ZIF‐8 Membrane Separation Performance Tuning by Vapor Phase Ligand Treatment
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Abstract Vapor phase ligand treatment (VPLT) of 2‐aminobenzimidazole (2abIm) for 2‐methylimidazole (2mIm) in ZIF‐8 membranes prepared by two different methods (LIPS: ligand induced permselectivation and RTD: rapid thermal deposition) results in a notable shift of the molecular level cut‐off to smaller molecules establishing selectivity improvements from ca. 1.8 to 5 for O 2 /N 2 ; 2.2 to 32 for CO 2 /CH 4 ; 2.4 to 24 for CO 2 /N 2 ; 4.8 to 140 for H 2 /CH 4 and 5.2 to 126 for H 2 /N 2 . Stable (based on a one‐week test) oxygen‐selective air separation performance at ambient temperature, 7 bar(a) feed, and 1 bar(a) sweep‐free permeate with a mixture separation factor of 4.5 and oxygen flux of 2.6×10 −3 mol m −2 s −1 is established. LIPS and RTD membranes exhibit fast and gradual evolution upon a 2abIm‐VPLT, respectively, reflecting differences in their thickness and microstructure. Functional reversibility is demonstrated by showing that the original permeation properties of the VPLT‐LIPS membranes can be recovered upon 2mIm‐VPLT.Keywords:
Air separation
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The separation of gas mixture is a major operation in the petrol-chemical industry. The
separation of oxygen/nitrogen is one of the main applications. The most common
applied commercial gas separation processes are cryogenic distillation, adsorption and
classical membrane permeation. Depending on the specific requirements for the
process, i.e., process scale and product purity, one or more of the available gas
separation techniques will be economically preferable. Under economic considerations
membrane systems have been proven to be loss energy intensive and thus more cost
effective to operate than the traditional techniques mentioned. Classical polymeric
membrane materials used so far possessing high selectivities for specific gas pairs show
generally low permeabilities, which is referred to as an “upper bound” relationship.
Facilitated transport of a specific gas molecule can therefore be suggested as one of the
promising methods to improve single bulk material (polymer) properties.
The main goal of this project was to synthesise a new class of gas separation
membranes based on carrier facilitated transport for oxygen/nitrogen separation.
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Air separation
Membrane Technology
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Industrial gas
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The purification of different components of air, such as oxygen, nitrogen, and argon, is an important industrial process. Pressure swing adsorption (PSA) is surpassing the traditional cryogenic distillation for many air separation applications, because of its lower energy consumption. Unfortunately, the oxygen product purity in an industrial PSA process is typically limited to 95% due to the presence of argon which always shows the same adsorption equilibrium properties as oxygen on most molecular sieves. Recent work investigating the adsorption of nitrogen, oxygen and argon on the surface of silver‐exchanged Engelhard Titanosilicate‐10 (ETS‐10), indicates that this molecular sieve is promising as an adsorbent capable of producing high‐purity oxygen. High‐purity oxygen (99.7+%) was generated using a bed of Ag‐ETS‐10 granules to separate air (78% N 2 , 21% O 2 , 1% Ar) at 25°C and 100 kPa, with an O 2 recovery rate greater than 30%. © 2012 American Institute of Chemical Engineers AIChE J, 59: 982–987, 2013
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A novel unsteady-state membrane permeation process was proposed based on steady-state permeation.The process comprises three basic cyclic steps: feed pressurized,permeate withdrawn and residue vented. The effects of time of feed pressurizing,permeate sucking and residue venting on the average purity,flow rate and recovery of oxygen were studied experimentally for air separation to produce oxygen enriched air by this process.The effectiveness of the process was compared with that of steady-state permeation.The results show that the proposed process in this paper can achieve significantly higher oxygen recovery and flow rate than those of the conventional steady-state permeation when the oxygen purities of both are the same.
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The separation of gases is a commercial process conducted primarily via cryogenic distillation. An alternative method involves the use of solvent cast polymer membranes. Unlike distillation, membrane processes are energy efficient, easy to scale-up, and require only electrical energy in their operation. Current membrane separation applications include oxygen or nitrogen enrichment of air, the separation of carbon dioxide from natural gases, and the recovery of hydrogen from refinery and purge streams. In our laboratory, gas separation membranes are being developed based on conducting, soluble and processable polymers such as poly(3-n-alkylthiophene)s. The chemistry of these membranes is being altered by changing the R group (e.g. octyl, dodecyl), the oxidation state, and by incorporating zeolites and molecular sieves to facilitate gas transport. An important aspect of this project concerns establishing the relationship(s) between the structure of poly(3-alkylthiophene) membranes and their bulk properties, specifically permeabilities and selectivities for various gases. It is anticipated that this understanding will help to elucidate the mechanism by which gas separation occurs in these membranes.
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This is the first in a series of papers presenting new concepts for the development of membranes for gas separation. In this paper two new cost parameters, which are useful for costing and optimization of membrane gas separation systems, are described. The new parameters, cost permeability and effective selectivity, can be used to show the direction to be taken in membrane research and development. The new parameters are shown to predict accurately the cost of membrane separation plant by correlating bids from membrane plant suppliers using the new parameters with cross-flow design equations. The parameters are used to optimize the membrane gas separation of hydrogen and carbon monoxide for two commercially available membrane systems in a process to manufacture acetic acid. The membrane separation is compared with the currently used method, cryogenic flash distillation. Economic evaluation methods are developed to compare different separation methods so that the process as a whole can be optimized. The evaluation shows that, for membrane gas separation, it is important to find the optimum degree of separation; when membrane separation is evaluated at the separation specification for the established cryogenic method, membranes are not competitive; however, when the process is optimized for membrane separation, the cost of separation reduces to less than 60% of the cryogenic separation.
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For gas separation, commonplace processes have been controlled-pressure distillation or separations using a solvent. Now, other separation techniques are available. These include: membrane separation, cryogenic separation and unconventional separations, such as dephlegmation and controlled freezing. This article reviews these later processes and discusses plant operations using solvents for acid gas removal. Membrane gas separation is an emerging technology for industrial gas separation. Membrane separation systems have gained worldwide acceptance in areas of gas processing such as helium recovery, hydrogen recovery and purification, acid gas removal, inert gas generation, air separation, and enhanced oil recovery. Membrane gas separation uses asymmetric semi-permeable polymeric material for separating different constituents of a feed gaseous stream. The major industrial membrane separators can be classified into the spiral wrap type and the hollow fiber type. This article depicts these two types of industrial membrane separators.
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Gas separation has been an important process in chemical industries. Polymer membranes have been widely developed for gas separation due to the main advantages such as light weight, low cost, and easy preparation procedure. However, a trade-off between permeability and selectivity of the polymer membrane becomes a significant hurdle for its application in gas separation. Moreover, in the case of air separation where the oxygen and nitrogen molecules have a very close diameter, a high selectivity is more difficult to be achieved. Therefore, numerous researches were directed to improve the performance of the polymer membrane. Surface modification is an attractive way to enhance the selectivity while maintaining the high permeability of the base membrane. This paper provides a review of surface modification of polymer membrane which aims to enhance the air separation performance. The discussion includes surface engineering strategies of polymer membrane and their performances in air separation. In the end, the conclusions and future directions are pointed out.
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