Process Modelling, Characterization and Dynamic Analysis of Centralized Distillation System for Separation of Methanol and Water

Abstract

Over the past few decades, there has been an increased awareness and concern about the rising pollution, global warming, sudden climatic changes, and the accelerating CO2 emissions that have coerced people to shift to renewable resources and sustainable fuels. Zero Emission Fuels (ZEF) is a leading startup company that focuses on developing a sustainable methanol micro-plant that is powered by solar energy. CO2 and H2O are absorbed from the air (DAC system) and the extracted H2O is split into H2 (AEC system) in order to produce a methanol-water mixture where the mixture is then separated using a Distillation System (DS system). Each micro-plant operates for 7 hours per day and produces 600 g of methanol. with a distillate methanol purity of 99.8% (AA grade methanol) and in the future ZEF is planning to develop a ‘methanol farm’ with 13500 micro-plants.
Based on a previous study at ZEF, fractional distillation was seen as a potential system for separating methanol and water mixture with desired purity. On the other hand, it was also suspected that implementing multiple decentralized separation units could be expensive with higher energy consumption and therefore alternative methods of implementing the DS system were analyzed. For this purpose, 3 different schemes namely Decentralized (1 DS per micro-plant), semi-centralized (1DS per 1000 micro-plant) and centralized (1 DS per 13500 micro-plant) were analyzed. Furthermore, heat integrations were introduced as various literature pointed out that a significant improvement in the total cost and energy consumption was observed and hence they were adapted to ZEF requirements. Therefore, six different systems namely conventional distillation (base case), Feed pre-heating, Vapour recompression, Bottom flashing, External heat integrated distillation and Vapour compression systems were suggested. The primary focus of this research will be to analyze these different distillation designs for all the three schemes and recommend a feasible design for the DS sub-system that is both energy-efficient and economically viable along with analyzing the impact of varying ambient conditions on the DS system.
For this purpose, each of these systems was modelled in COCO and the modelling results obtained were used in analyzing the overall energy consumption and the total cost of each design. Different scenarios were analyzed, preliminary equipment designs were estimated. Each system was compared and evaluated in terms of energy and cost-saving and a quantitative and qualitative analysis was performed. Based on this analysis a centralized VRC and base case systems powered by solar PV operating for 7 hours were recommended. In order to understand the impact of fluctuating solar radiation and ambient temperature on the DS system, a MATLAB model was developed. From the results, it was evident that the fluctuating ambient conditions affected the product purities and the startup and shutdown of the DS system. A feed mass flow-temperature control was implemented for Centralized base case system, where the feed mass flow rate into the DS system was adjusted depending on the fluctuating power input. The control structure was able to effectively control the distillate and bottom purities by varying the feed mass flow rate.