Methanol synthesis reactor types

Fig.3.3a-d. Various types of methanol synthesis reactors, (a) Cold gas quench (b) cooling by evaporation - multistage, adiabatic (c) cooling by evaporation - tubular, near isothermal (d) liquid entrained system using heat carrier liquid... 

Let us - after these rather extensive introductory remarks -turn to the specific problems encountered in design of ammonia and methanol synthesis reactors. We shall not endeavour to treat all of the above mentioned aspects, but mainly concentrate on the initial steps and on the basis of this illustrate how the various principal types of reactors can be applied in these syntheses. We shall discuss the reaction kinetics for the reactions and the calculation of reactor performance and some of the problems encountered in the calculation of reactor performance. The mathematical procedure used for the computer calculations is discussed by Christiansen and Jarvan W in a separate presentation in this volume. 

Retrofitting features of the more efficient reactor types have been the principal thmst of older methanol plant modernization (17). Conversion of quench converters to radial flow improves mixing and distribution, while reducing pressure drop. Installing an additional converter on the synthesis loop purge or before the final stage of the synthesis gas compressor has been proposed as a debotdenecking measure. 

This type of operation with heat exchange between the hot product stream and the cold incoming reactants is employed in many industrial reactors. Important examples include ammonia synthesis, methanol synthesis, the oxidation of sulphur dioxide, the water gas shift reaction and the synthesis of phthalic anhydride. 

The synthesis loop consists of a recycle compressor, feed/effluent exchanger, methanol reactor, final cooler and crude methanol separator. Uhde s methanol reactor is an isothermal tubular reactor with a copper catalyst contained in vertical tubes and boiling water on the shell side. The heat of methanol reaction is removed by partial evaporation of the boiler feedwater, thus generating 1-1.4 metric tons of MP steam per metric ton of methanol. Advantages of this reactor type are low byproduct formation due to almost isothermal reaction conditions, high level heat of reaction recovery, and easy temperature control by... 

Several types of reactors have been proposed such as the liquid entrained reactor(l) and the Trickle bed reactor(2). The authors have been studying a liquid-phase methanol synthesis process in order to develop a new technology as an alternative for a gas-phase process, and reported that a new process employing liquid-liquid separation of the products from the solvent has several advantages in practical methanol synthesis(3). 

Chem Systems Inc. has been developing a three phase reaction system for methanol synthesis since the mid 1970 s (ref. 28). The original concept incorporated a liquid-fluidized-bed reactor. This research, which was funded by the Electric Power Research Institute, used particles of a heterogeneous catalyst, obtained by crushing pellets of a commercial Cu-ZnO-AljO type catalyst, which was fluidized by a circulating inert hydrocarbon liquid such as a mineral oil. One of the major benefits of the process over conventional synthesis is claimed (ref. 28) to be excellent temperature control of the reactions so that higher per pass conversions can be achieved, thereby reducing... 

An important relatively recent achievement is the development of the so-called liquid entrained reactor (LER) for liquid phase methanol synthesis. To assess the performance of these reactors, much simultaneous work has been done on MARs, comparing the performance of the two types of reactors for this reaction. A good discussion of this can be found in a paper by Vijayaraghavan et a. (1993). An equation developed for the overall gas-liquid mass transfer coefficient (see Ko, 1987 Lee et al., 1988 Parameswaran et al., 1991) is... 

Bulk catalysts comprise mainly active substances, but some binder is often added to aid the forming/shaping operation. This is the case for iron oxide for the water-gas shift (WGS) reaction, iron molybdate for the oxidation of methanol to formaldehyde, and vanadyl pyrophosphate for butane oxidation to maleic anhydride. However, in some cases, bulk catalysts are used as prepared, without the need for addition of the binder. Typically, this involves catalysts prepared by high temperature fusion (eg, the iron-based ammonia synthesis catalyst). The need for the addition of binder, or the requirement for pelleting, solely depends on the strength required for the catalyst under the reaction conditions and the reactor type that is used in. This requires consideration of attrition resistance, and oxide... 

In recent years this type of reactor has been considered or selected for methanol synthesis [Tijm et al, 2001], Fischer-Tropsch synthesis [Krishna and Sie, 2000 Davis, 2002] and dimethylether synthesis [Fleisch et al, 2002, 2003 Fleisch and Sills, 2004 Ogawe et al, 2003],... 

All three types of reactors have been used in many different versions in large industrial units, both in ammonia and methanol synthesis. Some of the reactor types, which have been used, are described in [l and [l ... 

Fig. 6A, B and C show operating lines for the three types of reactors in methanol synthesis (confer Table 4, case 2). The situation is the same here. The internally cooled reactor gives the best approach to the optimum operating line and may as a consequence be designed for the smallest catalyst volume, whereas the quench cooled reactor requires the largest volume. It is not, however, possible to base a choice between the reactor types solely on the required catalyst volume. As indicated in Table 1, a number of other considerations must be taken into account.

In Haldor Tops0e s ammonia and methanol synthesis processes a series of adiabatic beds with indirect cooling between the beds is normally used, at least in large plants. In smaller plants internally cooled reactors are considered. In ammonia synthesis, the Tops0e solution is today the so-called S-200 converter (Fig. 7) and L6j. This converter type, which is a further development of the S-100 quench-type converter, was developed in the mid seventies the first industrial unit was started up in 1978, and today about 20 are in operation or on order. Both the S-100 and the S-200 reactors are radial flow reactors. The radial flow principle offers some very specific advantages compared to the more normal axial flow. It does, however, also require special catalyst properties. The advantages of the radial flow principle and the special requirements to the catalyst are summarized in Table 5. 

In methanol synthesis, the case for radial flow converters is less obvious. Tops0e has earlier proposed, the use of one-bed radial flow converters in large methanol plants. Later analyses, partly based on a change of catalyst type, have, however, led to the conclusion that axial flow should be preferred even in very large methanol synthesis converters. The reasons for this difference in reactor concept between ammonia synthesis and methanol synthesis are in the differences between the properties of the catalysts. As mentioned above, the ammonia synthesis catalyst is ideally suited for the radial flow principle. This is not true for the methanol synthesis catalyst. The reasons for not using the radial flow principle in methanol synthesis are the following ... 

Rahimpour, M.R., Rahmani, F. and Bayat, M. (2010) Contribution to emission reduction of CO2 by a fluidized-bed membrane dual-type reactor in methanol synthesis process. Chemical Engineering and Processing Process Intensification, 49, 589-598. 

Significant development has occurred within the industry over the last several years with respect to liquid-phase processes. One example of this process that is reasonably close to commercialization is that developed by Air Products. A pilot unit has been operated for several years at their La Porte, Texas location. The process is characterized briefly as using an inert hydrocarbon reaction medium in the liquid phase to absorb the synthesis heat of reaction conventional copper-zinc catalyst is fed to the reactor system as a slurry. This type of process appears to be particularly well suited to substoichiometric feeds (hi earbon eontent), such as those produced by partial oxidation or coal gasification. The Air Products process has been extensively deseribed in patent literature [14]. Kinetie data and liquid-phase reaetion systems have also been extensively diseussed by Lee in Methanol Synthesis Technology [15]. 

Adesina [14] considered the four main types of reactions for variable density conditions. It was shown that if the sums of the orders of the reactants and products are the same, then the OTP path is independent of the density parameter, implying that the ideal reactor size would be the same as no change in density. The optimal rate behavior with respect to T and the optimal temperature progression (T p ) have important roles in the design and operation of reactors performing reversible, exothermic reactions. Examples include the oxidation of SO2 to SO3 and the synthesis of NH3 and methanol CH3OH. 

Figure 17.24. Types of reactors for synthetic fuels [Meyers (Ed.), Handbook of Synfuels Technology, McGraw-Hill, New York, 1984], (a) ICI methanol reactor, showing internal distributors. C, D and E are cold shot nozzles, F = catalyst dropout, L = thermocouple, and O = catalyst input, (b) ICI methanol reactor with internal heat exchange and cold shots, (c) Fixed bed reactor for gasoline from coal synthesis gas dimensions 10 x 42 ft, 2000 2-in. dia tubes packed with promoted iron catalyst, production rate 5 tons/day per reactor, (d) Synthol fluidized bed continuous reactor system for gasoline from coal synthesis gas.

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