Experimental Characterization of High-Pressure Natural Gas Scrubbers

Scrubber design practice today is largely based on experimental data generated at ambient conditions with model fluid system such as air-water. Though good efficiency is often measured in the lab, real natural gas scrubbers often fail to meet the requirements. Mal-functioning scrubbers can lead to a series of operational and mechanical problems further downstream such as compressor breakdown, fouling, breakdown of gas cleaning processes and off-spec gas quality. Therefore one of the most common scrubber configurations has been put to the test in three different test rigs –a low-pressure rig, a high-pressure rig and a large scale high-pressure rig at an onshore gas plant. The scrubber configuration consisting of a vane-type inlet, a mesh pad and a bank of axial flow cyclones, has been tested at wide range of operating conditions and fluid properties. For the first time scientific laboratory scrubber measurements have been reported for live hydrocarbon fluids at pressures up to 113 barg. The separation efficiencies and pressure drops have been measured and reported for all tests. In addition some few unique measurements of the droplet size distribution in a high pressure scrubber have been carried out. The results show that the commonly used K-value (Souders-Brown equation) is a good design criterion for conditions where the mesh pad is below flooded condition, while it fails at more compact conditions i.e. K > 0.15 m/s. In most cases the largest droplets were larger than 400 microns and efficient droplet separation could therefore be expected. Though, droplets formed from condensation at highpressure natural gas where found to be in the size range 1- 10 microns which is hard to separate under these conditions. The performance degradation of the axial flow cyclone used in this study was totally governed by a general re-entrainment process rather than insufficient separation of small droplets. A new approach to predict cyclone efficiency was therefore needed. By mathematically modelling cyclone geometry, flow and fluid properties a brand new relationship was derived that correlates extraordinary well with separation efficiency –the dimensionless reentrainment number. In general the results showed that the efficiency decreased with increasing pressure. Also, it was seen that the efficiency dropped considerably when a live hydrocarbon fluid system was used instead of a model fluid system where only minor amounts of the gas is dissolved in the liquid phase and vice versa. Also, differences where found between the large scale and the small scale scrubber due to the uneven distribution of the fluids in the scrubber cross section. This work has therefore revealed the importance of carrying out tests with “real fluids”, at relevant pressure and at large scale in order to predict the performance of real natural gas scrubbers