Biogas conversion process

Abstract

The present invention relates to a biogas conversion process.

Description

[0001] BIOGAS CONVERSION PROCESS

[0002] Dimethyl ether (DME, also known as methoxymethane or methyl oxide) is an ether which, due to its chemical / physical characteristics, is similar to liquefied petroleum gas. Due to these characteristics, DME is widely used worldwide, especially in industrial and consumption applications. This is a gas that is easy to handle and store in liquid form; it has the further environmental advantage lying in the fact that any accidental leakage of DME does not contribute to the feared greenhouse effect because the gas degrades upon contact with atmospheric oxygen.

[0003] DME has various uses, including:

[0004] - mixing with propane: in current heating, cooking and industrial applications not connected to the network;

[0005] - use as 100% renewable fuel in industrial applications, especially for the high temperature heating, which is difficult to electrify;

[0006] - use as diesel additive in engines, but also as substitute only requiring an adaptation of the vehicle.

[0007] In an energy transition context, DME from renewable carbon can be produced from biogenic and non-biogenic raw materials, such as urban waste, cellulose and biogas materials.

[0008] Biogas is a mixture of gases, mainly methane and carbon dioxide, produced from anaerobic bacterial fermentation of plant or animal organic residues. The biogas industry has strengthened over the last 20 years, with particular reference to European countries. The produced biogas may be exploited in various ways; including, also the conversion of biogas for the production of high added value chemical compounds, such as dimethyl ether (DME).

[0009] The current plants for producing DME from biogas reveal some drawbacks, such us non-optimised yield and selectivity, and difficult DME purification step.

[0010] Advantageously and unexpectedly, the present invention instead allows to convert biogas streams into dimethyl ether (DME) with maximum yield and selectivity in product and with high process simplification of the purification sections. Furthermore, the present invention advantageously leads to a full removal of C02 (compound that is known to be difficult to separate from DME) downstream of the methanol synthesis section, therefore allowing to easily purify the DME obtained at the end of the process.

[0011] Below is the detailed description of the present invention.

[0012] DESCRIPTION OF THE FIGURES

[0013] Figure 1: Block Flow Diagram (BFD) of a plant for producing DME from biogas: 1: gas;

[0014] 2: compression and tri-reforming;

[0015] 3: Syngas;

[0016] 4: HEN and dehydration;

[0017] 5: water;

[0018] 6: compression and synthesis;

[0019] 7: separation 1;

[0020] 8: syngas;

[0021] 9: hydrogen recovery;

[0022] 10: purging - to the tri-reforming firebox;

[0023] 11: H2;

[0024] 12: full removal of CO2;

[0025] 13: methanol and water;

[0026] 14: DME synthesis;

[0027] 15: separation 2;

[0028] 16: DME;

[0029] 17: methanol and water;

[0030] Figure 2: Block flow diagram (BFD) relating to Example 1: 18: gas;

[0031] 19: compression and tri-reforming;

[0032] 20: syngas; 21: HEN and dehydration;

[0033] 22: water;

[0034] 23: compression and synthesis;

[0035] 24 separation 1;

[0036] 25: syngas;

[0037] 26: hydrogen recovery;

[0038] 27: purging (to the tri-reforming firebox);

[0039] 28: H2;

[0040] 29: electrolyzer;

[0041] 30: 02;

[0042] 31: methanol and water;

[0043] 32: full removal of C02;

[0044] 33: DME synthesis;

[0045] 34: Separation 2;

[0046] 35: DME;

[0047] 36: methanol and water.

[0048] Figure 3: Block flow diagram (BFD) relating to Example 2: 37: gas;

[0049] 38: compression and tri-reforming;

[0050] 39: syngas;

[0051] 40: HEN and dehydration;

[0052] 41: water;

[0053] 42: compression and synthesis;

[0054] 43: separation 1;

[0055] 44: syngas;

[0056] 45: hydrogen recovery;

[0057] 46: purging;

[0058] 47: H2;

[0059] 48: electrolyzer;

[0060] 49: 02 to tri-reforming;

[0061] 50: methanol and water;

[0062] 51: full removal of C02;

[0063] 52: DME synthesis;

[0064] 53: separation 2;

[0065] 54: DME; 55: Methanol and water.

[0066] Figure 4: Layout of a process simulated with AspenHysys vll system: 56: gas; 57: SR IN; 58: steam reformation; 59: SR OUT; 60: HP biogas; 61: water recycling; 62: methanol synthesis; 63: methanol stream; 64: syngas recycling; 65: CO2 purging; 66: concentrate; 67: DME synthesis; 68: DME; 69: Methanol and water recycling.

[0067] Figures 5A and 5B: Current tables, compositions and duties relating to figure 4.

[0068] Figure 6: Layout of a process simulated with AspenHysys vll system (version 2: H2 make-up from electrolyzer, CO2 recycling): 70: gas; 71: HP biogas; 72:SR IN; 73: SR OUT;

[0069] 74: syngas

[0070] 75.a: H2O recycling, 75.b: methanol synthesis; 76: methanol stream; 77: syngas recycling; 78: concentrate;

[0071] 79: membrane permeate; 80: CO2 recycling; 81: steam; 82: SOEC;

[0072] 83: 02;

[0073] 84: H2;

[0074] 85: DME synthesis;

[0075] 86: DME;

[0076] 87: methanol and water recycling.

[0077] Figures 7A and 7B: Current tables, compositions and duties relating to figure 6.

[0078] Figure 8: Layout of a process simulated with AspenHysys vll system (version 3: H2 and 02 make-up from electrolyzer , C02 recycling): 88: gas;

[0079] 89: HP biogas;

[0080] 90:SR IN;

[0081] 91: SR OUT;

[0082] 92: fresh syngas;

[0083] 93: methanol synthesis;

[0084] 94: water recycling;

[0085] 95: syngas recycling;

[0086] 96: C02 recycling;

[0087] 97: concentrate;

[0088] 98: 02;

[0089] 99: H2;

[0090] 100: DME synthesis;

[0091] 101: DME;

[0092] 102: methanol and water recycling.

[0093] 103: steam reformation.

[0094] Figures 9A and 9B: Current tables, compositions and duties relating to figure 8.

[0095] DETAILED DESCRIPTION OF THE INVENTION

[0096] The present invention relates to a process for converting gas to dimethyl ether, wherein said gas (18) is selected from the group consisting of: biogas, biomethane and methane, said process comprising the steps of: a) Tri-reformation (19) of said gas to syngas (20) according to the following reactions

[0097] [Rl] CH4 + H2O = CO + 3 H2

[0098] [R2] CH4 + CO2 = 2 CO + 2 H2

[0099] [R3] CH4 + 0.502 = CO + 2 H2

[0100] - at a temperature comprised in the range from 800-950°C; and

[0101] - at a pressure comprised in the range from 10- 30 bar; b) cooling said syngas (21) of step a) and recycling water (22) upstream of said step a); c)production of methanol (23) according to the reactions of [R4] CO + 2 H2 = CH3OH

[0102] [R5] CO2 + 3 H2 = CH3OH + H2O

[0103] - on a heterogeneous catalyst CZA;

[0104] - at a temperature comprised in the range from 240-280°C;

[0105] - at a pressure comprised in the range from 60- 100 bar; said conversion process characterised in that it further comprises the following steps: d)separating (24), through a separation section, the product exiting from c) in: d.i) a syngas current (25); d.ii) a current rich in CO2 (32), containing more than 65 mol%; d.iii) a current comprising methanol and water (31); wherein d.i) the residual syngas from said product exiting from c) and not yet converted to methanol is separated with said separation section consisting of at least one purification unitary operation, said at least one purification unitary operation being selected from the group consisting of: a flash, a distillation column, a filling column, a membrane, a molecular sieve, or combinations thereof; and wherein said syngas is subsequently recycled upstream of the step c) of said process, after splitting and purging (27); d.ii) said current rich in C02 is removed from said conversion process (32); and d.iii) said fluid current comprising methanol and water and sent to further purification; e) DME synthesis (33) between methanol and water coming from the separation d) according to reaction

[0106] [R6] 2 CH3OH = CH3OCH3 + H20

[0107] - at a pressure comprised in the range from 10 - 20 bar;

[0108] - at a temperature comprised in the range from 280 - 400 °C;

[0109] - on an acidic gamma-alumina or zeolite-based heterogeneous catalyst; f) DME separation (34) obtained from step e) and recycling of said methanol and residual water (36) from step e) upstream of said step a) of said conversion process, said separation being carried out on a dedicated distillation column, wherein in said column:

[0110] - the head section operates between 9 - 19 bar and 40 - 50°C and

[0111] - tail section operates between 10 - 20 bar and 155 - 165°C and. Preferably, in said conversion process (26), said at least one purification unitary operation of d.i) consists of two removal flashes in series, said removal flashes in series being

[0112] ■ one at high pressure, wherein said high pressure is comprised in the range from 50 - 90 bar, and

[0113] ■ one at low pressure wherein said low pressure is comprised in the range from 20 - 40 bar, and both said removal flashes in series being capable of operating at a temperature comprised in the range from 3 - 50°C.

[0114] Preferably, said purging step d.i) may take a value comprised between 5 - 15% of the recirculation flow rate of said residual syngas recirculated in step c).

[0115] Preferably, said syngas, optionally moist, obtained from said purification unitary operation of said step d.i) is purified with a membrane and split system and separated in:

[0116] - a purging current poor in hydrogen, containing less than 45 mol% H2; and

[0117] - a recirculation current rich in hydrogen recirculated upstream of step c), containing more than 60 mol% H2.

[0118] Preferably, said recirculation current rich in hydrogen is purified with a membrane selective separation system, wherein in said membrane selective separation system

[0119] - the upstream pressure is comprised in the range from 60-100 bar; and

[0120] - the downstream pressure is comprised in the range from 15-45 bar. Preferably, said current rich in C02 of said step d.ii) is separated through stripping assisted by column boiling, wherein

[0121] - the bottom temperature varies in the range from 120- 150°C; and

[0122] - the head temperature varies in the range from 30-40 °C.

[0123] Preferably, in said conversion process of the present invention there is integrated an electrolyzer (29) upstream of step c), said electrolyzer being capable of:

[0124] - receiving the steam produced in step c) (23); and

[0125] - producing pure oxygen (30) and hydrogen from steam, wherein said hydrogen from steam is recycled upstream of step c).

[0126] Preferably, said pure oxygen (49) produced by said electrolyzer is conveyed upstream of step a) of said conversion process (38).

[0127] The splitting system is preferably based on selective polymeric membranes, or on other hydrogen purification solutions through molecular sieves, with PSA (pressure swing adsorption) systems.

[0128] The electrolyzer is preferably based on technologies available on the market, such as alkaline electrolyzer or PEM (polymer electrolyte membrane) electrolyzer, or on developing technologies such as SOEC (solid oxide elctrolyzer cell) electrolyzers .

[0129] The present invention also relates to plants comprising the gas conversion process according to the present invention.

[0130] Preferably, said plants are selected from the group consisting of: gasification plants, pyrolysis plants, gas plants provided with flaring system, plants with co-presence of methane and carbon dioxide, methanation plants. As outlined above, the process of the present invention provides for a tri-reformation step (Vita et al., 2018; Pastorele et al., 2020) according to said reactions: [Rl] of Steam methane reforming [R2] of Dry reforming [R3] of Partial oxidation

[0131] The reaction Rl is used for the conversion of methane for producing hydrogen and syngas.

[0132] The reactions Rl and R2 lead to the bi-reforming operation, typically used for reducing CO2 (and therefore also valid for biogas) by using cerium catalysts.

[0133] Advantageously, the simultaneous reactions Rl, R2 and R3 allow to operate the unit as tri-reforming, with the benefit lying in the fact that part of the energy to be provided is developed in the tubes due to the exothermicity of the reaction R3.

[0134] In the co-presence of steam and CO2, the reaction R2 seems effective provided that the oxygen is present in modest fractions. The operating conditions are usually in the range of 10-30 bar and 800-950 °C.

[0135] As reported above, step b) of the present invention provides for that the syngas obtained is cooled and the water contained therein is condensed and recycled upstream. The stream is conveyed to the methanol synthesis which operates according to reactive processes (Park et al., 2014; Bozzano, Manenti, 2016; Bisotti et al., 2021): [R4] called CO path [R5] called C02 path

[0136] As outlined above, the synthesis is carried out on heterogeneous catalysts of the CZA type (Graaf et al., 1990) in catalytic reactors at 60-100 bar Previtali et al., 2020) and 240-280 °C.

[0137] Advantageously, the present invention allows to convert gas into dimethyl ether (DME) with high yield and selectivity and with significant process simplification of the purification sections.

[0138] In particular, the present invention provides for recycling all liquid and gaseous streams (including purging), at least their fraction of interest, to maximise productivity. Furthermore, the purification sections are not pushed and complete as required by conventional synthesis plants, but, except for the end DME, they are limited to partial purifications which are then fully recycled upstream of the various units which act as re-converters.

[0139] In order to simplify the purification of DME, which requires a significant number of operations and / or severe conditions (such as for example refrigeration), the present invention operates in two separate synthesis stages:

[0140] - the conversion of syngas to methanol and subsequently

[0141] - the conversion of methanol to DME.

[0142] Therefore, C02 can be easily separated from the methanol / water mixture prior to proceeding with the DME synthesis, given that purifying DME from C02 is conventionally complex.

[0143] On the other hand, the current of the products is subjected to a further column treatment to fully remove the dissolved C02; purity and pressure allow to store C02 in cylinders at the market specifications.

[0144] The methanol and water stream is conveyed to the synthesis of DME without further distillations which is carried out at temperatures that are slightly higher than the methanol synthesis and it is insensitive to pressure (but however operated at 10 bar or above to minimise recycling costs): [R6] 2 CH3OH = CH3OCH3 + H2O Methanol dehydration

[0145] DME is then separated in a single unitary operation from the residual methanol and water, which are recycled upstream of the reformer for their reconversion to syngas. Such recycling also allows to convert all possible superior alcohols and other organic compounds deriving from the parasitic reactions present in both synthesis reactors into syngas.

[0146] Advantageously, the process of the present invention allows the production of DME with small-size plants, high simplification of the purification sections, high yield and selectivity.

[0147] Furthermore, the present invention advantageously allows environmental and energy self-sustainability, and carbonnegative plants.

[0148] Below are some non-limiting exemplifying experiments aimed at better describing the technical aspects and advantages of the present invention.

[0149] EXAMPLES

[0150] EXAMPLE 1

[0151] The detailed process simulation was carried out using AspenHysys vll (Polytechnic University of Milan).

[0152] The process diagram is reported in Figure 2 and the spreadsheet of the currents, of the compositions and of the duties is reported in Table 1.

[0153] The present invention attains a conversion to pure DME equal to 46% by weight with respect to the inflowing biogas (770 kg / h of biogas at 53%v of methane).

[0154] EXAMPLE 2

[0155] In a special embodiment, an electrolyzer is integrated to the system of the present invention to produce pure oxygen and hydrogen starting from steam (Figure 3). The steam may be self-generated by the methanol synthesis section and in particular by the jacket of the synthesis tubular reactor in which cooling water is stored.

[0156] Other sources of steam are allowed in any case. The oxygen may be sold at market value, while hydrogen is used for fully decarbonising the current of C02 upon exiting from the first separation unit. The C02 current removed from the methanol and water mixture is therefore recycled to the methanol synthesis section together with the appropriate amount of hydrogen to be produced using the electrolyzer as it occurs with the reaction R5.

[0157] The detailed process simulation is reported in Figure 4 and the operating conditions, the currents and the compositions are reported in the Tables of Figures 5A and 5B

[0158] EXAMPLE 3

[0159] In an embodiment, the present invention also uses the oxygen produced by the electrolyzer to facilitate the tri-reforming section (Figure 8).

[0160] The oxygen may be used both on the tubes side and on the firebox side. The Figure and the Table show the process layout and the operating conditions for re-use of the firebox side oxygen.

Claims

CLAIMS1) Process for converting gas to dimethyl ether, wherein said gas (18) is selected from the group consisting of: biogas, biomethane and methane, said process comprising the steps of: a) Tri-reformation (19) of said gas to syngas (20) according to the following reactions[Rl] CH4 + H2O = CO + 3 H2[R2] CH4 + CO2 = 2 CO + 2 H2[R3] CH4 + 0.502 = CO + 2 H2- at a temperature comprised in the range from 800-950°C; and- at a pressure comprised in the range from 10- 30 bar; b) cooling (21) said syngas of step a) and recycling water (22) upstream of said step a); c)production of methanol (23) according to the reactions of[R4] CO + 2 H2 = CH3OH[R5] CO2 + 3 H2 = CH3OH + H2O- on a heterogeneous catalyst CZA;- at a temperature comprised in the range from 240-280°C;- at a pressure comprised in the range from 60- 100 bar; said conversion process characterised in that it further comprises the following steps: d)Separating (24) - through a separation section - the product exiting from c) in: d.i) a syngas current (25);d.ii) a current rich in C02 (32), containing more than 65 mol% C02; d.iii) a current comprising methanol and water (31); wherein d.i) the residual syngas from said product exiting from c) and not yet converted to methanol is separated with said separation section consisting of at least one purification unitary operation, said at least one purification unitary operation being selected from the group consisting of: a flash, a distillation column, a filling column, a membrane, a molecular sieve, or combinations thereof; and wherein said syngas is subsequently recycled upstream of the step c) of said process, after splitting and purging (27); d.ii) said current rich in C02 is removed from said conversion process (32); and d.iii) said fluid current comprising methanol and water (31) and sent to further purification; e) DME synthesis (33) between methanol and water coming from the separation d) according to reaction[R6] 2 CH3OH = CH3OCH3 + H20 at a pressure comprised in the range from 10 - 20 bar; at a temperature comprised in the range from 280 - 400 °C; on an acidic gamma-alumina or zeolite-based heterogeneous catalyst; f) DME separation (34) obtained from step e) and recycling of said methanol and residual water (36) from step e) upstream of said step a) of said conversion process, said separation being carried out on a dedicated distillation column, wherein in said column:- the head section operates between 9 - 19 bar and 40 - 50°C and- tail section operates between 10 - 20 bar and 155 - 165°C and.2) Conversion process according to claim 1, wherein said at least one purification unitary operation of d.i) consists of two removal flashes in series, said removal flashes in series being■ one at high pressure, wherein said high pressure is comprised in the range from 50 - 90 bar, and■ one at low pressure wherein said low pressure is comprised in the range from 20 - 40 bar, and both said removal flashes in series being capable of operating at a temperature comprised in the range from 3 - 50°C.3) Conversion process according to one or more of the preceding claims, wherein said purging of step d.i) has a variation comprised in the range from 5- 15% of the recirculation flow rate of said residual syngas recirculated in step c).4) Conversion process according to one or more of the preceding claims, wherein said syngas, optionally moist, obtained from said purification unitary operation of said step d.i) is purified with a membrane and split system and separated in:- a purging current poor in hydrogen, containing less than 45 mol% H2; and- a recirculation current rich in hydrogen recirculated upstream of step c), containing more than 60 mol% H2.5) Conversion process according to claim 4), wherein said recirculation current rich in hydrogen is purified with a membrane selective separation system, wherein in said membrane selective separation system- the upstream pressure is comprised in the range from 60-100 bar; andThe downstream pressure is comprised in the range from 15-45 bar.6) Conversion process according to one or more of the preceding claims, wherein said current rich in CO2 of said step d.ii) is separated through stripping assisted by column boiling, wherein- the bottom temperature varies in the range from 120- 150°C; and- the head temperature varies in the range from 30-40 °C.7)Conversion process according to one or more of the preceding claims, wherein in said process there is integrated an electrolyzer (48) upstream of step c), said electrolyzer being capable of:- receiving the steam produced in step c) (23); and- producing pure oxygen and hydrogen from steam (30), wherein said hydrogen from steam is recycled upstream of step c).8) Conversion process according to claim 7), wherein the pure oxygen produced by said electrolyzer (48) is conveyed upstream of step a) of said conversion process (38).9) Plants comprising the gas conversion process according to one or more of the preceding claims.10) Plants according to claim 9, wherein said plants are selected from the group consisting of: gasification plants, pyrolysis plants, gas plants provided with flaring system,plants with co-presence of methane and carbon dioxide, methanation plants.

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