Solvent recovery is very imperative in pharmaceutical and chemical synthesis, fine chemistry, food additive, and electrical manufacturing industries. There are many different processes used for solvent recovery.
Pervaporation (Here we abbreviate it as PV) is a new cutting-edge green technology that has been applied successfully. In this article, we will clearly show pervaporation membrane separation and its practical process performance with zeolite membrane.
Pervaporation is derived from two energy-efficient processes: permeation and evaporation. It achieves continuous separation of the permeable component on the properties of different dissolution and diffusion rates in the membrane module.
The process is especially suitable for the separation of near-boiling point, constant-boiling point mixtures, isomeric substrates, and compounds with poor thermal stability that are difficult or impossible to separate by the ordinary process of distillation, extraction, or adsorption.
It has obvious economic and technical advantages for the removal of trace water from organic solvents and mixed solvents, and for the separation of small amounts of organic pollutants in wastewater.
Compared with the traditional solvent recovery process, PV has outstanding features of high efficiency, low energy consumption, high recovery, convenient operation, safety, environment friendly, etc.
Keep on reading this article for more valuable information on PV separation. We have outlined its process flow, work principle, purpose, membranes material used, types of pervaporation, industrial application with zeolite membrane, and the approach to further improve PV performance.
Process Steps of Pervaporation
The separation mechanism of PV is mainly in a difference of dissolution-diffusion properties through the inorganic or organic material membrane.
Briefly speaking, it involves three process steps. Permeate sorption at the solution feed/membrane interface, diffusion across the membrane due to a chemical potential gradient (also called a rate-determining step), and vapor phase desorption at the permeate side of the membrane.
The permeate selectivity is mostly determined by the first two processes, and the membrane temperature and pressure gradient have a big impact on how well the separation works.
The amount of energy that must be supplied is at least as much as the permeate’s heat of vaporization. The permeate’s pressure needs to be kept below the process temperature’s saturation vapor pressure to ensure that the permeate boils at the vacuum side of the membrane.
The separation is determined by the physical-chemical interactions between the membrane phase and the permeating molecules, with no regard for vapor/liquid equilibrium. The chemical potential or partial pressure gradient across the membrane of the volatile organic compound (VOC) is the driving force for the mass transfer of permeants from the feed to the permeate side of the membrane, not the volatility of the VOC.
In addition to the above, we also need to consider both the azeotropic composition and the volume of the feed liquid when designing an appropriate PV process. If the azeotropic material contains very little water, the PV process can be connected to the outflow of the recovery distillation tower. If the content of water is similar to inorganic materials in the azeotropic material, the PV process unit shall be placed between the two recovery distillation towers, and its key role is to decompose an azeotropic material into two non-azeotropic solutions, and then sent to the distillation tower for fractionation and obtain high purity material.
How Does Pervaporation Membrane System Work?
The organic mixture to be separated is prefiltered and heated to a certain temperature and vaporized, the vapor phase flows into the membrane component on one side of the membrane at atmospheric pressure, and the permeable components are selectively dissolved and diffuse through the membrane under the drive of the differential vapor pressure (or chemical gradient) on both sides of the membrane.
Since the chemical level on the raw stream side is high and low on the downstream, so the components will continuously permeate through the membrane layer to the downstream. Meanwhile, permeate vapor will be quickly removed by a vacuum pump or inert gas purging so that the permeation process will continue. We will get outlet products of high content and purity from the membrane module residue.
Let’s take a look at the molecular sieve inorganic membrane PV process as an example, it includes operating units such as the raw material evaporator, preheater, gas-liquid separator, membrane module (including make-up heat), vacuum condensation (extraction and filtration) system, product condenser.
The downstream side of the membrane uses vacuum extraction plus condensation to recover the condensed permeate (mainly water), and the finished product is gradually dehydrated through the membrane module in series to get the required anhydrous organic solvent.
PV membrane separation system with zeolites membrane has outstanding features. They are strong hydrophilic membranes. Its separation factor is ≥10000, flux rate of ≥10000 g/(㎡·h), and purity of final product solution > 99.99%. They also have high-temperature resistance up to 150℃, and a service life that last for 4 – 6 years.
What Is the Purpose of Pervaporation?
The PV process is used to filter out volatile substances from solutions using a selective membrane. Volatile components in a liquid flow diffuse across a dense membrane by establishing a vacuum or adding a flow of purge gas on one side of the membrane.
In most cases, the PV process is used to dehydrate organic solutions and remove organic impurities from aqueous solutions by utilizing highly selective membranes. In addition to having high energy efficiency, PV separation has no azeotropic restrictions thanks to the use of dense inorganic membranes.
Furthermore, the PV process as an independent modular component can be easily integrated and coupled with other processes for alignment and to intensify the complete process performance.
Which Membrane Material Shall be Used?
For a given mixture, the permeate rate depends largely on the nature of the membrane. Appropriate materials and fabrication processes can produce membranes with high permeation rates for specific one component and relatively low or even near zero permeation rates for the other component so that the PV process can separate liquid mixtures with high efficiency.
According to the material fabricated, the membrane can be divided into categories of inorganic zeolite membrane, polymer composite membrane, and organic/inorganic composite membrane.
An ideal high performance PV membrane should be high throughput, efficient separation, and have long term of chemical and mechanical stability.
The inorganic ceramic membrane has a prosperous prospect in the pharmaceutical, chemical, food, fermentation, and lipid industry because of its stable character with thermal, chemical, solvent, and microbiological resistance and long span life compared with organic ones.
Ceramic membranes are asymmetric membranes which are made of metal oxide materials such as alumina, zirconium oxide, titanium oxide and silicon oxide, it is sintered by a coating process. Ceramic membranes with small pore size and narrow pore size distribution have the advantages of high separation accuracy and high efficiency. It can resist acid and alkali, organic solvent, high temperature, biological pollution, and high mechanical strength.
We use NaA-type zeolite as the best inorganic membrane layer material and the core separation membrane layer, a homogeneous and dense molecular sieve layer that is synthesized on porous ceramic as a carrier. Using its regular 8-ring pore channel structure to achieve the separation of molecular size between different components.
The molecular sieve crystals grow regularly on tubular ceramic support bodies to form a layer of closely packed membrane layer, the pore diameter is about 0.41nm, which is larger than the kinetic diameter of water molecules of 0.29nm and smaller than most of the organic solvents’ kinetic diameter. The silica-aluminum content (Si/Al=1) in the molecular sieve skeleton makes it extremely hydrophilic, which makes the zeolite pervaporation inorganic membrane especially suitable for organic solvent dehydration.
However, the polymer composite membrane is gradually fading out due to service life limit and dehydration only on ethanol solution.
Organic-inorganic composite membrane owns the merits of both organic membranes and inorganic membranes, so this membrane material process is very attractive and popular.
Types of Pervaporation
PV membrane can be divided into: water preferential permeation membrane, organic liquid preferential permeation membrane, organic liquid/organic liquid selective separation membrane according to the composition of the separation stream to the process.
Hydrophilic Pervaporation
Hydrophilic pervaporation, in particular, necessitates the use of hydrophilic membrane materials to promote water molecule dispersion and penetration across the membrane. This membrane can be exceedingly selective, owing to the fact that the materials used in these membranes have sorption and diffusion selectivities that are significantly more than unity.
They are therefore frequently used when water is a small component in the dehydration of organic solvents. Since water is a by-product of a reversible reaction or synthesis reaction, water removal is crucial to make sure process management and reaction efficiency. In this instance, the constant removal of water during its reaction process has the effect of allowing for a final conversion that can be close to unity while avoiding the thermodynamic restrictions of the equilibrium reaction.
Hydrophobic Pervaporation
The hydrophobic moiety of the membranes aids in the separation of non-polar organic molecules from water in hydrophobic pervaporation. Even though it is technically on a scale with dehydration, it performs the selective removal or recovery of organic molecules from an aqueous solution through the PV process. It is technically possible but not economically practical to remove organics from wastewater for instance, the VOC portion, including the application of hydrophobic pervaporation for water-chloroform and water-methyl-isobutyl ketone streams in industrial effluent.
Organic-Organic Pervaporation
Finally, both hydrophilic and hydrophobic membrane materials can be employed in organic–organic pervaporation. Given its critical role in the chemical and petrochemical sectors, the separation of organic-organic mixtures utilizing membrane separation techniques is a topic of intense research.
Pervaporation is now regarded as a fundamental unit operation for the separation of organic-organic liquid mixtures due to its efficiency in separating azeotropic and close-boiling mixtures, isomers, and heat-sensitive molecules. The materials utilized to make the membranes, which are employed to separate the four main types of organic-organic mixes, aromatic/alicyclic mixtures, aromatic/aliphatic mixtures, and aromatic isomers are both organic and inorganic in origin.
Pros and Cons of Pervaporation
Pervaporation process can be simple and automatic control with one key start-up, it is modular skid mounted unit with small space and easy for maintenance. It has several benefits, including reduced latent heat compared to evaporation methods, which is beneficial for substances that are sensitive to temperature.
Cons:
The PV separation process also requires precise and mature operation technique and timely feed stream quality control. It shall be strictly controlled with precise flow rate, water flux and working conditions for stable recovery and separation of components.
Take the zeolite membrane PV process as an example, the raw material shall be an organic solvent mixture, colorless, clear, and transparent. There shall be no stratification, and water content is less than 30%. The feed stream shall be detected with pH = 6.5-8.5 range, electric conductivity < 5us/cm, and chloride ion content shall below 20ppm. No pigment, salt crystals, sugar, colloid, acidic, alkaline component, or other viscous macromolecules that may pollute the equipment and membrane material.
Industrial Application of Zeolite Membrane on Pervaporation
In the process of pharmaceutical synthesis, extraction, and chemical reaction, organic solvents such as ethanol, Isopropyl alcohol, butanol, acetone, butyl acetate, methyl acetate, MTBE, THF tetrahydrofuran, etc. are largely used in the production operation.
The traditional processes are relatively low efficiency, and the quality of recovered solvents is not guaranteed, especially when there is azeotropy in the system or the water content of the material is high, the treatment technology will be cost and energy-consuming.
Pervaporation, as a new environmental protection and energy-saving separation technology, especially with the feature of not introducing the third component, can perfectly solve the problem of organic solvent dehydration and recycling.
At present, the zeolite pervaporation membrane equipment has been comprehensively used for the recovery of isopropanol in the production process of natural ingredient products and has achieved great economic benefits.
The pervaporation separation process has been successfully applied in ethanol dehydration, tetrahydrofuran dehydration, acetone dehydration, isopropanol dehydration, and the list including below.
Following are successful reference project application target:
- Dehydration target of tetrahydrofuran (THF) solution : moisture from 10% → 0.05%.
- Dehydration target of MTBE solution: moisture from 2% → 0.05%.
- Dehydration target of Acetone solution : moisture from 1.5% → 0.05%.
- Dehydration target of Mixed Solvents solution: moisture from 10% → 0.5%.
- Dehydration target of Ethanol solution: moisture from 5% → 0.5%.
- Dehydration target of Isopropyl alcohol solution: moisture from 10% → 0.1%.
- Dehydration target of mixed alcohol solution: moisture from 20%→0.5%.
- Dehydration target of Ethyl acetate solution: moisture from 15% →0.5%.
- Dehydration target of Toluene and ethyl ether solution: moisture from 1%→0.01%.
- Dehydration target of Acetonitrile solution: moisture from 50%→ 0.03%.
Note: Successful applications of pervaporation membrane dehydration plants are not limited to the list above, please contact us for detailed process engineering and solutions to gauge your project.
Conclusion:
Pervaporation is an innovative technology for the solvent recovery process. It has been applied comprehensively in the pharmaceutical and chemical industries. Its advanced technology and advantages have been more widely recognized and used in recent years, but it still needs continuous development and process flow optimization and intensification, so as to improve the overall performance of the PV membrane separation.
PV membrane with their low energy consumption, good quality, less pollution, simple process, easy operation, and other characteristics deserves envision bright prospects.
If you have any questions about the pervaporation membrane technology and project requirements, please feel free to contact us and discuss them with us.
references:
https://www.sciencedirect.com/topics/chemical-engineering/pervaporation
