167- Design scheme for the renovation of the mesh-type separators at the inlet and outlet of the mixed refrigerant cycle compressor in LNG plants using vane separators

In natural gas liquefaction projects, a mixed refrigerant cycle compression process is often used to liquefy the feed gas, thereby making the mixed refrigerant cycle compression a core component of LNG plants. However, in the actual operation of the mixed refrigerant cycle compression unit, fluctuations in the source of the upstream feed gas, or variations in the ratio of the components in the mixed refrigerant, can cause the inlet and outlet streams of the mixed refrigerant compressor to contain large amounts of liquid. This results in a significant deviation from the designed operating conditions, thereby severely affecting the proper functioning of the core equipment, namely the mixed refrigerant cycle compressor. As a consequence, the operational and maintenance costs of the facility increase, the LNG production capacity fails to meet the design requirements, and the efficiency of the facility’s operation is compromised.

This post was last edited by luoli519 on 2020-10-20 at 12:14. Taking as an example the project to upgrade the outlet separator of the mixed refrigerant compressor unit provided by our company, a coastal firm, for an LNG enterprise in Sanmenxia, Henan, using vane separation technology, this article discusses the problems faced by the separators of cycle chiller compressors in many LNG plants today and the technical solutions available. The flow diagram of the mixed refrigerant cycle compression unit designed by this company in Yangzhou for an LNG facility in Sanmenxia, Henan, is as follows:

This company in Yangzhou designed and supplied for an LNG facility in Sanmenxia, Henan, a mixed refrigerant cycle compressor unit with an outlet separator model F616A51; previously, a traditional and simple mesh separator was used. However, in actual operation, the separation efficiency of separator F616A51 is very low, resulting in the discharged refrigerant vent gas and refrigerant separation gas containing a significant amount of liquid.

The original separator F616A51 at the compressor outlet of the mixed refrigerant cycle was a horizontal mesh separator. Its dimensions are ID1600mm*SM/SM3950mm; the gas-liquid mixture inlet at the top right has a size of DN600mm, while the gas outlet at the top left has a size of DN400mm. See the attached diagram for details:

In actual operation, due to the large amount of liquid carried in the gas supplied by this separator, and the unstable supply of feed gas upstream resulting in a lower gas flow rate, the original process design company and the owner adjusted the components of the refrigerant used in the design, reducing the content of isopentane and increasing the content of propane, based on the cooling capacity required by the methane deep cryogenic separation unit. The flow rate variations under these design conditions and actual operating conditions far exceed the operational flexibility allowed by traditional screen separators; these are conditions that equipment based on gravity separation, filter element separation, or screen separation cannot handle.

Please take a look at the original design parameters for this separator: 1. Inlet flow rate (Kg/h), 212203.58； 2. Inlet temperature (°C), 20 ； 3. Inlet pressure (MpaG), 3.39 ； 4. Composition at the inlet (mol%): C2H4, 19.81 ； C2H6，2.20 ； N2，11.86 ； CH4，52.03 ； Isopentane, 4.69 ； C3H8，8.99 ； Amyl pentane, 0.19 ； CO，0.12 ； H2，0.12。

Please take a look at the actual operating parameters of this separator: 1. Inlet flow rate (Kg/h), 275202.9； 2. Inlet temperature (°C), 20 ； 3. Inlet pressure (MpaG), 2.725 ； 4. Composition at the inlet (mol%): C2H4, 18.37 ； C2H6，1.75 ； N2，12.16 ； CH4，48.2 ； Isopentane, 3.28 ； C3H8，16.24。

By comparing the aforementioned actual operating conditions with the design conditions, it can be seen that there are significant changes in these conditions: the inlet flow rate increases by 29.69%, while the pressure decreases by 19.62%. The combination of this positive increase in flow rate and this negative decrease in pressure leads to a sharp deterioration of the operating conditions. Therefore, the original screen separator is absolutely unable to meet the requirements. Moreover, the owner’s request this time to reuse the existing separator housing poses significant difficulties and challenges in terms of technical modification; only by using vaned separation technology components can an effective and targeted technical upgrade be achieved while still making use of the original separator housing.

I’m going out right now for a meeting. We’ll continue the analysis and discussion when we get back.

We discussed with the device designer and the owner why there is such a large difference between the designed operating conditions of this separator and its actual operating conditions. In fact, addressing this issue properly would put the designers in an awkward and embarrassing position. However, the designers were also fairly honest with us, stating that due to inadequate consideration during the design phase and adjustments to the refrigerant composition during actual operation, severe liquid carryover in the gas phase of the gas-liquid separator occurred, which affected the stable operation of the system; therefore, a gas-liquid separator with high separation accuracy was required.