PRESSURE DROP AT GAS-DISTRIBUTION STATIONS USING FOR NATURAL GAS LIQUEFACTION

Abstract

Liquefied natural gas share in the world consumption of natural gas increases annually. Theoretically, the minimum expenditure of energy to produce LNG in amount 1 kg is 0,26 ... 0,29 kWh from the initial state at standard conditions. It is advisable to use the potential energy, which is lost when gas reduces from main pipeline pressure to distributive pipeline pressure at gas stations for LNG production. The Ukrainian gas transportation system has significant energy reserves in the form of gas excess pressure that is delivered to consumers at low pressure - 0.3, 0.6 and 1.2 MPa. Low-tonnage LNG plants that operate through pressure drop could be constructed on many of more than 1,500 gas-distribution stations. The LNG production process flowsheet at gas-distribution stations on the base of expander cycle with production stream recirculation is proposed. In conventional expander schemes liquefaction coefficient does not exceed 14 % and the cost of gas purification system from carbon dioxide can make up 40-60% of plant cost. Due to the fact that production flow is closed in provided scheme, the costs of gas purification from CO2 can be reduced by 7-10 times. The optimal ratios of production and technological flows that are within 0,16-0,25 and depending on the pressure in the main (from 3,5 to 5,5 MPa) and distributive (0,3 to 0,6 MPa) pipelines has been determined. At these ratios, gas liquefaction is carried out with no additional energy outlay. In order to exclude the possibility of carbon dioxide crystallization after gas expansion in expander, CO2 crystallization temperatures in natural gas with CO2 percentage up to 2 % are calculated using software system HYSYS. It is shown that by using the main gas at temperature 15 °C and pressure from 3,5 to 5,5 MPa, the gas temperature after expansion to pressure in distribution network 0,3 and 0,6 MPa exceeds the calculated CO2 crystallization temperature. Thus, carbon dioxide removal of technological stream is not required. The reliability of calculations in software system HYSYS has been confirmed by comparison with experimental data for hydrocarbon systems containing carbon dioxide.