Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture
Reexamination Certificate
1998-01-15
2002-11-19
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
C423S245100, C423S245300, C423S651000, C423S652000, C423SDIG006
Reexamination Certificate
active
06482375
ABSTRACT:
BACKGROUND
1. Field of Invention
The invention relates to a method for carrying out endothermic and exothermic chemical reactions.
2. Description
A great many chemical reactions are characterized by a positive heat effect (exothermic reaction) or a negative heat effect (endothermic reaction). To enable chemical reactions to proceed in the desired manner, an efficient supply or removal of the reaction heat is indispensable. In some exothermic reactions, the thermodynamic equilibrium shifts in an undesired direction if the temperature rises. Examples are the synthesis of ammonia and methanol, the oxidation of sulfur dioxide to sulfur trioxide in the production of sulfuric acid, the reaction of sulfur dioxide with hydrogen sulfide in the Claus process, the selective oxidation of H
2
S to elementary sulfur and the reaction of carbon monoxide with hydrogen to methane. Since in the course of these reactions thermal energy is released, the temperature of the reaction mixture will rise and the thermodynamic equilibrium will shift in an unfavorable direction, unless the reaction heat released is removed fast and efficiently from the reactor.
In endothermic reactions too, a shift of the thermodynamic equilibrium in an undesired direction can occur, now by the consumption of thermal energy. Examples are methane-steam reforming and the dehydrogenation of ethylbenzene to styrene. A problem may also arise in that as a result of the consumption of energy by the reaction, the temperature of the reaction mixture decreases unduly, so that the desired reaction no longer proceeds.
Not only can a temperature change cause a shift of the thermodynamic equilibrium in an unfavorable direction, it can also adversely affect the selectivity of catalytic reactions.
Examples of reactions where the temperature affects the selectivity are the production of ethylene oxide from ethylene (the undesired reaction is the formation of water and carbon dioxide), the selective oxidation of hydrogen sulfide to elementary sulfur (the undesired reaction is the formation of SO
2
) and the Fischer Tropsch synthesis. In all cases, a temperature rise occurs as a result of the release of the reaction heat. If this temperature rise is not prevented through a rapid removal of the reaction heat, the selectivity decreases greatly.
In most conventional catalytic reactors, use is made of a fixed bed of catalyst particles. In such a catalyst bed, porous bodies of catalyst particles have been poured or piled.
In order to avoid an undesirably high pressure drop across such a catalyst bed, it is preferred to use bodies or particles of dimensions of at least 0.3 mm. These minimum dimensions of the catalyst bodies are necessary to keep the pressure drop that occurs upon the passage of a stream of reactants through the catalyst bed, within technically acceptable limits. While the dimensions are limited at the lower end of the range by the permissible pressure drop, the necessary activity of the catalyst imposes an upper limit on the dimensions of the catalytically active particles. The high activity required for a number of types of technical catalysts can mostly be achieved only with a surface of the active phase of 25 to 500 m
2
per ml catalyst volume. Surfaces of such an order of magnitude are possible only with very small particles, for instance with particles of 0.05 &mgr;m. Since particles with such dimensions no longer allow a liquid or gas mixture to flow through them, the primary, extremely small particles have to be formed into high-porous bodies with dimensions of at least about 0.3 mm, which can possess a large catalytic surface. An important task in the production of technical catalysts is to combine the required high porosity with a sufficiently high mechanical strength. The catalyst bodies cannot be allowed to disintegrate upon filling of the reactor and upon exposure to sudden temperature differences (thermal shock).
Under the conditions of the thermal pretreatment and/or catalytic reaction to be carried out, nearly all catalytically active materials soon sinter to form large conglomerates with a negligibly small active surface. Therefore, the active component (finely divided) is generally applied to a so-called support. This support exhibits the necessary thermal stability and hardly sinters, if at all, at high temperatures. Often used as supports are silicon dioxide, aluminum oxide or activated carbon.
As appears from the above examples, there is a very great need for a fast supply or removal of thermal energy in catalytic reactors, but the total heat transfer coefficient is mostly very low in a fixed catalyst bed. According to the present state of the art, it is virtually impossible to supply thermal energy to or remove it from a fixed catalyst bed in an efficient manner. This is indeed evident from the manner in which chemical reactions are carried out in fixed catalyst beds.
It is possible that of an exothermic reaction only the thermodynamic equilibrium shifts in adverse direction upon a temperature rise, without the selectivity decreasing unallowably. In that case, the reaction in a fixed catalyst bed can be made to proceed adiabatically. After passage through the reactor, the stream of reactants is cooled off in a separate heat exchanger. Because the conversion of the reactants is now thermodynamically limited by the temperature rise in the reactor, the unconverted reactants have to be reacted again upon cooling. The reaction product can be separated and the reactants can be recycled through the fixed catalyst bed. This occurs, for instance, in the ammonia and methanol synthesis. If the reaction product cannot be easily separated, downstream of the heat exchanger a second fixed bed reactor with a heat exchanger must be linked up. This is for instance the case in the oxidation of sulfur dioxide to sulfur trioxide. Sometimes, to prevent emission of harmful compounds, even a third reactor with heat exchanger is necessary. If connecting a number of reactors and heat exchangers in series is not properly possible and the separation of the reaction product is not either, the reaction product is sometimes recirculated through the catalyst bed. Per passage through the reactor, so little of the reactants is added to the circulating reaction product that it is converted completely. Because the rise of the temperature must then be properly controlled, per passage through the reactor only very little can be converted. In cases where the reaction must be carried out at a greatly increased pressure, the problems with the supply or removal of the reaction heat are extra large.
In the ammonia synthesis and the methanol synthesis, a catalyst bed is employed in which reactants are injected at different points at a relatively low temperature.
Such an implementation of the method, whereby gas streams must be passed through high pressure reactors in a complicated manner, obviously also requires high investments.
In a number of technically important cases, it is desired in catalytic reactions to work with a high to very high spatial throughput rate, with a great pressure drop across the reactor being considered a less serious drawback. In the conventional fixed bed reactors, a high pressure drop with the corresponding high spatial throughput rate is not properly possible. If the pressure at the reactor inlet is increased, the catalyst may be blown (gaseous reactants) or washed (liquid reactants) out of the reactor. It is also possible that at a particular critical value of the pressure at the reactor inlet “channeling” occurs. In that case, the catalyst particles in a particular part of the reactor are going to move. In that case, the reactants are found to flow virtually exclusively through the part of the catalyst bed that is in motion.
With the fixed bed reactors current at present, the catalyst bed clogs up. Therefore the reactor must be regularly opened and the cumulated layer of dust removed. It would be favorable if a pulse of gas of high pressure could be sent through the reactor in a direction opposite to that of the stream o
Mulder Alette
Polman Erik Alexander
van der Wal Willem Johan Jacob
van Waveren Anthony
van Wingerden Antonius Johannes Maria
Gastec N. V.
Silverman Stanley S.
Vanoy Timothy C
Weingarten Schurgin, Gagnebin & Lebovici LLP
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