Treating Coalbed Natural Gas Produced Water for Beneficial Use By MFI Zeolite Membranes

Desalination of brines produced from oil and gas fields is an attractive option for providing potable water in arid regions. Recent field-testing of subsurface sequestration of carbon dioxide for climate management purposes provides new motivation for optimizing efficacy of oilfield brine desalination: as subsurface reservoirs become used for storing CO{sub 2}, the displaced brines must be managed somehow. However, oilfield brine desalination is not economical at this time because of high costs of synthesizing membranes and the need for sophisticated pretreatments to reduce initial high TDS and to prevent serious fouling of membranes. In addition to these barriers, oil/gas field brines typically contain high concentrations of multivalent counter cations (eg. Ca{sup 2 } and SO{sub 4}{sup 2-}) that can reduce efficacy of reverse osmosis (RO). Development of inorganic membranes with typical characteristics of high strength and stability provide a valuable option to clean produced water for beneficial uses. Zeolite membranes have a well-defined subnanometer pore structure and extreme chemical and mechanical stability, thus showing promising applicability in produced water purification. For example, the MFI-type zeolite membranes with uniform pore size of {approx}0.56 nm can separate ions from aqueous solution through a mechanism of size exclusion and electrostatic repulsion (Donnan exclusion). Such a combination allows zeolite membranes to be unique in separation of both organics and electrolytes from aqueous solutions by a reverse osmosis process, which is of great interest for difficult separations, such as oil-containing produced water purification. The objectives of the project 'Treating Coalbed Natural Gas Produced Water for Beneficial Use by MFI Zeolite Membranes'are: (1) to conduct extensive fundamental investigations and understand the mechanism of the RO process on zeolite membranes and factors determining the membrane performance, (2) to improve the membranes and optimize operating conditions to enhance water flux and ion rejection, and (3) to perform long-term RO operation on tubular membranes to study membrane stability and to collect experimental data necessary for reliable evaluations of technical and economic feasibilities. Our completed research has resulted in deep understanding of the ion and organic separation mechanism by zeolite membranes. A two-step hydrothermal crystallization process resulted in a highly efficient membrane with good reproducibility. The zeolite membranes synthesized therein has an overall surface area of {approx}0.3 m{sup 2}. Multichannel vessels were designed and machined for holding the tubular zeolite membrane for water purification. A zeolite membrane RO demonstration with zeolite membranes fabricated on commercial alpha-alumina support was established in the laboratory. Good test results were obtained for both actual produced water samples and simulated samples. An overall 96.9% ion rejection and 2.23 kg/m{sup 2}.h water flux was achieved in the demonstration. In addition, a post-synthesis modification method using Al{sup 3 }-oligomers was developed for repairing the undesirable nano-scale intercrystalline pores. Considerable enhancement in ion rejection was achieved. This new method of zeolite membrane modification is particularly useful for enhancing the efficiency of ion separation from aqueous solutions because the modification does not need high temperature operation and may be carried out online during the RO operation. A long-term separation test for actual CBM produced water has indicated that the zeolite membranes show excellent ion separation and extraordinary stability at high pressure and produced water environment.

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