Methanol synthesis reactor

Methanol. Methanol is produced by stoichiometric reaction of CO and H2. The syngas produced by coal gasification contains insufficient hydrogen for complete conversion to methanol, and partial CO shifting is required to obtain the desired concentrations of H2, CO, and CO2. These concentrations are expressed in terms of a stoichiometric number, ((H2 — CO)/(H2 + CO2), which has a desired value of 2. In some cases CO2 removal is required to achieve the stoichiometric number target. CO and H2 are then reacted to form methanol in a catalytic methanol synthesis reactor. 

Synthesis gas is compressed to pressures of about 250 psig before entering the methanol synthesis reactor and conversion to methanol. Also, after methanol synthesis, any unreacted synthesis gas is again compressed and recycled back through the reactor. 

Methanol synthesis plants utilizing the low-pressure process currently operate at capacities of 2 x 105 to 2 x 106 metric tons per year [15]. Such installations are composed of a synthesis gas production unit, the actual methanol synthesis reactor, and a separation and purification section. The production and purification of synthesis gas accounts for 50%-80% of the total cost of methanol production, with the remaining cost associated with the actual synthesis and purification of methanol [2, 8], Although a variety of carbonaceous feedstocks can be transformed into synthesis gas, the steam reforming of natural gas (Equation [4]) is by far the most common option, especially for large plants [2, 15-16] ... 

After the condensed crude methanol is recovered in the high-pressure separator, it is sent to a methanol purification column. Typically, methanol purification requires two columns, one to remove the light ends (mainly by-products generated in the methanol synthesis reactor such as dimethyl ether and dissolved gases) and another to separate methanol and water and any other by-products with a lower volatility than methanol. Specification-grade methanol (greater than 99.85 wl% methanol) is recovered as the overhead product of the heavy ends column and sent to storage. 

TABLE 11.2 Methanol Synthesis Reactor Comparison of simulations and experimental data (from Wu and Gidaspow, 2000)... 

Most syngas is produced captively for the manufacture of methanol from natural gas. Natural gas is reformed with steam to produce a raw syngas which enters a methanol synthesis reactor and is converted directly to methanol. Methanol is not regarded as a petrochemical, as it is usually produced from natural gas rather than a petroleum-derived hydrocarbon, but it is used as feedstock to produce a great many petrochemicals. Syngas is also used as feedstock in the 0x0 process to produce a wide variety of aldehydes and alcohols. 

This figure clearly illustrates that the range within which multiple steady states can occur is very narrow. It is true that, as Hlavacek and Hofmann calculated, the adiabatic temperature rise is sufficiently high in ammonia, methanol and oxo-synthesis and in ethylene, naphthalene, and o-xylene oxidation. None of the reactions are carried out in adiabatic reactors, however, although multibed adiabatic reactors are sometimes used. According to Beskov (mentioned in Hlavacek and Hofmann) in methanol synthesis the effect of axial mixing would have to be taken into account when Pe < 30. In industrial methanol synthesis reactors Pe is of the order of 600 and more. In ethylene oxidation Pe would have to be smaller than 200 for axial effective transport to be of some importance, but in industrial practice Pe exceeds 2500. Baddour et al. in their simulation of the TVA ammonia synthesis converter found that the axial diffusion of heat altered the steady-state temperature profile by less than 0.6°C. Therefore, the length of... 

Figure / Fauser-Montecatini methanol synthesis reactor (from Cappelli, et al. [95]).

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... 

Rahimpour MR, Parvasi P, Setoodeh P. Dynamic optimization of a novel radial-flow, spherical-bed methanol synthesis reactor in the presence of catalyst deactivation using differential evolution (DE) algorithm. International Journal of Hydrogen Energy 2009 34 6221-6230. 

Examples of a simultaneous numerical solution of molar and energy balance equations for the gas bulk and catalyst particles are introduced in Figure 5.27, in which the concentration and temperature profiles in a methanol synthesis reactor are analyzed. The methanol synthesis reaction, CO -F 2H2 CH3OH, is a strongly exothermic and diffusion-limited reaction. This implies that concentration gradients emerge in the catalyst particles, whereas the heat conductivity of the particles is so good that the catalyst particles are practically isothermal. 

In the methanol synthesis reactor, the equilibrium composition is attained (Figure 5.27b) and the temperature in the reactor increases to the adiabatic temperature (Figure 5.27c). 

Dybkjaer, I. Design of Ammonia and Methanol Synthesis Reactors. This course. 

The above does, of course, not mean that it is not of prime importance to be able to predict as accurately as possible how the performance of an ammonia or methanol synthesis reactor will be at given process conditions. The synthesis reactor is the heart of the plant, and if it does not perform as expected then the plant will be unable to reach capacity. 

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. 

It has been found that the potential advantages by applying the radial flow principle in methanol syntheses cannot justify the potential risks imposed by especially the last point mentioned above, and it has as a consequence been decided to design the methanol synthesis reactor system as a series of normally three axial beds with cooling between the beds. In large plants, the reactors will be separate vessels with boilers or heat exchangers between the reactors. In smaller plants, it may be possible to have more than one catalyst bed inside the same pressure shell. 

When the calculation procedure and the mathematical model described above and in the appendix is applied for calculation of the performance of a methanol synthesis reactor, interesting results are obtained because of the special reaction system. In the mathematical model, the following 2 reactions are specified, each with its own kinetic expression ... 

Typically, the copper-based famify of methanol synthesis catalysts are extremely selective. Methanol yields are hi relative to or nic byproducts, with generalfy over 99.5% of the converted CO + C02 present as methanol in the crude product stream. H20, of course, is normally a by-product, with a resultant concentration in the crude product that is influenced by the ratio of C02 to CO in the methanol synthesis reactor feed stream. Hydrocarbon by-products typically are present in concentrations of less than 5000 ppm(w) and consist of such compounds as the following ... 

Heat is needed in the distillation steps, and steam is required for reforming and shift reactions. Heat is liberated in the reformer and the methanol synthesis reactors. The more efficient processes integrate the heat for distillation with the process waste heat, but even these processes must find a use for the excess process heat. Usually more steam is raised than is required for the process, and the excess may be exported or used to generate electrical power. 

The equilibrium for Reaction (2) is relatively unfavorable. The methanol synthesis reactor must be operated at relatively high pressures to obtain a reasonable conversion of CO and H2. 

Normally, the conversion per pass is quite low, so recycling is needed (Wender, 1996 Olah et al., 2011 Bermudez et al., 2013). One of the main challenges for this process is the efficient removal of the heat generated in the methanol synthesis reactor, which is usually a fixed-bed reactor. The integration of these reactors with other process streams can play a key role in the global efficiency of the process (HameUnck and Faaij, 2002 Bermudez et al., 2013 Olah et al., 2011). 

The unreacted outlet gas from the methanol synthesis reactor contains mainly CO and H2O. By adding steam, the CO is converted to CO2 according to CO H2O H2 + CO2 in the shift reactors. Like in an IGCC power plant case, a 95 percent conversion of CO has been assumed. The CO2 content in the gas then increases from 14 vol% to 44 vol%. CO2 is captured in a Selexol plant. The removal efficiency has been assumed to be 87 percent, which is close to the assumption for the IGCC power plant case. 

---