Control of the temperature of reactions within an acceptable range has been widely investigated and the chemical industry has devised several arrangements, those commonly used being discussed in standard references and texts, e.g. one might consider the general teachings by Octave LEVENSPIEL in Chapter 19 of Chemical Reaction Engineering (published by John Wiley & Sons).
The prior art includes a conventional reactor designed to offer more control over the reactant temperature and this is known as the staged adiabatic packed bed reactor. This system uses an arrangement wherein a number of discrete, spaced apart zones of reaction are provided with means therebetween to control the temperature of the products leaving a first zone of reaction prior to entering the next reaction zone. No heat exchanging means is provided to control the temperature of the reaction in the zones of the reaction. Thus the reactant fluid entering the reactor at a desired temperature passes through a packed bed containing catalyst. Upon exiting this first stage, the reactant gas and any products will have a temperature higher or lower than that of the initial temperature depending upon the reaction thermal characteristics. A heat exchanger then heats or cools the reactant gas to a second desired temperature, which may or may not be equivalent to the temperature of the first, before passing to the next packed bed i.e. the second stage. This sequence is repeated until the desired conversion is obtained. Thus the temperature profile of the reaction will be stepped within an acceptable range of temperature, and will therefore not be truly isothermal.
The preferred heat exchanger panel for the purposes of the invention is one formed from a plurality of plates superposed and diffusion bonded to form a stack of plates, wherein fluid passages are defined in said stack by virtue of a pre-treatment of said plates wherein each plate is selectively configured to provide channeled or blank surfaces according to the desired pattern of fluid passages by a treatment to remove surface material e.g. by chemical etching, or hydraulic milling, or the like process to a desired depth. Optionally the chemical treatment may be augmented by a mechanical treatment using a suitable tool.
Such a pre-treatment of the plates is conducted in a manner analogous to manufacture of printed circuit boards (PCBs) and for this reason the heat exchanger design can be described as a printed circuit heat exchanger (PCHE). The application of the diffusion bonding technique for metal plates is well understood in the art of metal working and is applied for a variety of purposes e.g. in medical prosthesis manufacture.
This design of the PCHE has been proven by the designers of the proposed PCR system since 1985 when these compact heat exchangers were first introduced.
A PCR type of reactor was designed by the present applicants and is the subject of a separate patent application (Ref:32 46271 WO-). Such a reactor is formed to provide at least one reaction zone, bounded by a heat exchanger formed from a plurality of plates superposed and diffusion bonded to form a stack of plates, wherein fluid channels are defined in said stack by virtue of pre-treatment of said plates wherein each plate is selectively configured according to the desired pattern of channels by a chemical treatment to remove surface material e.g. by chemical etching, to a desired depth. The fluid channels defined in the stack provide the opportunity to arrange for various reactant fluids to be conveyed in channels arranged in a heat exchange relationship with discrete channels containing at least one auxiliary fluid for controlling the temperature of the reactants.
Considering the example of a known ammonia converter, for a given ammonia content in the reactants there is a temperature for which the desired reaction rate is at a maximum. This is because the rate of synthesis of ammonia is the net result of the competing rates of the forward and reverse reactions. Consequently, by monitoring and controlling the temperature, it is possible to determine a temperature that favours the forward production reaction more than the reverse product dissociation reaction. In fact it is found that whilst increased temperature generally causes an increase in reaction, and indeed the desired forward product formation reaction rate increases favourably with temperature, at a certain approach to equilibrium conditions the concurrent increased rate of the reverse reaction begins to dominate, and to slow the overall synthesis rate. Maximum conversion in a bed of given size is therefore achieved if conditions remain on the maximum rate line. This is shown graphically in FIG. 1.
Ammonia synthesis typically takes place at high pressures, greater than 100 bar and therefore creating bed volume is relatively expensive. Furthermore, the catalyst itself is costly. Consequently, an important element of ammonia synthesis reactor design is to attempt to maintain the bed conditions as closely as possible to the maximum reaction rate line, and hence to maximise the rate of ammonia synthesis in a bed of a given size.
Various approaches have been taken to achieving this result, mainly falling into the categories of:
1. Quench-cooled multi-bed converters, in which cold reactant feed is injected between beds of catalyst,
2. Tube-cooled converters, in which tubes carrying cold reactant are embedded in the catalyst bed, and
3. Indirectly cooled multi-bed exchangers, in which heat is extracted from the hot reactants passing between beds in heat exchangers cooled by cold reactant feed.
An example of a quench type converter is disclosed in U.S. Pat. No. 3,663,179 wherein there is provided a vertically oriented container or reactor vessel, which is provided with an internal catalyst basket in which a bed or charge of catalyst particles is disposed. The basket is spaced from the container wall, and the feed fluid stream such as synthesis gas is passed onto the lower portion of the container and external to the basket. The feed fluid rises through the annular space between the basket and the container wall, and thus serves to cool the container and act as insulation against the thermal effects or hot spots generated within the catalyst bed. The rising warmed feed fluid stream is then heated to a suitable catalysis temperature by an internal heat exchanger. The hot fluid then flows to the catalyst bed in which the reaction takes place.
A perforated pipe is disposed in the catalyst bed, and a cold quench fluid, which may be of a composition comparable to the feed stream is passed through the pipes and distributed into the hot reacting gas within the catalyst bed, to provide a cooling effect and thereby moderate the catalytic reaction.
Ammonia synthesis is exothermic, typically operating in the 350° C. to 500° C. range, and the conversion factor is relatively low, typically less than 20%. As a result, it is possible for the cold feed stream to the reactor to be used to extract the reaction heat, at the same time preheating the cold feed stream to the required reaction temperatures. It is conventional practice to use the feed stream as a cooling medium in this way in ammonia converters. Related documents U.S. Pat. No. 4,230,669 and U.S. Pat. No. 4,230,680 describe such a converter in which the cold feed stream is used to extract the reaction heat and also includes a cold feed bypass line such that a controllable volume of the cold feed stream can by-pass the heat exchangers and therefore better control of the reactant temperature can be achieved.
This converter is an example of an indirectly cooled multi-bed exchanger as briefly outlined at point 3 above. It is an approach to bed cooling that is generally preferred by those in the art. In prior art converters though, only a very crude tracking of the maximum reaction rate curve is achievable in the reactor, due to the difficulties in cost-effectively arranging for more than two beds with inter-cooling by the feed stream. The general form of the temperature profile achieved with the common two-stage arrangement is illustrated in FIG. 2. The proposal outlined in U.S. Pat. Nos. 4,230,669 and 4,230,680 mentioned above actually has three catalyst beds, but even so performance leaves room for improvement and the design cannot be considered compact.
Other examples of ammonia converters are described in the publication “Ammonia and Synthesis Gas, Recent and Energy-saving Processes” edited by F J Brykowski and published by Noyes Data Corporation in 1981.
An object of the present invention is to provide an improved reactor design, particularly one that is useful for the purposes of ammonia conversion and the like reactions, and is furthermore of a relatively compact design.