Gas Separation by Membrane Operations
The separation of mixtures of gases and vapors is required in manufacturing processes across various industries. In the last years, membrane systems are gaining a larger acceptance in industry for gas separation and are recognized as a cost-efficient separation able to compete with consolidated processes such as pressure swing absorption and cryogenic distillation (Bernardo et al. 2009; Sanders et al. 2013). Membrane processes have several advantages over conventional separation techniques (e.g., distillation, extraction, absorption, and adsorption), including modularity and compactness, operational flexibility, and no need for energy-intensive phase changes or potentially expensive adsorbents and/or difficult to handle solvents. The features of membrane operations allow implementing the process intensification strategy in different production cycles. Their versatility represents a decisive factor to impose membrane processes in most gas separation fields.
The first membrane units were installed in ammonia plants for the separation of hydrogen from nitrogen more than 30 years ago. Today, the production of nitrogen from air is the largest application of membrane systems, owing to the demand for nitrogen to inert fuel tanks, also aboard aircrafts, and for blanketing chemical and liquefied gas shipments. Membrane systems are also applied to enrich oxygen for medical uses, for hydrogen recovery and purification in refineries, for air and gas dehydration, and for ratio adjustment of gas mixtures. Natural gas processing represents an important emerging application field (Baker and Lokhandwala 2008). The relatively low volume flow and the relatively high inlet carbon dioxide content are strong drivers for the implementation of the membrane technology in the biogas upgrading that it is at a developing stage (Makaruk et al. 2010). The challenging olefin/paraffin separation, not yet commercial, is attracting a lot of interest from the scientific community (Rungta et al. 2013).
Membrane separation allows recovering and recycling valuable compounds, such as hydrogen and light hydrocarbons (ethylene, propylene, and LPG), present in different off-gas streams (Baker et al. 1998). Polymeric membranes, cheap and with an easy processability, are typically used in the commercially available membrane system for gas separation (Yampolskii 2012). Commercial modules employ composite membranes (Pinnau et al. 1988), mainly in the form of compact hollow fibers. These membranes typically operate the separation based on a solution-diffusion transport mechanism: sorption of the permeant into the membrane, permeation by diffusion through the membrane, and desorption at the low-pressure side of the membrane.
The experimentally observed upper bound, based on various polymeric membranes, was reported by Robeson in 1991 and then updated in 2008 (Robeson 1991, 2008), thanks to the efforts to improve the gas separation performance of ultrahigh free volume and perfluoropolymers.
Glassy polymers are chosen for their size-selective behavior (e.g., in O2/N2 or H2 separations). However, when applied to mixtures and/or at high gas activities, these materials are prone to plasticization, which causes swelling of the polymer matrix and results in a higher permeability coupled with a loss of selectivity. Strategies to overcome plasticization include thermal curing and chemical cross-linking, which reduce the polymer free volume (Wind et al. 2002). The addition of nanofillers to a polymer matrix represents an interesting solution to overcome the trade-off of the polymeric membranes and the inherent brittleness issues of inorganic membranes (Goh et al. 2011).
Rubbery polymers, instead, present a solubility-controlled permeation and preferentially allow the permeation of large gas or vapor molecules in a gaseous mixture containing also smaller molecules (Grinevich et al. 2011). Their permeability, much higher than in conventional glassy polymers, increases with the critical volume of the penetrant (Matteucci et al. 2006). These materials are applied to the separation of organic vapors from non-condensable gases, treating petrochemical vent and process streams to recover valuable feedstocks (Baker 1999).
An interesting new concept is the use of water-swollen thin film composite membranes for biogas purification, taking advantage of the large difference in solubility in water to become selective for CO2 (Kárászová et al. 2012).
Facilitated transport membranes contain carrier agents that can react reversibly with the target gas component. Therefore, the reaction in the membrane creates another transport mechanism, in addition to the simple solution–diffusion mechanism (Huang et al. 2008). However, carrier poisoning and short life span of the polymeric membranes are typically reported (Rungta et al. 2013). Ionic liquids were considered as additives for facilitated transport membranes. Indeed, their negligible vapor pressure avoids solvent losses by evaporation, providing stability to the metallic cation dissolved inside, and acting as a medium for facilitated transport with mobile carrier (Fallanza et al. 2013). Ionic liquid gel membranes based on conventional polymers (Jansen et al. 2011) or on polymer ionic liquids (Bara et al. 2008) were proposed to increase the stability compared to supported liquid membranes.
The key for new applications of membranes in challenging and harsh environments (e.g., petrochemistry) is the development of new tough, high-performance materials. In the field of inorganic membranes, metal organic frameworks were recently considered for preparing membranes to be applied to the olefin/paraffin separation (Bux et al. 2011) or as additive to a polymer matrix (Bushell et al. 2013).
High free volume polymers have been investigated as gas separation membranes, combining their ease of processing and mechanical stability with the potential to surpass the polymeric upper bound for different gas pairs (Budd and McKeown 2010). Novel PIMs, characterized by a significant shape persistence, were developed, showing interesting performance for the O2/N2 separation (Carta et al. 2013).
Properly designed hybrid processes, combining a membrane system with a conventional one (e.g., PSA or absorption), represent technically and economically viable solutions, able to reduce energy consumption and total costs (Esteves and Mota 2007).
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