ZA200200571B - Natural gas conversion to hydrocarbons and ammonia. - Google Patents

Description

® 01/09038 1 PCT/ZA00/00125

NATURAL GAS CONVERSION TO HYDROCARBONS AND AMMONIA

The present invention relates to a process for the production of hydrocarbons and ammonia, and more particularly to a process for the optimisation of the production of hydrocarbons and ammonia using a combined Fischer-Tropsch reactor based process and an ammonia synthesis process. It relates also to a combined hydrogen synthesis plant and ammonia synthesis plant.

In the specification and claims which follow, the term "synthesis gas” is intended to refer to a gas containing predominantly CO and H, which is intended to be used to manufacture liquid hydrocarbons. Unless it is apparent from the context, the term is not intended to refer to a gas used to manufacture ammonia.

BACKGROUND OF THE INVENTION

Ammonia is the second largest chemical synthetic product with approximately 400 ammonia plants operating globally. H, and N, are reacted at a temperature of between 400 and 500°C and a pressure greater than 100 bar over an iron based catalyst. The production of the nitrogen and the hydrogen is the single most expensive step in the synthesis process. Much effort has hence been devoted to reducing the cost of synthesis gas production. Liquified Petroleum Gas (LPG), naptha, petroleum coke, coal and natural gas have been used as feedstocks, although the vast majority of processes utilise natural gas as feedstock and fuel. CO, CO; and H,O are all considered poisons for the catalyst even at parts per million concentration levels and hence great effort is made to remove them from the synthesis gas. Sulphur, particularly in the form of H,S also acts as a permanent poison and has to be removed from the synthesis gas to very low levels.

State of the art ammonia synthesis processes use a two step reforming process. The primary reformer is an indirectly heated tubular reactor filled with a Ni catalyst. The natural gas is passed through this reactor after being mixed with steam. The reaction is controlled so that there is about 15% methane remaining in the exit stream. The partly reformed gas is then transferred to the secondary reactor. Air is added in a burner, and the oxygen and methane react exothermically. The hot gas passes adiabatically through a catalyst bed and exits at approximately 1 000°C. The resultant synthesis gas contains CO, CO, Haz, Na,

H,0 and small quantities of CH, and other unconverted hydrocarbons.

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® 01/09038 2 PCT/ZA00/00125

The synthesis gas is cooled and passed through first a high temperature shift converter operated at 320-350°C, then further cooled and passed through a low temperature shift converter. The combination converts almost all of the CO into CO, and H; by reaction with water. The gas is then treated to remove CO, using a suitable solvent. A number of commercially available technologies can be utilised for this step. The solvent is regenerated by flashing, and the CO, is vented. Methanation is used as the final treatment step. In this unit, almost all of the remaining carbon oxides are reacted with H, to form methane and water. The water is removed using molecular sieve absorbers.

Some ammonia synthesis technology vendors (eg. Linde and KTI) do not use a secondary reformer as described above and use pressure swing adsorption (PSA) to separate Ha from the other synthesis gas constituents. The N, is supplied from a cryogenic air separation unit. Overall efficiencies for this process are reported to be as good as conventional technology.

Synthesis gas comprising predominantly CO and H, can be used for the manufacture of liquid hydrocarbons utilising Fischer-Tropsch Synthesis. Hydrocarbons are typically produced by contacting synthesis gas with a preferred selective catalyst such as Co or Fe at 200-260°C and 10-50 bar. Although it is known that the Fischer-Tropsch reaction can be performed in the presence of Ng, it is in general not preferred. In the process, N; acts as an inert gas that lowers the reactant partial pressures, and thus larger reactors or more catalyst is required. The selectivity to heavier hydrocarbons is also negatively affected by large concentrations of inert gases. As with ammonia synthesis, the synthesis gas for hydrocarbon production is typically produced from a natural gas feedstock utilising steam methane (tubular) reforming, autothermal reforming, or a combination of the two.

The process in which synthesis gas is produced for hydrocarbon production utilises a high purity O, stream in the reforming step rather than air, because the addition of inert gases (particularly Ny) is generally considered detrimental to the process economics. In stand alone Gas-to-Liquids (GTL) plants, nitrogen is an unused by-product of the air separation step. GTL plants are very intensive oxygen users, with between 0.2 and 0.3 tonnes of oxygen consumed per barrel of product. Consequently between 0.5 and 0.7 tonnes of nitrogen per barrel of hydrocarbons is made available.

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As with the ammonia synthesis catalyst, the Fischer-Tropsch catalyst is highly sensitive to poisoning by sulphur compounds and these have to be removed so that only extremely low levels remain to ensure economic catalyst life. itis known in the art that under most conditions, the Fischer-Tropsch process requires a synthesis gas that contains H, and CO in a ratio at, or below, about 2.5, and more preferably at or below 2.0. This is because when certain catalysts are used, for example a

Co catalyst, better selectivity for heavy hydrocarbons is achieved when the above ratio is at or below 2.0. Conventional steam reforming and autothermal reforming technologies produce synthesis gas at a ratio greater than this ideal. Various alternatives have been proposed to obtain the correct ratio. These involve recycling of CO, which can be extracted from various points in the synthesis loop or recycling the Fischer-Tropsch tailgas back to the reforming section. Such methods are useful not only because they reduce the H/CO ratio, but also because they increase the overall carbon utilisation in the process.

Conversely, ammonia synthesis requires a very high H,/CO ratio. This is adjusted even further after the reforming section by using shift converters, which convert CO and water into CO and

H,. The CO, is extracted using known methods and is vented to the atmosphere.

The ammonia synthesis process has been used in combination with a Fischer-Tropsch process commercially only in one case (by Sasol). In this process, the tail gas exiting the

Fischer-Tropsch reactor is used as the source of hydrogen. After treatment of this gas to remove hydrocarbons, a portion is sent to a shift converter. H, is recovered and this is then reacted with N,, obtained from a cryogenic oxygen plant, in the ammonia synthesis process. This process is useful when: 1) the synthesis gas enters the Fischer-Tropsch reactor with a H./CO ratio greater than the stoichiometric ratio. The stoichiometric ratio is the ratio of H, used to CO used in the Fischer Tropsch reactor. When the synthesis gas enters the reactor with a high ratio, H, builds up and the tail gas contains a higher proportion of H, than the feedgas. (A number of reactions influence the ultimate stoichiometric ratio, for example, the production of various hydrocarbons and the water gas shift reaction: 2n+1)H; + nCO — C Han+2 + NHO (paraffin production) 2nH; + nCO — CHs[(CH,),s]CH=CH, + nH,O (olefin production)

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® 01/09038 4 PCT/2ZA00/00125 2nH, + nCO — C H21:20 + (n-1)H0 (alcohol production)

CO +H,0 > CO, +H; (water gas shift))

Each specific catalyst and the particular process conditions determine the ultimate stoichiometry of H, and CO utilisation because the relative rates of each of the reactions varies. In general, however, it is well known in the art that Fe based catalysts are active for the water gas shift reaction, while Co based catalysts are not. Thus the stoichiometric ratio for Co catalyst is close to 2.0, whereas it is somewhat lower for Fe based catalysts; 2) the conversion is low so that not all of the available H, is utilised and can thus be extracted from the tail gas for ammonia synthesis.

Modern GTL facilities are designed to primarily produce liquid fuel. This is achieved in a three step process involving a) synthesis gas generation, b) hydrocarbon synthesis and c) hydroprocessing. The processes are designed to be highly efficient with high conversions and good selectivity so that the liquid fuel product can compete economically with conventional fuels derived from crude oil. Thus in a modern GTL facility, the above Sasol process will not be suitable as options (1) and/or (2) above are not considered to be viable.

There therefore remains a need for optimising the conversion of natural gas to synthesis gas so that desired H./CO ratios are obtained for use in both the production of hydrocarbons and the production of ammonia.

SUMMARY OF THE INVENTION

The present invention provides a novel way for overcoming problems in the operation of existing ammonia — Fischer-Tropsch integrated plants. :

The present invention relates to an optimisation process for the production of hydrocarbons and ammonia that significantly reduces the capital and operating costs for the combined processes.

In particular, the present invention provides a process for combined hydrocarbon and ammonia production which reduces emission of CO; into the atmosphere. In the process, hydrogen is extracted from a reforming section of a hydrocarbon synthesis process rather

AMENDED SHEET

® 01/09038 5 PCT/ZA00/00125 than being extracted from the tail gas stream, and is then fed into an ammonia synthesis reactor. Previously, hydrogen necessary for ammonia synthesis was produced using shift reactions which also produced large amounts of CO,, which were subsequently vented to the atmosphere. The hydrogen may be extracted from the reforming section by using known methods in the art.

According to a first aspect of the invention there is provided a process for the production of hydrocarbons and ammonia, the process including the steps of: by means of air separation means, separating air into oxygen and nitrogen; in a reforming section, reacting natural gas, steam and oxygen from the air separation means, to form synthesis gas; in a hydrogen extraction unit, extracting hydrogen from at least a portion of the synthesis gas; thereafter feeding the synthesis gas into a Fischer-Tropsch reactor in which hydrocarbons are produced from the synthesis gas, with the reforming section, the hydrogen extraction unit and the Fischer-Tropsch reactor forming part of a hydrocarbon synthesis process; and feeding at least a portion of the extracted hydrogen into an ammonia synthesis process.

The hydrogen may be extracted from the reforming section of the hydrocarbon synthesis process until a Hy/CO ratio of the synthesis gas is lower than or equal to a preselected value, the value typically being 2.5, and more preferably being 2.0 CO, may be removed from the synthesis gas prior to its entry into the Fischer-Tropsch reactor.

The synthesis gas exiting the reforming section may be split into at least two streams, the first stream entering the Fischer-Tropsch reactor and the second stream being sent to the hydrogen extraction unit. The synthesis gas may be divided into a hydrogen-rich stream and a hydrogen-poor stream in the hydrogen extraction unit, and at least a portion of the high purity hydrogen-rich stream may be fed into the ammonia synthesis process, thereby reducing or eliminating the danger of CO poisoning.

The hydrogen-poor stream may be returned to the hydrocarbon synthesis process or may be used as fuel gas. CO and/or CO, may be removed from the hydrogen-poor stream.

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® 01/09038 6 PCT/ZA00/00125

A portion of the Fischer-Tropsch tail gas may be returned to the reforming section of the hydrocarbon synthesis process.

A combined air separation means may be used for both the hydrocarbon synthesis process and the ammonia synthesis process, and similarly a combined reforming section may also be used for both the hydrocarbon synthesis process and the ammonia synthesis process.

According to yet a further aspect of the invention, there is provided a combined hydrogen synthesis plant and ammonia synthesis plant, which includes air separation means for separating air into oxygen and nitrogen; a reforming section in which natural gas, steam and oxygen from the air separation means can be reacted to form synthesis gas; means for extracting hydrogen from at least a portion of the synthesis gas; a Fischer-Tropsch reactor in which hydrocarbons can be produced from synthesis gas from which hydrogen has been removed, with the reforming section, the means for extracting hydrogen and the Fischer-Tropsch reactor forming part of a hydrocarbon synthesis plant; an ammonia synthesis plant; and means for feeding at least a portion of the hydrogen extracted from the synthesis gas into the ammonia synthesis plant.

The means for extracting hydrogen may separate a portion of the synthesis gas into a hydrogen-rich stream and a hydrogen-poor stream with at least a portion of the hydrogen- rich stream being fed into the ammonia synthesis plant.

The combined hydrocarbon synthesis plant and ammonia synthesis plant may also include means for returning the hydrogen-poor stream to the reforming section of the hydrocarbon synthesis plant.

The combined hydrocarbon synthesis plant and ammonia synthesis plant may further include means for returning at least a portion of the Fischer-Tropsch tail gas to the reforming section.

AMENDED SHEET

Means for feeding at least a portion of the extracted hydrogen into a hydroprocessing section of the hydrocarbon synthesis plant may aiso be provided.

According to yet a further embodiment of the invention there is provided a hydrocarbon produced according to the process described above.

According to yet a further embodiment of the invention there is provided ammonia produced according to the process described above.

The invention will now be illustrated further by way of the following non-limiting examples and with reference to the accompanying drawings.

In the drawings,

FIGURE 1 shows a simplified flow diagram of the process of Example 1;

FIGURE 2 shows a simplified flow diagram of the process of Example 2; and

FIGURE 3 shows a simplified flow diagram of the process of Example 3.

EXAMPLE 1 AND FIGURE 1

In Figure 1, reference numeral 10 generally indicates a process for the production of hydrocarbons and ammonia, in accordance with a first embodiment of the invention.

The process 10 includes an autothermal reformer 12, with a natural gas flow line 14 and a steam flow line 16 leading into the autothermal reformer 12, as does an oxygen line 18 which leads from an air separation plant 20.

A synthesis gas flow line 22 leads from the autothermal reformer 12 to a H, removal unit 24. A flow line 26 leads from the H, removal unit 24 to a Fischer-Tropsch reactor 28, with a flow line 30 leading from Fischer-Tropsch reactor 28 to a hydroprocessing stage 32. A diesel and naptha withdrawal line 34 leads from the hydroprocessing stage 32.

An air feed line 36 leads into the air separation plant 20, with the oxygen line 18 and a nitrogen line 38 leading from the air separation plant 20. The nitrogen line 38 leads to an ammonia synthesis process 40 as does a hydrogen line 42 which leads from the H, removal unit 24. A hydrogen line 44 leads from the hydrogen line 42 to the hydroprocessing stage 32.

AMENDED SHEET eo 01/09038 8 PCT/2ZA00/00125

An ammonia withdrawal line 46 leads from the ammonia synthesis process 40.

A Fischer-Tropsch reactor tail gas recycle 48 is also provided.

Natural gas, passing along flow line 14, is passed through one or more means (not shown) of removing sulphur compounds from the gas so that the level of sulphur no longer poses a poisoning risk for either a Fischer-Tropsch or an ammonia synthesis catalyst. The desulfurized natural gas is then combined with oxygen and steam, and reacted in the autothermal reformer 12. The oxygen is supplied from a cryogenic air separation facility or plant 20. The reformer unit 12 produces a synthesis gas with an Hx/CO ratio of greater than 2.0, the actual ratio being determined by the steam/reformable carbon ratio, operating conditions and composition of the feed gas. The hydrogen extraction unit 24 is placed on the exit stream 22 from the reformer 12 or a portion thereof so as to adjust the H,/CO ratio to a value desired for optimal hydrocarbon synthesis, typically below 2.1. The hydrogen removal step may be accomplished using, for instance, pressure swing adsorption (PSA) or a membrane unit, or any other known method. Optimally, the H extraction will be performed on a slip stream. A high purity H, product (withdrawn along the flow line 42) and a H, poor effluent stream (not shown) are produced. The effluent stream may be recompressed and returned to the synthesis gas stream at any point in the loop or it may be used as fuel gas. The synthesis gas is then sent to the Fischer-Tropsch reactor 28 that operates at a total conversion level above approximately 80%.

The excess hydrogen that has been removed above is further purified if required, compressed and sent to the ammonia synthesis process or reactor 40. Nitrogen, which is available at high purity from the cryogenic air separation unit 20 is either compressed in gaseous form or more conveniently produced in liquid form and pumped to the appropriate pressure. The nitrogen is treated to remove trace quantities of oxygen, then combined with the hydrogen and also sent to the ammonia synthesis process.

A plant of this type is used to produce 30 000 barrels per day (bpd) of liquid hydrocarbons and a further 1 000 tons per day of ammonia. Natural gas with the composition given in Table 1 is utilised for the purposes of this illustration. Process conditions may alter depending on the composition of natural gas in different situations. Process conditions will also alter depending on the relative quantities of ammonia and hydrocarbon products desired.

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® 01/09038 9 PCT/ZA00/00125

Table 2 compares the results of two individual plants operated at optimal conditions, one to give 30 000 bpd of liquid fuels and the other plant is used to produce 1 000 t/d ammonia, with the combined plant or process 10.

Plant 1 uses 300 000 m*,/h of natural gas fed to an autothermal reformer operating with a steam/reformable carbon ratio of 0.6 and an exit temperature of 1 050°C. A CO; recycle is used to adjust the H,/CO ratio of the resuiting synthesis gas to 1.90. The synthesis gas is fed directly into a Fischer-Tropsch based synthesis plant after cooling and water knock- out. 30 000 bpd of liquid fuel is produced.

Plant 2 uses 28 500 m’y/h of natural gas fed to a steam reformer operating with a steam/reformable carbon ratio of 4.0 and an exit temperature of 850°C. The steam reformer requires fuel gas to provide the heat required to drive the reaction. The resulting synthesis gas is cooled and transferred first to a high temperature shift reactor operating at 370°C, then to a low temperature shift reactor operating at 230°C. The gas is cooled and water is knocked-out from the stream. The dry gas is passed to a Benfield unit which removes the majority of CO,. The gas is then passed to a methanation reactor which converts the remaining carbon oxides to CH,. Finally, the H; rich gas is compressed and combined with N, from a cryogenic air separation plant and fed to an ammonia synthesis reactor, where 90% of the H; is converted. 1 000 t/d of ammonia is produced.

The combined plant or process 10 utilises a combination of the above process schemes. 336 700 m’/h of natural gas is fed to the autothermal reformer 12 operating with a steam/reformable carbon ratio of 0.9 and an exit temperature of 1 000°C. A similar quantity of recycled CO, is added to the feed gas as for the first plant. The resultant synthesis gas is cooled and a portion split off and transferred to a Pressure Swing

Adsorption (PSA) unit 24. The resulting H, stream (withdrawn along flow line 42) is greater than 99% pure. It is compressed and mixed with N, derived from the cryogenic air separation plant 20 and transferred to the ammonia synthesis reactor 40, where 90% of the H, is converted to ammonia. 1 000 t/d of ammonia is produced. The majority of the offgas from the PSA is recompressed and returned to the main synthesis gas stream. The combined stream is again split, with a portion of the synthesis gas stream being sent to a

Benfield unit where CO, is removed. The partially CO,.depleted synthesis gas is sent to a

Fischer-Tropsch synthesis plant 28. 30 000 bpd of liquid fuel is produced.

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® 01/09038 10 PCT/ZA00/00125

Table 1: Natural gas composition used for purpose of illustrating the invention

C, hydrocarbons

Ca. hydrocarbons o6 ~~]

Table 2: Comparison between stand alone plants and combined plant

Stand Alone Stand Alone Plant 1+ Combined

Gas-to-Liquids | Ammonia Plant 2 Plant 10

Plant Synthesis

NG Feed 300 000 28 500 328 500 337 000 m>./h

Fuel Gas 8 500 8 500 m>,/h) [note 1

Total NG Used | 300 000 37 000 337 000 337 000 m’y/h t/h

H, Used For 4 085 4 085 4 085

Ammonia

Synthesis kmol/h

Nz Used For 1362 1362 1362

NH; synthesis kmol/h

NH; Produced | 1.000 1 000 1 000 t/d

Liquid fuel 30 000 30 000 30 000

Produced (bpd

Total CO, 1467 1 467 1218

Produced kmol/h [note 3

Note 1: This includes only the additional fuel gas required to fire the tubular steam reformer.

Note 2: Although O, is not used in the reforming section, it is produced as a by-product of the N2 production.

Approximately 11 t/h of by-product O; is produced.

Note 3: The production of CO, makes the comparison between conventional ammonia production and the increase in CO; production in the combined plant over the stand alone Gas-to-Liquids plant.

The advantages of the combined plant 10 are: scale economies can be used so that less expensive synthesis gas generation can be achieved;

AMENDED SHEET

PY 01/09038 11 PCT/ZA00/00125 an air separation plant for ammonia synthesis is not required; the ammonia plant process scheme is very much simplified because pure hydrogen is produced in one step and no clean-up steps to remove carbon oxides is required. The carbon oxides are used in the Fischer-Tropsch unit; and 17% less CO; is consequently produced compared to a conventional ammonia plant.

Example 2 shows a slight modification of the process described in example 1.

EXAMPLE 2 AND FIGURE 2

In Figure 2, reference numeral 50 generally indicates a process for the production of hydrocarbons and ammonia, in accordance with a second embodiment of the invention.

Parts of the process 50 which are the same or similar to those of the process 10 of Figure 1, are indicated with the same reference numerals.

In the process 50, a steam methane reformer 52 is provided. The natural gas flow line 14 splits into two flow lines 54, 56, with the flow line 54 leading into the steam methane reformer 52 and the flow line 56 by-passing the reformer 52. A flow line 55 leads from the steam reformer 52. The flow lines 55, 56 recombine into a flow line 58 downstream of the reformer 52, with the flow line 58 leading into the autothermal reformer 12. The flow line 56 thus by-passes the steam methane reformer 52.

The process 50 includes, in addition to the internal recycle 48, also an external recycle 60 which recycles tail gas from the Fischer-Tropsch reactor 28 to upstream of the steam methane reformer 52.

Natural gas (along flow line 14) mixed with a recycle stream (along external recycle 60) from the Fischer-Tropsch reactor 28 is sent to the reforming section of a Fischer-Tropsch based hydrocarbon synthesis process or plant. The reforming section consists of the steam reformer 52 followed by the autothermal reformer 12. The natural gas stream is split, with a portion entering the steam reformer 52 (along the flow line 54), while the majority enters (along the flow line 56) the autothermal reformer 12. Hot synthesis gas from the steam reformer 52 (along the flow line 55) mixes with the bypass natural gas before entering the autothermal reformer 12.

AMENDED SHEET o 01/09038 12 PCT/ZA00/00125

The hydrogen extraction unit 24 is placed on the exit stream from the autothermal reformer 12 or a portion thereof so as to adjust the H,/CO ratio to a value below 2. The synthesis gas is then sent to the Fischer-Tropsch reactor 28 that operates at a total conversion level above 80%. Some of the Fischer-Tropsch tail gas is returned to the front of the reforming section, along the external recycle 40. This can be utilised to further adjust the H»/CO ratio as well as increasing the carbon utilisation of the process.

The excess hydrogen that has been removed is further purified, compressed and sent to the ammonia synthesis reactor 40. Nitrogen, which is available at high purity from the cryogenic air separation unit 20 is combined with the hydrogen and also sent to the ammonia synthesis loop.

EXAMPLE 3 AND FIGURE 3

In Figure 3, reference numeral 100 generally indicates a process for the production of hydrocarbons and ammonia, in accordance with a third embodiment of the invention.

Parts of the process 100, which are the same or similar to those of the process 10 of

Figure 1, and of the process 50 of Figure 2, are indicated with the same reference numerals.

The process 100 includes an external recycle 102 for recycling tail gas from the Fischer-

Tropsch reactor 28 to upstream of the autothermal reformer 12.

The process 100 does not have a H, removal unit between the autothermal reformer 12 and the Fischer-Tropsch reactor 28. Instead, it includes a shift reactor 104, with a flow line 106 leading from the flow line 55 to the shift reactor 104. A gas flow line 108 leads from the shift reactor 104 to a pressure swing adsorption unit 110, in which hydrogen removal is effected.

An offgas withdrawal line 112 leads from the unit 110 to a CO, removal stage 114, with a

CO; gas withdrawal line 116 as well as an offgas withdrawal line 118 leading from the stage 114. The line 118 leads to the autothermal reformer 12.

A hydrogen line 120 leads from the unit 110, with a hydrogen line 122 leading from the line 120 to the ammonia synthesis process or plant 40. The line 120 leads to the hydroprocessing stage 32.

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® 01/09038 13 PCT/ZA00/00125

Natural gas (along flow line 14) mixed with a recycle stream (along external recycle 102) from the Fischer-Tropsch reactor 28 is sent to the reforming section of the Fischer-Tropsch based hydrocarbon synthesis process or plant. The reforming section consists of the steam reformer 52 followed by the autothermal reformer. The natural gas stream is split, with a portion entering the steam reformer 52 along the flow line 54, while the majority enters the autothermal reformer 12 along the flow line 56. The synthesis gas from the steam reformer 52 is cooled and sent to the shift reactor 104 which converts most of the

CO and H,O into CO, and H,. The H; is then separated from the other gases in the unit 110. The hydrogen poor offgas (consisting predominantly of CO.) may be compressed and returned to the front end of the autothermal reformer, along the line 118. The split ratio between the steam reformer 52 and autothermal reformer 12 is adjusted so that the

H./CO ratio of the synthesis gas leaving the autothermal reformer is below 2.1. The synthesis gas is then sent to the Fischer-Tropsch reactor 28 that operates at a total conversion level above 80%. Some of the Fischer-Tropsch tailgas is returned to the front of the reforming section. This can be utilised to further adjust the H./CO ratio as well as increasing the carbon utilisation of the process.

The excess hydrogen that has been removed is further purified, compressed and sent to the ammonia synthesis reactor or plant 40, along the flow line 122. Nitrogen, which is available at high purity from the cryogenic air separation unit 20, is combined with the hydrogen and also sent to the ammonia plant 40.

A plant of this type is used to produce 30 000 barrels per day (bpd) of liquid hydrocarbons and a further 1 000 tons per day of ammonia. Natural gas with the composition given in

Table 1 of example 1 is again utilised for the purposes of this illustration. Process conditions may alter depending on the composition of natural gas in different situations.

Process conditions will also alter depending on the relative quantities of ammonia and hydrocarbon products desired. 328 000 m*/h of natural gas is fed to the reforming section of a combined hydrocarbon synthesis and ammonia synthesis plant 100 as exemplified above. 29 000 m’/h is sent to the steam reformer 52, while the rest bypasses the steam reformer. The majority of the product stream from the steam reformer is sent (along the lines 55, 106) to a water gas shift converter (WGSC) 104 operating at 230°C, which converts almost all of the CO into

H, and CO, by reaction with water. The product stream is dried and sent to the hydrogen

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® 01/09038 14 PCT/ZA00/00125 recovery unit 110. The offgas from the H, recovery unit 110 is split, with approximately 50% passing through the CO, adsorption unit 114. The remaining stream (not shown) is recompressed, if required, and mixed with the remaining natural gas. This, together with a recycle stream (along the recycle 102) from the Fischer-Tropsch reactor 28 is fed into the autothermal reforming reactor 12. This reformer is operated at a steam/reformable carbon ratio of 0.6, and an exit temperature of 1 000°C. The product from this step is fed the

Fischer-Tropsch reactor 28. Table 3 shows the results from this type of plant

Table 3: Comparison between stand alone plants and combined plant

Stand Alone Stand Alone Plant 1+ Combined

Gas-to-Liquids | Ammonia Plant 2 Plant 100

Plant Synthesis

NG Feed (m“,/h) | 300 000 28 500 328 000 328 000

Fuel Gas (m°./h) 8 500 8 500 8 705 note 1

Total NG Used 300 000 37 000 337 000 336 705 m-/h t/h

H, Used For 4 085 4 085 4 085

Ammonia

Synthesis kmol/h

Nz Used For NH; 1362 1362 1362

Synthesis kmol/h

NH; Produced | 1 000 1 000 1 000 t/d

Liquid Fuel 30 000 30 000 30 000

Produced (bpd

Total CO; 1467 1467 1352

Produced kmol/h note 3

Note 1: This includes only the additional fuel gas required to fire the tubular steam reformer.

Note 2: Although O; is not used in the reforming section, it is produced as a byproduct of the Nz production.

Approximately 11 t/h of byproduct O; is produced.

Note 3: The production of CO; makes the comparison between conventional ammonia production and the increase in CO; production in the combined plant over the stand alone Gas-to-Liquids plant.

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® 01/09038 15 PCT/ZA00/00125

The advantages of this process scheme are that less oxygen is required for the combined plant than for even the stand-alone Gas-to-Liquid plant, with 7% less oxygen required than for the sum of the individual stand alone plants. Also 7% less CO; is emitted to the atmosphere compared with the two stand-alone plants. This translates to more than 40 000 tonnes fewer CO, emissions per annum.

The applicant believes that the invention is advantageous in that apart from the high total conversion obtained, substantial economic benefits also result, for example: 1) a separate reforming section for an ammonia plant is not required; 2) H, can be produced in excess of the requirement of hydrocarbon synthesis, and the excess H; can be cost-efficiently used in ammonia production; 3) an H, stream is extracted from the synthesis gas stream and thus CO, which would be vented to atmosphere in a conventional ammonia facility remains in the Fischer-

Tropsch loop, and is further used to adjust the synthesis gas ratio to the desired value; and 4) conventional Fischer-Tropsch processes that aim to produce motor fuels additionally require a hydroprocessing section to upgrade the primary Fischer-

Tropsch products into diesel. This requires a hydrogen source that incurs additional capital expenditure. However, the present invention does not require this additional hydrogen source, thus resulting in a substantial monetary saving. In addition, the operating pressure of the hydroprocessing section is greater than 70 bar, and the operating pressure of the ammonia synthesis loop is greater than 100 bar. Hydrogen can be removed after the second stage of the ammonia feed compressor and sent to the hydroprocessing section, thereby eliminating the expensive feed compressor from the hydroprocessing section of the GTL plant.

The capital and operating costs for the combined processes are therefore significantly reduced.

The invention is not limited to the precise constructional details as herein described.

AMENDED SHEET

Claims (27)

Claims

1. A process for the production of hydrocarbons and ammonia, the process including the steps of: by means of air separation means, separating air into oxygen and nitrogen; in a reforming section, reacting natural gas, steam and oxygen from the air separation means, to form synthesis gas; in a hydrogen extraction unit, extracting hydrogen from at least a portion of the synthesis gas, thereafter feeding the synthesis gas into a Fischer-Tropsch reactor in which hydrocarbons are produced from the synthesis gas, with the reforming section, the hydrogen extraction unit and the Fischer-Tropsch reactor forming part of a hydrocarbon synthesis process; and feeding at least a portion of the extracted hydrogen into an ammonia synthesis process.

2. A process as claimed in claim 1, wherein the hydrogen is extracted from the reforming section until a H,/CO ratio of synthesis gas fed to the Fischer-Tropsch reactor is lower than or equal to 2.5.

3. A process as claimed in claim 2, wherein the H,/CO ratio is lower than or equal to 2.

4. A process as claimed in any one of claims 1 to 3, wherein only a portion of the synthesis gas is sent to the hydrogen extraction unit and is divided into a hydrogen-rich stream, at least a portion of which is fed into the ammonia synthesis process, and a hydrogen-poor stream.

5. A process as claimed in claim 4, wherein the hydrogen-poor stream is returned to the hydrocarbon synthesis process.

6. A process as claimed in either of claims 4 or 5, wherein the hydrogen-poor stream is used as fuel gas.

7. A process as claimed in any one of claims 4 to 6, wherein CO and/or CO; are removed from the hydrogen-poor stream. AMENDED SHEET

® 01/09038 17 PCT/ZA00/00125

8. A process as claimed in any one of claims 1 to 7, wherein at least a portion of CO, is removed from the synthesis gas stream prior to the synthesis gas stream entering the Fischer-Tropsch reactor.

9 A process as claimed in any one of claims 1 to 8, wherein a portion of a Fischer-Tropsch tail gas is returned to the reforming section of the hydrocarbon synthesis process.

10. A process as claimed in any one of claims 1 to 9, wherein one air separation means is used for both the hydrocarbon synthesis process and the ammonia synthesis process.

11. A process as claimed in any one of claims 1 to 10, wherein one reforming section is used for both the hydrocarbon synthesis process and the ammonia synthesis process.

12. A hydrocarbon produced according to the process described in any one of claims 1 to 11.

13. A diesel product produced according to the process described in any one of claims 1 to 11.

14. A naptha product produced according to the process described in any one of claims 1 to 11.

15. Ammonia produced according to the process described in any one of claims 1to 11.

16. A combined hydrogen synthesis plant and ammonia synthesis plant, which includes air separation means for separating air into oxygen and nitrogen; a reforming section in which natural gas, steam and oxygen from the air separation means can be reacted to form synthesis gas; means for extracting hydrogen from at least a portion of the synthesis gas; a Fischer-Tropsch reactor in which hydrocarbons can be produced from synthesis gas from which hydrogen has been removed, with the reforming section, the means for AMENDED SHEET

. ® 01/09038 18 PCT/ZA00/00125 extracting hydrogen and the Fischer-Tropsch reactor forming part of a hydrocarbon synthesis plant; an ammonia synthesis plant; and means for feeding at least a portion of the hydrogen extracted from the synthesis gas into the ammonia synthesis plant.

17. A combined hydrocarbon synthesis plant and ammonia synthesis plant as claimed in claim 16, wherein the air separation means is shared by both the hydrocarbon synthesis and ammonia synthesis plants.

18. A combined hydrocarbon synthesis plant and ammonia synthesis plant as claimed in either of claims 16 or 17, wherein the reforming section is shared by both the hydrocarbon synthesis and ammonia synthesis plants.

19. A combined hydrocarbon synthesis plant and ammonia synthesis plant as claimed in any one of claims 16 to 18, wherein the means for extracting hydrogen separates a portion of the synthesis gas into a hydrogen-rich stream and a hydrogen-poor stream, with at least a portion of the hydrogen-rich stream being fed into the ammonia synthesis plant.

20. A combined hydrocarbon synthesis plant and ammonia synthesis plant as claimed in claim 19, which includes means for returning the hydrogen-poor stream to the hydrocarbon synthesis plant.

21. A combined hydrocarbon synthesis plant and ammonia synthesis plant as claimed in any one of claims 16 to 20, which includes means for returning at least a portion of the hydrocarbon synthesis tail gas to the reforming section.

22. A combined hydrocarbon synthesis plant and ammonia synthesis plant as claimed in any one of claims 16 to 21, which includes means for feeding at least a portion of the extracted hydrogen into a hydroprocessing section of the hydrocarbon synthesis plant.

23. A process for the production of hydrocarbons and ammonia according to the invention, substantially as hereinbefore described and exemplified. AMENDED SHEET

® 01/09038 19 PCT/2ZA00/00125

24. A hydrocarbon, diesel product, naptha product or ammonia as claimed in any one of claims 12 to 15, substantially as hereinbefore described and exemplified.

25. A combined hydrocarbon synthesis plant and ammonia synthesis plant according to the invention, as hereinbefore generally described.

26. A combined hydrocarbon synthesis plant and ammonia synthesis plant as specifically described with reference to or as illustrated in the accompanying examples.

27. A process for the production of hydrocarbons and ammonia, a hydrocarbon, a diesel product, a naptha product, ammonia, or a combined hydrocarbon synthesis plant and ammonia synthesis plant including any new and inventive integer or combination of integers, substantially as herein described. AMENDED SHEET