Process for producing acrylic acid from propylene

A process for producing acrylic acid from propylene through acrolein as an intermediate by catalytic vapor phase oxidation, which comprises passing a starting reactant gas mixture containing propylene, a molecular oxygen-containing gas and steam through a first-stage reactor packed with a molybdenum-containing multi-component catalyst, passing the resulting acrolein-containing gas through a second-stage reactor packed with a multi-component catalyst containing vanadium and molybdenum, introducing the resulting acrylic acid-containing gas to an acrylic acid collector thereby to recover acrylic acid in the form of an aqueous solution, and incorporating a part of the exhaust gas from the collector in the starting reactant gas mixture.

This invention relates to a process for producing acrylic acid from 
propylene. More specifically, it relates to an improved process for 
producing acrylic acid with good commercial efficiency by the catalytic 
vapor phase oxidation of propylene in the presence of steam via acrolein 
as an intermediate. 
Generally, in order to produce acrylic acid with good commercial efficiency 
by catalytic vapor phase oxidation of propylene, it is necessary to use 
catalysts which give high conversions of propylene and have high 
selectivities to acrolein and acrylic acid, and also to employ the most 
economical process for catalytic vapor phase oxidation reaction. The 
catalytic vapor phase oxidation of propylene to acrylic acid usually 
consists of two stages. In the first stage, acrolein and a small amount of 
acrylic acid is formed from propylene. In the second stage, acrylic acid 
is formed from acrolein. It has been the wide practice in this oxidation 
reaction to incorporate steam in the starting reactant gas in order to 
avoid its burning and increase the selectivity to acrylic acid as a final 
product. 
For example, U.S. Pat. No. 3,970,702 discloses that in a reaction of 
oxidizing propylene to acrolein, it is desirable to incorporate steam in 
the starting reactant gas in an amount of about 1 to 15 moles per mole of 
propylene. German Patent Publication No. 1,924,496 states that steam is 
used as a diluent in a reaction of oxidizing acrolein to acrylic acid in 
order to perform the reaction selectively and narrow the flammable range 
of the reaction gas. 
Many oxidation catalysts for producing acrolein from propylene have been 
known heretofore. For example, by the process disclosed in U.S. Pat. No. 
3,825,600, acrolein is obtained in one-pass yield of 80 to 90 mole% by 
using catalytic oxides containing cobalt, iron, bismuth, tungsten, 
molybdenum, silicon and alkali metals as constituent elements. The process 
disclosed in U.S. Pat. No. 3,855,308 affords acrolein in a one-pass yield 
of 84 to 89 mole% when using catalytic oxides containing cobalt, iron, 
bismuth, tungsten, silicon and thallium as constituent elements. According 
to the process disclosed in Japanese Laid-Open Patent Publication No. 
47917/75, acrolein is obtained in a one-pass yield of 80% by using 
catalytic oxides containing cobalt, iron, bismuth, tungsten, molybdenum, 
zinc, indium and silicon as constituent elements. Elsewhere, the process 
idsclosed in Japanese Laid-Open Patent Publication No. 92006/74 gives 
acrolein in a one-pass yield of 92.4 mole% when using catalytic oxides 
containing cobalt, iron, bismuth, molybdenum, potassium, chromium, silicon 
and phosphorus as constituent elements. 
Known catalysts for producing acrylic acid from acrolein are also many. For 
example, U.S. Pat. No. 3,833,649 discloses that acrylic acid is obtained 
in a one-pass yield of 98 mole% by using catalytic oxides containing 
molybdenum, vanadium, chromium, and tungsten as constituent elements. The 
process disclosed in U.S. Pat. No. 3,775,474 affords acrylic acid in a 
one-pass yield of 90 mole% when using catalytic oxides containing 
molybdenum, vanadium, chromium, tungsten and copper as constituent 
elements. According to the process of U.S. Pat. No. 3,954,855, acrylic 
acid is obtained in a one-pass yield of 91.7 to 97.5 mole% by using 
catalytic oxides containing molybdenum, vanadium, tungsten, copper and 
alkaline earth metals as constituent elements. Furthermore, by the process 
disclosed in U.S. Pat. No. 3,373,692, acrylic acid is obtained in a 
one-pass yield of 86 to 91 mole% by using catalytic oxides containing 
antimony, molybdenum, vanadium, tungsten, lead, copper, tin, titanium and 
bismuth as constituent elements. 
All of these prior art techniques, however, are directed to the improvement 
of catalysts for producing acrylic acid from propylene through acrolein, 
namely the development of high-performance catalysts which give high 
yields of high selectivities. These prior art references are quite silent 
on the improvement of the manufacturing process itself, namely on the 
development of a process which can use high concentrations of propylene 
and avoid a danger of explosion (combustion), or a process which can 
maintain the performance of the catalyst over long periods of time, which 
are factors of utmost importance for commercial practice. 
Furthermore, in commercial operations, it is extremely important to attempt 
at process advantages, such as the reduction of the consumption of steam 
the recovery of a high concentration aqueous solution of acrylic acid in a 
step of collecting reaction products, and the reduction of the amount of 
the waste water, while maintaining high level reaction performance (the 
reaction conditions for maintaining the conversion of propylene and the 
selectivity to acrolein and acrylic acid in the first-stage reaction and 
the one-pass yield of acrylic acid in the second-stage reaction at high 
levels and also for maintaining the productivity of acrylic acid high). 
Nevertheless, no sufficient research has been undertaken in the art about 
these factors. 
It is an object of this invention therefore to provide an improved process 
for producing acrylic acid from propylene through acrolein as an 
intermediate by a two-stage catalytic vapor phase oxidation reaction. 
The object of the invention is achieved by a process which comprises 
passing a starting reactant gas mixture containing propylene, a molecular 
oxygen-containing gas and steam through a first-stage reactor packed with 
a molybdenum-containing multi-component catalyst, passing the resulting 
acrolein-containing gas through a second-stage reactor packed with a 
multi-component catayst containing vanadium and molybdenum, introducing 
the resulting acrylic acid-containing gas to an acrylic acid collector 
thereby to recover acrylic acid in the form of an aqueous solution, and 
incorporating a part of the exhaust gas from the collector in the starting 
reactant gas mixture; wherein 
(1) the starting reactant gas mixture contains 4 to 30% by volume of steam, 
3 to 9% by volume of propylene and 1.6 to 4.0 moles, per mole of 
propylene, of oxygen, 
(2) the reaction conditions in the first-stage reactor are controlled so as 
to maintain the reaction temperature at 250 to 450.degree. C., the contact 
time at 1.0 to 7.2 seconds, the conversion of propylene at at least 80 
mole%, and the total one-pass yield of acrolein and acrylic acis at at 
least 70 mole%, 
(3) the reaction conditions in the secondstage reactor are controlled so as 
to maintain the reaction temperature at 180 to 350.degree. C., the contact 
time at 1.0 to 7.2 seconds, and the one-pass yield of acryli acid based on 
propylene at at least 70 mole%, and 
(4) the amount of the exhaust gas to be incorporated in the starting 
reactant gas mixture is adjusted so that the acrylic acid content of the 
starting reactang gas mixture is not more than 0.5% by volume. 
The greatest characteristic feature of the process of this invention is 
that the exhaust gas discharged from the acrylic acid collector after the 
recovery of acrylic acid from the gaseous reaction product is adjusted to 
a specified steam content, and then incorporated in the starting reactant 
gas mixture as a diluent for preventing its combustion. 
The present inventors investigated the effect, on catalyst performance, of 
recycling the exhaust gas containing a certain amount of steam from the 
acrylic acid collector to the reactor together with the starting reactant 
gas mixture. In the course of this investigation, the inventors found that 
the performance of catalyst is reduced with time. As a result of searching 
for its cause, it was found that the reduction of the catalyst performance 
is ascribable to impurities (e.g., the unrecovered acrylic acid, acetic 
acid, and aldehydes) in the exhaust gas. It has not yet been known which 
of these impurities causes catalyst degradation. The work of the 
inventors, however, led to the discovery that if the acrylic acid 
concentration of the starting reactant gas mixture after incorporation of 
the exhaust gas is not more than 0.5% by volume, preferably not more than 
0.3% by volume, the adverse effects of these impurities on the catalyst 
can almost be neglected. 
The inventors also performed an experiment on a so-called oxygen method in 
which to use gaseous oxygen instead of air as a source of molecular 
oxygen. It was found that almost all of the exhaust gas can be recycled, 
but light-boiling impurities (e.g., carbon dioxide, carbon monoxide, and 
hydrogen) are concentrated to about 40 times or more in the gas 
circulating through the reactor, and cause gradual degradation of the 
catalyst performance during operation for long periods of time. Thus, it 
was ascertained that acrylic acid cannot be produced generally in high 
yields by the complete recycle method in accordance with the oxygen 
method. It is necessary to avoid the accumulation of impurities which 
cause the degradation of catalyst performance. Hence, conditions for 
obtaining the exhaust gas and conditions for recycling it to the reactor, 
namely, the operating conditions in the acrylic acid collector and the 
recycling rate of the exhaust gas to the reactor, are important, and the 
present invention has offered a solution to this problem. 
The acrylic acid collector is a device which cools the pre-cooled gaseous 
reaction product, and using water, collects acrylic acid in the form of an 
aqueous solution, and may, for example, be a packed tower, a plate tower, 
a bubble cap tower, or a sieve tower. 
In these types of acrylic acid collectors, the temperature of the tower top 
is set within the range of temperatures at which acrylic acid is recovered 
from the gaseous reaction product with good efficiency as a high 
concentration aqueous solution of acrylic acid and at which the 
concentration of steam in the starting reactant gas mixture reaches a 
predetermined value. The tower top temperature so set is 35.degree. to 
80.degree. C., preferably 40.degree. to 70.degree. C. If the tower top 
temperature is set at a lower point, that is below 35.degree. C., the 
amount of the recycle gas increases as a result of supplying a specified 
amount of steam, and the amount of oxygen to be supplied is insufficient. 
Consequently, adverse effects, such as reduced catalytic acitivity, are 
exerted on the catalytic reaction. Furthermore, because of the low 
temperatures, light-boiling aldehydes such as acrolein or other 
by-products tend to be collected at the same time as the recovery of 
acrylic acid, and this will cause various troubles to subsequent steps for 
purification of acrylic acid. On the other hand, when the tower top 
temperature exceeds 80.degree. C., acrylic acid and other impurities are 
fed to the reactor together with the recycle exhaust gas, and adversely 
affect the catalytic reaction. Moreover, the rate of recovering acrylic 
acid decreases. Attempts to increase the recovery rate of acrylic acid at 
this time inevitably involves the reduction of the concentration of the 
aqueous solution of acrylic acid. Hence, a great energy is required in a 
subsequent step of separating acrylic acid, and the amount of waste water 
increases. These are economically disadvantageous. 
The proportion of that part of the exhaust gas from the acrylic acid 
collector which is to be recycled to the reactor is determined according 
to the concentrations of propylene, steam and oxygen in the starting 
reactant gas mixture, and the tower top temperature of the acrylic acid 
collector. Usually, the amount of the recycle exhaust gas is 15 to 85%, 
preferably 18 to 70%, based on the exhaust gas. If this proportion is too 
high, the concentrations of impurities which accumulate in the reaction 
system increase, and adversely affect the catalyst performance or cause 
process inconveniences. Furthermore, troubles tend to occur owing to the 
insufficiency of oxygen in the reaction system. If, on the other hand, the 
proportion of the recycle gas is small, the tower top temperature of the 
acrylic acid collector should be extremely increased in order to secure a 
sufficient amount of steam required for the reaction. Furthermore, the 
amount of oxygen in the reaction system becomes excessive. Hence, this 
causes the defect that the concentration of propylene cannot be increased 
in order to avoid a danger of combustion. 
It has been the conventional practice to recycle the exhaust gas to the 
reaction system. For example, in the process disclosed in U.S. Pat. No. 
3,801,634, propylene is oxidized in two stages to produce acrylic acid, 
and the exhaust gas is recycled to the first-stage reaction after removing 
all condensable substances, such as acrylic acid or steam, from gaseous 
reaction products by cooling. According to the process of this U.S. 
Patent, the exhaust gas is used only as an inert diluting gas for the 
reaction, and is not used additionally as a source of steam essential for 
the reaction, as is done in the process of the present invention. Since 
the process of the U.S. Patent does not intend the substantial inclusion 
of steam in the exhaust gas, the conditions for re-using the exhaust gas 
as an inert diluting gas, are not important, and the U.S. Patent does not 
at all disclose such conditions. 
U.S. Pat. No. 3,717,675 also discloses a process in which the exhaust gas 
is recycled to the reaction system. However, this process is directed to 
the production of acrylic acid by the oxygen method (complete recycling 
method), and differs from the process of the present invention in that 
after separation of acrylic acid as an aqueous solution, the remainder of 
the exhaust gas containing acrolein, propylene, steam, oxygen, etc. is all 
recycled back to the reaction system. 
The present inventors extensively worked on the re-use of the exhaust gas 
an an inert diluting gas for the reaction, and found that the conditions 
for obtaining the exhaust gas and the conditions for re-using the exhaust 
gas (the proportion of the recycle gas) are of utmost importance. Further 
investigations into these conditions led to the discovery that acrylic 
acid can be obtained in high yields over long periods of time with 
commercial advantage only when the temperature of the tower top of the 
acrylic acid collector is adjusted to 35.degree.-80.degree. C., and the 
proportion of the recycle gas is adjusted to 15 to 85%. 
In the process of U.S. Pat. No. 3,801,634 cited above, for example, in 
Example 13, the conversion of propylene is as low as 79%, and the yield of 
acrylic acid is also as low as 50%. This is presumably because the 
conditions for the overall process of recycling the exhaust gas and the 
reaction conditions are outside the range of the essential conditions used 
in the process of the present invention. When calculated on the basis of 
the Examples of Belgian Patent Nos. 738,250 and 746,202 cited in this U.S. 
Patent, the conversion of propylene must be at least 90%, and the yield of 
acrylic acid (the first and second stages inclusive) must be 77%, in the 
reactions of the first and second stages in Example 13 of the U.S. Patent. 
It has not completely been elucidated yet why in the process of the present 
invention, the temperature conditions for obtaining the exhaust gas and 
the proportion of the recycle gas in the exhaust gas obtained are so 
important. The present inventors, however, assume that unidentifiable 
impurities formed in the oxidation reaction are concentrated in the 
recycle system when the conditions specified in the invention are not met, 
or acrylic acid or by-product acetic acid and other impurities are again 
fed into the reactor together with the exhaust gas when they are not 
sufficiently collected, with the result that the catalytic reaction is 
impaired. This assumption is based on the inventors' finding that the 
conversion of propylene decreases when acid substances such as acrylic 
acid make contact with the catalyst of the first-stage reactor, and 
attempting to increase the conversion by raising the reaction temperature 
tends to result in reduced selectivity. 
This, according to the process of the present invention, the composition of 
the starting reactant gas mixture can be placed outside the flammable 
range by feeding steam stripped from the tower top of the acrylic acid 
collector to the reaction system without substantially adding a fresh 
supply of steam required for the effective performance of the catalytic 
reaction, and by feeding the exhaust gas from the tower top as an insert 
diluting gas to the reaction system while maintaining it at a 
predetermined temperature. 
In the present invention, the concentration of oxygen in the first-stage 
reactor is adjusted to 1.6-4.0 moles, preferably 1.7-3.0 moles, per mole 
of propylene. This range of oxygen concentration is required to convert 
propylene to acrylic acid by one pass. If the oxygen concentration is less 
than 1.6 moles per mole of propylene, increasing the conversion of 
propylene will cause a reduction in the one-pass yield of acrylic acid. 
Moreover, when the oxygen concentration exceeds 4.0 moles per mole of 
propylene, the concentration of propylene must be reduced to avoid 
explosion or combustion and the process is necessarily low in productivity 
and commercial value.

Air is fed from a blower 101, passed through a line 1, heated at a 
preheated 102, and then mixed in a line 2 with a recycle gas from a line 
13. If desired, steam for adjustment purposes may come into the line 13 
from a line 19. The reactant gas mixture obtained is mixed in a line 3 
with propylene gas fed through a line 4. The starting reactant gas mixture 
then enters a first-stage reactor 103 through a line 5. The reactor 103 is 
of a multi-tubular heat exchanger type having a catalyst packed inside the 
tubes and a heat-transfer medium for removal of the heat of reaction being 
circulated outside the tubes. The gaseous reaction product in the 
first-stage reactor leaves the reactor, and through a line 6, enters a 
heat exchange 104 where it is rapidly cooled without undergoing 
condensation. The cooled gas passes through a line 7 and enters a 
second-stage reactor 105 which is of the same type as the first-stage 
reactor 103. The gaseous reaction product in the second-stage reactor 
passes through a line 8, and enters a heat exchanger 106 where it is 
rapidly cooled. The cooled gaseous product passes through a line 9, and 
enters an acrylic acid collector 107. (The gaseous reaction product does 
not undergo condensation by rapid cooling until it reaches the line 9.) 
The acrylic acid collector 107 consists of a lower portion and an upper 
portion having different functions. The lower portion is of a structure of 
a multi-tubular heat exchanger, or a packed tower or plate tower having a 
heat exchanger either inside or outside. In the lower portion, the gaseous 
product fed is cooled indirectly by a cooling medium, or directly cooled 
by contact with a cooled aqueous solution of acrylic acid, and also 
humidified. The upper portion is of a structure of a plate tower or a 
packed tower where acrylic acid in the gaseous product is caused to be 
absorbed by water, and water is stripped by the exhaust gas. Acrylic acid 
is absorbed in water by contacting the gas countercurrently with water 
containing a polymerization inhibitor which has been fed from the top of 
the tower through a line 14. 
The acrylic acid collector 104 should be operated in such a manner that 
acrylic acid is collected as a high concentration aqueous solution of 
acrylic acid with good efficiency, the absorption of impurities such as 
acrolein is prevented to the greatest possible extent, and all the steam 
required for the reaction is included in the exhaust gas which is 
discharged from the top of the tower. Of the operating conditions 
required, the operating temperature is especially important. Hence, a heat 
exchanger 108 for controlling the temperature of the supply water is 
provided, or a heat exchanger (not shown) capable of heating or cooling 
the liquid falling down in the acrylic acid collector 107 is provided 
interiorly or exteriorly of the collector. 
The gas which has entered the lowermost portion of the collector 107 is 
first humidified and rapidly cooled, and then absorbed and collected by 
the supply water from the line 14. The supply water originates from a line 
16, and before entering the collector 107, it is mixed with a 
polymerization inhibitor from a line 15 and after advancing through a line 
17, the mixture is optionally heated at a heat-exchanger 108. Ordinary 
water is used as the water from the line 16. The waste water from the 
process of acrylic acid purification (for example, the waste water 
resulting after separating acrylic acid from the aqueous solution of 
acrylic acid, and removing light-boiling substances from the residue) can 
also be used with a care taken, however, not to have the impurities such 
as acrylic acid returned to the reaction system. 
The aqueous solution of acrylic acid obtained in the collector 107 is 
withdrawn through a line 18, and subjected to a separating and purifying 
procedure. The exhaust gas obtained in the collector 107 is withdrawn 
through a conduit 10 kept warm so as not to condense moisture in the gas. 
The exhaust gas is then divided into two portions, one to be reused in the 
reaction, and the other to be discharged. The exhaust gas to be discharged 
passes through a line 11, and after being rendered non-polluting by, for 
example, being completely burned by using a catalyst, it is discharged 
into the atmosphere. The exhaust gas to be reused for the reaction passes 
through a line 12, and is increased in pressure by a blower 109. Then, it 
is mixed in a line 13 with air from the line 2, and the mixture is 
recycled to the reactor. 
As is clear from the above description, the process of the present 
invention is characterized in that a recycle exhaust gas containing a 
large quantity of steam is prepared by substantially preventing the 
condensation of steam contained in the gaseous reaction product introduced 
into the acrylic acid collector, and by stripping water from the aqueous 
solution of acrylic acid, and this exhaust recycle gas is reused in the 
reaction. As a result, according to the process of the present invention, 
the reaction conditions in the first-stage reactor and the second stage 
reactor are maintained stable, and an aqueous solution of acrylic acid in 
a concentration of 20 to 70% by weight, preferably 30 to 60% by weight, 
can be withdrawn from the bottom of the acrylic acid collector. 
The molybdenum-containing multi-component catalyst used in the first-stage 
reaction is preferably a catalyst containing molybdenum, iron and bismuth, 
more preferably a catalyst containing molybdenum, cobalt, iron, bismuth 
and at least one element selected from the group consisting of alkali 
metals, alkaline earth metals, thallium, tungsten and silicon. These 
catalysts are disclosed, for example, in U.S. Pat. Nos. 3,639,269, 
3,778,386, 3,799,978, 3,970,702, and 3,972,920, German Laid-Open Patent 
Publications 2,165,335 and 2,203,710, Japanese Patent Publications 
42813/72, 4762/73 and 4764/73, and Japanese Laid-Open Patent Publication 
30308/74. In addition to the catalysts disclosed in these prior art 
references, any other catalysts can be used which can meet the conditions 
in the first-stage reaction, namely which can achieve a propylene 
conversion of at least 80 mole%, preferably at least 90 mole%, and a total 
one-pass yield of acrolein and acrylic acid of at least 70 mole%, 
preferably at least 80 mole%, when a starting reactant gas mixture 
containing 4 to 30% by volume, preferably 5 to 25% by volume, of steam, 3 
to 9% by volume, preferably 4 to 8% by volume, of propylene and 1.6 to 4.0 
moles, preferably 1.7 to 3.0 moles, per mole of propylene, specifically, 6 
to 18% by volume, preferably 8 to 16% by volume) of oxygen is used, and 
the reaction is carried out at a reaction temperature of 250.degree. to 
450.degree. C., preferably 270.degree. to 370.degree. C., with a contact 
time of 1.0 to 7.2 seconds, preferably 1.8 to 3.6 seconds. 
The multi-component cartalyst containing vanadium and molybdenum used in 
the second-stage reaction is preferably a catalyst containing vanadium, 
molybdenum, and at least one element selected from the group consisting of 
copper, tungsten, chromium and alkaline earth metals. Such catalysts are 
disclosed, for example, In U.S. Pat. No. 3,766,265, and German Laid-Open 
Patent Publication Specification Nos. 2,164,905, 2,337,510, 2,344,956, 
2,448,804, and 2,459,092. In addition to these catalysts, any other 
catalysts can be used which meet the conditions of the second-stage 
reaction, namely which can achieve a one-pass yield of acrylic acid based 
on propylene of at least 70 mole% when the reaction is carried out at a 
reaction temperature of 180.degree. to 350.degree. C., preferably 
200.degree. to 300.degree. C. with a contact time of 1.0 to 7.2 seconds, 
preferably 1.6 to 3.0 seconds. 
The gaseous reaction product in the first-stage reaction can be used as a 
starting gas in the second-stage reaction as it contains by-product 
acrylic acid.. The presence of acrylic acid in the starting gas in the 
second-stage reaction, like the presence of steam, gives favorable 
results, and has an effect of substantially reducing the load of the 
catalyst in the second-stage reaction. 
The following Examples and Comparative Examples illustrate the present 
invention in greater detail. 
EXAMPLE 1 
Preparation of a catalyst for the first-stage reaction 
Ammonium molybdate (10.62 kg) and 3.24 kg of ammonium paratungstate were 
added to 15 liters of heated water, and the mixture was vigourously 
stirred (the solution obtained is designated solution A). 
Separately, 7.00 kg of cobalt nitrate was dissolved in 2 liters of water; 
2.43 kg of ferric nitrate, in 2 liters of water; and 2.92 kg of bismuth 
nitrate, in a mixture of 0.6 liter of conc. nitric acid and 3 liters of 
water. The three nitrate solutions were mixed, and the mixture was added 
dropwise to the solution A. Then, 2.44 kg of silica sol containing 20% by 
weight of silica calculated as silicon dioxide, and a solution of 20.2 g 
of pottasium hydroxide in 1.5 liters of water were added to the mixture. 
The resulting suspension was evaporated by heating, molded, and calcined 
under a stream of air at 450.degree. C. for 6 hours to form a catalyst. 
The composition of this catalyst excepting oxygen, in terms of atomic 
ratio, is as follows: 
EQU Co.sub.4 Fe.sub.1 Bi.sub.1 W.sub.2 Mo.sub.10 Si.sub.1.35 K.sub.0.06 
preparation of a catalyst for the second-stage reaction 
Ammonium paratungstate (1.254 kg), 1.03 kg of ammonium metavanadate, 4.06 
kg of ammonium molybdate, and then 0.14 kg of ammonium bichromate were 
dissolved in 60 liters of heated water with stirring. Separately, an 
aqueous solution of 1.03 kg of copper nitrate in 0.72 liter of water was 
prepared. The two solutions were mixed, and the mixture was placed in a 
stainless steel evaporator equipped with a steam heater, and 12 liters of 
an .alpha.-alumina carrier in the form of granules with a diameter of 3 to 
5 mm which had a surface area of less than 1 m.sup.2 /g and a porosity of 
42%, and contained pores, 92% by volume of which consisted of pores having 
a pure diameter of 75 to 250 microns, was added. With stirring, the 
mixture was evaporated to dryness, followed by calcining at 400.degree. C. 
for 5 hours to form a catalyst. The composition of this catalyst excepting 
oxygen, in terms of atomic ratio, was as follows: 
EQU Mo.sub.12 V.sub.4.6 Cu.sub.2.2 Cr.sub.0.6 W.sub.2.4 
reactions and the collection of acrylic acid 
A multi-tubular reactor including 10 steel reaction tubes with an inside 
diameter of 25 mm and a length of 3,000 mm was used in which heat exchange 
was possible on the shell side by circulating molten salts. The catalyst 
for the first-stage reaction (12.0 liters) was packed uniformly into the 
tube of the reactor, and heated to 325.degree. C. 
Separately, 9.0 liters of the catalyst for the second-stage reaction was 
packed uniformly into the tube of the same type of multi-tubular reactor 
as the first-stage reaction, and heated to 260.degree. C. 
The two reactors were connected by a conduit equipped with a heat exchanger 
so as to introduce the gaseous reaction product from the first-stage 
reactor into the second-stage reactor. 
The acrylic acid collector used was a stainless steel tower with an inside 
diameter of 200 mm. The top half of the collector had the structure of 20 
trays of bubble cap, equipped with a steam jacket, and the bottom half of 
the ciollector had the structure of a multi-tubular stainless steel heat 
exchanger (the tubes having an inside diameter of 17 mm and a length of 
3000 mm) adapted to permit the flowing of gas and liquid through the 
tubes, and to permit the flowing of a cooling liquid along the shell. The 
gaseous product from the second-stage reactor was introduced into the 
collector from below, and the acrylic acid in the gaseous product was 
collected as an aqueous solution by flowing down water containing a 
polymerization inhibitor from the topmost part of the tower. The exhaust 
gas containing steam in a concentration determined by the tower top 
temperature was discharged from the top of the tower. The exhaust gas was 
not condensed, and a part of it was prged. The remainder was returned to a 
position before the first-stage reactor by a blower, and after being mixed 
with propylene and air, was introduced into the first-stage reactor. 
During the operation, a gaseous mixture consisting of 5.5% by volume of 
propylene, 10.0% by volume of steam, 12.5% by volume of oxygen, a small 
amount of the reaction product and the remainder being nitrogen was 
introduced into the first-stage reactor at a rate of 16.2 m.sup.3 /h 
(calculated on NTP). At this time, the temperature of the tower top of the 
acrylic acid collector was 64.degree. C., and the proportion of the 
recycle gas was 42.4% based on the exhaust gas obtained. The flow rate of 
water flowing down from the tower top of the collector was 3.5 kg/hr, and 
the rate of acrylic acid collection was 98 to 99%. 
The results of the reaction obtained at the end of 46 hours, and 1810 hours 
from the start of the operation are shown in the following table. 
COMATIVE EXAMPLE 1 
The procedure of Example 1 was repeated except that the oxygen 
concentration in the starting reactant gas mixture to be introduced into 
the first-stage reactor was charged to 8.25% by volume (as a result, the 
oxygen/propylene molar ratio was setat 1.5), the tower top temperature of 
the acrylic acid collector was set at 58.degree. C., the proportion of the 
recycle gas was adjusted to 59.2% based on the exhaust gas, and the flow 
rate of the water from the tower top was 3.5 kg/hr. The results are shown 
in the following table. 
EXAMPLE 2 
The procedure of Example 1 was repeated except that the starting reactant 
gas mixture to be introduced into the first-stage reactor consisted of 6% 
by volume of propylene, 15% by volume of steam, 13.7% by volume of oxygen, 
a small amount of the reaction product and the remainder being nitrogen, 
the tower top temperature of the acrylic acid collector was changed to 
79.degree. C., the proportion of the recycle gas was adjusted to 25.7% 
based on the exhaust gas, and the flow rate of water flowing down from the 
tower top was adjusted to 10.0 kg/hr. The results obtained at the end of 
520 hours from the start of the reaction are shown in the following table. 
During this time, an aqueous solution of acrylic acid in a concentration of 
30 to 32% by weight was obtained, but the rate of acrylic acid collection 
decreased to 88%. The starting reactant gas mixture to be introduced into 
the first-stage reactor contained 0.13% by colume of acrylic acid. When a 
large amount of flowing water was used in order to raise the rate of 
acrylic acid collection to 98-99%, the concentration of the resulting 
aqueous solution of acrylic acid decreased drastically. 
COMATIVE EXAMPLE 2 
Acrylic acid was produced using the below-specified starting reactant gas 
and the same catalysts and reactors as used in Example 1. The results are 
tabulated hereinbelow. 
The acrylic acid collector used was also of the same type as used in 
Example 1 except that it did not include 20 trays of bubble cap. The 
gaseous reaction product was introduced into the collector from its 
bottom, and acrylic acid was collected by water containing a 
polymerization inhibitor which was cooled by a cooler and flowed down from 
the top of the tower at a rate of 2.5 kg/hr. The tower top temperature was 
adjusted to 64.degree. C., and the exhaust gas was obtained. 
A portion (6,790 liters/hr; 42.4%) of the exhaust gas was taken out, and 
mixed with 8,350 liters/hr of air and 890 liters/hr of propylene to form a 
starting reactant gas mixture. After 100 hours from the start of the 
reaction, the conversion of proplyene decreased to 85%, and the starting 
reactant gas at the inlet of the first-stage reactor contained 0.7% by 
volume of acrylic acid. 
EXAMPLES 3 to 7 
The procedure of Example 1 was repeated except that the composition of the 
starting reactant gas mixture, the reaction pressure, the tower top 
temperature of the acrylic acid collector, and the proportion of the 
recycle gas was changed so as shown in the following table. The results 
are shown in the following table. 
The amount of the flowing water was adjusted so as to obtain an acrylic 
acid collection rate of 98-99%. 
EXAMPLE 8 
In the same way as in Example 1, a catalyst (I) for the first-stage 
reaction, and a catalyst (II) for the second-stage reaction were prepared. 
The compositions of these catalysts excepting oxygen, in terms of atomic 
ratio, were as follows: 
Catalyst (I): 
EQU co.sub.5 Fe.sub.0.35 Bi.sub.1 W.sub.2 Mo.sub.10 Si.sub.1.35 K.sub.0.06 
catalyst (II): 
EQU mo.sub.12 V.sub.4.6 W.sub.2.2 Cu.sub.3.0 
Using 10.8 liters of the catalyst (I) and 9.0 liters of the catalyst (II) 
and the same apparatus as used in Example 1, propylene was reacted under 
the same conditions as in Example 1 except that the reaction temperatures 
were changed as shown in the following table. 
The results are tabulated hereinbelow. 
EXAMPLE 9 
In the same way as in Example 1, a catalyst (I) for the first-stage 
reduction and a catalyst (II) for the second-stage reaction was prepared. 
The compositions of the catalysts excepting oxygen, in terms of atomic 
ratio, were as follows: 
Catalyst (I): 
EQU co.sub.4 Fe.sub.1 Bi.sub.1 W.sub.2 Mo.sub.10 Si.sub.1.35 Tl.sub.0.05 
Catalyst (II): Mo.sub.12.4 V.sub.4.8 Sr.sub.0.5 W.sub.2.4 
in the catalyst preparation, thallium nitrate was used as a source of 
thallium, and strontium nitrate, as a source of strontium. 
Using 12.0 liters of the catalyst (I) and 9.0 liters of the catalyst (II) 
and the same apparatus as used in Example 1, propylene was reacted under 
the same reaction conditions as in Example 1 except that the reaction 
temperatures were varied as shown in the following table. 
The results are tabulated hereinbelow. 
EXAMPLE 10 
In the same way as in Example 1, a catalyst (I) for the first-stage 
reaction and a catalyst (II) for the second-stage reaction were prepared. 
The compositions of these catalysts excepting oxygen, in terms of atomic 
ratio, were as follows: 
Catalyst (I): 
EQU co.sub.4 Fe.sub.1 Bi.sub.1 W.sub.2 Mo.sub.10 Si.sub.1.35 Mg.sub.0.04 
Catalyst (II): 
EQU mo.sub.12 V.sub.4.8 Ba.sub.0.5 Cu.sub.2.2 W.sub.2.4 
in the catalyst preparation, magnesium nitrate was used as a source of 
magnesium, and barium nitrate, as a source of barium. 
Using 12.0 liters of the catalyst (I) and 9.0 liters of the catalyst (II) 
and the same apparatus as in Example 1, propylene was reacted under the 
same reaction conditions except that the reaction temperatures were 
varied. The results are tabulated below. 
The "conversion," "one-pass yield," and "proportion of recycle gas" as used 
in the present application, are defined as follows: 
##EQU1## 
__________________________________________________________________________ 
Reaction 
Composition of the starting 
temperature 
reactant gas mixture Oxygen/ 
Reaction 
(.degree. C.) 
(% by volume) propylene 
time that 
1st 
2nd Acrylic 
(mole elapsed 
stage 
stage 
Propylene 
Steam 
Oxygen 
acid ratio) 
(hr) 
__________________________________________________________________________ 
Example 1 
325 
260 5.5 10.0 12.5 0.03 2.28 46 
1810 
Comparative 
325 
260 5.5 10.0 8.25 0.03 1.50 30 
Example 1 
Example 2 
325 
260 6.0 15.0 13.7 0.13 2.28 520 
Comparative 
325 
260 5.5 10.0 12.5 0.7 2.25 2 
Example 2 100 
Example 3 
325 
260 3.0 10.0 8.1 0.03 2.70 264 
Example 4 
325 
260 6.0 12.0 13.7 0.04 2.28 328 
Example 5 
325 
265 7.0 5.0 14.0 0.02 2.00 430 
Example 6 
325 
260 4.0 20.0 10.0 0.04 2.50 241 
Example 7 
325 
260 5.5 10.0 12.5 0.02 2.28 500 
Example 8 
325 
250 5.5 10.0 12.5 0.03 2.28 208 
Example 9 
335 
245 5.5 10.0 12.5 0.02 2.28 120 
Example 10 
310 
245 5.5 10.0 12.5 0.03 2.28 145 
__________________________________________________________________________ 
Acrylic acid collector 
Concentration of 
Tower top 
Proportion 
Amount of 
the resulting 
Conversion 
One-pass yield 
tempera- 
of the re- 
absorbing 
aqueous solution 
of pro- 
(mole %) 
ture cycle gas 
water of acrylic acid 
pylene 
Acrylic 
Acro- 
(.degree. C.) 
(%) (kg/hr) 
(wt. %) (%) acid lein 
__________________________________________________________________________ 
Example 1 
64 42.4 3.5 49.5 95.2 83.7 0.5 
50.0 95.7 84.0 0.4 
Comparative 
58 59.2 3.5 37.5 93.5 66.4 20.3 
Example 1 
Example 2 
79 25.7 10.0 30.0-32.0 
94.2 82.0 0.5 
Comparative 
64 42.4 2.5 50.1 95.0 83.5 0.5 
Example 2 41.5 85.1 69.2 1.3 
Example 3 
53 80.0 3.5 50.0 94.1 83.3 0.4 
Example 4 
73 29.4 3.5 40.0 95.3 83.0 0.5 
Example 5 
56 31.8 3.5 55.0 95.5 82.2 0.5 
Example 6 
73 58.5 3.5 33.0 95.7 83.2 0.4 
Example 7 
64 42.4 3.5 50.4 96.5 84.3 0.3 
Example 8 
64 42.4 3.5 50.0 95.9 84.1 0.4 
Example 9 
64 42.4 3.5 48.0 92.1 81.0 0.6 
Example 10 
64 42.4 3.5 49.0 93.2 82.1 0.4 
__________________________________________________________________________