Abstract:
As the world moves towards renewable energy generation, biogas and
biomethane play a significant role in meeting energy requirements shortly. The
study aimed to design a biogas upgrading process to produce higher-purity
biomethane while producing Carbon dioxide as a by-product. The design and
simulation were performed in Aspen Plus V.12 software. Three different
concentrated biogas feed systems were analyzed, with typical biogas
concentrations of 50% (mol mol-1), 60% (mol mol-1), and 75% (mol mol-1)
methane concentrations. The water removal from raw biogas was carried out by
designing a flash drum unit, and a low-temperature double-distillation approach
was designed and analyzed as the primary biogas upgrading step. The
refrigeration cycle was designed to maintain a low temperature for the
purification process, and Nitrogen was used as the working fluid in the design.
The simulation was carried out to find the sensitivity analysis and optimizations
of the operation conditions in each processing unit. Based on sensitivity analysis
results, the maximum water removal can be achieved at around -35 0C at 8 bar
pressure in Flash drum operation, and the 10 and 9 stages for the series-operated
distillation columns were required to perform a smooth distillation process
without freezing the carbon dioxide at any stage. The optimal distillate-to-feed
flow ratios in the first distillation column were 0.54, 0.62 and 0.78 and the
corresponding ratios for the second distillation column were 0.922, 0.920 and
0.915. These ratios were determined for systems 01, 02, and 03, which utilized
50%, 60% and 75% CH4 methane feed biogas respectively. All three designed
systems, operating at a feed rate of 1000Kmol hr-1, consistently produced
methane with a purity of 98.5% (mol mol-1) while generating a high-purity
Carbon dioxide stream as a valuable by product. The major limitation of the
system was the freezing conditions of Carbon dioxide, and the simulation was
optimized to maintain a Carbon dioxide frozen-free environment up tp the
optimum operating level.
