WO2009150678A1

WO2009150678A1 - Externally heated membrane reforming

Info

Publication number: WO2009150678A1
Application number: PCT/IT2008/000395
Authority: WO
Inventors: Gaetano Iaquaniello
Original assignee: Technip KTI SpA
Current assignee: KT Kinetics Technology SpA
Priority date: 2008-06-12
Filing date: 2008-06-12
Publication date: 2009-12-17
Anticipated expiration: 2010-12-12
Also published as: EP2300362A1

Abstract

A system for converting natural gas or heavy hydrocarbons into hydrogen comprises: a heat exchanger where air is heated up by molten salts or other fluids as hot helium, a feed hydrodesulfuration section, two or more post-combustion chambers wherein the preheated air temperature is adjusted, membrane reforming based on three steps reaction/separation, a retentate recirculator, a CO2 separation section, hydrogen cooler and compressor, and a final PSA. Such system can be fed by hydrocarbons, as methane, gasoline, naphtha or alternative fuel as bio- diesel, in the last case CO2 overall mass balance could be equal to zero.

Description

EXTERNALLY HEATED MEMBRANE REFORMING

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Description Field of invention

The present invention is related generally to the hydrogen production from hydrocarbons and more specifically to hydrogen production via membrane reforming wherein a large part of the reaction heat is supplied not throughout the combustion of any hydrocarbons or off-gas, only a little amount of hydrocarbons (purge gas and make up fuel) being burned in the post combustion chambers.

Hydrogen is largely used today in refinery application to convert sulfur into H₂S, to reduce aromatics content and to crack heavier compounds into lighter ones. Hydrogen may play also an important role in future as energy carrier. The efficiency in transforming the feed into H₂ and consequently production costs is the main driver for process selection. Environmental impact is going to play more and more in such a transformation process. It is known that membrane reforming may play an important role in converting natural gas or heavy hydrocarbons into hydrogen in a very efficient way.

US Patent 6348278, US Patent 6802876, US Publication 2004/0049982, for instance, disclose various hydrogen production systems provided with a membrane reformer or a reformer including a reformer body and a hydrogen separation unit using a hydrogen separation membrane. It is an object of the present invention to further increase the overall energy efficiency of a system for producing hydrogen via membrane reforming and pave the way to produce large amount of hydrogen with the minimum environmental impact.

According to the invention it is provided a system for producing hydrogen which integrates membrane reforming of hydrocarbons with an external source of heat as a separated nuclear reactor or solar heated molten salts.

In a preferred embodiment of the invention, the integration is done by an intermediate stream, air for instance, which is heated at least up to 550-600 ⁰C in a heat exchange, by cooling down a hot medium stream as molten salts or hot helium coming from a nuclear reactor or heated by the sun.

Brief Description of the Drawings

These and other features and advantages of the invention will become apparent from the accompanying drawings, in which

Figure 1 is a schematic view of the proposed system;

Figure 2 is a schematic process flow diagram;

Figure 3 is a schematic view of membrane reforming step; Figure 4a is a schematic view of the membrane module separation;

Figure 4b is a cross section of the figure 4a; Figure 5 is the effect of the recirculation stream versus the natural gas conversion; Figure β is the schematic view of the two reforming steps;

Figure 7a is the schematic cross-section of a suitable configuration for membranes.

Figure 7b is a SEM image with a detail of the membrane. Detailed description and best mode of the disclosure

Our system is schematically illustrated in figure 1 and generally indicated at 100.

System 100 includes a heat exchanger, 110, that recovery the heat from the external sources to preheat air, 10, up to 550-600⁰C at least. The preheated air stream is delivered to the post-combustion chamber, 120, where it is further pre-heated to 700-750⁰C, by burning the off-gas, 11, from PSA unit, 190, and part of the retentate, 12, from the third separator module, 136. The stream 10 is used to provide all the reaction heat, pre-heat the process streams, produce medium pressure steam, 13, to be used for the process and low pressure stream, 14, to be used as sweeping gas in the separation module. After the recovery section, 140, stream 10 is discharged to the stack, 180.

Device 130, the membrane reformer see also figure 3, consists of at least two steps, more typically three, each step has its own reforming reactor, 131, 132 and 133 and its own separation module, 134 135 and 136. Natural gas or other hydrocarbons feedstock, 20, is desulfurized in the device 170. Following is mixed with process steam, 13, recirculation flow, 25, and heated up in the device 140 before entering reactor 131. Syngas at the outlet, 15, is cooled down in 202 and routed to the 134 separation module, where a first ritentate, 16, and an almost pure hydrogen stream, 21, are obtained. Stream 16, after a pre-heating, in 203, enters into the second reforming step, 132, where part of remaining natural gas is reformed to syngas, 17. Upstream at the second reformer reactor, a second combustion chamber 137 provides a temperature rise of the stream 10 in order to supply the heat of the reaction at 700-750⁰C. Stream 17 is routed to the second separation module, 135, where the syngas is divided in a second retentate stream, 18, and another hydrogen stream 22. Stream 18, after a pre-heating in 205, enters in the third reformer reactor, 133, obtaining stream 19. This stream is cooled down in 206 till 40⁰C and sent to the CO₂ separation section 150. Device 150 consists of a condensate separator (151) and an adsorption column 152 where the CO₂ is captured and stored. The outlet stream is then heated up 450 ⁰C in 209 and sent to the third separation module 136.

The retentate stream 24 from unit 136 is partially send to device 120, the remaining flow, 25, is recycled back to device 131 through a reciculator. Effect of recirculation flow versus the feed conversion is shown in figure 5.

Almost pure hydrogen streams, 21 and 22, are mixed, 23, cooled down and sent to the separator 160, where the sweeping steam is condensate.

One of separation modules is represented in figures 4a, 4b, where syngas 15 is flowing on the shell side and sweeping steam, 14, flows on the tube side from one side. The low pressure streams, which contain hydrogen, water vapor and hundreds of ppm of CO+CO₂+CH₄, after the steam condensation and water removal, are compressed and fed to the PSA unit, 190, where hydrogen is finally purified and off-gas, 11, is produced.

Device 130 is typically operated at elevated temperature and pressure. For example devices 132, 133 and 134 may be operated at temperature in the range of 500-650°C, much less than a conventional reformer. Pressure may range from 5 to 25 barg. Devices 134, 135 and 136 may be operated at temperature much lower than the relative reforming step, in the range 420-470⁰C.

The permeates pressure are ranging from 0.5 to 2 barg. The figure 2 depicts a simplified process scheme where all the streams have been indicated.

A suitable structure for reforming steps, 131, 132 and 133, is made by a bundle of tubes, externally finned, and filled with a proper catalyst active at such low reforming temperature.

Tubes diameter may ranges form 3 to 5" and heated length from 5 to 10 meter. Due to the lower tubes metal temperature, compared to the conventional reforming technology, such tubes are made of common stainless steel and not in the exotic material as the cast 25-35 micro alloy material.

In figure 6 an illustrative example of a suitable configuration for the convective reforming reactor is reported. A suitable structure for the separation module, 134, 135 and 136, includes a bundle of tubular membranes 40 assembled in a shell 42 like tube configuration as in fig.4. Tubular membranes diameter may range from 20 to 100 mm, and length from one to five meters . Alternatively such separation module can be build with planar membrane configuration, having up to 1 meter dimension.

The membranes may be made of any material hydrogen-permeable suitable to be used at the operating environments in which devices 134, 135 and 136 operate. Examples of suitable materials for membranes include palladium and palladium alloys, particularly thin films of such metals and metal alloys. Palladium alloys have proven particularly effective, especially palladium with at least 23% of silver (see fig.7b) . These membranes are typically formed from a thin foil 44 that is approximately 1 to 10 micron thick. Examples of suitable mechanism for reducing the thickness of the membranes include sputtering and etching. The membrane is supported by an under-laying support 46 made of porous ceramic and metallic material. Typically porous stainless steel is considered to be a good candidate for such application.

To minimize interferences between membranes 44 and support structure 46 a ceramic layer 48 is inserted between the previous two. Titanium or zirconium oxides with a proper grain size have been used for such application. In figures 7a and 7b illustrative example of a suitable configuration for membranes is shown. It should be understood that the membrane configurations discussed above have been illustrated schematically as per figure 7, are not intended to represent every possible configuration within the scope of invention. For example, although membrane illustrated in figures 4a, 4b as having a tubular configuration, it is within the scope of the invention that such a membrane may have a planar configuration as well. The fact that membranes are not inserted into the reforming reactor, integrally formed, allows optimizing the design of the reactor independently from the one of the separator module. Conversely in the integrated design, the limiting flux of the membranes is going to limit the heat flux of the reforming tubes with the result to install more surface of the minimum required.

In Figures 1 and 2, device 110 is a heat exchanger where air is heated by cooling down a hot medium stream. Such a stream may be melted salts or hot helium stream coming from a nuclear reactor or heated by the sun throughout peculiar devices. It is within the scope of the invention the use of any intermediate hot fluid suitable to heat up air in the range of 550-700°C which in turn will be used to provide the heat of reforming reactions . In figures 1 and 2, device 120 and 137 are two post-combustion chambers where air from device 110 is heated to the required level for reforming in two or more steps in order to optimize the level of heat exchange for each reforming step. This may imply more than two post-combustion sections or also only one.

Claims

1. A hydrogen production system consisting of: a heat exchanger (110) where air is heated up by- molten salts or other fluids as hot helium, a feed hydrodesulphuration section (170), two or more post- combustion chambers (120,137), membrane reforming (130) based on three steps reaction/membrane hydrogen separation, a retentate recirculator (138), a CO₂ separation section (152), hydrogen cooler and compressor (200-210) , and a final purification with PSA (190) .

2. The system of claim 1, wherein heat of reaction is not provide by the combustion of the hydrocarbons, but is supplied from a separated unit, nuclear reactor or solar heated molten salts. (105) .

3. The system of claim 2, wherein the integration of the two unit: the provider of the heat (105) and the membrane reactor (130) is done by an intermediate stream, air for instance, which is heated up to 550- 600⁰C or more in the heat exchanger (110) .

4. The system of the claim 3 wherein the intermediate stream temperature (10) is adjusted in a post combustion step (120,137) wherein part of retentate (12), make-up fuel gas (27) and PSA off gas (11) are burned.

5. The system of claim 1 wherein the hydrocarbons are desulfurized in the hydrodesulfuration section (170) which consists of a catalytic converter of sulfur to H₂S and a zinc oxide bed for H₂S adsorption.

6. The system of claim 1 wherein the membrane reformer (130) consists of two or more steps, preferably three.

7. The system of claim 1 wherein each reforming step is made by a reforming reactor (131,132,133) and a separation module (134,135,136), physically separated.

8. The system of claim 7 wherein the reforming outlet temperature is higher than the separation module operative temperature and controllable in order to optimize the operation of the separation module.

9. The system of claim 7 wherein the operation of the separation module has no impact with design and operation of reforming step.

10. The system of claim 7 wherein the reforming temperature is occurring at much lower temperature (300 0C or more lower) than conventional reforming.

11. The system of claim 7 wherein each reforming reactor (131,132,133) is made by a bundle of tubes in stainless steel and where heat is mainly exchanged by convection mode.

12. The system of claims 10 and 11 wherein such a low reforming temperature in the reforming reactor (131,132,133) is also achieved filling said tubes with a noble based catalyst installed on a porous foam used as mechanical support .

13. The system of claim 1 wherein the hydrogen partial pressure in the separation module is reduced by means of sweeping (14) gas as steam or nitrogen.

14. The system of claim 7 wherein the separation modules (134,135,136) can be realized either by a bundle of tubular membranes or a laminar ones; each tubular or planar membrane being a multilayer structure made of a ceramic or metallic (porous stainless steel) substrate and a metallic Pd-alloy hydrogen selective layer.

15. The system of claim 14 wherein the membrane thickness ranges form 1 to 10 micron.

16. The system of claim 1 wherein hydrogen purity is achieved not through the separation module but with a Pressure Swing Absorber (PSA) (190) .

17. The system of claim 1 wherein the feed conversion is also controlled by the amount of retentate recirculated to the first step.

18. The system of claim 17 wherein the convective reformer tubes are externally finned in order to increase the external heat coefficient relevant to inside internal tube diameter.

19. The system of claim 17 wherein the catalyst used inside the convective reformer tubes is made of a porous foam metallic structure to improve the inside tubes heat transfer coefficient.

20. The system of claim 7 wherein the not-integrated design allows to find the optimum design for both equipment independently.

21. The system of claim 4 and 6 wherein the post- combustion step may be split in one or more steps inserted just before the reforming devices.

22. The system of claim 21 wherein the post-combustion steps are realized as duct burner and installed just up-stream of devices (131 and 132) .

23. The system of claim 1 wherein CO₂ separation section consists of a condensate separator (151) and an adsorption columns (152) for the CO₂ capture and store.