1. Dynamics of crystal sedimentation in a pilot scale MEG reclamation rig
- Author
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Lopez Rodriguez, Rafael and Martin, Peter
- Subjects
660 ,Sedimentation ,Pilot plant ,Mixing in stratified fluids ,Monoethylene glycol ,MEG ,MEG recovery unit - Abstract
The formation of natural gas hydrates inside the pipelines can plug or block them, which can cause significant economic losses. The addition of monoethylene glycol(MEG) is a common method to prevent their formation in field operations. MEG is recovered and recycled in MEG Recovery Units, which separate the MEG from water and salts. The University of Manchester has a pilot plant scale MEG recovery unit, which boils the MEG and water from the inlet in a flash tank and the salts (usually NaCl) are separated by sedimentation through a glass column (the downcomer). The MEG recovery is enhanced in the downcomer with the use of brine, which forms a density stratified fluid with the MEG and affects the settling of the crystals. Additionally, the MEG is dragged by the crystals and mixes with the brine during the process, which decreases the efficiency of the recovery. The phenomena occurring in the pilot plant were characterised. Conductivity profiles were analysed for several NaCl fluxes and several heights. These profiles showed faster changes at higher NaCl fluxes. The MEG was mixed and transported faster as the NaCl flux increased in the experiments conducted. Samples were collected from the downcomer after 2.5 hours of continuous addition of the feed. The settling crystals were uniform and with cubic shape, their average size (D32) was between 50.5 and 62.1 micron and they did not present significant variations despite the differences in the inlet conditions and the sampling height. The settling process was scaled down and analyses were conducted at laboratory scale varying key process parameters: the particle size of the particles, the solids concentration and the MEG concentration in the media. The settling speed decreased as the MEG concentration and the solids concentration increased in the samples; the settling speed increased as the particle size increased. The experimental settling speeds were compared with the theoretical settling speed using the Richardson-Zaki equation with three different diameters. The theoretical settling speed using the Stokes diameter was in good agreement with the experimental results. The MEG transport was successfully scaled down with the design of the laboratory scale downcomer. The MEG concentration profiles and solids concentration profiles were obtained at different NaCl fluxes and several heights. The MEG transport was faster as the NaCl flux increased in the laboratory scale downcomer experiments. The final concentration was around 32% wt. MEG in the downcomer. MEG concentration profiles were estimated from the conductivity profiles. These profiles were in good agreement with the experimental results from the samples. MEG flux was calculated along the downcomer using mass balances from the samples obtained in the experiments. The characterisation of the settling speed and the MEG transport in the pilot plant and in the laboratory could be used to improve the design and operation of the MEG recovery units.
- Published
- 2016