Process for the production of synthesis gas

Abstract

The invention relates to a process for the production of liquid hydrocarbons by Fischer-Tropsch synthesis in which the reforming section of the plant comprises a process line comprising autothermal reforming (ATR) (5) or catalytic partial oxidation (CPO), and a separate process line comprising steam methane reforming (SMR) (8).

Claims

1. A process for the production of synthesis gas comprising: (a) passing a first hydrocarbon feedstock, a tail gas from a Fischer-Tropsch (FT) synthesis stage, and an oxidant gas to an autothermal reforming (ATR) stage or catalytic partial oxidation (CPO) stage to form a raw synthesis gas; (b) passing a second hydrocarbon feedstock through a primary reforming stage in the form of steam methane reforming (SMR), to form a primary reformed gas; (c) combining part or all of the primary reformed gas of step (b) with the raw synthesis gas of step (a) to form a synthesis gas according to a R.sub.SMR ratio between 1 and 20%, where R.sub.SMR is defined as the volumetric flow rate of carbon monoxide and hydrogen in the primary reformed gas to the volumetric flow rate of the hydrogen and carbon monoxide in the synthesis gas.

2. The process according to claim 1 in which said first hydrocarbon feedstock and said second hydrocarbon feedstock are split from a single hydrocarbon feedstock and wherein prior to split the single hydrocarbon feedstock is subjected to pre-reforming.

3. The process according to claim 1 wherein naphtha formed or synthesised in the FT-synthesis stage is not added to the first hydrocarbon feedstock or the second hydrocarbon feedstock or to the single hydrocarbon feedstock.

4. The 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 autothermal reforming stage or primary reforming stage.

5. The process according to claim 1 further comprising subjecting primary reformed gas to the sequential stages of water gas shifting, separating a hydrogen-rich stream in a separation means, and mixing all or a portion of the hydrogen-rich stream with the produced synthesis gas of step (c).

6. The process according to claim 1 wherein the produced synthesis gas of step (c) has a molar ratio of hydrogen to carbon monoxide of 1.7-2.3.

7. The process according to claim 1 wherein in step (a) the autothermal reforming (ATR) stage is combined with heat exchange reforming (HER) arranged in series or in parallel.

8. The process according to claim 1 wherein the process further comprises previous to steps (a) or (b) a step of passing the first or second hydrocarbon feedstock through a desulfurization stage.

9. The process according to claim 1 wherein the process steam-to-carbon molar ratio (S/C) in step (a) is in the range 0.4-1.0, while the process steam-to-carbon molar ratio in step (b) is in the range 1.5-4.0.

10. The process according to claim 1 wherein the process further comprises subjecting primary reformed gas to the sequential stages of water gas shifting, separating a hydrogen-rich stream in a separation means, and mixing all or a portion of the hydrogen-rich stream with the raw synthesis gas of step (a).

11. The process according to claim 1 wherein the process further comprises converting the synthesis gas to liquid hydrocarbons by Fischer-Tropsch synthesis.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The invention is further illustrated by reference to the attached figures, in which

(2) FIG. 1 shows a schematic of the surprising increase in plant efficiency.

(3) FIG. 2 shows a general embodiment of the invention in which tail gas recycle is used in the ATR line.

(4) FIG. 3 shows a specific embodiment of the invention in which the first and second hydrocarbon feedstock streams are split from a single hydrocarbon feedstock which is subjected to pre-reforming.

DETAILED DESCRIPTION OF THE FIGURES

(5) FIG. 1 illustrates the plant efficiency as function of R.sub.SMR. As R.sub.SMR is increased from zero, the plant efficiency also increases. The plant efficiency further increases with increasing values of R.sub.SMR up to a certain value of R.sub.SMR (called R.sub.sMR,max=R.sub.max) with a corresponding maximum efficiency, Eff.sub.MAX. As the value of R.sub.SMR is further increased from R.sub.max, the plant efficiency decreases to values below Eff.sub.MAX. At R.sub.SMR=1 corresponding to synthesis gas production by only an SMR-line, the plant efficiency is lower than for the reference case with synthesis gas production by only an ATR-line.

(6) In FIG. 2 first hydrocarbon feedstock 1 is mixed with tail gas 2 from the F-T synthesis to form mixture 3. The mixture 3 is sent into the ATR 5 and reacted together with oxygen 4 into raw synthesis gas 6 that exits the ATR. In parallel with the ATR train is the second hydrocarbon feedstock 7 which is sent into a SMR 8 and reacted into primary reformed gas 9. Streams 6 and 9 are mixed into a synthesis gas (10) that is sent to F-T synthesis.

(7) In FIG. 3 hydrocarbon feedstock 1 is mixed with steam 2 to form stream 3 before it is sent into a pre-reformer 4. The pre-reformed gas 5 is split into a first hydrocarbon feedstock 6 and a second hydrocarbon feedstock 12. The first hydrocarbon feedstock 6 is mixed with tail gas 7 from the F-T synthesis into 8 before it is sent into the ATR 10. In the ATR 10, the mixture 8 is reacting with the added oxygen 9 into raw synthesis gas 11. The second hydrocarbon feedstock 12 is sent to a SMR 13 and reacts into a primary reformed gas 14. The raw synthesis gas 11 is mixed with the primary reformed gas 14 into a synthesis gas 15 which is sent to F-T synthesis.

Example

(8) Calculations were made to simulate the operation of a complete GTL facility including synthesis gas production according to the invention and Fischer-Tropsch (FT) synthesis in a cooled reactor according to process scheme as described by Landoli and Kjelstrup (Energy & Fuels 2007, 21, 2317-2324). The calculation model includes recycle of unconverted synthesis gas (tail gas) to the FT reactor as internal recycle and to the synthesis gas production as external recycle. The FT reactor is simulated as a series of three converters. The first converter converts synthesis gas to a product of linear, saturated hydrocarbons assuming the Anderson-Schulz-Flory distribution with an alpha-value (chain growth probability) of 0.94. Hydrocarbons with up to 52 C-atoms are considered. The second converter converts part of the saturated hydrocarbons to olefins, and the third converter converts part of the olefins to oxygenates. The conversions to olefins and oxygenates are adjusted to approximately match compositions given in the open literature, e.g. by Dieter Leckel, Upgrading of Fischer-Tropsch Products to produce Diesel, in Haldor Topsoe Catalysis Forum, Munkerupgaard, 19-20 Aug. 2010. The per pass conversion of H.sub.2 in the FT reactor is specified to be approximately 60% and the internal recycle of tail gas is adjusted to obtain an overall conversion of H.sub.2 in the FT synthesis loop of 90%. The external recycle of tail gas is adjusted to obtain a H.sub.2/CO molar ratio in the synthesis gas of 2.0. Excess tail gas is used as fuel for the burners in the SMR and in the fired heaters heating process streams in the ATR line to the required temperatures and superheating steam from the waste heat boilers downstream the ATR and the SMR. Remaining tail gas after this is flared. All components with more than 2 carbon atoms are considered to be products (wax, diesel naphtha, and LPG) and are assumed to be recovered with 100% efficiency.

(9) In concordance with the definition of the present invention, the plant efficiency is calculated as carbon in FT-product divided by the carbon in the hydrocarbon feedstock. The hydrocarbon feedstock is natural gas. Natural gas consumed in the plant as fuel is not included.

(10) The superheated steam from the waste heat boiler downstream the ATR and the SMR and the saturated steam from the FT-reactor are assumed to be expanded for power production (after extraction of steam required for process purposes) with typical efficiencies. The consumption of power for the process including the power required for production of oxygen for the ATR in the Air Separation Unit (ASU) is included in the calculation of power import/export. In all examples cases below, the power produced exceeds the power consumed by the process. The excess power is considered of no value.

(11) The conditions in the ATR are assumed to be process steam to carbon molar ratio (S/C) equal to 0.40 or 0.60 and the exit temperature 1025 C. The SMR in the parallel line operates at S/C ratio of 3.0 and an exit temperature of 870 C. The feed for the SMR is natural gas. The product gases from ATR and the SMR are cooled, and condensate is separated, before the two streams are mixed and used as feed for the FT synthesis unit. Carbon dioxide is removed to 1 vol % from the product gas from the SMR line before combining with the exit gas from the ATR. The pressure in each of the two synthesis gas production lines is adjusted to obtain a pressure at inlet FT-synthesis reactor of 30 bar g.

(12) Overall S/C in the ATR Line 0.60:

(13) (R.sub.SMR %, Plant efficiency %)=(0, 76.75), (3.49, 78.81), (6.23, 78.79), (11.43, 78.72), (15.62, 78.56)

(14) Overall S/C in the ATR Line 0.40:

(15) (R.sub.SMR %, Plant efficiency %)=(0, 74.28), (6.82, 79.70), (10.17, 79.65), (13.34, 79.32), (14.76, 79.23)

(16) It is observed that, for both values of the overall steam to carbon ratio, the plant efficiency increases significantly when introducing synthesis gas production in an SMR-line. When the capacity of the SMR line increases, the plant efficiency reaches a maximum value. However, when the capacity of the SMR line is further increased beyond the point leading to the maximum efficiency, the efficiency slowly decreases. This behavior of the system is both surprising and counterintuitive. The differences in plant efficiency values are highly significant, not least when considering daily productions in order of 10000 barrels of product per day.