Content uploaded by Enrique J. Martinez de la Ossa
Author content
All content in this area was uploaded by Enrique J. Martinez de la Ossa on Jun 16, 2016
Content may be subject to copyright.
Simulation of a counter current refrigeration system for a
SCWO reactor
J.M. Benjumea*, J. Sánchez-Oneto, J.R. Portela, E. J. Martínez de la Ossa
Department of Chemical Engineering and Food Technologies
Faculty of Sciences.University of Cádiz
11510 Puerto Real (Cádiz), Spain
E-mail: josemanuel.benjumea@uca.es. Phone: +34 956016458. Fax: +34 956016411
ABSTRACT
Several types of reactors have been designed in the last years to avoid or reduce the
drawbacks derived from the Supercritical Water Oxidation (SCWO) process. For example,
the high pressure and temperature achieved in SCWO reactors require materials with special
characteristics to resist those severe conditions, as nickel based alloys such Inconel 625 and
Hastelloy. But even using those alloys, it is always needed to prevent temperatures above
600ºC and a refrigeration system is required, especially to treat high concentration
wastewaters. According to those premises, different reactors have been designed to work at
supercritical conditions, for example transpiring wall reactor, cool wall reactor or tubular
reactor with cooling water injections.
This work proposes the design of a SCWO reactor with a counter current refrigeration system.
The set consists of two concentric pipes, being the inner tube the reactor itself, where
exothermic reactions take place and the heat produced is transferred through the reactor wall
by heat conduction. In the external concentric tube, a cross current flow of water will
dissipate part of the energy transferred, preventing an excess in the maximum value of
temperature allowed for the material. Depending on the wastewater concentration fed, and
therefore the heat produced by its oxidation, the flowrate of cooling water is controlled to
maintain stable temperature profiles along the reactor (always below 600ºC).
In order to design the reactor, a model is built up to simulate the desired conditions as a
previous step to the experimental system construction. Simulation allows us to know easily
the behaviour of the system at different conditions with the aim of optimize the reactor
design. The software used in this work have been Engineering Equation Solver (EES) and
Matlab, both widely used in many engineering problems and simulations involving
thermodynamical processes. The main development of the model has been carried out with
Matlab, while EES, that counts on a huge thermodynamical properties and models database
for many compounds, has been used to determinate the properties of compounds.
INTRODUCTION
Supercritical water oxidation (SCWO) is a high temperature and pressure process whose
operational conditions are above the critical point of the pure water (Tc=374ºC and
Pc=221 bar). Above the critical point, water exhibits unique physical–chemical properties that
make it an effective reaction medium for the oxidation of organic and inorganic compounds
[1], being possible to carry out all oxidation reactions in a single reaction phase (no mass
transfer limitations), with very high reaction rates (removal efficiencies >99.99) and non-
harmful products, allowing the effective treatment of a wide variety of industrial wastes [2,3].
In a conventional SCWO waste treatment system, dilute aqueous organic waste is combined
with oxidizer at elevated pressure and temperature in a reactor for residence times in the order
of 10 to 15 seconds. Several steps are needed to work at those conditions, including
pressurization, heating, reaction, cooling, depressurization and phase separation. From an
environmental perspective, the resulting effluent complies with the strictest environmental
regulations and can be disposed of without further treatment [4]. In fact, it is a technique that
is superior to conventional disposal technologies. This feature is especially useful when
treating highly toxic or radioactive wastes.
In the last decade, significant advancements have been made in areas related to efficient
reactor configurations [5]. Different types of reactor as transpiring wall reactor [6, 7], cool
water reactor [8], double shell SCWO reactor [9], tubular reactor with oxidant and cooling
water injections [10]…, have been studied with the aim of enhance this technology. However,
despite those improvements, SCWO has not been fully developed at industrial scale and it is
necessary to know perfectly how the reactor behaves to make an efficient design.
That is where simulation tools play an important role in order to design and scale-up this
technology at industrial scale. The build up of a previous reactor model allow us to know his
behaviour before the construction, to ensure that the operation conditions designed are
optimized. Many attempts have been carried out successfully in the simulation of SCWO in
stationary state. Several authors have used commercial computational fluid dynamics (CFD)
software such as MODAR®, FEMLAB® and FLUENT® to describe the flow characteristics
of SCWO reactor with different configurations [11-13].
In this work, we focus in the model of a counter current refrigeration system for a SCWO
tubular reactor compared with a conventional tubular reactor, where the exothermic reactions
increasing the temperature quickly, being necessary to limit the feed concentration of waste to
reduce the heat produced. With the aim of increasing the concentration of wastewater to be
fed, a counter current refrigeration system is added to the conventional tubular reactor. In this
way, along the inner pipe reactions take place at the same time that, in a concentric pipe cool
water dissipate the produced heat. A comparison has been made with a conventional tubular
reactor in order to show the advantages of the new system, especially the increasing of the
concentrations of the waste fed.
A Cutting oil emulsion has been chosen as a model wastewater to carry out the simulation due
to its excellent representation of water–soluble organic compounds at a high concentration.
MATERIALS AND METHODS
The simulated counter current refrigeration system consist on two concentric pipes, where the
inner pipe is the reactor itself, with 3 meters length and an external diameter of ¼ in. The
external concentric pipe is a refrigeration system to prevent an increasing of the temperature
profile above 600ºC and its diameter is ½ in. As can be seen in Figure 1, besides the reactor,
the system consist on two tanks, one of them containing the water and waste mixture that is
being continuously stirred, and the other one containing a commercial hydrogen peroxide
solution with a purity of 30% w/w, that is used as a source of oxygen. Both wastewater and
oxidant solutions are pressurized and preheated independently before being mixed.
At those conditions, the oxidant stream is decomposed in H2O and O2. At the entrance of the
reactor, both streams are mixed reaching a temperature around 430ºC, and then reactions take
place along the reactor increasing the temperature. The system also include another tank with
the refrigeration water. After being pressurized until supercritical pressure, this stream is fed
to the refrigeration system at ambient temperature in order to dissipate the heat produced and
decreasing the temperature profile reached in the reactor. Due to the high temperature
achieved in external pipe, an isolation covering is necessary.
Depending of the operation conditions, both reactor effluent and exit refrigeration water
stream can change their temperatures. In both cases, residual heat can be used to generate
high-pressure vapour.
Figure 1: Counter Current Refrigeration System for a SCWO
The software used have been both MATLAB® [14] and Engineering Equation Solver (EES)
[15]. MATLAB® is a high-level technical computing language and interactive environment
for algorithm development, data visualization, data analysis, and numeric computation. The
main programming of the model has been developed with MATLAB in order manage all
numerical dates obtained. EES has been used to determine the properties of the present
compounds and solve the equation simultaneously. As distinguished from other commercial
software, EES provides many built-in mathematical and thermophysical property functions
useful for engineering calculations. In addition, EES counts on a thermodynamic and
transport properties of many substances, including steam, air, refrigerants, cryogenic fluids,
JANAF table gases, hydrocarbons and psychrometrics.
MODEL RESOLUTION
To solve the complete system, the finite element method has been used, dividing the system
into different slices, each one with a thickness of 0.1 m. Initial conditions are known in both
sides of the system, that is, temperature at the entrance of the reactor and refrigeration system.
It is necessary a simultaneous resolution of mass and energy equations balance, both the hot
fluid (reactor) and the cool fluid (refrigeration system).
The mathematical expressions needed to represent the process in stationary state are shown
below. The momentum equation is not taken into account because pressure remains constant
and it can be neglected. The equation system is simplified for compressible and Newtonian
fluid with respect to a control volume.
Governing equations
Global Mass Balance.
0)( fs
m
(1)
where fs
m
(kg/s) is the mass flow of the stream. This equation is applied to the reactor (hot
fluid) and refrigeration system (cool fluid).
Species Mass Balance in the reactor.
0)( i
rm fsi
(2)
where fs
i
m
(kg/s) is the mass flowrate of component i and i
r(kg/s) is the reaction velocity of
component i.
Energy Balance.
Reactor: tR
fs
QQmzgvH
)
2
1
(
2 (3)
Refrigeration System: hlt
fs
QQmzgvH
)
2
1
(
2 (4)
where H is the specific enthalpy (J/kg), v is the velocity (m/s), z is the elevation above a
datum level (m), g is the local acceleration of gravity (m/s2), QR is the reaction heat produced
(W), Qt is the heat transferred from the reactor to refrigeration system (W) and Qhl is the heat
losses transfer from the cool fluid to ambient (W).
In this case, the potential and kinetic energy can be neglected because both are much smaller
than enthalpy.
Heat transfer
In Figure 2, the heat transferred from the hot fluid to ambient air in radial direction can be
seen. The reaction heat produced is transferred by convection from the hot fluid to the reactor
pipe (eq. 5), by conduction through the reactor thickness (eq. 6) and by convection to the cool
fluid (eq. 7). In the refrigeration system side, heat is transferred from the cool fluid to the pipe
by convection (eq. 8), through the pipe and isolation material respectively
by conduction (eq. 9, 10) and finally, again by convection to the ambient air (eq. 11).
Conventional correlations were used to estimate of the heat transfer coefficients for different
fluids.
Figure 2: Heat transfer in radial direction
Heat transferred from the reactor
)( 11 whct TTAhQ (5)
1
2
21
ln
)(2
D
D
TTxk
Qwwpipe
t
(6)
)( 22 cwht TTAhQ (7)
Heat transferred from the exchanger
)( 33 wcchl TTAhQ (8)
3
4
43
ln
)(2
D
D
TTxk
Qwwpipe
hl
(9)
4
5
54
ln
)(2
D
D
TTxk
Qwwiso
hl
(10)
)( 54 ambwambhl TTAhQ (11)
where h is the convective heat transfer coefficient in the different cases (W/m2K), A is the
transfer area (m2) and k is the conductivity of the insulating and pipe material (W/mK).
Kinetic model of the organic compound
In order to simulate the oxidation process, a well-known model wastewater has been used,
that is, a cutting fluid with a COD of 2.264±0.041 (gO2/g concentrated cutting fluid). The
kinetic model used was obtained in a previous experimental work [16] and can be expressed
as follows.
2
exp OCOD
RT
E
A
t
CODd
ra
COD
(12)
where A is the pre-exponential constant (35 (mg O2/l)1-β s
-1), Ea is the activation energy
(70000 J/mol), R is the universal gas constant (8.314 J/mol K), T is the temperature in Kelvin,
β is the reaction order for oxygen (0.579) and [COD] and [O2] are the concentrations in
kg /m3. The heat of reaction for the oxidation of cutting fluid is given with
ΔHcom=-39200 kJ/kg.
Thermodynamical properties
The thermodynamical and transport properties of the organic compounds are only known at
pressures and temperatures far from critical conditions. However, the mass percentage of
organic compounds in the wastewater is always lower than 15% of the total mass flow for all
the conditions studied, so the fluid properties were considered to be the same as for water.
This assumption is consistent with most SCWO simulations reported in the literature [17-19].
For each pressure and temperature considered, the properties of all pure chemical species
were calculated with the code EES. For those analyses where a unique fluid property is
required, the corresponding magnitude was evaluated through a mass average using the
follow expression:
j
jij
im
TpBm
TpB
),(
),( (13)
where Bi is the property i of the pure chemical species j evaluated at pressure p and
temperature T, and mj is the mass flow of j.
RESULTS
In the reactor, the operating pressure is 250 bar and the temperature at the entrance is 430ºC.
The flowrates are 1.85 kg/h of a wastewater with different COD concentrations, and 2.5 kg/h
of hydrogen peroxide solution in water.
In the refrigeration system entrance, the water is at ambient temperature and the work
pressure in the concentric pipe is 250 bar. The flowrate of water is varied in order to analyse
the behaviour of the temperature control.
In Figure 3, the effect of the concentration of waste in the fed solution can be seen. It made a
comparison between a conventional reactor (without refrigeration system) and the system
studied with initial COD concentrations of 11.2, 14.7 and 18 kg/m3.
Figure 3: Temperatures profiles with different initial COD concentrations
The Figure 4 shows the behaviour of the system with an initial COD concentration of
18 kg/m3 and refrigeration water flowrates of 2, 2.5 and 3 kg/h. It is clear that, with an
increasing of water flowrate, the temperature achieved in the reactor decreasing, allowing a
better control of the profiles.
Figure 4: Temperatures profiles with different flowrates of water
00.5 11.5 22.5 3
0
100
200
300
400
500
600
700
Length (m)
Temperature (ºC)
Case 3
Case 1: 11.2 k g/m3 COD Concentration
Case 2: 14.7 k g/m3 COD Concentration
Case 3: 18 k g/m3 COD Concentration
Case 2 Case 1
Case 3 Case 2
Case 1
Case 3
Case 2
Case 1
Conventional Reactor
Cool Fluid
Hot Fluid
00.5 11.5 22.5 3
0
50
100
150
200
250
300
350
400
450
500
550
Length (m)
Temperature (ºC)
Case 1: 2 k g/h refrigeration water
Case 2: 2. 5 kg/h refrigeration wat er
Case 3: 3 k g/h refrigeration water
Case 2
Case 3
Case 1
Case 1
Case 2
Case 3
Cool Fluid
Hot Fluid
CONCLUSION
The simulations carried out can be considered consistent, being the first step to design a
counter current refrigeration system for a SCWO tubular reactor. Matlab and EES software
have been used in combination as a powerful tool that makes possible to predict the behaviour
of the fluids in the internal and external parts of the system, and to optimize the process.
As the results show, in comparison with a convectional reactor without refrigeration, the new
system studied would be capable of controlling the temperature profile with an appropriate
flowrate of cooling water according to the refrigeration requirements of the reactor. In this
way, the treatment of solution with high waste concentration is possible, allowing at the same
time the generation of a high-pressure vapour that can be used to power generation.
In futures works, the construction of the studied system will be carried out to contrast the
model built up, try to fit experimental and simulated data and optimize the process with the
aim of maximize the efficiency.
REFERENCES
[1] SHAW, R.W., BRILL, T.B., CLIFFORD, A.A., ECKERT, C.A., FRANK, E.U., C&EN, 1991, p. 26–39.
[2] GLOYNA, E.F., LI, L., Encyclopedia of Environmental Analysis and Remediation, Wiley, 1998.
[3] STASZAK, C.N., MALINOWSKI, K.C., KILLILEA, W.R., Environ. Prog., Vol. 6, 1987, p. 39–43.
[4] TESTER, J.W., HOLGATE, H.R., ARMELLINI, F.J., WEBLEY, P.A., KILILEA, W.R., HONG, G.T.,
BARNERN, H.E., ACS Symp. Ser., Vol. 518, 1993, p. 35–76.
[5] KRITZER, P., DINJUS, E., Chem. Eng. J., Vol. 83, 2001, p. 207–214
[6] DONGHAI, X., SHUZHONG, W., CHUANBAO, H., XINGYING, T., YANG, G., Chem. Eng. Research
and Design, 2014, In Press.
[7] BERMEJO, M.D., COCERO, M.J., J. of Hazardous Materials, Vol. 137, 2006, p. 965–971
[8] COCERO, M.J., MARTINEZ, J.L., J. Supercrit. Fluids, Vol. 31, 2004, p. 41–45
[9] MOUSSIERE, S., ROUBAUD, A., BOUTIN, O., GUICHARDON, P., FOURNEL, B., JOUSSOT-DUBIEN,
C., J. Supercrit. Fluids, Vol. 65, 2012, p. 25–31
[10] COCERO, M.J., Industrial Chemistry Library, Vol. 9, 2001, p. 509-526.
[11] ABELN, J., KLUTH, M., BÖTTCHER, M., SENGPIEL, W., Env. Eng. Sci., Vol. 21, 2004, p. 93-96.
[12] CHEN, P., LI L., GLOYNA, E. F., J. Supercrit. Fluids. Vol. 24, 1995, p. 348-353.
[13] ZHOU, N., KRISHNAN, A., VOGEL, F., PETERS, W. A., Adv. Environ. Res., Vol. 4, 2000, p. 79-95.
[14] MATLAB. http://www.mathworks.es/
[15] ENGINEERING EQUATION SOLVER. http://www.fchart.com/ees/ees.shtml
[16] SÁNCHEZ-ONETO, J., MANCINI, F., PORTELA, J. R., NEBOT, E., CANSELL, F., MARTÍNEZ DE LA
OSSA, E. J., Chem. Eng. J., Vol. 144, 2008, p. 361–367[16]
[17] CHKOUNDALI, S.; ALAYA, S.; LAUNAY, J. C.; GABSI, S.; CANSELL, F., Environ. Eng. Sci. Vol. 25,
2008, p. 173–180.
[18] DUTOURNIE ´, P.; MERCADIER, J., J. Supercrit. Fluids, Vol. 42, 2007, p. 234–240.
[19] VIELCAZALS, S.; MERCADIER, J.; MARIAS, F.; MATEOS, D., BOTTREAU, M.; CANSELL, M.,
MARRAUD, C., J. Hazard. Mater., Vol. 52, 2001, p. 95–106.