Pervaporation using ceramic HybSi® membranes can deliver 30% to 50% energy savings relative to conventional distillation.
It is particularly striking that this ceramic membrane can be used up to a temperature of 150°C, while the maximum temperature for polymeric membranes is around 100°C. As a result, ceramic membranes enable a fourfold increase in output, thereby cutting the cost of materials up to 75%. That means the proper application and use of ceramic HybSi membranes for pervaporation offers significant advantages, as demonstrated by a recently completed ISPT/NL GUTS techno project.
Pervaporation is frequently used to separate water from organic liquids. Pervatech, an SME in Enter, the Netherlands, develops tubular pervaporation membranes that transport vapour from inside to outside. The company previously developed two generations of silica membranes on its own, but these did not differ sufficiently from existing technologies. Reason enough for the company to create a third generation based on its license for the HybSi membrane it developed in partnership with ECN and the universities of Twente and Amsterdam.
The goal of the ISPT/NL GUTS techno project was to explore whether Pervatech’s HybSi membrane can operate in a number of industrial processes at ISPT partners Huntsman and DSM. The two chemical companies participated in the study as potential future users of the technology.
At Huntsman, the membrane was tested on the separation of a water-methanol-ammonia flow into its constituent fractions. The pervaporation process ran smoothly, but it did not succeed in separating the three fractions. Methanol and ammonia largely flowed through the membrane along with the water. A different procedure could enable ammonia separation technologically, but is not feasible in operational terms.
AT DSM, the ceramic membrane was tested on the separation of water from a condensation reaction mixture. The preliminary conclusion is that pervaporation using these ceramic membranes has significant advantages compared with polymeric membranes. DSM is looking into opportunities to conduct pilot testing later this year.
Was this techno project really necessary? After all, the HybSi membrane had already operated at 150°C without loss of performance in a previous three-year test. A shorter test of several months at 190°C also completed without problems.
"In feeling out the market, we had run into the issue that though the membrane had already been tested, it hadn’t yet been tested on industrial liquids," said Frans Velterop, Pervatech director. "That would have been a significant stumbling block for its market introduction."
A textbook example
The Dutch company believes that a completely different issue is also at play.
"Discarding false modesty, I can say that this membrane technology is ahead of the rest of the world. But Pervatech is a small company. Membranes are expected to form a significant percentage of a chemical plant’s core in the next twenty to thirty years.
"You can imagine that investors planning to build a new chemical plant on the order of several billion Euros feel uneasy about an SME providing the key component. It’s in line with ISPT’s mission to lower or completely break down barriers like these.
"The ISPT organisation has created a number of instruments that enable SMEs to demonstrate the advantages of their technology to large companies, and to show how they can minimise possible risks. This techno project is a textbook example of that."
Pervatech produces four different ceramic pervaporation membranes: silica, modified silica, titania and HybSi. The advantages of HybSi are thermal stability (>150°C; tested 190°C), chemical stability (NMP, MEK, 2 < pH < 8), large application window (up to 30% water, removal of methanol and NH3), and resistance against condensation.
The membranes consist of a single-tube ceramic carrier with the separating membrane layer on the inside, so the permeate (vapour) flows from inside to outside. The liquid, from which a volatile component needs to be removed, flows on the inside of the tube under atmospheric or low pressure. A vacuum is maintained on the outside of the tube.
The difference in partial chemical potential of the permeating component in the feed compared to the permeate chamber is the driving force for the volatile component to evaporate through the membrane.
The membrane tubes are mounted in a stainless steel vessel to make a module to collect the permeated vapour and route this to a condenser and a vacuum pump. The condensed vapour is recovered as liquid. The original feed passes through several tubes/modules in series and is then routed for further processing.
Contrary to distillation, the only heat required in pervaporation is to evaporate the component to be removed, which leads to considerable energy savings. The membrane may also act as a barrier, which results in a purer condensed vapour.
The membranes and modules are also suitable for separating the same components in the vapour/gas phase, which is called gas separation or vapour permeation. Applications include breaking azeotropes, dewatering (bio)ethanol and other liquids, recovering solvents (ethyl acetate, NMP, phenol, THF, ACN), recovering aprotic solvents (DMAc, DMSO, DMF), methanol removal, and in situ dehydration of condensation reactions.