Increased liquid extraction in the Kollsnes dew point process by using mixed glycols

Abstract

Liquid extraction is an important and challenging part of natural gas processing. Removing heavy hydrocarbon components from the gas is necessary in obtaining sales gas specifications. In addition, liquefied heavy gas components, commonly termed condensate, is an energy dense petroleum product, with high commercial value. Optimized condensate production requires low temperatures and elevated pressure, which corresponds with an increasing tendency to solid formation and operational problems. Producing more liquid from natural gas is desirable due to high energy content and sales price. At the same time, it is imperative to maintain process safety in both current and future gas processing scenarios.

Currently, an aqueous solution of pure MEG and water is injected as hydrate inhibitor on Kollsnes. The dew point separator operates with [78 wt%, 77 wt%] MEG in/out, at a temperature around -23℃. This work investigates the potential for increased liquid extraction in the gas plant, by reducing operating temperatures in the dew point process. Hydrate formation and MEG solidification conditions, as well as plant capacity limitations, are taken into account. The objective is to derive conditions for maximized safe and feasible liquid extraction, and quantify the optimization gain, both in terms of condensate production and increased income.

Two simulation models of the Kollsnes process have been developed, and implemented with two different equations of state. Hydrocarbon-dominated calculations are performed in HYSYS (8.6), implemented with SRK equation of state. NeqSim, implemented with the CPA EoS, is used when polar components dominate, particularly for predicting MEG freeze out and hydrate formation conditions, as well as VLE calculations in the MEG regeneration system. The models are validated against field and design data from Kollsnes, and used to derive conditions for maximum allowable liquid extraction. In addition, experiments are preformed to investigate MEG solidification temperatures under the influence of condensate. Measured MEG freezing points correspond well with NeqSim simulations, adding credibility to the derived optimized conditions.

The work shows considerable potentials for optimizing the Kollsnes dew point process in terms of increased liquid extraction. Changing the injected MEG-water concentration to [83 wt%, 81 wt%] MEG in/out, allows safe reduction of the dew point separator temperature to process minimum of -26℃. This increases condensate production by roughly 20%, adding up to a net income increase of around 25 MNOK/year pr. dew point train. New MEG concentrations reduce the energy required for regeneration. However, the savings are relatively moderate, adding up to 0,1 MNOK/year for the amount of MEG in one dew point train.

Aqueous solutions of mixed glycols (MEG and TEG) have lower freezing points than pure MEG-water. Injected as hydrate inhibitor, a mixture of 80 mole% MEG 20 mole% TEG, (62 wt% MEG 38 wt% TEG) allows safe reduction of the dew point separator temperature to -30℃. This increases the condensate production by 24% compared to the optimized MEG case, giving a net growth in income of 37 MNOK/year from one dew point train. However, the capacity of the export compressors rather than solid formation limits temperature reduction in the dew point separator. Reduction to process minimum is obtained with pure MEG-water as hydrate inhibitor, and as for today, there is no direct utilization for mixed glycols on Kollsnes. There is however a considerable potential for increased liquid extraction by optimizing current operations with MEG.