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Highlights • A new subsea natural gas dehydration process with membrane technology is evaluated. • Three different process configurations with staging of the regeneration are investigated. • Optimization is performed with respect to membrane sizes and TEG flow rate. • Stating of the regeneration with heating between the stages is favorable. Abstract Subsea processing enables broader exploration of oil and gas reservoir, giving an increased focus on developing alternative processes for subsea oil and gas treatment. This work provides a first evaluation of a new proposed subsea natural gas dehydration process with the use of a membrane contactor with triethylene glycol (TEG) for dehydration of the natural gas in combination with thermopervaporation for regeneration of the TEG. Simulation models are developed in Aspen HYSYS V8.6 and process optimization is performed on three different process designs with respect to staging of the regeneration. By introducing two thermopervaporation units in series the TEG flow rate is reduced by 55%, the membrane volume by 14.6% and the energy demands by 37.8%, compared to a design with one thermopervaporation unit. However, increasing the number of regeneration stages increases the complexity as additional heaters are introduced. [ABSTRACT FROM AUTHOR]

The petroleum industry is receiving increased interest in subsea oil and gas processing as it is running out of easily accessible oil and gas reservoirs. Membrane processes fulfill subsea design requirements with a simple and compact design. However, today no application has been used subsea. This paper reviews the advances in membrane separation to date in view of the industrial needs for subsea separation. Some potential applications of membranes and membrane processes in subsea processing are proposed based on the topside experience. Two subsea natural gas treatment processes, namely natural gas dehydration and acid gas removal, are discussed in details with respect to the advantages and challenges in the implementation of membrane technology subsea, including future research perspectives. This study can be a starting point in connecting the two research areas (subsea separation and membrane technology) together. [ABSTRACT FROM AUTHOR]

Flexible thermal power plants integrated with CO 2 capture systems can balance the intermittent power generation of renewable energy sources with low-carbon electricity. Among these power systems, natural gas combined cycles will play a fundamental role because of their faster operation and higher efficiency. Optimisation-based control strategies can enhance the flexible power dispatch of these systems and improve their performance during transient operation. This work proposes a model predictive control (MPC) strategy to stabilise these power plants with post-combustion CO 2 capture based on temperature swing chemical absorption and provide offset-free reference tracking. A delta-input formulation with disturbance modelling is proposed, as it provides more efficient computation with offset-free control. Data-based models were developed to replicate the performance of the actual power and capture plants. Prediction of nonlinear behaviour was accomplished by creating a network of local linear models, which allowed the formulation of the dynamic optimisation program in the MPC strategy as a convex quadratic programming problem. A case study demonstrated the effectiveness of the proposed MPC to balance drastic changes on power demand and keep specified capture ratios. Furthermore, the reduced deviations achieved in the reboiler temperature suggest that the nominal value of this parameter could be increased to improve the desorption process without risks of reaching temperatures where the solvent would degradate. [ABSTRACT FROM AUTHOR]

A mathematical model of a membrane contactor using triethylene glycol (TEG) for dehydration of natural gas at subsea operation conditions has been developed. The goal for the membrane contactor is to dehydrate the natural gas to transport pipeline specifications, which are −19 °C at 69 barg. The membrane contactor model is based on a hollow fiber configuration with the gas in the shell side and the liquid absorbent inside the fiber. The shell side gas is modelled as one-dimensional on the assumption of turbulent flow, while the fiber/lumen side is modelled as two-dimensional flow due to the laminar flow. Orthogonal collocation is applied to solve the two-point boundary value problem. The developed model is validated against proprietary high pressure experimental data for dehydration of natural gas with TEG in a membrane contactor. The H 2 O molar flow removed from the gas phase is predicted by the model with a mean absolute error range of 3–7% compared to the experimental results, based on two methods of calculation due to uncertainty in experimental basis. A sensitivity study was performed to evaluate the effect of membrane properties and operation conditions on the dehydration performance. The main findings are that preferred membrane properties from a separation performance point of view are thin membrane, small fiber diameter, long membrane module, high porosity and high permeability. But, the production of the membrane, stability and long term operation also need to be considered in the selection of membrane material and module parameters. For high pressure subsea operation it is found that the use of a dense layer on the top of the porous support could be favourable to prevent wetting of the membrane. It is found that a wetting of 1% already provides a significant drop in separation performance. For the operation conditions the main findings are that high pressure and low temperature are favourable for the separation. Increasing the gas and liquid flow gives increased flux over the membrane, but also results in increased pressure drop in the module. However, as the membrane contactor will be placed in a system with regeneration of the solvent, the operation conditions should be optimized considering the whole system. [ABSTRACT FROM AUTHOR]

• A nonporous membrane contactor effectively removes water vapor from natural gas. • Considerable water flux and the dewpoint of < -20 °C in gas outlet were documented. • A 1D-2D model estimates simultaneously the permeance and overall mass transfer coefficient. • The effect of operating conditions was evaluated experimentally and by modeling. • Axial dispersion correlations were obtained and validated with experimental data. Subsea natural gas reservoirs are usually saturated with water vapor that causes corrosion in pipelines and forms hydrates blocking pipes and installations. In this work, the viability of the subsea natural gas dehydration in a non-porous membrane contactor was evaluated experimentally and by modeling. A flat sheet composite membrane is employed as the membrane interface and a structured packing turbulence promoter was installed at the gas side. The water flux and the outlet dew point were measured at different operating conditions (e.g., pressures, liquid and gas flow rates). To estimate simultaneously the permeability, overall mass transfer coefficient, and outlet water content, a systematic approach was applied and a 1D-2D model was developed bypassing the liquid phase mass transfer resistance and solved numerically coupled with a developed correlation for axial dispersion coefficient. The results show an increase in transmembrane water flux to the highest values of 72 g/m2h and 53 g/m2h with increasing gas flow rate and pressure to 500 ml/min and 11 bara, respectively. The effect of using a turbulence promoter was evaluated, and a good agreement between the simulated and experimental results was obtained. The mass transfer resistance of a dense layer was found to be dominating compared to that of gas and liquid phase and the high pressure and high gas flow rate are favored in this process. This study shows that membrane contactor enables effective water separation under subsea conditions. Installation of a turbulence promoter in the gas phase significantly reduced the water content in the outlet gas stream. [ABSTRACT FROM AUTHOR]

• Hybrid membrane system for natural gas dehydrating and sweetening is designed. • Captured CO 2 is used as sweep gas in the dehydration unit. • Process parameters are investigated to identify the optimal operation condition. • Future research direction on membrane materials development is proposed. Subsea natural gas processing attracts increased interest due to the smaller environmental footprint. Natural gas (NG) dehydration and sweetening are the main processing steps to avoid pipeline plugging and corrosion caused by the presence of water and CO 2. Triethylene glycol (TEG) and amine absorption are the commercial technologies for these applications. However, membrane technology is considered as promising solutions for alternative subsea gas processing technologies, which provides unmanned operations without the requirements for rapidly periodical maintenance. In this work, a hybrid membrane process was designed for integrated dehydration and sweetening of a saturated natural gas containing 10 mol% CO 2 , and the process operating parameters such as inter-stage feed and permeate pressures are investigated. The simulation results indicated that the optimal permeate pressure in the 2nd -stage unit is 4 bar, and the optimal 3rd-stage feed and permeate pressures are15bar and 2 bar, respectively. The minimum specific cost of <2.71 × 10−3 $/m3 sweet natural gas was estimated to achieve the separation requirement of <2.5 mol% CO 2 in purified NG together with captured high purity CO 2 (>95 mol%) for enhanced gas recovery. However, due to the relatively low water selectivity of the dehydration membranes at high pressure of 60 bar used in the simulation, the hydrocarbon loss is still quite higher. Thus, advanced membranes with high H 2 O/CH 4 selectivity at high pressure should be pursued to promote the application of the designed membrane system for subsea natural gas dehydration and sweetening. [ABSTRACT FROM AUTHOR]

• A membrane model is developed and solved with orthogonal collocation. • The developed model is validated and applied for efficient hollow fiber module design. • A process parametric study is conducted on membranes for natural gas sweetening. Hollow fiber membranes show a great potential in natural gas sweetening by removing CO 2 to meet gas grid specifications. A membrane model with high prediction accuracy is developed to model multicomponent gas transport through hollow fiber modules. The influences of hollow fiber diameter and length, and packing density on module efficiency related to pressure drops in both sides were systematically investigated based on the developed model. The total pressure drop along the length is less than 1% if the inner diameter of hollow fibers (0.6 m length) is larger than 200 μm. Moreover, the highest module packing density was found to be dependent on hollow fiber dimension, and too high packing density will cause extreme high pressure drops. Both feed CO 2 concentration and pressure were found to significantly influence membrane module performance related to the required specific membrane area and hydrocarbon loss based on process parametric study of CO 2 removal from natural gas. Larger pressure drops along fiber length was found for the more-permeable polyimide membranes compared to the less-permeable cellulose acetate and carbon membranes. The developed model can be used for guiding the design of efficient hollow fiber membrane modules and potentially process simulation of membrane gas separation. [ABSTRACT FROM AUTHOR]

Carbon hollow fiber membranes (CHFMs) were fabricated based on cellulose hollow fiber precursors spun from a cellulose/ionic liquid system. By a thermal treatment on the precursors using a preheating process before carbonization, the micropores of the prepared CHFMs were tightened and thus resulting in highly selective carbon molecular sieve (CMS) membranes. By increasing the drying temperature from RT to 140 °C, the cellulose hollow fiber precursors show a substantial shrinkage, which results in a reduction of average pore size of the derived CHFMs from 6 to 4.9 Å. Although the narrowed micropore size causes the decrease of gas diffusion coefficient, stronger resistance to the larger gas molecules, such as CH 4 , eventually results in an ultra-high CO 2 /CH 4 ideal selectivity of 917 tested at 2 bar for CHFM-140C due to the simultaneously enhanced diffusion and sorption selectivity. The CHFM-140C was further tested with a 10 mol%CO 2 /90 mol%CH 4 mixed gas at 60 °C and feed pressure ranging from 10 to 50 bar. The obtained remarkable CO 2 /CH 4 separation factor of 131 at 50 bar and good stability make these carbon membranes great potential candidates for CO 2 removal from high-pressure natural gas. • Highly CO 2 selective CMS membranes made from cellulose precursors. • CMS membranes present ultra-high CO 2 /CH 4 selectivity of 917 at 2 bar. • CO 2 /CH 4 mixed gas testing up to 50 bar shows potential for natural gas sweetening. [ABSTRACT FROM AUTHOR]