SE1851239A1 - Process and apparatus for the production of methanated gas - Google Patents

Abstract

:A process for the production of a methane-rich product gas from a syngas feed comprises (a) that recycle of part of the effluent from the methanation reactor(s) back to the feed stream to the reactor inlet comprises an ejector, (b) that said ejector functions with superheated steam, (c) that liquid water is removed downstream the throttling valve, (d) that the steam from the steam drum is split into a recycle stream and a stream to be exported, and (e) that isenthalpic throttling of at least a part of the steam from a steam drum is used followed by re-heating the steam with itself upstream the throttling valve without the need of a process-fired superheater.

Description

Process and apparatus for the production of methanated gas The present invention relates to a process for the produc- tion of a methane-rich product from a syngas feed. Further, the invention relates to an apparatus for carrying out the process.

The low availability of fossil liquid and gaseous fuels such as oil and natural gas has revived the interest in de- veloping technologies capable of producing combustible gas synthetically from widely available resources such as coal, biomass as well as off-gases from coke ovens. The produced gas goes under the name substitute natural gas or synthetic natural gas (SNG) having methane as its main constituent.

The present invention relates to a process and an apparatus for the production of methanated gas. In particular, the methanated gas is SNG, and the feed for the process origi- nates from coke ovens, from gasification of coal, biomass and/or waste or from biogas or pyrolysis gas. Preferably the feed is coke oven gas (COG).

Coke is a solid fuel produced from coal by baking the coal in an airless furnace. During coke production, various vol- atile coal constituents are driven off and purified, and an off-gas comprising i.e. one or both of carbon dioxide and carbon monoxide as well as hydrogen and hydrocarbons is produced. This coke oven off-gas is energy rich, and it is often combusted for generation of heat, e.g. for heating the coke furnace, when coke is produced in relation to steel works. However, especially when coke is produced as a lO solid fuel in a plant without other requirements for en- ergy, excess off-gas may be available.

In methanation processes, the formation of methane from carbon oxides and hydrogen proceeds quickly to equilibrium in the presence of a catalyst and in accordance with either or both of the following reaction schemes: CO + 3H2 <=> CH4+ H20 (l) CO2+ 4H2 <=> CH4+ 2H2O (2) It is not very important to know which of the above two re- actions is the faster, since there will at the same time be an approach to equilibrium between carbon monoxide and car- bon dioxide as follows: CO + H20 <=> C02-+ H2 (3) The net reaction of methane formation, whether by reaction (l) or reaction (2) or both, will be highly exothermic. Therefore, the temperature of the reactants and products will increase during the passage through a catalyst bed in an adiabatic reactor. On the other hand, such increasing temperature will tend to shift the equilibrium towards lower methane concentration. Consequently, complete or close to complete will only be possible if the temperature increase is limited by cooling the reacting gas in one way or another, for instance by recycling of cooled product gas. lO Coke ovens can be stand-alone plants, or they can be part of a steel production plant. Stand-alone plants (merchant coke ovens) have little or no use for the COG produced. COG is mostly used locally as a low grade fuel, or it is simply flared. However, since COG mainly consists of CH4 and syn- gas (CO + H2), it can be converted into various valuable chemicals (such as hydrogen, ammonia, methanol and dimethyl ether), SNG, liquefied natural gas (LNG) or synthetic gaso- line.

COG can be used for producing SNG in a method developed by the applicant, said method comprising a recycle of part of the effluent from the first methanation reactor and, if ap- plicable, also from the second methanation reactor back to the feed stream to said first reactor. This recycle can be driven by a compressor, or it can be driven by an ejector.

In a previous application (WO 2012/084076) by the present applicant it was found that by careful analysis of thermo- dynamics and reaction conditions, it is possible to iden- tify an optimal operation window, by combination of temper- ature control and steam addition. It was also found that the use of an ejector for driving the recycle of product gases is especially beneficial in the case of presence of C2+ hydrocarbons, as the effect of increased steam addition via an ejector will have an effect of increased recycle, and the combined increase in steam addition and recycle will have a synergistic effect in reducing the carbonaceous material formation.

In said previous application, the operation window is de- fined by the operating temperature T obtained by equili- brating the feed gas according to the methanation reaction and the steam to carbon in higher hydrocarbons molecular ratio S/HHC of the methanation equilibrated gas with uncon- verted higher hydrocarbons. In the broadest form, the oper- ating window for methanation covers operation in the pres- ence of at least 1% C2+ hydrocarbons at temperatures above 460°C, an S/HHC ratio below 25 and a temperature below T = (30-S/HHC + 425)°C.

The temperature of the reactants and products will increase during the passage through a catalyst bed in an adiabatic reactor, if the reaction is exothermic. On the other hand, such increasing temperature will tend to shift the equilib- rium towards lower methane concentration. Consequently, complete or close to complete reaction is only possible if the temperature increase is limited by cooling the reacting gas in one way or another, for instance by recycling of cooled product gas, as it is disclosed in US 4,l30,575.

It is well known that the temperature of the methanation reaction may be controlled by addition of steam to the syn- thesis gas, as disclosed e.g. by application EP 2 110 425. Such a steam addition, especially in the case of a feed comprising higher hydrocarbons (C>l), has the effect of re- ducing whisker carbon formation, which otherwise may poten- tially damage the catalyst.

Thus, the present invention relates to a process for the production of a methane-rich product gas from a syngas feed originating from a coke oven, from gasification of coal, lO biomass and/or waste or from biogas or pyrolysis gas, said process comprising (a) that the recycle of part of the effluent from the first methanation reactor and, if applicable, also from the sec- ond methanation reactor back to the feed stream to said first reactor comprises an ejector configured for having a steam feed as motive gas and a recycled methane rich prod- uct gas as driven gas, said steam being produced in a boil- ing water reactor or in a boiler downstream the first methanation reactor, (b) that the ejector functions with superheated steam, (c) that liquid water is removed downstream the throttling valve, (d) that the steam from the steam drum is split into a re- cycle stream and a stream to be exported, and (e) that isenthalpic throttling of at least a part of the steam from a steam drum is used followed by re-heating the steam with itself upstream the throttling valve without having a process-fired superheater.

Re-heating the steam with itself means that the steam is re-heated separately with steam from the steam drum.

A process-fired superheater is a superheater which is fired by the process heat. If a process-fired superheater is used, it is e.g. located inside the steam drum or connected to the steam drum. lO The recycle of part of the effluent from the first methana- tion reactor back to the feed stream to the first reactor comprises an ejector configured for having a steam feed as motive gas and a recycled product gas rich in methane as driven gas, with the associated benefit of providing a re- cycle without requiring any energy for pumping or requiring a pump with moving parts. Particularly the use of recycling by addition of steam via an ejector is attractive, since steam can be used to drive the ejector recycling the prod- uct stream, without additional consumption of energy. Thus, the use of an ejector allows for a combined adjustment of temperature and steam content in the feed in order not to exceed a critical combination of operating temperature and the critical steam to higher hydrocarbon ratio, when higher hydrocarbons are present in the feedstock.

The design of an ejector operating at high temperatures and pressures and at varying capacities is rather simple, and such an ejector is relatively cheap. Consequently, in addi- tion to an increase of the energy economy, the use of an ejector also contributes to an improvement of the overall economy of the methanation process.

However, the function of an ejector clearly is best with superheated steam because saturated steam may give rise to erosion problems, and a plant based on a boiling water re- actor (BWR) often produces saturated steam only, because process-fired superheating is not possible within the SNG unit. This is a problem for the ejector. lO It has now surprisingly turned out that this problem can be solved by so-called isenthalpic throttling of the steam from the steam drum followed by a re-heating of the steam 'with itself', and this fact constitutes the crux of the present invention.

An isenthalpic process (or isoenthalpic process) is defined as a process that proceeds without any change in enthalpy or specific enthalpy.

In a steady-state, steady-flow process, significant changes in pressure and temperature can occur to the fluid, and yet the process will be isenthalpic if there is no transfer of heat to or from the surroundings, no work done on or by the surroundings and no change in the kinetic energy of the fluid.

The throttling process is a good example of an isenthalpic process. If we consider the lifting of a relief valve or safety valve on a pressure vessel, then the specific en- thalpy of the fluid inside the pressure vessel is the same as the specific enthalpy of the fluid as it escapes from the valve. With knowledge of the specific enthalpy of the fluid and the pressure outside the pressure vessel, it is possible to determine the temperature and speed of the es- caping fluid.

The invention also comprises an apparatus to be used for carrying out the process, said apparatus comprising: lO a first methanation reactor in the form of a boiling water reactor, which can be preceded by a sulfur guard, and op- tionally a second adiabatic methanation reactor, and also comprising a superheater, a steam drum, a knock out drum and an ejector, wherein isenthalpic throttling of at least a part of the steam from the steam drum is carried out, followed by re- heating the steam with itself upstream the throttling valve, thereby establishing the superheated steam which is needed for the function of the ejector.

A knock out drum is a vapor-liquid separator often used in several industrial applications to separate a vapor-liquid mixture.

In the process of the invention, saturated steam is pro- duced at around 85 bar, preferably 30 bar and most prefera- bly 4O bar higher than the process pressure in the reactor. Superheating is preferably achieved by using a dedicated heat exchanger or by inserting a tube bundle or coil into the steam drum.

The invention is further explained with reference to the figures l-6. Of these, figures l-4 and 6 illustrate possi- ble ways of arranging the heating and the ejector in an ap- paratus for the production of a methane-rich product gas by the process according to the invention, while figure 5 shows a known design with a traditional fired superheater.

More specifically, Fig. 1 shows a possible embodiment of the apparatus of the invention, wherein some of the steam (116) produced in a steam drum, which is fed with boiling water (102), heats up the gas phase (144) from a knock out drum (140) in a heat exchanger (120), while the rest of the steam (124) produced in the steam drum is exported. The cooled steam (122) is fed to said knock out drum (140) via a valve (130). Boiling water (112) from the steam drum is fed to the methanation reactor and returns to the steam drum via line (104). The heated gas phase (146) is used to feed an ejector.

While this embodiment works satisfactorily, it has the mi- nor drawback that the valve (130) will have to be cleaned regularly.

Fig. 2 illustrates another embodiment of the apparatus of the invention, in which a heater (220) located inside the steam drum (210), fed with boiling water (202), heats up the gas phase (244) from the knock out drum (240). The cooled steam (222) is partly fed to said knock out drum (240) via a valve (230) and partly exported via line (224).

The heated gas phase (246) is used to feed an ejector.

In still another embodiment of the apparatus according to the invention, shown in Fig. 3, the heat exchanger (320) is located outside the steam drum much the same way as in Fig. 1, but this time part of the steam (315) produced in the steam drum (310) fed with boiling water (302) is passed through the heat exchanger (320) and then fed back to the steam drum via line (322). This way, the steam from the steam drum is re-heated 'with itself' while the heated gas phase (346) is used to feed an ejector.

Fig. 4 shows a more complete apparatus layout of the inven- tion comprising the steam drum/knock out drum arrangement of Fig. l but also including a boiling water methanation reactor (460) and an ejector (450) configured for having a steam feed as motive gas and a recycled product gas rich in methane as driven gas. More specifically, the ejector (450) is fed by the steam (464) from the heat exchanger (420) and further by part (466) of the methane-rich effluent (462) from the methanation reactor (460).

Fig. 5 illustrates a traditional fired superheater design for a methanation apparatus, said design comprising a methanation reactor (570), a heater (560), a heat exchanger (520) and an ejector (550) fed by the gas (522) from the heat exchanger (520) and further by a part (566) of the ef- fluent (564) from the methanation reactor.

Finally, Fig. 6 shows a more complete apparatus layout of the invention comprising the steam drum/knock out drum of Fig. 2 but also including a boiling water methanation reac- tor (670) and an ejector (650) configured for having a steam feed as motive gas and a recycled product gas rich in methane as driven gas. More specifically, the ejector (650) is fed by the steam (664) from the heater located inside the steam drum (610) and further by part (666) of the me- thane-rich effluent (672) from the methanation reactor (670). lO ll The following table shows a comparison between the known design with a traditional fired superheater (Fig. 5) and two embodiments of the novel design according to the inven- tion (Fig. 4, isothermic, and Fig. 6, adiabatic).

Tradition- Isothermal, Adiabatic, al (Fig. 5) inv* (Fig. 4) inv* (Fig. 6) Steam. flow 2559 kg/h 4740 kg/h 3599 kg/h export temp. 226°C 226°C 226°C pressure 25 bar 25 bar 25 bar Møtive fiøw 4501 kg/h 3778 kg/h 3697 kg/h steam temp. 400°C 280°C 270°C pressure 27 bar 29.5 bar 29.5 bar Steam. flow 706l kg/h 8783 kg/h 7523 kg/h drum temp. 259°C 300°C 300°C @XpOrt pressure 45 bar 85 bar 84.9 bar *) according to the invention It is seen that the process of the invention exports more steam, even though the motive steam has a lower temperature compared to the reference, because the pressure drop in the recycle loop is lower due to the fact that according to the invention there is no process steam superheater.

The process of the invention presents an alternative to a process steam superheater. It is especially useful for small process plants. Furthermore, the process remedies the lack of process heat for superheating in ejector-based BWR plants, and it can export both high pressure and medium pressure superheated steam.

Claims (5)

1. l. A process for the production of a methane-rich product gas from a syngas feed originating from a coke oven, from gasification of coal, biomass and/or waste or from biogas or pyrolysis gas, said process comprising (a) that the recycle of part of the effluent from the first methanation reactor and, if applicable, also from the sec- ond methanation reactor back to the feed stream to said first reactor comprises an ejector configured for having a steam feed as motive gas and a recycled methane rich prod- uct gas as driven gas, said steam being produced in a boil- ing water reactor or in a boiler downstream the first methanation reactor, (b) that the ejector functions with superheated steam, (c) that liquid water is removed downstream the throttling valve, (d) that the steam from the steam drum is split into a re- cycle stream and a stream to be exported, and (e) that isenthalpic throttling of at least a part of the steam from a steam drum is used followed by re-heating the steam with itself upstream the throttling valve without having a process-fired superheater, i.e. a superheater which is fired by the process heat.
2. 2. The process according to claim l, wherein saturated steam is produced at a pressure around 85 bar, preferably 30 bar and most preferably 40 bar higher than the process pressure in the reactor.
3. 3. The process according to claim l or 2, wherein super- heating is achieved using a dedicated heat exchanger.
4. 4. The process according to claim l or 2, wherein super- heating is achieved by inserting a tube bundle or coil into the steam drum.
5. 5. An apparatus to be used for carrying out the process according to any of the claims l-4, said apparatus compris- ing: a first methanation reactor in the form of a boiling water reactor, which can be preceded by a sulfur guard, and op- tionally a second adiabatic methanation reactor, and also comprising a superheater, a steam drum, a knock out drum, and an ejector, wherein isenthalpic throttling of at least a part of the steam from the steam drum is carried out, followed by re- heating the steam with itself upstream the throttling valve, thereby establishing the superheated steam needed for the function of the ejector.