Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2001-06-12
2003-12-02
Rotman, Alan L. (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S534000, C562S535000
Reexamination Certificate
active
06657080
ABSTRACT:
TECHNICAL FIELD TO WHICH THE INVENTION BELONGS
This invention relates to an improvement in a process for producing acrylic acid by vapor-phase catalytic oxidation of an acrolein-containing gas.
CONVENTIONAL TECHNOLOGY
In such production process of acrylic acid, for the purpose of enhancing productivity of acrylic acid, such means as increasing concentration of the starting material or increasing space velocity of the gaseous material are adopted in recent years. Under those heavy load conditions, however, the temperature at hot spots in the catalyst layers rises high because the vapor-phase catalytic oxidation is an extremely exothermic reaction, to induce over-oxidation. In consequence, acrylic acid yield drops and thermal degradation of the catalyst is accelerated, in the worst case even causing a run-away reaction. Therefore, currently the process is under considerable restrictions in respect of the reaction conditions.
Various methods have been proposed to solve this problem, which proposals include, for example: {circle around (1)} Japanese Patent Publication Sho 53(1978)-30688B1 (=U.S. Pat. No. 3,801,634) proposed a method comprising diluting the catalyst with an inert substance; {circle around (2)} Japanese Patent Publication Hei 9(1997)-241209A1 (=U.S. Pat. No. 5,719,318) proposed a method comprising changing the catalyst size; and {circle around (3)} Japanese Patent Publication Hei 7(1995)-10802A1, a method comprising changing the carriage ratio of catalytically active component (weight ratio of the active substance per unit weight of the catalyst).
These methods basically adopt a means of dividing reaction tubes into plural reaction zones in their axial direction and filling the reaction zones with catalysts such that the catalytic activity successively increases from the gas inlet side toward the outlet side. In these methods, however, reactivity of the starting material tends to decrease because activity of the catalyst disposed at the gas inlet side is set at a low level. For overcoming this defect and obtaining an industrially advantageous high reactivity, such countermeasures as increasing the total catalyst layer length or raising the reaction temperature are required.
Extending the catalyst layer length, however, invites a disadvantage that pressure loss at the catalyst layer increases, and moreover necessitates to enlarge the reactor. Thus the method cannot be economically advantageous. Where the reaction temperature is raised, on the other hand, thermal degradation of the catalyst is accelerated to adversely affect the catalyst life, similarly to the case wherein the hot spot temperature becomes high. Besides, there rises an additional problem that side products increase to reduce yield of the object product. In particular, where the reaction is carried out under such heavy load conditions as increased concentration of the starting material or higher space velocity, catalyst of still lower activity level must be disposed at the reactant gas inlet side to suppress the hot spot temperature, which renders these problems even more serious.
On the other hand, a reactant gas introduced into a catalyst layer in a reactor in industrial working of the vapor-phase catalytic oxidation generally has a temperature lower than the reaction temperature. For satisfactory and efficient catalytic performance, however, the temperature of the reactant gas which is introduced into the catalyst layer needs to be raised to the reaction temperature level. As a method of heating a reactant gas to a predetermined reaction temperature, it is known to provide a pre-heating zone formed of an inert substance at the reactant gas inlet side of the reaction tubes. However, provision of such an inert substance layer, which does not participate in the reaction, in the reaction tubes of a limited length is quite inefficient. Whereas, when no pre-heating zone is provided or the pre-heating zone is short, in the conventional process disposing a catalyst of the lowest activity level at the reactant gas inlet side, the catalyst takes nearly no part in the oxidation reaction during its contact with the reactant gas having a temperature lower than the reaction temperature. This amounts to utilization of costly catalyst simply as a pre-heating zone, which obviously is ineconomical.
PROBLEM TO BE SOLVED BY THE INVENTION
Accordingly, the object of the invention is to provide a process for producing acrylic acid at high yield stably over a prolonged period, by efficiently inhibit occurrence of hot spots during production of acrylic acid through vapor-phase catalytic oxidation of acrolein-containing gas.
MEANS FOR SOLVING THE PROBLEM
I have discovered: when each reaction tube in a shell-and-tube type fixed bed reactor is divided into at least three reaction zones (catalyst layers), forming sequentially the first, second, third . . . reaction zones from the gas inlet side toward the gas outlet side and filling the second reaction zone with a catalyst of the lowest activity level among plural catalysts exhibiting different activity levels which are advancely prepared, in other words, when a catalyst of a higher activity level is disposed in the reaction zone closest to the gas inlet, the reaction temperature can be lowered even in the reaction under heavy load conditions and hot spot temperature does not rise inconveniently, in consequence enabling the reaction to continue with stability. I have furthermore discovered that the disposition of a catalyst of higher activity level at the gas inlet side enables the catalysts to exhibit their performance with high efficiency even when the pre-heating zone is shorter than that in conventional processes or no pre-heating zone is provided, and in consequence gives the object product at high yield without lowering reactivity of the starting raw material.
Accordingly, therefore, the invention provides a production process of acrylic acid comprising vapor-phase catalytic oxidation of acrolein-containing gas using a shell-and-tube type fixed bed reactor, said process being characterized by dividing each of the reaction tubes into at least three reaction zones in its axial direction, filling the first reaction zone closest to the gas inlet with a catalyst having a higher activity than that of the catalyst filling the adjacent, second reaction zone and filling the subsequent reaction zones with catalysts of different activity levels such that the catalyst activity successively rises from the second reaction zone toward the gas outlet side.
WORKING EMBODIMENTS OF THE INVENTION
According to the invention, each reaction tube in a shell-and-tube type fixed bed reactor is divided into at least three reaction zones in the axial direction of said tube, and the reaction zones are filled with catalyst layers in the above-described mode. While the more number of the reaction zones (catalyst layers), the easier to control the temperature rise at hot spots, for industrial practice provision of three reaction zones (catalyst layers) is normally sufficient for achieving the intended effect.
The ratios between individual lengths of said three or more reaction zones (catalyst layers), which are formed by dividing individual reaction tube in its axial direction, and the total length of said plural reaction zones (catalyst layers) is variable depending on the reaction conditions, catalyst activity levels and the like and cannot be generally specified. Whereas, when the number of the reaction zones (catalyst layers) is three, for example, the ratios can be set up, for example, in the following manner: the ratio (L
1
/L) between the length (L
1
) of the first reaction zone (the first catalyst layer) and the total length (L) of the reaction zones (catalyst layers) is normally selected to satisfy the condition expressed by the following formula
0
<
L
1
L
≦
0.5
,
preferably
0
<
L
1
L
≦
0.2
,
inter
alia
0
<
L
1
L
≦
0.1
;
the ratio (L
2
/L) between th
Nippon Shokukai Co. Ltd
Oh Taylor V
Rotman Alan L.
Sherman & Shalloway
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