463765 Absorption of Mixed Refrigerant in Lubricant Oil
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University,P.R.China
Of all processes producing liquefied natural gas (LNG), mixed refrigerant cycle is considerably appealing for its high efficiency. In a small scale mixed refrigerant cycle, utilizing injected screw compressors makes the whole compression unit more compact. However mixed refrigerant may dissolve in lubricant oil existing in injected screw compressors. It is pretty essential to understand the detailed knowledge of mixed refrigerant absorption in refrigerant-lubricant oil mixtures, as the change in compositions of mixed refrigerant can directly influence some important operation parameters such as process energy consumption, heat exchangers performance, etc.
To determine the optimized components of mixed refrigerant in single mixed refrigerant (SMR) liquefaction process, without considering the absorption of refrigerant in oil, the Knowledge Based Optimization (KBO) method was adopted. During this optimization process, there were two limiting conditions for operation: the minimum heat transfer temperature difference in the LNG heat exchanger was set 3K and liquid entering compressors was not allowed. The P-R equation of state, which is suitable to calculate the hydrocarbon mixtures in particular, was adopted in the simulation. Typically, energy consumption of unite LNG product directly affects the running cost of the liquefaction system, so the minimum specific power consumption was set as objective function. Then refrigerant components in SMR LNG process were optimized for both four and five compositions by adjusting sub-flow of the components from low to high boiling point according to the KBO method. At the end of the optimization process, the optimal constitution was gained with methane 21%, ethylene 14%, propane 58%, nitrogen 7% in the four-composition mixed refrigerant and methane 19%, ethylene 30%, propane 4%, i-butane 39% and nitrogen 8% respectively. These optimal results were then used as basis of follow-up studies on the effect of single refrigerant component change on operation parameters as well as experimental investigation on refrigerant absorption in lubricant oil.
After the optimal constitution of mixed refrigerant was gained, composition change due to dissolving in lubricant oil was considered. In order to investigate the impact of the change of each component on the operation parameters referred to above, a variable-controlling approach based on HYSYS was adopted. More precisely, the influence of methane, ethylene, propane and isobutene was investigated respectively. Firstly, we investigated the effect of each component change on the specific power consumption. Different component reducing affect specific power consumption differently due to their thermophysical property difference. To study this effect more specifically, the content of each component was reduced gradually, each time 0.01kmol/h, while other components remained the same and the specific power consumption was recorded at the same time. Methane, ethylene, propane, and i-butane was individually operated in the same way until the minimum heat transfer temperature difference in the LNG heat exchanger was less than 3K. During this process, we can obtain that with the decrease of each kind of components, the specific power consumption declined in a linear trend. However, reducing the same amount of each component, the specific power consumption reduced in a different degree—reducing methane had the greatest impact, followed by ethylene, propane in the four-composition refrigerant process and the five-composition refrigerant shared the same trend with i-butane had the smallest impact. It is noteworthy that, nitrogen is hardly soluble in lubricant oil, so we did not consider its reduction. Secondly, the effect of each component change on the minimum temperature difference in LNG heat exchanger was investigated. Each component was reduce in the same way referred to above, during this process the minimum temperature difference was recorded. It can be obtained that in the four-composition refrigerant process, reducing propane impacted to the greatest extent, followed by methane and ethylene. However, the five-composition refrigerant process saw a different pattern with i-butane impacted most, followed by propane, ethylene and methane. This trend can be explained by temperature sensitive range of each component.
As well as single component analysis based on HYSYS, experiments focusing on overall absorption of mixed refrigerant in lubricant oil were also conducted.During this experiment process, we studied four-component gas, which was constituted by 21% methane, 14% ethylene, 58%propane, 7%nitrogen, entering lubricant oil. The model number and brand of lubricant oil were determined by actual LNG process. In this experiment, we tested three different kinds of lubricant oil (CP-1010-100, CP-1005-100, CP-1516-100) which were widely used in the SMR process compressors. In order to ensure the lubricant was sufficient, the tank was designed to be 10 liters.
Initially, the tank was evacuated, after which lubricant oil was added to the desired weight (10kg). Then turn on the heater until the oil reached 303K by regulating the regulator. After that, open the valve of the high pressure gas bottle, then the mixed gas was introduced into the lubricant oil. Regulating the valve to make sure the pressure in the tank was 0.5MPa initially. Finally, the component of the exhaust mixed gas was measured by gas chromatograph. When the component measured kept unchanged for two times, wrote down the specific percentage of each component and mixed gas mass flow of intake and exhaust. Apart from that, the instantaneous system pressure and temperatures in vapor and liquid regions were both recorded at temperatures of 303K, 313K, 323K, 333K, 343K and 353K. When the mixed gas in the bottle was used up, the lubricant oil should be discharged and the tank was cleaned for the next kind of lubricant oil. Because nitrogen is hardly soluble in lubricant oil, its mass flow kept unchanged, which could be used as a leak detection during the experiment process.
The mass of each component of mixed refrigerant absorbed in lubricant oil was calculated by the experiment data. When the calculation was completed, the results were then used to investigate the impact of mixed refrigerant absorbed in lubricant oil on operation parameters change in the LNG liquefied process through HYSYS.