Equilibrium, methanol synthesis

In the original announcement of the workshop the participants were told that everything was to be taken from methanol synthesis except the kinetics. Some may have interpreted this to mean that the known thermodynamic equilibrium information of the methanol synthesis is not valid when taken together with the kinetics. This was not intended, but... 

Fig ure 6-12. Profiles of equilibrium oonversion Xg versus temperature T for methanol synthesis. (Source Schmidt, L. D., The Engineering of Chemioal Reaotions, Oxford University Press, New York, 1998.)... 

A low-pressure process has been developed by ICl operating at about 50 atm (700 psi) using a new active copper-based catalyst at 240°C. The synthesis reaction occurs over a bed of heterogeneous catalyst arranged in either sequential adiabatic beds or placed within heat transfer tubes. The reaction is limited by equilibrium, and methanol concentration at the converter s exit rarely exceeds 7%. The converter effluent is cooled to 40°C to condense product methanol, and the unreacted gases are recycled. Crude methanol from the separator contains water and low levels of by-products, which are removed using a two-column distillation system. Figure 5-5 shows the ICl methanol synthesis process. 

Table 8.2. Coverages of the various intermediates in the methanol synthesis for a stoichiometric gas mixture at 500 K at 85 % equilibrium note that the surface is almost empty at low pressures, while H atoms and formate coverages become significant at high pressure.

CO in the synthesis gas mixture for the methanol synthesis does not seem to take part directly in the reaction, but it does influence the process through two effects First the water-gas shift reaction and, secondly, through its effect on the surface morphology (and possibly also composition). For thermodynamic reasons, however, it would be desirable if CO could be hydrogenated directly via Eq (18) instead of going through two coupled equations (3) and (19), since it would yield a higher equilibrium concentration of methanol at the reactor exit. 

The solution is illustrated in Fig. 8.15, which shows the equilibrium concentration of methanol for different initial gas mixtures. Note that the maximum methanol concentration occurs for the pure CO + H2 mixture. Hence, in principle, a mixture of just CO and H2 could be used, with minor amounts of CO2, to produce the maximum amount of methanol. However, it is not only the equilibrium constant that matters but also the rate of methanol formation, and one must remember that methanol forms from CO2 not CO. Hence, the rate is proportional to the CO2 pressure and this is why the methanol synthesis is not performed with the simple stoichiometric 3 1 mixture of H2 and CO2 that Eq. (19) suggests. 

Syngas (typically a mixture of CO, H, and CO ) reacts over the active catalyst (Cu/Zn/AljOj) dispersed in an inert oil medium. This process offers considerable advantages over the conventional vapor phase synthesis of methanol in the areas of heat transfer, exothermicity, and selectivity toward methanol. However, this process suffers from the drawback that the methanol synthesis reaction is a thermodynamically governed equilibrium reaction. 

Figiu 3-18 Plot of equilibrium conversion Xq versus temperatiwe for methanol synthesis starting witii stoichiomeh ic feed. While die equilibriimi is favorable at ambient temperature, die conversion rapidly decreases at higher temperature, and industrial reactors operate with a Cu/ZnO catalyst at pressures as high as 100 atm. 

An industrial process to produce methanol from carbon monoxide and hydrogen was developed by BASF in 1923 using a zinc oxide-chromia catalyst.361 362 Since this catalyst exhibited relatively low specific activity, high temperature was required. The low equilibrium methanol concentration at this high temperature was compensated by using high pressures. This so-called high-pressure process was operated typically at 200 atm and 350°C. The development of the process and early results on methanol synthesis were reviewed by Natta 363... 

Figure 17.20. Control of temperature in multibed reactors so as to utilize the high rates of reaction at high temperatures and the more favorable equilibrium conversion at lower temperatures, (a) Adiabatic and isothermal reaction lines on the equilibrium diagram for ammonia synthesis, (b) Oxidation of SOz in a four-bed reactor at essentially atmospheric pressure, (c) Methanol synthesis in a four bed reactor by the ICI process at 50 atm not to scale 35% methanol at 250°C, 8.2% at 300°C, equilibrium concentrations.

Another interesting example of reactive adsorption is the so-called gas-solid-solid trickle flow reactor, in which adsorbent trickles through the fixed bed of catalyst, removing selectively in situ one or more of the products from the reaction zone. In the case of methanol synthesis this led to conversions significantly exceeding the equilibrium conversions under the given conditions (67). 

The current primary feedstock for industrial methanol synthesis is synthesis gas a mixture of CO, C02> and hydrogen derived from the reforming of natural gas or other hydrocarbons [2], The interconversion of carbon oxides and methanol, central to methanol synthesis and steam reforming, is defined by the following three equilibrium equations ... 

Figure 5.3.2 The equilibrium constant of formation methanol synthesis by-products from 3 1 H2 C02 mixtures at 250°C.

Methanol synthesis from waste C02 streams has the potential to contribute to the limitation of worldwide C02 emissions and to serve as an alternative carbon source to fossil fuels if a renewable source of hydrogen is available (see Section 5.3.1). The main obstacle to methanol synthesis from C02-rich streams is thermodynamics. The equilibrium yield of methanol from 25% C0/C02 75% H2 mixtures of varying C0/C02 ratio is shown in Figure 5.3.5. For pure CO, a one-pass methanol yield of nearly 55% can be obtained at 525 K, while pure C02 would only yield 18%. Besides the addition of CO, this equilibrium limitation can be overcome by operating at lower temperatures (an option that requires more active catalysts), implementing higher recycle ratios, or product extraction (an option that requires higher capital investment) [8]. 

The thermodynamic equilibrium is most favourable at high pressure and low temperature. The methanol synthesis process was developed at the same time as NH3 synthesis. In the development of a commercial process for NH3 synthesis it was observed that, depending on the catalyst and reaction conditions, oxygenated products were formed as well. Compared with ammonia synthesis, catalyst development for methanol synthesis was more difficult because selectivity is crucial besides activity. In the CO hydrogenation other products can be formed, such as higher alcohols and hydrocarbons that are thermodynamically favoured. Figure 2.19 illustrates this. 

The synthesis gas reaction (I) is limited to an equilibrium and is therefore run in the presence of an excess of water, which has another beneficial role that of reducing the carbon formation on the catalyst. As reaction (1) produces too much hydrogen fur the methanol synthesis ( ). the process may be operated in the presence of CO2. in order to adjust the gas composition (5) ... 

Preheated natural gas is fed at about 600°C to the reformer and exits at about 880°C and 2.1 MPa. Methanol synthesis is then performed over copper-based catalysts at about 240-270°C and 10.3 MPa. The product gas contains about 5% methanol. By-products are 1-2% dimethyl ether and 0.3-0.5% higher alcohols. Because of equilibrium limitations, conversion of synthesis gas is only a few percent per pass in the catalytic reactor, and the product gas stream after... 

The yield of hydrocarbons (14.4%) were higher than that of equilibrium conversion of COj to methanol (ca 7% at 4001C, 50 atm). It means that methanol formation was accelerated by MTG reaction. When methanol synthesis catalyst was prepared by coprecipitation, the yield of hydrocarbons decreased (Run 2). This seems to be due to the deactivation of zeolite by the sodium remaining after 5 times wash. Similar tendency was observed on the hydrocarbon synthesis between two Cu-Zn/HY composite catalyst, in which one Cu-Zn catalyst was precipitated by Na2C03, and another Cu-Zn catalyst was precipitated by oxalic acid[3]. When methanol synthesis catalyst was prepared by sodium compound, remaining sodium deactivate an active site of zeolite on MTG reaction. 

The present test plant for methanol synthesis is beeing easily operated. Figure 3 shows the rate of production of methanol, i.e., space time yield of methanol (STY), as a function of time on stream over the multicomponent catalyst under the reaction conditions of 523 K, 5 MPa and SV = 5,000 h 10,000 h. The production rate of methanol at SV = 5,000 h was almost the same as that at reaction equilibrium. In the case of SV = 10,000 h the methanol production rate was 600 g/l-cat-h, which is 20% lower than that at reaction equilibrium. No significant difference was observed between data obtained from the present... 

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