Energy and water Recovery from Waste Heat Streams
M. Tavana1, S.N. Ashrafizadeh1, E. Tavana2
Research Laboratory for Advanced Separation Processes, Department of Chemical Engineering, Iran
University of Science and Technology, Narmak, Tehran 16846-13114, Iran (mousatavana@chemeng.iust.ac.ir)
(ashrafi@iust.ac.ir)
Department of Chemical Engineering, Islamic Azad University of Bushehr, Bushehr, Iran
(tavana.eisa@yahoo.com)
Abstract
A new waste heat and water recovery technology based on a nano-porous ceramic membrane
water vapor separation mechanism was developed, to extract the water vapor and its latent
heat from low temperature high moisture content waste gas streams. The purpose of this
paper is to introduce a model for condensing vapors transfer in porous structures. Such
models have the potential to provide a better understanding of the transform mechanism. We
are concerned with simulation of condensable vapors transfer in porous structures. The model
was designed to study the mass transfer in micro-pore ceramic membranes for water recovery
from wasted gas into the industry. A computer program for numerical simulations of water
(H2O) condensation in a flue gas condensing Membrane Condensers Exchanger was
developed. The main processes involved, including Knudsen penetration and capillary
condensation, are simulated in the related pressure range. The simulation results show that
the porous membrane has, respectively, the mass recover and heat 70 and 60 percent larger
than the steel converter. The modeling results were compared with experimental data
obtained to determine the modeling accuracy and it was displayed that the model presented is
highly accurate.
Keywords:
Membrane Condenser, Water Vapor, Energy Recovery, Numerical Simulations
Introduction
Power plants release a large amount of water vapor into the atmosphere through the stack.
The flue gas can be a Potential source for obtaining much needed cooling water and energy
for a power plant. If a power plant could recover and reuse a portion of this moisture, it could
reduce its total cooling water intake requirement. The power plant could also recover latent
heat due to condensation as well as sensible heat due to lowering the flue gas exit
temperature.
Today, the high price and the indiscriminate use of energy has become a challenge for
*Corresponding Author: ashrafi@iust.ac.ir
various industries. This is why the industry is looking for new technologies to recycle water
and heat energy. In industrial processes, it is essential to re-enter the cycle and reuse the
flows existed in process, especially water, to minimize the need for clean and fresh water [14]. On the other hand, the vapor discharge to the atmosphere is not only a waste of energy but
simultaneously carries environmental damages, because the water vapor is one major
greenhouse gas that could accelerate the climate change [5]. Currently, there is no
commercialized technology to recover water frm industrial processes' wasted vapors. The
membrane technology has the ability to separate the water vapor from the gas stream and to
produce the water with high purity. These advantages turn the membrane to an interesting
and promising option which could separate the water vapor from the gas stream produced by
the reactors[6].
So far, the compressed membranes and porous hydrophilic membranes have been
used for this purpose [7]. About compressed membranes we can say gases is dehumidified
and dried by influence [8] The fundamental problem of these membranes is they work in
high-pressure because for permeate flowing of steam through the membrane, the pressure
difference is necessary. This matter in turn means density, high energy consumption and high
cost. Dehumidifier systems that are based on hydrophilic membrane work by the wet
selectivity. On the other hand, membrane is a barrier between wet gas phase and coolant
liquid usually water phase. Temperature of coolant material accompany the pressure
difference between both sides of the membrane causes pressure of water from the wet gas to
the part of coolant material[9]. The high speed of steam influence through an ionic and non
porous membrane has been reported as a method for the recovery of high purity vapor at a
temperature close to 100° C or higher. [10] The nano-particle composite membranes PVA/
silica have been tested for vapor recovery [5]. and water vapor pressure 70-150 Kg/m²h was
attained in difference of pressure 6 Bar. The steam absorption device which worked by of
zeolite LTA membranes was used in sugar production factory. [11] And very high pressures
was reported: 11.9 , 14.9 , 17.6 , and 22.4 Kg/m²h respectively for 100° , 110° , 120° and
130° C. Zhang and his colleagues showed that the permeance ratio of air to water for
hydrophilic membranes has a span from 460 to 30,000. In other words, all gases except water
vapor could rarely pass through the membrane. Advantage of using this type of material in
separation of water vapor and the composite of vapor and gas, is their strong intermolecular
forces with water molecules. These strong intermolecular forces lead to a large difference of
permeation between water vapor and the other combinations there are in gas flows. So, in
recent years, more and more attention to this type of methods is raised.
Among the porous materials, the biphasic flow generally gets more attention due to its
abundant applications such as drying, moisture transfer in building materials, advanced
methods mechanism of oil recovery, assessment of nuclear waste disposal in underground
and cooling the microelectronic chips [7]. The condensable vapors stream in a porous
interface is a complex problem. Before the onset of capillary condensation, multilayer
adsorption occurs in cavities filled with vapor that reduces the pore volume level.
Kenvergiaks and colleagues [12], and also Yang and Renjynak proved that, the molecules
shallow absorbed and capillary condensation could increase the permeability of condensable
vapors, under the influence of a pressure gradient in a membrane [13].
Water separation in the state of Knudsen penetration is weak due to the low flux, but
when the gas flow of leakage part is sufficiently cooled by the heat transformer exhaust gases
and the relative humidity of exhaust gas is increased, the capillary transform will occur in
porous membranes. Then, the flux of water vapor transform is increased by more than 5 times
of the amount measured for Knudsen penetration and the separation ratio is improved by 100
times, and this causes the membrane capillary condensation mechanism more considerable
than the other ones (Fiq.1) [14-15].
Figure 1: Membrane Transport Mode Effect
This paper considers the heat and mass transfer modeling through condensation in ceramic
membrane tube which is used to recover heat and water from flue gas resulted from
combustion. Tube wall is designed from special porous material capable of extracting
condensed liquid from flue gas.
Development of Model
When one of the gas components is condensable and the pores are small, the capillary
condensation can occur. In this case, condensations could stop the gas phase penetration
through the pores and only let the condensation phase to pass. This theory proves that the
vapor separation in membrane in capillary condensation can also cause a high transfer and
breakdown rate [16].
This work considers two-dimensional mathematical model of the water vapor adsorption
through condensation technology in membrane pores using water absorbent. In this modeling,
the membrane was considered as a two-dimensional porous environment. All components
have been assumed as two phases – one phase including water vapor and the second phase
consisted of incondensable components which could penetrate in membrane pores along with
the vapor until the condensation time. The real 3-D geometry can be obtained from the 2-D
geometry under the hypothesis of axial symmetry. Fig.2 shows schematic diagram of studied.
Figure 2: 1st and 2nd Gen TMC Designs
Figure (3) displays a schematic of model studied in a cylindrical coordinate system. The
flow consisted of cold water from the low modulus (z = 0) (reactor or module) is fed. The
flue gas comprising condensable water vapor and other three components; dioxide, oxygen
and nitrogen, is fed from the shell side of modulus (z = L) and losses it's moisture during the
transfer along the module. As noted in the introduction, the membrane is modeled in such a
way that it is only able to absorb water.
Figure 3: TMC Concept Schematic
Numerical solution
For a detailed understanding of phenomena or transform and to find their impact on
process performance, the simultaneous impact of phenomena or transfer should be modeled.
Because the highly nonlinear differential equations are used, Method of producing a good is
as follows:
1. All parameters related to the operation conditions are placed within the considered
simulation area. The geometry of the simulation area (feed, membrane and exudates) will be
made and the meshing will be generated.
2. The solution gained in the previous step is implemented as the initial guess and the
equations for momentum mass transfer inside the membrane are solved.
3. The solution obtained in the previous step is applied as the initial guess and the
equations for heat transfer in whole module are solved to achieve the profile in the entire
module.
4. The solution obtained in the previous step is used as the initial guess and all the
dominant equations are solved and the resulted answer is considered as the initial guess for
the future calculations.
Schematic of the algorithm used to generate, displayed in Figure (4), is considerable.
Input mild operation
condition
producing module
geometery
Solving mass and
energy balance
equations based on
previos stage as an
initiall guess
store results as an
initiall guess of
operation condition for
next calculations
Solving mumentum
and mass balance
equations based on
previos stage as an
initiall guess
Figure 4: The simulating algorithm for water vapor separation
Results
Mass transfer mechanism is carried out in membrane contactor shell through movement
and diffusion mechanism. The mass transfer along with the direction r is only occurred
through diffusion, while the mass transfer along with the direction z occurs due to both mass
movement and diffusion phenomena. Although, the diffusion against mass movement along
with direction z could be neglected.
The shell dimensionless concentration distribution in different gas velocities (cold
fluid discharge 1/2 kg / min, cold fluid temperature 25oC, inlet flue gas temperature 80oC,
H2O volume fraction in feed gas 11 %)
Figure 5:
Of comparing the membrane module and thermal exchanger to recover the water vapor
and industries dissipation heat, given in fig (6), show that the membrane module has 60
percent, thermally, and 70 percent, regarding the mass, higher capacity to recover the
industrial waste vapors.
Figure 6: comparing the membrane module thermal recovery with thermal exchanger and experimental results (cold
fluid flow discharge 1.2 kg /min, inlet cold fluid temperature 20oC, flue gas discharge 60 m3 / h, H2o volume fraction in feed
gas 11%, TMC: Transport Membrane Condenser, HX: Heat Exchanger, EX: experimental )
Model designed
The model designed, using experimental results of Wang et al, was approved to validate
[1]. Membrane characteristics and operating conditions are given in Table 1. Fig 6 compares
the experimental parameters and the model and displays a high accuracy. As illustrated,
increasing the temperature may decrease the vapor recovery rate, because the increased
temperature would decrease the membrane capillary coefficient.
Table.1. Membrane characteristics and operating condition [1].
Table 1
Membrane characteristics and operating condition[1].
Membrane characteristics
Sy
Parameters
Material
Fiber inner radius
Value
mbol
Alu
mina
𝑟1
7
Fiber outer radius (mm)
𝑟2
Thickness (mm)
𝛿
1
Fiber porosity (%)
𝜀
0.3
Pore diameter (𝒏𝒎)
Fiber length (cm)
Number of membrane
fiber
Sectional area of
membrane contactor(𝒄𝒎𝟐 )
Parameters
𝑑𝑝,𝑖
L
10
43.2
𝑛𝑓𝑖𝑏𝑒𝑟
78
𝑊×𝐻
43×9
Operating conditions
Sy
Water inlet flux (𝒌𝒈/
𝒎𝒊𝒏)
water inlet
temperature(°𝑪)
Flue gas inlet flux (𝒎𝟑 /𝒉)
Flue gas
inlettemperature(°𝑪)
flue gas humidity %
11
Range
mbol
𝑄𝑙,𝑖𝑛
1-5
𝑇𝑤,𝑖𝑛
20-40
𝑄𝐹𝐺,𝑖𝑛
10-60
𝑇𝐹𝐺,𝑖𝑛
60-90
ℋ
11
Conclusion
In this paper, a model of two-dimensional mass transfer was presented as a porous network to
study the ceramic micro-pore membranes in order to recycle waste water and waste gas heat
in membrane contactor. This model is based on solving the solute component conservation
equations in two liquid and gas phases in a membrane contactor to simulate the water vapor
removal from flue gas through capillary condensation and cold water mechanisms in a
membrane contactor. The simulation results were compared with experimental data to
investigate the model accuracy. The effect of diffusion mechanisms Knudsen, capillary
condensation and operating parameters of temperature and flow discharge on the removal rate
were investigated. Results indicated that if carrying the removal only by diffusion
mechanism, small amount of water vapor would be removed from flue gas, but while
carrying capillary condensation in membrane pores, the removal rate will increase to 4.6
times. The membrane contactor has heat and mass transfer, respectively, 60 and 70 higher
than steel thermal exchanger. The results show that the numerical method and model
presented could predict the process of removing water vapor from gas in membrane
contactor. So, we can use this model to compare by the systems with chemical reaction in gas
or liquid phase.
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