In many industries, like the petrochemical and chemical industries for instance, the processes employ reactors in which chemical reactions are effected in the components of one or more reaction fluids by contact with a catalyst under given temperature and pressure conditions. Most of these reactions generate or absorb heat to various extents and are, therefore, exothermic or endothermic. The heating or chilling effects associated with exothermic or endothermic reactions can positively or negatively affect the operation of the reaction zone. The negative effects can include among other things: poor product production, deactivation of the catalyst, production of unwanted by-products and, in extreme cases, damage to the reaction vessel and associated piping. More typically, the undesired effects associated with temperature changes will reduce the selectivity or yield of products from the reaction zone.
One solution to the problem has been the indirect heating of reactants and/or catalysts within a reaction zone with a heating or cooling medium. The most well known catalytic reactors of this type are tubular arrangements that have fixed or moving catalyst beds. The geometry of tubular reactors poses layout constraints that require large reactors or limit throughput.
Indirect heat exchange has also been accomplished using thin plates to define alternate channels that retain catalyst and reactants in one set of channels and a heat transfer fluid in alternate channels for indirectly heating or cooling the reactants and catalysts. Heat exchange plates in these indirect heat exchange reactors can be flat or curved and may have surface variations such as corrugations to increase heat transfer between the heat transfer fluids and the reactants and catalysts. Although the thin heat transfer plates can, to some extent, compensate for the changes in temperature induced by the heat of reaction, not all indirect heat transfer arrangements are able to offer the complete temperature control that would benefit many processes by maintaining a desired temperature profile through a reaction zone. Many hydrocarbon conversion processes will operate more advantageously by maintaining a temperature profile that differs from that created by the heat of reaction. In many reactions, the most beneficial temperature profile will be obtained by substantially isothermal conditions. In some cases, a temperature profile directionally opposite to the temperature changes associated with the heat of reaction will provide the most beneficial conditions. An example of such a case is in dehydrogenation reactions wherein the selectivity and conversion of the endothermic process is improved by having a rising temperature profile or reverse temperature gradient through the reaction zone. A specific arrangement for heat transfer and reactant channels that offers more complete control can be found in U.S. Pat. No. 5,525,311; the contents of which are hereby incorporated by reference.
Heating reactants within a reaction zone poses a number of limitations on the reactor arrangement and the operation of the process. A heat exchange reactor typically needs to operate with a large fluid mass flow rate of the heat transfer fluid in order to provide adequate mass flux of the heat transfer fluid over the heat transfer surfaces. Failure to maintain the adequate heat transfer fluid mass flux across the heat transfer surfaces will result in inadequate heating and a loss of benefit from providing the internal heating within the reaction zone. The heat exchange reaction section may be divided into multiple heat exchange reactor sections. Nevertheless each heat exchange reactor section still requires a high mass flux rate to provide adequate heating across all the heat transfer surfaces.
Even with separate reaction zones or reaction stacks, as they are referred to herein, the maximum temperature for the heat transfer fluid also remains limited. Constraints on the temperature of the heat transfer fluid as providing heating to the reactants can typically apply to minimum or maximum values. Minimum allowable temperature must be high enough to induce a reaction rate that exceeds what would ordinarily be obtained from an adiabatic process. However, at the same time, the maximum temperature at which the heating fluid enters the heat transfer zone must not heat the reactants to a temperature that can cause a lack of selectivity in the products produced, or worse, a decomposition of the products already produced.
For example, in the dehydrogenation of ethylbenzene, the process requires that the endothermic heat of reaction be supplied internally or externally. In an adiabatic reactor operation, the sensible heat in the feedstream provides the endothermic heat of reaction. Mixing a large quantity of super heated steam to the ethylbenzene feed increases the available sensible heat. Limitations in the ability to provide heating by sensible heat restricts the maximum allowable temperature drop across the reactor. However, using excessive steam temperatures to maintain a minimum reaction temperature will exceed the maximum sensitivity temperature for the ethylbenzene and begin its decomposition. Furthermore, designing equipment for the more severe operating conditions significantly increases its cost. Again, the endothermic heat of reaction may also be supplied by indirect heat exchange from an appropriate heat transfer fluid. Nevertheless, providing sufficient mass flux to all of the heat transfer surfaces will require a high heat transfer fluid mass flow rate which would lead to larger equipment sizes and higher processing costs.
Accordingly, it is an object of this invention to reduce the heat transfer fluid mass flow rate required to provide the necessary heat transfer fluid mass flux to maintain a high reaction temperatures without exceeding the sensitivity temperature of the reactants or the products produced by their reaction.
It is a further object of this invention to provide a reactor apparatus for the indirect heating of a reactant stream in a reaction zone while conserving heat and reducing the necessary mass flow rate to provide a given mass flux over the heat transfer surfaces.