Mitigating and overcoming environmental problems brought about by the current worldwide fossil fuel-based energy infrastructure requires the creation of innovative alternatives. In particular, such alternatives must actively contribute to the reduction of carbon emissions via carbon recycling and a shift to the use of renewable sources of energy. Carbon neutral transformation of biomass to liquid fuels is one of such alternatives, but it is limited by the inherently low energy efficiency of photosynthesis with regard to the net production of biomass. Researchers have thus been looking for alternative, energy-efficient chemical routes inspired in the biological transformation of solar power, CO2 and H2O into useful chemicals; specifically, liquid fuels.
Methanol has been the focus of a fair number of publications for its versatility as a fuel, and its use as an intermediate chemical in the synthesis of many compounds. In some of these studies, (e.g. Joo et al., (2004), Mignard and Pritchard (2006), Galindo and Badr (2007)) CO2 and renewable H2 (e.g. electrolytic H2) are considered as the raw materials for the production of methanol and other liquid fuels. Several basic PFD diagrams have been proposed. One of the most promising is the so called CAMERE process (Joo et al., 1999 ). In this process, carbon dioxide and renewable hydrogen are fed to a first reactor and transformed according to:
H2 + CO2 <=> H2O + CO Reverse Water Gas Shift (RWGS)
After eliminating the produced water the resulting H2/CO2/CO mixture is then feed to a second reactor where it is converted to methanol according to:
CO2 + 3.H2 <=> CH3OH + H2O Methanol Synthesis (MS) CO + H2O <=> CO2 + H2 Water Gas Shift (WGS)
The approach here is to produce enough CO to eliminate, via WGS, the water produced by MS. This is beneficial since water has been proven to block active sites in the MS catalyst.
In this work a different process alternative is presented: One that combines the CO2 recycling of the CAMERE process and the use of solar energy implicit in some of the biomass-based process, but in this case with the potential high energy efficiency of thermo-chemical transformations.
PROCESS DESCRIPTION The proposed process is composed of 5 different subsystems:
1. Dish-CR5 solar array: This system takes the main raw material CO2 and partially converts it into CO, finally generating a CO2-CO mixture. It consists of a parallel arrangement of many identical parabolic dishes. Each one concentrates solar power on a small thermo-chemical splitting reactor (called CR5) where CO2 is transformed into CO and O2. From the thermodynamic point of view, the array uses solar energy to transform the low energy molecule CO2 into the higher energy molecule: CO. In fact, it is the energy stored in the resulting CO that is later used to drive the rest of the chemical transformations in this process.
2. WGS reaction system: This system takes the CO2-CO mixture produced by the solar array (later adjusted for CO2 content in the CO2 absorption system) along with H2O to generate H2 and CO2 via WGS. This system is responsible for supplying the H2 required by the MS reaction system. The reaction is carried out on a commercial Cu/ZnO/Al2O3 catalyst used as well in the MS reaction system, but at conditions that favor WGS specifically. Only a fraction of the feed CO is used to produce the H2 leaving enough to control water poisoning in the MS reaction system.
3. MS reaction system: This system takes the CO-CO2-H2 mixture produced in the WGS reaction system (later adjusted for CO2 content in the CO2 absorption system) to generate MeOH and H2O. This H2O tends to poison the catalyst but it is continuously removed from the surface via WGS reaction with CO, producing extra H2 and CO2 which end up as additional MeOH. As mentioned before, this reactor uses a commercial Cu/ZnO/Al2O3 catalyst, while the reaction conditions are adjusted to favor both the WGS and the MS reaction.
4. Amine based CO2 absorption system: Both the methanol synthesis and the water gas shift reaction systems are fed with gas mixtures having significant content of CO2 and CO. However, each one of their reactors achieves an optimum performance at specific feed CO2/CO molar ratios. To adjust these ratios, an amine based CO2 absorption system is included. The system uses an aqueous solution of monoethanolamine (MEA) to absorb CO2 in two different columns connected in a loop with a solvent regeneration column. The first column absorbs part of the CO2 in the stream leaving the Dish-CR5 array, producing a rich CO mixture that is later fed to the WGSR system. The objective here is to drive the WGS equilibrium to the production of H2. The second column absorbs part of the CO2 generated in the WGS system, producing a mixture with only the required amount of CO2 to optimally drive the methanol synthesis reaction. The objective here is to drive the WGS equilibrium to counter the water poisoning while still favoring the MS equilibrium as to obtain an optimum carbon to methanol yield.
5. Methanol purification system: This system is responsible for the separation incondensable materials (i.e. CO2, CO, H2) and water from the main product MeOH. The system includes a simple flash vessel to eliminate the light materials and a standard distillation column to perform MeOH-H2O separation.
ECONOMIC ANALYSIS This work presents an assessment on the current economic feasibility of the process for a plant capacity of 83000 MT/y (i.e. 319 kmol/hr) of MeOH. Nominal prices of CO2, H2O and MeOH were used according to recent technology analysis and market trends. Detailed Net Present Value (NPV) sensitivity analysis studies were also performed. The main conclusions of this analysis are:
- The main economic bottlenecks appear to be the high utility consumption for the CO/CO2 separation and the high capital investment associated primarily with the Dish-CR5 array. - In terms of capital cost, the Dish-CR5 array is the dominating factor, accounting for more than 90% of the capital expenditure. - In terms of operating costs, the low pressure steam consumption of the amine CO2 absorption system is the higher contributor followed by the compression electricity consumption for the CR5 system. - The sensitivity analysis indicate that the NPV sensitivity to the CO2 price is much lower that than its sensitivity to the price of MeOH; in fact, modifying the CO2 price alone it is not possible to reach a breakeven point. However, it is also important to remember that if emission regulations or emission trading schemes are introduced CO2 consumers will get credits, which means an improvement in the process economics.
Our analysis indicates that the process has potential, and points towards future research efforts. Particularly, it is clear that there is much room for improvement in the development of a less expensive and more efficient CR5 system, work that is currently under way at Sandia National Labs. In addition, the use of advance process synthesis techniques can bring much improvement in terms of energy efficiency to the preliminary process design proposed here.
"This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, in the form of a Grand Challenge project entitled Reimagining Liquid Transportation Fuels: Sunshine to Petrol. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000".
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