Description of the New Technology to Convert Synthesis Gas to Liquid Petrochemicals
The new technology offered by Epurga to be used in a process to convert municipal solid waste (MSW) and construction debris/waste (CD&T) to high value petrochemicals. The overall process begins with sorting and conditioning of the feed waste to produce a uniform product known as refuse derived fuel (RDF). This RDF is gasified in a conventional gasifier to produce an equimolar mixture of carbon monoxide and hydrogen at a mole fraction of 0.3 for each component. This equimolar mixture is the feed to the novel MSU technology that produces liquid petrochemicals.
We begin this discussion with the hypothesis driving our research. This hypothesis asserts that a conversion technology can be developed that is superior to the competing technology: Fischer-Tropsch. The two main weaknesses of the F/T technology are: 1) high heat release rate which requires a complicated reactor design, and 2) liquid products having a wide-distribution of molecular weight which requires a second conversion step to crack the high-molecular weight wax products into lower molecular weight species. These two weaknesses can be addressed by the following: 1) use a catalyst that is slower than the F/T catalyst, such as a molybdenum oxide (MoO3); and 2) use a reaction pathway that is amenable to shape-selective control of the reaction intermediates, such as alcohols as reaction intermediates. Molybdenum oxide is a catalyst to convert synthesis gas into alcohols such as methanol, ethanol, propanol, etc. Such alcohols can be converted to hydrocarbons having a narrow distribution of molecular weights over a catalyst such as a zeolite.
Another weakness of the F/T synthesis is the requirement that the feed gas have a composition that is rich in hydrogen, i.e., the H2/CO molar ratio by greater than or equal to 2 mol/mol. Since most gasifiers produce a syngas mixture that is equimolar in hydrogen and carbon monoxide, this requirement by the F/T synthesis places an extra cost on the process since another unit operation is required to meet the H2/CO requirement: the water gas shift reaction. We eliminate this extra unit operation by executing the water gas shift reaction inside the same catalyst that converts the synthesis gas to alcohols and that converts the alcohols to liquid petrochemicals. Molybdenum oxide is also a water gas shift catalyst that operates in the same range of temperatures as the alcohol synthesis catalyst (MoO3) and the alcohol conversion catalyst (zeolite). Thus, by placing a molybdenum oxide component inside a zeolite, such as H-ZSM-5, we are able to address all of the major weaknesses of the F/T technology and thus reduce the high operating costs associated with the F/T process by eliminating some of the unit operations required by the F/T process: 1) water gas shift conversion and 2) wax conversion to lighter hydrocarbons. In addition, the choice of zeolite affords to us the flexibility to alter the types of hydrocarbons produced by this process. When H-ZSM-5 is the zeolite, the products are rich in petrochemicals, namely mixed xylenes, methyl-substituted benzenes, and methyl-substituted naphthalenes.
The data to support this description is shown in part in the US patent along with the claims of the patent. The two Applied Catalysis General publications describe the enabling science for the conversion of alcohols to hydrocarbons at high pressure, 70 bar, and the conversion of synthesis gas to hydrocarbon liquids at high pressure, 70 bar. The conversion of methanol to gasoline at low pressure (5 bar or less) over zeolite H-ZSM-5, was described in a series of patents in the late 1970’s as the MTG process. The conversion of syngas to alcohols at high pressures, ca. ~ 70 bar, over molybdenum oxide was described in the literature as early as 1991.
Thus, the first step in our research was to confirm that the low pressure MTG process for converting methanol to gasoline could be operated at a pressure of 70 bar where the molybdenum oxide converts syngas to alcohols. This technology was confirmed by the work published in volume 363 of Applied Catalysis, General. Here we showed that higher molecular weight alcohols were converted more quickly to hydrocarbons than methanol at the same reaction conditions and that zeolite H-ZSM-5 converted these alcohols to petrochemicals! Finally, the close confinements offered by the zeolite H-ZSM-5 restricted the range of molecular weights in the liquid products from 78 to ~ 160. With such a narrow distribution of aromatic products, it was not necessary to affect catalytic cracking of these products as was required by the F/T process.
Having demonstrated that alcohols could be converted to petrochemicals inside H-ZSM-5 at high pressures, we moved on to develop a zeolite that contained the alcohols synthesis agent: MoO3. This catalyst, described in volume 357 of Applied Catalysis: General, shows how it is necessary to match up the silica/alumina ratio of the zeolite with the loading of Mo metal oxide. The maximum yields of liquid hydrocarbons were obtained with a silica/alumina ratio = 50 when the Mo loading was 5 weight percent; where the ratio of zeolite ion exchange capacity is nearly equal to the loading of Mo (~0.5 mmol Mo/g zeolite). This result showed the importance of matching the amount of Mo ions in the zeolite with the number of acid sites in the zeolite. The choice of zeolite regulates the types of hydrocarbons produced by this technology from alkanes to aromatics such as benzene, toluene, xylenes, substituted benzenes and naphthalenes.
The PowerPoint presentation emphasizes these strengths of our technology along with providing some of the information needed to practice this technology, such as the effect of particle size and the variation in yields with space-time and with temperature.