Process for start-up of an autothermal reformer

Abstract

The invention relates to a process for the start-up of an autothermal reformer, wherein syngas is produced in the autothermal reformer during start-up through steam reforming. To facilitate autoignition in the autothermal reformer reactor of the autothermal reformer, the reformed syngas is recycled to an upstream section of the autothermal reformer reactor and is mixed with process steam and a hydrocarbon containing process stream. As soon as a minimum hydrogen threshold concentration at the upstream section of the autothermal reformer reactor is reached in the mixed process stream, oxygen is added to the burner of the ATR reactor to obtain autoignition of the mixed process stream. Due to the process of the invention, an external hydrogen source for facilitating autoignition of the mixed stream can be omitted. The invention further relates to a plant configured to carry out the process of the invention.

Claims

1. A process for the start-up of an autothermal reformer, the autothermal reformer comprising an autothermal reformer reactor with a burner, the process comprising: heating an inert gas by an inert gas heating means, and introducing the heated inert gas in a circulation loop containing the autothermal reformer reactor; heating a mixture of inert gas and process steam by a mixture heating means, and introducing the heated mixture of inert gas and process steam in the circulation loop containing the autothermal reformer reactor; introducing heated process steam in the circulation loop as soon as a minimum outlet threshold temperature of the autothermal reformer reactor is reached, wherein inert gas is removed from the circulation loop; heating a hydrocarbon containing process stream by a hydrocarbon stream heating means, and introducing the heated hydrocarbon containing process stream into the autothermal reformer reactor, whereby a syngas stream containing carbon oxides and hydrogen is produced in the autothermal reformer reactor through steam reforming; cooling the produced syngas stream to separate water from the syngas stream as process condensate, whereby a dry syngas stream is obtained; recycling a fraction of the dry syngas stream from a downstream section of the autothermal reformer reactor to an upstream section of the autothermal reformer reactor; whereby a mixed process stream is obtained at the upstream section of the autothermal reformer reactor, the mixed process stream containing the process steam, the hydrocarbon containing process stream and the recycled dry syngas stream; introducing the mixed process stream to the burner of the autothermal reformer reactor; introducing an oxygen containing process stream to the burner of the autothermal reformer reactor as soon as a minimum hydrogen threshold concentration in the mixed process stream at the upstream section of the autothermal reformer reactor is reached; whereby autoignition of the mixed process stream is obtained; and operating the autothermal reformer to produce syngas through autothermal reforming.

2. The process according to claim 1, wherein the minimum hydrogen threshold concentration in the mixed process stream is up to 15.0 mol-% on a wet basis.

3. The process according to claim 1, wherein the minimum outlet threshold temperature of the autothermal reformer reactor is between 500° C. to 650° C.

4. The process according to claim 1, wherein the fraction of the dry syngas stream is compressed by means of a gas compressor prior to being introduced into the autothermal reformer reactor.

5. The process according to claim 4, wherein the hydrocarbon containing process stream is compressed by the gas compressor.

6. The process according to claim 1, wherein the fraction of the dry syngas stream is compressed by means of a steam ejector prior to being introduced into the autothermal reformer reactor.

7. The process according to claim 6, wherein the hydrocarbon containing process stream is compressed by the steam ejector.

8. The process according to claim 6, wherein the heated process steam is used as a motive fluid for the steam ejector.

9. The process according to claim 1, wherein the fraction of dry syngas recycled from the downstream section of the autothermal reformer reactor to the upstream section of the ATR is 40 to 70% of the volumetric flow of the dry syngas stream.

10. The process according to claim 1, wherein the fraction of dry syngas recycled to the upstream section of the autothermal reformer reactor is 55 to 65% of the volumetric flow of the mixed process stream obtained at the upstream section of the ATR.

11. The process according to claim 1, wherein the mixed process stream is heated to 500 to 650° C. before introducing it to the main burner of the autothermal reformer reactor.

12. The process according to claim 1, wherein the autothermal reformer reactor is operated at 950° C. to 1050° C. after autoignition to produce syngas through autothermal reforming.

13. The process according to claim 1, wherein the autothermal reformer is operated in series with a steam reformer, whereby syngas is produced by the autothermal reformer through autothermal reforming, and syngas is produced by the steam reformer through steam reforming.

14. The process according to claim 13, wherein the syngas produced by the steam reformer is added to the upstream section of the autothermal reformer, whereby the mixed process stream contains the process steam, the hydrocarbon containing process stream, the recycled dry syngas stream and the added syngas produced by the steam reformer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

(2) FIG. 1 depicts a simplified schematic representation of a process or plant 10 according to a first embodiment of the invention and

(3) FIG. 2 depicts a simplified schematic representation of a process or plant 20 according to a second embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(4) In the figures and the accompanying description, equivalent elements are each provided with the same reference marks.

(5) FIG. 1 shows a simplified schematic of a first specific embodiment of the invention. The process or plant 10 comprises a circulation loop, which comprises a gas compressor 11, a desulphurisation unit 12, a pre-reformer 13, an autothermal reformer 14, the autothermal reformer 14 comprising an ATR reactor and a burner (not shown), and a cooling unit 15, whereby the aforementioned elements are fluidly interconnected. Nitrogen is introduced to the circulation loop via conduit 20, process steam via conduits 21 and 22, oxygen via conduit 23 and a natural gas (feedstock) stream via conduit 24.

(6) In an initial process step, nitrogen as an inert carrier gas is pre-heated by a fired heater (not shown) and introduced to the circulation loop via conduit 20. The circulation loop is entirely flushed with nitrogen, before process steam, pre-heated by a fired heater (not shown) is introduced via conduit 22 to autothermal reformer 14. Thereby a heated mixture of process steam and nitrogen in the circulation loop results. As soon as a minimum outlet threshold temperature of the ATR reactor of the autothermal reformer 14 is reached, such as 500° C. or more, supply of nitrogen is stopped. As a result, nitrogen is removed gradually from the circulation loop, whereby pure steam remains in the system in the final analysis.

(7) In a subsequent step, natural gas as the hydrocarbon containing process stream, pre-heated by a fired heater (not shown), is introduced via conduit 24 and subsequently compressed by gas compressor 11 to e.g. 30 bar. The compressed natural gas is sent to desulphurisation unit 12 via conduit 25. In the desulphurisation unit 12, sulphur containing contaminants of natural gas are hydrogenated to convert said contaminants to hydrogen sulfide (H.sub.2S). The hydrogen sulfide is subsequently eliminated by absorption to a ZnO fixed bed contained in the desulphurisation unit. The desulphurised, i.e. cleaned natural gas is then sent via conduit 26 to pre-reformer 13, to which process steam is supplied via conduit 21. In the pre-reformer 13, higher hydrocarbons of the cleaned natural gas are pre-reformed and reacted with steam to obtain a natural gas stream which mainly contains methane as the hydrocarbon source. The pre-reformed natural gas is then sent to autothermal reformer 14 via conduit 27.

(8) The ATR reactor of the autothermal reformer 14 comprises a Ni based catalyst bed, where the compressed and cleaned pre-reformed natural gas is mixed with process steam supplied via conduit 22, and thus converted to synthesis gas. The synthesis gas thereby obtained at the outlet of the ATR reactor is sent to cooling unit 15 via conduit 28 to separate excess, i.e. not reacted water. The condensed water is discharged via conduit 29.

(9) A fraction of the dry synthesis gas obtained in the cooling unit and discharged via conduit 30 is recycled via conduit 31 to an upstream section of the ATR reactor. Remaining synthesis gas, which is not recycled to an upstream section of the autothermal reformer 14, is discharged via conduit 32 and is sent to flare.

(10) In the embodiment according to FIG. 1, the upstream section of the ATR reactor is before (upstream to) the gas compressor 11, where the dry synthesis gas is mixed with natural gas of conduit 24 to form a mixture of natural gas and synthesis gas in conduit 33, which subsequently passes the gas compressor 11, the desulphurisation unit 12 and the pre-reformer 13. The compressed, desulphurised and pre-reformed mixture of natural and synthesis gas is supplied to autothermal reformer 14 via conduit 27, and steam is added via conduit 22 so that a mixed process stream containing the process steam, the hydrocarbon containing process stream and recycled dry synthesis gas is introduced to the ATR reactor of the autothermal reformer 14. As soon as a minimum hydrogen threshold concentration in the mixed stream at an upstream section of the ATR reactor is reached, oxygen is supplied to the burner of the ATR reactor of autothermal reformer 14, by what autoignition of the mixed process stream in the autothermal reformer 14 is obtained. The minimum hydrogen threshold concentration at an upstream section of the ATR reactor according to the embodiment of FIG. 1 is the hydrogen concentration in the mixed stream obtained by the streams of conduits 27 and 22.

(11) After adding oxygen to the mixed process stream and autoignition of said process stream, synthesis gas is produced in the autothermal reformer 14 through autothermal reforming.

(12) The embodiment according to FIG. 2 differs from the embodiment of FIG. 1 in that a steam ejector 16 for compressing a mixture of already desulphurised natural gas (of conduit 26) and recycled dry synthesis gas (of conduit 31) is used. A mixture of recycled dry syngas and natural gas is obtained in conduit 34, which is sent to pre-reformer 13 for further processing as described for the embodiment of FIG. 1. The steam reformer supplied with steam via conduit 35 as motive fluid. The steam used as a motive fluid can also be used, advantageously, as process steam (conduits 21 and/or conduit 22).

(13) In the following numerical example (table), typical compositions and flow rates of the different streams of the foregoing embodiments are presented.

(14) TABLE-US-00001 Natural Natural Recycle gas + Outlet Inlet to gas stream recycle pre- ATR Outlet of dry (cond. (cond. stream reformer (cond. ATR syngas 24) 31) (cond 33) (cond. 27) 27 + 22) (cond. 28) (cond. 30) Water mol-frac. 0.000 0.003 0.002 0.441 0.493 0.477 0.003 CO.sub.2 mol-frac. 0.014 0.069 0.048 0.027 0.024 0.036 0.069 CO mol-frac. 0.000 0.004 0.003 0.001 0.001 0.002 0.004 H.sub.2 mol-frac. 0.000 0.236 0.144 0.081 0.073 0.124 0.236 Ar mol-frac. 0.000 0.000 0.000 0.000 0.000 0.000 0.000 N.sub.2 mol-frac. 0.002 0.001 0.002 0.001 0.001 0.001 0.001 CH.sub.4 mol-frac. 0.968 0.686 0.796 0.446 0.404 0.360 0.686 molar kmol/hr 1320.0 2071.3 3391.3 6056.6 6676.0 7316.9 3835.7 flow mass kg/hr 22082.0 30552.0 52634.0 100646.3 111805.2 119305.2 56577.9 flow

(15) According to the numerical example, the hydrogen threshold concentration in the mixed stream (conduits 27+22, inlet to ATR) is 7.3 mol-% on a wet basis to achieve autoignition after admixture of oxygen via conduit 23.

LIST OF REFERENCE SIGNS

(16) 10, 20 process/plant

(17) 11 gas compressor

(18) 12 desulphurisation unit

(19) 13 pre-reformer

(20) 14 autothermal reformer

(21) 15 cooling unit

(22) 16 steam ejector

(23) 20-35 conduit