The present invention is directed to purification of raw gas. In particular, the invention concerns removal of ni- trogen from the raw gas .

Industrial raw gasses arise typically from carbonaceous raw materials such as gasification of coal, oil petroleum coke, biomass and the like, as a reformed hydrocarbon feed or as coke oven gas.

As used herein in the following, "raw gas" shall comprise any gas containing hydrogen, at least one carbon oxide and nitrogen .

Typically, such a raw gas is obtained by the above mentioned gasification process or as off-gas from the production of coke, the so called coke oven gas. These gases contain hydrogen, which inter alia is a valuable reactant for use as alternative fuel or for use in the preparation of a number of bulk chemicals and of liquid or gaseous fuels . As an example, a gasification raw gas and coke oven gas may be employed in the preparation of substitute natural gas (SNG) .

SNG must have a high content of methane and residual amounts of impurities arising during preparation of SNG must be low if a high calorific value of SNG is required, when using SNG as replacement of natural gas.

CONFIRMATION COPY A raw gas may also be converted into a liquid fuel, such as gasoline or diesel by the Fischer Tropsch or an oxygenate to gasoline process. In this conversion a gas stream may be recycled to recover reactants, possibly with reforming of undesired hydrocarbons such as methane into carbon monoxide. In order to avoid a build up of undesired components in the recycle stream, a purge gas is withdrawn. One such undesired component is inert nitrogen, and therefore specific removal of nitrogen is beneficial as it may reduce the purge stream, which remove raw material from the process .

Although the concentration of nitrogen in the raw gas from gasification may be minimised by operating the gasifier with high purity oxygen, nitrogen is still a component of coal, biomass, pet coke and other feedstocks being employed in the gasification process and will thus be released from the feedstock and transferred to the raw gas produced by gasification .

Similar considerations apply for the coking process.

Carbon dioxide and optionally hydrogen sulphide, if present in the raw gas may be removed by means of a conventional acid gas removal process, wherein the gas is treated with a physical carbon dioxide and hydrogen sulphide sorbent, like the known "Rectisol" or "Selexol" process or a chemical sorbent such as the known amine wash. Common solutions to reduce the content of nitrogen in the raw gas are cryogenic processes, in which the gas is cooled below the boiling point of nitrogen. Cryogenic processes are expensive to establish and to operate. Thus, the present disclosure provides a method for the reduction of undesired impurities from a raw gas, including nitrogen, by conversion of nitrogen to ammonia, which is more convenient to remove from the raw gas compared to the known methods and which at the same time may be used as feed-stock for the production of valuable chemicals, e.g. ammonium thiosulphate or urea.

In order to allow the catalytic conversion of nitrogen to ammonia, it is necessary to remove essentially all of the carbon monoxide and carbon dioxide (carbon oxides) and water in the raw gas prior to introduction into the ammonia conversion reaction. To remove carbon oxides the raw gas is treated in a

methanation step, wherein carbon monoxide and carbon dioxide react with hydrogen to methane in presence of a

methanation catalyst. The methanated process gas, having little or no presence of carbon oxides then enters into the commonly known ammonia conversion reaction, and the produced ammonia may be separated as a liquid from the purified gas stream. Ammonia in its liquid form shall herein be understood as either con- densed ammonia or ammonia dissolved in water. The invention is in its broadest embodiment a process for converting a carbonaceous raw material to a liquid or gaseous carbon based fuel, comprising the steps of

a) converting said carbonaceous raw material to a carbon based fuel

b) withdrawing a process gas to be purified comprising hydrogen, a carbon oxide, such as carbon monoxide or carbon dioxide and nitrogen, from a position of the process of step a)

c) subjecting the process gas to be purified to a methana- tion reaction in which the carbon oxide contained in the process gas to be purified is converted to methane providing a methanated process gas

d) directing the methanated process gas to ammonia synthe- sis for converting nitrogen in the gas to ammonia, providing an ammonia containing process gas;

e) withdrawing liquid phase ammonia from the ammonia containing process gas of step d) producing a purified gas with a reduced content of nitrogen and

f) directing an amount of the purified gas to a position of the process of step a) ,

wherein the molar flow of carbon in the carbon based fuel is greater than the molar flow of the ammonia withdrawn. In a specific embodiment the molar flow of carbon in the carbon based fuel is at least two times greater than the molar flow of the ammonia withdrawn, with the associated benefit of an increased yield of fuel.

In a specific embodiment for the production of Fischer Tropsch products, the process gas to be purified is a Fischer-Tropsch (FT) tail gas and the purified gas is recycled either as a feed to FT synthesis or as a feed to partial oxidation or a reforming step upstream the FT synthesis. The Fischer-Tropsch conversion is well known by the skilled person, and may be conducted in the presence of a catalyst such as a group VIII metal, preferably Fe, Co, Ru and Ni, or Mo as it is commonly known.

In a specific embodiment for synthesis of methanol or gaso- line, the process gas to be purified is withdrawn from a synthesis loop for synthesis of methanol or for synthesis of gasoline from an oxygenate and the purified gas is recycled as a feed to partial oxidation or as a feed to a reforming step upstream said synthesis loop. The synthesis of methanol is well known by the skilled person, and may be conducted in the presence of a catalyst such as Cu, as it is commonly known.

Similarly the synthesis of gasoline from oxygenates, such as methanol or dimethyl ether is also well known by the skilled person, and may be conducted in the presence of a catalyst such as a zeolite, preferably ZSM5 zeolite as it is commonly known. A specific embodiment, for production of synthetic natural gas comprises the following sub-steps;

u) withdrawing a first split stream from the raw gas, v) reacting the carbon monoxide of said first split stream to form hydrogen by a water gas shift process, providing a shifted gas stream w) treating the shifted gas stream in an acid gas removal step for providing said process gas to be purified according to steps b to f,

x) treating the remainder of raw gas in an acid gas removal step for obtaining a feed stream;

y) directing the purified gas to be combined with at least a part of the feed stream

z) converting the combined stream to substitute natural gas by a process for synthesis of synthetic natural gas.

The synthesis of synthetic natural gas is well known by the skilled person, and may be conducted in the presence of a catalyst such as a group VIII metal, preferably Ni or Ru, as it is commonly known.

Nitrogen is converted to ammonia by catalytic reaction with hydrogen further contained in the raw gas by means of the known catalytic ammonia conversion process. Depending on the concentration of nitrogen in the gas, the ammonia con- version reaction may be carried out in a once-through operation or in an ammonia synthesis loop. The synthesis of ammonia is well known by the skilled person, and may be conducted in the presence of a catalyst such as a Fe or Ru, as it is commonly known.

Where ammonia is synthesized in an ammonia synthesis loop, this loop comprises the steps of

i) contacting an amount of the ammonia synthesis loop gas with a catalytic material for reacting N ₂ and H ₂ to form NH ₃ forming the ammonia containing gas

ii) separating the ammonia containing process gas into liquid phase ammonia and the purified gas iii) directing a further amount of the purified gas to the process of step a) wherein the methanated process gas to be purified is added to a position of the ammonia synthesis loop, such as immediately upstream or immediately down- stream the catalytic material. It may be beneficial to add the methanated gas, which contains water, upstream the position for withdrawal of ammonia, as the presence of even small amounts of water aids the removal. Synthesizing ammonia in a synthesis loop has the benefit of increasing the yield of ammonia, and of being able to combine the methanated process gas with the ammonia containing process gas, such that water in the methanated process gas may removed with the ammonia.

Produced ammonia is in an embodiment removed from the raw gas by conventional condensation or conventional aqueous ammonia wash, which has the benefit of being a cost effective process for collection of ammonia.

The methanated process gas may subsequently be dried by e.g. cooling and separation of condensed water and/or by contact with molecular sieves being able to remove water molecules from a gas stream. Such molecular sieves are known in the art and conventionally employed in the drying of moist gases .

In an embodiment a part of the carbon monoxide in the raw gas is converted by reaction with water, forming hydrogen and carbon dioxide according to the water gas shift reaction and thereby the concentration of hydrogen in the gas is increased and the carbon monoxide content is decreased, with the benefit of producing a raw gas with a more optimal balance between hydrogen and carbon oxides prior to

methanation. The carbon monoxide content will be further reduced in the subsequent methanation step, as mentioned hereinbefore.

Where a shift reaction step is included, a step of acid gas removal is required for removing carbon dioxide, to avoid consumption of the produced hydrogen during methanation, by reaction with carbon dioxide. In the absence of a shift reaction step, the treatment of the gas in an acid gas removal step, whereby the amount of carbon dioxide and hydrogen sulphide, if further present in the raw gas, are reduced by chemical or physical absorption or adsorption ac- cording to known methods as already mentioned above. This has the effect of providing the possibility of using catalysts which are more effective, but subject to poisoning by the presence of carbon dioxide and sulphur. In a further embodiment a methanation step prior to acid gas removal may be included, for converting carbon oxides to methane, which will not be withdrawn in the acid gas removal step. This has the associated benefit of providing a higher level of carbon in the purified gas.

SNG is produced by catalytic methanation of carbon oxides with hydrogen. As already discussed hereinbefore, carbon oxides are converted to methane during purification of the process gas to be purified. If a certain content of nitro- gen can be tolerated in the final SNG product, it is according to a further embodiment of the invention possible to increase methane production, by withdrawing a split stream from the raw gas prior to its purification and treating the split stream solely in an acid gas removal step to reduce or remove the content of carbon dioxide from the split stream. For SNG production the ratio (H ₂ - C0 ₂ ) / (CO+C0 ₂ ) in the feed may optionally be around 3, where H ₂ , CO, and C0 ₂ represents the molar flow of the given components .

In the acid gas removal, hydrogen sulphide which also may be present in the raw gas is also adsorbed in the chemical or physical wash. Spent washing solution may be regenerated by desorbing carbon dioxide and hydrogen sulphide.

A part of the desorbed carbon dioxide can be recycled to the purified raw gas for use in the synthesis of carbon based fuel, with the benefit of increasing the fuel yield.

A part of the desorbed carbon dioxide can also be used in the synthesis of urea from ammonia, with the benefit of urea being a more valuable product than ammonia.

Desorbed hydrogen sulphide may be utilized for the preparation of ammonium thiosulphate by reaction with ammonia being obtained in the purification of the raw gas as e.g. de- scribed in European patent no.1 375 422.

The method of the invention is also useful to reduce the nitrogen concentration in recycle streams, thus affording a reduction of the recycle and, possibly, of the volume to purge, particularly for processes in which the additional amount of methane being formed in the methanation step is beneficial or can be converted downstream e.g. in a reform- ing process, such as autothermal reforming or steam reforming.

Fig.l represents an embodiment in which a carbonaceous raw material is converted to a liquid fuel, employing the disclosed purification process.

Fig.2 represents an embodiment in which a carbonaceous raw material is converted to synthethic natural gas, employing the disclosed purification process.

Figure 1 shows a specific embodiment of the invention, in which a carbonaceous raw material is provided to a process for synthesis of a liquid fuel 104 as a raw gas 100. The raw gas containing nitrogen 100 is converted into a fuel 104 such as methanol, gasoline, diesel and naphtha by a catalytical reaction in a reactor 102. Hydrogen and carbon monoxide are consumed during the production of fuels creating a stream of process gas to be purified which is rich in nitrogen.

The process gas to be purified 106 may be directed to a water-gas shift stage 108, if it is required to convert CO into hydrogen to ensure a surplus of hydrogen after CO and C0 ₂ are converted into methane in the methanation step 116 according to the following reactions:

CO + 3H ₂ = H ₂ 0 + CH _4/

C0 ₂ + 4H ₂ = 2H ₂ 0 + CH ₄

A reasonable surplus of hydrogen shall also ensure a partial conversion of nitrogen into ammonia in a downstream ammonia synthesis 118. The optimum molar flow of molecular hydrogen into the downstream methanation reactor 116 may be calculated as follow: H ₂ > 3*CO + 4*C0 ₂ + 3*N ₂ , where H ₂ , CO, C0 ₂ , N ₂ represents the molar flow of the given components. However, the process may also work by partly removing N ₂ by formation of NH ₃ with less hydrogen down to a level of H ₂ = 3*CO + 4*C0 ₂ where all hydrogen will be used for methanation, and no re- moval of N ₂ will take place.

Water may be removed from the shifted gas by conventional methods, including cooling down shifted gas to about 40°C in a condenser 110 and then separating the condensed water.

The C0 ₂ content is adjusted according to the equation above to ensure a surplus of hydrogen into the methanation reactor, by removal of excess C0 ₂ 114 in an acid gas wash 112. The partly purified process gas is sent to the methanation step 116 to convert CO and C0 ₂ into methane, as these two components are poisons to ammonia catalyst.

A methanated process gas comprising hydrogen, nitrogen and methane is sent to an ammonia synthesis loop 118 to at least partly convert nitrogen into ammonia which may be separated from the stream of ammonia containing gas, in a condenser operating around -5°C, forming a purified gas stream with a reduced concentration of nitrogen, and a liq- uid stream of ammonia 119. An additional aqueous ammonia wash 120 may be used to remove remaining ammonia 122. The purified gas stream is then reformed in 124 to convert methane into synthesis gas, i.e. CO and H ₂ , creating a reformed purified gas for the fuel synthesis in 102. A build up of other impurities such as Ar in the synthesis gas is avoided by a purge stream.

If the C0 ₂ concentration is too high in the reformed purified gas then it can be removed by means of a acid gas removal unit 126.

Figure 2 shows an alternative embodiment of the invention, in which a carbonaceous raw material is provided to a process for production of synthethic natural gas 204 as a raw gas 200, containing nitrogen.

A first part of the raw synthesis gas is sent to a water- gas shift stage 202, in order to convert CO and water into hydrogen and C0 ₂ to ensure a surplus of hydrogen when CO and C0 ₂ are converted into methane at a later stage accord- ing to the following reactions:

CO + 3H ₂ = H ₂ 0 + CH ₄

C0 ₂ + 4H ₂ = 2H ₂ 0 + CH ₄ A reasonable surplus of hydrogen shall also ensure a partial conversion of nitrogen into ammonia in a downstream ammonia synthesis. The minimum molar flow of molecular hydrogen into the downstream methanation reactor 216 may be calculated as follow:

H ₂ > 3*CO + 4*C0 ₂ + 3*N ₂ , where H ₂ , CO, C0 ₂ , N ₂ represents molar flow of the given components, but as discussed above the process will remove some N ₂ as long as H ₂ > 3*CO + 4*C0 ₂ .

Water is removed from the shifted gas in a condenser 210 by cooling down shifted gas to about 40°C and followed by separation the condensed water.

The C0 ₂ content is adjusted according to the equation above to ensure a surplus hydrogen into the methanation reactor, by removal of excess C0 ₂ 214 in an acid gas wash 212.

The partly purified process gas to be purified is sent to the methanation step 216 to convert CO and C0 ₂ into methane, as these two components are poisons to ammonia cata- lyst.

A methanated process gas comprising hydrogen, nitrogen and methane is sent to an ammonia synthesis loop 218 to partly convert nitrogen into ammonia which may be separated in ara- monia wash 220 on from the gas stream forming a purified gas stream having a reduced concentration of nitrogen, and a stream of ammonia 222.

The part of the synthesis gas which was not directed to wa- ter-gas shift is treated in an Acid Gas Removal stage 226 to remove sour gasses, such as H ₂ S and C0 ₂ forming a treated synthesis gas.

This treated synthesis gas stream is then combined with the purified gas stream to form a feed gas for a SNG synthesis unit 230, wherein (H ₂ -C0 ₂ ) / (C0 ₂ +CO) ~3. Examples

In a first example of the disclosure in relation to a

Fischer Tropsch process, a process gas to be purified with a content of 24% N ₂ was purified according to an embodiment of the present disclosure, according to Figure 1. The gas compositions in the process will be as shown in table 1.

In a second example, the composition of the gas to be purified from was similar, but the requirements to CO content in the purified gas was higher. Therefore a further

methanation reactor was included upstream the shift reactor. The resulting gas compositions are shown in table 2.

Finally a third example with a lower concentration of nitrogen in the process gas to be purified is provided in table 3.

Table 1

Feed Outlet Outlet AGR & Gas outlet Ammonia Outlet

Shift compression ammonia stream ATR loop

100 108 112 118 122 124

Composition, mole % dry

Ar 2 00 1.60 2 .04 4.06 2.41

CH4 18 90 15.09 19 .31 40.91 0.08 1.43

CO 26 49 1.00 1 .28 17.49

C02 2 12 21.84 0 .01 4.98

H2 26 49 41.61 52 .84 28.50 0.03 58.38

N2 24 .00 19.16 24 .52 25.79 0.02 15.31

NH3 0 00 0.00 0 .00 0.74 99.86 0.00

Nm3/h 100, 000 125, 238 97890 49232 22265 84, 316

Table 2

Feed Outlet pre- Outlet Outlet AGR & Gas outlet Ammonia Outlet methanation Shift compression ammonia stream ATR loop

100 108 112 118 122 124

Composition, mole % dry

Ar 2 00 2.30 2.01 2. 56 3.72 2.02

CH4 18 90 29.31 25.61 32. 51 49.14 0.09 1.59

CO 26 49 15.56 1.00 1. 27 19.57

C02 2 12 9.86 21.22 0. 01 5.04

H2 26 49 15.32 26.00 32. 98 11.93 0.1 53.21

N2 24 00 27.65 24.16 30. 67 34.46 0.03 18.6

NH3 0 00 0.00 0.00 0. 00 0.75 99.86 0.00

Nm3/h 100, 000 86,814 99, 330 78,263 53788 10537 100, 975

Table 3

Feed Outlet pre- Outlet Outlet AGR & Gas outlet Ammonia Outlet met anation Shift compression ammonia stream ATR loop

100 108 112 118 122 124

Composition, mole % dry

Ar 2 00 2.39 2.02 2. 74 4.07 1.99

CH 22 70 36.90 31.09 42. 31 64.87 0.12 3.45

CO 32 84 19.88 1.00 1. 36 22.74

C02 2 63 12.79 26.52 0. 01 4.74

H2 32 83 19.67 32.32 43. 98 26.84 0.03 65.20

N2 7 00 8.37 7.05 9 .6 3.47 1.88

NH3 0 00 0.00 0.00 0. 00 0.75 99.84 0.00

Nm3/h 100, 000 83,611 99, 237 72922 49084 10239 102, 934