Method for production of methacrylic acid

This invention relates to a improved method for the production of methacrylic acid by the steps of subjecting isobutylene and/or tertiory hutanol to catalytic vapor-phase oxidation with molecular oxygen in a first reactor, then supplying the resultant gax mixture to a second reactor, a rodlike or plate like insert set being placed in the empty space of gas inlet part of the tube of said second reactor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for the production of methacrylic acid 
by the catalytic vapor-phase oxidation of isobutylene and/or tertiary 
butanol. More particularly, this invention relates to a method for 
producing methacrylic acid from isobutylene and/or tertiary butanol by the 
two-step continuous reaction, namely a first step reaction which subjects 
isobutylene and/or tertiary butanol to catalytic vapor-phase oxidation 
with molecular oxygen and mainly produces methacrolein and a second step 
reaction which converts this methacrolein into methacrylic acid. Owing to 
this invention two-step continuous reaction, the trouble possibly 
encountered in the second step reaction, i.e. the occlusion of the 
catalyst bed with an occlusive substance contained in the gas formed by 
the first step reaction, is precluded and the production of methacrylic 
acid is obtained smoothly and technically stably. 
2. Description of the Prior Art 
In the production of methacrylic acid by the catalytic vapor-phase 
oxidation method from isobutylene and/or tertiary butanol, the two-step 
oxidation reaction is generally adopted which comprises converting 
isobutylene and/or tertiary butanol by the catalytic vapor-phase oxidation 
into methacrolein (hereinafter this reaction will be referred to as the 
"first step reaction" and the catalyst used therein as the "first step 
catalyst") and subsequently converting the methacrolein by the catalytic 
vapor-phase oxidation into methacrylic acid (hereinafter this reaction 
will be referred to as the "second step reaction" and the catalyst used 
therein as the "second step catalyst"). 
The catalyst to be used in the first step reaction is generally a 
multi-element type oxide catalyst containing molybdenum, bismuth, an iron. 
When the catalytic vapor-phase oxidation of isobutylene and/or tertiary 
butanol is carried out in the presence of a catalyst of this type, the 
reaction in addition to forming methacrolein as a main product producer by 
product compounds of relatively high boiling points such as maleic acid 
and terephthalic acid and further entails evolution of a gas containing 
polymers and tarry substances. When the reaction gas containing such 
substances is supplied in the unmodified form to the second step reaction, 
these substances induce clogging of the piping and occlusion of the packed 
bed of the second step catalyst and consequently cause aggravation of 
pressure loss, degradation of catalytic activity, and impairment of the 
selectivity of the reaction for methacrylic acid. These troubles occur 
frequently when the feed volume or feed rate of isobutylene and/or 
tertiary butanol is increased or the concentration of isobutylene and/or 
tertiary butanol is increased for the purpose of ensuring a large output 
of methacrylic acid. 
Among the methods heretofore proposed and adopted for the prevention of 
these troubles, a method which comprises periodically discontinuing the 
reaction, removing from the gas inlet side of the second step catalyst an 
inert substance such as, for example, ceramic balls packed therein for 
preventing the catalyst bed from occlusion or precluding the loss of 
catalytic activity, and replacing a new supply of the inert substance, a 
method which comprise separating methacrolein from the gas produced by the 
first step reaction and elaborately feeding the separated methacrolein to 
the second step reaction thereby optimizing the process of oxidation, and 
a method which comprises diluting the feed gas to a concentration lower 
than normally required and ensuring a desired decrease in the 
concentration of by-products in the product of reaction are well known. 
These methods, however, are invariably not close to proving satisfactory 
from the economic point of view because they are complicated and 
expensive. Further, a method which recycles as an inert gas the waste gas 
emanated from the reaction gas for the purpose of lowering the oxygen 
contents of the reaction gases formed in the first step and second step 
reactions to the fullest possible extent for the purpose of preventing 
excessive oxidation has found popular acceptance. In this connection, a 
method which, for the purpose of preventing the catalyst bed from 
occlusion and precluding the loss of catalytic activity, lowers the 
concentration of minute solid particles in the waste gas being recycled to 
the first step reaction to the fullest possible extent (Japanese Patent 
Laid-Open SHO 56(1981)-113,732), a method which, for the purpose of 
preventing the piping intervening between the sites of the first step and 
second step reactions from clogging, retains the portion of the piping at 
a temperature exceeding the boiling point of maleic anhydride or devises 
means for extremely increasing the linear speed of gas (Japanese Patent 
Laid-Open SHO 50(1975)-126,605), and a method which represses the 
occlusion of the second step catalyst with a solid matter from the first 
step reaction vessel by giving a specific form to the catalyst used in the 
second step reaction and consequently increasing the void ratio in the 
catalyst bed (Japanese Patent Laid-Open SHO 61(1986)-221,149) have been 
proposed. These methods also are not close to proving fully satisfactory 
for commercial operation. 
Now that the development of the second step catalyst has advanced so much 
as to promise a reduction in the reaction temperature and an addition to 
the magnitude of load, the measure for preventing the trouble of clogging 
of the second step reaction vessel on the gas inlet side has been gaining 
all the more in significance. 
An object of this invention, therefore, is to provide for the production of 
methacrylic acid an economically advantageous method free of the drawbacks 
of the prior art described above. 
The inventors have found that, in the production of methacrylic acid from 
isobutylene and/or tertiary butanol by the catalytic vapor-phase oxidation 
using molecular oxygen and comprising a first-step reaction and a second 
step reaction, the occlusion of the second step catalyst bed with the 
by-products entrained by the gas produced in the first step reaction can 
be efficiently precluded and consequently the reaction performed on a 
commercial scale can be performed smoothly and stably by keeping a rodlike 
or platelike insert set in the reaction tube of the second step reactor on 
the gas inlet side thereof. This invention has been perfected as the 
result. 
SUMMARY OF THE INVENTION 
To accomplish the object described above in accordance with this invention, 
there is provided a method for the production of methacrylic acid, which 
is characterized by subjecting at least one member selected from the group 
consisting of isobutylene and tertiary butanol to catalytic vapor-phase 
oxidation with molecular oxygen in a first heat-exchanger type multitube 
reactor packed with an oxide catalyst containing bismuth, molybdenum, and 
iron thereby mainly forming methacrolein, then supplying the gas 
consequently formed by the reaction to a second heat-exchanger type 
multitube reactor connected directly to the aforementioned first reactor 
and packed with an oxide catalyst containing molybdenum and phosphorus 
thereby subjecting the methacrolein to catalytic vapor-phase oxidation 
with molecular oxygen and consequently forming methacrylic acid and, 
during the formation of methacrylic acid, keeping a rodlike or platelike 
insert set in the empty space of the gas inlet part of the tube of the 
second reactor packed with the catalyst. 
The advantages of the method of this invention are that the possibility of 
entailing autoxidation and explosion is rare because the temperature of 
the gas to be introduced into the second step reaction is not required to 
be unduly elevated, that the catalyst which is highly active even at low 
temperatures can be adopted as the second step catalyst, that the empty 
space part is not required to be excessively elongated for the purpose of 
accommodating a gas-preheating layer therein, that the inevitable 
necessity for removing the inert carrier otherwise entailed when occlusion 
occurs in the inlet part of the second step catalyst bed can be obviated, 
and that the production of methacrylic acid can be carried out by a 
continuous operation under economically advantageous conditions without 
entailing either the occlusion with high-boiling compounds or the decline 
of yield even when isobutylene and/or tertiary butanol is used in a 
heightened concentration.

DESCRIPTION OF PREFERRED EMBODIMENT 
Now, the present invention will be described more specifically hereinafter. 
In the production of methacrylic acid by the catalytic vapor-phase 
oxidation of isobutylene and/or tertiary butanol, the two-step oxidation 
method is adopted frequently. This two-step oxidation method comes in two 
versions; a method which produces methacrylic acid by separating 
methacrolein from the methacrolein-containing mixed gas formed chiefly in 
the first step catalyst bed, supplying the separated methacrolein to the 
second step catalyst bed, and subjecting it to catalytic vapor-phase 
oxidation therein and a method which produces methacrylic acid by 
supplying the methacrolein-containing mixed gas directly in its unaltered 
form to the second step catalyst bed and subjecting it to catalytic 
vapor-phase oxidation therein. Optionally, either of these versions may 
further incorporate therein a step of recycling a combustion gas from the 
waste gas remaining after the recovery of useful components. 
Incidentally, when the multi-element type catalyst containing bismuth, 
molybdenum, and iron is used as the first step catalyst, the occurrence of 
such high-boiling compounds as maleic acid and terephthalic acid, 
polymers, or tarry substances is not avoidable. Further, polymers, tarry 
substances, or fumy solids are thermally formed from the reaction product 
even within the piping. They can be formed by collision with the wall of 
the piping. 
When the production of methacrylic acid is effected by such a process as 
described above, these polymers and by-product substances are inevitably 
circulated through the system and introduced into the reactor in a gaseous 
form, in the form of minute solid particles, or in a fumy form and they 
have strong possibility of being entrained in a large volume into the 
second step reactor in particular. Under these conditions, the possibility 
of such by-products as high-boiling compounds assuming a solid state and 
inducing occlusion of the inlet side of the second step catalyst bed 
increases in proportion as the temperature of the mixed gas introduced 
into the latter-step reactor decreases. 
The trouble of this occlusion is liable to occur when the concentration of 
isobutylene and/or tertiary butanol is heightened and consequently the 
amount of such secondary products as high-boiling compounds and tarry 
substances in the gas produced by the reaction is increased. For the 
avoidance of the trouble of this occlusion, in addition to the methods 
enumerated above, an idea of elevating the temperature of the gas on the 
verge of entering the second step reactor or an idea of providing for the 
second step reactor with a preheating bed part preceding the inlet part of 
the catalyst bed may be conceived. 
These conventional methods have problems of their own. The elevation of the 
temperature of the gas being supplied to the second step catalyst bed has 
its limit because it is subject to the restrictions imposed for the sake 
of curbing the autoxidation of methacrolein formed in the first step 
catalyst bed and avoiding entering an explosion range. When the preheating 
layer part for the gas is provided in the form of an empty space en route 
to the catalyst bed, the layer in the empty space part must be elongated 
to some extent and consequently the reactor must be enlarged 
proportionately. When the inert carrier is placed in the preheating layer 
part, the portion of the preheating layer part accommodating the inert 
carrier possibly entails the phenomenon of occlusion. The inventors have 
examined the troubles of occlusion caused under these conditions in search 
of a solution and have consequently acquired the following knowledge. 
When the two-step oxidation method is adopted in the production of 
methacrylic acid by the catalytic vapor-phase oxidation of isobutylene 
and/or tertiary butanol, the gas produced by the reaction in the first 
step reactor is generally cooled enough for the prevention of the 
autoxidation and the avoidance of the explosion limit. When the 
temperature of the gas is lowered excessively, the by-products are 
converted into solids or fumy substances, which in the unaltered form are 
destined to induce occlusion of the second step reactor on the inlet side 
of the catalyst bed. For the elimination of this occlusion, the inventors 
have adopted a theory that the occlusion of the second step catalyst bed 
ought to be avoided by heightening linear speed of the gas and, in the 
meantime, elevating the temperature of the gas at the highest possible 
rate thereby enabling the by-products of the first step reaction to 
undergo excessive reaction readily on the second step catalyst and 
allowing the gas in a harmless state to pass the second step catalyst bed. 
After a further study on this theory, they have found that though the 
trouble of occlusion is not fully avoided by the packing with the inert 
carrier or by the preheating as in the second step catalyst bed, the 
preheating effect of the gas for the second step reaction is manifested, 
the occlusion is avoided, the reaction is continued stably for a long 
time, and the autoxidation of methacrolein during the course of 
temperature elevation can be curbed unexpectedly by simply keeping a 
rodlike or platelike insert set in the empty space part of the second step 
catalyst on the gas inlet side. This knowledge has led to perfection of 
this invention. 
As regards the shape of the insert for use in this invention, the rodlike 
insert may be in the form of a straight bar, a zigzag bar, a spiral bar, a 
polygonal prism, or a circular column, for example, and the platelike 
insert in the form of a ribbon, a zigzag plate, or a spiral plate, for 
example. The platelike insert need not be in the form of a perfect plate 
but may be in the form of a reticular plate. As regards the size of the 
insert, the overall length is desired to be in the range of 200 to 1,000 
mm, preferably 250 to 500 mm and the width is desired to be such that the 
void ratio may fall in the following range. The void ratio of the portion 
of the empty space in the gas inlet part of the second step catalyst bed 
is selected to suit the shape of the insert to be adopted. Generally, the 
void ratio is desired to be in the range of 30 to 99%. Preferably, the 
void ratio is in the range of 40 to 99% where the insert is in the form of 
a rod or in the range of 50 to 99% where the insert is in the form of a 
plate. By the observance of this void ratio, the occlusion of the layer of 
insert with solids can be precluded, the preheating effect can be 
attained, and the reaction in the second step catalyst bed can be 
performed smoothly. 
The term "void ratio" as used in the present invention refers to what is 
defined by the following expression. 
##EQU1## 
The materials which are usable for the insert include metals of high 
thermal conductivity such as, for example, iron, nickel, aluminum, and 
alloys thereof. The insert made of stainless steel proves particularly 
desirable. The insert may be made of a metal which has undergone a 
chemical treatment for rust proofing the surface thereof. The ceramic 
insert may be obtained by forming zirconia or alumina in the shape of 
sheet, for example. 
The present invention is desired to be worked specifically as follows. A 
feed gas comprising 1 to 10% by volume of isobutylene and/or tertiary 
butanol, 3 to 20% by volume of molecular oxygen, 0 to 60% by volume of 
steam, and inert gases such as nitrogen and carbon dioxide is supplied at 
a reaction temperature (temperature of the heat medium in the reactor) in 
the range of 250.degree. to 450.degree. C. at a space velocity in the 
range of 300 to 5,000 hr.sup.-1 (STP), preferably 500 to 3000 hr.sup.-1, 
to the bismuth-molybdenum-iron-containing multi-element type first step 
catalyst bed capable of converting isobutylene and/or tertiary butanol 
into methacrolein. Then, the gas produced by the first step reaction is 
replenished with secondary air, secondary oxygen, or steam. The resultant 
mixed gas is adjusted to a temperature in the range of 100.degree. to 
350.degree. C., preferably 150.degree. to 300.degree. C. (namely the 
temperature at which no occlusion is allowed is suffered to occur within 
the piping and neither autoxidation nor explosion limit is suffered to 
ensure), and supplied to the second step catalyst having a rodlike or 
platelike insert set in place in the empty space of the gas inlet part 
thereof. 
The catalyst to be used in the first step reaction is an oxide catalyst 
having bismuth, iron, and molybdenum as its main components. The catalyst 
of the following composition proves particularly desirable. 
EQU Mo.sub.a W.sub.b Bi.sub.c Fe.sub.d A.sub.e B.sub.f C.sub.g D.sub.h O.sub.x 
wherein Mo stands for molybenum, W for tungsten, Bi for bismuth, Fe for 
iron, A for at least one element selected from the group consisting of 
nickel and cobalt, B for at least one element selected from the group 
consisting of alkali metals, alkaline earth metals, and thallium, C for at 
least one element selected from the group consisting of phosphorus, 
tellurium, antimony, tin, cerium, lead, niobium, manganese, and zinc, D 
for at least one element selected from the group consisting of silicon, 
aluminum, titanium, and zirconium, and O for oxygen. Then, a, b, c, d, e, 
f, g, h, and x respectively stand for the numbers of atoms of the elements 
of Mo, W, Bi, Fe, A, B, C, D, and O such that, where a is assumed to be 
12, b is in the range of 0 to 10, c in the range of 0.1 to 10, d in the 
range of 0.1 to 20, e in the range of 2 to 20, f in the range of 0 to 10, 
g in the range of 0 to 4, h in the range of 0 to 30, and x assumes a 
numerical value to be fixed by the states of oxidation of the elements. 
The oxide catalyst may be in the form of pellets produced by the use of a 
tableting machine or an extrusion molder, for example, in the form of 
beads, or in the form of rings containing a through hole. It may be 
effectively used in the form of a composite having a catalytic substance 
deposited on a refractory carrier. 
The second step catalyst is only required to be an oxide catalyst 
containing molybdenum and phosphorus as main components. It is desired to 
contain a phosphomolybdic acid type heteropolyacid or a metal salt 
thereof. The catalyst of the following composition proves particularly 
desirable. 
EQU Mo.sub.a P.sub.b A.sub.c B.sub.d C.sub.e D.sub.f O.sub.x 
wherein Mo stands for molybdenum, P for phosphorus, A for at least on 
element selected from the group consisting of arsenic, antimony, 
germanium, bismuth, zirconium, and selenium, B for at least one element 
selected from the group consisting of copper, iron, chromium, nickel, 
manganese, cobalt, tin, silver, zinc, palladium, rhodium, and tellurium, C 
for at least one element selected from the group consisting of vanadium, 
tungsten, and niobium, D for at least one element selected from the group 
consisting of alkali metals, alkaline earth metals, and thallium, and O 
for oxygen. Then, a, b, c, d, e, f, and x respectively stand for the 
atomic ratio of Mo, P, A, B, C, D, and O such that, where a is assumed to 
be 12, b is in the range of 0.5 to 4, c in the range of 0 to 5, d in the 
range of 0 to 3, e in the range of 0 to 4, f in the range of 0.01 to 4, 
and x assumes a numerical value to be fixed by the states of oxidation of 
the component elements. The form in which the catalyst is used is not 
critical. This catalyst may be in the form of cylinders, in the form of 
hollow spheres, or in the form of beads. Of course, this catalyst may be 
used in the form of a composite having a catalytic substance deposited on 
a refractory carrier. 
Now, the present invention will be described more specifically hereinafter 
with reference to working examples. 
As a preliminary test, the insert specified by the present invention was 
tested for the effect manifested in the preheating of the gas, to obtain 
the following results. A steel pipe 30 mm in inside diameter and 2 mm in 
wall thickness was prepared, molten salt was used as a heat source, and 
air preheated to 255.degree. C. was used as a feed gas. The flow volume of 
the air was fixed at 1.9 m3 per hour as reduced to standard conditions. 
When the temperature of the molten salt was fixed at 290.degree. C., the 
steel pipe used in its empty form was required to have a length of about 
800 to 900 mm in order to preheat the air to 280.degree. C. When a 
stainless steel plate 18 mm in width corrugated with a zigzaging angle of 
about 90 degrees was inserted (void ratio 98%) in the steel pipe, however, 
the length of about 300 mm was sufficient for the pipe to preheat the air 
up to 280.degree. C. 
EXAMPLE 1 
(Preparation of first step catalyst) 
In 15 liters of water which was kept heated and stirred, 9.5 kg of ammonium 
molybdate and 4.9 kg were dissolved. Separately, 7.0 kg of cobalt nitrate 
was dissolved in 2 liters of distilled water, 2.4 kg of ferric nitrate in 
2 liters of distilled water, and 2.9 kg of bismuth nitrate in 3 liters of 
distilled water acidified in advance by addition of 0.6 liter of 
concentrated nitric acid. The mixture of these three nitrate solutions was 
added dropwise. Then, a liquid obtained by dissolving 2.4 kg of 20% silica 
sol and 76 g of sodium nitrate in 1.5 liters of distilled wter was added 
to the mixed aqueous solution obtained as described above. The suspension 
consequently produced was heated and stirred for evaporation. The 
resultant residue of evaporation was molded and then calcined under a 
current of air at 450.degree. C. for six hours, to prepare a catalyst. The 
metal composition of this catalyst in atomic ratio was as follows. 
EQU Co.sub.4 Fe.sub.1 Bi.sub.1 W.sub.3 Mo.sub.9 Si.sub.1.35 Na.sub.0.1 
(Preparation of second step catalyst) 
In 40 liters of heated water, 17.7 kg of ammonium paramolybdate and 1.9 kg 
of ammonium metavandate were stirred and dissolved. To the resultant 
solution, 4 kg of pyridine and 1.25 kg of phosphoric acid (85% by weight) 
were added and then a mixed solution obtained by dissolving 11 kg of 
nitric acid. 1.8 kg of strontium nitrate, 2.5 kg of calcium nitrate, and 
0.4 kg of copper nitrate in 220 liters of water was added. The resultant 
mixture was stirred and heated to be concentrated. The clayish substance 
consequently obtained was molded in a Cylindrical form of 5 mm 
.phi..times.5 mm L (.phi.: diameter, L: long), dried at 250.degree. C., 
and calcined under a current of nitrogen at 450.degree. C. for four hours 
and under a current of air at 400.degree. C. for two hours. Consequently, 
there was obtained a catalyst oxide. The composition of this catalyst 
except for oxygen in atomic ratio was as follows. 
EQU P.sub.1.3 MO.sub.12 V.sub.2 Sr.sub.1.0 Ca.sub.1.5 Cu.sub.0.2 
(Method of reaction) 
In a reactor formed of one stainless steel reaction tube 25.4 mm in inside 
diameter and 5,000 mm in length and adapted to effect exchange of heat 
through circulation of molten salt, the aforementioned first step catalyst 
was packed in the form of a bed 1,700 mm in height and heated to 
340.degree. C. 
In a separate reactor formed of one stainless steel reaction tube 25.0 mm 
in inside diameter and 5,000 mm in length and adapted to effect exchange 
of heat through circulation of molten salt, a stainless steel gauze was 
set in place at a position, 1,800 mm above the lower end of the reaction 
tube so as to serve as a catalyst retainer and the aforementioned second 
step catalyst was packed in the form of a bed 2,700 mm in height and 
heated to 280.degree. C. 
The two reactors thus prepared were interconnected with a conduit provided 
with nozzles for introduction of a molecular oxygen-containing gas and 
steam and further provided with a heat-exchanger, so as to permit 
introduction of the gas formed by the reaction in the reactor containing 
the first step catalyst into the reactor containing the second step 
catalyst. In this case, the temperature of the gas kept at 220.degree. C. 
en route to the entrance to the second step reaction tube inside the 
second step reactor. 
Further, in the upper part (the reaction gas inlet side) of the catalyst 
bed in the second step reaction tube, a metallic plate 300 mm in overall 
length formed by corrugating a stainless steel plate (SUS 304) 0.4 mm in 
wall thickness and 17 mm in width with a zigzag angle of about 90 degrees 
and a zigzag pitch of 35 mm was inserted so as to extend from the point 
220 mm from the inlet part of the reaction tube to the upper end of the 
second step catalyst bed. In this case, the void ratio in the portion 
accommodating the corrugated metallic plate was 98%. 
Through the gas inlet part of the first step catalyst bed, a mixed gas 
consisting of 4.5% by volume of isobutylene, 10.0% by volume of oxygen, 
15.0% by volume of steam, and the balance of nitrogen gas was supplied to 
the first step catalyst at a flow rate of 1,100 N.liters per hour. Then, 
at the inlet to the second step catalyst bed, the feed gas was replenished 
with secondary air in such an amount as to adjust the molar ratio of 
oxygen (O.sub.2) to methacrolein (MAL), O.sub.2 /MAL, to 2.5. At this 
time, the pressure difference between the inlet and outlet of the second 
step reactor was 160 mmHg. This reaction was continued for 2,000 hours. In 
this while, the temperatures of the molten salt in the first step and 
second step reactors were required to be elevated respectively by 
3.degree. C. and 2.degree. C. 
The results of the reaction at the outset of the reaction and after 2,000 
hours' reaction were as shown in Table 1. The conversion of isobutylene 
shown in the table was calculated from the amount of isobutylene consumed 
on the way from the inlet part of the first step reactor to the outlet 
part of the second step reactor and the one-pass yield of methacrylic acid 
was expressed as the ratio of the amount of methacrylic acid formed at the 
outlet of the second step reactor to the amount of isobutylene supplied to 
the first step reactor. 
Control 1 
The same reactors as used in Example 1 were prepared, except that the use 
of the insert on the inlet side of the second step catalyst bed of the 
second step reactor was omitted and the second step catalyst was packed 
after the pattern of Example 1 so that the upper surface of the catalyst 
bed might rises to 500 mm from the inlet side of the reactor. 
The performance of the reaction at the outset thereof was as shown in Table 
1. During a protracted continuance of this reaction, the pressure 
difference between the outlet and inlet of the second step reactor rose to 
243 mmHg after about 800 hours' reaction (Control 1--1). When the reaction 
was discontinued and the second step reactor was examined, it was found 
that the inlet side catalyst bed of the second step catalyst was occluded 
with polymers. To avoid this occlusion, the empty space part on the inlet 
side of the second step catalyst bed was given a length of 1,500 mm. The 
results at the outset of the reaction and after 2,000 hours reaction' were 
as shown in Table 1. The reaction was carried out by following the 
procedure of Example 1, except for the point mentioned above (Control 
1-2). 
It is clearly noted from Table 1 that the length of the empty space part 
was elongated, it induced autoxidation (evidenced increase in the amounts 
of carbon monoxide and acetic acid) and lead to decline of yield. In spite 
of the empty space, the pressure loss was increased, though slightly, 
after 2,000 hours' reaction. 
EXAMPLE 2 
A reaction was carried out by faithfully following the procedure of Example 
1, except that tertiary butanol was used in place of isobutylene. The 
results at the outset of the reaction and after 2,000 hours' reaction were 
as shown in Table 1. 
EXAMPLE 3 
A reaction was carried out by following the procedure of Example 1, except 
that the temperature of the molten salt was set to 345.degree. C. in the 
first step reactor and 285.degree. C. in the second step reactor and a 
mixed gas consisting of 7.0% by volume of isobutylene, 14.0% by volume of 
oxygen, 15% by volume of steam, and the balance of nitrogen gas was 
supplied to the first step catalyst bed. The results at the outset of the 
reaction and after 2,000 hours' reaction were as shown in Table 1. It is 
clearly noted from Table 1 that no increase of pressure loss was 
recognized when the concentration of isobutylene was increased. 
EXAMPLE 4 
A spiraled insert having a width 23 mm and a pitch of 45 mm was prepared of 
a metal plate of the same material and dimensions as the metal plate used 
in Example 1. This insert had an overall length of 300 mm (void ratio 
97.5%). The reaction was carried out by following the procedure of Example 
1. The results were as shown in Table 1. 
EXAMPLE 5 
A reaction was carried out by following the procedure of Example 1, except 
that a corrugated insert having a zigzag angle of about 90 degrees and a 
zigzag pitch of about 35 mm and measuring 300 mm in overall length which 
was formed of a cylindrical metallic bar made of stailess steel (SUS 304) 
and possessed of an outside diameter of 5 mm was used instead. The void 
ratio in this case was about 96%. The reaction results were as shown in 
Table 1. 
Control 2 
A reaction was carried out by following the procedure of Example 1, except 
that the reaction tube of the second step reactor was packed with 
stainless steel Raschig rings 10 mm in diameter and 10 mm in length 
instead of using the insert. The packing was so made as giving a length of 
200 mm to the empty space part on the inlet side of the reactor and a 
height of 300 mm to the layer of the Raschig ring inserts beneath the 
empty space part and form a bed of the second catalyst thereunder. The 
void ratio of the packed bed of Raschig rings was 91%. The reaction 
results were as shown in Table 1. 
The pressure loss between the outlet and inlet of the second step reactor 
increased as the reaction advanced. When the reaction was discontinued 
after 800 hours' operation and the second step reactor was examined. it 
was found that the packed bed of Raschig rings was occluded rather 
conspicuously with solid matters such as polymers. It is noted that the 
state of occlusion was heavily affected by the shape of the packing in 
spite of the ampleness of the void ratio. 
EXAMPLE 6 
In the same apparatus as used in Example 5, except that the outside 
diameter of the cylindrical metallic bar was increased to 19 mm and the 
length of the insert was set to 300 mm, with the insert disposed in the 
empty space part. The void ratio in this case was about 43%. The reaction 
was carried out by following the procedure of Example 1. The results were 
as shown in Table 1. 
After 2,000 hours' operation, the pressure loss increased to a slight 
extent and not to such an extent as to impede the reaction. When the 
reaction was discontinued and the portion of the reactor accommodating the 
insert was examined, it was found that polymers were deposited only 
slightly on the inlet side portion of the insert. 
EXAMPLE 7 
A reaction was carried out by faithfully following the procedure of Example 
1, except that a plate of alumina measuring 0.4 mm in wall thickness, 17 
mm in width, and 300 mm in length was used as an insert. 
In this case, the void ratio in the portion accommodating the alumina 
insert was 98.3%. The method of reaction and the method of catalyst 
packing were similar to those of Example 1. The result were as shown in 
Table 1. 
TABLE 1 
__________________________________________________________________________ 
Conversion of 
Insert used in second step 
isobutylene (tertiary 
One-pass yield of 
Pressure loss in second 
reaction tube butanol) (mol %) 
methacrylic acid (mol %) 
step reactor (mmH 
__________________________________________________________________________ 
g) 
Example 1 
SUS plate (zig zag) 
at the outset 99.0 68.5 160 
after 2,000 hours' operation 
98.9 68.7 158 
Control 1-1 
Empty space 500 mm 
at the outset 99.2 68.4 156 
after 800 hours' operation 
99.0 68.5 243 
Control 1-2 
Empty space 1,500 mm 
at the outset 98.9 67.8 161 
after 2,000 hours' operation 
99.1 67.6 167 
Example 2 
SUS plate (zig zag) 
at the outset 100 68.7 163 
after 2,000 hours' operation 
100 68.4 164 
Example 3 
SUS plate (zig zag) 
at the outset 98.5 67.2 180 
after 2,000 hours' operation 
98.4 67.1 178 
Example 4 
SUS plate (spiral) 
at the outset 98.8 68.3 165 
after 2,000 hours' operation 
98.9 68.3 167 
Example 5 
SUS rod (zig zag) 
at the outset 99.2 68.4 158 
after 800 hours' operation 
99.3 68.7 160 
Control 2 
Rasching ring 
at the outset 99.0 68.1 168 
after 800 hours' operation 
98.8 67.7 210 
Example 6 
SUS rod (zig zag) 
at the outset 98.6 68.5 165 
after 2,000 hours' operation 
98.7 68.2 182 
Example 7 
Alumina sheet (plate) 
at the outset 98.9 68.4 162 
after 2,000 hours' operation 
99.0 68.5 165 
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SUS: Stainless steel