Dynamics of crystal sedimentation in a pilot scale MEG reclamation rig

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 isrecovered and recycled in MEG Recovery Units, which separate the MEG fromwater and salts. The University of Manchester has a pilot plant scale MEG recoveryunit, 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 (thedowncomer). The MEG recovery is enhanced in the downcomer with the use ofbrine, which forms a density stratified fluid with the MEG and affects the settling ofthe crystals. Additionally, the MEG is dragged by the crystals and mixes with thebrine during the process, which decreases the efficiency of the recovery.The phenomena occurring in the pilot plant were characterised. Conductivityprofiles were analysed for several NaCl fluxes and several heights. These profilesshowed faster changes at higher NaCl fluxes. The MEG was mixed and transportedfaster as the NaCl flux increased in the experiments conducted. Samples werecollected 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 variationsdespite the differences in the inlet conditions and the sampling height.The settling process was scaled down and analyses were conducted at laboratoryscale varying key process parameters: the particle size of the particles, the solidsconcentration and the MEG concentration in the media. The settling speeddecreased as the MEG concentration and the solids concentration increased in thesamples; the settling speed increased as the particle size increased. Theexperimental settling speeds were compared with the theoretical settling speedusing the Richardson-Zaki equation with three different diameters. The theoreticalsettling speed using the Stokes diameter was in good agreement with theexperimental results.The MEG transport was successfully scaled down with the design of the laboratoryscale downcomer. The MEG concentration profiles and solids concentrationprofiles were obtained at different NaCl fluxes and several heights. The MEGtransport was faster as the NaCl flux increased in the laboratory scale downcomerexperiments. The final concentration was around 32 % wt. MEG in the downcomer.MEG concentration profiles were estimated from the conductivity profiles. Theseprofiles were in good agreement with the experimental results from the samples.MEG flux was calculated along the downcomer using mass balances from thesamples obtained in the experiments.The characterisation of the settling speed and the MEG transport in the pilot plantand in the laboratory could be used to improve the design and operation of theMEG recovery units.

Date of Award	1 Aug 2016
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Peter Martin (Supervisor)