Jules Sibilio and Shari Wilbarger
March 7, 1997
ENVE 436

INCINERATION AND HAZARDOUS WASTE

Thermodynamics Problems and REACT!



INTRODUCTION
The focus of this research is to provide the reader with a basic relationship between incineration of hazardous waste and thermodynamics problems. This relationship is demonstrated with various thermodynamic equations culminating with an example of a computer program, REACT!, which acts similarly to a hand calculator in solving thermodynamics problems. Thermodynamics problems solved by REACT! in relation to incineration, include; chemical equilibria equations and adiabatic flame temperature. This paper will include the relationship between thermodynamics of incineration, some sample equations and a description of REACT!.

THERMODYNAMICS AND INCINERATION
"Incineration systems are designed to destroy only organic components of waste…and most hazardous waste will contain both combustible organics and non-combustible inorganics" (LeGrega, et. al., 1994:705). Incineration reduces the volume of hazardous waste by destroying the organic fraction of the waste, and converting it to carbon dioxide and water vapor. The organic components are what generally contain the toxic components (LaGrega, et. al., 1994:705).

When combustion occurs with excess oxygen in the stoichiometric requirements, it is termed "excess air combustion" (Tchobanoglous, et. al., 1993:611). Combustion is good when there is complete oxidization of the carbon and hydrogen. This good combustion will most likely occur if there is excess air in the reaction. In order to have good combustion, air must be thoroughly mixed with the organic components of the fuel (or waste). The nitrogen in the air is inert and goes out in the combustion process (LaGrega, et. al., 1994:706). "Combustion calculations are often based upon the change in the amount of heat content between the reactants and the products of the combustion" (LaGrega, et. al., 1994:719). The content of heat that occurs in the reaction is enthalpy.

Incinerators tend to operate under adiabatic processes "wherein no heat enters or leaves the system" (Hewitt, 1989:308). Adiabatic processes usually happen so fast that there is little time for heat to enter or leave the system. For example, if work is done on a system, the internal energy will increase and the temperature raises and it heats up. If work is done by a system, the internal energy decreases and it cools (Hewitt, 1989:308).

Gibbs free energy equation calculates the process that is driven spontaneously toward equilibrium by decreasing energy and increasing randomness (Brown, et. al., 1991:683). The DG will determine the maximum amount of energy that a system, such as an incinerator, will be able to do work. "As a reaction proceeds, its capacity to perform work, as measured by G, diminishes until finally equilibrium is reached and the system can no longer supply work" (Brady, et. al., 1982:386).

THERMODYNAMIC EQUATIONS
The standard heat of reaction is the change in enthalpy that occurs during a reaction when the reactants and the products of the reaction are at the same temperature. When the tempurature changes (i.e. combustion), the heat of reaction also changes. The equation is given by:

dDH = DCp dT..................................................……….(1)

where
dDH = the change in the heat of reaction
DCp = the change in the heat capacity
dT = The change in temperature
Integrating both sides of equation 1 gives the equation

DHT = DH298 + DCpmh(T - 298)......................................(2)

where
DHT = the heat of reaction at tempurature T
DCpmh = the change in the mean value heat capacity
T = the final temperature


The final or flame tempurature can now be found by rearranging equation (2) to solve for T. DCpmh in equation (2) is also dependent on final tempurature, therefore iterations will need to be carried out in order to obtain a value for T.

The equilibrium constant K can be found using the following relationship:

Ln K = -DG/RT

where
K = equilibrium constant at tempurature T
DG = The change in Gibbs free energy
R = Ideal gas equation constant
T = the final temperature


REACT!
The program REACT! was developed primarily to perform chemical equilibria equations. REACT! has many functions such as; exploring equilibrium compositions, exploring various constraints such as temperature and pressure, verifying LeChatelier's principle, computing adiabatic flame temperatures, balancing chemical reactions and calculating equilibrium constants (REACT!:Help). This program eliminates the time consuming process of hand calculations of the formulas and equations previously mentioned. In addition to computing the equilibrium state for selected chemicals and thermodynamic equations, REACT! will determine the adiabatic flame temperature as needed for incineration problems.

With REACT!, chemicals can be selected as reactants, and constraints such as pressure and temperature can be applied to solve thermodynamic equations and form products. In order for the program to solve equations, trace components of chemicals need to be added as products. REACT! will then calculate the equilibrium state (K) and a set of independent reactions as well as any other reaction which may occur. "REACT! operates by minimizing the Gibbs free energy of a set of chemical species" (REACT!:Help). As an end result, REACT! gives a balanced equation, final temperature, final pressure, the final moles, total enthalpy change and the equilibrium constant (K).

REACT! has thermodynamic limitations such as: only gases and solids are allowed, solid species are immiscible so that each solid species is a phase, and all gases are ideal. In addition, no more than 20 chemicals or 10 elemental species are allowed (REACT!:Help). There is a list of chemicals available for selection and chemicals can be added.

Thermodynamic data in REACT! that are stored for each chemical are described as follows (REACT!: Help):


To experiment with this program, various equations were run on REACT! to determine fluctuations in adiabatic flame temperatures and equilibrium constants. For example, a chemical equation in which Toluene was burned (with pure oxygen) resulted in a final temperature of 5581.58 K and an equilibrium constant of 3.515 X 1047. The burning of Toluene (CH3C6H5) resulted in extremely high temperatures. The reactants and products of this equation, and the constraints ,were taken from LeGrega, et. al., Example 12-1, and are as follows (1994: 707):

CH3C6H5 + 9O2 Þ 7CO2 + 4H2O


The constraints used in this equation, which were stated in the sample problem, were a temperature of 60 degrees F, 1 atm of pressure, with adiabatic and isobaric features. See Sample Problem #1 in Attachment A.

Sample Problem #2 (Attachment B) burns Toluene with air (O2/N2). The constraints are set as they are for Sample Problem #1, however in this equation, N2, as an inert constituent, goes out completely, resulting in a lower final temperature than that of Sample Problem #1.

In Sample Problem #3 (Attachment C), Methane is burned with pure oxygen with the same constraints as the first two problems. In this equation, the final temperature remains constant and there is a negative enthalpy change as well as a very small equilibrium constant. In contrast to the first two problems, this suggests that there is no heat generation, rather a net loss of heat from this equation and that the reaction is spontaneous and fast.

CONCLUSION
To get the best combustion equation, excess air is necessary for complete oxidization of the chemical reaction. When the Gibbs free energy is maximized, and the reaction reaches equilibrium, there is no more energy to be expended, and the reaction is complete. Using REACT! is a fast and convenient way of finding chemical equilibriums and adiabatic flame temperatures for incineration problems and allows experiments with fluctuating conditions such as temperature and pressure to determine efficient burning of hazardous waste.

REFERENCES

Brady, James E and Humiston, Gerard E. 1982. General Chemistry. Principles and Structures. Third Edition. John Wiley & Sons. New York.

Brown, Theodore L., LeMay, H. Eugene Jr. And Burston, Bruce E. 1991. Chemistry. The Central Science. Prentice Hall, Englewood Cliffs, NJ.

Hewitt, Paul G. 1989. Conceptual Physics. Sixth Edition. Scott, Foresman and Company. Glenview, Illinois.

LaGrega, Michael D., Buckingham, Phillip L., and Evans, Jeffrey C. 1994. Hazardous Waste Management. McGraw-Hill, Inc. New York

React 2.5. A Chemical Equilibrium Calculator. CopyrightÓ 1992 by UnitOOPS Software, P.O. Box 4422, Stamford, CT 06907-0422, U.S.A. All Rights Reserved.

Smith, J.M. and Van Ness, H.C. 1987. Introduction to Chemical Engineering Thermodynamics. McGraw-Hill, Inc. New York.

Tchobanoglous, George, Theisen, Hilary, and Vigil, Samuel. 1993. Integrated Solid Waste Management. Engineering Principles and Management Issues. McGraw-Hill, Inc. New York.

Attachment A
Sample Problem #1

Burning Toluene with Oxygen

Initial temperature = 288.00 [K] ; Final temperature = 5581.58 [K]
Initial pressure = 1.00 [atm]; Final pressure = 1.00 [atm]

Chemical   Initial moles   Final moles   Final y

CH_3C_6H_5 (g)   1.000E+0000   1.111E-0001   1.124E-0002
CO_2 (g)   1.000E-0008   6.222E+0000   6.292E-0001
H_2O (g)   1.000E-0008   3.556E+0000   3.595E-0001
O_2 (g)   8.000E+0000   3.758E-0005   3.800E-0006

Total (gas only)   9.000E+0000   9.889E+0000   1.000E+0000


Total enthalpy change = 2.923E-0001 [J]

There is 1 independent chemical reaction.
There are 3 independent chemical species.

Rxn # 1: CH_3C_6H_5 (g) + 9 O_2 (g) = 7 CO_2 (g) + 4 H_2O (g) ; K(5581.58) = 3.515E+0047


Attachment B
Sample Problem # 2

Burning Toluene with Air (O2/N2)
Initial temperature = 288.00 [K] ; Final temperature = 3771.44 [K]
Initial pressure = 1.00 [atm]; Final pressure = 1.00 [atm]

Chemical   Initial moles   Final moles   Final y

CH_3C_6H_5 (g)   1.000E+0000   4.444E-0001   3.846E-0002
CO_2 (g)   1.000E-0008   3.889E+0000   3.365E-0001
H_2O (g)   1.000E-0008   2.222E+0000   1.923E-0001
N_2 (g)   5.000E+0000   5.000E+0000   4.327E-0001
O_2 (g)   5.000E+0000   1.444E-0006   1.249E-0007

Total (gas only)   1.100E+0001   1.156E+0001   1.000E+0000

Total enthalpy change = 1.624E-0002 [J]

There is 1 independent chemical reaction.
There are 4 independent chemical species.

Rxn # 1: CH_3C_6H_5 (g) + 9 O_2 (g) = 7 CO_2 (g) + 4 H_2O (g) ; K(3771.44) = 2.347E+0057

Attachment C
Sample Problem #3

Burning Methane with Pure Oxygen

Initial temperature = 288.00 [K] ; Final temperature = 288.00 [K]
Initial pressure = 1.00 [atm]; Final pressure = 1.00 [atm]

Chemical   Initial moles   Final moles   Final y
CH_3C_6H_5 (g)   1.000E+0000   1.000E+0000   3.333E-0001
CH_4 (g)   1.000E-0008   ~0.0   <= 3.330E-0021
H_2O (g)   1.000E-0008   2.429E-0008   8.095E-0009
O_2 (g)   2.000E+0000   2.000E+0000   6.667E-0001

Total (gas only)   3.000E+0000   3.000E+0000   1.000E+0000

Total enthalpy change = -2.638E-0003 [J]

There is 1 independent chemical reaction.
There are 3 independent chemical species.

Rxn # 1: CH_3C_6H_5 (g) + 10 H_2O (g) = 7 CH_4 (g) + 5 O_2 (g) ; K(288) = <1.00000E-38