FIELD OF THE INVENTION Embodiments of the invention generally relate to a cooling medium reactor and more specifically to a boiling water reactor for an exothermal reaction. More specifically, the invention relates to ensure cooling of a catalyst arranged above the upper tube sheet of a cooling medium reactor. BACKGROUND

The typical design of a cooling medium 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 cool- ing medium than water may also be used if the boiling point is appropriate. Furthermore, the cooling medium does not have to be in liquid phase or partly in liquid phase, the cooling may also be convective cooling with gas, liquid or a mixture as cooling medium. 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 cooling medium 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 con- siderations of explosion limits and catalyst stability to mention some.

To increase the capacity of a cooling medium 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, or damage to the top of the reaction tubes or a welding between the top of the reaction tubes and the upper 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 exothermal reaction in the cooling medium 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 in some embodiments 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:

US5000926 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.

US5759500 discloses a fluid-reactor, heat exchange device and method of re- acting 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. The term "reactor shell" shall be construed as covering the casing or the walls of the reactor, whilst the term "reactor shell volume" is to be construed as covering the rooms or spaces within the reactor.

In the following, tubes shall be construed as enclosures of any cross sectional 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.

SUMMARY OF THE INVENTION

Embodiments of the invention generally relate to a cooling medium reactor for an exothermal reaction. The reactor comprises a reactor shell having a reactor shell volume, the reactor shell volume comprising at least one cooling medium inlet and at least one cooling medium outlet. The reactor shell volume is ar- ranged to hold a cooling medium under pressure. The reactor comprises a reac- tant inlet and a product outlet. The cooling medium reactor further comprises a reaction enclosure embedded within the reactor shell volume. The reaction enclosure comprises a reaction zone with a plurality of reaction tubes, an inlet manifold extending between the reactant inlet and the reaction zone, and an outlet manifold extending between the reaction zone and the product outlet. The cooling medium is arranged to flow between the cooling medium inlet and the cooling medium outlet, around the reaction tubes, so that the reaction tubes are in thermal contact with the cooling medium. The reactor shell volume is dimensioned so as to allow boiling of a liquid phase of the cooling medium within the reactor shell volume. A mixture of cooling medium in gas phase and liquid phase may pass through at least one cooling medium outlet to an external drum for separation of the liquid and the gas phase. In one embodiment the mixture of gas phase and liquid phase comprises steam and water.

In general, temperature of the cooling medium is controlled by control of its pressure, and the cooling medium is typically kept at a temperature proximate to the boiling point of the cooling medium.

In an embodiment of the invention a cooling medium reactor for an exothermal reaction comprises a reactor shell with a reactor shell volume arranged to hold a cooling medium under pressure. The cooling medium may be water or another liquid with a boiling point suitable for the process. The reactor shell volume comprises at least one cooling medium inlet and at least one cooling medium outlet and a reactant inlet and a product outlet. A reaction enclosure is embedded within the reactor shell volume and comprises a reaction zone with a plurality of reaction tubes for holding catalyst within the tubes. The catalyst may in some embodiments be SNG catalyst or a methanol catalyst, but any catalyst suitable for the process may be used in the present invention. The reactor further comprises an inlet manifold extending between the reactant inlet and the reaction zone, and an outlet manifold extending between said reaction zone and said product outlet. Within the reactor there is at least an upper and a lower tube sheet, comprising holes for connection of each end of the reaction tubes to said tube sheets. The reaction tubes may be connected to the tube sheets in any suitable way known in the art. The cooling medium is arranged to flow between the cooling medium inlet and the cooling medium outlet, around the reac- tion tubes, so that the reaction tubes are in thermal contact with the cooling medium. The cooling medium reactor is adapted to hold catalyst within the tubes and above the upper tube sheet. Thus, the heat developed by the exothermal reaction in the reaction tubes is transferred through the reaction tube wall to the cooling medium. The cooling medium reactor further comprises at least one reactant tube insert mounted at the top of said upper tube sheet adapted hold an amount of catalyst within the insert and comprising guide means for the reactant flow, to ensure that at least a part of the total flow of the reactant passes between at least a part of the insert and the upper tube sheet before it starts to react with the catalyst and therefore is relative cold. This ensures that the reactant which is passed between the catalyst in the insert and the upper tube sheet serves as a cooling or thermally insulating curtain between the upper tube sheet and the catalyst within the insert which is relatively hot as compared to the cooling me- dia and the reactant entering the reactor. Thus the tube sheet is protected from critical high temperatures which may otherwise damage the tube sheet or at least decrease its strength. Thereby the capacity of the reactor is increased due to the possibility of a arranging an amount of catalyst above the upper tube sheet without compromising the strength of the upper tube sheet. Another ad- vantage 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 in an embodiment of the invention will sink into the reaction tubes in case of catalyst shrinkage. It is not a precondition for the cooling that the cooling media is boiling. As mentioned before, the cooling medium may also be in gas phase.

In an embodiment of the invention, the guide means comprise a plurality of walls of the insert with different perimeters, whereby one or more annuluses is created in between the walls where the reactant can flow. In an embodiment there are three walls, forming two annuluses within each other which ensures that at least a part of the reactant flows in a zig-zag motion, first downwards in the outermost annulus adjacent to at least a part of the upper tube sheet, and then upwards in the innermost annulus adjacent to the catalyst within the reaction tube insert. The walls may be of any shape, including circular which forms concentric annuluses. In an embodiment of the invention, the at least one reaction tube insert comprises a plurality of single tube inserts with a lower end diameter smaller than the inner diameter of the top end of the reaction tubes. This ensures that the inserts may fit into the top end of the reactor tube in the upper tube sheet. The tube inserts can have means for connection to the top end of the reaction tubes. In an embodiment, the connection means comprises a top part of the tube inserts with an outer dimension larger than the inner diameter of the top end of the reaction tubes. Accordingly, the tube inserts are fixed to the top end of the reaction tubes by means of gravity as the inserts rests on the top part of the tube inserts. In an embodiment of the invention, the inserts have a connection section between the lower and the upper part of the inserts which is conical in shape, the inserts rests on the upper part of the reaction tubes on the part of this connection section which has a diameter approximately equal to the inner diameter of the reaction tubes.

In a particular embodiment of the invention, the top part of the tube inserts is hexagon or square in shape, which ensures only a small gap between the upper part of the inserts. This may be an advantage when loading catalyst into the re- actant tubes and the inserts as it minimizes the risk of spilling catalyst between the inserts. In an embodiment, the gap between the top part of each of the inserts is below 2 mm.

The reaction tube inserts may in one embodiment have a closed top section, which ensures that all the reactant passing through the insert is forced to flow along at least a part of the perimeter of the reaction tube insert as described above, before it passes by the catalyst comprised within the insert. The top section may be detachable to allow for loading of catalyst into the insert. In another embodiment of the invention, there is a reactant by-pass in the top section of the reaction tube insert. The by-pass allows for some of the reactant flow in the insert to flow directly to the catalyst without passing along at least a part of the perimeter of the insert. The size of the by-pass and thus the amount of the reactant flow which by-passes directly to the catalyst may vary according to the activity of the catalyst, process conditions among other parameters. In an embodiment of the invention, the amount of flow which flows through the by-pass is 0% to 95% of the total reactant flow through the reaction tube inserts. It is noted that the connection between the reaction tube insert and the reaction tubes is not necessarily gas tight, and therefore also an amount of reactant may by-pass the catalyst in the reaction tube inserts and flow directly from the reactant inlet to the reaction tubes.

In another embodiment of the invention, the reactant tube inserts comprise a plurality of assembled reaction tube inserts for holding catalyst and insertion into the top end of the reaction tubes. In an embodiment of the invention the reaction tubes have an inner diameter in the range of 20 - 150 mm, preferably 40 - 80 mm. Further the reaction tube inserts may in one embodiment have a height of 100 - 2000 mm. But the invention is not limited to only these ranges, since the invention principle works for other tube sizes as well.

In a further embodiment of the invention, the reaction tube inserts comprise a grid arranged within the insert to support the catalyst comprised within the insert. FEATURES OF THE INVENTION

1 . A cooling medium reactor for an exothermal reaction, said reactor comprising,

• a reactor shell having a reactor shell volume arranged to hold a

cooling medium under pressure, • said reactor shell volume comprising at least one cooling medium inlet and at least one cooling medium outlet,

• said reactor comprising a reactant inlet for a reactant flow and a product outlet,

• a reaction enclosure embedded within said reactor shell volume, said reaction enclosure comprising a reaction zone with a plurality of reaction tubes for holding catalyst within the tubes,

• an inlet manifold extending between said reactant inlet and said reaction zone, and an outlet manifold extending between said reaction zone and said product outlet,

• an upper and a lower tube sheet, comprising holes for connection of each end of the reaction tubes to said tube sheets,

wherein said cooling medium is arranged to flow between said cooling medium inlet and said cooling medium outlet, around said reaction tubes, whereby said reaction tubes are in thermal contact with said cooling medium,

wherein the cooling medium reactor is adapted to hold catalyst within the tubes and above the tubes and the upper tube sheet,

wherein the chemical reaction taking place for the reactant flow when contacting the catalyst is exothermal, causing the temperature of the flow to rise if not cooled, and

wherein the cooling medium reactor further comprises at least one reaction tube insert mounted above and with its lower end at least partly within the holes of said upper tube sheet and adapted to contain at least a part of said catalyst, said reaction tube inserts comprise guide means to guide at least a part of the cold unreacted reactant in a flow along at least a part of the perimeter of the reaction tube inserts thereby thermally insulating the upper tube sheet from the hot catalyst within the reaction tube inserts and then guide said reactant flow into the reaction tube inserts where it reacts with the catalyst contained in said reactant tube inserts. Cooling medium reactor according to feature 1 , wherein the cooling medium is a boiling medium.

Cooling medium reactor according to feature 1 or 2, wherein said guide means comprises a plurality of concentric walls with an annulus between the walls where said reactant can flow.

Cooling medium reactor according to feature 3, wherein said guide means comprises two concentric walls.

Cooling medium reactor according to any of the preceding features, wherein said at least one reaction tube insert has a lower end diameter smaller than the inner diameter of the top end of the reaction tubes and means for sealing said reaction tube inserts to the top end of the reaction tubes.

Cooling medium reactor according to any of the preceding features, wherein at least one reaction tube insert comprises a connection section, to connect each of the reaction tube inserts to the top of a reaction tube.

Cooling medium reactor according to feature 6, wherein said connection section comprises a conical wall concentric to and arranged outside the reaction tube inserts.

Cooling medium reactor according to any of the preceding features, wherein said at least one reaction tube insert has a closed top section enabling said at least part of the reactant to flow only along at least part of the outside of the reaction tube inserts and into the reaction tube inserts where it reacts with the catalyst contained in said reaction tube inserts. Cooling medium reactor according to any of the features 1 - 7, wherein said at least one reaction tube insert has a by-pass in the top section, enabling a part of the reactant to by-pass the guide means and flow directly into the reaction tube inserts where it reacts with the catalyst contained in said reaction tube inserts. Cooling medium reactor according to any of the preceding features, wherein said at least one reaction tube insert has a removable top section, enabling said catalyst to be loaded into the reaction tube inserts or enabling said catalyst to be loaded into the reaction tube inserts and into the reaction tube connected to the reaction tube inserts. Cooling medium reactor according to any of the preceding features, wherein the outer dimensions of said at least one reaction tube insert are large enough to ensure that the space between adjacent reaction tube inserts is minimized. Cooling medium reactor according to any of the preceding features, wherein the outer dimensions of at least a part of said at least one reaction tube inserts are large enough to ensure that the space between adjacent reaction tube inserts is below 2 mm. Cooling medium reactor according to any of the preceding features, wherein said at least one reaction tube insert has a hexagonal shape or a square shape. Cooling medium reactor according to any of the preceding features, wherein said at least one reaction tube insert has a height of 100 - 2000 mm. Cooling medium reactor according to any of the preceding features, wherein the guide means are dimensioned so that the part of the reactant which is guided in a flow along at least a part of the perimeter of the reaction tube inserts is 5% to 100% of the total reactant flow through the reaction tubes. Cooling medium reactor according to any of the preceding features, wherein said at least one reaction tube insert comprises an assembly of a plurality of assembled reaction tube inserts. Cooling medium reactor according to any of the preceding features, wherein the reaction tubes have an inner diameter in the range of 20 - 150 mm, preferably 40 - 80 mm. Cooling medium reactor according to any of the preceding features, wherein said reaction tube inserts comprise a grid to support the catalyst contained within the reaction tube inserts. Cooling medium reactor according to any of the preceding features, wherein the catalyst can be a methanol catalyst or an SNG catalyst. Process for a cooling medium reactor for an exothermal reaction, said reactor comprising a reactor shell having a reactor shell volume arranged to hold a cooling medium under pressure, said reactor shell volume comprising at least one cooling medium inlet and at least one cooling medium outlet, said reactor shell comprising a reactant inlet for a reactant flow and a product outlet, a reaction enclosure embedded within said reactor shell volume, said reaction enclosure comprising a reaction zone with a plurality of reaction tubes for holding catalyst within the tubes, an inlet manifold extending between said reactant inlet and said reaction zone, and an outlet manifold extending between said reaction zone and said product outlet, an upper and a lower tube sheet, comprising holes for connection of each end of the reaction tubes to said tube sheets, at least one reaction tube insert comprising guide means and mounted above and with its lower end at least partly within the holes of said upper tube sheet and adapted to contain at least a part of said catalyst, said process comprising the steps of

• guiding at least a part of the flow of said reactant from the reac- tant inlet in a flow along at least a part of the perimeter of the reaction tube inserts, thereby thermally insulating the upper tube sheet from the catalyst within the reaction tube inserts and then

• guiding said reactant flow into the reaction tube inserts where it reacts with the catalyst contained in said reactant tube inserts.

Process according to feature 20, wherein a part of the reactant by-passes the guide means and flows directly from the reactant inlet into the reaction tube inserts.

Process according to any of the features 20 - 21 , wherein a part of the reactant by-passes the reaction tube inserts and flows directly from the reactant inlet into at least a part of the reaction tubes.

23. Process according to any of the features 20 - 22, wherein 5% to 100% of the total reactant flow is guided in a flow along at least a part of the perimeter of the reaction tube inserts.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention are explained, by way of example, and with reference to the accompanying drawings. It is to be noted that the appended drawings illustrate only examples of embodiments of this invention and they are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

Fig. 1 shows a cross sectional view of a cooling medium reactor according to an embodiment of the invention,

Fig. 2 shows a cross sectional view of a reaction tube insert and a section of the top of a cooling medium reactor according to an embodiment of the invention, Fig. 3 shows a cross sectional view of a reaction tube insert and a section of the top of a cooling medium reactor according to a further embodiment of the inven- tion

Fig. 4 shows a cross sectional view of a plurality of reaction tube inserts and a section of the top of a cooling medium reactor according to an embodiment of the invention, and

Fig. 5 shows a cross sectional view of a reaction tube insert and a section of the top of a cooling medium reactor according to another embodiment of the invention.

Position numbers:

0 1 . Cooling medium reactor

02 . Reactor shell

03 . Reactor shell volume

04 . Cooling medium

05 . Cooling medium inlet

0 6 . Cooling medium outlet

07 . Reactant inlet

0 8 . Reactant flow 0 9 . Product outlet

10 . Product flow

11 . Reaction tubes

12 . Catalyst

13 . Inlet manifold

14 . Outlet manifold

15 . Upper tube sheet

1 6 . Lower tube sheet

17 . Tube sheet holes

18 . Reaction tube insert

1 9 . Reaction tube insert lower end

20 . Guide means

21 . Annulus

22 . Connection section

23 . Closed top section

24 . By-pass

25 . Grid

DETAILED DESCRIPTION

As known in the art, the capacity of a cooling medium reactor 01 as shown on Fig. 1 can be increased by loading catalyst 12 not only within the reaction tubes 1 1 of the reactor, but also arranged on the top of the upper tube sheet 15. Arranging catalyst on top of the tube sheet does not make the reactor as a whole more expensive, since this space within the top dome of the reactor shell 02 would otherwise just remain void. But because some reactions may be strongly exothermal, an amount of heat will then be generated above and on the upper tube sheet and in the tube sheet holes 17, which heat cannot be efficiently removed, since the cooling medium 04 is in the section of the cooling medium re- actor below the upper tube sheet. Thus, a risk emerges that the upper tube sheet may be damaged and/or critically weakened by the higher temperature. The upper tube sheet is dimensioned to withstand the thermal tensions and process pressure within a certain temperature range. If the temperature rises above this range, the strength of the upper tube sheet decreases, which may lead to a critical malfunction and defect.

The present invention offers a solution to this problem by providing reaction tube inserts 18 which are arranged on top of the upper tube sheet and adapted to hold catalyst within them, but providing a thermal insulation between the reacting hot catalyst within the inserts and the upper tube sheet. A principle draw- ing of a cooling medium reactor seen in cross sectional view is shown on Fig 1 . Reaction tubes are arranged within the reactor shell 02, fixed in both ends to the upper tube sheet and the lower tube sheet 16. A cooling medium inlet 04 allows a cooling medium to be provided to the reactor shell volume 03, where it is in contact to the outer side of the reaction tubes, before it exits the reactor via the cooling medium outlet 06. Above the upper tube sheet, an inlet manifold 13 provides transfer of the reactant flow 08 from the reactant inlet 07 to the reaction tube inserts which are arranged within each top of the reaction tubes. The reactant reacts with the catalyst within the inserts and the reaction tubes and exits the lower end of the reaction tubes as product flow which is provided to the product outlet 09 by the outlet manifold 14.

The reaction tube inserts allow for increase of the reactor capacity by allowing for catalyst not only within the reaction tubes but also in the void above the upper tube sheet, but at the same time also providing at thermal insulation be- tween the hot catalyst above and within the tube sheet holes and the tube sheet itself. As can be seen on Fig. 2 according to an embodiment of the invention, this is achieved by guide means 20 which are arranged around the perimeter of the inserts. The guide means forces the reactant to flow in an annulus 21 between the catalyst contained in each insert and the inner side of each corre- sponding tube sheet hole. The reactant flow has a downwards pass in the annulus, is guided in a U-turn in the reaction tube insert lower end 19 and then has an upwards pass still in the annulus in the perimeter of the insert, before the unreached reactant finally inters the inner part of the inserts where it starts to react with the catalyst contained there. When the reactant enters the reactor via the reactant inlet, it is cold relative to the product flow which exits from the exother- mal reaction taking place in the reaction tubes and the reaction tube inserts where the catalyst is contained. Therefore, the flow of relative cold reactant between the exothermal reacting catalyst and reactant within the inserts and the upper tube sheet provides a thermal insulation of the upper tube sheet from the relative high temperature within the inserts.

The guide means of the reaction tube inserts comprises a double wall around the entire length of the insert and in the lower end a triple wall, which as seen on Fig. 1 provides for the described U-turning or zig-zag flow path of the relative cold reactant around the perimeter of the insert. A closed top section 23 of the insert ensures that all the reactant passing through the reaction tube insert has to perform a "cooling pass" around the perimeter of the insert in its entire length before it can enter the inner part of the insert and start reacting with the catalyst contained there. The lower end of the insert has an outer diameter which is smaller than the inner diameter of the reaction tube top section. The connection of the insert to the upper tube sheet and the reaction tube is therefore simply done by inserting the insert into a hole in the upper tube sheet and the reaction tube mounted within said hole. A conical connection section 22 of the insert forms connection between the lower part of the insert and the upper part of the insert which has an outer diameter larger than the inner diameter of the reaction tube. Thus, by means of gravity, the insert simply rests on the inner edge of the reaction tube in contact with the outer part of the connection section which has an outer diameter similar to the inner diameter of the reaction tube. If care is taken to tolerances, the connection between the insert and the reaction tube is gas tight. But due to tolerances and also the temperature expansion and con- traction of the materials, there may be gaps in the connection between the in- sert and the reaction tube. This is not critical however, since it may simply reduce the reaction of the catalyst in the insert to a small extent, which may be countered by the embodiment shown in Fig. 3. The embodiment of the invention shown in Fig. 3 is completely similar to the embodiment shown in Fig. 2 (with similar position numbers) with the exception that the top section of the reaction tube insert is not completely closed. Instead there is a by-pass 24 where a part of the total flow of the reactant through the insert can pass. The amount of reactant passing through the by-pass may vary in accordance with process parameters of the reaction. Some processes and catalysts are highly exothermal and may require a large extent of cooling for the process temperature to be kept stable, hence no or only a small by-pass is prudent. Other processes may require less cooling, and then a larger by-pass can be an advantage compensating for an amount of reactant which may flow through leaks in the connection between the insert and the reaction tube and also lowering the total pressure loss for the reactant flow. The closed top section of the insert may simply be a blind or a lid covering the by-pass opening. Also catalyst may be loaded into the insert through the by-pass or through a removable top closure of the top section.

Only the connection between one reaction tube insert and a reaction tube is shown in Fig. 2 and Fig. 3, but as can be seen to some extent in Fig. 1 , all reaction tubes of the reactor may have an insert to increase the capacity of the reactor. This is shown in more detail on Fig. 4, where a cut picture of the upper tube sheet with three holes is shown. Otherwise similar to Fig. 3, Fig. 4 shows three reaction tube inserts (18 A, 18 B and 18 C) arranged in three reaction tubes side by side. The space between two adjacent inserts may be minimized to achieve the largest capacity increase for the reactor. However, some spacing between the insert may also be prudent to allow for mounting and reactant flow to the area adjacent the upper side of the upper tube sheet. The cross sectional shape of the reaction tube inserts (not shown on the Figs.) may be circular which is easy and cheap to produce, or to increase capacity any other shape which minimizes the space between the inserts, e.g. hexagonal or square. Also the inserts may be arranged in groups of a plurality of inserts which are connected (not shown).

In Fig. 5, an embodiment of the invention is shown which is identical to the embodiment of Fig. 2 (with identical position numbers) except for the difference that a grid 25 has been arranged within the insert, forming a support for the catalyst contained in the insert. The position of the grid may vary according to the process or other consideration. The higher up in the insert the grid is mounted, the smaller the capacity increase of the reactor; and the smaller the heat generation of the exothermal reaction within the insert. In the embodiment shown in Fig. 5, the grid is located sufficiently high in the insert to have the exothermal reaction within the insert entirely above the upper tube sheet. Also the grid form- ing a support for the catalyst may render the mounting of the inserts in the reactor easier and less time consuming, with less down-time of the reactor, since the catalyst may be pre-loaded into the inserts before mounting them in the reactor.