The present invention relates to a novel method for metha ^¬ nol synthesis. More specifically, the invention concerns a novel treatment of the make-up gas used in a methanol syn ^¬ thesis loop.

Methanol is synthesized from a synthesis gas, which con ^¬ sists of ¾ and carbon oxides, i.e. CO and CO2. The conver- sion from syngas can be formulated as a hydrogenation of either carbon monoxide or carbon dioxide, accompanied by the reverse shift reaction, and can be summarized by the following reaction sequence: CO + 2H ₂ <-> CH3OH

C0 ₂ + 3H ₂ <-> CH3OH + H ₂ 0

C0 ₂ + H ₂ <-> CO + H ₂ 0

The conversion is performed over a catalyst, which is most often a copper-zinc oxide catalyst on an alumina support.

Examples of this catalyst include applicant's catalysts MK- 121 and MK-151 FENCE™.

Producing methanol theoretically requires a synthesis gas (syngas) with a module M equal to 2. The module M is de ^¬ fined as

M = ( H2 - CO2 ) / (CO+CO2 ) . As syngas typically also contains inert compounds, the op ^¬ timum module may become slightly higher than 2, typically 2.05, allowing purge of the inert compounds which inevita ^¬ bly also will result in purge of reactants ¾, CO and CO2 . For a syngas with a module less than the optimum module as defined above, surplus carbon oxides are present, and the module must be adjusted to the required level, e.g. by re ^¬ covery of ¾ from the purge stream and recycle of the re ^¬ covered ¾ to the synthesis section. In known processes this is done by recovering ¾ from the purge in a separa ^¬ tion unit, e.g. a PSA unit or a membrane unit, which pro- duces a ¾-enriched gas for recycle and a ¾-depleted waste gas .

In a typical methanol production process, make-up gas is mixed with ¾-rich recycle gas and passed to the synthesis reactor, optionally via a sulfur guard if the make-up gas contains enough sulfur to impact the lifetime of the metha ^¬ nol synthesis catalyst. After mixing the make-up gas with the recycle gas, the combined gas is sent to the methanol reactor, in which hydrogen and carbon oxides react to form methanol as shown in the above reaction sequence.

Until now it has been normal practice to add CO2 to the make-up gas in the methanol synthesis loop in order to maintain a sufficient selectivity of the methanol synthesis catalyst. This is because, in general, the selectivity of the methanol synthesis catalyst decreases when operating at too high CO/ CO2 ratios, which can be compensated for by in ^¬ creasing the CO2 content in the make-up gas. However, this addition of CO2 to the make-up gas can be a problem, especially in coal-based methanol plants, because the CO2 normally will originate from a CO2 removal step, where the resulting CO ₂ is received at ambient pressure. Moreover, this CO ₂ will normally be contaminated with sul ^¬ fur . It has now surprisingly turned out that the problem mentioned above can be solved by adding water to the make-up gas instead of CO2.

A number of prior art documents deal with the synthesis of methanol. Thus, EP 1 080 059 Bl describes a process wherein methanol is synthesized in a synthesis loop in at least two synthesis stages from a synthesis gas comprising hydrogen and carbon oxides. With said process, the problem of using a preliminary synthesis step or operating at low circula- tion ratios, leading to relatively high partial pressures, which in turn lead to excessive reaction and heat evolution in the catalyst bed, can be avoided.

Use of more than one methanol reactor is described in US 2010/0160694 Al, which concerns a process for the synthesis of methanol comprising passing a syngas mixture comprising a loop gas and a make-up gas through a first synthesis re ^¬ actor containing a methanol synthesis catalyst to form a mixed gas containing methanol, cooling said mixed gas con- taining methanol and passing it through a second synthesis reactor containing a methanol synthesis catalyst, where further methanol is synthesized to form a product gas stream. This product gas stream is cooled to condense out methanol, and unreacted gas is returned as the loop gas to said first synthesis reactor. This set-up includes the use of a combination of a steam raising converter (SRC) cooled by boiling water under pressure as the first methanol reac- tor and a tube cooled converter (TCC) as the second metha ^¬ nol reactor.

The use of more than one methanol reactor is also disclosed in US 8.629.190 B2. Synthesis gas is passed through a first, preferably water-cooled reactor, in which a part of the carbon oxides in the gas is catalytically converted to methanol, and the resulting mixture of synthesis gas and methanol vapor is supplied to a second, preferably gas- cooled reactor in series with the first reactor. In said second reactor, a further part of the carbon oxides is converted to methanol. The mixture withdrawn from the first reactor is guided through a gas/gas heat exchanger in which the mixture is cooled to a temperature below its dew point. Subsequently, methanol is separated from the gas stream and withdrawn, while the remaining gas stream is fed to the second reactor.

US 2009/0018220 Al describes a process for synthesizing methanol, wherein a make-up gas with a stoichiometric num ^¬ ber or module M (M = ( [H ₂ -C0 ₂ ] ) / ( [C0 ₂ ] + [CO] ) ) of less than 2.0, preferably less than 1.8, is combined with unreacted synthesis gas to form a gas mixture, which is used to pro ^¬ duce methanol in a single synthesis reactor. The make-up gas is obtained by reforming a hydrocarbon feedstock, such as methane or natural gas, and removing water from the re ^¬ sulting reformed gas mixture.

US 5.079.267 and US 5.266.281 both describe a process for the production of methanol from synthesis gas produced in a steam reformer. The synthesis gas is cooled followed by re ^¬ moval of C0 ₂ and H ₂ O from the gas. Then ¾0 is removed to obtain a residual level of H ₂ 0 of 10 ppm or lower, and CO ₂ is removed to obtain a residual level of CO ₂ of 500 ppm, preferably 100 ppm or lower. The synthesis gas undergoes H ₂ /CO stoichiometric adjustment before it is sent to the methanol synthesis reactor.

Finally, US 7.019.039 describes a high efficiency process for producing methanol from synthesis gas, wherein the stoichiometric number or module M = ( [H ₂ -C0 ₂ ] ) / ( [C0 ₂ ] + [CO] ) of the make-up gas has been increased to about 2.05 by re ^¬ jecting CO ₂ from the gas mixture for a series of single- pass reactors.

In none of the prior art documents, the possibility of re- placing the CO ₂ addition to the make-up gas with an addi ^¬ tion of water is suggested.

Thus, the present invention relates to a process for metha ^¬ nol production from synthesis gas, said process comprising the following steps:

- providing a make-up gas containing hydrogen and carbon monoxide, in which the content of carbon dioxide is less than 0.1 mole% ,

- mixing the make-up gas with a hydrogen-rich recycle gas and passing the gas mixture to a methanol synthesis reac ^¬ tor, optionally via a sulfur guard, and - subjecting the effluent from the synthesis reactor to a separation step, thereby providing crude methanol and the hydrogen-rich recycle gas, wherein the customary addition of carbon dioxide to the make-up gas is replaced by addition of water in an amount to obtain a water content of 0.1 to 5 mole% in the make-up gas .

The amount of added water preferably corresponds to a con ^¬ tent of 0.5 to 2.5 mole%, most preferably 0.8 to 1.2 mole% in the make-up gas.

By adding water to the make-up gas instead of adding carbon dioxide, the otherwise necessary compression of CO ₂ is omitted and thus a CO ₂ compressor is saved to the benefit of the process economy.

At the same time, the amount of poisonous sulfur in the make-up gas is markedly reduced.

The presence of sufficient CO ₂ in the make-up gas is still necessary. The improvement over the prior art lies in the fact that the water addition will ensure sufficient CO ₂ for the methanol synthesis via the shift reaction

CO + H ₂ 0 <-> C0 ₂ + H ₂

In the following the invention will be further described with reference to the appended figure, which is exemplary and not to be construed as limiting for the invention.

The figure shows a plant which can be used according to the present invention. The make-up gas, to which water has been added, is mixed with ¾-rich recycle gas and passed to the methanol reactor. From this reactor a product stream and a purge stream are withdrawn. The purge stream is heated in a preheater and mixed with the process steam to obtain a mixed stream, which is passed to a shift conversion unit, where steam and CO react to ¾ and CO ₂ . The reacted gas is cooled to below its dew point in a cooler. The cooled stream is passed to a process condensate separator, and the vapor stream from the condensate separator is passed to a hydrogen recovery unit. From this unit a hydrogen-enriched stream and a hydrogen-depleted waste gas stream are with- drawn. The hydrogen-enriched gas may be compressed in a re ^¬ cycle compressor to form the hydrogen-enriched recycle stream, which is added to the make-up gas as described above .

The invention is illustrated further in the examples 1-4, which follow. The examples illustrate four different cases with constant converter pressure drop and various make-up gas (MUG) compositions, viz.

Case No C0 ₂ ; no H ₂ 0 in MUG

Case 1 mole% C0 ₂ ; no H ₂ 0 in MUG

Case No C0 ₂ ; 1 mole% H ₂ 0 in MUG

Case No C0 ₂ ; 2 mole% H ₂ 0 in MUG The carbon loop efficiency listed in the examples is a di ^¬ rect measure of the methanol synthesis efficiency.

In case 1 the carbon loop efficiency is significantly lower than in cases 2 to 4. This illustrates the necessity of the presence of CO ₂ or a CO ₂ generator in the make-up gas. Cases 2 to 4 illustrate that CO ₂ in the make-up gas can be replaced by H ₂ 0 as it is possible to obtain similar carbon loop efficiencies.

Example 1

This example shows the impact of the MUG composition on the synthesis loop performance in the base case: 29% CO, 67% H ₂ , 3% N ₂ and 1% CH ₄ ; no C0 ₂ and no H ₂ 0 in the MUG.

The following results were found:

Gas compositions, measured as recycle gas composition

(RGC) , converter inlet gas composition (CIGC) and converter outlet gas composition (COGC) were as follows: RGC CIGC COGC

H ₂ , mole% 66.69 66.77 66.06

CO, mole% 28.04 28.29 27.78

C0 ₂ , mole% 0.126 0.093 0.13

N ₂ , mole% 3.400 3.295 3.37

CH ₄ , mole% 1.132 1.097 1.12

Data for the boiling water reactor (BWR) :

Example 2 This example shows the impact of the MUG composition on the synthesis loop performance in case 2: 1 mole% CO ₂ and no H ₂ 0 in the MUG.

The following results were found:

Gas compositions, measured as RGC, CIGC and COGC were as follows :

Data for the boiling water reactor (BWR) :

Space-time yield, kg MeOH/kg catalyst/h 1.139

BWR inlet bed pressure, kg/cm -g 81.475

BWR outlet bed pressure, kg/cm -g 79.475

Pressure drop, kg/cm ² 2.00 Number of tubes 4405

Total catalyst mass, kg 5.412

Duty of BWR, MW 42.449

Temperatures :

BWR temperature, °C 230

Approach temperature to MeOH equilibrium, °C 49.67

BWR inlet temperature, °C 208.00

BWR outlet temperature, °C 240.95

Maximum catalyst temperature (hot spot) , °C 247.85 Example 3

This example shows the impact of the MUG composition on the synthesis loop performance in case 3: No CO ₂ and 1 mole% H ₂ 0 in the MUG.

The following results were found:

Purge 2.677 NmVh

Total purge 2.737 NmVh

Gas compositions, measured as RGC, CIGC and COGC were as follows :

Data for the boiling water reactor (BWR) :

Space-time yield, kg MeOH/kg catalyst/h 1.101

BWR inlet bed pressure, kg/cm -g 81.475

BWR outlet bed pressure, kg/cm -g 79.475

Pressure drop, kg/cm ² 2.00

Number of tubes 4405

Total catalyst mass, kg 5.412

Duty of BWR, MW 40.778

Temperatures :

BWR temperature, °C 230

Approach temperature to MeOH equilibrium, °C 58.97

BWR inlet temperature, °C 208.00

BWR outlet temperature, °C 240.70

Maximum catalyst temperature (hot spot) , °C 245.90 Example 4

This example shows the impact of the MUG composition on the synthesis loop performance in case 4: No CO ₂ and 2mole% ¾0 in the MUG.

The following results were found:

Gas compositions, measured as RGC, CIGC and COGC were as follows :

RGC CIGC COGC

H ₂ , mole% 75.94 73.88 70.36

CO, mole% 2.098 7.84 1.95

C0 ₂ , mole% 1.121 0.863 1.06

N ₂ , mole% 15.341 12.497 14.22

CH ₄ , mole% 4.894 3.997 4.55 Data for the boiling water reactor (BWR) :

Space-time yield, kg MeOH/kg catalyst/h 1.084

BWR inlet bed pressure, kg/cm -g 81.475

BWR outlet bed pressure, kg/cm -g 79.475

Pressure drop, kg/cm ² 2.00

Number of tubes 4405

Total catalyst mass, kg 5.412

Duty of BWR, MW 40.270

Temperatures :

BWR temperature, °C 230

Approach temperature to MeOH equilibrium, °C 44.05

BWR inlet temperature, °C 208.00

BWR outlet temperature, °C 237.36

Maximum catalyst temperature (hot spot) , °C 246.67