METHANOL SYNTHESIS CONVERTER AND PROCESS

[0001] The invention is in the field of methanol synthesis. In particular, the invention relates to a methanol synthesis converter and to a process for the synthesis of methanol.

[0002] The industrial preparation of methanol starts in a front-end section of a methanol plant wherein a make-up gas containing carbon oxides CO, CO₂ and hydrogen H₂ is obtained via reforming or partial oxidation of a hydrocarbon feedstock. The hydrocarbon feedstock subjected to reforming can come from different sources but it usually comprises one or more of the following: natural gas, coal, pet-coke, waste and/or biomass.

[0003] The conversion of the make-up gas into crude methanol is carried out in one or more methanol converter(s) which is/are part of a methanol synthesis loop. The converters are complex equipment because the reaction that leads to the synthesis of methanol is highly exothermic and the converter must be able to withstand high temperature and high pressure typically from 200° C. to 300° C. and from 50 to 100 bar.

[0004] Methanol converters include one or more catalytic bed(s) containing a catalyst and they can be provided with one or more cooling element(s) immersed in the catalytic mass and/or with one or more heat exchanger(s) arranged downstream the catalytic bed(s) to remove the heat developed by the methanol synthesis reaction.

[0005] In the art, the catalytic converters provided with cooling elements arranged in the catalytic mass are referred to as isothermal converters in view of the fact that the temperature of the reaction that leads to the synthesis of methanol is kept within a suitable temperature range. On the other hand, converters not provided with cooling elements within the catalytic mass are referred to as adiabatic converters.

[0006] Commonly a methanol synthesis loop includes, together with an isothermal converter and/or an adiabatic converter, additional processing units e.g. one or more heat exchangers upstream or downstream of the converter. Said one or more processing units are not part of the catalytic converter.

[0007] The performance of a catalytic converter can be evaluated by taking into account an active volume of the converter which in simple terms is the volume of the converter that actively contributes to the synthesis of methanol. The remainder volume can be termed inactive volume. The active volume is the sum of several contributions including the volume occupied by the catalyst bed(s) and the volume occupied by the cooling elements immersed in the catalyst.

[0008] In practice, for assessing the performance of the converter, the active volume of the converter is divided by the internal volume of the converter to provide a parameter which is referred to as actively utilized volume ratio. Said ratio is an indirect indication of the proper design of the methanol converter and the higher is the ratio, the lower are CAPEX and the better is for the optimisation of the cost.

[0009] Unfortunately, in the adiabatic converters and isothermal converters of the prior art not all the internal volume of catalytic converters is active towards the synthesis of methanol. This is the consequence of mechanical and fluid dynamic constraints and/or maintenance requirements.

[0010] In practice, the catalytic converters of the current art are designed with a central cavity which is substantially an empty volume not filled with catalyst and which is inactive towards the synthesis of methanol. It follows that, in view of the current design, the converters are characterised by a low actively utilized volume ratio which results in an unoptimized design.

[0011] U.S. Pat. No. 4,714,592 discloses a radial flow reactor including a heat exchange apparatus within the catalytic bed. WO 2019/121155 discloses a multi-bed catalytic converter; JP S63 283741 discloses a contact reactive device for ammonia, methanol or the like.

[0012] The invention aims to overcome the above drawbacks of the prior art. Among others, the invention faces the problem of designing a methanol converter having a higher active volume or higher actively utilized volume ratio compared to the converters of the current art.

[0013] Accordingly, one aspect of the present invention is a catalytic converter for the synthesis of methanol according to claim 1.

[0014] The catalytic converter for the synthesis of methanol comprises a pressure vessel which includes a catalytic bed having an annular configuration and an effluent heat exchanger. Said effluent heat exchanger is installed in an empty central cavity of the converter and is arranged to transfer heat from a hot methanol-containing gas exiting the catalytic bed to a cooling medium.

[0015] The converter of the invention may include one or more catalyst bed(s). Said catalyst bed(s) may include one or more adiabatic beds or isothermal beds. An isothermal catalytic bed is equipped with one or more cooling elements, preferably in the form of heat exchanger plates, arranged within the catalyst bed to remove reaction heat from the catalyst. An adiabatic bed has no cooled heat exchange elements immersed in the catalyst; a heat exchanger is preferably provided downstream an adiabatic catalytic bed to remove heat of reaction from the effluent.

[0016] The effluent heat exchanger is preferably a feed-effluent heat exchanger arranged to transfer heat from said hot methanol-containing gas effluent from the catalytic bed to a syngas feed, so that said syngas feed is preheated before entering said catalytic bed.

[0017] The volume occupied by the feed effluent heat exchanger within the converter is countable for the definition of the active volume.

[0018] A further aspect of the invention is a process for the synthesis of methanol according to the claims.

[0019] The advantages of the invention are the following.

[0020] The inactive volume of the converter is reduced by the installation in the central cavity of the feed-effluent heat exchanger. It follows that also the actively utilized volume ratio of the converter is increased which is particularly beneficial for the investment cost of the synthesis loop.

[0021] A compact, simplified and more practical converter can be obtained particularly in view of the plurality of processing functions which can be integrated into a single pressure vessel. For instance, the converter of the invention can be configured to carry out a series of functions including: cooling, heating and in some embodiments also condensation.

[0022] The converter of the invention is particularly suited to be applied in small-size methanol plants and methanol plants powered at least in part by renewable energy sources.

[0023] The vessel of the converter can be designed with a slightly bigger diameter than the traditional converters. This marginal increment in the diameter of the converter enables to include of one or more additional processing unit(s) within the volume of the converter. It follows that the costs of external equipment and piping together with the CAPEX are reduced. Further, the pressure losses and the energy consumption (OPEX) are also reduced.

[0024] In a broad aspect, the invention concerns a catalytic converter for the synthesis of methanol comprising a pressure vessel which includes a catalytic bed having an annular configuration and an effluent heat exchanger.

[0025] The effluent heat exchanger is installed in the empty central cavity of the converter and is arranged to transfer heat from a hot methanol-containing gas effluent from the catalytic bed to a cooling medium.

[0026] The cooling medium may be unreacted syngas feed to the converter itself (process stream) or an external medium. Suitable external cooling fluids may include: cooling water, demi-water, boiler feed water which can be used in the process for recovery of energy or for steam production.

[0027] In a highly preferred embodiment said effluent heat exchanger is a feed-effluent heat exchanger arranged to transfer heat from a hot methanol-containing gas effluent from the catalytic bed to an unreacted syngas feed to the converter, so that said converter feed is preheated before entering the catalytic bed.

[0028] Said effluent heat exchanger comprises a first side for the syngas feed and a second side for the hot gas effluent. An output of said first side is in communication with an inlet of said catalytic bed and an inlet of said second side is in communication with an outlet of said catalytic bed.

[0029] Said effluent heat exchanger, according to various application, may be one of the following.

[0030] In a first option said effluent heat exchanger is a feed-effluent heat exchanger, as above defined, or a unit of the same, in case the feed-effluent heat exchanger is a multi-unit apparatus or is part of a heat exchanger train. In general terms, a feed-effluent heat exchanger is understood as an apparatus including one or more units arranged to cover a given thermal duty, i.e. heat that must be transferred from the hot effluent to the incoming gas. The heat exchanger arranged in the pressure vessel and within the cavity of the annular bed may be, according to various embodiments, the entire feed-effluent heat exchanger, arranged to fully cover the thermal duty, or a unit thereof arranged to cover a percentage of said duty.

[0031] In an interesting embodiment, said effluent heat exchanger is a plate type heat exchanger with heat exchange elements in the form of plates. Said heat exchange plates are relatively thin, flat heat exchange bodies having a length and width predominant over the thickness.

[0032] In a second option said effluent heat exchanger includes a liquid-gas separator arranged downstream of the feed-effluent heat exchanger.

[0033] In a third option said effluent heat exchanger includes a feed-effluent heat exchanger and a further heat exchanger arranged to remove additional heat from the hot reaction effluent with an external cooling medium such as cooling water.

[0034] In another option said effluent heat exchanger includes a feed-effluent heat exchanger, an additional heat exchanger and a liquid-gas separator.

[0035] The converter of the invention can comprise one or more catalytic bed(s).

[0036] Preferably the catalytic converter comprises only a single catalytic bed (single-bed converter). A particularly preferred embodiment is a single-bed isothermal converter having only one catalytic bed which is provided with cooling elements immersed in the catalytic mass to keep the temperature of the reaction zone within a target range. More preferably said cooling elements are in the form of heat exchange plates internally traversed by a cooling medium. The cooling medium is preferably water.

[0037] In some embodiments the catalytic converter comprises a single or multiple adiabatic bed(s) wherein no cooling element is provided within the catalytic mass.

[0038] In some embodiments the catalytic converter comprises multiple catalytic beds wherein dedicated cooling elements are immersed into each of the catalytic beds so to keep the reaction temperature in the respectively catalytic beds within a target range (isothermal or pseudo-isothermal range).

[0039] According to an alternative embodiment the converter is provided with multiple catalytic beds wherein some of the catalytic beds are isothermal beds whilst others are adiabatic beds without cooling elements within the catalytic mass.

[0040] According to an embodiment the converter comprises two catalytic beds arranged in series one downstream of the other wherein the first catalytic bed is an isothermal catalytic bed and the second catalytic bed is an adiabatic bed.

[0041] According to an embodiment the converter comprises two catalytic beds arranged in series one downstream of the other wherein the first catalytic bed is an adiabatic bed and the second catalytic bed is an isothermal catalytic bed.

[0042] According to various embodiments, said feed-effluent heat exchanger is a shell and tube heat exchanger comprising tubes housed within a cylindrical shell or said effluent heat exchanger is a plate type heat exchanger wherein the cooling elements are in the form of plates.

[0043] The converter can further comprise a methanol condenser contained in said pressure vessel, wherein said methanol condenser is arranged downstream of and in fluid communication with said feed-effluent heat exchanger, so to receive a cooled methanol-containing gas effluent from said second side of said heat exchanger and to produce a liquid methanol-containing product.

[0044] The invention can be applied to the making of a new converter and/or in revamping of pre-existing chemical plants especially when catalytic converters of compact sizes are required and limited space is available for their installation. In some embodiments, a methanol converter according to the invention can replace an existing methanol converter during a revamping procedure.

[0045] In a further aspect of the invention, a hydrocarbon or a biomass feedstock is subjected to a reforming step to yield a syngas feed comprising a mixture of carbon oxides and hydrogen and the syngas feed is then subjected to a heating step and subsequently reacted over a catalyst to produce a hot methanol containing gas. In some embodiments, the production of the syngas feed may include gasification of a suitable feedstock, preferably a biomass feedstock. The reforming step may be partially or fully substituted by said gasification step.

[0046] The hot methanol containing gas is then cooled and transferred to a condensation step and said heating of the syngas feed and the cooling of said hot methanol containing gas are carried out in an effluent heat exchanger so that the heat retained by the hot methanol containing gas is indirectly transferred to the syngas feed.

[0047] The reaction of the syngas feed over the catalyst, the heating of said syngas feed and the cooling of said hot methanol containing gas are carried out in the above-disclosed catalytic converter which can be configured to operate under isothermal conditions or under adiabatic conditions.

[0048] The isothermal conditions are preferably obtained by transferring the heat developed by the reaction of the syngas feed to a cooling medium. Any suitable cooling medium can be used but preferably pressurized water is used.

[0049] In some embodiment, additional hydrogen and carbon oxides can be prepared in the process and then reacted in the catalytic converter to increase the production of methanol. In an embodiment, additional hydrogen is produced via water electrolysis and carbon dioxide can also be obtained from a carbon dioxide recovery section.

[0050] In an embodiment, condensation of the raw methanol containing gas is carried out within the same pressure vessel of the converter, preferably after cooling in the feed effluent heat exchanger.

[0052] The converter 1 shown in FIG. 1 comprises a pressure vessel 3 provided with a nozzle 16 for a syngas feed 14 and an outlet 17 for a raw methanol gas 2.

[0053] The pressure vessel further includes an annular catalytic bed 4, heat exchanger plates 5 and a feed-effluent heat exchanger 6.

[0054] The feed-effluent heat exchanger 6 is arranged centrally in the converter 1 along an axis of symmetry (A-A) of the pressure vessel 3. The catalytic bed 4 has an annular shape and is arranged concentrically around said feed-effluent heat exchanger 6 so to enclose the heat exchanger 6.

[0055] The exchanger plates 5 are immersed in the catalytic bed 4 and are in charge of removing the heat developed by the methanol synthesis reaction to keep the reaction temperature within a target temperature range.

[0056] FIG. 1 discloses a preferred embodiment of a single-bed, plate-cooled isothermal reactor.

[0057] The feed-effluent heat exchanger 6 comprises a first side 7 and a second side 8 wherein said first side 7 is in fluid communication with an inlet 9 of said catalytic bed 4 and said second side 8 is in fluid communication with an outlet 10 of said catalytic bed 4.

[0058] The feed-effluent heat exchanger 6 is a shell and tube heat exchanger provided with multiples tubes 11 arranged within a shell of the heat exchanger. The shell-tube heat exchanger is arranged vertically within the converter so that the multiples tubes 11 are parallel to the axis of symmetry A-A of the reactor. The tubes 11 are in fluid communication with the outlet 10 of said catalytic bed 4 and with the outlet 17 of the pressure vessel.

[0059] In a variant (not shown), also the feed-effluent heat exchanger is a plate heat exchanger with heat exchange elements in the form of plates instead of tubes.

[0060] The outlet 10 of the catalytic bed 4 communicates with the tubes 11 of the heat exchanger 6 via a collector 21. The collector 21 is part of the shell of the feed-effluent heat exchanger 6 and extends along the entire length of said exchanger 6.

[0061] The shell side of the feed-effluent heat exchanger 6 is in fluid communication with a nozzle 16 of the pressure vessel 3 and with the inlet 9 of the catalytic bed 4.

[0062] An opening can be present on the pressure vessel 3 of the converter 1 to allow the extraction of the feed-effluent heat exchanger 6 from the pressure vessel 3 for maintenance and replacement purposes.

[0063] The heat exchanger plates 5 are also provided with a distribution system 18 arranged to supply a cooling medium 13, such as pressurized water, to the above-mentioned exchanger plates 5 and with a collector 19 arranged to collect the cooling medium 20 outside the converter 1.

[0064] The catalytic converter 1 works as follows: a syngas feed 14 enters the converter via nozzle 16 and crosses the shell side of the feed-effluent heat exchanger 6 wherein it absorbs heat from a hot methanol containing gas 12 which flows into the tubes 11 of the heat exchange 6. As a result of this heat transfer process, the syngas feed is pre-heated. The pre-heated synthesis gas is then transferred through the first side 7 toward the inlet 9 of the catalytic bed and in sequence into the catalytic bed 4 itself.

[0065] Herein the pre-heated synthesis gas reacts over the catalytic bed 4 to generate the hot methanol containing gas 12 which is then collected and conveyed to the second side 8 of the feed-effluent heat exchanger 6. Further the hot methanol containing gas 12, after crossing the collector 21, is fed to the tubes 11.

[0066] It can be noted that the hot methanol containing gas crosses the tubes 11 counter currently with the syngas feed 14 before being discharged outside the converter 1 via the outlet 17 nozzle as raw methanol gas 2. As above disclosed, heat is indirectly transferred from the hot methanol containing gas 12 to the syngas feed 14 in said shell and tube heat exchanger 6.