Membrane contactors modelled for process intensification post combustion solvent regeneration

Highlights

• Simulation of two existing membrane contactors for solvent regeneration by two competing models.

• Calculated module lengths for two membrane contactors at different regeneration temperatures.

• Membrane contactors achieve process intensification compared to a column above 90 °C.

• Membrane contactors have lower energy demand than a column above 95 °C.

Abstract

Membrane gas-solvent contactors are potentially more efficient at undertaking solvent regeneration in carbon capture applications compared to traditional desorber columns. This potential is investigated here through modelling of two experimentally reported membrane contactors based on Teflon AF1600 and polydimethylsiloxane active layers, by a simple approximate model and a one-dimensional mass transfer model. The respective membrane contactors could be operated at temperatures below that corresponding to the vaporisation of the solvent, due to the separation of the solvent and gas phase by the membrane. This required a steam sweep operating under vacuum conditions. The calculated module length for the Teflon AF1600 membrane varied between 8.9–70.2 m with decreasing regeneration temperature. This increase in required length was due to reductions in overall mass transfer coefficient and mass transfer driving force as temperature is lowered. It was determined that at 110 °C a PDMS contactor of length 1.1 m was required to regenerate the solvent, achievable with commercial modules. A comparison of the equipment volume footprint, known as process intensification, revealed that both the Teflon AF1600 and PDMS membranes required a lower volume than a standard packed column when operating at temperatures above 90 °C. Temperatures higher than 95 °C also designated the transition above which membrane contactors have a lower energy duty than the corresponding solvent column approach. This energy duty is a trade-off between the reduction in latent heat required to produce CO₂ and regenerate the solvent at lower temperatures, countered by the work duty of the vacuum pump needed to operate the steam sweep at pressures below atmospheric. The investigation demonstrated that membrane contactors are a viable alternative technology for solvent regeneration.

Introduction

Membrane gas solvent contactors for CO₂ capture in a post-combustion scenario are able to combine the high selectivity for CO₂ of a solvent absorption process within the compact module of a membrane (Simons et al., 2009; Zhao et al., 2016). This is achieved by having a semi-permeable membrane separating the solvent and gas phases, and hence the mass transfer area per unit volume is significantly higher than a traditional solvent column. Furthermore, the two phases’ flows are strictly controlled within the membrane module to avoid flooding and entrainment. These advantages have made membrane contactors attractive alternatives to traditional solvent absorption processes, with a number of pilot plants trialled (Scholes et al., 2014, 2012). The majority of membrane contactor research for CO₂ capture has focused on the absorption process using both porous and non-porous membranes with various solvents (Kosaraju et al., 2005; Scholes et al., 2015a). Importantly, this research has demonstrated that membrane contactors’ can reduce the volume of the equipment needed to undertake CO₂ absorption compared to traditional solvent columns, known as process intensification (Falk-Pedersen and Dannstrom, 1997; Favre and Svendsen, 2012; Mendez et al., 2017; Nguyen et al., 2011). This is important in many carbon capture situations where space is limited, such as off-shore platforms. Regeneration of the CO₂ loaded solvent (i.e. stripping) through a membrane contactor is less reported in the literature, though a number of recent studies have demonstrated the feasibility (Dibrov et al., 2014; Khaisri et al., 2011; Koonaphapdeelert et al., 2009; Simioni et al., 2011). The major advantage of membrane contactors in solvent regeneration is the ability to remove the CO₂ from the solvent below the vaporization temperature of the loaded solvent. This can be achieved through either the gas phase being under vacuum or applying a sweep gas, generally steam. This enables the CO₂ partial pressure above the solvent to be reduced below the vapour-liquid equilibrium, resulting in CO₂ liberation from the solvent, and hence CO₂ desorption without full vaporization of the solvent. Critically, for membrane contactors undertaking solvent regeneration, a full scale process has not been studied or modelled to date to determine the viability of replacing a traditional desorber column with a membrane contactor. This is addressed in this investigation by modelling a large scale membrane contactor process for solvent regeneration and comparing the relative performance to a traditional solvent desorber column in terms of module length, process volume and therefore process intensification, as well as energy duty.

A number of reliable mass and energy transfer models have been developed for membrane contactors undertaking CO₂ absorption. Here, two models are presented and utilised for CO₂ desorption from a solvent; a simple mass transfer approximation adapted from packed columns and a more detailed one-dimensional model for mass transfer through hollow fibers. The contactors modelled are based on a non-porous composite membranes consisting of Teflon AF1600 on porous polypropylene (PP) and an asymmetric polydimethylsiloxane (PDMS), given that their performance in the literature has previously been reported for solvent regeneration over a range of temperatures (Scholes et al., 2016, 2015b). The loaded solvent undergoing regeneration is 30 wt% monoethanolamine (MEA), given it is the generic standard for CO₂ capture and the properties of the loaded and non-loaded solvent are well known. The evaluated modelled performance of these two contactors will provide insight into the viability of membrane contactors to achieve process intensification for CO₂ desorption.