(239d) Design and Economical Evaluation of Polygen Process to Co-Produce Synthetic Natural Gas (SNG), Methanol and Ethylene Glycol

Design and Economical Evaluation of POLYGEN process to co-produce Synthetic Natural Gas (SNG), Methanol and Ethylene Glycol

Bor-Yih Yu and I-Lung Chien*

Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan

The steady state design and economical evaluation of poly-generation (POLYGEN) process to co-produce synthetic natural gas (SNG), methanol and ethylene glycol (EG) is studied in this work. POLYGEN has been a widely studied topic recently, in which several different chemicals can be produced parallel at the same time. In POLYGEN process, the relative production rates for different chemicals can be adjusted based on different market demands, daily usages, and also changing political strategies. This character enhances the flexibility of plant-wide operation and also the economic performance. Among the products, SNG holds very similar composition and heat value to typical natural gas, and can be used as a replacement in industrial and home usages. Methanol is one of the most important industrial feedstocks, which can be the basis of various kinds of chemicals. Ethylene glycol is also an important organic chemical, which can be used to synthesize polyesters, antifreezes, solvents, and so on.

The items covered by this work are illustrated in the following. In coal-based POLYGEN processes, syngas is first generated from coal gasification in a CO2-rich atmosphere, and the raw syngas is then passes the treating stage. Syngas treating stages contains two section, namely, sour water-gas-shift-reaction (SWGSR) and acid-gas-removal (AGR). In SWGSR, excess amount of steam is added to adjust the H2/CO ratio. After that it is cooled to low temperature to enter AGR. In AGR, SELEXOL method is used to capture 99.7% of H2S and 92% CO2. Part of the captured CO2 is sent back to gasification section in order to reduce the carbon emission, and the other part is compressed and sequestrated. With proper arrangement in configuration, three streams with qualified composition to be the feedstocks for downstream chemical plants are produced.

For downstream chemical plants, the products produced will meet the industrial standard. The SNG has heat value higher than 965 (BTU/scf), the methanol purity is 99.5 (mol%), and EG purity is 99.8 (mol%). EG is traditionally synthesized through petroleum route, a novel route from coal-derived syngas has been widely considered a potential alternative as the oil shortages continuing. Thus in this work, the production of EG from syngas is adopted. Firstly, CO and H2 in syngas are separated, and are used in different reaction stages. In the first stage, CO is reacted to form dimethyl oxalate (DMO), and in second stage, DMO is further reacted with H2 to form ethylene glycol.

After the design, optimization works are studied based on minimized total annual cost (TAC), and the economical evaluation is performed to investigate the overall economic performance. Besides, some waste heat could be recovered by generating steam, and the steam is used to drive turbine to produce electricity for usage inside the plant-wide process. Thus through this work, a better understanding of POLYGEN process, and the design and operation of coal to ethylene glycol route is reached.