Energy efficient methanol-to-olefins process
Introduction
Olefins are essential building blocks for the organic synthesis. Today the olefins are manufactured almost exclusively from fossil oil and gas resources. Modern technologies, as methanol-to-olefin (MTO) and methanol-to-propylene (MTP), are available by employing renewable raw materials, namely biogas and biomass, as well as coal, which in this way becomes a clean resource for chemical industries. Today methanol is emerging as the most important building block for sustainable chemical industries (Olah et al., 2009, Schmidt and Pätzold, 2014).
The availability of low-cost methanol is a requirement for profitable MTO business. However, a supply-chain of higher-value end-products, namely speciality polymers, allows getting profitable margins even with higher methanol cost, as it is the case in Europe.
The starting point for methanol production is syngas (CO and H2 mixture), in turn available from various sources, as natural gas, biogas, biomass, or coal. Fig. 1 shows the large applicability of methanol, which embraces a wide range of chemicals and commodity polymers, today known for the most part as essential petrochemical products. From a strategic viewpoint, the MTO process may contribute to decoupling the olefins from the petroleum market, allowing countries without hydrocarbon resources to develop an independent infrastructure of basic chemicals.
The presentation of MTO technology may be found in several articles (Pujado and Anderson, 2004, Chen et al., 2012, Ye et al., 2015, Tian et al., 2015). Conceptual process design is handled only in a recent paper (Yu and Chien, 2016a, Yu and Chien, 2016b). Here the energy saving is treated by a method that takes advantage from the potential of the reactor effluent. However, this is not fully exploited, as we will demonstrate. Their flowsheet is taken as reference case in our analysis (see Appendix A). Note that the olefins separation makes use of seven distillation columns.
In this paper, we propose a novel approach that results in a highly efficient MTO process, almost neutral as energy requirements and employing a minimum number of separation units. In the first place, the energy generated in the chemical reactor is fully recovered for methanol feed conditioning. The novel method employs Mechanical Vapour Compression (MVC) for upgrading both the temperature and enthalpy of the reactor effluent. The method is highly efficient, since the spent mechanical energy helps recovering about ten times more thermal energy. The considerable amount of heat developed in the reactor is used for running a combined heat and power cycle, which supplies utilities, namely for gas compressors and an ammonia-refrigeration plant. The olefin separation employs a minimum number of five columns, integrated with the reaction section, which results in an almost neutral process from energy viewpoint.
This paper deals with the conceptual design of a process for manufacturing ethylene and propylene from methanol with the nominal capacity of 100 tph methanol. Product specifications are olefins of polymer grade, with over 99% purity. The location is a harbour site in Europe.
The paper is organised as following. The first section deals with the bases of design, including catalysis, thermodynamics, kinetics, and HSE issues. It follows preliminary material balance and process profitability. Then Process Synthesis is developed, with focus on the key innovative aspects compared with previous works. The treatment is split in reaction, preliminary separation and olefins separation sections. The solution is assessed by computer simulation, by paying attention to suitability and accuracy of the thermodynamic methods. The results are material and heat balances, as well as sizing of key equipment. Rigorous methods are employed for designing heat exchangers and distillation columns. The final part handles Process Integration issues, as combined heat and power, refrigeration, heat-pump assisted distillation, thermal coupling and management of utilities. Conclusions highlight the novelty aspects and the energetic performance of the proposed design.
Section snippets
Chemistry and catalysis
The transformation of methanol to olefins (MTO) is a complex catalytic process (Alwahabi and Froment, 2004, Chen et al., 2012). In the first step methanol dehydrates to dimethyl-ether (DME) by a reversible reaction, followed by conversion to olefins, as ethylene, propylene, and butenes. Sub-products are methane, ethane, propane, heavier hydrocarbons and aromatics.
The MTO reaction takes place in vapor phase in the presence of a suitable catalyst at 350–500 °C,
Preliminary material balance
Fig. 3 presents the input/output structure of a MTO process. Inputs are methanol, catalyst, and air for catalyst regeneration. Outputs are light olefins ethylene and propylene, heavier olefins and hydrocarbon, LPG fuel including propane fraction, gaseous emissions and solid waste resulting from catalyst regeneration, as well as water resulting from reaction.
Table 2 presents data for determining a preliminary material balance. The analysis is focused on process synthesis issues, namely on
Preliminary economic assessment
A preliminary economic analysis is done in the conditions of Western Europe. The methanol price is 250 $/t, while for the ethylene, propylene, butene and fuel hydrocarbon the prices are 1300, 1200, 900 and 500 $/t respectively. We assume the catalyst cost 5% with respect to feedstock. A crude estimation of the process profitability can be obtained by calculating the index rate of return on fixed capital ROIC:In the above relation, NP is the net profit before taxes (revenue — all
Process synthesis
The process synthesis follows the methodology outlined in Dimian et al. (2014). The results presented in the next sections have been obtained by employing the process simulator Aspen Plus version 9.0.
Conclusions
Getting ethylene and propylene from methanol by the MTO process represents a major progress in chemical technology in recent years. MTO is fully sustainable since it allows the valorisation of renewable resources, as biogas and biomass, as well as of coal resources by a clean technology. The highly exothermic reaction takes place at 470 °C and 2 bar in a reaction set-up consisting of fluid bed reactor and regeneration device. The hot reactor effluent has enough enthalpy for ensuring feed
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