OF HYDROGEN

The present invention relates to an apparatus and a process for producing hydrogen from natural gas or methane.

Fuels cells consuming hydrogen and oxygen (from the air) offer the promise of providing a clean, efficient electrical power source. However this leads to a requirement for an efficient and correspondingly clean process for the production of hydrogen. It would be convenient if this could be produced from hydrocarbons that are currently widely available, for example through the existing natural gas pipeline network. Natural gas as produced from a gas well consists primarily of methane, but may also contain up to about 10% of ethane and propane. Natural gas as supplied to domestic users has chemical odours added to it to ensure that any gas leak can be detected by smell, and may contain small quantities of higher hydrocarbons such as C2 to C4 compounds. Hydrocarbons up to C4 are referred to herein as short chain hydrocarbons .

A process is described in WO 01/51194 and WO 03/048034 (Accentus pic) in which methane is reacted with steam, to generate carbon monoxide and hydrogen in a first catalytic reactor; the resulting gas mixture is then used to perform Fischer-Tropsch synthesis in a second catalytic reactor. The first stage of this process produces hydrogen, but it is inevitably produced in conjunction with carbon monoxide, and so this is not a satisfactory source of hydrogen, because for example pressure swing absorption materials are deleteriously affected by carbon monoxide if it is present at above about 10%.

The present invention accordingly provides a plant for producing hydrogen from a feed gas comprising short chain hydrocarbons, the plant comprising reactor means defining channels for treating the feed gas in three successive stages, the channels for treating the feed gas in each case being adjacent to respective heat transfer channels, and the successive treatment stages being as follows :

(a) pre-heating the feed gas;

(b) subjecting a mixture of steam and the heated feed gas to reforming in the presence of a reforming catalyst, while providing heat from adjacent channels in which catalytic combustion occurs; and

(c) subjecting the gases produced by reforming to the water gas shift reaction in the presence of a shift catalyst, whilst removing heat into adjacent channels to generate steam.

Preferably in stage (a) , the feed gas is heated by heat exchange from the exhaust gases from the catalytic combustion channels.

It will thus be appreciated that the overall process is thermodynamically efficient, in that the heat from the combustion exhaust stream is used to preheat the feed gas, and the heat from the gases produced by reforming is used to generate the steam. Preferably the reactor means is a single integrated reactor in which all three of the successive treatment stages occur. Such a reactor may be formed by a stack of plates between which the treatment channels and heat transfer channels are defined alternately in the stack.

The hydrogen-producing plant also incorporates other components. The feed gas may be natural gas, and if the feed gas contains sulphur-containing compounds, then the plant will incorporate a desulphurising unit, which may for example be a de-sulphurisation bed, and this will typically be arranged to treat the feed gas after it has been preheated in stage (a) but before it is reformed in stage (b) . Clearly means are required to mix the hot steam produced in stage (c) with the preheated feed gas produced by stage (a) , and this mixing means may either be integral with the integrated reactor, or may be a separate mixing unit .

The gas stream emerging from stage (c) after the water gas shift reaction contains a high proportion of hydrogen, typically between 60 and 90%, and smaller proportions of carbon monoxide, carbon dioxide, steam and methane. This gas stream is subjected to a hydrogen separation treatment. This may require a preliminary cooling step, to a temperature suitable for the hydrogen separation process, and so as to condense some of the steam. The hydrogen separation treatment may utilise membrane separation or pressure swing absorption, and preferably removes at least 50% of the hydrogen, more preferably at least 70% but no more than 90%. The resulting tail gas contains hydrogen, methane and some carbon monoxide and carbon dioxide, and may be used as the fuel for the combustion channels.

The hydrogen separation treatment typically requires the gases to be at an elevated pressure in the range 4 to 14 atmospheres (0.4 to 1.4 MPa), and preferably all the treatment stages for the feed gas are at such an elevated pressure, so that no compression of the gas mixture produced by stage (c) is necessary. If necessary, the feed gas is compressed to such an elevated pressure

before being supplied to the reactor means, but in many contexts the feed gas will already be at a suitable elevated pressure.

The invention also provides a process for producing hydrogen from a feed gas comprising short chain hydrocarbons, and making use of such a plant.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 shows a flow diagram of the plant of the invention;

Figure 2 shows a sectional view of a reactor suitable for use in the plant of figure 1; and

Figure 3 shows a sectional view of a stack of plates, shown separated, for making the reactor of figure 2.

Referring now to figure 1, a plant 10 for generating pure hydrogen is provided with natural gas from a pipeline 12 at a pressure of 1.4 MPa. The plant 10 includes a compact reactor 14 in which the natural gas is subjected to three successive treatment stages 16, 18 and 20, these stages being shown separately for clarity in the diagram. The reactor 14 is also provided with air, compressed to a sufficient pressure by a compressor 22 and passed through a filter 24. The reactor 14 consists of a stack of flat plates separated by plates with rectangular corrugations so as to define flow channels for different fluids between successive flat plates, as described below in relation to figures 2 and 3.

The first stage 16 is a preheating stage, and this part of the reactor 14 has channels for three different fluids arranged successively in the stack: combustion exhaust gases 16a (flowing axially from top to bottom as shown), natural gas 16b (flowing diagonally), and air 16c (flowing diagonally) such that the natural gas and the air are both preheated by heat exchange with the exhaust gases. There might for example be thirteen of each type of flow channel, the outermost flow channel carrying exhaust gases 16a. The flow directions may differ from those shown here, and for example the natural gas and the air might flow along serpentine paths made up as a series of cross-flow passes between appropriate headers.

The natural gas is then passed through a desulphurisation bed 26 in which any sulphur-containing odours are removed by chemisorption. It is then mixed with steam at the same elevated pressure at a mixer 28 at a controlled mole ratio with respect to the carbon in the feed gas. This ratio is in the range 2 to 4:1 of steam relative to carbon, in order to promote not only steam reforming but also the water gas shift reaction.

This hot mixture of natural gas and steam is then passed into the second stage 18 in which it exchanges heat with a catalytic combustion reaction in adjacent channels, while contacting a reforming catalyst. The combustion channels 18a (extending axially from bottom to top as shown) contain a combustion catalyst, while the reforming channels 18b (extending transversely) contain the reforming catalyst. For the oxidation reaction (catalytic combustion) several different catalysts may be used, for example palladium, platinum or copper on a ceramic support; for example copper or platinum on an alumina support stabilised with lanthanum, cerium or barium, or palladium on zirconia, or palladium on a metal

hexaaluminate such as magnesium, calcium, strontium, barium or potassium hexaaluminate. For the reforming reaction also several different catalysts may be used, for example nickel, platinum, palladium, ruthenium or rhodium, which may be used on ceramic coatings; the preferred catalyst for the reforming reaction is rhodium or platinum on alumina or stabilised alumina. In each case the catalyst is provided in the form of a corrugated metal foil coated with the ceramic coating, typically no more than 100 μm thick, and impregnated with the catalytic metal, such a corrugated foil being inserted into each flow channel in which a reaction is to occur. There may for example be thirteen combustion channels 18a alternating with twelve reforming channels 18b.

By virtue of heat exchange with the combustion process, the reforming channels are at an elevated temperature that may for example be 800 ⁰ C. The methane reacts with water vapour, by the reaction:

H ₂ O + CH ₄ → CO + 3 H ₂ .

The resulting gas mixture, which may be referred to as synthesis gas, is then supplied to the third stage 20 in which it loses heat to form the high-temperature steam in the adjacent channels 20a (the steam channels being shown extending axially from bottom to top) , the channels 20b carrying the synthesis gas (extending transversely as shown) containing a catalyst for the water gas shift reaction:

CO + H ₂ O → CO ₂ + H ₂

the resulting steam being supplied to the mixer 28. The effect of the water gas shift reaction is to increase the

proportion of hydrogen, and to decrease the proportion of carbon monoxide. Preferably the carbon monoxide proportion is reduced to no more than about 5% (by volume) .

The resulting gas mixture comprises hydrogen, unreacted methane, carbon monoxide and carbon dioxide, and steam. It is first passed through a heat exchanger 30 in which it is cooled by air to the optimal value for the subsequent hydrogen removal. After cooling, the excess steam condenses, and is separated in a separator 32 and may be returned to the steam generating channels 20a. Alternatively this heat exchanger 30 might use water as coolant .

The cooled gas mixture is then supplied to the hydrogen recovery unit 34. This may utilise pressure swing absorption to separate a pure hydrogen stream 36. This is operated in such a way that about 75% of the hydrogen is removed. The remaining tail gases 38 are then fed, via a detonation arrester 40 and a mixer 42 in which they are mixed with the preheated air, into the combustion channels 18a.

A water supply 44 is required by the plant 10 at least for start-up and for process water make-up and this is supplied through a pressurising pump 45. It must be pure water to ensure the integrity of the steam raising circuit and the reforming catalyst, so this water supply is treated by an absorption column 46. During operation, most of the water for generating steam is actually provided by condensing the steam in the heat exchanger 30. To initiate operation of the plant 10, a proportion of the combustion air may be heated electrically so that when mixed with the natural gas combustion will occur in the presence of the catalyst in the combustion channels

18 a .

It will be appreciated that the materials of which the reactor 14 are made are subjected to a severely corrosive atmosphere in use, for example the temperature in the combustion channels 18a may be as high as 900°, although more typically around 85O ⁰ C. The plates forming the reactor 14 may be made of an iron/nickel/chromium alloy for high temperature use, such as Haynes HR-120 or Inconel 800HT (trade marks) . The catalyst substrates do not have to resist pressure differences, and may be of an aluminium-bearing ferritic steel such as Fecralloy (trade mark) which is iron with up to 20% chromium, 0.5 - 12% aluminium, and 0.1 - 3% yttrium. For example it might comprise iron with 15% chromium, 4% aluminium, and 0.3% yttrium. When this metal is heated in air it forms an adherent oxide coating of alumina which protects the alloy against further oxidation; this oxide layer also protects the alloy against corrosion under conditions that prevail within for example a methane oxidation reactor or a steam/methane reforming reactor.

Where such an aluminium-bearing steel is used as the corrugated foil for a catalyst substrate, and is coated with a ceramic layer into which a catalyst material is incorporated, the alumina oxide layer on the metal is believed to bind with the oxide coating, so ensuring the catalytic material adheres to the metal substrate. The substrate may be a wire mesh or a felt sheet, which may be corrugated, dimpled or pleated, but the preferred substrate is a thin metal foil. The metal substrate of the catalyst structure within the flow channels enhances heat transfer and catalyst surface area. The catalyst structures are removable from the channels in the stack, so they can be replaced if the catalyst becomes spent; consequently headers or covers must be provided on the

stack which can be removed to provide access to the channels, for this removal and replacement.

The stack of plates forming the reactor 14 are bonded together either by diffusion bonding or by brazing.

Referring now to figures 2 and 3, the reactor 14 comprises a stack of rectangular flat plates 50 (see figure 3) each 1 mm thick, spaced apart by corrugated plates. Depending on the scale of the plant 10, these plates 50 might be of length between 100 and 1200 mm, and of width between 75 and 600 mm. Axial flow channels 52 (such as the flow channels 16a, 18a or 20a) are defined by plates 54 corrugated with rectangular castellations 5 mm high and 20 mm wide; each such plate 54 is provided with an edge rib 55 of the same height as the overall height of the castellated plate 54 along each side. Transverse flow channels 56 (such as the flow channels 18b or 20b) are defined by plates 58 with similar rectangular castellations, for example 5 mm high and 20 mm wide, and with similar edge ribs 59, each plate 58 having castellations extending from one side to the other of the stack, but the width of each plate 58 is only a third of the total length of the stack, so that there are three such plates 58 side-by-side. As shown in figure 2, appropriate headers 60 are welded onto each end of the stack to communicate with the axial flow channels 52. Similarly, for the transverse flow channels 56, an inlet header 62 is welded to the stack at one end of one side, and an outlet header 63 at the other end of the other side, each being of the same width as one of the castellated plates 58. Headers 64 of twice that width are then welded on to the remainder of each side, so as to communicate with the other pair of castellated plates 58 on that side. Hence the castellated plates 58 and the

headers 62, 64 and 63 ensure that the gases flowing in these transverse flow channels 56 follow a generally zigzag path, as shown by the arrows.

Diagonal flow channels such as the flow channels 16b and 16c may be defined by similar castellated plates whose castellations extend in an appropriate diagonal direction between respective inlet headers and outlet headers attached to the sides of the stack.

The axial flow channels 18a contain catalyst foils as described earlier, as do the transverse flow channels 18b and 20b. When it is necessary to replace the catalyst, this may be done by cutting off one of the headers 60, or both the headers 64, extracting the foils on which the catalyst is spent, and replacing with fresh catalyst-carrying foils. The headers 60 or 64 can then be re-welded into position.

It will be appreciated that the reactor 14 shown in figures 2 and 3 is by way of example only, and that a hydrogen production plant of the invention might utilise a different design of compact catalytic reactor. For example the channels, or at least some of the channels, might be defined by grooves in thick plates, rather than by castellations in thin plates.