WO2018019512A1

WO2018019512A1 - Syngas production

Info

Publication number: WO2018019512A1
Application number: PCT/EP2017/066270
Authority: WO
Inventors: Niels Ulrik Andersen; Anne Krog JENSEN; Peter Mølgaard Mortensen; Steffen Spangsberg Christensen; Tommy Lykke WIND
Original assignee: Haldor Topsøe A/S
Priority date: 2016-07-29
Filing date: 2017-06-30
Publication date: 2018-02-01

Abstract

The invention relates to a system for the production of a synthesis gas. The system comprises: -a pressure swing adsorption unit (PSA unit) arranged to receive a heavy process gas comprising higher hydrocarbons and/or alcohols and one or more of the flowing gas components: methane, hydrogen, carbon monoxide, carbon dioxide, said PSA unit being arranged to separate the components of said heavy process gas into a first process gas and a tail gas, where most of the higher hydrocarbons of the heavy process gas are in the tail gas, -a reformer unit downstream the pressure swing adsorption unit and downstream the pre-reformer unit, said reformer unit being arranged to receive the first process gas and a second process gas for reforming thereof. The invention also relates to a corresponding process.

Description

Title: Syngas production

FIELD OF THE INVENTION

Embodiments of the invention generally relate to a system and a process for the production of a synthesis gas from a process gas comprising higher hydrocarbons and one or more of the following gas components: methane, hydrogen, carbon monoxide, and carbon dioxide.

BACKGROUND

Many industrial plants, e.g. those producing Gas To Liquid (GTL) products such as lubrication oils, synthetic transport fuels or higher alcohols, apply synthesis gas, consisting of mainly hydrogen (H₂) and carbon monoxide (CO), as the main feedstock. This synthesis gas, typically having a specific H₂/CO ratio, may be produced in a syngas generation reformer, such as a SMR (steam methane reformer), SPARG SMR (sulfur passiv- ated reformer) or ATR (autothermal reformer). Due to selectivity issues, the GTL reactor downstream the syngas generation reformer will produce some undesired byproducts that must be purged to maintain the desired process conditions in the GTL reactor. Such a purge gas typically comprises a mixture of CO, CO2, CH₄, H2, higher hydrocarbons (olefins/paraffins) and possibly hydrogen sulfide and/or alcohols. To ensure high carbon efficiency, it is advantageous to recycle the heavy process gas back to the syngas generation reformer to supplement the carbon rich feedstock, e.g. natural gas, supplied to the syngas generation reformer.

However, the catalytic material, e.g. typically nickel catalyst material, provided in the syngas generation unit, can only tolerate low concentrations of higher hydrocarbons (especially higher alcohols, higher olefins and to some extent higher paraffins) since carbon may be formed on the catalysts leading to either increased pressure drop over the catalyst bed or loss of catalytic activity.

For a syngas generation unit utilizing an ATR, the problem of metal dusting may be in- duced by olefins (unsaturated higher hydrocarbons).

Finally, if the heavy process gas contains sulfur components, such as H₂S, nickel reforming catalysts will quickly be poisoned and deactivate if exposed directly to the heavy process gas.

Therefore, traditionally such heavy process gasses from the industrial plant have been treated with special means to handle the higher hydrocarbons, higher alcohols and/or H₂S: a) H₂S must be removed by a sulfur guard, e.g. a sulfur absorber in the form of ZnO, to avoid sulfur poisoning.

b) Olefins must be hydrogenated in a separate reactor to reduce the risk of carbon accumulation on downstream nickel catalysts.

c) Higher paraffins and higher alcohols must be converted in a pre-reformer which will require relatively high steam to carbon ratio to avoid carbon formation.

For a heavy process gas comprising higher hydrocarbons and/or sulfur components and/or higher alcohols, e.g. for a recycled purge gas from a GTL plant, the above desul- furization, hydrogenation and pre-reforming reactors will have to operate at non- standard conditions and therefore it becomes relatively expensive to handle the aggressive heavy process gas. SUMMARY OF THE INVENTION

One embodiment of the invention relates to a system for the production of a synthesis gas from a heavy process gas. The system comprises:

- a pressure swing adsorption unit (PSA unit) arranged to receive a heavy process gas comprising higher hydrocarbons and/or alcohols and one or more of the following gas components: methane, hydrogen, carbon monoxide, carbon dioxide, where the PSA unit is arranged to separate the components of the heavy process gas into a first process gas and a tail gas, where the tail gas comprises substantially all of the higher hy- drocarbons and/or higher alcohols of the heavy process gas,

- a reformer unit downstream the pressure swing adsorption unit, where the reformer unit is arranged to receive the first process gas and a methane rich second process gas for reforming thereof. In this system two feed gasses are provided to the reformer unit: a heavy process gas, e.g. a recycle gas from an industrial plant, e.g. a GTL plant, and a methane rich second process gas.

In comparison with the traditional preconditioning of a heavy process gas, the inven- tion provides a simpler and cheaper solution for removing the higher hydrocarbons by a pressure swing adsorption unit (PSA unit) prior to reforming in a syngas reformer unit. By treating the heavy process gas from e.g. a GTL plant in a PSA unit, the treated gas becomes purified with respect to the higher hydrocarbons (down to ppm level) and may subsequently be sent directly to the reformer unit - together with a methane rich gas, viz. the second process gas - for production of synthesis gas. Since the first process gas comprises methane, hydrogen, carbon monoxide and/or carbon dioxide, the first process gas is well suited for reforming in the reforming unit.

If the higher hydrocarbons, higher olefins and/or higher alcohols of the heavy process gas were not removed in the PSA, then the higher hydrocarbons/olefins/alcohols would have to be removed in another way. Pre-reforming would be the most likely candidate for removal of higher hydrocarbons in a process gas for reforming, but large amounts of water are needed to convert such higher hydrocarbons without risking carbon formation. The current invention therefore gives the benefit that the co-feed of water to the reforming section can be decreased.

The PSA unit itself may be a well-known PSA unit arranged for adsorbing higher hydrocarbons. Typically, more than 99% (on molar basis) of the higher hydrocarbons in the heavy process gas entering into the PSA unit are separated into the tail gas. Typically, less than 0.1 mol% higher hydrocarbons are in the second process gas led to the reformer unit.

Even though a PSA unit is added upstream the reformer unit, the system is still advantageous compared to a system where the pretreatment section (sulfur guard, hydro- genation reactor and pre-reformer) should be designed for processing higher hydrocarbons in a heavy process gas. The pretreatment section may thus be smaller than in a case where this unit should precondition a process gas comprising higher hydrocarbons and/or alcohols and/or hydrogen sulfide, e.g. a recycle gas from a GTL plant, prior to letting it into a reformer.

The term "heavy process gas" is meant to denote a process gas comprising a mixture of one or more of the following gasses: CO, C0₂, CH₄, and H₂, as well as higher hydrocarbons (higher olefins/higher paraffins) and possibly hydrogen sulfide and/or higher alcohols. As used herein, the term "higher hydrocarbons" is meant to denote hydrocar- bons having two or more carbon atoms, such as ethane, ethene, propane, propene, butane, butane, etc. Moreover, the term "higher alcohols" is meant to denote alcohols having two or more carbon atoms, such as ethanol, propanol, butanol, etc. The term "substantially all the higher hydrocarbons and/or higher alcohols are in the tail gas" is meant to denote that after removing the tail gas from the heavy process gas, the re- maining first process gas contains a very small amount of higher hydrocarbons and/or higher alcohols, such as less than 1-2 vol%, but preferably less than a few 100s ppmv or even less than 100 ppmv.

In an embodiment, the PSA unit is also arranged to separate any sulfur comprising gas- ses in said heavy process gas into said tail gas. This is in particular advantageous when the reformer unit is a steam methane reformer unit or an ATR.

In an embodiment, the reformer unit is a fired steam methane reformer (SMR reformer) having one or more burners. In this case, heated steam is added to the gas to be reformed, either upstream the SMR reformer or into the SMR reformer.

In another embodiment, the reformer unit is a sulfur assisted reformer (SPARG reformer) having one or more burners. In this case, heated steam may be added to the gas to be reformed. This addition of heated steam to the second process gas will typi- cally take place upstream the SPARG reformer. Moreover, a gas comprising sulfur is added to the SPARG reformer for avoiding carbon formation at the catalyst of the SPARG reformer.

In an embodiment, the system comprises a steam methane reformer or a SPARG re- former. In an embodiment, the tail gas is input for use as a fuel gas for the one or more burner(s) of the fired steam methane reformer or the SPARG reformer. Hereby, the combustion value of the tail gas is used.

In an embodiment, the reformer unit is an autothermal reformer (ATR). In this case, the tail gas is typically purged, or, for example, is used in a fired heater to utilize the heating value.

In an embodiment, the system further comprising a pre-reformer unit for receiving a hydrocarbon rich second process gas, where the pre-reformer unit is arranged for pre- reforming the hydrocarbon rich second process gas to a methane rich second process gas, and where the methane rich second process gas is led to the reformer unit.

In this embodiment, the methane rich second process gas has undergone pre-reform- ing in a pre-reformer unit prior to entering the reformer, whilst the content of higher hydrocarbons in the heavy process gas is reduced considerably in the PSA unit upstream to the reformer, before the resultant first process gas is led to the reformer unit. The reformer unit is thus arranged for reforming the first process gas together with the pre-reformed second process gas. Heated or superheated steam is typically added upstream the pre-reformer unit or into the pre-reformer. In a case, where the system comprises a steam methane reformer or a SPARG reformer, additional heated or superheated steam may be added upstream the steam methane reformer or SPARG reformer or into the steam methane reformer or SPARG reformer. In conclusion, pre-reforming is only performed on the second process gas and the size of the pre-reformer is therefore significantly decreased compared to a case where pre- reforming of the combined first and second process gasses has to be carried out. Thus, even though a PSA unit is added upstream the reformer unit, the system is still advantageous compared to a system where the pretreatment section (sulfur guard, hydro- genation reactor, and pre-reformer) should be designed for processing higher hydrocarbons and/or higher alcohols and/or sulfur components in a heavy process gas. The pretreatment section may thus be smaller than in a case where such a pretreatment section should precondition a process gas comprising higher hydrocarbons and/or higher alcohols and/or hydrogen sulfide, e.g. a recycle gas from a GTL plant, prior to letting it into a reformer.

By removal of the higher hydrocarbons and/or higher alcohols in the PSA and sending the first process gas directly to the reformer unit instead of adding the heavy process gas, e.g. a recycle gas, upstream the pre-reformer together with the second process gas, the requirements of steam to the pre-reformer is reduced significantly. Overall, this concept will enable operation of the reformer unit at a lower steam to hydrocarbon-carbon ratio which both reduces the size of the pretreatment section and reduces the operating costs. Additionally, this enables better control of the H2/CO ratio of the produced synthesis gas.

In an embodiment, the system further comprises a sulfur removal unit upstream the pre-reformer unit. This is relevant in case the methane rich gas to be fed to the pre-re- former unit contains sulfur in order to protect catalyst within the pre-reformer from sulfur.

Another aspect of the invention relates to a process for production of a synthesis gas from a heavy process gas comprising higher hydrocarbons and one or more of the following gas components: methane, hydrogen, carbon monoxide, carbon dioxide. The process according to the invention has similar advantages as the system according to the invention and will not be described further here.

The following is a detailed description of embodiments of the invention depicted in the accompanying drawings. The embodiments are examples and are in such detail as to clearly communicate the invention. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. BRIEF DESCRIPTION OF THE DRAWING

Figure 1 illustrates a schematic view of an embodiment of the system according to the invention. DETAILED DESCRIPTION OF THE DRAWING

Figure 1 shows a system 100 for the production of a synthesis gas from a heavy process gas 11 comprising higher hydrocarbons and/or higher alcohols and one or more of the following gas components: methane, hydrogen, carbon monoxide, carbon dioxide, nitrogen, and from a second process gas 21, 22, 32. The system 100 comprises a pressure swing adsorption unit (PSA unit) 10 arranged to receive the heavy process gas 11 and separate the components thereof into a first process gas 12 and a tail gas 13, where substantially all of the higher hydrocarbons and/or higher alcohols of the heavy process gas 11 are in the tail gas 13. The first process gas 12 thus mainly consists of methane, hydrogen, carbon monoxide, carbon dioxide and/or nitrogen. However, if the heavy process gas 11 comprises other components, that are not higher hydrocarbons, higher alcohols or sulfur comprising gas, e.g. an inert gas, such other components will be in the first process gas.

The first process gas 12 outlet from the PSA unit is a gas that contains very low concentrations of higher hydrocarbons and/or higher alcohols and/or sulfur comprising gas- ses. The system 100 comprises a reformer unit 20 downstream the pressure swing adsorption unit 10 and the first process gas 12 is sent directly to a reformer unit 20 with- out further treatment, for reforming thereof. The tail gas 13 from the PSA unit may be utilized as fuel in the reformer or it may be disregarded.

In the embodiment shown, the reformer unit 20 is a fired steam methane reformer (SMR reformer) having one or more burners. A fuel stream 36 is led to the burners; in this example the tail gas 13 may be supplied to the burners of the SMR reformer 20 as a secondary fuel stream as indicated by the dotted line in the figure. This is also the case if the reformer unit 20 of the system 100 is a SPARG reformer. However, in the case where the reformer unit of the system is an ATR unit, the tail gas from the PSA unit may be discarded. In the case, where the reformer 20 is a SPARG reformer, a stream 15 of heated or superheated steam and a sulfur containing stream 18 are added to the SPARG reformer 20 or to the first process gas 12 upstream of the SPARG reformer 20 in order to assist in the reforming. In the case where the reformer 20 is a steam methane reformer, a stream 15 of heated or superheated steam may be added to the reformer 20 and/or to the first process gas 12 upstream of the reformer. In this case, no sulfur stream 18 is added. In the case where the reformer unit 20 is an autothermal reformer, steam may be added upstream the prereformer, if it is present, or upstream the ATR reformer 20. The system 100 shown in figure 1 also comprises an optional desulfurization unit 25, also denote "sulfur removal unit" and an optional pre-reformer unit 30. In the case where the second process gas introduced into the system 100 is a methane rich second process gas 32 without neither sulfur, higher hydrocarbons nor higher alcohols, the methane rich second process gas 32 may be inlet directly into the reformer unit 20. However, in the case where the second process gas is a hydrocarbon rich second process gas 22 containing methane and higher hydrocarbons or higher alcohols, the hydrocarbon rich second process gas 22 is led through a pre-reformer unit 30 for pre-re- forming the hydrocarbon rich second process gas 22 to a methane rich second process gas 32 prior to reforming in the reformer unit 20. Moreover, if the second process gas 21 contains more than trace amounts of sulfur, it is advantageous that the system 100 comprises a sulfur removal unit or desulfurization unit 25. This sulfur removal unit 25 is arranged to remove substantially all sulfur components of the second process gas 21. It is also conceivable that the system 100 comprises a sulfur removal unit 25, but no pre-reformer unit 30 (not shown in figure 1).

Heated or superheated steam 15 may be introduced into the pre-reformer unit 30 or into the hydrocarbon rich second process gas 22 upstream the pre-reformer unit 30 (not shown in figure 1). Additional heated or superheated gas may be introduced into the reformer unit 20, in order to adjust the steam/carbon ratio within the reformer unit 20 or the composition of the resulting product gas 40. The second process gas 32 led to the reformer unit 20 is thus a methane rich second process gas 32. In the case that the second feed stream led to the system 100 was originally a methane rich second process gas 32 without sulfur and without higher hydro- carbons or higher alcohols, the sulfur removal unit 25 and the pre-reformer 30 would be superfluous. However, in order to ensure that the second process gas 32 reaching the reformer unit 20 is substantially without sulfur, higher hydrocarbons and higher alcohols, the sulfur removal unit may be included for second process gasses 21 with sulfur and/or the pre-reformer unit 30 may be added for second process gasses 22 with higher hydrocarbons and/or higher alcohols.

The result of the reforming in the reformer unit 20 is a product gas 40. This product gas 40 may subsequently undergo further purification and conditioning steps, e.g. removal of C0₂ and/or other gas components in a further PSA unit, a water gas shift operation and/or changing the ratio between CO and H₂ in the gas, e.g. in a membrane unit.

If a system without a PSA unit 10 was to be used for a heavy process gas 11 comprising higher hydrocarbons, customized and costly desulfurization and/or hydrogenation and/or pre-reforming treatment of the heavy process gas would be needed before it could be introduced to the reforming unit. In the system 100, the desulfurisation unit 25 and the pre-reformer unit 30 may be considerably smaller when the heavy process gas is led through a PSA unit compared to a case where the heavy process gas comprising higher hydrocarbons were to be led through the desulfurization unit 25 and the pre-reformer unit 30. EXAMPLE:

As a specific example, synthesis production for a GTL plant for producing mixed alcohols is described below. Reference numbers refer to figure 1. A heavy process gas 11 recycled from the GTL plant for producing mixed alcohols is used as feed for the system of the invention. The composition of this heavy process gas is shown in Table 1, in the column titled "Heavy process gas".

The heavy process gas 11 is separated into a first process gas 12 and a tail gas 13. The compositions of the first process gas and the tail gas are also shown in Table 1.

The first process gas 12 is to be led to the reformer unit 20, whilst the tail gas 13 may be used as secondary fuel for burner(s) of the reformer unit 20 to drive the endother- mic steam reforming, or it may be purged. As seen in table 1, the first process gas 12 only comprises totally 50 ppmv of higher hydrocarbons. This corresponds to 99.6% removal of the higher hydrocarbons from the first process gas. This purification step renders it possible to utilize the first process gas 12 from the PSA unit 10 directly in the reformer unit and thus avoiding customized and costly desulfurization/hydrogena- tion/pre-reforming treatment of the heavy process gas 11.

Table 1:

While the invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant's general inventive concept.

Claims

CLAIMS:

1. A system for the production of a synthesis gas, said system comprising:

- a pressure swing adsorption unit (PSA unit) arranged to receive a heavy process gas comprising higher hydrocarbons and/or higher alcohols and one or more of the following gas components: methane, hydrogen, carbon monoxide, carbon dioxide, said PSA unit being arranged to separate the components of said heavy process gas into a first process gas and a tail gas, where substantially all of said higher hydrocarbons and/or higher alcohols of the heavy process gas are in the tail gas,

- a reformer unit downstream the pressure swing adsorption unit, said reformer unit being arranged to receive the first process gas and a methane rich second process gas for reforming thereof.

2. A system according to claim 1, wherein PSA unit is also arranged to separate any sul- fur comprising gasses in said heavy process gas into said tail gas.

3. A system according to claim 1 or 2, wherein said reformer unit is a fired steam methane reformer (SMR reformer) having one or more burners.

4. A system according to claim 1 or 2, wherein said reformer unit is a sulfur assisted reformer (SPARG reformer) having one or more burners.

5. A system according to claim 3 or 4, wherein said tail gas is input for use as a fuel gas for said one or more burner(s).

6. A system according to claim 1 or 2, wherein said reformer unit is an autothermal reformer (ATR).

7. A system according to any of the claims 1 to 6, said system further comprising a pre- reformer unit for receiving a hydrocarbon rich second process gas, said pre-reformer unit being arranged for pre-reforming said hydrocarbon rich second process gas to a methane rich second process gas, where said methane rich second process gas is led to said reformer unit.

8. A system according to claim 7, said system further comprising a sulfur removal unit upstream said pre-reformer unit.

9. A process for production of a synthesis gas, said process comprising the steps of:

- providing a heavy process gas comprising higher hydrocarbons and/or alcohols and one or more of the following gas components: methane, hydrogen, carbon monoxide, carbon dioxide into a pressure swing adsorption unit,

- in said pressure swing adsorption unit, separating the components of the recycled process gas into a first process gas and a tail gas, where substantially all of the higher hydrocarbons and alcohols of the recycled process gas are in the tail gas,

- providing said first process gas to a reformer unit,

- in said reformer unit, reforming said first process gas and a methane rich second process gas into a product gas.

10. A process according to claim 9, wherein the step of separating also comprises sepa- rating any sulfur comprising gasses in said heavy process gas into said tail gas.

11. A process according to claim 9 or 10, wherein the reformer unit is a fired steam methane reformer (SM R) having one or more burners.

12. A process according to claim 9 or 10, wherein the reformer unit is a sulfur assisted reformer (SPARG) having one or more burners.

13. A process according to claim 11 or 12, wherein the method comprises the step of using the tail gas as a fuel gas for said one or more burner(s).

14. A process according to claim 9 or 10, wherein the reformer unit is an autothermal reformer (ATR).

15. A process according to any of the claims 9 to 14, further comprising the step of: - in a pre-reformer, pre-reforming a hydrocarbon rich second process gas to a methane rich second process gas, and

- providing said methane rich second process gas to the reformer unit for reforming.

16. A process according to claim 15, further the step of:

- providing the hydrocarbon rich second process gas to a sulfur removal unit upstream said pre-reformer unit.