Chemistry of inorganic compounds – Sulfur or compound thereof – Oxygen containing
Reexamination Certificate
1997-01-24
2003-06-03
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Sulfur or compound thereof
Oxygen containing
C423S522000, C423S533000, C423S543000, C422S110000, C422S129000, C422S160000, C422S161000, C422S198000, C422S198000, C422S198000, C422S200000
Reexamination Certificate
active
06572835
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to chemical processing methods and equipment and more particularly to a method and apparatus for producing gaseous sulfur trioxide.
Gaseous sulfur trioxide (SO
3
) has many uses. In one such use, gaseous sulfur trioxide is reacted with an organic reactant (e.g. alkyl benzene) to produce a sulfonate which is used to make detergents. Gaseous sulfur trioxide is also used to condition flue gas (e.g. from power generating boilers) to facilitate the removal of fly ash from the flue gas.
Typically, SO
3
is produced by reacting sulfur and air in a sulfur burner to produce a first mixture consisting essentially of sulfur dioxide (SO
2
) and air. This first gaseous mixture is then flowed into a catalytic converter where the SO
2
in the first mixture is converted to SO
3
to produce a second mixture consisting essentially of SO
3
and air which is withdrawn from the converter and directed to a location where the SO
3
in the second mixture is reacted with an organic reactant to produce a sulfonate (in one example of a use) or where the SO
3
is used to condition flue gas to facilitate the removal of fly ash (in another example).
There is a temperature range (e.g. 780-850° F. (416-454° C.)) which is favorable to initiate the catalytic conversion of SO
2
to SO
3
. When the temperature of SO
2
in the first mixture is either above or below this temperature range, it is difficult if not impossible to initiate the catalytic conversion of SO
2
to SO
3
. Generally, the first mixture (SO
2
and air) has a temperature above the favorable temperature range when the first mixture exits the sulfur burner. As a result, the first mixture is conventionally subjected to cooling between the sulfur burner and the converter. Cooling is typically accomplished by flowing the first mixture through either a radiant cooler or a heat exchanger, for example. A mixture of SO
2
and air which has been thus cooled enters the converter at a temperature within the range favorable for initiating the conversion of SO
2
to SO
3
.
The minimum temperature for initiating catalytic conversion of SO
2
to SO
3
(the threshold or ignition temperature) varies with the catalyzing agent employed in the conversion process and can be in the range 380 to 420° C. (715-788° F.), for example. Once the conversion reaction is initiated (ignition), it can be sustained at temperatures which may drop below the ignition temperature.
The conversion of SO
2
to SO
3
is an equilibrium reaction (SO
2
+½O
2
⇄SO
3
). In a typical commercial process, the oxygen required to convert SO
2
to SO
3
is provided by the air in the first mixture (SO
2
and air). The percentage of SO
2
which can be converted to SO
3
varies with temperature and with the concentration (partial pressure) of the gaseous initial reactants (SO
2
and O
2
). The lower the temperature in the temperature range at which the conversion reaction occurs, the greater the conversion of SO
2
to SO
3
. For a given concentration of reactants and assuming the conversion reaction proceeds to equilibrium, there is a theoretical conversion percentage of SO
2
to SO
3
at each temperature within the range at which conversion can be sustained. The conversion temperature range has maximum and minimum temperatures. Maximum theoretical conversion occurs at the minimum temperature at which conversion can be sustained. Depending upon the concentration of the reactants, maximum theoretical conversion can be 99% or more, at a minimum sustaining temperature of 400° C. (752° F.), for example. In conventional commercial processes, the actual conversion percentage (yield) is usually an approximation of the theoretical conversion percentage, i.e., slightly below the theoretical conversion percentage; the closeness of the approximation is influenced by a variety of parameters such as gas distribution in the porous bed containing the catalyzing agent, gas velocity through that bed, and the activity of the catalyzing agent.
As noted above, there is a maximum temperature at which the conversion reaction can be sustained, and the maximum sustaining temperature decreases as the conversion percentage increases. For example, depending upon the concentration of the initial reactants, at a temperature of about 600° C. (1112° F.) the conversion reaction reaches equilibrium when the theoretical SO
3
percentage is about 70%; a lower temperature, e.g., about 480° C. (896° F.) or below, may be required to obtain a theoretical conversion of 95%, and a temperature of about 400° C. (752° F.) may be required to obtain a theoretical conversion of 99%. An example of the concentrations of reactants, for achieving the results described in the preceding part of this paragraph, comprises about 10.5 vol. % SO
2
and 10.4 vol. % O
2
. Generally, at a given temperature, the theoretical conversion percentage increases as the initial SO
2
percentage decreases and the initial O
2
percentage increases.
The conversion of SO
2
to SO
3
is an exothermic reaction which generates a substantial amount of heat in turn raising the temperature of the gases flowing through the converter to a temperature close to or above the temperature at which conversion can be sustained. In addition, as the conversion reaction proceeds, the percentage of SO
3
in the gaseous stream increases, in turn requiring a decrease in the temperature of the gaseous stream in order for further conversion to occur. These two factors, i.e., increasing temperature and increasing SO
3
percentage, require cooling of the gaseous stream in order to further increase the percentage of SO
3
in the gaseous stream.
Therefore, in order to convert all or substantially all of the SO
2
to SO
3
, it has been conventional to conduct commercial conversion processes in two or more conversion stages with the partially converted gaseous mixture from one stage being subjected to cooling between that stage and the next stage. Typically, cooling has been accomplished by flowing the partially converted gaseous mixture through either a radiant cooler or a heat exchanger located outside of the converter vessel. Alternatively, the partially converted mixture is diluted between stages with a cooling fluid, such as cool air, which, in addition to cooling the partially converted gaseous mixture, necessarily reduces the concentration of SO
2
and SO
3
in the partially converted gaseous mixture and increases its volume.
Cooling between stages reduces the temperature of the gaseous stream to a temperature at which catalytic conversion can be initiated and then sustained for awhile, keeping in mind that as conversion once again proceeds, the temperature of, and the percentage of SO
3
in, the gaseous stream both increase, eventually again producing impediments to further conversion, as described above.
A converter employing two conversion stages together with a single cooling stage therebetween can, under appropriate circumstances, convert up to about 97% of the SO
2
to SO
3
. A gaseous mixture in which up to about 97% of the SO
2
has been converted to SO
3
is acceptable for use in the conditioning of flue gas. However, when the SO
3
is to be employed as a sulfonating agent, it is oftentimes desirable to employ a gaseous mixture in which 99% (or more) of the SO
2
has been converted to SO
3
. In such a case, the converter employs three conversion stages (or more) with a cooling stage between the first and second conversion stages and another cooling stage between the second and third conversion stages, etc.
A gaseous mixture consisting essentially of air and SO
3
is usually cooled after it exits the converter and before the SO
3
therein is employed as a sulfonating agent. Typically, the gaseous mixture exiting the converter would not be cooled, to any substantial extent, when the SO
3
is employed as a flue gas conditioning agent.
An example of a conventional process for producing SO
3
for use as a sulfonating agent is described in UK published patent application GB 2 088 350 A. An example of a conven
Chittenden John C.
Jessup Walter A.
MacArthur Brian W.
Bos Steven
Marshall Gerstein & Borun
The Chemithon Corporation
Vanoy Timothy C
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