Technology

What is pervaporation?

Pervaporation is the selective evaporation of one of the components of a liquid mixture using a membrane. The word pervaporation itself is a combination of the words permeation, the transport through a membrane, and evaporation, the phase transition from the liquid to the vapor phase. It is not a pressure driven process such as several filtration processes and reverse osmosis. Instead, the driving force is due to the fact that on the feed side the chemical potential is higher than on the permeate side, similar to what is found in gas separation membranes. The gradient in chemical potential is maximized by using high feed temperatures and low pressures on the permeate side. Alternatively to pervaporation, membranes can often be used in vapor permeation mode as well. In this case the feed is fully vaporized. Often a detailed analysis of the full process is required to determine which option offers the highest benefits to the end user. These kinds of membrane processes are beneficial for the separation of e.g. azeotropes. In distillation the complete liquid feed is evaporated leading to high energy demands. By replacing distillation by the pervaporation membranes or combining the two processes large energy savings are possible.

Membrane stability

A fair number of membranes are available on the market today. These polymeric and ceramic membranes all have their advantages and disadvantages, and a careful selection has to be made based on detailed knowledge of the process. The HybSi® team has been working over the years to overcome common disadvantages and offer membranes for a wider application window. This has been achieved by enhancing the stability towards

  • hydrothermal attack,
  • the presence of acids,
  • the presence of aggressive organic solvents

All of this results in a membrane system that is applicable in a wide range of solvents and is stable in the presence of water and acid at high temperatures. 

Details of the HybSi® material

HybSi® is an organic-inorganic hybrid silica-based amorphous material. The hybrid nature of this material lies in the fact that each silicon atom is not only connected to oxygen atoms as in pure silica, but also to an organic fragment. The special feature of HybSi® is that the organic fragments are acting as integral bridging fragments of the structure, and not just as end standing groups as in the methylated silica developed by De Vos. The result is a true hybrid silica pore network in which organic and inorganic fragments cooperate. It is prepared by a sol-gel process from so-called bis-silyl precursors, such as BTESE ((EtO)3Si–CH2CH2–Si(OEt)3) and BTESM ((EtO)3Si–CH2–Si(OEt)3).


BTESE BTESM

Cross-sectional SEM micrograph of the layered structure of a hybrid membrane, showing the supporting layers and the ~150 nm thick selective hybrid silica top layer.

Explanations

Several explanations have been proposed for the remarkable stability of the HybSi®. These include

  • More stable bonds: the hydrothermally instable Si-O-Si bonds are replaced by Si-(CH2)n-Si bonds
  • Higher crack propagation energy resulting in more ductile material in which initiated nano-cracks do not easily grow to a defect.
  • Increased connectivity number of the basic building block from four to six siloxane bridges, resulting in a lower surface diffusion coefficient
  • Lower solubility, the larger bis-silyl units are expected to have a much lower solubility leading to suppressed mobility of silica fragments.

Specific applications

Examples of water and methanol azeotropes that can be separated with the HybSi® membrane system include:

Permeating species

Retained species

Azeotrope (wt% of retained species)

Water

Acetonitrile

83.7

Water

Ethanol

95.5

Water

n-Propanol

71.7

Water

t-Butanol

88.3

Water

Ethylene chloride

91.8

Water

Methyl acetate

95.0

Water

Methyl ethylketone

89.0

Water

Tetrahydrofuran

95.0

Methanol

Toluene

31.0

Methanol

Methyl acetate

81.3

Methanol

Tetrahydrofuran

69.0

 

Other dewatering examples for which HybSi® can be used include complex distillations and processes like:

  • Acetone/phenol in e.g. the oxidation of. cumene
  • Acrylates
  • Bisphenol A
  • Carbonates
  • Diols
  • EDC/VCM/PVC
  • Isocyanates
  • Propylene oxide
  • Terephthalate compounds and terephthalic acid