Study On Natural Gas Purification And Liquefaction Process Of Small Scale LNG Project

In the field of natural gas liquefaction, the small-scale natural gas liquefier has been attached importance to by China inside and outside, due to its small bulk, mobile transportation, convenient open-up and shut-down, as well as skid-mounted package. This paper will put forward the concept of modularized gas purification and a new type energy saving liquefaction process of integral incorporated cascade of mixed refrigerant (SP-MRC), aiming at the implementary project of small-scale natural gas liquefier and its key technical research. The main contents are as follow.Firstly, the overall technical process is selected, which comprise the optimized selection of pretreatment way and liquefaction process. It is proposed that dehydration and desulfurization of molecular sieve adsorption should be adopted for small-scale natural gas liquefier. The concept of modularized gas purification is put forward for the first time. The mixed refrigerant cycle maybe is the best choice of minitype plant. The mathematical modeling of pretreatment equipment, heat exchanger, revolving machine, separator and pipe accessory are done.Secondly, the natural gas pretreatment tester is designed and built. The tester is employed to measure the dehydration and purification data for simulated natural gas mixtures, which consist of the following sections: (1) testing gas feeding system; (2) gas mixer; (3) adsorption system; (4) measurement system; (5) sampling analysis system; (6) safety.Thirdly, the adsorption technics of modularization has been researched experimentally in the purification of natural gas by molecular sieve for the first time. The main task contents comprise: (1) breakthrough curve of H2O or CO2 in a column of zeolite 3A, 4A and 13X separately; (2) breakthrough curve of H2S in a column of zeolite RK-38; (3) breakthrough curve of natural gas in a layered column of zeolite 3A and 13X, and optimizing combination and scheme of the various adsorption modules which adopts competitive adsorption theory; (4) analysis of experimental figures of CO2 removal for two molecular sieves, and the comparison result indicates the separation effect of zeolite 13X is better than 4A; (5) mathematical model for fixed bed behavior used for multi-component separation, confirmation of adsorption equilibrium and kinetics parameter using the test curves, and mathematical simulation for the breakthrough curves of natural gas in a duality adsorbent column, as well as the evolution of temperature and adsorption loading; (6) research of the dynamic adsorption for molecular sieve at different pressure and temperature; (7) adsorption order of the pretreatment module is dehydration - desulfurization - decarbonization, and regeneration order is decarbonization– desulfurization– dehydration; (8) pretreatment technics of modularization can meet the rigorous liquefaction requirement of natural gas for different producing area and gas property.Fourthly, the mixed refrigerant cycle and N₂-CH₄ expander cycle are simulated. A novel mixed refrigerant cycle of integral incorporated cascade is put forward first used for small-scale natural gas liquefaction process in skid-mounted package (SP-MRC). The research involves: (1) Two typical types of small-scale natural gas liquefaction process in skid-mounted package are designed and simulated. The key parameters of the two processes are compared, and the matching of the heating and cooling curves in heat exchangers is also analyzed. (2) Three types of liquefaction process (C₃/MRC, N₂-CH₄ Expander and SP-MRC) are simulated, and the key parameters are also compared. The result indicates that specific power consumption and specific refrigerant flowrate of SP-MRC are close to C₃/MRC, and specific cooling water charge of SP-MRC is between C₃/MRC and N₂-CH₄ Expander cycle. (3) The new SP-MRC adopts the technology of integral incorporated cascade which combines heavy and light hydrocarbon. It can reduce the quantity and volume of compressors and heat exchangers in the guarantee of higher liquefaction rate and lower specific power consumption.Fifthly, how the key parameters of SP-MRC affect the process performance (specific power consumption, liquefaction rate, specific refrigerant flowrate and specific cooling water charge) is labored from thermodynamic point of view. The Optimizer of Original mode in HYSYS is used to optimize SP-MRC and find the operating conditions which minimize LNG specific power consumption (objective function). On the basis of objective function optimization, the performance parameter analysis of plate-fin heat exchanger is carried out to get the performance curve of multi-component mixed refrigerant existing phase change coupling in the heat exchanger. The research reveals that the change of temperature, pressure and component fraction at the same time will influence remarkably enthalpy, heat flow and UA of multi-component refrigerant in the heat exchanger.Sixthly, the feasibility research of small-scale liquefied natural gas plant in skid-mounted package is carried through based on No.1 station outgoing gas of 23 gas field of Zhongyuan Petroleum, which gets temperature, pressure, enthalpy, entropy, flow rate, vapour fraction and component mole fractions in liquid and gas phase of every node of the whole process that adopts the complex modularized adsorption technology and SP-MRC refrigeration. The outcome shows LNG recovery is more than 90% and power consumption cost is only 0.379 kWh/ Nm³, which corresponds to 0.19 RMB/ Nm³, reaching international one-up level.