Evaluation of new ideas for offshore oil and gas processing - Evaluering av nye ideer for offshore olje- og gassprosessering

Abstract

Water removal is one of the most important factors for safe transport of rich gas through pipelines. Water dew-point specifications at the Norwegian Continental Shelf are set to a temperature of -18°C at 69 barg by Gassco AS to insure against hydrate formation. This thesis is focusing on the dehydration process of natural gas with internal regeneration of glycol. Internal regeneration of glycols is a process where glycols are injected in an early stage of the process. By utilizing heat from compressed natural gas, it is possible to regenerate glycols by evaporating water from the rich glycol. The process is evaluated with both MEG and TEG.

Each glycol is evaluated for two different well stream compositions. One with a light methane-rich composition and the other with a composition that resembles a traditional well stream with heavy hydrocarbons. The regeneration process is fixed to give 32-mole ppm water in the rich gas and is obtained by adjusting pressure and temperature in the regeneration column. To obtain 32-mole ppm, it is necessary to adjust the temperature to acquire a sufficient glycol regeneration. For this thesis, a sustainable process is defined as a process with less than 10 kg/h glycol losses. A high-temperature increases glycol losses as glycols evaporate and follow the gas. By applying modifications to the process, it is possible to enhance glycol recovery. This thesis has been focusing on three types of modifications. Applying a second regeneration column to condensate evaporated glycols from the gas. Expanding the well stream before the process to reduce operation pressure. Splitting the regeneration stream to reduce the actual volume for regeneration equipment.

To evaluate processes are thermodynamic models used to calculate properties and interactions between fluids-fluids and fluid-equipment. Therefore is a review of oil and gas related thermodynamic models analyzed in relation to experimental data for dew-point related interactions. Most of the calculations are conducted through Aspen HYSYS v8.8 and some calculated using NeqSim. The evaluation of systems is carried out by simulation with Peng-Robinson.

The results of simulated shows that TEG is well suited for use in an internal regeneration process. The best results for glycol recovery is obtained by applying adjustments to the regeneration section. By combining several modifications is the process optimized. For the heavy hydrocarbon rich composition glycol losses are too high with no alterations to the process. The rich methane composition has few losses and is hence better suited for and internal regeneration process. A system with MEG is not able to obtain a sufficient glycol recovery for either of the compositions. It is therefore necessary to implement a glycol recovery system by treating condensed water.

The thesis ends with an analysis of weight and footprint for processes selected from evaluated modifications of internal glycol regeneration. The total weight and footprint are based on compressors, heat exchangers and separator column. Footprint is mainly affected by the actual volume flow rate through the equipment. Weight is determined by equipment size and pressure. Separators contribute to a majority of the total weight and footprint.