Practicum Guide (V1.5)
Membrane Distillation (MD)
The aim of this instruction is to briefly describe the membrane distillation, direct contact
membrane distillation and the effect of process conditions on membrane permeate flux
for the purpose of the process engineering I lab course. The preliminary discussion takes
place in the morning at 8:00 o’clock of the same day as you do the experiment.
1. Introduction
Membrane technologies such as microfiltration (MF), ultrafiltration (UF), nanofiltration
(NF), reverse osmosis (RO), and membrane distillation (MD) have become more attractive when compared to conventional separation. Among membrane technologies, MF,
UF, NF, and RO are the pressure-driven separation technologies and only MD is a thermal-driven separation process. MD is an emerging non-isothermal technology, in which
only vapour molecules are able to pass through a porous hydrophobic membrane. This
separation process is driven by the vapour pressure difference existing between the porous hydrophobic membrane surfaces. Using MD has many attractive features, such as
low operating temperatures (in the range of 40-80 °C) in comparison to those encountered in conventional process; the solution (mainly water) is not necessarily heated up
to the boiling point. Moreover, the hydrostatic pressure encountered in MD is lower than
that used in pressure-driven membrane processes like reverse osmosis (RO), therefore
MD requires less demanding membrane mechanical properties and high rejection factors of non-volatile solutes is achievable. Finally, the vapour-liquid interface is developed at the pore inlet of a hydrophobic membrane, resulting in a high contact area and
allowing very compact installations and reduced footprint. These characteristics make
MD attractive within the academic community as well as industrial sectors. MD can be
used for various applications (desalting highly saline waters, environmental/waste
clean-up, water- reuse, food, medical, etc.). The possibility of using waste heat and renewable energy sources also enables MD technique to be used in conjunction with other
processes in an industrial scale. In MD process, both heat and mass transfer through porous hydrophobic membranes are involved simultaneously. The mass transfer occurs
through the pores of the membrane whereas heat is transferred through both the membrane matrix and its pores. The heat transfer within the membrane is due to the latent
heat accompanying vapour or gas flux and the heat transferred by conduction across
both the membrane material and the gas-filled membrane pores.
2. Theory
2.1. Concept of MD
MD involves the transport of vapours from a liquid stream through the pores of a hydrophobic membrane. Since the hydrophobic membrane is not wetted, water vapour passes
through the membrane pores but the aqueous solution is prevented from passing
VT1-SS15-MD
Mohammad Rezaei, M.Sc.
Page 1
Practicum Guide (V1.5)
through the pores. Volatile vapours transfer across the hydrophobic membrane and are
condensed or removed as a vapour from the permeate side of the membrane module.
Since liquid does not transport across hydrophobic membrane, dissolved ions (with virtually no vapour pressure) are completely rejected by the membrane.
2.2. Direct contact membrane distillation (DCMD)
A variety of methods have been employed to impose a vapour pressure difference across
the membranes for MD. These types are varying in a way of permeate collection, feed
and permeate compositions and characteristics, the mass transfer mechanism, the reason for driving force formation as well as productivity. One of the possibilities is that an
aqueous solution colder than the feed solution is maintained in direct contact with the
permeate side of the membrane giving rise to the configuration known as Direct Contact
Membrane Distillation (DCMD). Both the feed and permeate aqueous solutions are circulated tangentially to the membrane surfaces by means of circulating pumps. In this case
the transmembrane temperature difference induces a vapour pressure difference. Consequently, volatile molecules evaporate at the hot liquid/vapour interface, cross the
membrane pores in vapour phase and condense in the cold liquid/vapour interface inside the membrane module (Fig. 1).
Figure 1 DCMD process configuration
The term DCMD comes from its similarity to conventional distillation (i.e. simple and
multi-effect distillation) because evaporation and condensation take place at the liquidvapour interfaces formed at the pore entrances on the feed and the permeate side, respectively (Fig. 2). The hydrophobic porous membrane is maintained in direct contact
with the feed aqueous solution to be treated and permeate aqueous solution that is usually drinkable water or distilled water. Furthermore, both DCMD and conventional distilVT1-SS15-MD
Mohammad Rezaei, M.Sc.
Page 2
Practicum Guide (V1.5)
lation technologies require latent heat of evaporation to be supplied to the aqueous feed
solution in order to produce mass flux.
Figure 2 Schema of heat and mass transfer through a hydrophobic membrane used in
DCMD process.
2.3. Model
Concentrations of both permeate and feed solutions are determined at a temperature of
20 °C, by a previously calibrated conductivity meter (i.e. electrical conductivity vs. solute
concentration). The final separation factor, a, is calculated using the following expression:
𝛼 = (1 −
𝐶𝑏,𝑝
) × 100
𝐶𝑏,𝑓
Equation 1
where Cb,p and Cb,f are the final solute concentration in permeate and in the bulk feed
solution, respectively. Keep this in mind, just in case of increase in permeate electrical
conductivity you will make the conductivity vs. concentration calibration curve. Otherwise theoretically the separation factor is 100%. The final concentration factor is also
defined as follows:
VT1-SS15-MD
Mohammad Rezaei, M.Sc.
Page 3
Practicum Guide (V1.5)
𝑓=
𝑀𝐹
(𝑀𝐹 − 𝑀𝑝 )
Equation 2
where MF = initial feed mass and Mp = final obtained permeate mass. The final term water recovery (WR) is used as:
𝑊𝑅 = 100 × 𝑀𝑃 /𝑀𝐹 = 100(1 − 1/𝑓)
Equation 3
3. Experimental
3.1. Materials
A shell-and-tube membrane module with diameter of 15mm is used in this practice to
conduct the DCMD experiments. It consists of one tubular polypropylene microporous
hydrophobic membrane with an effective filtration area of 0.015 m2. The internal and
external tubular diameters are 5.5 mm and 8.4 mm, respectively. The membrane pore
size and porosity, as supplied by the manufacturer are 0.2 µm and 45%, respectively.
DCMD experiments are performed at different experimental conditions using pure water
in permeate side and 3 l sodium sulphate aqueous solutions with defined concentrations
as feed. Please note that although sodium sulphate is generally regarded as non-toxic, it
should be handled with care. The dust can cause temporary asthma or eye irritation; this
risk can be prevented by using eye protection and a paper mask. For more information
you can click here. Please look at sodium sulphate solubility-temperature relation here.
3.2. Method
DCMD experiments were conducted using the system presented in Fig. 3.
Figure 3 Experimental setup.
VT1-SS15-MD
Mohammad Rezaei, M.Sc.
Page 4
Practicum Guide (V1.5)
In order to reduce the heat lost to the surroundings, the feed solution is circulated
through the lumen side of the membrane module in all the experiments. The concentration in the feed container is defined in each experimental test for each group. The temperature of the feed and permeate solutions is measured at both the inlet and outlet of
the membrane module. The feed and permeate pressures are close to the atmospheric
pressure. The absence of pore wetting can be checked by determining the permeate concentration by measuring the electrical conductivity on the permeate side. In all MD configurations the permeate flux is calculated by measuring the condensate collected on the
permeate side of the membrane module for a pre-determined period of time.
3.3. Effects of process conditions
Effect of feed temperature, feed and permeate circulating flow rates on permeate flux
are investigated within the practicum. Each process parameters should acquire at least 3
different values. Data is measured in every 10 minutes intervals, in which for every process parameter variations at least 5 points should be recorded. Remember to make the
balance zero after each parameter changes. On the day of practicum you will get a complementary plan for investigating the effect of process conditions.
3.3.1. Effects of Feed Temperature
In this DCMD process the applied feed temperature (Tb,f) ranges between 40 °C and 60
°C. The highest temperature used is commonly below the boiling point of the feed aqueous solution. Can you guess how feed temperature would affect the transmembrane vapour pressure difference and ultimately the permeate flux? If you are not sure we will
find out its effect on permeate flux in the practicum.
3.3.2. Effects of Feed and permeate flow rates
To reduce temperature and concentration polarization effects, the feed and permeate
flow rates (i.e. feed circulation velocity) must be increased. When the flow rate is increased the temperature and non-volatile solute concentration at the membrane surface
become closer to the corresponding bulk temperature and bulk concentration, resulting
in higher transmembrane temperature difference and greater DCMD permeate flux.
However, the flow rate must be varied with due precautions in order to avoid membrane
pore wetting as the transmembrane hydrostatic pressure must be lower than the liquid
entry pressure (LEP) and, at the same time, to assure working under turbulent flow regime in order to obtain high productivity. In this practicum you need to change these
parameters at least 2 times to note their effect on permeate flux. Moreover, you need to
calculate the Reynolds numbers for feed and permeate flow to understand in which flow
regimes the experiment is running.
3.4. Sensitivity analysis
VT1-SS15-MD
Mohammad Rezaei, M.Sc.
Page 5
Practicum Guide (V1.5)
Sensitivity of permeate flux based on process parameters should be calculated as follows:
𝛿𝑖 =
(𝑋𝑖 − 𝑋1 ) (𝐽𝑖 − 𝐽1 )
×
𝑋1
𝐽1
where i is sensitivity coefficient, Xi is the step process parameter value, X1 is the base
process parameter value, Ji is permeate flux at Xi and J1 is the permeate flux at X1.
4. Cleaning procedure
Remember to clean up all the tubes and devices with distilled water and empty them at
the end, making them ready for the use of the next group. For clean-up procedures
please ask the instructor for help.
5. Evaluation and protocol
The protocol should be written in English and should include the following structure:
1.
2.
3.
4.
5.
6.
Task
Theoretical principles / calculations
Experimental implementation
Presentation of results
Discussion of the Results / Summary
Appendix (measured values / calculated data)
In the discussion of the results the following additional points and questions should be
answered:
1. How is the influence of process conditions on the permeate flux based on sensitivity analysis? Which process parameter has the main influence on the permeate
flux?
2. Depict the permeate flux vs. time and process parameters and explain which
trends do you see and why?
3. Can you suggest any theoretical equations that calculate the permeate flux based
on membrane characteristics and process parameters and is in agreement with
your experiments?
The protocol should be submitted via e-mail (Microsoft Word format) with the file
name GrXX_MD_SS15.docx to ([email protected]) no later than two weeks following completion of the experiment.
I am looking forward to doing the experiment with you.
Till then, have a great time.
VT1-SS15-MD
Mohammad Rezaei, M.Sc.
Page 6