WO2017054932A1 - Process and plant for producing synthesis gas with variable composition - Google Patents

Process and plant for producing synthesis gas with variable composition Download PDF

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Publication number
WO2017054932A1
WO2017054932A1 PCT/EP2016/025102 EP2016025102W WO2017054932A1 WO 2017054932 A1 WO2017054932 A1 WO 2017054932A1 EP 2016025102 W EP2016025102 W EP 2016025102W WO 2017054932 A1 WO2017054932 A1 WO 2017054932A1
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Prior art keywords
gas
rich
hydrogen
gas mixture
plant
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PCT/EP2016/025102
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French (fr)
Inventor
Bernd Polster
Antoine Hernandez
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LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
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LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
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Application filed by LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude filed Critical LAir Liquide SA pour lEtude et lExploitation des Procedes Georges Claude
Priority to US15/764,489 priority Critical patent/US11235974B2/en
Priority to CN201680054771.6A priority patent/CN108025913B/en
Publication of WO2017054932A1 publication Critical patent/WO2017054932A1/en
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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    • C01B3/02—Production of hydrogen; Production of gaseous mixtures containing hydrogen
    • C01B3/32—Production of hydrogen; Production of gaseous mixtures containing hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide or air
    • C01B3/34—Production of hydrogen; Production of gaseous mixtures containing hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide or air by reaction of hydrocarbons with gasifying agents
    • C01B3/38—Production of hydrogen; Production of gaseous mixtures containing hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide or air by reaction of hydrocarbons with gasifying agents using catalysts
    • C01B3/384—Production of hydrogen; Production of gaseous mixtures containing hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide or air by reaction of hydrocarbons with gasifying agents using catalysts with external heating of the catalyst
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    • C01B3/32—Production of hydrogen; Production of gaseous mixtures containing hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide or air
    • C01B3/34—Production of hydrogen; Production of gaseous mixtures containing hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide or air by reaction of hydrocarbons with gasifying agents
    • C01B3/48—Production of hydrogen; Production of gaseous mixtures containing hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide or air by reaction of hydrocarbons with gasifying agents followed by reaction of water vapour with carbon monoxide
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    • C01B2203/0283—Processes for making hydrogen or synthesis gas containing a CO-shift step, i.e. a water gas shift step
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    • C01B2203/0811—Methods of heating the process for making hydrogen or synthesis gas by combustion of fuel
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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F25J2205/00—Processes or apparatus using other separation and/or other processing means
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    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2205/00—Processes or apparatus using other separation and/or other processing means
    • F25J2205/60—Processes or apparatus using other separation and/or other processing means using adsorption on solid adsorbents, e.g. by temperature-swing adsorption [TSA] at the hot or cold end
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    • F25J2215/00—Processes characterised by the type or other details of the product stream
    • F25J2215/02—Mixing or blending of fluids to yield a certain product
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
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    • F25J2245/00—Processes or apparatus involving steps for recycling of process streams
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    • F25J2260/00—Coupling of processes or apparatus to other units; Integrated schemes
    • F25J2260/60—Integration in an installation using hydrocarbons, e.g. for fuel purposes
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Definitions

  • This invention relates to a process and a plant for the continuous conversion of a hydrocarbonaceous feed gas into a synthesis gas comprising carbon monoxide and hydrogen.
  • this invention relates to a process and a plant for the continuous production of synthesis gases with variable molar ratio of hydrogen to carbon monoxide.
  • the molar ratio of hydrogen to carbon monoxide (H 2 /CO ratio, mol/mol) of a synthesis gas produced by means of such process or in such plant substantially is determined by the following chemical equilibrium reactions:
  • H 2 /CO ratio in the synthesis gas is determined by the height of the molar ratio between carbon and steam in the feed gas, by the efficiency of the reforming catalyst, and by the height of the pressures and temperatures existing during the reforming reactions.
  • Another known possibility for setting the H 2 /CO molar ratio consists in setting the carbon dioxide content in the feed gas, as it is likewise taught in the patent specification US 8,888,873 B2.
  • setting or varying the H 2 /CO ratio only is possible within relatively narrow limits.
  • a process for the continuous conversion of a hydrocarbonaceous feed gas into a synthesis gas comprising carbon monoxide and hydrogen comprising the following process steps carried out one after the other:
  • a feed gas mixture which comprises hydrocarbonaceous gas and steam, as well as a heating gas mixture which comprises a fuel gas and a gas rich in oxygen,
  • step d) at least a part of the gas rich in methane obtained according to step d) is admixed to the feed gas mixture, and that
  • a process for the continuous conversion of a hydrocarbonaceous feed gas into a synthesis gas comprising carbon monoxide and hydrogen comprising the following process steps carried out one after the other:
  • a feed gas mixture which comprises hydrocarbonaceous gas and steam, as well as a heating gas mixture which comprises a fuel gas and a gas rich in oxygen,
  • step d) at least a part of the gas rich in methane obtained according to step d) is admixed to the feed gas mixture, and that
  • a plant for the continuous conversion of a hydrocarbonaceous feed gas into a synthesis gas comprising carbon monoxide and hydrogen comprising the following means and plant sections:
  • a feed gas mixture which comprises hydrocarbonaceous gas and steam, as well as a heating gas mixture which comprises a fuel gas and a gas rich in oxygen, from external sources to the plant, as well as means for the transfer of the gas or gases between the plant sections and out of the plant for their further use,
  • catalyst tubes which are arranged in the housing and are filled with a catalyst bed and through which the feed gas mixture is passed and is reformed into raw synthesis gas containing hydrogen, carbon monoxide, carbon dioxide and methane,
  • burners which are operated with the heating gas mixture and which are installed in the housing such that their flames reach into the housing and thereby produce the heat necessary for the reformation and transmit the same to the catalyst tubes and hence indirectly to the feed gas mixture
  • plant section for separating the carbon dioxide from the raw synthesis gas
  • plant section for fractionating the raw synthesis gas into a gas rich in hydrogen, which is supplied either to the treatment in step e) or to the further use outside the plant, furthermore into a gas rich in carbon monoxide which is supplied to the further use outside the plant, and into a gas rich in methane which is admixed to the feed gas mixture and/or to the heating gas mixture,
  • h means for controlling the mass flows of the gases to be admixed to the feed gas mixture and to the heating gas mixture.
  • Normal operation is understood to be a mode of operation of the reformer plant known from the prior art, in which neither gas rich in methane is recirculated to the feed gas mixture nor H 2 product gas, gas rich in hydrogen or CO product gas is recirculated to the heating gas mixture.
  • Reforming conditions are understood to be the operating conditions of the reformer plant well known to the skilled person, which ensure a technically and economically expedient degree of conversion of the feedstocks to synthesis gas components.
  • a set of operating conditions chosen therefor also is referred to as operating point.
  • at least a part of the gas rich in methane obtained according to step d) is admixed to the feed gas mixture.
  • At least a part of the H 2 product gas and/or a fraction of the gas rich in hydrogen increased with respect to the normal operation of the process can be admixed to the heating gas mixture, so that the H 2 /CO ratio V1 is lowered with respect to the base H 2 /CO ratio VB to a target value V1 ⁇ VB, or alternatively
  • At least a part of the CO product gas and/or a fraction of the gas rich in carbon monoxide increased with respect to the normal operation of the process can be admixed to the heating gas mixture, so that the H 2 /CO ratio V1 is increased with respect to the base H 2 /CO ratio VB to a target value V1 > VB.
  • a preferred aspect of the invention is characterized in that the fraction of gas rich in methane, which is admixed to the feed gas mixture, is at least 50 %, preferably at least 70 %, most preferably at least 90 % of the gas rich in methane obtained according to step d).
  • At least 20 %, preferably at least 30 %, most preferably at least 40 % of the H 2 product gas or the gas rich in hydrogen are admixed to the heating gas mixture or, in an alternative aspect, at least 20 %, preferably at least 30 %, most preferably at least 40 % of the CO product gas are admixed to the heating gas mixture. It has been found that within the aforementioned ranges taking account of the caloric equivalence of the gas streams admixed to the feed gas mixture and to the heating gas mixture the H 2 /CO ratio can be set within wide limits, while at the same time a stable and trouble-free operation of the reformer plant becomes possible with a proper heat balance.
  • Step 1 The H 2 product gas or the gas rich in hydrogen or the CO product gas is completely admixed to the heating gas mixture.
  • Step 2 The H 2 product gas or the gas rich in hydrogen or the CO product gas for one part is admixed to the heating gas mixture and for the other part is discharged for the further use outside the process, wherein the discharged fraction is increased or reduced continuously or step by step, until the target value V1 of the H 2 /CO ratio is achieved.
  • This procedure provides for realizing the change of the quantity of CO and/or H 2 delivered to the external consumer more quickly than is possible in a conventional way.
  • the carbon dioxide separated from the raw synthesis gas is admixed to the feed gas mixture at least in part.
  • the separated carbon dioxide can be utilized materially at least in part. Furthermore, it thus is possible to further influence the H 2 /CO ratio.
  • the H 2 /CO ratio can be influenced to an even greater extent by admixing carbon dioxide produced outside the process to the feed gas mixture.
  • a sink for carbon dioxide present in excess or undesired, foreign to the process thus is created at the same time.
  • the separation of the raw synthesis gas into a gas rich in hydrogen, a gas rich in carbon monoxide and a gas rich in methane is carried out by means of a cryogenic distillation process, wherein a further gas stream containing combustible gas components (coldbox flashgas) is obtained, which is at least partly admixed to the heating gas mixture and/or the feed gas mixture.
  • Coldbox flashgas combustible gas components
  • cleaning of the gas rich in hydrogen is carried out by separation of gaseous impurities by means of a pressure swing adsorption process (PSA), wherein the gaseous impurities are obtained as separate gas stream (PSA offgas) and are at least partly admixed to the heating gas mixture.
  • PSA pressure swing adsorption process
  • Another preferred aspect of the invention is characterized in that separating the carbon dioxide from the raw synthesis gas by means of an absorption process, carried out in a corresponding absorption plant, is effected by using e.g. methyldiethanolamine (MDEA) as absorbent. This process or such plant is described e.g.
  • MDEA methyldiethanolamine
  • the carbon dioxide can efficiently be separated from the raw synthesis gas and can be used further, for example for the recirculation to the feed gas mixture.
  • the same also comprises means for the recirculation of the carbon dioxide separated from the raw synthesis gas to the feed gas mixture and/or means for supplying carbon dioxide produced outside the process to the feed gas mixture.
  • the plant section for separating the raw synthesis gas into a gas rich in hydrogen, a gas rich in carbon monoxide and a gas rich in methane is designed as cryogenic distillation process, wherein there are furthermore comprised means for discharging a further gas stream containing combustible gas components (coldbox flashgas).
  • the plant section for cleaning the gas rich in hydrogen by separation of gaseous impurities is designed as pressure swing adsorption process (PSA), wherein there are furthermore comprised means for discharging a further gas stream containing gaseous impurities (PSA offgas) and for at least partly admixing the same to the heating gas mixure.
  • PSA pressure swing adsorption process
  • Fig. 1 shows a block diagram of a process according to the prior art
  • Fig. 2 shows a block diagram of an alternative process according to the prior art with carbon dioxide recirculation and supply of external carbon dioxide
  • Fig. 3 shows a block diagram of a first exemplary aspect of the invention, based on Fig. 2,
  • Fig. 4 shows a block diagram of a second exemplary aspect of the invention, based on Fig. 3, with additional CO shift stage.
  • Fig. 1 comparative example according to the prior art.
  • the feed gas mixture 1 comprising hydrocarbonaceous gas and steam, as well as the heating gas mixture 2, comprising a fuel gas and a gas rich in oxygen, are charged to a reformer 3.
  • the feed gas mixture 1 is converted into a raw synthesis gas 4, comprising hydrogen, carbon monoxide, carbon dioxide and methane as well as water, possibly nitrogen and impurities known to the skilled person.
  • the raw synthesis gas 4 is transferred from the reformer 3 into a plant 5 for separating the carbon dioxide 6 from the raw synthesis gas 4.
  • This separation in plant 5 can be effected for example by means of an absorption process by using methyldiethanolamine (MDEA).
  • MDEA methyldiethanolamine
  • the raw synthesis gas 7 liberated from carbon dioxide is passed from plant 5 into a plant 8 for fractionating the raw synthesis gas into a gas 9 rich in hydrogen, a gas 10 rich in carbon monoxide, and a gas 1 1 rich in methane.
  • This fractionation in plant 8 can be effected for example by means of a cryogenic distillation process.
  • the gas 1 1 rich in methane is added to the heating gas mixture 2.
  • the gas 10 rich in carbon monoxide is discharged for the further use as product gas and the gas 9 rich in hydrogen in this example is supplied to a plant 12 for the separation of gaseous impurities.
  • a pressure swing adsorption process can be employed.
  • the gaseous impurities separated in this process are admixed to the heating gas mixture 2.
  • the cleaned gas 14 rich in hydrogen is discharged from the plant for the further use as H 2 product gas.
  • the carbon dioxide 6 separated from the raw synthesis gas 7 in plant 5 by means of an absorption process is added to the feed gases 1 , in addition with externally obtained carbon dioxide 15.
  • This example shows a plant according to Fig. 2, in which a part of the product gas 1 6 rich in carbon monoxide and/or of the gas 18 rich in hydrogen or of the H 2 product gas 17 is recirculated in addition and admixed to the heating gas mixture 2. Furthermore, a part of the gas 1 1 a rich in methane is recirculated from the cryogenic distillation plant (coldbox) 8 into the feed gas mixture. The remaining fraction of gas 1 1 rich in methane is admixed to the heating gas mixture.
  • the gas stream 18 rich in hydrogen provided from the cryogenic distillation plant already has a very high hydrogen content, the same can also be admixed to the heating gas mixture in accordance with the invention, in order to lower the H 2 /CO ratio. Hence, it is not absolutely necessary to admix a partial stream of the purified H 2 product stream to the heating gas mixture.
  • Fig. 4 exemplary embodiment of the invention
  • This example shows a plant according to Fig. 3, but without the recirculation of C0 2 separated from the raw synthesis gas or imported from external sources. Furthermore, this example contains a CO shift stage 20 which is charged with a partial stream of the raw synthesis gas 19 and from which a gas stream 21 enriched in hydrogen is discharged and together with the gas stream 9 rich in hydrogen from plant 8 is supplied to the pressure swing adsorption plant 12.
  • H 2 /CO molar ratio A possibility known to the skilled person for setting the H 2 /CO molar ratio consists in setting the carbon dioxide content in the feed gas, as it is illustrated in the following Tables 1 to 3. Without addition of carbon dioxide into the feed gas the H 2 /CO molar ratio in the synthesis gas can be set in the range from 3.8 to 5.4 or as ratio of the H 2 and CO product streams in the range from 3.3 to 4.7 by selection of the operating conditions (see Table 1 ). Operating conditions which influence the H 2 /CO molar ratio above all include the reformer temperature and the ratio of steam to carbon (S/C ratio) in the feed gas and to a smaller extent also the reformer pressure and the efficiencies of the CO and H 2 purification devices.
  • S/C ratio steam to carbon
  • Table 1 Comparison of the H 2 /CO ratios for the product and synthesis gas streams for various operating conditions (reformer temperature, S/C ratio) at a reformer pressure of 25 bara, for a feed of 100% CH 4 , with a CO and H 2 efficiency of 97% for the cryogenic distillation plant and a H 2 efficiency of 87% for the pressure swing adsorption process
  • ratios of 2.6 to 3.3 can be set, based on the product streams (see Table 2), or of 3.8 to 3.0 in the synthesis gas.
  • the ratio can be lowered to the range of 1 .4 to 2.6, based on the product streams (see Table 3), or of 1 .6 to 3.0 in the synthesis gas, when in addition to the complete carbon dioxide recirculation carbon dioxide originating from an external source is admixed to the feed gas, wherein the quantity of the carbon dioxide admixed to the feed gas, originating from an external source, decisively influences the ratio.
  • Table 2 Comparison of the H 2 /CO ratios for the product and synthesis gas streams for the same operating conditions as for Table 1 (derived cases A, D, F, H) and with an additional C0 2 admixture of 95% of the carbon dioxide separated from the raw synthesis gas to the feed gas mixture
  • Plants for the production of CO and H 2 are designed such that a specified H 2 /CO molar ratio of the H 2 and CO product streams is achieved in that the operating conditions are adapted and for example a partial or complete recirculation of the carbon dioxide separated from the synthesis gas is carried out.
  • H 2 /CO molar ratio achieved is too high even with complete recirculation of CO 2 , an admixture of externally obtained CO 2 to the feed gas mixture can be taken into consideration.
  • a recirculation of the carbon dioxide separated from the synthesis gas also can possibly be omitted, when externally obtained CO 2 e.g. is present already in a sufficient quantity at feed pressure.
  • the external carbon dioxide should be present in sufficient purity, wherein impurities with carbon, nitrogen and hydrogen compounds possibly are acceptable, and should not contain any compounds which are harmful for the catalysts of the steam reformation plant (such as e.g. arsenic, sulfur, chlorine, metals).
  • impurities with carbon, nitrogen and hydrogen compounds possibly are acceptable, and should not contain any compounds which are harmful for the catalysts of the steam reformation plant (such as e.g. arsenic, sulfur, chlorine, metals).
  • the feed parameters chosen for the plant design can be made during operation of the plant, which then also allow only small changes of the H 2 /CO product ratio as compared to the original design. Deviations of maximally 10 % of the H 2 /CO molar ratio then are possible. In the real plant operation, however, load cases sometimes occur, in which the H 2 /CO product ratio differs distinctly from the possible changes. Since the fluctuations of such H 2 /CO product ratios in part are very large, but normally only last for a short time, these cases normally are not taken
  • a partial stream of the raw synthesis gas might be supplied to a CO shift stage, in order to increase the H 2 /CO ratio or provide a plant with a high flexibility with regard to the H 2 /CO ratio.
  • the H 2 /CO ratio of the products as specified during the plant design substantially is constant at various load conditions of the plant.
  • a plant for the production of H 2 and CO normally can be run down to a plant load of approximately 20 % to 40 % of the design load. In operation of a plant under partial load a higher flexibility of the H 2 /CO product ratio is given. At a plant load of approximately 50 % deviations of about 20 % of the H 2 /CO molar ratio are possible.
  • One aspect of the invention is characterized in that at least a part of the gas rich in methane obtained according to step d) of claim 1 is admixed to the feed gas mixture.
  • the recirculation of the gas rich in methane to the feed gas mixture can be effected in conjunction with the recirculation of the carbon dioxide separated from the raw synthesis gas or independent thereof.
  • measures of pressure increase generally are provided, such as e.g. compressors.
  • the carbon dioxide separated from the raw synthesis gas normally is obtained at ambient pressure or at a pressure of up to about 3 bar.
  • the stream rich in methane can be discharged from the cryogenic distillation process with a pressure comparable to the carbon dioxide, but possibly also under pressure. In the latter case, it can then be introduced directly into the feed gas or it requires an only small pressure increase as compared to the carbon dioxide, which can also be executed together with the carbon dioxide, in e.g. one or more common last pressure boosting stages of a carbon dioxide compressor.
  • the carbon dioxide preferably is admixed to the feed gas after a possible pre reformat! on unit (prereforming), while the admixture of the methane-containing gas can be chosen freely. Since the methane-containing gas contains no sulfur, if it is permanently admixed to the feed gas, the desulfurization unit of the steam reforming plant can be reduced in size correspondingly. If the methane-containing gas is admixed to the feed gas with the carbon dioxide after a possible prereformer, steam must also be added to the feed gas mixture in this region of the plant, in order to establish a required S/C ratio in the feed gas mixture.
  • the quantity of waste gases (offgases) from the cryogenic distillation process for the CO purification corresponds to approximately 25 % to 50 % of the total quantity of heating gas.
  • These off gases comprise a methane-containing gas (CH 4 ) and a flashgas (FG) rich in H 2 .
  • CH 4 methane-containing gas
  • FG flashgas
  • plants with a nitrogen removal in the cryogenic distillation plant there is also obtained a nitrogen-containing offgas which normally, however, has no significant share in the energy content of the heating gas mixture.
  • the offgas of the pressure swing adsorption (PSA) has a share of about 20 % to 35 % in the total heating gas.
  • Table 4 Heating gas distribution of synthesis gas plants (derived cases F from Tables 1 to 3) for an air preheating temperature of 340 °C and a steam export at 30 barg and 300 °C.
  • Table 6 Heating gas distribution of synthesis gas plants (derived case F2 of Table 2) for an air preheating temperature of 340 °C and a steam export at 30 barg and 300 °C - Influence of the admixture of gas rich in hydrogen to the heating gas mixture on the H 2 /CO product ratio for plants with recirculation of the carbon dioxide separated from the synthesis gas into the feed gas and without use of externally obtained carbon dioxide
  • Table 7 Heating gas distribution of synthesis gas plants (derived case F2-II of Table 3) for an air preheating temperature of 340 °C and a steam export at 30 barg and 300 °C - Influence of the admixture of gas rich in hydrogen to the heating gas mixture on the H 2 /CO product ratio for plants with recirculation of the carbon dioxide separated from the synthesis gas into the feed gas and with use of
  • Tables 5, 6 and 7 are based on recirculation rates of 90 % of the methane- containing stream separated by cryogenic distillation into the feed gas mixture. This allows to keep the methane-containing stream recirculated into the feed gas constant independent of fluctuations.
  • the external heating gas quantity is varied. The same is preset to about 5 %, which allows to compensate fluctuations in parts of the heating gas quantity and at the same time in the case of the loss of a heating gas quantity (e.g. by sudden shut-down of the pressure swing plant due to a valve malfunction) allows to compensate the required amount of heating gas by rapid increase of the external heating gas quantity, so that the steam reforming plant can be operated further.
  • this ratio can also be increased temporarily by feeding CO product gas into the heating gas mixture. This is the case in particular when e.g. in a CO pipeline system (CO complex) a CO consumer suddenly fails and the combustion of excess CO in a flare system should then be avoided, until the steam reforming plant has been run down to the current need of CO or until the failed CO consumer again withdraws CO from the CO complex. During scheduled maintenance work on a CO consumer, the discharged amount of CO also can be reduced without having to change the operating point of the synthesis gas plant.
  • the temporary feeding of CO or H 2 into the heating gas mixture also is advantageous when the run-up rate (from a defined plant operating point, which at least corresponds to the minimum operating point of the plant - which normally corresponds to between 20 % and 40 % of the design quantity of a product stream - to another plant operating point with higher product stream) of a CO or H 2 consumer is higher than the run-up rate of the entire steam reforming plant including the CO and/or H 2 purification plant.
  • Typical values lie in the range from 0.1 %/min to maximally 2 %/min or preferably in the range from 0.2 %/min to 1 %/min, based on the design point of a product stream.
  • a design for 10000 Nm 3 /h of CO product stream for example corresponds to a run-up rate of 1 %/min of a load increase of 100 Nm 3 of CO per minute.
  • a part of the product gas can be admixed to the heating gas mixture.
  • the same problem does exist when, for example due to the loss of a consumer, the quantity of the product streams must be reduced.
  • the invention provides a process and a plant in which the H 2 /CO molar ratio can be changed beyond the limits known from the prior art or beyond the plant design.
  • the production hence can even further be adapted to the demand of the gas consumers.
  • temporally fluctuating operating conditions within the plant complex or fluctuating demands of the consumers withdrawing H 2 or CO within this complex can be satisfied without having to leave the previously fixed operating point of the reformer plant.

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Abstract

There is indicated a process and a plant for the continuous conversion of a hydrocarbonaceous feed gas into a synthesis gas comprising carbon monoxide and hydrogen, wherein the H2/CO molar ratio of the product gases can be varied within a wide range. According to the invention this is achieved in that at least a part of a gas rich in methane obtained during the fractionation of the raw synthesis gas, for example by cryogenic separation, is admixed to the feed gas mixture, and that in the alternative - at least a part of the H2 product gas and/or a fraction of a gas rich in hydrogen increased with respect to the normal operation of the process is admixed to the heating gas mixture, in order to lower the H2/CO ratio, or - at least a part of the CO product gas and/or a fraction of a gas rich in carbon monoxide increased with respect to the normal operation of the process is admixed to the heating gas mixture, in order to increase the H2/CO ratio.

Description

AIR LIQUIDE File Number: S1 0314
Process and Plant for Producing Synthesis Gas with Variable Composition
Field of the Invention
This invention relates to a process and a plant for the continuous conversion of a hydrocarbonaceous feed gas into a synthesis gas comprising carbon monoxide and hydrogen. In particular, this invention relates to a process and a plant for the continuous production of synthesis gases with variable molar ratio of hydrogen to carbon monoxide.
Prior art
Processes and plants as described above are known from the prior art and described e.g. in the American patent specification US 8,888,873 B2.
The molar ratio of hydrogen to carbon monoxide (H2/CO ratio, mol/mol) of a synthesis gas produced by means of such process or in such plant substantially is determined by the following chemical equilibrium reactions:
CH4 + H20 = CO + 3 H2
CO + H2O = CO2 + H2 The height of the H2/CO ratio in the synthesis gas is determined by the height of the molar ratio between carbon and steam in the feed gas, by the efficiency of the reforming catalyst, and by the height of the pressures and temperatures existing during the reforming reactions. Another known possibility for setting the H2/CO molar ratio consists in setting the carbon dioxide content in the feed gas, as it is likewise taught in the patent specification US 8,888,873 B2. However, setting or varying the H2/CO ratio only is possible within relatively narrow limits. At the same time there is a need to produce synthesis gases with very different H2/CO ratios preferably with the same production plant, in order to satisfy the needs of various downstream processes within integrated production networks.
It therefore is the object of the invention to provide a process and a plant which provides for setting the H2/CO molar ratio within wider limits than is known so far from the prior art.
Description of the Invention
This object is solved by a process according to the features of claims 1 or 2 and by a plant according to the features of claim 1 1 .
Process according to the invention:
A process for the continuous conversion of a hydrocarbonaceous feed gas into a synthesis gas comprising carbon monoxide and hydrogen, comprising the following process steps carried out one after the other:
a) providing a feed gas mixture which comprises hydrocarbonaceous gas and steam, as well as a heating gas mixture which comprises a fuel gas and a gas rich in oxygen,
b) splitting the feed gas mixture under reforming conditions by catalytic steam reformation in a tubular reformer fired by burners into a raw synthesis gas containing hydrogen, carbon monoxide, carbon dioxide and methane, wherein the heat necessary for splitting, which is indirectly transmitted to the feed gas mixture, is produced by combustion of the heating gas mixture, wherein in normal operation of the process a base H2/CO ratio VB is obtained,
c) separating the carbon dioxide from the raw synthesis gas,
d) fractionating the raw synthesis gas into a gas rich in hydrogen, a gas rich in carbon monoxide and a gas rich in methane, wherein the gas rich in hydrogen optionally is supplied to a further cleaning step and otherwise is discharged from the process as H2 product gas, and the gas rich in carbon monoxide is supplied to the further use outside the process as CO product gas,
e) optionally cleaning of the gas rich in hydrogen by separating gaseous impurities and admixing the same to the heating gas mixture and discharging the cleaned gas rich in hydrogen as H2 product gas for the further use outside the process,
characterized in that
f) at least a part of the gas rich in methane obtained according to step d) is admixed to the feed gas mixture, and that
g) at least a part of the H2 product gas and/or a fraction of the gas rich in hydrogen increased with respect to the normal operation of the process is admixed to the heating gas mixture, so that the H2/CO ratio V1 is lowered with respect to the base H2/CO ratio VB to a target value V1 < VB.
Process according to the invention in an alternative configuration:
A process for the continuous conversion of a hydrocarbonaceous feed gas into a synthesis gas comprising carbon monoxide and hydrogen, comprising the following process steps carried out one after the other:
a) providing a feed gas mixture which comprises hydrocarbonaceous gas and steam, as well as a heating gas mixture which comprises a fuel gas and a gas rich in oxygen,
b) splitting the feed gas mixture under reforming conditions by catalytic steam reformation in a tubular reformer fired by burners into a raw synthesis gas containing hydrogen, carbon monoxide, carbon dioxide and methane, wherein the heat necessary for splitting, which is indirectly transmitted to the feed gas mixture, is produced by combustion of the heating gas mixture, wherein in normal operation of the process a base H2/CO ratio VB is obtained,
c) separating the carbon dioxide from the raw synthesis gas,
d) fractionating the raw synthesis gas into a gas rich in hydrogen, a gas rich in carbon monoxide and a gas rich in methane, wherein the gas rich in hydrogen optionally is supplied to a further cleaning step and otherwise is discharged from the process as H2 product gas, and the gas rich in carbon monoxide is supplied to the further use outside the process as CO product gas,
e) optionally cleaning of the gas rich in hydrogen by separating gaseous impurities and admixing the same to the heating gas mixture and discharging the cleaned gas rich in hydrogen as H2 product gas for the further use outside the process,
characterized in that
f) at least a part of the gas rich in methane obtained according to step d) is admixed to the feed gas mixture, and that
g) at least a part of the CO product gas and/or a fraction of the gas rich in carbon monoxide increased with respect to the normal operation of the process is admixed to the heating gas mixture, so that the H2/CO ratio V1 is increased with respect to the base H2/CO ratio VB to a target value V1 > VB. Plant according to the invention:
A plant for the continuous conversion of a hydrocarbonaceous feed gas into a synthesis gas comprising carbon monoxide and hydrogen, comprising the following means and plant sections:
a) means for supplying a feed gas mixture which comprises hydrocarbonaceous gas and steam, as well as a heating gas mixture which comprises a fuel gas and a gas rich in oxygen, from external sources to the plant, as well as means for the transfer of the gas or gases between the plant sections and out of the plant for their further use,
b) plant section for the steam reformation of the feed gas mixture to a raw synthesis gas, with a tubular reformer fired by burners, comprising:
a housing,
catalyst tubes which are arranged in the housing and are filled with a catalyst bed and through which the feed gas mixture is passed and is reformed into raw synthesis gas containing hydrogen, carbon monoxide, carbon dioxide and methane,
burners which are operated with the heating gas mixture and which are installed in the housing such that their flames reach into the housing and thereby produce the heat necessary for the reformation and transmit the same to the catalyst tubes and hence indirectly to the feed gas mixture,
c) plant section for separating the carbon dioxide from the raw synthesis gas, d) plant section for fractionating the raw synthesis gas into a gas rich in hydrogen, which is supplied either to the treatment in step e) or to the further use outside the plant, furthermore into a gas rich in carbon monoxide which is supplied to the further use outside the plant, and into a gas rich in methane which is admixed to the feed gas mixture and/or to the heating gas mixture,
e) optionally a plant section for cleaning of the gas rich in hydrogen by separating gaseous impurities, means for admixing the same to the heating gas mixture, and means for discharging the cleaned gas rich in hydrogen as H2 product gas for the further use outside the process,
characterized by
f) means for admixing at least a part of the gas rich in methane obtained in plant section d) to the feed gas mixture, and
g) means for admixing at least a part of the H2 product gas and/or the gas rich in hydrogen to the heating gas mixture or means for admixing at least a part of the CO product gas to the heating gas mixture,
h) means for controlling the mass flows of the gases to be admixed to the feed gas mixture and to the heating gas mixture.
Normal operation is understood to be a mode of operation of the reformer plant known from the prior art, in which neither gas rich in methane is recirculated to the feed gas mixture nor H2 product gas, gas rich in hydrogen or CO product gas is recirculated to the heating gas mixture.
Reforming conditions are understood to be the operating conditions of the reformer plant well known to the skilled person, which ensure a technically and economically expedient degree of conversion of the feedstocks to synthesis gas components. A set of operating conditions chosen therefor also is referred to as operating point. In one aspect of the invention at least a part of the gas rich in methane obtained according to step d) is admixed to the feed gas mixture. In an alternative aspect either
- at least a part of the H2 product gas and/or a fraction of the gas rich in hydrogen increased with respect to the normal operation of the process can be admixed to the heating gas mixture, so that the H2/CO ratio V1 is lowered with respect to the base H2/CO ratio VB to a target value V1 < VB, or alternatively
- at least a part of the CO product gas and/or a fraction of the gas rich in carbon monoxide increased with respect to the normal operation of the process can be admixed to the heating gas mixture, so that the H2/CO ratio V1 is increased with respect to the base H2/CO ratio VB to a target value V1 > VB.
It here is important that the fraction of gas rich in methane, which according to the invention as compared to the normal operation of the process is not admixed to the heating gas mixture, but to the feed gas mixture, is replaced by calorically equivalent fractions of H2 product gas, gas rich in hydrogen or CO product gas, i.e. fractions equivalent with regard to their calorific value, in order to ensure a stable operation of the reformer plant. Preferred Aspects of the Invention
A preferred aspect of the invention is characterized in that the fraction of gas rich in methane, which is admixed to the feed gas mixture, is at least 50 %, preferably at least 70 %, most preferably at least 90 % of the gas rich in methane obtained according to step d).
It furthermore is favorable when at least 20 %, preferably at least 30 %, most preferably at least 40 % of the H2 product gas or the gas rich in hydrogen are admixed to the heating gas mixture or, in an alternative aspect, at least 20 %, preferably at least 30 %, most preferably at least 40 % of the CO product gas are admixed to the heating gas mixture. It has been found that within the aforementioned ranges taking account of the caloric equivalence of the gas streams admixed to the feed gas mixture and to the heating gas mixture the H2/CO ratio can be set within wide limits, while at the same time a stable and trouble-free operation of the reformer plant becomes possible with a proper heat balance.
Another preferred aspect of the invention is characterized in that the process is carried out with the following temporally successive steps:
Step 1 : The H2 product gas or the gas rich in hydrogen or the CO product gas is completely admixed to the heating gas mixture.
Step 2: The H2 product gas or the gas rich in hydrogen or the CO product gas for one part is admixed to the heating gas mixture and for the other part is discharged for the further use outside the process, wherein the discharged fraction is increased or reduced continuously or step by step, until the target value V1 of the H2/CO ratio is achieved.
This procedure provides for realizing the change of the quantity of CO and/or H2 delivered to the external consumer more quickly than is possible in a conventional way.
Preferably, the carbon dioxide separated from the raw synthesis gas is admixed to the feed gas mixture at least in part. In this way, the separated carbon dioxide can be utilized materially at least in part. Furthermore, it thus is possible to further influence the H2/CO ratio.
The H2/CO ratio can be influenced to an even greater extent by admixing carbon dioxide produced outside the process to the feed gas mixture. Advantageously, a sink for carbon dioxide present in excess or undesired, foreign to the process thus is created at the same time.
In a preferred aspect of the process according to the invention the separation of the raw synthesis gas into a gas rich in hydrogen, a gas rich in carbon monoxide and a gas rich in methane is carried out by means of a cryogenic distillation process, wherein a further gas stream containing combustible gas components (coldbox flashgas) is obtained, which is at least partly admixed to the heating gas mixture and/or the feed gas mixture. This provides for the thermal and/or material utilization of the coldbox flashgas. This procedure and such plants are proven in practical operation and in the current language are referred to as Coldbox (CB).
When carrying out the process according to the invention it furthermore is preferred that cleaning of the gas rich in hydrogen is carried out by separation of gaseous impurities by means of a pressure swing adsorption process (PSA), wherein the gaseous impurities are obtained as separate gas stream (PSA offgas) and are at least partly admixed to the heating gas mixture. This provides for the thermal utilization of the PSA offgas. Another preferred aspect of the invention is characterized in that separating the carbon dioxide from the raw synthesis gas by means of an absorption process, carried out in a corresponding absorption plant, is effected by using e.g. methyldiethanolamine (MDEA) as absorbent. This process or such plant is described e.g. in Ullmann's Encyclopedia of Industrial Chemistry, Sixth Edition, Vol. 15, page 410. These are long-established processes or plants. It is advantageous that the carbon dioxide can efficiently be separated from the raw synthesis gas and can be used further, for example for the recirculation to the feed gas mixture. In a preferred aspect of the plant according to the invention the same also comprises means for the recirculation of the carbon dioxide separated from the raw synthesis gas to the feed gas mixture and/or means for supplying carbon dioxide produced outside the process to the feed gas mixture. In a further preferred aspect of the plant according to the invention the plant section for separating the raw synthesis gas into a gas rich in hydrogen, a gas rich in carbon monoxide and a gas rich in methane is designed as cryogenic distillation process, wherein there are furthermore comprised means for discharging a further gas stream containing combustible gas components (coldbox flashgas). In a further preferred aspect of the plant according to the invention the plant section for cleaning the gas rich in hydrogen by separation of gaseous impurities is designed as pressure swing adsorption process (PSA), wherein there are furthermore comprised means for discharging a further gas stream containing gaseous impurities (PSA offgas) and for at least partly admixing the same to the heating gas mixure.
Exemplary embodiments and numerical examples
Further features, advantages and possible applications of the invention can also be taken from the following description of exemplary embodiments and numerical examples as well as the drawings. All features described and/or illustrated form the subject-matter of the invention per se or in any combination, independent of their inclusion in the claims or their back-reference.
The invention will now be explained with reference to the drawings, in which:
Fig. 1 shows a block diagram of a process according to the prior art,
Fig. 2 shows a block diagram of an alternative process according to the prior art with carbon dioxide recirculation and supply of external carbon dioxide,
Fig. 3 shows a block diagram of a first exemplary aspect of the invention, based on Fig. 2,
Fig. 4 shows a block diagram of a second exemplary aspect of the invention, based on Fig. 3, with additional CO shift stage. Fig. 1 , comparative example according to the prior art.
The feed gas mixture 1 , comprising hydrocarbonaceous gas and steam, as well as the heating gas mixture 2, comprising a fuel gas and a gas rich in oxygen, are charged to a reformer 3. In the reformer 3 the feed gas mixture 1 is converted into a raw synthesis gas 4, comprising hydrogen, carbon monoxide, carbon dioxide and methane as well as water, possibly nitrogen and impurities known to the skilled person. The raw synthesis gas 4 is transferred from the reformer 3 into a plant 5 for separating the carbon dioxide 6 from the raw synthesis gas 4. This separation in plant 5 can be effected for example by means of an absorption process by using methyldiethanolamine (MDEA). The separated carbon dioxide 6 is discharged from the plant 5 and supplied to a further use or discharged into the environment. The raw synthesis gas 7 liberated from carbon dioxide is passed from plant 5 into a plant 8 for fractionating the raw synthesis gas into a gas 9 rich in hydrogen, a gas 10 rich in carbon monoxide, and a gas 1 1 rich in methane. This fractionation in plant 8 can be effected for example by means of a cryogenic distillation process. In this example, the gas 1 1 rich in methane is added to the heating gas mixture 2. The gas 10 rich in carbon monoxide is discharged for the further use as product gas and the gas 9 rich in hydrogen in this example is supplied to a plant 12 for the separation of gaseous impurities. For carrying out this separation a pressure swing adsorption process can be employed. The gaseous impurities separated in this process are admixed to the heating gas mixture 2. The cleaned gas 14 rich in hydrogen is discharged from the plant for the further use as H2 product gas.
When using a cryogenic distillation plant for the separation of the raw synthesis gas 7 liberated from carbon dioxides, a high purity of the gas 9 rich in hydrogen is achieved already, which typically lies in the range from 95 % to 99 %, based on the molar fraction of hydrogen. Depending on the required purity of the H2 product stream a further purification of this stream normally is necessary, in order to achieve a hydrogen purity in the range from 99.9 % to 99.999 %, based on the molar fraction of hydrogen. In rare cases, this purification is not necessary. Fig. 2, comparative example according to the prior art.
In this example, the carbon dioxide 6 separated from the raw synthesis gas 7 in plant 5 by means of an absorption process is added to the feed gases 1 , in addition with externally obtained carbon dioxide 15.
Fig. 3, exemplary embodiment of the invention
This example shows a plant according to Fig. 2, in which a part of the product gas 1 6 rich in carbon monoxide and/or of the gas 18 rich in hydrogen or of the H2 product gas 17 is recirculated in addition and admixed to the heating gas mixture 2. Furthermore, a part of the gas 1 1 a rich in methane is recirculated from the cryogenic distillation plant (coldbox) 8 into the feed gas mixture. The remaining fraction of gas 1 1 rich in methane is admixed to the heating gas mixture.
Since the gas stream 18 rich in hydrogen provided from the cryogenic distillation plant already has a very high hydrogen content, the same can also be admixed to the heating gas mixture in accordance with the invention, in order to lower the H2/CO ratio. Hence, it is not absolutely necessary to admix a partial stream of the purified H2 product stream to the heating gas mixture. Fig. 4, exemplary embodiment of the invention
This example shows a plant according to Fig. 3, but without the recirculation of C02 separated from the raw synthesis gas or imported from external sources. Furthermore, this example contains a CO shift stage 20 which is charged with a partial stream of the raw synthesis gas 19 and from which a gas stream 21 enriched in hydrogen is discharged and together with the gas stream 9 rich in hydrogen from plant 8 is supplied to the pressure swing adsorption plant 12.
Numerical examples of the prior art (comparative examples), Tables 1 to 3
A possibility known to the skilled person for setting the H2/CO molar ratio consists in setting the carbon dioxide content in the feed gas, as it is illustrated in the following Tables 1 to 3. Without addition of carbon dioxide into the feed gas the H2/CO molar ratio in the synthesis gas can be set in the range from 3.8 to 5.4 or as ratio of the H2 and CO product streams in the range from 3.3 to 4.7 by selection of the operating conditions (see Table 1 ). Operating conditions which influence the H2/CO molar ratio above all include the reformer temperature and the ratio of steam to carbon (S/C ratio) in the feed gas and to a smaller extent also the reformer pressure and the efficiencies of the CO and H2 purification devices.
Table 1 : Comparison of the H2/CO ratios for the product and synthesis gas streams for various operating conditions (reformer temperature, S/C ratio) at a reformer pressure of 25 bara, for a feed of 100% CH4, with a CO and H2 efficiency of 97% for the cryogenic distillation plant and a H2 efficiency of 87% for the pressure swing adsorption process
Figure imgf000013_0001
With partial to complete recirculation of the carbon dioxide separated from the synthesis gas into the feed gas ratios of 2.6 to 3.3 can be set, based on the product streams (see Table 2), or of 3.8 to 3.0 in the synthesis gas. The ratio can be lowered to the range of 1 .4 to 2.6, based on the product streams (see Table 3), or of 1 .6 to 3.0 in the synthesis gas, when in addition to the complete carbon dioxide recirculation carbon dioxide originating from an external source is admixed to the feed gas, wherein the quantity of the carbon dioxide admixed to the feed gas, originating from an external source, decisively influences the ratio. Table 2: Comparison of the H2/CO ratios for the product and synthesis gas streams for the same operating conditions as for Table 1 (derived cases A, D, F, H) and with an additional C02 admixture of 95% of the carbon dioxide separated from the raw synthesis gas to the feed gas mixture
Case A-2 D-2 F-2 H-2
S/C ratio 2.5 2.0 2.0 1.8
Reformer temperature (°C) 850 880 920 940
CO product stream (Nm3/h) 10000 10000 10000 10000
H2 product stream (Nm3/h) 27380 26920 26715 26570
CH4 feed stream (Nm3/h) 14670 14405 12910 12735
H2/CO in the synthesis gas 3.19 3.14 3.1 1 3.09
C02 content in the 7280 4740 3570 2760 synthesis gas (Nm3/h)
CH4 content in the 3990 3860 2420 2290 synthesis gas (Nm3/h) Table 3: Comparison of the H2/CO ratios for the product and synthesis gas streams for the same operating conditions as for Tables 1 and 2 (derived case F-2 from Table 2) with additional admixture of C02 originating from an external source to the feed gas mixture
Figure imgf000015_0001
Plants for the production of CO and H2 are designed such that a specified H2/CO molar ratio of the H2 and CO product streams is achieved in that the operating conditions are adapted and for example a partial or complete recirculation of the carbon dioxide separated from the synthesis gas is carried out. When the H2/CO molar ratio achieved is too high even with complete recirculation of CO2, an admixture of externally obtained CO2 to the feed gas mixture can be taken into consideration. A recirculation of the carbon dioxide separated from the synthesis gas also can possibly be omitted, when externally obtained CO2 e.g. is present already in a sufficient quantity at feed pressure. The external carbon dioxide should be present in sufficient purity, wherein impurities with carbon, nitrogen and hydrogen compounds possibly are acceptable, and should not contain any compounds which are harmful for the catalysts of the steam reformation plant (such as e.g. arsenic, sulfur, chlorine, metals). To keep the plant costs low, only small adaptations of the feed parameters chosen for the plant design can be made during operation of the plant, which then also allow only small changes of the H2/CO product ratio as compared to the original design. Deviations of maximally 10 % of the H2/CO molar ratio then are possible. In the real plant operation, however, load cases sometimes occur, in which the H2/CO product ratio differs distinctly from the possible changes. Since the fluctuations of such H2/CO product ratios in part are very large, but normally only last for a short time, these cases normally are not taken into account in the plant design and covered by flaring of the excess product stream, which can lead to high costs for extended periods.
Setting H2/CO molar ratios outside these ranges would lead to an uneconomic design of individual plant components or a plant design intended for normal operation is limited with regard to the temperature levels to be set by the loadability of the construction materials of the plant components, or over- dimensioning of plant components would have to be effected, whose economy is not given in normal operation.
Possibly, additional plant components also would have to be used; for example, a partial stream of the raw synthesis gas might be supplied to a CO shift stage, in order to increase the H2/CO ratio or provide a plant with a high flexibility with regard to the H2/CO ratio.
The H2/CO ratio of the products as specified during the plant design substantially is constant at various load conditions of the plant. A plant for the production of H2 and CO normally can be run down to a plant load of approximately 20 % to 40 % of the design load. In operation of a plant under partial load a higher flexibility of the H2/CO product ratio is given. At a plant load of approximately 50 % deviations of about 20 % of the H2/CO molar ratio are possible. Numerical examples of the invention, Tables 4 to 7
One aspect of the invention is characterized in that at least a part of the gas rich in methane obtained according to step d) of claim 1 is admixed to the feed gas mixture. By this measure the consumption of external hydrocarbonaceous feed gas is reduced and at the same time the admixture of CO product gas and/or of H2 product gas to the heating gas mixture becomes possible, as it is explained in Tables 4, 5, 6 and 7.
The recirculation of the gas rich in methane to the feed gas mixture can be effected in conjunction with the recirculation of the carbon dioxide separated from the raw synthesis gas or independent thereof. In these recirculations measures of pressure increase generally are provided, such as e.g. compressors. The carbon dioxide separated from the raw synthesis gas normally is obtained at ambient pressure or at a pressure of up to about 3 bar. The stream rich in methane can be discharged from the cryogenic distillation process with a pressure comparable to the carbon dioxide, but possibly also under pressure. In the latter case, it can then be introduced directly into the feed gas or it requires an only small pressure increase as compared to the carbon dioxide, which can also be executed together with the carbon dioxide, in e.g. one or more common last pressure boosting stages of a carbon dioxide compressor.
The carbon dioxide preferably is admixed to the feed gas after a possible pre reformat! on unit (prereforming), while the admixture of the methane-containing gas can be chosen freely. Since the methane-containing gas contains no sulfur, if it is permanently admixed to the feed gas, the desulfurization unit of the steam reforming plant can be reduced in size correspondingly. If the methane-containing gas is admixed to the feed gas with the carbon dioxide after a possible prereformer, steam must also be added to the feed gas mixture in this region of the plant, in order to establish a required S/C ratio in the feed gas mixture.
As shown in Table 4, the quantity of waste gases (offgases) from the cryogenic distillation process for the CO purification (coldbox, CB) corresponds to approximately 25 % to 50 % of the total quantity of heating gas. These off gases comprise a methane-containing gas (CH4) and a flashgas (FG) rich in H2. In plants with a nitrogen removal in the cryogenic distillation plant there is also obtained a nitrogen-containing offgas which normally, however, has no significant share in the energy content of the heating gas mixture. The offgas of the pressure swing adsorption (PSA) has a share of about 20 % to 35 % in the total heating gas. The air preheating temperature above all influences the steam export quantity.
By recirculating the gas rich in methane from the cryogenic distillation plant to the feed gas, and by also reducing the external heating gas quantity as required, it is possible to admix a large amount of gas rich in hydrogen to the heating gas mixture and thus reduce the H2/CO molar ratio of the H2 and CO product streams to a high extent, without the feed parameters being changed or plant sections being overdimensioned. This is shown in Tables 5, 6 and 7.
Table 4: Heating gas distribution of synthesis gas plants (derived cases F from Tables 1 to 3) for an air preheating temperature of 340 °C and a steam export at 30 barg and 300 °C.
Case F F2 F2-I F2-II F2-III
Air preheating temperature °C 340 340 340 340 340
Steam export quantity t/h 14.2 10.8 9.4 7.7 3.5
Total heating gas quantity Gcal/h 58.9 48.39 44.74 41.14 34.22
• PSA residual gas content % 33.1 29.52 27.93 25.95 20.82
• CB CH4 content /o 41 .8 35.00 31.64 27.44 17.00
• CB FG content /o 7.0 6.32 6.02 5.65 4.69
• External heating gas o //o 18.1 29.17 34.41 40.96 57.50 content Table 5: Heating gas distribution of synthesis gas plants (derived case F of Table 1 ) for an air preheating temperature of 450 °C and a steam export at 30 barg and 300 °C - Influence of the admixture of gas rich in hydrogen to the heating gas mixture on the H2/CO product ratio for plants without carbon dioxide recirculation and without use of externally obtained carbon dioxide
F with CH4 recirculation
F into the feed with CH4 gas and gas recirculation rich in into the feed hydrogen into
Case F gas heating gas
Air preheating temperature °C 450 450 450
Feed gas stream Nm3/h 16850 14266 14266
H2 product stream Nm3/h 36250 36250 26250
CO product stream Nm3/h 10000 10000 10000
Steam export quantity t/h 8.9 9.5 8.1
Total heating gas quantity Gcal/h 55.26 55.58 54.59
• PSA residual gas % 35.28 35.08 35.72 content
• CB CH4 content /o 44.57 4.42 4.50
• CB FG content /o 7.45 7.41 7.54
• External heating gas /o 12.69 53.09 5.01 content
• Gas rich in hydrogen o //o 0.00 0.00 47.23
Table 6: Heating gas distribution of synthesis gas plants (derived case F2 of Table 2) for an air preheating temperature of 340 °C and a steam export at 30 barg and 300 °C - Influence of the admixture of gas rich in hydrogen to the heating gas mixture on the H2/CO product ratio for plants with recirculation of the carbon dioxide separated from the synthesis gas into the feed gas and without use of externally obtained carbon dioxide
F2
with CH4 recirculation
F2 into the feed with CH4 gas and gas recirculation rich in into the feed hydrogen into
Case F2 gas heating gas
Air preheating temperature °C 340 340 340
Feed gas stream Nm3/h 12910 1 1 136 1 1 136
H2 product stream Nm3/h 26715 26715 16715
CO product stream Nm3/h 10000 10000 10000
Steam export quantity t/h 10.8 1 1 .2 9.3
Total heating gas quantity Gcal/h 48.39 48.55 47.25
• PSA residual gas % 29.52 29.42 30.23 content
• CB CH4 content /o 35.00 3.49 3.49
• CB FG content /o 6.32 6.30 6.30
• External heating gas /o 29.17 60.79 5.02 content
• Gas rich in hydrogen o //o 0.00 0.00 53.10 Table 7: Heating gas distribution of synthesis gas plants (derived case F2-II of Table 3) for an air preheating temperature of 340 °C and a steam export at 30 barg and 300 °C - Influence of the admixture of gas rich in hydrogen to the heating gas mixture on the H2/CO product ratio for plants with recirculation of the carbon dioxide separated from the synthesis gas into the feed gas and with use of
2000 Nm3/h of externally obtained carbon dioxide
Figure imgf000021_0001
Tables 5, 6 and 7 are based on recirculation rates of 90 % of the methane- containing stream separated by cryogenic distillation into the feed gas mixture. This allows to keep the methane-containing stream recirculated into the feed gas constant independent of fluctuations. For compensation, the external heating gas quantity is varied. The same is preset to about 5 %, which allows to compensate fluctuations in parts of the heating gas quantity and at the same time in the case of the loss of a heating gas quantity (e.g. by sudden shut-down of the pressure swing plant due to a valve malfunction) allows to compensate the required amount of heating gas by rapid increase of the external heating gas quantity, so that the steam reforming plant can be operated further.
Except for the reduction of the H2/CO product ratio by feeding hydrogen- containing gas into the heating gas mixture, this ratio can also be increased temporarily by feeding CO product gas into the heating gas mixture. This is the case in particular when e.g. in a CO pipeline system (CO complex) a CO consumer suddenly fails and the combustion of excess CO in a flare system should then be avoided, until the steam reforming plant has been run down to the current need of CO or until the failed CO consumer again withdraws CO from the CO complex. During scheduled maintenance work on a CO consumer, the discharged amount of CO also can be reduced without having to change the operating point of the synthesis gas plant. The description of the above cases, which lead to the feeding of CO product gas into the heating gas mixture and hence to an increase of the H2/CO product ratio, also are possible for the feeding of gas rich in hydrogen or H2 product gas, so that for example upon loss of a H2 consumer excess H2 is passed to the heating gas mixture.
The temporary feeding of CO or H2 into the heating gas mixture also is advantageous when the run-up rate (from a defined plant operating point, which at least corresponds to the minimum operating point of the plant - which normally corresponds to between 20 % and 40 % of the design quantity of a product stream - to another plant operating point with higher product stream) of a CO or H2 consumer is higher than the run-up rate of the entire steam reforming plant including the CO and/or H2 purification plant. Typical values lie in the range from 0.1 %/min to maximally 2 %/min or preferably in the range from 0.2 %/min to 1 %/min, based on the design point of a product stream. A design for 10000 Nm3/h of CO product stream for example corresponds to a run-up rate of 1 %/min of a load increase of 100 Nm3 of CO per minute. In this case, a part of the product gas can be admixed to the heating gas mixture. The same problem does exist when, for example due to the loss of a consumer, the quantity of the product streams must be reduced.
Industrial Applicability
The invention provides a process and a plant in which the H2/CO molar ratio can be changed beyond the limits known from the prior art or beyond the plant design. The production hence can even further be adapted to the demand of the gas consumers. Furthermore, temporally fluctuating operating conditions within the plant complex or fluctuating demands of the consumers withdrawing H2 or CO within this complex, can be satisfied without having to leave the previously fixed operating point of the reformer plant.
List of Reference Numerals
1 feed gas mixture
2 heating gas mixture
3 reformer
4 raw synthesis gas
5 plant for separating carbon dioxide
6 carbon dioxide
7 raw synthesis gas, free from carbon dioxide
8 plant for fractionating raw synthesis gas
9 gas rich in hydrogen
10 gas rich in carbon monoxide (CO product gas)
1 1 gas rich in methane to the heating gas mixture
1 1 a gas rich in methane to the feed gas mixture
12 pressure swing adsorption plant
13 gaseous impurities
14 gas rich in hydrogen, purified (H2 product gas)
15 externally obtained carbon dioxide
16 gas rich in carbon monoxide to the heating gas mixture
17 gas rich in hydrogen after purification
18 gas rich in hydrogen before purification
19 raw synthesis gas to the CO shift stage
20 CO shift stage
21 shifted synthesis gas for hydrogen purification

Claims

Claims:
1 . A process for the continuous conversion of a hydrocarbonaceous feed gas into a synthesis gas comprising carbon monoxide and hydrogen, comprising the following process steps carried out one after the other:
a) providing a feed gas mixture which comprises hydrocarbonaceous gas and steam, as well as a heating gas mixture which comprises a fuel gas and a gas rich in oxygen,
b) splitting the feed gas mixture under reforming conditions by catalytic steam reformation in a tubular reformer fired by burners into a raw synthesis gas containing hydrogen, carbon monoxide, carbon dioxide and methane, wherein the heat necessary for splitting, which is indirectly transmitted to the feed gas mixture, is produced by combustion of the heating gas mixture, wherein in normal operation of the process a base H2/CO ratio VB is obtained,
c) separating the carbon dioxide from the raw synthesis gas,
d) fractionating the raw synthesis gas into a gas rich in hydrogen, a gas rich in carbon monoxide and a gas rich in methane, wherein the gas rich in hydrogen optionally is supplied to a further cleaning step and otherwise is discharged from the process as H2 product gas, and the gas rich in carbon monoxide is supplied to the further use outside the process as CO product gas,
e) optionally cleaning of the gas rich in hydrogen by separating gaseous impurities and admixing the same to the heating gas mixture and discharging the cleaned gas rich in hydrogen as H2 product gas for the further use outside the process,
characterized in that
f) at least a part of the gas rich in methane obtained according to step d) is admixed to the feed gas mixture, and that
g) at least a part of the H2 product gas and/or a fraction of the gas rich in hydrogen increased with respect to the normal operation is admixed to the heating gas mixture, so that the H2/CO ratio V1 is lowered with respect to the base H2/CO ratio VB to a target value V1 < VB.
2. A process for the continuous conversion of a hydrocarbonaceous feed gas into a synthesis gas comprising carbon monoxide and hydrogen, comprising the following process steps carried out one after the other:
a) providing a feed gas mixture which comprises hydrocarbonaceous gas and steam, as well as a heating gas mixture which comprises a fuel gas and a gas rich in oxygen,
b) splitting the feed gas mixture under reforming conditions by catalytic steam reformation in a tubular reformer fired by burners into a raw synthesis gas containing hydrogen, carbon monoxide, carbon dioxide and methane, wherein the heat necessary for splitting, which is indirectly transmitted to the feed gas mixture, is produced by combustion of the heating gas mixture, wherein in normal operation of the process a base H2/CO ratio VB is obtained,
c) separating the carbon dioxide from the raw synthesis gas,
d) fractionating the raw synthesis gas into a gas rich in hydrogen, a gas rich in carbon monoxide and a gas rich in methane, wherein the gas rich in hydrogen optionally is supplied to a further cleaning step and otherwise is discharged from the process as H2 product gas, and the gas rich in carbon monoxide is supplied to the further use outside the process as CO product gas,
e) optionally cleaning of the gas rich in hydrogen by separating gaseous impurities and admixing the same to the heating gas mixture and discharging the cleaned gas rich in hydrogen as H2 product gas for the further use outside the process,
characterized in that
f) at least a part of the gas rich in methane obtained according to step d) is admixed to the feed gas mixture, and that
g) at least a part of the CO product gas and/or a fraction of the gas rich in carbon monoxide increased with respect to the normal operation of the process is admixed to the heating gas mixture, so that the H2/CO ratio V1 is increased with respect to the base H2/CO ratio VB to a target value V1 > VB.
3. The process according to claim 1 or 2, characterized in that the fraction of gas rich in methane, which is admixed to the feed gas mixture, is at least 50 %, preferably at least 70 %, most preferably at least 90 % of the gas rich in methane obtained according to step d).
4. The process according to claim 1 or 3, characterized in that at least 20 %, preferably at least 30 %, most preferably at least 40 % of the H2 product gas or the gas rich in hydrogen are admixed to the heating gas mixture.
5. The process according to claim 2 or 3, characterized in that at least 20 %, preferably at least 30 %, most preferably at least 40 % of the CO product gas are admixed to the heating gas mixture.
6. The process according to any of the previous claims, characterized in that it is carried out with the following temporally successive steps:
Step 1 : The H2 product gas or the gas rich in hydrogen or the CO product gas is completely admixed to the heating gas mixture.
Step 2: The H2 product gas or the gas rich in hydrogen or the CO product gas for one part is admixed to the heating gas mixture and for the other part is discharged for the further use outside the process, wherein the discharged fraction is increased or reduced continuously or step by step, until the target value V1 of the H2/CO ratio is achieved.
7. The process according to any of the previous claims, characterized in that the carbon dioxide separated from the raw synthesis gas is admixed to the feed gas mixture at least in part.
8. The process according to any of the previous claims, characterized in that carbon dioxide produced outside the process is admixed to the feed gas mixture.
9. The process according to any of the preceding claims, characterized in that the separation of the raw synthesis gas into a gas rich in hydrogen, a gas rich in carbon monoxide and a gas rich in methane is carried out by means of a cryogenic distillation process, wherein a further gas stream containing combustible gas components (coldbox flashgas) is obtained, which is at least partly admixed to the heating gas mixture and/or the feed gas mixture.
10. The process according to any of the preceding claims, characterized in that the purification of the gas rich in hydrogen is carried out by separation of gaseous impurities by means of a pressure swing adsorption process (PSA), wherein the gaseous impurities (PSA offgas) are at least partly admixed to the heating gas mixture.
1 1 . A plant for the continuous conversion of a hydrocarbonaceous feed gas into a synthesis gas comprising carbon monoxide and hydrogen, comprising the following means and plant sections:
a) means for supplying a feed gas mixture which comprises hydrocarbonaceous gas and steam, as well as a heating gas mixture which comprises a fuel gas and a gas rich in oxygen, from external sources to the plant, as well as means for the transfer of the gas or gases between the plant sections and out of the plant for their further use,
b) plant section for the steam reformation of the feed gas mixture to a raw synthesis gas, with a tubular reformer fired by burners, comprising:
a housing,
catalyst tubes which are arranged in the housing and are filled with a catalyst bed and through which the feed gas mixture is passed and is reformed into raw synthesis gas containing hydrogen, carbon monoxide, carbon dioxide and methane,
burners which are operated with the heating gas mixture and which are installed in the housing such that their flames reach into the housing and thereby produce the heat necessary for the reformation and transmit the same to the catalyst tubes and hence indirectly to the feed gas mixture,
c) plant section for separating the carbon dioxide from the raw synthesis gas, d) plant section for separating the raw synthesis gas into a gas rich in hydrogen, which is supplied either to the treatment in step e) or to the further use outside the plant, furthermore into a gas rich in carbon monoxide which is supplied to the further use outside the plant, and into a gas rich in methane which is admixed to the feed gas mixture and/or to the heating gas mixture,
e) optionally a plant section for cleaning of gas rich in hydrogen by separating gaseous impurities, means for admixing the same to the heating gas mixture, and means for discharging the cleaned gas rich in hydrogen as H2 product gas for the further use outside the process,
characterized by
f) means for admixing at least a part of the gas rich in methane obtained in plant section d) to the feed gas mixture, and
g) means for admixing at least a part of the H2 product gas and/or the gas rich in hydrogen to the heating gas mixture or means for admixing at least a part of the CO product gas to the heating gas mixture,
h) means for controlling the mass flows of the gases to be admixed to the feed gas mixture and to the heating gas mixture.
12. The plant according to claim 1 1 , characterized by means for the recirculation of the carbon dioxide separated from the raw synthesis gas in plant section c) to the feed gas mixture.
13. The plant according to claim 1 1 or 12, characterized by means for supplying carbon dioxide produced outside the process to the feed gas mixture.
14. The plant according to any of the previous claims, characterized in that the plant section d) for separating the raw synthesis gas into a gas rich in hydrogen, a gas rich in carbon monoxide and a gas rich in methane is designed as cryogenic distillation process, wherein there are furthermore comprised means for discharging a further gas stream containing combustible gas components (coldbox flashgas).
15. The plant according to any of the previous claims, characterized in that the plant section e) for the purification of the gas rich in hydrogen by separation of gaseous impurities is designed as pressure swing adsorption process (PSA), wherein there are furthermore comprised means for discharging a further gas stream containing gaseous impurities (PSA offgas) and for at least partly admixing the same to the heating gas mixure.
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