The Oman Methanol Company (OMC) plant in Suhar, Oman, is designed to produce more than 3,000 tons premium-grade methanol per day. Three large storage tanks buffer flow from the process to the shipping outlet, and three smaller tanks are used for process and quality control functions. All tanks are of a fixed roof design and, as a result, fluctuations in atmospheric pressure and gas volumes are accommodated by cyclically venting to atmosphere or pressurising the tanks with nitrogen. Releases from the tanks due to this operation account for only a tiny fraction of the total annual plant production, but OMC needed to quantify the losses and explore options to reduce or eliminate the emissions.
STS carried out a site survey of the facility, obtaining information on the plant design and production rates, as well as drawings of the tank layout. As retrofitting the tanks with floating roofs would have been far too costly, the scheme investigated was to dewpoint the methanol and nitrogen mixture discharged from the tanks. This approach considered one dewpoint heat exchanger local to each tank, and the design and selection of the heat exchangers was carried out to determine the required cooling load and dewpoint temperature.
A budget cost for the proposed installation was then generated using manufacturer quotations for major equipment (packaged heat exchanger skids, glycol chiller) and contractor estimates for the insulated glycol and methanol site pipework elements. In addition, annual energy usage for the chiller, glycol pumps and methanol condensate pumps was assessed, and the total simple payback cost for the project was calculated. This information package provided OMC with the information they needed to clarify their approach to managing the emissions in future. It was also pointed out that the tanks, while previously painted white, had degraded to the orange colour of the undercoat. Calculations showed that this could increase the tank emissions by a factor of two due to the increased solar absorption, and thus maintaining tank paint condition was highlighted as the lowest cost option to reduce emissions as far as possible without resorting to expensive dewpointing infrastructure.