The main difference in the process described so far and the conventional ICI MeOH synthesis process is the inclusion of N2 due to the thermodynamically allowed full conversion per pass at low temperature conditions. As mentioned earlier, cryogenic production of O2 from air is capital intensive and the possibility to operate a MeOH plant with an air-blown ATR process is cost effective [20, 22]. Even though, some level of air purification is needed in the enriched air-blown system, the use of a PSA unit will be cheaper than the use of cryogenic air separation [171] considering the 70 % O2 (in air) purity estimated for the partial oxidation in this work. Furthermore, the exothermic partial oxidation in the ATR which resulted in 1356 and 1439 oC outlet temperature raises material suitability concerns. However, a typical ATR reactor burner operates above 2000 oC [168], therefore we expect that a gas outlet stream with a good heat transfer will not pose more challenges than present in existing ATR reactor technologies.
When the heat component of the process was considered, the excess energy produced was sufficient to cover the energy demand for heating in the overall process. Besides covering the energy demand, a surplus of 7.68 x 108 and 6.10 x 108 kJ/h in the normal and enriched air-blown systems respectively were realized. This implies that, if the surplus energy is not recovered, the energy lost in normal air-blown system will be higher than that of the enriched air-blown system. The surplus energy can be recovered for power generation in steam turbines [172] for example, which can be used subsequently to power the compressing units. In terms of compression, 2.80 x 108 kJ/h and 1.23 x 108 kJ/h energies were required for the normal and enriched air-blown process respectively. This implies that the enriched air-blown system also requires lower energy to power the compressing unit compared to the normal air-blown system.
The calculations revealed that temperature and partial pressure of the syngas are important for full conversion per pass. Below 120 oC, with the right syngas partial pressure, full conversion per pass could be achieved. However, the relative amount of N2 in the reactant gas affected the pressure required for full conversion. In a normal
air-___
117
blown system for example, 100 bar pressure was required while similar conversion could be attained at 60 bar in the enriched air-blown system. The relatively high amount of N2
present in the syngas requires larger reactor volume size with it consequent capital cost for the MeOH synthesis. Hence, the use of the enriched air-blown system, which reduces the 36 % N2 composition in syngas (from normal air) to 7 % (from enriched air) minimizes such added capital cost on the larger reactor size. Furthermore, with the same starting amount of CH4 the energy demand due to compression, relative to MeOH production was estimated to be 2270 and 983 MJ/ton MeOH product for the normal air-blown and the enriched air-blown systems respectively. This implies that a relatively high energy is required to compress the normal air-blown system mainly due to compressing excess unreactive N2. Overall, the enriched air-blown system has considerable advantages over the normal air-blown system.
Figure 8.9 shows a simplified flow design of the overall air-blown LTMS process. The proposed process, involves a possible pre-heating of the ATR feed by heat exchanges from the exiting syngas from the ATR. Even though full conversion is possible at 60 bar and 100 oC in the enriched air-blown system, one potential drawback is the presence of H2O and CO2 in the syngas feed. Experimentally, H2O and CO2 levels are required to be less than 10 ppm [26, 27], otherwise they would react with the methoxide catalyst component to render it inactive. Even though in the process simulation, conditions to minimize H2O and CO2 production were adopted, the inclusion of H2O/CO2 absorbers will further reduce their content below 1 ppm. Aside this, a system for regular recycling of the methoxide catalyst can be included in the process. Methoxide for example is produced from NaOH and MeOH reaction with either evaporation or drying of water [40, 47, 48]. Hence recycling of the catalyst system due to deactivation from NaOH and MeOH formation to restore methoxide activity is expected to be highly feasible.
Ultimately, the air-blown ATR LTMS process presents a cheaper and promising alternative to current MeOH production.
___
118
Figure 8.9: Simplified flow diagram of a complete air-blown LTMS process design
8.4 Summary
The Aspen HYSYS program has been used to simulate and optimize a complete air-blown LTMS process. Two components of the process, namely syngas production and MeOH synthesis were individually simulated to obtain optimized reaction conditions. The presence of N2 was observed to influence overall MeOH production, such that both normal air-blown and O2 enriched air-blown systems were simulated for the complete process. The N2 composition was 39 and 7 % for the normal and enriched air-blown systems respectively, while the CH4/O2 ratio was kept at 2. In the ATR, 20 bar and 1200
oC were selected as optimal conditions for high conversion and low side reactions. Our calculations indicated that more than 99 % feed conversion per pass could be attained at 100 and 60 bar for the normal and enriched air-blown systems respectively at 100 oC MeOH synthesis reaction. In both air-blown systems, the total energy generated in the process was enough to cover the energy demand for heating with a surplus energy.
However, when the surplus energy is not recovered, the heat lost in the normal air-blown system will be higher than the enriched air-air-blown system. The estimated energy required for compression per MeOH production was estimated to be 2270 and 983 MJ/ton MeOH product for the normal air and enriched air-blown systems respectively.
Hence, the enriched blown system has considerable advantage over the normal
air-___
119
blown system. Finally, an overall process design was proposed based on optimized conditions for the enriched air-blown process.
___
120
___
121