The typical design of a boiling water reactor involves a multitude of tubes inside a reactor shell. A confined part of the reactor shell is filled with a cooling medium under pressure. Often water is used as cooling medium, but other cooling media than water may also be used if the boiling point is appropriate. The pressure of the confined part of the reactor shell controls the boiling point of the cooling medium, which then, if operating at the boiling point, may act as a heat sink with substantially constant temperature, to the extent that liquid cooling medium is present in the reactor. The cooling medium may be provided to the reactor shell from an external cooling medium container, such as e.g. a steam drum.
Common chemical processes where boiling water reactors are of interest include methane, methanol and formaldehyde production from synthesis gas, i.e. a gas comprising hydrogen and carbon oxides and possibly other constituents. The synthesis gas may originate from a variety of sources, including gasification of carbonaceous materials, such as coal, (typically heavy) hydrocarbons, solid waste and biomass, from reforming of hydrocarbons, from coke oven waste gas, from biogas or from combination of streams rich in carbon oxides and hydrogen—e.g. of electrolytic origin. Methane and methanol production are limited by an equilibrium involving a condensable component and for formaldehyde production it is desired to maintain the methanol concentration low due to considerations of explosion limits and catalyst stability, a.o.
To increase the capacity of a boiling water reactor, the catalyst is in some cases loaded not only into the reaction tubes, but also further up above the upper tube sheet wherein the reaction tubes are mounted. Regarding exothermic reactions, this increases the reactant gas temperature even before the reactant reaches the reaction tubes which are in thermal contact with the cooling medium. Thus there is a risk that the temperature of the tube sheet gets too high with risk of damage to the tube sheet. The present invention relates to a solution to this problem, avoiding a critically high temperature of the upper tube sheet even with catalyst loaded above the tube sheet for an exothermic reaction in the boiling water reactor. Another advantage of the present invention is that it can compensate for shrinkage of the catalyst within the reaction tubes, as the layer of catalyst arranged above the upper tube sheet will sink into the reaction tubes in case of catalyst shrinkage.
Known art offers little solution to this problem, as can be seen in the following references, where:
U.S. Pat. No. 5,000,926 describes a catalyst layer-fixed reactor for an exothermic reaction which comprises a plurality of reaction tubes disposed within a shell of the reactor, an inner tube disposed in the middle portion of each of the reaction tubes, catalyst layers formed by catalyst charged in the space inside the reaction tubes and outside the inner tubes, and a cooling medium charged between each of the reaction tubes and the shell, and in which a feed gas is flowed in each of the inner tubes in co-current to feed gas flowing in the fixed catalyst layer.
U.S. Pat. No. 5,759,500 discloses a fluid-reactor, heat exchange device and method of reacting a fluid in the device. The device embodies a bundle of heat-exchange tubes mounted internally of an elongated reactor shell to a stationary tube sheet attached to the reactor shell near one end of the shell. The heat-exchange tubes are also mounted to a floating tube-sheet which is located near the other end of the shell. Attached to the floating tube sheet is a catalyst basket which when the device is in operation will contain catalyst. The catalyst is supported in the basket, and the fluid to be reacted will enter the shell near the point of attachment to the stationary tube sheet, where it will contact the heat exchange tubes. The fluid will flow along the outside of the tubes and into the catalyst basket where it will contact the catalyst and react. The fluid will then pass into the heat exchange tubes and finally be removed from the device near the end of the reactor where it was introduced.
In EP1048343A2 a heat-exchanger type reactor is described, which has a plurality of tubes holding a catalyst, a shell section through which a heat-transfer medium is passed to carry out heat-transfer with a reaction fluid in said tubes, and upper and lower tube sheets, the upper ends of said tubes being joined to said upper tube sheet by way of first expansion joints fixed to the upper side of said upper tube sheet, the lower ends of said tubes being fixed directly to the floatable lower tube sheet, a floatable room being formed which is partitioned by said lower tube sheet and an inner end plate (inner head) joined to the lower side thereof and has an opening in the lower part, and said opening being joined by way of a second expansion joint to a tube-side outlet to the outside of the reactor.
None of the above known art references offer a solution to the mentioned problem as described in the following.
In the following, a section of the reactor is called reaction enclosure. However this shall not necessarily be construed as implying that a reaction takes place, since a reaction enclosure may simply have the function of a heat exchanger.
In the following, tubes shall be construed as enclosures of any circumferential shape, only characterized by being longer than the cross sectional distance. Typically tubes are cylindrical, but they may also have non-circular cross sectional shapes and varying cross sectional shape over the tube length.