WO2008082312A1

WO2008082312A1 - Hydrogen production

Info

Publication number: WO2008082312A1
Application number: PCT/NO2008/000004
Authority: WO
Inventors: Bjørnar ARSTAD; Ivar Martin Dahl; Richard Blom
Original assignee: Sinvent AS
Current assignee: Sinvent AS
Priority date: 2007-01-05
Filing date: 2008-01-07
Publication date: 2008-07-10
Anticipated expiration: 2009-07-05

Abstract

The present invention comprises a method for continuous production of hydrogen gas and simultaneously separation of at least one gas product component, in which at least one feed gas is conveyed to a first reactor comprising of a sorbent powder, the hydrocarbon rich feed is converted into at least one gas product component which is substantially absorbed by the sorbent powder, wherein said sorbent is conveyed substantial vertically into a loop-seal and through a substantially S-shaped tube, further said sorbent is conveyed substantial vertically out of said loop-seal and into a riser, said sorbent is conveyed via said riser into a second reactor for regeneration, said sorbent is conveyed substantial vertically into a second loop-seal and through a substantially S-shaped tube, said sorbent is conveyed substantial vertically out of said second loop-seal and further to said first reactor. Further, the present invention comprises a system for continuous production of hydrogen gas and simultaneously separation of at least one gas product component, comprising at least two fluidized bed reactors, at least two loop seals and a riser.

Description

Hydrogen production

Introduction

The present invention comprises a method and system for continuous production of hydrogen gas and simultaneously separation of at least one gas product component.

State of the art

Hydrogen will be an important energy carrier in the future and much research is focused on various production methods. Hydrogen production may be completed by use of renewable energy sources, via e.g. hydrolysis of water into H₂ and O₂, or from fossil fuels like, coal, oil and gas. The major way to produce hydrogen/synthesis gas from fossil fuel today is by steam reforming (SR) of natural gas which is a two step reaction:

CH₄(g) + H₂O(g) = CO(g) + 3H₂(g) Reforming (1)

CO(g) + H₂O(g) = CO₂(g) + H₂(g) Water-gas shift (2)

CH₄(g) + 2H₂O(g) = CO₂(g) + 4 H₂(g) Overall

(3)

The highly endothermic SR reaction (3) takes place at 700-900⁰C using a nickel based catalyst. The second reaction is the water-gas shift reaction in which CO reacts with water and produces CO₂ and H₂. These reactions are reversible and by removing CO₂ with an internal sorbent during reaction more hydrogen will be produced. This is favorable since a typical equilibrium hydrogen concentration in steam reforming of methane is only ca. 70-75 %. Use of an internal sorbent in steam reforming is commonly termed sorption enhanced steam reforming (SESR). This principle is quite old and in 1984 Rostrup-Nielsen reported that the concept of an added sorbent for CO₂ absorption in a hydrocarbon-steam reactor was already published in 1868 [1]. In 1933 Williams patented a process where steam and methane react in the presence of a mixture of lime and catalyst to produce hydrogen [2], and in 1963 Gorin and Retallick patented a fluidized-bed process using reforming catalyst and a CO₂ acceptor [3]. When CO₂ is produced in the reactor it reacts with the sorbent and the reaction is driven towards the products. It is thus possible to exceed the sorbent free thermodynamic equilibrium for the reactions. In addition, the product stream will be depleted of CO₂ compared to a regular steam reforming reaction. The sorbent is regenerated in another process step and produce a CO₂ rich stream suitable for other uses like sequestration or similar.

Sorbents for SESR

Several sorbents may be suitable for SESR, both natural and synthetic ones. Some common sorbents that have been used, or have a potential use in SESR is natural dolomite, (a mixed carbonate of MgCO₃ and CaCO₃), lime (CaO), and potassium promoted hydrotalcite [4-7]. Dolomite is a very common mineral in nature and together with two other calcium carbonates (calcite and aragonite) it makes up -2% of the earth's crust [8]. Calcium oxide based sorbents may react with CO₂ and form carbonates in an exothermic reaction like:

CaO(s) + CO₂(g) = CaCO₃(S) ΔH_r(298 K) = - 178 kJ/mol (2)

To obtain fast absorption kinetics for calcium oxide sorbents the temperature must typically be around 500-600 ⁰C. CO₂ desorbs from the material at ca. 800 ⁰C, and higher, depending on composition of the atmosphere. In a pure CO₂ atmosphere at 1 atm. CO₂ desorbs from the solid material at ca. 900 ⁰C and at lower temperatures by reducing the partial pressure of CO₂.

Ideally, the material should last for many absortion/desorption cycles but in reality it will eventually detoriate and become less effective. Several studies have addressed these problems for natural sorbents like dolomite and lime. These materials may deactivate due to sintering, because of impurities and minerals in the feed, or from the formation of hydroxyl groups on the surface [5, 9-11]. Loss of structure and mechanical strength may also take place due to cracking and breaking down of the material during use. Silaban et al. compared dolomite and lime and found that with CaO obtained from CaCO₃, the first-cycle fractional recarbonation was limited to about 0.80, a value which decreased by 15 to 20% in each subsequent cycle. In contrast, the first-cycle fractional recarbonation of CaO in calcined dolomite was typically 0.90 to 0.95 and this value decreased by only 1 to 2% in each subsequent cycle. Dolomite's advantages are attributed to the "excess" pore volume created by the original decomposition of MgCO₃, and by a reduction in the rate of CaCO₃ sintering in the presence of MgO [10]. In contrast, Silaban et al. suggested that for lime there is a pore closure after just a few absortion/desorption cycles [10]. Calcinations kinetics of calcium-based sorbents was studied in more detail by Garcia-Labiano et al. [12]. They proposed different kinetic equations for CO₂ desorption under various conditions like e.g. partial pressure of CO₂ and total pressure.

Other reported promising sorbents are based on lithium zirconates [13-16] and lithium silicates [17, 18]. Ochoa-Fernandez et al. recently reported promising stability of lithium zirconates based materials during several adsorption/desorption cycles [16].

There have been some studies using quantum chemical modelling of reactions of CO₂ with MgO and CaO. Such studies give valuable information on molecular details of surface reaction of CO₂ with the oxides. A recent paper by Jensen et al. investigates CO₂ ¹S reaction with MgO and CaO clusters [19]. One finding is that

CO₂ generally adsorbs better on CaO surfaces than MgO surfaces. This finding is in line with general trends where acidic gases react better with more basic oxides. Oxides of group 2 metals become more basic by increasing atomic number, MgO

< CaO < SrO < BaO where BaO is most basic [20]. Another interesting result from quantum chemical calculations is molecular structures for the investigated species.

Calculated structure of CO₂ adsorption on MgO/CaO edges can be seen in [19]. Note that the oxides are modelled as cluster, i.e. that only a small part of a particle is used to model the reaction. However, adsorption structures should probably not change much by increasing the particle models more than actually employed in these calculations. SESR

Sorption enhanced steam reforming is a process suitable for a cyclic reactor unit but most works so far have mostly used systems that switch gases over a catalyst bed during the experiments thus simulating the cyclic process of absorption/desorption of CO₂. Both fixed beds and fluidized bed reactor systems have been used [4, 21-24].

In a recent work by Johnsen et al., SESR of methane using dolomite as an internal sorbent was studied in a fluidized bubbling bed reactor [4].

They used a nickel catalyst at 600 ⁰C during reforming and they regenerated the dolomite batchwise in nitrogen atmosphere at 850 ⁰C. They obtained, as expected, a hydrogen concentration of 98-99 vol% at a dry basis for a period of 150-180 minutes before CO2 breakthrough.

The hydrogen concentration remained at 98-99 % after four reforming/calcination cycles, however, the production time before regeneration decreased with increasing cycle number due to loss in activity of the dolomite. The overall reaction rate did not change due to deactivation of the dolomite. They also concluded that bubbling fluidized bed reactors are very attractive for SESR.

Modelling of SESR processes

Modelling of SESR is sparse but a recent work by Johnsen et al. models the reaction in a dual fluidized bubbling bed reactor and cites some other relevant modelling studies [28]. In their modelling work they find that dry hydrogen concentrations of more than 98 % can be achieved for temperatures of ~600°C and superficial gas velocities of 0.1 m/s, using a simple two-phase bubbling bed model for the reformer, coupled with reaction kinetics of steam reforming of methane, water-gas shift and carbonation of Arctic dolomite. They also find that operating at a high circulation rate gives the highest efficiencies because the reaction rate of natural sorbents is slow at high conversions. Other related sorption enhanced processes.

The principle of sorption enhanced steam reforming has also been used in other processes. It has been reported that during gasification of coal, oil and other hydrocarbons lime (CaO) has been used to enhance product formation and to fixate CO₂ [29].

In a cyclic continuously operating reactor configuration the reforming and absorption take place in one reactor and after some reaction time the sorbent is transported away to another reactor for regeneration. After regeneration, the sorbent is transported back into the reformer again.

Since the sorption/desorption process take place at two different temperatures there must be a transport of powder between the two temperature zones/reactors. Such powder transport between the two reactor units may be accomplished via several different technologies. It may be done mechanically or by a circulating fluidized system. In a circulating fluidized system the two reactors are fluidized bed reactors, and two powders transport lines goes via loop seals that each is also fluidized. These loop seals ensure that the gases in the two reactors are separated during the whole process. Professor Lyngfelt at Chalmers in Gothenburg and his group has constructed cyclic fluidized reactors for chemical looping combustion (CLC) [25-27]. Lyngfelt et al. describes combustion systems (CLC) comprising a reactor and a riser in which regeneration of the catalyst is performed in addition to conveying the catalyst/powder. Regeneration of the catalyst in the CLC process is fast.

Summary of the invention

The present invention comprises a method and system for SESR or other sorbent enhanced reactions, e.g. sorbent enhanced watergas shift reactions. The present invention comprises two reactors that operate in a bubbling fluidization regime. In addition the present invention has a riser that is used for transporting powder in the system. In the present invention there are no reaction taking place in the riser. In the present invention a separate fluidized bed reactor is required in order to regenerate the catalyst due to the relative long regeneration time of the present sorbent. The present invention is a continuous process for the production of H₂ and simultaneously separation of CO₂ and not a combustion process.

The objective of the present invention is to provide a method and system for continuous production of hydrogen gas and simultaneously separating produced CO₂ away from the effluent gas.

The feed to be converted in the reactor could be natural gas, gasified coal or biomass or it could be suitable oil fractions. In addition the feed into the reactor could also be CO and steam for sorbent enhanced watergas shift reactions.

The present invention comprises a method for continuous production of hydrogen gas and simultaneously separation of at least one gas product component, in which at least one feed gas is fed to a first reactor, comprising a sorbent powder, said feed gas is converted into at least one gas product component in which at least one gas product component is substantially absorbed by the sorbent powder, and hydrogen gas produced thereby is separated off, wherein said sorbent is conveyed into a loop-seal, further said sorbent is conveyed out of said loop-seal and into a riser, said sorbent is conveyed via said riser into a second reactor for regeneration, and said at least one gas product component is separated off, said sorbent is conveyed into a second loop-seal, said sorbent is conveyed out of said second loop-seal and further to said first reactor.

Further, the present invention comprises an embodiment in which a method for continuous production of hydrogen gas and simultaneously separation of at least one gas product component, in which at least one feed gas is fed to a first reactor, comprising a sorbent powder, said feed gas is converted into at least one gas product component in which at least one gas product component is substantially absorbed by the sorbent powder, and hydrogen gas produced thereby is separated off, wherein said sorbent is conveyed into a loop-seal, further said sorbent is conveyed out of said loop-seal and into a second reactor for regeneration, and said at least one gas product component is separated off, said sorbent is conveyed into a second loop-seal, further said sorbent is conveyed out of said second loop-seal and into a riser, said sorbent is conveyed via said riser into said first reactor.

A feature of the present invention is to convey the sorbent substantially vertically into and out of said loop seals. Further, said loop seal comprises a substantially S- shaped tube, and at least one of the following gases is fed to said loop seals: steam and inert gas. Further, the following gases are fed to the reactors according to the invention: steam, inert gas, CO, CO₂, H₂ and hydrocarbon containing gas. Concerning the riser of the invention at least one of the following gases are fed to said riser: steam, inert gas and CO₂.

The temperature of said first and second reactor is at least 100 ^°C. Further, the temperature of said second reactor is in the range of from 300 - 1000⁰C. The temperature of said second reactor is higher than the temperature of said first reactor. Further, the present invention may comprise a catalyst. The sorbent may be conveyed from said riser via a cyclone to a second reactor in the present invention. Another embodiment of the invention is the sorbent being conveyed from said riser via a cyclone to a first reactor. A feature of the present invention is the possibility of conveying the sorbent directly from said riser to said second reactor. The use of the expression directly in the present invention should be understood as directly with no opportunity for other apparatus between the mentioned riser and the mentioned reactor. Further, said sorbent is conveyed directly from said riser to said first reactor.

Another aspect of the present invention is a system for continuous production of hydrogen gas and simultaneously separation of at least one gas product component, comprising at least two fluidized bed reactors, at least two loop seals and a riser. The inlet to and an outlet from said loop seals are arranged substantially vertically, and the loop seals are substantially S-shaped. An embodiment of the present invention is a system in which said second reactor and second loop seal is succeeded by said first reactor and said first loop seal and further succeeded by said riser. Further, a cyclone is arranged previous to said second reactor. Another feature of the invention is that the riser is directly connected to said second reactor.

A further embodiment of the system according to the invention is the first reactor and first loop seal being succeeded by said second reactor and said second loop seal and further succeeded by said riser. The embodiment can be arranged with a cyclone previous to said first reactor. Further, the mentioned riser is directly connected to said first reactor.

An even further embodiment of the system for continuous production of hydrogen gas and simultaneously separation of at least one gas product component, comprises the following: a first fluidized bed reactor and a first loop seal substantially vertically thereunder, said first fluidized bed reactor and first fluidized loop seal constituting a first reactor group, further a second fluidized bed reactor, and a second loop seal substantially vertically thereunder, said second fluidized bed reactor and second loop seal constituting a second reactor group, one of said groups being a top group arranged substantially vertically above the other lower group for conveying said sorbent powder substantially vertically downward from the top group to the lower group, and a riser for returning said sorbent powder from a bottom exit of the lower group to an entrance on top of the top group, completing a sorbent powder cycle arrangement, one of said reactors being a main reactor and comprising a H₂ exit, and the other reactor being a regeneration reactor and comprising an exit for said at least one gas product component.

The temperature of said first and said second reactor is at least 100 ^°C.

The present invention comprises a method for continually production of H₂ and simultaneously separation of a combustion gas, wherein at least one feed gas is conveyed to a first reactor optionally together with steam and/or other reactive gases, said first reactor is a fluidized bed reactor comprising a sorbent in which at least one reaction takes place, said sorbent is conveyed into a loop seal and further through a riser, optionally into a cyclone and further into a second reactor for regeneration, said sorbent is conveyed into a second loop seal and further to said first reactor. Further, said loop seal comprises a U shaped tube. The expression U-shape can also be regarded as part of a S-shape. The steam are conveyed to said loop seals and reactors. Further, a combustion gas, steam, CO₂ and/or combustion gas is conveyed to said riser. The temperature of said first and second reactors are at least 100 ⁰C. The system for continually production of H₂ and a combustion gas, comprises at least two fluidized bed reactors, at least two loop seals, a riser and optionally a cyclone. The loop seals has a U like shape.

Brief description of drawings

Figure 1 shows equilibrium pressure of CO₂ over a CaO/CaCO₃ mixture as a function of temperature [28].

Figure 2 shows the stability of lithium zirconate during capture/regeneration cycles. The figure shows high stability and fast kinetics during several sorption/desorption processes [16].

Figure 3 shows two selected structures of adsorbed CO₂ on MgO/CaO particles

[29]. Note the two different adsorption modes of CO₂, a) bidentate and b) tridentate. Red atoms are oxygen and green atoms are Mg/Ca [19]. Figure 4 is a schematic drawing of the fluidized bed reformer used by Johnsen et al. [4].

Figure 5 shows outlet composition (dry basis) as a function of time [4].

Figure 6 shows a process diagram of the SESR concept. The sorbent, in this case

CaO, and the catalyst is transported between the two reactor units. The effluent gases the reactors do also contain inert gases and unconverted feed.

Embodiment of the invention will be described with reference to the following drawings, where:

Figure 7 shows a reactor system according to the present invention in which two connected fluidized beds perform continually two different chemical reactions in sequence. The sorbent is conveyed via a cyclone before regeneration. Figure 8 shows a reactor system according to the present invention in which two connected fluidized beds perform continually two different chemical reactions in sequence.

Figure 9 shows a reactor system according to the present invention in which a first reactor and first loop seal is succeeded by a second reactor and a second loop seal in which the sorbent is conveyed via a riser to a cyclone previous to said first reactor.

Figure 10 shows a reactor system according to the present invention in which a first reactor and first loop seal is succeeded by a second reactor and a second loop seal via a riser to said first reactor.

Detailed description

The objective of the present invention may be obtained by the features as seth forth in the following description of the invention.

The shape and mentioning of the loop seal as S-shaped in the present invention should be understood according to figure 7 and 8. The shape and expression U- shaped should in the present invention also be understood as mentioned previously as U-shaped, or rather as S-shaped. Further, the loop seal should also be understood as a powder lock.

The present invention comprises a process for continuous production of H₂ from natural gas or a hydrocarbon containing gas and simultaneously separation of CO₂. Further, said invention also comprises a system in which the reactor unit is designed with two connected fluidized bed reactors that makes it possible to perform continually two different chemical reactions in sequence, like e.g. sorption enhanced steam reforming or other sorbent enhanced reactions. Examples:

Reactor 1 :

~4 cm diameter, height ca. 10-15. Used for the sorption enhanced steam reforming of natural gas. The sorbent (Dolomite or others) and the catalyst (Ni-based or a common reforming catalyst) has a residence time for some minutes in this reactor. The feed is a mixture of CH₄ and steam and the total feed gas flow is from 2-5 L/min. The outlet is enriched of hydrogen. After some residence time in the reactor the powder falls, via a tube, into a chamber in a loop seal.

Loop seal 1 :

2x2 cm areal and 10 cm high.

1-1.5 L/min gas (nitrogen/steam) is used for fluidizing the powder in the loop seal. The loop seal has a S like shape, and function to separate the two reactor gas streams from each other. Without these two loop seals the reactors would be connected and the outlet gases mixed. The powder is transported through the S- shaped loop seal and falls into a pipe that is connected with the riser. The particular design shown in the figure ensures that the powder is transported together with the loop seal feed gas into the riser easily and without problems.

Riser:

6 mm inner diameter of the riser

The riser is a tube used for transporting used powder to the top of the reactor unit and into reactor 2, via a cyclone. The riser gas (N₂/CO₂) is fed at a rate of 2-3 L/min.

Cyclone:

~ 4 cm in diameter A cyclone is able to separate the particles from the riser gas stream. In this reactor unit this may be done but it is not necessary. At present the cyclone is used as an intermediate unit before the riser gas and powder is transported into reactor 2. Reactor 2:

Similar dimensions as rector 1 , but that is not required. The ultimate size would depend on the kinetics of the actual reaction taking place. In sorption enhanced steam reforming, Reactor 2 is used for regeneration of the sorbent at 300-1000 ^°C, preferably 800-900°C. The fluidizing gases in reactor 2 may be CO₂/N₂/steam. In sorption enhanced steam reforming, the sorbent carries CO₂ that is liberated when heated. The outlet of reactor two is the carrier gas and CO₂. Recirculation of parts of the CO₂ gas may be used for the riser carrier gas. After some minutes in reactor 2, the powder is transported via loop seal 2 (similar to loop seal 1) and into reactor 1 for another cycle of the sorption enhanced steam reforming.

The presented reactor system is a design that shows a technical solution for coupling two fluidized beds that can be used for e.g. sorption enhanced steam reforming.

References

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Claims

C l a i m s

1. Method for continuous production of hydrogen gas and simultaneously separation of at least one gas product component, in which at least one feed 5 gas is fed to a first reactor, comprising a sorbent powder, said feed gas is converted into at least one gas product component in which at least one gas product component is substantially absorbed by the sorbent powder, and hydrogen gas produced thereby is separated off, wherein said sorbent is conveyed into a loop-seal, further said sorbent is conveyed out of said loop- io seal and into a riser, said sorbent is conveyed via said riser into a second reactor for regeneration, and said at least one gas product component is separated off, said sorbent is conveyed into a second loop-seal, said sorbent is conveyed out of said second loop-seal and further to said first reactor.

i5 2. Method for continuous production of hydrogen gas and simultaneously separation of at least one gas product component, in which at least one feed gas is fed to a first reactor, comprising a sorbent powder, said feed gas is converted into at least one gas product component in which at least one gas product component is substantially absorbed by the sorbent powder, and hydrogen gas0 produced thereby is separated off, wherein said sorbent is conveyed into a loop- seal, further said sorbent is conveyed out of said loop-seal and into a second reactor for regeneration, and said at least one gas product component is separated off, said sorbent is conveyed into a second loop-seal, further said sorbent is conveyed out of said second loop-seal and into a riser, said sorbent is5 conveyed via said riser into said first reactor.

3. Method according to any of the claims 1 -2, wherein said sorbent is conveyed substantially vertically into said loop seals. 0

4. Method according to any of the claims 1 -2, wherein said sorbent is conveyed substantially vertically out of said loop seals.

5. Method according to any of the claims 1 -2, wherein said loop seal comprises a substantially S-shaped tube.

6. Method according to any of the claims 1 -2, wherein at least one of the following gases is fed to said loop seals: steam and inert gas.

7. Method according to any of the claims 1 -2, wherein at least one of the following gases are fed to said reactors: steam, inert gas, CO, CO₂, H₂ and hydrocarbon containing gas.

8. Method according to any of the claims 1-2, wherein at least one of the following gases are fed to said riser: steam, inert gas and CO₂.

9. Method according to any of the claims 1-2, wherein the temperature of said first and said second reactor is at least 100 ^°C.

10. Method according to any of the claims 1-2, wherein the temperature of said second reactor is in the range of from 300 - 1000^°C.

11. Method according to any of the claims 1 -2, wherein the temperature of said second reactor is higher than the temperature of said first reactor.

12. Method according to any of the claims 1-2, wherein said first reactor comprises a catalyst.

13. Method according to claims 1 , wherein said sorbent is conveyed from said riser via a cyclone to a second reactor.

14. Method according to claim 1 , wherein said sorbent is conveyed directly from said riser to said second reactor.

15. Method according to claims 2, wherein said sorbent is conveyed from said riser via a cyclone to a first reactor.

16. Method according to claim 2, wherein said sorbent is conveyed directly from said riser to said first reactor.

17. System for continuous production of hydrogen gas and simultaneously separation of at least one gas product component, comprising at least two fluidized bed reactors, at least two loop seals and a riser.

18. System according to claim 17, wherein an inlet to and an outlet from said loop seals are arranged substantially vertically.

19. System according to claim 17, wherein said loop seals are substantially S- shaped.

20. System according to claim 17, wherein said second reactor and second loop seal is succeeded by said first reactor and said first loop seal and further succeeded by said riser.

21. System according to claim 20, wherein a cyclone is arranged previous to said second reactor.

22. System according to claim 20, wherein said riser is directly connected to said second reactor.

23. System according to claim 17, wherein said first reactor and first loop seal is succeeded by said second reactor and said second loop seal and further succeeded by said riser.

24. System according to claim 23, wherein a cyclone is arranged previous to said first reactor.

25. System according to claim 23, wherein said riser is directly connected to said first reactor.

26. System for continuous production of hydrogen gas and simultaneously separation of at least one gas product component, comprising the following: a first fluidized bed reactor and a first loop seal substantially vertically thereunder, said first fluidized bed reactor and first fluidized loop seal constituting a first reactor group, further a second fluidized bed reactor, and a second loop seal substantially vertically thereunder, said second fluidized bed reactor and second loop seal constituting a second reactor group, one of said groups being a top group arranged substantially vertically above the other lower group for conveying said sorbent powder substantially vertically downward from the top group to the lower group, and a riser for returning said sorbent powder from a bottom exit of the lower group to an entrance on top of the top group, completing a sorbent powder cycle arrangement, one of said reactors being a main reactor and comprising a H₂ exit, and the other reactor being a regeneration reactor and comprising an exit for said at least one gas product component.

27. System according to claim 26, wherein the temperature of said first and said second reactor is at least 100 ^°C.