WO2009150679A1

WO2009150679A1 - Method of and apparatus for manufacturing hydrogen and produce power

Info

Publication number: WO2009150679A1
Application number: PCT/IT2008/000396
Authority: WO
Inventors: Alberto Santalucia
Original assignee: Processi Innovativi Srl
Priority date: 2008-06-12
Filing date: 2008-06-12
Publication date: 2009-12-17

Abstract

A system to co-generate hydrogen and power using natural gas or a hydrocarbon stream as feedstock through a membrane reforming. The system of the present invention produces hydrogen by a membrane steam reforming reactor and power b a gas turbine. All the heat required for the steam reforming is provided by the exhaust gases coming out from the gas turbine. Most of the fuel used in the gas turbine is taken from the retentate stream. Pressure of retentate and then by of the overall system is such that retentate can be fed to the gas turbine combustion chamber without further compression. The membrane steam reforming reactor consists of at least of a convective type reformer reactor and two steps of membrane separations. Hydrocarbon feed, steam and a retentat recycle stream are fed to the reforming reactor, reformed gas is fed to the first membrane module which generates a retentate stream and a purified hydrogen stream. The purified hydrogen is cooled down to condensate the steam used as sweeping gas and then is compressedbefore the final purity control by PSA and delivery. Retentate stream is partially recycled with the fresh feed and partially goes to a second membrane module which generate in turn another retentate stream and a second purified stream which is mixed with the one generated by the first module. Retentate stream being set at a proper pressur is fed to the combustion chamber of a gas turbine to extract mechanical work before routing the exhaust gases into the reforming reactor.

Description

METHOD OF AND APPARATUS FOR MANUFACTURING HYDROGEN AND PRODUCE POWER

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Description Field of the invention

The present invention relates to a system for manufacturing hydrogen and producing power for industrial applications, particularly for refinery application, using natural gas or a hydrocarbon stream as feedstock.

More particularly, the present invention relates to an integrated and energy efficient apparatus and related method to cogenerate hydrogen by a membrane steam reforming reactor and power by a gas turbine, wherein said membrane steam reforming reactor consists of a convective type reformer reactor and two membrane separation modules.

In the present application, the term "integrated " means that most of the feed to the gas turbine is taken from the retentate recycle. Background of the invention

Steam reforming natural gas is a well-established technology in the petrochemical and fertilizer industries for the production of hydrogen. The process requires high temperature in the range of 850-900⁰C and high pressure, 15 to 40 barg. As result of such operating condition, steam for export must be produced to not penalize strongly the efficiency. Talking into account the steam credit, the feedstock + fuel requirement ranges from 3200-3500 kcal (LHV) /Nm³ of hydrogen produced. The schematic representation of the conventional steam reforming is depicted in fig.l.

A part from the steam reforming (SR) , the process include the feed hydrodesulphurization (HDS) upstream, the CO water gas shift (CO Shift) downstream and the pressure swing absorption (PSA) for the final hydrogen purification.

Recently, the MMR technology using metallic Pd-Ag alloy membrane has introduced major changes in the above architecture, which are: the reforming reaction is performed at a temperature of about 600⁰C; CH₄ conversions up to 80% are obtained because the hydrogen is continuously removed and recovered at high purity through the Pd-Ag membrane separation; the retentate obtained as result of the hydrogen removal has a high Calorific Value and is available under a pressure slightly lower than the inlet reforming one; the shift reaction is practically performed due to the low reforming temperature, strongly reducing the overall duty of the steam reforming (up to -35%) .

A new architecture is then emerging from the above concepts, where only two process steps are required: hydrodesulphurization of the feedstock and membrane reforming.

Purge gas or retentate is routed to the gas turbine whom exhaust are used for providing the reforming duty and to preheat the other process services with minimum or nil post-combustion. The block flow diagram in the fig. 2 represents said hybrid process scheme with the membrane reforming reactor (MRR) .

The integration of MRR into a topping cycle allows to produce simultaneously power and hydrogen in a highly efficient way against actual process scheme which makes steam and hydrogen or use exhaust as preheat combustion air [1,5]. Table 1 compares many design parameters for the conventional steam reforming technology with the recent architecture.

Table 1 : Comparison between conventional steam reforming technology and integrated MRR configuration for a 10000 Nm³/h hydrogen production plant. A further evolution of the above concept has led to realize a MRR formed by two distinctive parts: a convective reformer and a external membrane separating module to recover 99.95% pure hydrogen. The installation of the membrane outside the reforming section as shows in figure 3, has the following advantages: it makes possible to perform the separation at a temperature lower than the reforming temperature and this increase the stability and life of the membranes; the reforming temperature can be optimized independently by the membrane constrains; it strongly simplifies the mechanical design of membrane tubes embedded into a catalyst tube; it makes easier the maintenance of the Pd/Ag membrane module by installing a spare module without shutting down the plant.

However, this solution slightly penalizes the overall system efficiency. Using the concept of MRR unintegrated reactor and hybrid system, a complete process scheme has been developed by the Applicant in order to cogenerate hydrogen by a membrane steam reforming and power by a gas turbine with an overall efficiency higher than 60 %. Performance has been analyzed by means of detailed simulation. Optimized scheme was selected throughout pinch and sensitivity analysis. Sensitivity of process performances has been analyzed by typical operating parameters such as temperature outlet, S/C ratio and recycle ratio.

Therefore, it is an object of the present invention, an integrated and energy efficient system for manufacturing hydrogen and producing power comprising a membrane reforming reactor and a gas turbine, wherein all the heat required for the steam reforming is provided by the exhaust gases coming out from the gas turbine. Most of the fuel used in the gas turbine is taken from the retentate stream.

The membrane steam reforming reactor consists of a convective type reformer reactor and two steps of membrane separations. Such architecture allows to reduce the operating temperature in the range of 550- 600⁰C and pressure form 10 to 30 barg.

In the present invention, hydrocarbon feed, steam and a retentate recycle stream are fed to the reforming reactor, reformed gas is fed to the first membrane module which generates a retentate stream and a purified hydrogen stream. The purified hydrogen is cooled down to condensate the steam used as sweeping gas and then is compressed before the delivery and final purity control.

Retentate stream is partially recycled with the fresh feed and partially goes to a second membrane module which generate in turn another retentate stream and a second purified stream which is mixed with the one generated by the first module.

Retentate stream being at high pressure is fed to the combustion chamber of a gas turbine to extract mechanical work before routing the exhaust gases into the reforming reactor. Thus, the system of the present invention realizes:

(a) a low temperature reforming reaction; (b) minimize the reforming duty by combining the steam reforming reaction with the water gas shift (WGS) reaction;

(c) transforming the fired reforming reactor into one convective type;

(d) use the exhaust gases form the gas turbine to provide the required reforming duty;

(e) use of high pressure retentate into a gas turbine to generate power and heat to be used in the reforming section and the down-stream heat recovery system.

With the above integration, hydrogen and power are produced with an overall efficiency higher than 60% or hydrogen is produced with an heat requirement, kcal per Nm³ of H₂ which is at least 10% better than conventional technology (around 3250 kcal per Nm³) . Brief description of the drawings

The invention will be more clearly understood from the following description of some embodiments thereof given by way of example only with reference to the accompanying drawings in which:

Fig.l is a schematic representation of the conventional steam reforming ;

Fig.2 is a schematic representation of membrane reforming reactor (MRR): process hybrid scheme. Fig.3 is a schematic representation of MRR reactor

Fig.4 shows a simplified process scheme according to the present invention;

Figg.5A/5B/5C/5D show the results of the calculations to determine the effects of the recycle rate in the process of fig.4;

Figg.6A/6B/6C/6D show the results of the calculations to determine the effects of the steam/carbon ratio in the process of fig.4;

Fig.7 shows how reducing the MRR outlet temperature strongly affects the reformer duty. Brief process description

The entire process scheme is represented in Fig.4. The system of the present invention uses streams coming out from the MRR, Membrane Reforming Reactor and gas turbine to supply feeds and heat to other components.

According to a preferred embodiment, such components include an HDS reactor, MRR based on a two convective reforming sections, a two membrane separation modules, a retentate recycle compressor, not always required, a gas turbine with a recovery section, a final cooling section and hydrogen compressor and a final purification unit based on PSA.

Natural gas feed from battery limits is mixed with hydrogen recycle and preheated before entering the HDS reactor, where sulphur is hydrogenated, transformed in H₂S and removed over a zinc-oxide absorbing bed.

The desulphurized feed is then mixed with steam in a ratio from 2.5 to 4. The steam to carbon ratio (S/C) used to reform the hydrocarbon feeds depends on the type and quantity of hydrocarbons heaver than methane present in the natural gas feed. The presence of the retentate recycle increases H₂ contents in the feed mixture and allows operating with S/C lower than traditional technology with the same feed. Reformed gas feed enters the first membrane module which in turn originates a retentate and H₂ + sweeping gas (steam for instance) . Steam/Hydrogen is ranging from to 1/2 to 1/3.

A retentate portion is cooled down and goes to second membrane separator. The second membrane module generates another H₂ + sweeping gas steam and a second retentate which is routed to the gas turbine as fuel.

Operating conditions around the MRR are set mainly through the recycle rate to satisfy the heat requirement of the gas turbine with same external natural gas make-up.

Hydrogen streams are cooled down to condensate steam that is separated and recycled back to the degasifier. The hydrogen stream is then compressed, further cooled, before final purification with the PSA. Although Pd/Ag tube membrane have assumed, experimental data have been considered not conclusive to avoid a final purification step. Effect of the recycle rate and steam/carbon ratio

The results of the calculations to determine the effects of the recycle rate are shown on fig. 5A/5B/5C/5D, where methane conversion, membrane surface, export power and overall plant efficiency have been plotted against different outlet reforming temperature ranging from 530⁰C to 580⁰C. Reducing the recycle rate is advantageous as regard the overall plant efficiency although the methane conversion is reduced to. At minimum plant efficiency, CH₄ global conversion is limited to 40%. In such a case, the retentate routed to the gas turbine has a substantial calorific value to provide all the process services downstream the gas turbine. At the same recycle rate, steam/carbon ratio has also been analyzed, results are show on Fig. 6A, 6B,6C and 6D for same parameters. Increase the steam to carbon ratio increases the efficiency moving the heat requirement to less than 2900 kcal/Nm³H₂ produced. As a matter of fact by pushing CH₄ conversion with S/C ratio it is possible to avoid the recycle installation.

Effect of pressure and temperature at the outlet of MRR

Pressure of retentate is fixed in order to route part of it directly to the gas turbine where pressure in turn is determined by calculating the required exhaust flue gases temperature at the inlet of the MRR and downstream the waste heat boiler.

Fig. 7 allows to see how reducing the MRR outlet temperature strongly affects the reformer duty. A 30- 35% reduction compared with the conventional technology can be easily achieved working at a temperature as low as 550-560 ⁰C. Effect of shift conversion Although the presence of a shift converter further reduces the CO content, its impact on H₂ production is quite limited, less then 3%.

As result of this, overall its influence is marginal as becomes clear from table 2. Shift converter has then been eliminated from the proposed scheme.

Table 2 Comparison with or without shift converter

Detailed description of the invention Now referring to figure 4, which shows the schematic representation of the invention related to the proposed integrated system, (where the term "integrated " means that most of the feed to the gas turbine is taken from the retentate recycle) , the MRR are embedded into the heat recovery section of the gas turbine which provide all the reaction heat.

Natural gas or any hydrocarbon feasible for the steam reforming reaction is mixed with recycled hydrogen and enters HDS reactor 100, where sulphur compounds are removed from the feed. Feed is then mixed with steam raised in the heat recovery section, 400, and pre-heated also in 400, before entering the first MRR module, 200, where is also mixed with the retentate recycle coming from the second MRR module, 201. Part of the retentate is used as fuel in the gas turbine, 300, from whom the exhaust gases are used to provide heat for the reforming reaction and the other process services. Product hydrogen from the separation modules, 500 and 501, are cooled down in 590 and compressed in 600 before the final purification step in 700.

Although a solid foam structure has been assumed as support for the reforming catalyst in our scheme, there are various type of catalytic supports, for example ceramic or metallic monolith open matrix, which could be used to have a pressure drop across the reactor limited to 0.1 1 bar g, preferably 0.5 barg.

The embedded reformer is heated only by the exhaust gases from the gas turbines which is clearly depicted also in figure 4, without any direct combustion. This implies an even distribution of heat around the reforming catalyst tubes, the lower tube skin temperature is such that high alloy centrifugal tubes are not any more required. The absence of burners results in a very compact and inexpensive configuration of the MRR.

Preferably the operating reforming- temperature at the outlet of the MRR is at least 500⁰C, more preferably in the range of 540-560 ⁰C, but not higher than 600⁰C.

Operating reforming pressure is at least 8 barg, preferably in the range of 10-15 barg.

The amount of steam used to reform the hydrocarbon feed stream depends upon the feed type, reaction conditions and the desired degree of conversion of the feed. With higher S/C ratio it is also possible to eliminate the recycle. Increasing the S/C ratio in the reactor it is also possible to minimize the risk of coke formation. Presence of the recycle stream allows to lower the S/C ratio in comparison to when using no recycle. The composition of the reformate streams 21 and 26 are listed in table 3. Such compositions represent typical values, depending on the reforming conditions the composition may largely change.

Component: Stream 21 Stream 26

H₂O 0 . 6116 0. 6186

H₂ 0 . 2104 0 . 1821

CO 0 . 0478 0 . 0858

CO₂ 4 . 6E-3 7 . 07E-3

CH₄ 0 . 1256 0. 1064

Table 7 : The composition of the reformate streams 21 and 26

Membranes are chosen so that mainly hydrogen passes. The hydrogen leaves the permeate side of the membrane as a purified stream that contains less than 1% of impurities, more preferable less than 500 ppm. By impurities it is meant mainly CO and CO₂ which adversely affect the catalyst performance of the hydro- treaters or hydro-crackers operated in refinery.

The hydrogen separation membrane is preferable operated to have a hydrogen partial pressure of at least 1 barg.

Final purification of such "almost" pure hydrogen is achieved via a Pressure Swing Absorption (PSA) unit.

Any hydrogen separating membrane may be used that is effective in separating hydrogen from the other reaction products in the reforming stream. Preferably the hydrogen separating membrane is selected from palladium, or alloys of palladium with silver and/or copper. Other suitable hydrogen separating membranes are made of ceramic material, as silica-alumina ones or mixture of ceramic and metallic materials.

A portion of retentate is burn into the combustion chamber of a gas turbine. Pressure is selected in order to obtain at the turbine exit a temperature in line with the requirements of the MRR. Such a temperature is preferably in the range of 700-800⁰C.

Temperature in the combustion chamber is fixed by the mechanical resistance of the turbine rotor and is in the range of 1200-1300 ⁰C. Presence of large amount of steam in the retantate allows to maintain the required temperature with limited excess air, much less than the 250-300 % used in a traditional application and drastically decrease the nitrogen oxides generation. Flow of retantate and hydrocarbons' conversion in the MRR, are determined by the duty in the MRR and other process services downstream.

With a stack temperature of 140-160⁰C, the overall system efficiency is higher that 65% defined as the energy in the products (hydrogen + electricity) versus the energy input.

The MRR may be designed in various ways, in one case such reactor could include a bundles of tubes disposed in a shell. Such bundles are made of stainless steel material and not anymore of an exotic material as K40 or HP microalloy used in conventional reformer reactor and are normally contained inside a casing flue-gas-tight steel plate, made with a minimum thickness of 6 mm, all welded and internally insulated. Internal lining is made of ceramic fiber and mineral wool with an internal liner made of stainless steel.

Spacing, or pitch between tubes, is preferably from about 1.1 to about 2 times the tube diameters, when measured form the center of the adjacent tubes. In order to minimize the tube surface, finned tubes are normally used. Extended surface is preferably limited to 197 fins per m and fin height of 25.4 mm. Fin material will be selected according to the maximum tip temperature. The reformate streams 21 and 26 exiting the two reforming stages are preferably passed through heat exchangers to cool the reformate streams at a temperature which is compatible with the hydrogen separating membrane with palladium alloy, such a temperature is ranging form 400 to 500⁰C, preferably around 450⁰C.

The purified hydrogen streams 22 and 24 exiting the separation modules are cooled to condense and remove water at a temperature compatible with compressor operation, at least 100⁰C, but preferably lower. After the hydrogen compression, the stream is cooled to ambient temperature to be compatible with PSA operation for the final purification to 99,999%. Pressure at battery limits is in the range of 10 to 30 barg, preferably around 15-20 barg.

Claims

1) A method for co-generating hydrogen and power for industrial application consisting at least of the following steps: providing hydrocarbon feed and steam to a first convective type reforming reactor, containing low pressure drop reforming catalyst; routing reformed gas to a first membrane separation module which generates a retentate stream and a purified hydrogen stream; cooling down and routing the retentate stream from the first membrane separation module to a second convective type reforming reactor from which the reformed stream is cooled down before entering the second membrane separation module which provides a second purified hydrogen stream which is mixed with the one generates by the first module and a second retentate stream which is routed to a gas turbine as fuel; cooling down the final purified hydrogen stream to condensate the steam used as sweeping gas and then compressing the purified hydrogen before the delivery and final purity control.

2) A method of the preceding claim wherein the reformer steps are part of the same enclosure, meanwhile the hydrogen membrane separation modules consist of separate apparatus.

3) A method of the preceding claims wherein the most of the feed to the gas turbine is taken from the retentate recycle provided by the second membrane separation module. 4) A method of the preceding claims wherein the second retentate stream being at high pressure is fed to the combustion chamber of the gas turbine to extract mechanical work before routing the exaust gases into the reforming reactor.

4) A method of the preceding claims wherein the reforming reactor is heated only by the exhaust gases from the gas turbine.

5) A method of the preceding claims wherein the retentate stream from the first membrane separation module is partially recycled with the fresh feed and partially goes to the second membrane module

6) A method of the claim 6 wherein the retentate recycle to the reforming reactor may vary from 1:1 to 1:10 of the incoming feed, and the second retentate stream may vary from 1:1 to 1:10 of the first retentate stream.

7) A method of claim 1 wherein the hydrogen separation is conducted in two stages to: -a) minimize the overall membrane surface to be installed ;

-b) allow to operate the membrane separator to a temperature lower than the reforming temperature; - c) allow to operate with a sweeping gas as steam or CO2 to lower the partial pressure of purified hydrogen.

8) A method of claim 1 where the reforming catalyst is based on monolith porous structure.

9) A method of claim 1 where the reforming catalyst is: -a) selected from the group consisting of noble metals or high Ni content; -b) supported on low pressure drop matrix as metallic sponge or solid foam ;

-c) able to maximize the internal heat transfer coefficient minimizing the reformer wall temperature. 9) A method of claim 1 where the integrated system works without providing export steam.

10) A method of claim 1 where the gas turbine consists of:

-a) a pressurized combustion chamber fed with recycle retentate;

- -b) an expander where gases from the combustion chamber are expanded to produce mechanical work; -c) a waste heat recovery system based in two sections: -cl) a reforming section;

-c2) a convective section to pre-heat all the process streams, to generate and superheat steam and to preheat BFW.

11) A method of claim 1 wherein from the retentate of the second membrane module, CO₂ is removed before the stream is fed to the gas turbine

12) A method of claim 1 wherein hydrogen and power are produced without NOx emissions.

13) A method of claims 1 and 12 wherein hydrogen and power are produced without Nox and minimal CO₂ emissions

14) An apparatus for co-generating hydrogen and power for industrial application comprising: at least a two-step membrane reforming reactors each comprising a convective type reforming reactor containing low pressure drop reforming catalyst and a membrane separation module in order to provide a retentate recycle to the second reforming reactor from the first module, a retentate stream from the second module to the gas turbine and two purified hydrogen streams; a gas turbine fed by the high pressure recycle retentate, whose exhaust gases are routed to the reforming reactor and subsequent to a heat recovery system; a cooling and final purification system to takeout from the product hydrogen before its delivery.

15) An apparatus according to claim 14 wherein the convective type reforming reactors work at a temperature ranging from 500 to 650⁰C and where the operating temperature of the membrane separation modules is indipendent from the reforming temperature and is ranging from 400 to 450⁰C.

16) An apparatus according to claim 14 further comprising an HDS reactor where sulphur compounds are removed from the feed before the first convective type reforming reactor.

17) An apparatus according to claim 14, wherein said cooling and final purification system consists of a final cooling section and hydrogen compressor and a final purification unit based on a Pressure Swing Absorption (PSA) unit.

18) An apparatus according to claim 14 wherein retentate stream is partially recycled with the fresh feed and partially goes to a second membrane module which generate in turn another retentate stream and a second purified stream which is mixed with the one generated by the first module.

19) An apparatus for cogenerating hydrogen and power for industrial application consisting of: a HDS reactor where sulphur compounds are removed from the feed: at least a membrane reforming reactor MMR, based on a two convective reforming sections in one common enclosure, a two membrane separation modules, operating at a temperature which is indipendent from the refoming one, a gas turbine with a recovery section where the reforming steps are placed, ; a final cooling section and hydrogen compressor; and a final purification unit based on PSA. 20) An apparatus according to claims 14 and 18 where the gas turbine consists of:

-a) a pressurized combustion chamber fed with recycle retentate;

-b) an expander where gases from the combustion chamber are expanded to produce mechanical work;

-c) a waste heat recovery system based in two sections :

-cl) a reforming section;

21) An apparatus according to claims 14 and 18 wherein the purified hydrogen is cooled down to condensate the steam used as sweeping gas and then is compressed before the final purity control by a PSA (Pressure Swing Adsorption) and delivery. 22) An apparatus according to claims 14 and 18 wherein the catalytic supports for the reforming catalyst is selected from a solid foam structure or a ceramic or metallic monolith open matrix, in order to have a pressure drop across the reactor limited to 0.1 -1 barg, preferably 0.5 barg.

23) An apparatus according to claims 14 and 18 wherein the hydrogen separating membrane is selected from palladium, alloys of palladium with silver and/or copper, ceramic material, as silica-alumina ones or mixture of ceramic and metallic materials.

24) An apparatus according to claims 14 and 18 wherein the reformer is heated only by the exhaust gases from the gas turbine without any direct combustion. 25) An apparatus according to claims 14 and 18 wherein the temperature in the combustion chamber of the gas turbine is in the range of 1200-1300 ⁰C. 26) An apparatus according to claims 14 and 18 wherein the MRR include a bundles of tubes disposed in a shell. 27) An apparatus according to claims 14 and 19 wherein the operating reforming pressure is at least 8 barg, preferably in the range of 10-15 barg.