DESCRIPTION

Field of application The invention relates to a chemical reactor containing a tubular heat exchanger. Prior art

Chemical reactors containing tubular heat exchangers are known. For example, chemical reactors are known containing a catalytic bed and a tube bundle immersed in said bed for controlling the catalyst temperature. For example, in reactors carrying out exothermic reactions, the tubes remove heat producing steam, with the advantage of controlling the temperature of the bed and recovering useful heat. Reactors of this type are used, inter alia, for the synthesis of ammonia, the synthesis of methanol, as shift reactors in ammonia plants. The conventional design provides a bundle of straight tubes fixed to two tube plates. By means of said two plates, the heat exchange fluid circulating inside the tubes is respectively distributed and collected.

Said tube plates, however, constitute expensive items; furthermore they have the drawback of limiting significantly the accessibility for the catalyst charge and discharge, in the case of reactors where the catalyst is around the tubes. Therefore, with the aim of reducing costs, attempts have been made to find alternative solutions. A known method for eliminating the tube plates is that of connecting the tubes to a heat exchange fluid distributor and collector, for example having a toroidal shape. A distributor or collector so provided has a lower cost than a tube plate, but has the drawback of modifying the geometrical configuration of the bundle, with a substantial deviation from the design pitch. In fact, tube ends must be bent to converge towards the toroidal body, such that said tube ends are no longer parallel. The distance between the tubes is thus modified with negative consequences: a greater spacing between the tubes may create zones of reduced control over the catalyst bed temperature, which are thus more prone to overheating. A dense arrangement of the tubes may create zones where it is difficult to operate, for example to weld the tubes, and said zones are also difficult to inspect or repair. Moreover, the execution of complex welding operations provides greater risk of welding defects and therefore leaks. Moreover, modification of the geometrical arrangement of the tube bundle may perturb the gas flow passing through the shell side and result in poorer heat exchange.

US 4,636,365 discloses a reactor comprising a tube bundle with a helical configuration.

Summary of the invention

The invention proposes an embodiment of straight-tube tubular chemical reactor which is able to reduce the manufacturing costs by eliminating the tube plates, while managing at the same time to avoid or reduce to a minimum any alteration in the geometrical configuration of the tube bundle.

These objects are achieved with a chemical reactor according to claim 1 . The chemical reactor comprises a shell with an axis and comprises an internal tube- bundle heat exchanger of straight and parallel tubes, and further comprises at least one fluid distributor or fluid collector, respectively, for said tube bundle. The distributor or collector has an ogival form including a roundly tapered end facing said tube bundle, said distributor or collector extending axially along said axis of the reactor, the tubes of the bundle being directly connected to said distributor or collector.

Advantageously, said distributor or collector comprises a cylindrical or substantially cylindrical portion and a closing bottom. Said closing bottom is directed towards the tube bundle. Said bottom is preferably elliptical or hemispherical.

More preferably, the outer tubes of the bundle, which form a ring around the periphery of the bundle, are advantageously connected to said cylindrical portion, while the inner tubes of the bundle, i.e. arranged in the central part of the bundle and close to the axis, are connected to said bottom. Even more advantageously, the outer tubes of the bundle are connected to said cylindrical part of the distributor or collector by means of tube terminations curved substantially at 90 degrees.

The straight parallel tubes include a straight portion and tube terminations (tube ends) which may be curved to connect to the distributor and/or collector. Said straight portion accounts for the large majority of the length of each tube, e.g. at least 80% and preferably at least 90%.

In a preferred embodiment, the reactor is vertical and the aforementioned distributor or collector is positioned in the upper part of the reactor, with its bottom directed downwards, i.e. facing the tube bundle.

The distributor or collector so realized costs significantly less than a conventional tube plate. Moreover, the invention has the advantage that the geometrical configuration of the tubes and their pitch are not significantly altered. The outer tubes can be connected to the cylindrical body with 90-degree bending, remaining substantially parallel and therefore keeping the design pitch. The welding, even though carried out on a cylindrical surface, is also facilitated by the fact that the tubes are perpendicular to said surface.

The inner tubes of the bundle are instead advantageously connected to an elliptical or semicircular bottom, this requiring only a minimum deviation from the parallel direction.

These advantages are felt during the exchanger manufacturing, in particular when performing the welds, and also during operation. In preferred applications, the tube bundle is immersed in a catalytic bed and the invention allows maintain a uniform distribution of the tubes even in the proximity of the collector or the distributor, also resulting in more uniform control of the bed temperature. For example, in the case of exothermic reactions, zones with excessive spacing of the tubes, which may result in local overheating of the bed, are avoided.

The invention also allows easy access to the tube bundle and the catalytic bed, in particular for the catalyst charge and discharge.

Advantageously, the reactor according to the invention is used for the synthesis of ammonia or methanol or for the shift conversion which, as is known, converts carbon monoxide (CO) and water into carbon dioxide (CO ₂ ) and hydrogen (H ₂ ). A reactor according to the invention may be used to provide shift reactors operating in accordance with the known processes which also comprise: sour shift, high-temperature shift, medium or low temperature shift. The invention may be also applied to axial flow or radial flow reactors, this term indicating the direction of the flow passing through the catalytic bed. More advantageously the invention is applied to an axial-flow reactor, which can be termed as tubular-axial reactor.

The advantages of the invention will emerge even more clearly with the aid of the detailed description and with reference to the figures, in which:

Fig. 1 shows a schematic cross-sectional view of a reactor according to a preferred embodiment of the invention, and

Fig. 2 shows a detail of Fig. 1 . Detailed description Figure 1 shows a vertical reactor 1 essentially comprising: a shell 2, a tube bundle 3, a gas inlet nozzle 4, a gas outlet nozzle 5. The tube bundle 3 is immersed in a catalytic bed. The nozzle 4 allows entry of a charge of reactant gases and the nozzle 5 allows the extraction of reaction products. Advantageously, the gas flow is axial, i.e. directed mainly along the axis of the reactor 1 . The tube bundle 3 is supplied with a heat exchange fluid, for example evaporating water. In this way the bundle 3 allows control over the temperature of the bed in which it is immersed, removing the reaction heat.

In greater detail, the tube bundle 3 is supplied via a water inlet flange and a toroidal-shaped distributor 7. The steam or water-steam mixture produced in the tubes is collected in a collector 8 and exported through one or more steam outlet flanges 9.

The collector 8 has essentially an ogival form and extends axially along the axis of said reactor 1 . Said collector comprises a cylindrical portion 10 and a bottom 1 1 which is preferably elliptical or hemispherical. The tubes of the bundle 3 are connected directly to said collector 8. More particularly (Fig. 2), the bundle 3 comprises a first set of peripheral tubes 3', which are distanced from the axis A of the reactor 1 and are connected to the cylindrical part 10 of the collector 8 by means of an end part 12 of the tubes which substantially follows a 90-degree curve. The remaining tubes 3", which are located on the inside, i.e. close to the axis A of the reactor, connect to the bottom 1 1 .

As can be seen from the figures, the outer tubes 3' remain substantially parallel over most of their length and the distance between the tubes remains practically unchanged even in the zone of the curved terminations 12. The inner tubes 3" are instead connected to the bottom 1 1 by means of terminations 13 which are bent by a small angle. It can be understood therefore that both in the zone of the outer tubes 3' and in the zone of the inner tubes 3", the deviation of the bundle geometrical configuration from the design pitch is limited. In Fig. 2 it can be also seen that the terminations 12 are substantially perpendicular to the cylindrical surface of the portion 10 of the collector.

The invention thus achieves the aforementioned objects, in particular the collector 8 is a much less expensive component than a conventional tube plate.