PROCESS FOR PRODUCING SYNTHESIS GAS

Abstract

Process for the production of synthesis gas by catalytic steam reforming of a hydrocarbon containing feedstock in parallel in an autothermal steam reformer and heat exchange reformer, the heat for the steam reforming reactions in the heat exchange reformer being provided by indirect heat exchange with the combined effluent of the heat exchange reformer and a portion of the autothermal steam reformer.

Claims

1. Process for the production of synthesis gas by catalytic steam reforming of hydrocarbon feedstock by parallel arrangement of heat exchange reforming (HER) and autothermal reforming (ATR) comprising: passing a first hydrocarbon feedstock through an autothermal reforming stage and withdrawing an effluent gas of raw synthesis gas; dividing this raw synthesis gas into at least a first and second portion of raw synthesis gas; passing a second hydrocarbon feedstock through the catalyst side of a heat exchange reforming stage and withdrawing a primary reformed synthesis gas; combining a portion or all of the primary reformed gas with the first portion of raw synthesis gas to form a synthesis gas, and passing the synthesis gas through the non-catalyst side of the heat exchange reforming stage to provide heat for the steam reforming reactions in said catalyst side by indirect heat exchange with said synthesis gas; withdrawing from the heat exchange reforming stage a cooled synthesis gas.

2. Process according to claim 1 further comprising combining a second portion of the raw synthesis gas with all or a portion of said cooled synthesis gas.

3. Process according to claim 1, wherein said first hydrocarbon feedstock and said second hydrocarbon feedstock are split from a single hydrocarbon feedstock and prior to split the single hydrocarbon feedstock is subjected to pre-reforming.

4. Process according to claim 1, wherein each individual stream in the form of first hydrocarbon feedstock, or second hydrocarbon feedstock, or both, are subjected to pre-reforming prior to passing through the autothermal reforming or heat exchange reforming.

5. Process according to claim 1, in which tail gas from downstream synthesis of diesel, methanol or gasoline, is combined with the first or second hydrocarbon feedstock.

6. Process according to claim 1, in which the steam-to-carbon molar ratio of the first hydrocarbon feedstock is lower than the steam-to-carbon molar ratio of the second hydrocarbon feedstock.

7. Process according to claim 6 in which the steam-to-carbon molar ratio of the first hydrocarbon feedstock is less than 1.20, preferably below 1.0.

8. Process according to claim 1, in which the volumetric flow rate of the second hydrocarbon feedstock is 1-30% of the volumetric flow rate of the first and second hydrocarbon feedstock combined.

9. Process according to claim 1, in which the ratio between the volumetric flow rate of the second portion of the raw synthesis gas stream to the volumetric flow rate of the effluent gas of raw synthesis gas is 50-95%.

10. Process according to claim 1, in which carbon dioxide is removed completely or partly from the second portion of raw synthesis gas, the cooled synthesis gas, or from the synthesis gas resulting from combining said second portion of raw synthesis gas and said cooled synthesis gas.

11. Process according to claim 1, in which catalytic partial oxidation (CPO) is used instead of autothermal reforming (ATR).

12. Process according to claim 1, in which the ratio between the volumetric flow rate of the second portion of the raw synthesis gas stream to the volumetric flow rate of the effluent gas of raw synthesis gas is 20-49%.

Description

BRIEF DESCRIPTION OF THE DRAWING

[0053] The accompanying figure shows a specific embodiment of the invention comprising the parallel arrangement of autothermal reforming and heat exchange reforming with tail gas addition to the autothermal reforming stage.

DETAILED DESCRIPTION

[0054] In the accompanying figure single hydrocarbon feedstock 1, such as natural gas, which may be pre-reformed, is split into a first hydrocarbon feedstock 2 and second hydrocarbon feedstock 3. The latter is combined with steam 4 to form gas stream 5 which is then passed through heat exchange reformer 20 having one or more catalyst tubes or regions containing catalyst 21 (catalyst side). The gas stream 5 is reformed under contact with catalyst 21 to form primary reformed synthesis gas 6. The first hydrocarbon feedstock 2, optionally mixed with steam, is mixed with recycled tail gas 8 from downstream synthesis such as Fischer-Tropsch synthesis and is then subjected to autothermal reforming in ATR unit 22 containing reforming catalyst bed 23 under the addition of oxygen 9. An effluent gas of raw synthesis gas 7 is withdrawn from the ATR, a first portion of which is combined with the primary reformed gas 6 to form synthesis gas 10. This synthesis gas 10 is used to deliver heat to the reforming reactions in catalytic side 21 by indirect heat exchange. Hence the synthesis gas 10 passes through the non-catalytic side (e.g. shell side) of the heat exchange reformer 20 resulting in cooled synthesis gas 11. This cooled synthesis gas 11 may be combined with a second portion 12 of the raw synthesis gas 7 to produce synthesis gas 13 for downstream processes.

Example 1

[0055] Calculations have been made to simulate the synthesis gas section of a Gas-to-Liquids facility according to the invention as described herein and with reference to the accompanying figure. Natural gas (NG) mixed with steam is used as feed (stream 1). 10% of the feed is sent to the heat exchange reformer (line 3). Additional steam is added to give a steam-to-carbon ratio of the heat exchange reformer feed of 3.5. The remaining part of the feed is mixed with tail gas (line 8) from a Fischer-Tropsch synthesis section for production of liquid hydrocarbons and passed to the ATR. The amount of steam in the feed (line 1) is adjusted such that the S/C-ratio in the feed to the ATR is 0.6. The amount of tail gas added in line 8 is adjusted to provide an H.sub.2/CO-ratio downstream the ATR (line 7) of 2.00. Oxygen is also added to the ATR through line 9. The process conditions are designed to provide an exit temperature of 1025 C. from the ATR.

[0056] Part of the exit stream (line 7), i.e. raw synthesis gas from the ATR bypasses the heat exchange reformer through line 12 as the second portion of the raw synthesis gas. The remaining part is mixed with the effluent from the catalyst side of the heat exchange reformer (line 6) in the form of a primary reformed gas to give synthesis gas stream 10. Stream 10 provides by indirect heat exchange the heat to carry out the endothermic steam reforming reaction in the heat exchange reactor. The total production of H.sub.2+CO in the synthesis gas unit is the sum of the H.sub.2+CO in cooled synthesis gas stream 11 and the second portion of raw synthesis gas 12.

[0057] The table below shows the results of calculations for various values of the bypass ratio, i.e. volumetric flow of the second portion of the raw synthesis gas (stream 12) to the volumetric flow rate (kmol/hr) of the effluent gas of raw synthesis gas (stream 7).

[0058] Qa is the reaction quotient calculated from reaction (a) in the present specification. The thermodynamic potential for metal dusting increases with decreasing values of Qa.

TABLE-US-00001 CO reduction, Specific prod. Q.sub.a.sup.2) (CO + H.sub.2)/NG Bypass ratio.sup.1) % bar mol/mol 0 0.0627 3.1777 20 0.0659 3.1777 60 0.0810 3.1777 70 0.0903 3.1777 .sup.1)% of ATR effluent gas bypassing heat exchange reactor: stream 12/stream 7 .sup.2)Qa = P.sub.H2O/(PCO * P.sub.H2) for the CO reduction CO + H.sub.2 = C + H.sub.2O, reaction (a), on the shell side of the heat exchange reformer.

[0059] It is seen from the table that the thermodynamic potential for metal dusting decreases when the bypass ratio is increased and the same time it is possible to operate at low S/C-ratio in the ATR, here S/C=0.6. The production of synthesis gas per unit of natural gas feed is unaffected. For comparison the production has also been calculated for a scheme with only ATR (i.e. stand-alone ATR; with pre-reformer but without the use of a heat exchange reformer). A stand-alone ATR at same conditions results in lower specific prod: 2.9948 mol CO+H.sub.2 pr mol NG and higher specific oxygen consumption: 0.1952 mol O.sub.2 per mol CO+H.sub.2, compared to 0.1769 mol O.sub.2 per mol CO+H.sub.2, which is the same for all bypass ratios in the table. This also shows that the efficiency, in terms of energy consumption, is higher when a heat exchange reformer is included.

Example 2

[0060] Calculations have been made to simulate the synthesis gas section of a methanol plant according to the invention as described herein and with reference to the accompanying figure. The parameters have been set to the same values as in Example 1 with the exception that the S/C-ratio in the feed to the ATR is 0.4. The only other difference from the parameters given in Example 1 is that no tail gas is added, i.e. volumetric flow in stream 8 is zero.

[0061] In methanol it is desired to have a so-called module of ca. 2. The module is defined as:

[0062] (FH.sub.2FCO.sub.2)/(FCO+FCO.sub.2), where FX is the flow of component X.

[0063] The results of the calculations are shown in the table below. It is seen that the thermodynamic potential of metal dusting decreases when the bypass ratio is increased.

TABLE-US-00002 CO reduction Q.sub.a.sup.2) Bypass ratio.sup.1) % bar 0 0.0444 20 0.0485 60 0.0674 70 0.0790 .sup.1)% of ATR effluent gas bypassing heat exchange reactor: stream 12/stream 7 .sup.2)Qa = P.sub.H2O/(PCO * P.sub.H2) for the CO reduction CO + H.sub.2 = C + H.sub.2O, reaction (a), on the shell side of the heat exchange reformer

[0064] The methanol module for all of the cases is the same, namely 1.9514. In comparison the methanol module for a concept based only on ATR (with pre-reformer but without the heat exchange reformer) is significantly lower, namely 1.8582. This indicates that the produced synthesis gas from the scheme including a heat exchange reformer has a better stoichiometry for downstream methanol synthesis and is thereby more efficient than a synthesis gas produced without the heat exchange reformer. In addition, a stand-alone ATR at same conditions results in a higher specific oxygen consumption: 0.1802 mol O.sub.2 per mol H.sub.2+CO compared to 0.1687 mol O.sub.2 per mol CO+H.sub.2, which is the same for all bypass ratios in the table.