Technoeconomic assessment of lignocellulosic ethanol production via DME (dimethyl ether) hydrocarbonylation
Highlights
► We assess an innovative thermochemical indirect route to produce bioethanol. ► Minimum sale price is lower than present 2nd generation biochemical ethanol. ► Additional lab research is still necessary to elucidate some of the chemistry.
Introduction
Bioethanol is increasingly used as a transportation fuel in USA, Brazil and Europe. It can be used pure in vehicles with modified internal combustion engines (ICEV) or blended with gasoline in regular ICEV. Nowadays, most bioethanol is produced by first generation processes based on fermentation technologies for sugar and starchy crops. However, these crops have some drawbacks: a high value for food application and low sugar yield per hectare. Thus, currently, suitable processes for lignocellulosic biomass are being developed under the name “2nd generation bioethanol processes”.
This abundant and relatively cheap biomass can be converted to ethanol by hydrolysis and fermentation (biochemical pathway) but also by thermochemical processing, i.e. gasification of biomass to synthesis gas (syngas) followed by catalytic synthesis or syngas fermentation. Technoeconomic studies of the direct synthesis of alcohols from syngas produced by biomass gasification have been carried out by several researchers, either based on patented catalysts [1] or expected performance of these catalysts in the future [2], [3], [4]. From the results of this research, it is clear that significant development of current mixed alcohol catalysts is needed, in terms of conversion and selectivity to ethanol, to make the process economically feasible. Indirect synthesis processes (indirect routes) could be an interesting option if they were able to achieve higher ethanol productivity so as to outweigh their inherently greater complexity.
Routes to make ethanol from syngas, through one intermediate, have been reviewed recently by Subramani [5]. This review discusses three indirect routes to ethanol: the methanol bimolecular reaction route, the methanol homologation (reductive carbonylation) route and the acetic acid route (ENSOL process) [6]. Other current indirect routes not reviewed are: indirect acetic acid route [7], Enerkem's process via methyl acetate [8], [9], dry ethanol process [10] and ethanol via dimethyl ether (DME) hydrocarbonylation [11]. Enerkem Inc. has recently developed the ethanol via methyl acetate process with a demonstration plant and two commercial plants in project or under construction [8].
In this paper, a conceptual design of a thermochemical process for the production of ethanol via DME hydrocarbonylation route is developed and economically assessed by the ASPEN PLUS process simulator. The process makes use of commercial methanol and DME synthesis technologies. No commercial DME hydrocarbonylation reactor exists today but its development is not expected to be difficult as hydrocarbonylation reaction is slightly exothermic at mild conditions (15 bar, 220 °C), and only heterogeneous catalysts are involved.
The paper is organized as follows. First, a description of the DME hydrocarbonylation chemistry is presented which includes a short discussion of catalysts, reaction conditions and experimental kinetic data found in the literature [11], [12]. Then, the conceptual process design and modeling is described including the main design assumptions and parameters. Finally, the performance of the process is calculated for different conditions in the DME hydrocarbonylation reactor, which is modeled using the experimental kinetic data. Results of the simulation are used to assess and compare the economics of the process for each case.
Section snippets
Ethanol via DME hydrocarbonylation route
Recently, a new route to make ethanol from DME and syngas has been proposed [11], [12]. The innovation resides in the use of a dual catalytic bed reactor of H-Mordenite and Cu/ZnO. DME reacts with CO from the syngas to form methyl acetate in the presence of H-Mordenite catalyst (Eq. (3)). Formed methyl acetate is hydrogenated to ethanol and methanol by means of the Cu/ZnO catalyst (Eq. (4)).
H-Mordenite is a kind of acid zeolite that selectively catalyzes the carbonylation of DME to methyl
Conceptual design
Fig. 1 shows the conceptual block diagram of the process. Biomass is first converted to syngas in an atmospheric circulating fluidized bed gasifier. The producer gas is cleaned-up and conditioned in order to meet the requirements of the catalysts used in the reactor network. The configuration of the reactor network is largely dictated by the way in which DME is produced in the plant. DME for hydrocarbonylation can be produced from syngas by direct synthesis or indirectly by first synthesizing
Process economics
Results from the process simulations were used to estimate capital and operating costs. Purchase costs for equipment (PEC) are taken from published BTL studies and vendor quotes. First, the purchase costs are scaled and the effect of inflation is corrected by using the CEPCI (Chemical Engineering Plant Cost Index) index. Then, the installed equipment cost is calculated by multiplying the purchase cost by an installation factor. If the installation factor is unknown it is assumed 2.47 according
Process discussion: material and energy balance results
Fig. 3 shows that ethanol production varies from 255.8 for 49:1 case up to 262.6 ML/year for 10:1 case (a variation of only 2.6%), or equivalently, energy efficiency of biomass to ethanol ranges from 44.35 to 45.53% (HHV basis). The ethanol production is very similar for all cases despite differences in DME conversion and ethanol selectivity for different CO/DME ratios. These results also prove that using a H2/CO ratio different from 1 in the hydrocarbonylation reactor does not affect global
Conclusions
This technoeconomic assessment shows that the production cost of ethanol via the DME hydrocarbonylation route is lower than that reported in the literature for the production of lignocellulosic ethanol via biochemical pathway [37]. Thus, ethanol from the DME route looks promising. This study has raised some important issues with regard to catalyst performance which should be investigated in order to optimize the design of the process such as performance of the hydrocarbonylation catalyst at
Role of the funding source
This work has been partially funded by Abengoa Bioenergy New Technologies (ABNT) and partially carried out in the framework of the CENIT projects “I+DEA” (PI-0063/2007) and “BIOSOS” (CEN-20091040), in which the Spanish Ministry of Science and Innovation grants 50% of the project budget. The role of ABNT in the making of the paper is limited to providing technical information for the assessment.
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