Introduction to Pervaporation and Vapour permeation

Pervaporation and vapour permeation have been established over the past two decades as an improved technology for the dehydration of organic solvents, such as alcohols, ethers, esters and ketones, as well as for the removal of organics from aqueous streams or separating various types mixtures of organic compounds. The optimal use can be achieved if this technology gets part of a hybrid system, for example, in combination with distillation and rectification columns.

Introduction
Pervaporation and vapour permeation are membrane-based processes for separating binary or multi-component mixtures of miscellaneous organic fluids. The separation of the mixtures is effected by means of a membrane – the pervaporation membrane. These non-porous (‘dense’) pervaporation membranes made of polymeric or ceramic materials exhibit different permeabilities towards different components, resulting in the desired separation of the components.
In the process, the feed is first heated up to the operating temperature and then brought into contact with the active (feed) side of the pervaporation membrane. The better permeating component preferentially passes through the membrane and is continuously removed in the form of vapour from the back (permeate) side of the membrane. The continuous removal of the vaporous permeate creates a concentration gradient across both sides of the pervaporation membrane. This concentration gradient acts as a driving force for the process.

Basic principles
The concentration gradient is best expressed in terms of partial vapour pressure. A number of different models have been developed to describe the pervaporation process, but for the sake of simplification, the mass transfer across a pervaporation membrane can be divided into three major steps:

  • Sorption of permeating components at the feed side into the membrane.
  • Transport of components across the membrane by diffusion according to Fick´s law.
  • Desorption at the permeate side into vapour phase under vacuum.

Two values characterise a membrane:

  • Its selectivity (also called separation characteristic); and
  • The permeate flux (or mass transfer rate) across the membrane.

Pervaporation process
In the pervaporation process a liquid feed stream is first pre-heated to operating temperature and then routed to a membrane module. The permeate transported through the membrane is vapourised on the permeate side of the membrane and heat is dissipated from the feed. As the partial pressure of the transported component, and with it the driving force for mass transportation, decreases at declining temperature, the feed mixture has to be re-heated. In most cases, re-heating takes place outside the modules in separate heat exchangers. Therefore for larger plants and high permeate rates, it may be necessary to provide for a very large number of small membrane modules with upstream heat exchangers. The vaporous permeate leaving the membrane module is condensed in an external heat exchanger. The vacuum pump is only used for the removal of the inert gases, but has no other function in the process.

Vapour permeation process
In the vapour permeation process, saturated vapour instead of the liquid feed solution is passed through the module. This process – similar to gas separation – exhibits some other advantages over pervaporation. The series arrangement of modules and heat exchangers can be dispensed with because the necessary evaporation energy is supplied from outside the modules in a separate evaporator. Because of the more favourable fluid dynamics, overall larger modules may be used with an associated benefit of cost reduction. Moreover, vapour permeation is advantageous if the feed mixture contains non-volatile or undissolved constituents and any of its constituents that tend to precipitate out can be separated as bottom product in the evaporator.

Main advantages
The main advantages of a pervaporation or vapour permeation process may be summarised as follows:

  • Since only the properties of the membrane determine the distribution of a component in the permeate phase, mixtures which at normal distillation form azeotropes and/or require a large number of theoretical stages (like the dehydration of acetone), can easily and economically be separated even without the use of entrainers. Therefore, high product purity is obtained (no entrainer required) and no environmental pollution occurs (no entrainer emitted).
  • Multi component mixtures even with just small differences in boiling points can be dehydrated effectively and economically.
  • The feed mixtures to be treated may be supplied in either liquid (→ pervaporation) or vapour (→ vapour permeation) form.
  • Low energy consumption for pervaporation and vapour permeation processes.
  • Significantly reduced energy consumption for hybrid systems (pervaporation and vapour permeation in combination with rectification/distillation).
  • Due to the modular design of the membrane system even small units can operate economically.
  • High degrees of flexibility regarding the feed mixtures that may be accommodated (multi-purpose systems, various feed mixtures can be treated in one unit), throughputs, and final product qualities.
  • Modularly, compactly designed, and factory-preassembled systems simplify their adaptation to suit the desired performance parameters and shorten the time required for system installation and start-up.
  • Pervaporation and vapour permeation systems are simple to operate and can be started up and shut down rapidly.

Feed materials
The following summary presents an overview of some important substances that can be treated by pervaporation or vapour permeation:

  • Alcohols such as methanol, ethanol, propanols, butanols and higher linear alcohols, as well as higher alcohols such as glycol, glycerin and glycol ether on consultation.
  • Ketones such as acetone, methyl ethyl ketone (MEK) and methyl isobutyl ketone (MIBK).
  • Ethers such as diethyl ether, diisopropyl ether, tetrahydrofurane (THF) and dioxan.
  • Esters such as ethyl acetate, butyl acetate and isopropyl propionate.
  • Hydrocarbons such as benzene, toluene and xylene, (in most cases in mixtures with other solvents), as well as chlorinated hydrocarbons such as trichlorethylene.
  • Organic acids such as acetic acid, propionic acid and aqueous solvents higher acids.
  • Amines (on consultation) such as methylamine, ethylamine and pyridine.

Aprotic solvents like DMF, DMSO or NMP will attack polymeric pervaporation membranes, but can be dehydrated by means of ceramic pervaporation membranes.

Major applications
Some typical applications for pervaporation and vapour permeation include:

  • Removal of water from organics.
  • Removal of organics from water.
  • Separation of organic mixtures.
  • Concentration of aqueous solutions.

Examples of water and methanol azeotropes that can be separated with the Pervatech membranes 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 our membranes 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

Here you will find an overview of the applications that we have worked on.

Application overview