Adaptable Membrane
Figure 1. Adjusting the organic bridging group allows tailoring of separation performance.
Source: University of Amsterdam.The so-called HybSi membrane was developed by researchers at the University of Amsterdam, the University of Twente and the Energy Research Center of the Netherlands (ECN) (see:Hybrid membrane aims to displace distillation). It overcomes the limitations of polymer membranes (e.g., applicability only below around 100°C, and vulnerability to solvents and acids) and ceramic ones (low hydrothermal and acid stability), say the researchers. The membrane consists of silica atoms linked by organic bridging groups (Figure 1). This gives the material a polymeric character while retaining the ceramic backbone structure, they explain. Adjusting the size, flexibility, shape and electronic structure of these groups enables fine-tuning the membrane for specific separations, such as removal of water or carbon dioxide or alcohols, the researchers report. More details appear in a recent article in Advanced Functional Materials.
The membrane is particularly promising for pervaporation and vapor permeation, says Castricum, but research on gas separation is starting.
The pilot plant initially will use a membrane developed specifically for pervaporation at high temperature of liquids with high water content, he notes. It has short alkane bridges between the silicon atoms. This membrane is the version most ready for commercialization. It has proven its stability at 150°C for 1,000 days in the laboratory for dehydration of butanol. (In these trials, water flux reached 5.5 kg/m2hr, and water content in the permeate exceeded 95 wt %.)
The unit at Plant One contains 24 membrane tubes totaling about 1 m2 of surface area. Fluxes typically should run 5–10 kg/m2hr, although rates as high as 20–30 kg/m2hr are possible. The aim is to dehydrate the feed liquid, an organic waste stream from a plant, to 2% water from 30%. The trial should last 30–60 days.
A consortium that includes Air Products, Sulzer Chemtech and ECN is backing the piloting. If the test goes well, the next step would be supply of a small-scale commercial plant for dehydrating a side stream, removing water to the desired level from a high level in one pass, says Castricum. Depending upon its size, such a plant could be in operation within 18 months, he adds.
"We have identified a number of important applications. The dehydration of organic solvent covers a wide range of applications. We have been looking in[to] removal of water at esterification reactions, NMP [N-methylpyrrolidone] dehydration, and plenty [of] others. In general, any water containing azeotrope would be suitable. The highest advantages can be obtained at levels of 2-15 wt % water," says Castricum.
Producing membranes of a size suitable for industrial use isn't an issue, he notes. "We have estimated that the way we make membrane at this moment can be scaled up to 200 m2/yr with limited risks." In the longer term, an automated production process may allow larger scale production and price reduction, he adds.
Pervatech, Enter, the Netherlands, already has received a license to produce the membrane; agreement with a second licensee is expected this summer.
