio of oxygen to propylene or to isobutylene ranges from 1.5 to 2.5, and a molar ratio of water vapor to propylene or to isobutylene ranges from 0.8 to 2.0.

The introduced raw material gas Rg is distributed to the reaction tubes 1a, 1b, and 1c and then flows through each reaction tube to react by an oxidation catalyst packed inside each reaction tube. However, the distribution of the raw material gas Rg to each reaction tube is affected by the packing weight, packing density, and the like of a catalyst in a reaction tube. The packing weight, packing density, and the like are set at the time of packing a catalyst into a reaction tube. Therefore, it is essential to uniformly pack a catalyst into each reaction tube.

To uniformize the weight of a catalyst packed inside each reaction tube, it is important to set a rigorous tolerance of a reaction tube into which a catalyst is packed.

The raw material gas Rg flowing through each of the reaction tubes 1a, 1b, and 1c is initially heated to a reaction starting temperature while flowing through the inert substance layer packed at the inlet portion.

The raw material (propylene or isobutylene) is oxidized by the catalyst packed as the successive layer in the reaction tube, and the temperature of the raw material further increases by heat of reaction.

The reaction weight in the inlet portion of the catalyst layer is most. The heat of reaction generated increases the temperature of the raw material gas Rg when the heat of reaction is greater than the quantity of heat removal by the heat medium Hm. In such a case, hot spots may be formed. The hot spots are often formed at a position 300 to 1,000 mm from the inlet of each of the reaction tubes 1a, 1b, and 1c.

Here, the effect of the heat of reaction generated on a catalyst, the temperature of a heat medium and the allowable maximum temperature of hot spots when producing acrolein through an oxidation reaction of propylene with a molecular oxygen-containing gas, the type of heat medium used, and the effect of a fluid state of the heat medium on the heat removal efficiency of the heat medium will be described.

When the heat of reaction generated exceeds the heat removal capacity of the heat medium Hm on the outer periphery of the corresponding reaction tube, the temperature of the raw material gas Rg further increases, and the heat of reaction also increases. Finally, the reaction becomes out of control. In this case, the temperature of the catalyst exceeds the allowable maximum temperature, so that the catalyst undergoes a qualitative change. This change may be a main cause for the deterionation or breakage of the catalyst.

A description is given by taking as an example a former stage reactor (for instance, the portion of the reactor above the intermediate tube plate 9 in FIG. 5) in which acrolein is produced through an oxidation reaction of propylene with a molecular oxygen-containing gas. In this example, the temperature of the heat medium Hm is in the range of 250 to 350.degree. C. and the allowable maximum temperature of the hot spots is in the range of 400 to 500.degree. C.

In addition, the temperature of the heat medium Hm in a latter stage reactor (for instance, the portion of the reactor below the intermediate tube plate 9 in FIG. 5) in which acrolein is oxidized by a molecular oxygen-containing gas to yield acrylic acid is in the range of 200 to 300.degree. C. and the allowable maximum temperature of the hot spots is in the range of 300 to 400.degree. C.

Niter, a mixture of nitrates, is often used as the heat medium Hm that flows inside the shell 2 surrounding the reaction tubes 1a, 1b, and 1c. However, a phenyl ether heat medium of an organic liquid system may also be used.

The heat medium Hm flows to remove heat from the outer periphery of each of the reaction tubes 1a, 1b, and 1c. However, the heat medium Hm introduced into the shell 2 through the ring-shaped tube 3a for heat medium introduction flows toward the central portion from the outer peripheral portion of the shell 2 at a position and reverses its flow direction at another position. Heat removal effects at the positions were found to extremely differ from each other.

A heat transfer coefficient of the heat medium when the flow direction of the heat medium Hm is at a right angle with the axis of a reaction tube is in the range of 1,000 to 2,000 W/m.sup.2.degree. C. When the flow direction is not at a right angle with the axis of a reaction tube, the heat transfer coefficient varies depending on the flow rate and on whether the flow is an upflow or a downflow. However, the heat transfer coefficient often falls within a narrow range of 100 to 300 W/m.sup.2.degree. C. when niter is used as the heat medium.

On the other hand, the heat transfer coefficient of a catalyst layer in each of the reaction tubes 1a, 1b, and 1c naturally dependents on the flow rate of the raw material gas Rg, but is about 100 W/m.sup.2.degree. C. As a matter of course, the rate determining factor of heat transfer is a gas phase in a tube as usual.

Specifically, heat transfer resistance on the outer periphery of each of the reaction tubes 1a, 1b, and 1c when the flow of the heat medium Hm is at a right angle with the axis of the tube is 1/10 to 1/20 that of the gas Rg in the tube. Therefore, a change in flow rate of the heat medium Hm hardly affects the overall heat transfer resistance.

However, when niter flows in parallel with the axis of a tube, the heat transfer coefficient in each of the reaction tubes 1a, 1b, and 1c is comparable to that outside the reaction tubes 1a, 1b, and 1c. Therefore, the effect of the fluid state at the outer periphery of the tube on the heat removal efficiency is large. That is, when the heat transfer resistance at the outer periphery of a tube is 100 W/m.sup.2.degree. C., the overall heat transfer coefficient becomes half. Furthermore, half the change in heat transfer resistance at the outer periphery of the tube affects the overall heat transfer coefficient.

In each of FIGS. 1 and 5, the flow direction of the heat medium Hm in the shell 2 is represented as an upflow by arrows. However, the present invention can also be applied to the opposite flow direction.

In determining the direction of a circulation flow of the heat medium Hm, a phenomenon of engulfing, in the heat medium flow, a gas that may be present at top ends of the shell 2 and the circulating pump 7, in particular an inert gas such as nitrogen, must be prevented.

In the case where the heat medium Hm is an upflow as shown in FIG. 1, a cavitation phenomenon occurs in the circulating pump 7 when a gas is engulfed at an upper portion of the circulating pump 7. In the worst case, the pump may break.

In the case where the heat medium Hm is a downflow, a gas engulfing phenomenon occurs also at an upper portion of the shell 2. In this case, a stagnant portion of a gas phase is formed at an upper portion of the shell 2, and an upper portion of a reaction tube corresponding to the gas stagnant portion is not cooled by the heat medium Hm.

Prevention of gas stagnation must include: providing a degas line; and replacing a gas of the gas phase with the heat medium Hm. To achieve this, the pressure of the heat medium in the heat medium feed tube 8a is increased and the heat medium discharging tube 8b is provided at the highest position as possible to increase the pressure in the shell 2. The heat medium discharging tube 8b is provided at least above the upper tube plate 5a.

The raw material gas Rg can be an upflow or a downflow in the reaction tubes 1a, 1b, and 1c. However, the raw material gas Rg preferably in a countercurrent in relation to the heat medium flow.

Examples of a method of adjusting activity of a catalyst layer include: a method of adjusting catalyst compositions to use catalysts having different activities; and a method of adjusting activity by mixing catalyst particles with inert substance particles to dilute the catalyst.

A catalyst layer having a small ratio of the catalyst particles is packed into an inlet portion of each of the reaction tubes 1a, 1b, and 1c. A catalyst layer having a large ratio of the catalyst particles or catalyst layer not diluted is packed into a portion of the reaction tube, the portion located downstream with respect to the flow direction of the raw material gas. Although the degree of dilution varies by a catalyst, a (catalyst particles/inert substance particles) mixing ratio is preferably in the range of 7/3 to 3/7 in the former stage and in the range of 10/0 to 5/5 in the latter stage. 2 to 3 stages are typically adopted for the activity change or dilution of a catalyst.

Dilution ratios of the catalysts packed inside the reaction tubes 1a, 1b, and 1c do not need to be the same with each other. For example, the reaction tube 1a has a high maximum temperature so that there is a high possibility of catalyst deterioration. To prevent the deterioration, it is possible to lower the catalyst particle ratio in the former stage and to increase the catalyst particle ratio in the latter stage.

Differences in reaction conversions among the respective reaction tubes may affect the average conversion and yield of the entire reactor. Therefore, it is preferable that the dilution rate is set so that the same conversion is obtained in the respective reaction tubes even when dilution ratios are changed.

The present invention is suitably applied to a multitube reactor for oxidizing propylene or isobutylene with a molecular oxygen-containing gas or to a multitube reactor in which (meth)acrolein is oxidized with a molecular oxygen-containing gas to yield (meth)acrylic acid. A catalyst used in oxidation of propylene is preferably a multicomponent mixed metal oxide, mainly an Mo--Bi mixed metal oxide. A catalyst used in oxidation of acrolein to yield acrylic acid is preferably an Sb--Mo mixed oxide.

Propylene or isobutylene is typically oxidized in 2 stages and hence different catalysts may be packed inside 2 multitube reactors to carry out a reaction. Alternatively, the present invention can also be applied to the case of yielding (meth)acrylic acid in a single reactor with the shell of the reactor divided into 2 or more chambers by intermediate tube plates as shown in FIG. 5 and with the chambers packed with different catalysts.

In a multitube reactor for oxidizing propylene or isobutylene with a molecular oxygen-containing gas, when the reactor shown in FIG. 1 is adopted and the raw material gas Rg enters from 4a and is discharged from 4b, the concentration of the target product (meth) acrolein is high in the vicinity of the shell outlet Sb. In addition, the temperature of the raw material gas increases because the raw material gas is heated by the heat of reaction. Therefore, in this case, a heat exchanger is additionally provided in the course of the raw material gas Rg following 4b of the shell shown in FIG. 1, to thereby sufficiently cool the reaction gas to prevent (meth)acrolein from causing an autooxidation reaction.

In the case where the reactor shown in FIG. 5 is adopted, when the raw material gas Rg enters from 4a and is discharged from 4b, the concentration of the target product (meth)acrolein is high in the vicinity of the catalyst layer outlet 9 in the former stage. In addition, the temperature of the raw material gas increases because the gas is heated by the heat of reaction.

When a catalyst is packed only into 5a 6a 6b 6a 9, no reaction is carried out in the catalyst layer outlet portion (between 9 and 5b) in the latter stage of the reaction tubes 1a, 1b, and 1c. The raw material gas is cooled by the heat media Hm1 and Hm2 flowing through flow paths to the shell in order to prevent (meth) acrolein from causing an autooxidation reaction. In this case, the gas outlet portion (between 9 and 5b) of each of the reaction tubes 1a, 1b, and 1c is packed with no catalyst or with an inert substance without reaction activity. However, the latter is preferably packed for improving heat transfer characteristics.

In addition, in FIG. 5, in the case where the catalyst layer (5a 6a 6b 6a 9) in the former stage on the inlet side of the raw material gas Rg and the catalyst layer (9 6a' 6b' 6a' 5b) in the latter stage on the outlet side of the gas are packed with different catalysts to obtain (meth)acrolein and (meth)acrylic acid from propylene and isobutylene, the temperature of the catalyst layer in the former stage is higher than that of the catalyst layer in the latter stage. Therefore, no reaction is carried out around the catalyst layer outlet (6a-9) in the former stage and the catalyst layer inlet (9 6a') in the latter stage because a position around them has a high temperature. The raw material gas is cooled by the heat medium flowing through a flow path to the shell side in order to prevent (meth)acrolein from causing an autooxidation reaction.

In this case, a portion into which no catalyst is packed is provided among 6a 9 6a' of the reaction tubes 1a, 1b, and 1c to serve as a cavity. Alternatively, an inert substance without reaction activity is packed among 6a 9 6a' of the reaction tubes 1a, 1b, and 1c. However, the latter is preferably packed for improving heat transfer characteristics.

A vapor phase catalytic oxidation reaction involves: mixing propylene or isobutylene as a raw material with molecular oxygen and an inert gas such as nitrogen, carbon dioxide, or water vapor to prepare a raw material gas; and reacting the raw material gas in the presence of a solid catalyst to yield acrolein and acrylic acid or methacrolein and methacrylic acid. Any conventionally known catalyst is available for the catalyst. According to the present invention, it is also possible to yield acrylic acid by subjecting propane to vapor phase oxidation by using a Mo--V--Te mixed oxide catalyst, Mo--V--Sb mixed oxide catalyst, or the like.

The composition of a former stage reaction catalyst (for a reaction converting an olefin into an unsaturated aldehyde or an unsaturated acid) that can be preferably used in the present invention is represented by the following general formula (I). Mo.sub.aW.sub.bBi.sub.cFe.sub.dA.sub.eB.sub.fC.sub.gD.sub.hE.sub.iO.sub.j (I) (wherein, Mo represents molybdenum; W represents tungsten; Bi represents bismuth; Fe represents iron; A represents at least one type of element chosen from nickel and cobalt; B represents at least one type of element selected from the group consisting of sodium, potassium, rubidium, cesium, and thallium; C represents at least one type of element selected from alkaline earth metals; D represents at least one type of element selected from the group consisting of phosphorus, tellurium, antimony, tin, cerium, lead, niobium, manganese, arsenic, boron, and zinc; E represents at least one type of element selected from the group consisting of silicon, aluminum, titanium, and zirconium; O represents oxygen; a, b, c, d, e, f, g, h, i, and j represent atomic ratios of Mo, W, Bi, Fe, A, B, C, D, E, and O respectively; and if a=12, 0.ltoreq.b.ltoreq.10, 0.ltoreq.c.ltoreq.10, 0<d.ltoreq.10, 2.ltoreq.e.ltoreq.15, 0<f.ltoreq.10, 0.ltoreq.g.ltoreq.10, 0.ltoreq.h.ltoreq.4, and 0.ltoreq.i.ltoreq.30; and j is a value determined from oxidation states of the respective elements.)

In the multitube reactor of the present invention, c, d, and f in the above general formula (I) preferably satisfy 0.1.ltoreq.c.ltoreq.10, 0.1.ltoreq.d.ltoreq.10, and 0.001.ltoreq.f.ltoreq.10.

Further, the composition of a latter stage reaction catalyst (for a reaction converting an olefin into an unsaturated aldehyde or an unsaturated acid) that can be preferably used in the present invention is represented by the following general formula (II). Sb.sub.kMo.sub.l(V/Nb).sub.mX.sub.nY.sub.pSi.sub.qO.sub.r (II) (wherein, Sb represents antimony; Mo represents molybdenum; V represents vanadium; Nb represents niobium; X represents at least one type of element selected from the group consisting of iron (Fe), cobalt (Co), nickel (Ni), and bismuth (Bi); Y represents at least one type of element chosen from copper (Cu) and/or tungsten (W); Si represents silicon; O represents oxygen; (V/Nb) represents V and/or Nb; k, l, m, n, p, q, and r represent atomic ratios of Sb, Mo, (V/Nb), X, Y, Si, and O respectively; and 1.ltoreq.k.ltoreq.100, 1.ltoreq.1.ltoreq.100, 0.1.ltoreq.m.ltoreq.50, 1.ltoreq.n.ltoreq.100, 0.1.ltoreq.p.ltoreq.50, 1.ltoreq.q.ltoreq.100; and r is a value determined from oxidation states of the respective elements.) In the multitube reactor of the present invention, k, l, m, n, p, and q in the above general formula (II) preferably satisfy 10.ltoreq.k.ltoreq.100, 1.ltoreq.1.ltoreq.50, 1<m.ltoreq.20, 10.ltoreq.n.ltoreq.100, 1.ltoreq.p.ltoreq.20, and 10.ltoreq.q.ltoreq.100.

The shape of and molding method for a catalyst used are described. A catalyst used in the multitube reactor of the present invention may be a molded catalyst molded through extrusion molding or tablet compression or may be a catalyst prepared by supporting a mixed oxide composed of a catalyst component on an inert support such as silicon carbide, alumina, zirconium oxide, or titanium oxide.

The shape of a catalyst used in the present invention is not particularly limited and may be spherical, cylindrical, ring-shaped, amorphous, or the like.

In particular, the use of a ring-shaped catalyst has an effect of preventing heat storage in hot spot portions.

A catalyst packed into a reaction tube inlet may have the same or different composition and shape with a catalyst packed into an upper portion of the reaction tube.

An inert substance used for catalyst dilution in the present reaction is not limited as long as the inert substance is stable under the conditions of the present reaction and has no reactivity with a raw material substance and a product. Specific examples of the inert substance include those typically used as catalyst supports such as alumina, silicon carbide, silica, zirconium oxide, and titanium oxide. In addition, as in the case of the catalyst, the shape of the inert substance is not limited and may be spherical, cylindrical, ring-shaped, amorphous, or the like. The size of the inert substance may be set in consideration of the diameter of a reaction tube and differential pressure in a reaction tube.

In the case where a multitube reactor is used and a plurality of reaction zones are provided by dividing the inside of each reaction tube in its axial direction, the number of reaction zones may be appropriately selected in such a manner providing the maximum effect of the reaction zones. However, an excessively large number of reaction zones requires much effort for catalyst packing. Therefore, an industrially desirable number of reaction zones is about 2 to 5.

In addition, the length of a reaction zone may be appropriately selected in such a manner that the maximum effect of the present invention is exerted because the most suitable value of the length is determined by the catalyst type, the number of reaction zones, the reaction conditions, and the like. The length of each reaction zone is typically 10 to 80%, preferably 20 to 70%, of the total length.

According to the present invention, the catalytic activity of a catalyst packed into a plurality of reaction zones is controlled by altering mixing with an inert substance, a shape of the catalyst, a composition of the catalyst, and a burning temperature upon catalyst preparation, and, if the catalyst is a supported catalyst, the amount of a catalyst active ingredient supported.

<First vapor phase catalytic oxidation method>

The inventors of the present invention have devoted themselves to research and have confirmed that the above-mentioned problems such as yield reduction and reduced catalyst life arise when catalyst layer peak temperature sites,