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SIMULATION OF SCWO PROCESS IN TRANSITORY STATE
D. Roldán-Calbo
1
(*); J. Sánchez-Oneto
1
; J. R. Portela
1
; F. Jiménez-Espadafor
2
and E. J. Martínez de la Ossa
1
1
Department of Chemical Engineering, Food Technology and Environmental
Technologies. Faculty of Sciences. University of Cádiz.
11510 Puerto Real (Cádiz), Spain.
Phone: +34 956016458. Fax: +34 956016411. E-mail: daniel.roldan@uca.es
2
Departamento de Ingeniería energética y mecánica de fluidos.
Escuela Superior de Ingenieros, Universidad de Sevilla,
Camino de los Descubrimientos, S/N, 41092, Sevilla, Spain.
Phone: +34 954487245. Fax: +34 954487243
Abstract
Supercritical Water Oxidation (SCWO), is a high-efficiency, thermal oxidation
process capable of treating a wide variety of hazardous wastewaters. SCWO
processing systems are fully enclosed and do not produce hazardous air pollutants
(HAPS) or NOx.
The simulation of SCWO process is a crucial step in the optimization of the process
and the energy generation, to explore the scale-up of the process, to optimize
changes in operating conditions and to implement new improvement to the process
for a complex wastewater. Many attempts have been carried out successfully in
stationary state, however, a transitory state simulation is needed in order to achieve
an optimal simulation.
The software Engineering Equation Solver (EES) have been used to develop the
simulation in transitory state. This software is widely used in many engineering
problems and simulations involving thermodynamical processes in steady state,
because of its huge thermodynamical properties and models database for many
compounds.
In this work, a transitory state simulation is built-up for the tubular reactor of a
supercritical water oxidation plant using the SCWO of isopropanol to model the
system. The main problems are both that the transitory state is not a well-known
field and that the mathematical expressions involved are quite complex to solve out.
Keywords: SCWO, Supercritical hydrothermal oxidation, simulation, transitory
state, isopropanol.
1. INTRODUCTION
Supercritical Water Oxidation (SCWO), has proved to be an effective process to treat a wide variety of
industrial wastes [1,2]. SCWO essentially consists of an oxidation in an aqueous medium at both high
temperature and pressure, above the critical point for pure water, usually ranging from 673 to 923K and from 25
to 35MPa, respectively. Above its critical point (T
c
= 647 K, P
c
= 22.1MPa) water exhibits unique physical–
chemical properties that make it an effective reaction medium for the oxidation of organic and inorganic
compounds [3]. The reaction occurs in a homogeneous phase, meaning that interface mass transfer limitations
are avoided and high conversions efficiencies can be achieved in short residence times [4].
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 on the order of 10 to 15 seconds. For most
wastes, these conditions are sufficient to achieve ≥99,99% Destruction and Removal Efficiencies (DRE). Once
the reaction has achieved the desired DRE, the reactor effluent is cooled, depressurized and separated into
gaseous and liquid streams. The process is totally enclosed up to the point of final discharge to the environment,
facilitating post-processing and monitoring prior to release. From an environmental perspective, the resulting
effluent complies with the strictest environmental regulations and can be disposed of without further treatment
2
[5]. 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 many cases, high-quality excess heat of reaction can be recovered for use within the process, or for external
purposes. The SCWO process is self-sustaining for organic concentrations in excess of approximately 1%
concentration. Concentrated wastes can be diluted with water prior to injection into the process. Water-soluble
fuels, such as ethanol or isopropanol, can be added to the waste to help support the oxidation reaction.
Despite its advantages, SCWO has not been fully developed at industrial level because of a lack of
understanding on how some of the limitations of this technology can be avoided or controlled, and many
research efforts have been dedicated to address them [6]. Significant advancements have been made, for
example, in areas related to corrosion control [7], proper handling of insoluble salts [8], efficient reactor
configurations [7], and to determination of the oxidation mechanisms and kinetics for a variety of chemical
compound [9]. These advancements are being integrated into simulation tools for correlating experimental
results and predicting the performance of proposed SCWO processes [10]. Scaling SCWO processes up to
industrial level depends on the development and improvement of such simulation tools.
Many attempts have been carried out successfully in the simulation of SCWO in stationary state [11]. Several
authors [10,12-14] have used commercial computational fluid dynamics (CFD) software such as MODAR®,
FEMLAB® and FLUENT® to describe the flow characteristics of SCWO in tubular reactors. These studies are
based on unidimensional or bidimensional a steady state models and have had as purpose to determine the final
conversion and temperature profile that can be achieved in a reactor. Only a few papers [15,16] have been
reported on the response of a SCWO reactor to a transitory phenomena such as those present during the start-up,
or to a sudden change in the process conditions, which is more important than predicting the steady state reactor
profiles, because the possibility of reaching runaway conditions or the formation of hot spots inside the reactor
must be analyzed and avoided.
In this work, a tubular reactor of a pilot-scale supercritical water oxidation plant in transitory state is simulated.
In the recent literature only few publications have dealt with the simulation of the transitory behavior of SCWO
tubular reactors, whereas to the best of our knowledge, no experimental work on this topic has been reported.
The study of the transitory response is important for scaling-up the process because such study allows one to
determine the optimal temperature and feed concentration that guarantee a safe operation; that is, to avoid the
formation of hot spots that may arise because of the intense oxidation reaction, and to avoid the possibility of
reaching runaway conditions that may lead to a thermal failure of the reactor [6]. It is also important in order to
simulate changes in operating conditions or in the implementation of a new improvement to the process.
Isopropanol has been chosen as a model compound to carry out the simulation due to its excellent representation
of water – soluble organic compounds and because of its SCWO kinetics are well known [17].
2. MATERIALS AND METHODS
The supercritical water oxidation pilot plant of the University of Cádiz was design to be able to treat wastewaters
operating auto-thermally, that is, the heating capacity of the effluent of the reactor is used to increase the temperature
of the waste and the oxidant before they are mixing at the beginning of the reactor. Moreover, the SCWO pilot plant
includes an electrical heating system in order to supply energy demand as in the case of the staring-up of the
experiments. Due to the extreme work conditions, several safety components were included in the design and
construction of the pilot plant.
This pilot plant facility includes two independent feed streams: 1) An aqueous feed stream (with or without
organic contaminants) is pressurized by a high pressure liquid pump, preheated at supercritical temperature
(above 400ºC) and introduced into the system at a flow rate up to 25 kg/h. 2) A pressurized and preheated air
stream added before entering the tubular reactor. Once the oxidant and the organic compounds are mixed at high
temperature, the oxidation reactions take place at a high rate, releasing an important amount of heat [18]. The
main equipment is the continuous flow reactor which is made of stainless steel AISI 316L. The total volume of
the reactor is 1229,6 cm
3
. The inlet temperature in the reactor is around 400 ºC and the outlet temperature can be
up 550 ºC.
In order to minimize the lost of the heat produced by the oxidation reactions of the wastewater, this
reactor is surrounded with a thermal shield. In order to measure the temperature profile generated in the reaction
system there are seven thermocouples along the reactor. The reactor is not totally adiabatic and the simulation
may include the lost heating coefficient. Fig. 1 shows a schematic diagram of the pilot plant facility.
3
The pilot plant disposes a coaxial counter-current heat exchanger used to preheat the liquid feed with the excess
of energy of the effluent of the reactor. In this equipment the effluent of the reactor, at high temperature and
pressure, flows through the internal pipe giving the calorific power to the cold feed which flows through the
annular space between both coaxial pipes. In this way, The calorific power of the heat fluid is used to preheat
the liquid feed stream. Then it crosses a second coaxial counter-current which is used to preheat the air feed.
Both preheaters are insulated so that this system can operate auto-thermally. Once cooled, the effluent is
depressurized by a back pressure regulator and the product stream is then separated into liquid and vapour
phases in a gas-liquid separator.
3. THE SOFTWARE
The software, Engineering Equation Solver (EES), has been used to build-up the simulator. The basic function
provided by EES is the solution of a set of algebraic equations. EES can also solve differential equations,
equations with complex variables, do optimization, provide linear and non-linear regression, generate
publication-quality plots, simplify uncertainty analyses and provide animations.
There are two major differences between EES and other existing numerical equation-solving programs. 1) EES
automatically identifies and groups equations that must be solved simultaneously. Solves up to 12000
simultaneous non-linear equations, the equations can be entered in any order with built-in editor. 2) EES
provides many built-in Mathematical and thermophysical property functions useful for engineering calculations.
This difference is the most important for our purposes and the one that made us choose this software. Built-in
functions are provided for thermodynamic and transport properties of many substances, including steam, air,
refrigerants, cryogenic fluids, JANAF table gases, hydrocarbons and psychrometrics. Additional property data
can be added. EES also allows user-written functions, procedures, modules, and tabular data. EES can also
interface with REFPROP and other NIST fluid property programs. REFPROP provides the most advanced
methods for estimating the properties of mixtures.
4. THE WORK
As it was said in the abstract, the first of the main problems involved in our work is the lack of knowledge in
this field. The second main problem is that the mathematical expressions involved are quite complex to solve
out with the usual mathematical tools. Therefore, we must use both EES and mathematical and physical
simplifications that allow us to achieve a satisfactory solution of the problem. Our procedure, based mainly in
EES, consists in three stages as follows.
4.1. Collecting all the expressions that rule the tubular reactor in transitory state.
These expressions are usually differential equations, quite difficult to solve out, such as for the transitory
conduction for the tubular stainless steel shell, shown in eq. (1).
(1)
t
T
ρCq
z
T
k
zy
T
k
y
x
T
k
x
P
∂
∂
=+
∂
∂
∂
∂
+
∂
∂
∂
∂
+
∂
∂
∂
∂&
Fig.1. Schematic diagram of the supercritical water oxidation pilot plant of the University of Cádiz.
Phase separator
Water / aqueous
waste tank
High pressure
Pump
Tubular reactor
Electrical Preheater
Back pressure
regulator
Cooler
Liquid phase
Gas phase
Air Compressor
Liquid heat exchanger
Air heat exchanger
4
To solve them out we need both mathematical and physical simplifications and some tools EES provides us. By
example, eq. (1) is simplified to eq. (2) by considering uni-dimensional conduction and no heat generation.
As an example, we illustrate the procedure concerning the internal energy of the system, involved in the energy
balance to the tubular reactor. The variation of the internal energy of the system will be the sum of the variations
of internal energy of every compound implied in the process, as eq. (3) shows.
The internal energy for every compound j depends on the number of moles of j and its internal molar energy,
which depends on the pressure, the temperature and the time. This dependence is defined by Eq. (4).
The SCWO process is nearly isobaric (250 bar), so simplifying and deriving eq. (4) in time, we arrive to eq. (5).
To solve out this equation we relate every item in eq. (5) to the time and the temperature as shown in eq. (6) and
the following ones. Equation (6) relates the number of moles of the component j to its molar density and to the
system volume.
Thanks to the EES thermophysical property functions, we are able to get expressions for the internal energy and
the molar density related to the temperature for each component as is shown in eq. (7) and eq. (8).
These equations might be obtained from a relation provided and correlated by EES as Fig. 2 shows.
From this moment, it is already possible to solve out the eq. (3) and (5) by using a numerical method built-in in
EES, such as Runge-kutta.
4.2. Encoding these expressions in EES and building-up the modules.
(4) t)T,(P,U(t)n U
jjj
⋅=
(6) Vt)(T,ρ (t)n
jj
⋅=
(3)
dt
dU
dt
dU
jsystem
∑
=
(5)
dt
dT
T
U
(t)n t)(T,U(t)n
dt
dU
P
j
jjj
j
∂
∂
⋅+⋅
′
=
(7) Ta Ta Ta Ta Ta Ta a (T)U
6
6
5
5
4
4
3
3
2
210j
++++++=
(8) Tb Tb Tb Tb Tb Tb b t)(T,ρ
6
6
5
5
4
4
3
3
2
210j
++++++=
(2)
t
T
ρC
x
T
k
x
P
∂
∂
=
∂
∂
∂
∂
Fig.2. Variation of U with the temperature for water at 250 bars.
5
(9)
t
T
α
1
x
T
2
2
∂
∂
=
∂
∂
(10)
Fo
θ
x
θ
*
2*
*2
∂
∂
=
∂
∂
(11) )xcos(ζθ θ
*
1
*
o
*
=
(12) )xFo)cos(ζζexp(C
TT TT
θ
*
1
2
11
fluidi
fluido
*
o
−⋅=
−
−
=
As another example, we illustrate the procedure concerning the making of the transitory conduction module for
the stainless steel 316 shell tube. Eq. (2) showed the differential equation involved in this process. This equation
is simplified to eq. (9) by considering constant properties.
where α is the thermal conductive coefficient. To be able to solve this equation out, we transform and
accommodate it using adimensional numbers and relations, arriving to eq. (10)
where θ
*
is the adimensional temperature, x
*
is the adimensional length and Fo is the Fourier number. For Fo >
0,2 the estimative solution to eq. (10) is as shown in eq. (11) and eq. (12)
where θ
o*
represents the temperature in x
*
=0, that in our case it is in the external surface of the tube, T
fluid
is the
temperature of the supercritical fluid, T
i
is the initial tube temperature and T
o
is the tube temperature at t. ξ
1
y C
1
are correlated for different values of the number of Biot in the bibliography. Fig. 3 shows this relationship.
This module provides a graphical solution, as it is shown in Fig. 4, for several supercritical fluid temperatures.
Fig.3. Variation of ξ
1
y C
1
with Bi
Fig.4. Shell tube temperature versus time
6
4.3. Indexing the modules in a single program that simulates the tubular reactor transitory behaviour.
The last stage involves the index of all modules in order to create a single program that describes the behavior of
the tubular reactor in transitory state.
5. CONCLUSIONS
The development of simulation tools in transitory state is an important step for scaling-up SCWO processes,
because predictions of temperature and concentration profiles are useful for sizing the reactor and for making a
good description of its operation and its energy requirements.
EES has revealed itself as a very powerful tool to develop transitory simulations, due to its wide thermophysical
database and to its easy and predictive interface, as well as its powerful solve engine and its plotting features.
The simulations with the EES software of the SCWO of isopropanol agree well with the experimental results,
which makes possible to assure that the software can be used for modelling and simulation with the purpose of
optimizing the supercritical water processes.
6. REFERENCES
[1] E.F. Gloyna, L. Li, Encyclopedia of Environmental Analysis and Remediation, Wiley, New York, 1998.
[2] C.N. Staszak, K.C. Malinowski, W.R. Killilea, The pilot-scale demonstration of the MODAR process for the
destruction of hazardous organic waste materials, Environ. Prog. 6 (1987) 39–43.
[3] R.W. Shaw, T.B. Brill, A.A. Clifford, C.A. Eckert, E.U. Frank, Supercritical water: a medium for chemistry,
C&EN.December 23 Special Report (1991)26–39.
[4] J.W. Tester, J.A. Cline, Hydrolisis and oxidation in subcritical and supercritical water: connecting process
engineering science to molecular interactions, Corrosion (Houston) 51 (1999) 1088–1100.
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(1993) 35–76.
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toxic wastes: an analysis tool to understand a complex phenomena, I Iberoamerican Conference on Supercritical
Fluids, PROSCIBA (2007).
[7] Peter Kritzer, Eckhard Dinjus, An assessment of supercritical water oxidation (SCWO), Existing problems,
possible solutions and new reactor concepts, Chemical Engineering Journal 83 (2001) 207–214
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th
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