高分子概論
Membrane
distillation
班級:化材三乙
姓名:楊國倫
學號:49940062
一、 原理說明
The systems to be studied consist of a porous hydrophobic membrane
which is held between two symmetric channels. A hot aqueous solution
(non-volatile solute) is circulated through one of the channels and cold pure
water through the other one. The hot (feed) and cold (permeate) fluids,
referred in the following with the subindex 1 and 2, respectively, co-flow
tangentially to the membrane surface in a flat membrane module. The
temperature and solute concentration differences through the membrane give
rise to water vapour pressure difference and, consequently to water flux, J,
through the membrane. The heat requirements for water evaporation at the
membrane–liquid interface have to be supplied from the hot liquid phase. In
the same way, the condensation heat at the other membrane–liquid interface
has to be removed to the cold liquid phase. This creates temperature gradients
in the liquid films adjoining the membrane so that the temperatures at the limit
of the thermal boundary layers (Tb1 and Tb2) are different from the
corresponding values on the membrane surfaces (Tm1 and Tm2). The driving
force for water transport through the membrane is the water vapour pressure
difference across the membrane. Thus, the transmembrane water flux J is
usually expressed as: (1) 1 2 ( ) m m J = C p – p where pm1 and pm2 are a
function of temperature and concentration at the membrane surfaces, and C is
the vapour transfer coefficient of the membrane. According to the dusty-gas
model for gas transport through porous media, in the membrane system
studied, the vapour transport through the membrane pores takes place via a
combined Knudsen/molecular mechanism, and C may be written,
where gs is the superficial porosity of the membrane, δ the membrane
thickness, q the tortuosity factor of the membrane pores, M the molecular
weight of water, R the gas constant, T the temperature, pa the partial pressure
of the air entrapped in the pores, p the total pressure inside the pores, DK is
the Knudsen diffusion coefficient of water vapour and Dwa is the diffusion
coefficient of water in air. According to Eqs. (1) and (2), besides some
membrane characteristics, for the estimation of the permeate flux through the
membrane we have to known the temperature on both membrane surfaces
and the solute concentration on the feed surface of the membrane. Those
temperatures can be calculated taking into account that, at steady conditions,
the heat transported through the liquid boundary layers can be written:
where h1 and h2 are the film heat transfer coefficients, ΔHv is the heat of
water vaporization and km is the thermal conductivity of the membrane. In fact,
from these equations we obtain:
where H is the effective heat transfer coefficient for the membrane, H = (km/δ)
+ (JΔHv)/(Tm11!
Tm2), and h is the global film heat transfer coefficient, 1/h = (1/h1) + (1/h2). On
the other hand, the concentration on the feed surface of the membrane can be
calculated from a mass balance across the feed concentration boundary layer
as:
with K the film mass transfer coefficient, and ρ the density of feed solution.
The transfer coefficients h1, h2 and K are usually calculated in membrane
distillation assuming empirical correlations as:
where k is the fluid thermal conductivity; D the solute diffusion coefficient; μ is
the fluid viscosity, cp the fluid specific heat; v the fluid mean linear velocity on
membrane surface; A is a parameter including geometric characteristics of the
membrane module and the value of α is to be determined by the state of
development of the velocity, temperature and concentration profiles along the
flow channel in the membrane distillation module. The above equations
constitute a transport model that can be used to predict mass flux J for given
experimental operating conditions, Tb1, Tb2, xb1 and v once the values of the
parameters r, gs/qδ, km /δ, A and α are known. From the above discussion it
can be concluded that the temperature and concentration polarization
phenomena are always present in DCMD. They are usually measured from the
temperature
polarization coefficient, τ = (Tm1!Tm2)/(Tb1!Tb2), and the concentration
polarization coefficient, Γ = (xm1!xb1)/xb1. From the above written equations
it can be seen that τ and Γ are related to the film transfer coefficients h1, h2
and K. Temperature and concentration polarizations cause a reduction in the
effective driving force, which can be measured by the vapour pressure
polarization coefficient, defined as:
Taking into account Eq. (1) and all the above, it can be concluded that
adequate membranes
(with adequate transfer coefficient C) and modules (that allow adequate
transfer coefficients
h1, h2 and K) are necessary in order to obtain good performance, that is, high
flux accomplished of low loss of driving force by effect of polarization. The aim
of this work is to show the effects of improving membrane and module
characteristics on flux when different feeds are processed.
二、 應用/用途
Membrane distillation application
Membrane distillation (MD) hasmany applications. Table 3
summarisesome
of MD application such as fresh water production, heavy metal removal and
food industry. Most of current MD applications are still in the laboratory or
small scale pilot plant phase. Actually, there are some pilot plants that have
been recently developed to produce fresh water [17,58].
Membrane modules
5.1. Plate and frame
The membrane and the spacers are layered together between two
plates (e.g. flat sheet). The flat sheet membrane configuration is
widely used on laboratory scale, because it is easy to clean and replace.
However, the packing density, which is the ratio of membrane
area to the packing volume, is low and a membrane support is required.
Table 3 presents some characteristics for flat sheet membranes
that were used by some researchers. As can be seen in
Table 3, the flat sheet membrane is used widely in MD applications,
such as desalination and water treatment.
5.2. Hollow fibre
The hollow fibre module, which has been used in MD, has thousands
of hollow fibres bundled and sealed inside a shell tube. The
feed solution flows through the hollow fibre and the permeate is collected
on the outside of the membrane fibre (inside-outside), or the
feed solution flows from outside the hollow fibres and the permeate
is collected inside the hollow fibre (outside-inside) [9]. For instance,
Lagana et al. [38] and Fujii et al. [70] implemented a hollow fibre
module (DCMD configuration) to concentrate apple juice and alcohol
respectively. Also, saline wastewater was treated successfully in a
capillary polypropylene membrane [71]. The main advantages of the
hollow fibre module are very high packing density and low energy
consumption. On the other hand, it has high tendency to fouling
and is difficult to clean and maintain.
It is worth mentioning that, if feed solution penetrates the membrane
pores in shell and tube modules, the whole module should be
changed. [9,72].
5.3. Tubular membrane
In this sort of modules, the membrane is tube-shaped and inserted
between two cylindrical chambers (hot and cold fluid chambers). In
the commercial field, the tubular module is more attractive, because it
has low tendency to fouling, easy to clean and has a high effective
area. However, the packing density of this module is low and it has a
high operating cost. Tubular membranes are also utilized in MD. Tubular
ceramic membranes were employed in three MD configurations:
DCMD, AGMD and VMD to treat NaCl aqueous solution, where salt rejection
was more than 99% [30].
5.4. Spiral wound membrane
In this type, flat sheet membrane and spacers are enveloped and
rolled around a perforated central collection tube. The feed moves
across the membrane surface in an axial direction, while the permeate
flows radially to the centre and exits through the collection
tube. The spiral wound membrane has good packing density, average
tendency to fouling and acceptable energy consumption.
It is worth stating that there are two possibilities for flow in a
microfiltration system; cross flow and dead-end flow. For cross
flow, which is used in MD, the feed solution is pumped tangentially
to the membrane. The permeate passes through the membrane,
while the feed is recirculated. However, all the feed passes through
the membrane in the dead-end type. [72].
三、 參考文獻
1. 作者: Abdullah Alkhudhiri, Naif Darwish, Nidal Hilal
2. 文獻名稱: Membrane distillation: A comprehensive review
3. 年代: 15 February 2012
1. 作者: G.W. Meindersma , C.M. Guijt , A.B. de Haan
2. 文獻名稱: Desalination and water recycling by air gapmembrane distillation
3. 年代: 5 February 2006
1. 作者: L. Martínez , J.M. Rodríguez-Maroto
2. 文獻名稱: Effects of membrane and module design improvements on
flux in direct contact membrane distillation
3. 年代: 5 February 2007