CO₂ separation from humidified ternary gas mixtures using a polydecylmethylsiloxane composite membrane

Highlights

• Polydecylmethylsiloxane composite membrane as a pre-concentrator

• Permeability close to 1000 barrer and moderate CO₂/N₂ selectivity of 8.7

• Membrane behaviour independent of the feed pressure and composition

• Separation properties unvaried in presence of water vapour

• Performance maps as tool for separation performance prediction

Abstract

In this work, we investigated the use of a polydecylmethylsiloxane composite membrane as pre-concentrator unit for the separation of CO₂ from flue gas streams. The separation properties of the tailor-made membrane were evaluated with both a ternary gas mixture (CO₂:N₂:O₂ = 15:80:5) and pure gases (CO₂, N₂, O₂) at different temperatures (25, 50 °C) and trans-membrane pressure differences (1–6 bar). The membrane exhibited a high permeability close to 1000 barrer, and a moderate CO₂/N₂ selectivity of 8.7. The results obtained in mixed gas conditions revealed a certain reduction of CO₂ and O₂ permeability with respect to pure gas, whereas the presence of water vapour did not induce significant changes to mass transport properties.

Performance maps were thus developed for better understanding whether and in which conditions the membrane developed in this work can be suitable to be used as pre-concentrator.

Introduction

Carbon capture and storage is an important strategy that many Countries are pursuing to tackle climate change. The last reports of the International Energy Agency [1] clearly evidences that in 2018, the global CO₂ emissions of the energy sector rose up to 33.1 Gt CO₂. This was the highest rate of growth since 2013, 70% higher than the average increase since 2010. However, for the first time, in almost a decade, the number of plans to develop large-scale carbon capture, utilization and storage facilities increased significantly, with around 43 projects operating or under construction. In the last 30 years, membrane technologies have done impressive progresses, both in materials and processes development for separating gases, treating water, etc. Today, CO₂ separation by using membranes is considered an energy efficient separation process because of its low capital cost [[2], [3], [4]]. Thanks to the limited cost, easy fabrication, and straightforward scale-up, polymer membranes have been confirmed as a good alternative to traditional gas separation processes in large-scale industrial applications. The development of polymer membranes for CO₂ separation before emitting in the atmosphere has recently become significant and even if a plethora of new membrane materials has been studied at the bench scale, still today more than 90% of current commercial membranes are manufactured from less than ten polymers, most of which have been in use for decades [5]. A desired aspect for the application of polymeric membranes in CO₂ separation, is that they possess high CO₂ permeability and high CO₂ selectivity, as well as robust mechanical stability. For this purpose, several efforts have focused on identifying such membranes and improving gas separation properties and stability by controlling membrane preparation. Generally, there is a debate among the scientific community about the fact that it could be more convenient to have a membrane with high permeance and low selectivity, or vice versa [6]. A high CO₂ permeance minimizes membrane area requirements, whilst high CO₂/i selectivity improves the CO₂ permeate concentration. The benefits of higher selectivity are accentuated at higher feed-to-permeate pressure ratios, at the expense of increased energy cost, whereas the advantages of higher permeance are most pronounced at lower pressure ratios. Usually, for gaseous streams such as flue-gas, where the CO₂ concentration is relatively low (<15%), it has been demonstrated [2] how it is more convenient to have a first membrane stage highly permeable operating as pre-concentrator for retentate and permeate streams, which will be then purified in the successive stages, where much more selective membranes can be used. This approach has been recently proved by the pilot tests of the MTR's full-scale Polaris™ modules for the capture of 1 ton day⁻¹ of CO₂ from the flue gas of the coal-fired power plant at the National Carbon Capture Center in Wilsonville (Alabama, USA) [7]. As already done by MTR with the membranes used in Polaris™ modules, a strategy for having highly permeable membranes, that in the meantime are mechanically stable is to produce polymer thin-film composite (TFC) membranes comprising a thin selective layer deposited on a support. The development of TFC membranes has gained much attention owing to their potentiality of increasing flux and hence the energy efficiency for gas separation [[8], [9], [10], [11], [12], [13]]. The highly permeable PDMS was often used as intermediate acting as a protective coating which prevents the penetration of diluted polymer solution into the porous structure and renders the entire membrane surface smoother [14,15]. Fu et al. [16] developed a class of TFC membranes, based on a high molecular weight amorphous poly(ethylene oxide)/poly(ether-block-amide) selective layer supported on highly permeable polydimethylsiloxane intermediate layer which was pre-coated onto a polyacrylonitrile microporous support, obtaining outstanding CO₂ separation performance. Borisov et al. [17] prepared TFC membranes with high CO₂ permeance and superior CO₂/N₂ selectivity coupling a thin layer (0.29–0.42 μm) of PIM-1 with a cross-linked PTMSP intermediate layer (2.07–3.44 μm) on a porous backing material.

However, the selective layer of such membranes is quite often based on the glassy polymers that exhibit the reduction of their free volume fraction owing to polymer chains relaxation and internal fouling by different components from the feed stream during the long-term operation resulting in the drop of gas transport characteristics. Bearing this in mind, the rubber polymers can be considered as more robust membrane materials especially for the treatment of flue gas stream.

Usually, TFC membranes are prepared using ultrafiltration porous supports. A thin (<300 nm) polydimethylsiloxane (PDMS) film is generally used as an intermediate layer between the support and the selective layer. This PDMS layer, also called gutter layer, is highly permeable to gases and it acts as a protective coating, for avoiding the penetration of polymer solution into the porous structure and makes smoother the entire membrane surface. Recently, Bazhenov et al. [18] developed a preparation procedure that allows to use ultra-permeable microfiltration supports, for TFC membrane. This allows to significantly reduce the high resistance of the porous support to the gas transport.

Earlier, we proposed a simplified one-stage synthesis of polyalkylmethylsiloxanes with the increased hydrocarbon ideal separation selectivity [[19], [20], [21], [22]]. It was allowed to use bulk chemicals for such polymer modifications: 1-decen as a modifying component and 1,7-oc-tadiene as a cross-linking agent. In this work we developed a TFC membrane constituted of a separating layer of polydecylmethylsiloxane deposited on a microfiltration membrane (MFFK-1) used as support. Membranes coated on the MFFK-1 have a higher gas permeability and separation selectivity compared to membranes on an ultrafiltration support [22]. Thus, these membranes can be promising for the removal of CO₂ from flue gas. It is well known that flue gas is a mixture of CO₂, N₂ and O₂ in saturated conditions, also containing contaminants such as CO, NO_x, SO_x. In this work, we analysed the performance of these membranes for separating CO₂ from a ternary gas mixture (CO₂, N₂, O₂) in dry and wet conditions, devoting particular attention on the influence of water vapour on the mass transport properties and separation performance, but without considering contaminants.

Experimental results were used as input data of a simulation tool that we developed some year ago [[23], [24], [25]], able to generate performance maps to better understand whether and how the membrane developed in this work can be suitable to be used as pre-concentrator in a multistage gas separation membrane system.