The impact on the plant due to a decrease in the flow rate of gas entering the natural gas processing plant, a change in the gas composition toward a less rich profile, and an increase in the CO2 content
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The renovation and expansion project of the Third Gas Treatment Plant at Zhongyuan Oil & Gas is a medium-pressure cryogenic natural gas treatment facility, whose production process follows the design scheme provided by the German company LINDE. The main process units of the plant consist of raw material gas compression and drying, pressurized expansion refrigeration, propane-assisted precooling, NGL recovery, and NGL distillation. The designed capacity is 1 million cubic meters per day; however, due to a decrease in the pressure of the gas supply and an improvement in its quality, the amount of associated gas is around 500,000 cubic meters per day. It is necessary to conduct a study on operating under these conditions with reduced gas flow, focusing on determining the modification plans for the gas turbines, expansion compressors, cryogenic tanks, and distillation towers. I was wondering if any of you have relevant information on this topic and could offer some suggestions.
Based on the new process parameters, the dimensions of the equipment are recalculated, and future trends are taken into account to ensure that a single technical upgrade can extend its service life
The standard design calls for an operating capacity of 60%-110% of the maximum capacity. In the case described by the poster, there is a serious issue of overkill – the device is constantly operating at a low load. As time passes, the amount of associated gas will gradually decrease, the device’s adaptability will worsen, and the device will also face the need for modification. It is recommended to adopt a multi-unit design (for example, 300,000 cubic meters per compressor) to improve adaptability.
It is necessary to conduct a feasibility study for operation at low gas flow rates, primarily to determine the modification plans for the gas turbine, expansion compressor, cryogenic tank, and distillation tower under such conditions. The purpose of this study is to figure out whether it is still possible to rely on the existing equipment Should I consider replacing it? Indeed, the current processing volume is significantly deviated from the designed level, and it is difficult for the existing process to operate in a highly efficient and energy-saving manner. It is likely necessary to consider redesigning the equipment; as for the compressors, it may also be hard to ensure normal operation even by adjusting the reflux conditions.
Firstly, as mentioned above, the device operates under a condition of using excessive resources for a relatively minor task, which significantly increases its energy consumption. Secondly, as the composition of the feed gas becomes poorer, it is necessary to further reduce the cooling temperature; by increasing the pressure at the compressor outlet, the expander can be cooled even more, thereby achieving greater recovery of light hydrocarbons. Finally, the poster mentioned an increase in the CO2 content of the feed gas; this issue also plagues our cryogenic units. It is necessary to further calculate the lowest negative temperature that can be achieved by the demethanization tower under such CO2 levels. If the negative temperature is too low, CO2 can cause freezing and blockages at the exit of the expander or at the top of the demethanization tower, leading to unstable operation of the unit and even shutdown. This requires the negative temperature at the top of the demethanization tower to be reduced as low as possible without the formation of dry ice. Based on the above three points, the following approaches can be taken: first, determine the economic viability and energy consumption of the plant when operating with the existing equipment, by taking into account the volume and composition of the raw gas fed into the plant. Can it operate properly? Secondly, by accounting for the CO2 content in the raw feed gas, a negative temperature is applied at the top of the demethanization tower in order to recover as many light hydrocarbon resources as possible.