Process for producing hydrocarbons

Abstract

The invention relates to a process for the production of liquid hydrocarbons by the use of light-end fractions from downstream synthesis in the reforming section of the plant.

Claims

1. Process for the production of liquid hydrocarbons from a naphtha free hydrocarbon feedstock comprising: (a) passing a hydrocarbon feedstock through a hydrogenation stage to form a hydrogenated feedstock; (b) passing the naphtha free hydrogenated feedstock through a desulfurization stage to form a desulfurized feedstock; (c) passing the desulfurized feedstock through a pre-reforming catalytic nickel stage under the addition of steam to form a pre-reformed gas; (d) passing the pre-reformed gas through an autothermal reformer (ATR) or Catalytic Partial Oxidation unit (CPO) under the addition of an oxidant gas to form a synthesis gas; (e) passing the synthesis gas through a Fischer-Tropsch synthesis stage to form a tail gas stream and a raw product stream of hydrocarbons; (f) passing the raw product stream of hydrocarbons through an upgrading stage to form a final product stream of liquid hydrocarbons and a naphtha free light-end fraction stream, the naphtha free light-end fraction stream comprises a C1-C6 fraction and C6+ fraction containing paraffinic and olefinic hydrocarbons, but no naphtha, wherein said naphtha free light-end fraction stream is combined with a stream of natural gas to form the naphtha free hydrocarbon feedstock.

2. Process according to claim 1 in which the upgrading stage (f) comprises hydrocracking but no hydrotreating.

3. Process according to claim 1 in which the naphtha free light-end fraction stream is liquefied petroleum gas (LPG) constituted by a C2-C6 fraction.

4. Process according to claim 1 in which the hydrogenation of step (a) is conducted under the addition of hydrogen to the naphtha free hydrocarbon feedstock.

5. Process according to claim 1 in which the pre-reforming stage is conducted adiabatically in a fixed bed said catalytic of nickel catalyst.

6. Process according to claim 1 in which the ATR or CPO stage is conducted in a fixed bed of nickel catalyst in which the active component is not solely a metal of the group consisting or Rh, Ru, Ir, Pt and mixtures thereof.

7. Process according to claim 1 in which tail gas from the Fischer-Tropsch synthesis of step (e) is recycled to hydrogenation stage (a) , desulphurization stage (b), pre-reforming stage (c), reforming stage (d), or a combination thereof.

8. Process according to claim 1 in which step (d) further comprises passing the pre-reformed gas through a heat exchange reformer before the ATR or CPO, and using the hot effluent gas from the ATR or CPO as heat exchanging medium in the heat exchange reformer thereby cooling the hot effluent gas into said synthesis gas.

9. Process for the production of liquid hydrocarbons in the form of gasoline from a naphtha free hydrocarbon feedstock containing natural gas comprising: (i) combining a naphtha free light-end fraction stream with a stream of natural gas to form the naphtha free hydrocarbon feedstock; (ii) passing said naphtha free hydrocarbon feedstock through a hydrogenation stage to form a hydrogenated feedstock; (iii) passing the hydrogenated feedstock through a desulfurization stage to form a desulfurized feedstock; (iv) passing the desulfurized feedstock through a pre-reforming stage under the addition of steam to form a pre-reformed gas; (v) passing the pre-reformed gas through an autothermal reformer (ATR), secondary reformer or Catalytic Partial Oxidation unit (CPO) under the addition of an oxidant gas to form a synthesis gas; (vi) passing the synthesis gas through a methanol synthesis stage, dimethyl ether (DME) synthesis stage, or a combination of both, to form a raw product stream of oxygenates comprising methanol, DME or a mixture of both; (vii) passing the raw product stream of oxygenates through a gasoline reactor to form a raw product stream of gasoline and passing said raw product stream through an upgrading stage to form a final product stream of liquid hydrocarbons comprising gasoline and a naphtha free light-end fraction stream, in which the light-end fraction stream is liquefied petroleum gas (LPG) constituted by a C2-C6 fraction.

10. Process according to claim 9 in which step (v) further comprises passing the pre-reformed gas through: a heat exchange reformer before the ATR, secondary reformer or CPO, and using the hot effluent gas from the ATR, secondary reformer or CPO as heat exchanging medium in the heat exchange reformer thereby cooling the hot effluent gas into said synthesis gas, or a steam methane reformer (SMR) before the ATR, secondary reformer or CPO.

11. Process according to claim 9 in which the upgrading stage (vii) comprises hydrocracking but no hydrotreating.

12. Process according to claim 9 in which step (ii) is conducted under the addition of hydrogen to the hydrocarbon feedstock.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The invention is further illustrated by reference to the attached FIGURE which shows a specific embodiment of the invention in which LPG recycle is used in the syngas section (reforming section) of a GTL plant, upstream the hydrogenation stage and before steam addition.

DETAILED DESCRIPTION

(2) Referring to the appended FIGURE, a hydrocarbon feedstock 3 is formed by combining natural gas 1 with LPG recycle stream 2 from downstream upgrading unit of Fischer-Tropsch section for producing diesel or downstream synthesis section for production of gasoline (not shown). Hydrogen 4 is added to the hydrocarbon feedstock prior to heating in fired heater 30 using fuel source 7. The heated hydrocarbon feed is then passed through hydrogenation reactor 40 containing a fixed bed 41 of CoMo or NiMo catalyst, then hydrodesulphurization unit (HDS) 50 containing a fixed bed 51 comprising ZnO to capture sulphur. The desulphurized feedstock 5 is then passed through adiabatic pre-reformer 60 containing a fixed bed of nickel catalyst 61 under the addition of steam 6 and further heating via fired heater 30. The pre-reformed 8 gas is further heated and is combined with a CO2-rich recycle stream 9 such as Fischer-Tropsch tail gas to form stream 10. The pre-reformed gas stream 10 is then passed through autothermal reformer (ATR) 70 comprising a bed of nickel based catalyst 71. Oxygen 11 and steam 12 may be added to form a mixture 13 of oxygen and steam which is supplied to ATR 70. Oxygen 13 and steam 12 can also be supplied independently. The hot effluent gas 14 from the ATR is then cooled in waste heat boilers 80, 81 under the production of high pressure steam 15 using boiler feed water 16. The cooled synthesis gas 17 is then passed through a final cooling and separation stage 90, where water 18 (process condensate) is removed and synthesis gas stream 19 is produced for downstream process, such as Fischer-Tropsh synthesis for production of diesel, or methanol and/or DME followed by gasoline synthesis for production of gasoline.

EXAMPLE 1

(3) In one embodiment a light-end fraction stream containing LPG components and other light gasses are withdrawn from separation step which could be a stripper column in the upgrading section. An example of light-end fraction stream composition is listed below:

(4) TABLE-US-00003 H2 29 mole % CO 4 mol % CO2 11 mole % CH4 6 mol % N2 1 mole % C2 3 mole % C3 8 mole % C4 18 mole % C5 9 mole % C6+ 6 mole % H20 5 mole %

(5) A minor fraction of the hydrocarbons are olefins while the majority of the hydrocarbon fraction is paraffins. Further the stream could contain traces of sulphur components.

(6) The light-end stream containing LPG is recycled to the desulphurization reactor of the syngas section in which it is mixed with a natural gas stream. The olefins in the combined gas stream are hydrogenated over a hydrogenation catalyst under the addition of hydrogen (typical CoMo og NiMo type catalyst) thereby removing undesirable olefins and any sulphur components in the combined gas are then removed on the desulphurization catalysts of the subsequent desulphurization stage. The light-end stream containing LPG stream substitutes some of the natural gas fed to the process. The hydrogenated and sulphur depleted feed gas is then mixed with steam and sent to the pre-reformer followed by the ATR reformer.

(7) As an example from a GTL plant an light-end stream with a flow of totally of 224 Nm.sup.3/hr (with above composition) is recycled back to the syngas section, in particular upstream the hydrogenation stage. Despite of the low recycle ratio of the light-end stream, i.e. about 2% of the natural gas feed, the amount of natural gas (NG) import is reduced by 3% from 11378 Nm.sup.3/hr to 11035 Nm.sup.3/hr. The syngas section continue to produce the same amount of syngas with the desirable H.sub.2/CO molar ratio=2.0 for Fischer-Tropsch synthesis, in an amount of 33700 Nm.sup.3/hr which is equivalent to a liquid production of approx 1000 BPD while at the same time avoiding sulphur poisoning as well as carbon deposition of the pre-reformer. The pre-reformer and the autothermal reformer can be operated with conventional nickel catalysts, i.e. without the need of using expensive catalysts based on Ru, Rh, Ir, or Pt as the sole active constituents.

EXAMPLE 2

(8) In another embodiment the light-end gas from a separation step which could be a stripper column in the upgrading section has been further separated into a light end fuel gas and a LPG stream.

(9) The LPG stream has the following composition:

(10) TABLE-US-00004 C2 0.7 mole % C3 28 mole % C4 70 mole % C5 1 mole % C6 0.3 mole %

(11) A minor fraction of the hydrocarbons are olefins while the majority of the hydrocarbon fraction is paraffins.

(12) The LPG stream is recycled to the desulphurization reactor of the syngas section in which it is mixed with the other hydrocarbon feed stream, namely natural gas. The olefins in the LPG stream are hydrogenated over the hydrogenation catalyst (typically CoMo og NiMo type catalyst). The LPG stream substitutes some of the natural gas coming (or other hydrocarbon feed stream coming from outside feed source). The hydrogenated and sulphur depleted feed gas is then mixed with steam and sent to the pre-reformer followed by the ATR reformer.

(13) As an example from a GTL plant a LPG recycle stream of 224 Nm.sup.3/hr (with above composition) is recycled back to the syngas section, upstream the hydrogenation stage. Despite the low recycle ratio of LPG with respect to natural gas (about 2%), the amount of natural gas (NG) import is reduced by 5% from 11378 Nm3/hr to 10805 Nm3/hr. The syngas section continue to produce the same amount of syngas with the desired H.sub.2/CO molar ratio=2.0 for Fischer-Tropsch synthesis in an amount of 33796 Nm.sup.3/hr which is equivalent to a liquid production of approx 1000 BPD while at the same time avoiding sulphur poisoning as well as carbon deposition of the pre-reformer. As Example 1, the pre-reformer and the autothermal reformer can be operated with conventional nickel catalysts, i.e. without the need of using expensive catalysts based on Ru, Rh, Ir, or Pt as the sole active constituents.

EXAMPLE 3

Comparative

(14) In another embodiment a light hydrocarbon fraction which is separated from the main hydrocarbon fraction in a separation step in a fractionation column. The main hydrocarbon fraction is a diesel fraction and the light end fraction is a naphtha fraction. The naphtha fraction is recycled to the syngas section and mixed with the other hydrocarbon feed upstream the desulphurization stage.

(15) The naphtha stream contains long chain higher hydrocarbons and the syngas section must be operated at higher steam-to-carbon ratio to avoid carbon formation from the higher hydrocarbons in the reforming section and especially in the pre-reforming step. This will require a higher addition of high pressure steam to the hydrocarbon feed and thereby higher steam consumption. Most of the equipment will increase in size and thereby in cost due to the high steam-to-carbon molar ratios.

(16) The naphtha stream is a stream with initial boiling point of 30 C. and final boiling point of 170 C. with main hydrocarbon in the C4-C9 range.

(17) Naphtha Composition:

(18) TABLE-US-00005 Components Mole % C4 1.1 C5 12.5 C6 21.5 C7 32.6 C8 25.8 C9 7.6

(19) As an example from a GTL plant a naphtha recycle stream of 650 kg/hr (with above composition) is recycled back to the syngas section. The amount of natural gas (NG) import is reduced by 19% from 11378 Nm.sup.3/hr to 9193 Nm.sup.3/hr, which improves carbon utilization, yet at the same time the recycle of Fischer-Tropsch tail gas increases by a factor 1.5-2 because of the naphtha process require operating at a higher steam-to-carbon ratio. The increase in the flow of such tail gas is needed to compensate for the higher steam-to-carbon ratio in order to obtain the desired H.sub.2/CO molar ratio of 2 in the synthesis gas used for Fischer-Tropsch synthesis. Accordingly, the costs of the tail gas recycle compressor increases. Since the higher steam-to-carbon ratio leads to higher inlet flow to the pre-reformer and ATR, a higher oxygen requirement in the ATR is necessary. The amount of oxygen import increases by 7% resulting in an associated increase in the investment of the air separation plant. The total flow through the plan increases and thereby most equipment will increase in size by 9%, with an associated increase in equipment cost. The syngas section continue to produce the same amount of syngas with H.sub.2/CO ratio=2.0 in an amount of 33796 Nm.sup.3/hr. Even though the natural gas consumption is reduced by 19% the operation cost and investment increases. This example illustrates that the recycle of naphtha is not beneficial to the process economy or the investment in the syngas section of the GTL plant, despite savings in carbon utilization in the form of reduced NG import.