Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Sulfur or sulfur containing component
Reexamination Certificate
2000-10-03
2003-09-09
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Sulfur or sulfur containing component
C423S522000, C502S180000, C502S514000, C502S527190
Reexamination Certificate
active
06616905
ABSTRACT:
TECHNICAL FIELD
This invention relates to an active carbon catalyst for recovering and removing sulfur oxides contained in flue gas after transforming them into sulfuric acid by catalytic oxidation and also to a method of flue gas desulfurization by means of such an active carbon catalyst.
BACKGROUND ART
Methods are known for catalytically oxidizing sulfur dioxide gas contained in flue gas in the presence of a catalyst and oxygen at low temperature to eventually turn them into sulfuric acid and recovering the obtained sulfuric acid. Active carbon is the catalyst that is most popularly used with such methods. This is because, if a catalyst comprising ceramic type carriers such as alumina, silica, titania and/or zeolite is used, it does not provide a sufficient level of activity and hence catalytic components such as a metal or a metal oxide have to be carried on it but such catalytic components are prone to be attacked by sulfuric acid generated as reaction product and become dissolved or transformed to lose their catalytic effect so that it is highly difficult to make them stably remain catalytically active for a prolonged period of time. Active carbon, on the other hand, shows a substantive level of activity without carrying catalytic components such as a metal or a metal oxide and the level of activity is maintained for a prolonged period of time so that it is substantially free from the above identified problem.
However, from the viewpoint of using active carbon in a flue gas desulfurization plant running on a commercial basis, commercially available active carbon does not necessarily always maintain a high level of activity and therefore a large volume of active carbon will have to be supplied to constantly achieve the intended desulfurization efficiency. Thus, the use of active carbon will more often than not be costly if compared with other desulfurization processes such as a wet type flue gas desulfurization process. The reason why active carbon cannot maintain a high level of activity is generally believed to be that, while active carbon intrinsically shows a very high level of activity of adsorbing and oxidizing sulfur dioxide gas (hereinafter simply referred to as “activity”), once sulfur dioxide gas is adsorbed by the surface of active carbon and oxidized in the presence of moisture at low temperature, it absorbs moisture to become dilute sulfuric acid, which by turn covers or closes, if partly, the pores of active carbon to interfere with the diffusion of sulfur dioxide gas and the possible contact thereof with the active sites within active carbon so that consequently the active sites within active carbon will not be fully utilized. Thus, there have been proposed various techniques for fully exploiting the high activity level of active carbon by providing active carbon with water repellency so that the generated dilute sulfuric acid may be quickly expelled from the pores of active carbon to maintain the high activity level thereof.
For instance, there is a report in Chem. Eng. Comm. Vol. 60 (1987), p.253 that the rate constant of the reaction of adsorbing and oxidizing sulfur dioxide gas is tripled by spraying a solution of dispersed polytetrafluoroethylene (PTFE) to active carbon having an average grain diameter of 0.78 mm if PTFE is added by 8 to 20%. Japanese Patent Application Laid-Open No. 59-36531 describes that the effect of active carbon of adsorbing and oxidizing sulfur oxide gas is increased by treating active carbon for water repellency and, more specifically, granular active carbon with a grain size of 5 to 10 mm comes to show a remarkably high activity level as catalyst when it is impregnated with a solution of dispersed PTFE and heat treated at 200° C. for 2 hours if compared with untreated granular active carbon.
DISCLOSURE OF THE INVENTION
The inventors of the present invention conducted an experiment as described below in order to examine the effectiveness of the above known methods for improving the catalytic activity of active carbon. Firstly, according to the known techniques of providing active carbon with water repellency, commercially available granular active carbon having a grain size between 2.8 and 4.0 mm was made to be impregnated with PTFE by spraying or immersion to find that the activity was improved to a certain extent and retained for a prolonged period of time if compared with untreated active carbon. However, the improvement of activity to such an extent is not enough in view of the competition of a process using treated active carbon with other desulfurization processes to be adopted in a flue gas desulfurization plant running on a commercial basis and the inventors realized that a further improvement has to be achieved for the catalytic activity of active carbon.
As a result of additional research efforts, the inventors of the present invention came to find that the catalytic activity of active carbon can be effectively improved by providing only the macropores (minute pores with a diameter greater than 5nm) of active carbon with water repellency. More specifically, they found that the catalytic activity of granular active carbon is improved to a large extent by making the granular active carbon impregnated with polystyrene (PS) particles having a sphere equivalent diameter between 10 and 100 nm as water-repellent substance. However, when particles of fluororesin such as PTFE that is more water-repellent than PS are used, they cannot successfully make macropores of active carbon water-repellent by a known technique of impregnating active carbon with a water-repellent substance and making it carry the latter such as the spraying or immersion technique because commercially available fluororesin particles have a relatively large diameter of 100 nm or more. In order to make clear this fact, the inventors of the present invention prepared an active carbon catalyst by causing commercially available granular active carbon to be impregnated with and carry PTFE by means of the spraying or immersion technique using a PTFE-dispersed solution and then analyzed the fluorine distribution profile of the prepared catalyst by means of EPMA. As a result of the analysis, it was found that PTFE particles had not got to the inside of the granular active carbon and only remained adhering to the outer surface of the granules of active carbon. More specifically, since commercially available granular active carbon practically does not have pores with a diameter greater than 1 &mgr;m, it is highly difficult for PTFE particles with a diameter between 0.2 and 0.4 &mgr;m to enter any of the pores of commercially available active carbon. The result of experiment was similar when the PTFE-dispersed solution was replaced by a solution containing PS particles with an average particle diameter of 0.3 &mgr;m in a dispersed state. When the two active carbon catalysts containing respectively the two different types of water-repellent particles were used to test the activity, it was found that the one carrying PTFE particles was slightly more active than the one carrying PS particles, although neither of them did not show the expected level of activity.
The inventors of the present invention further looked into the macropore diameter of active carbon that can most improve the activity of active carbon when the latter is processed for water repellency. Firstly, five different specimens of latex (obtained by dispersing PS particles of relatively similar sizes into water by about 10 wt %) with respective average particle diameters of 10, 28, 55, 102 and 300 nm were prepared. Then, they were diluted to different concentrations between 0.1 and 5 wt % and different granular active carbon samples were immersed respectively into the obtained latex specimens and subsequently dried under reduced pressure to produce so many different active carbon catalysts. As a result, it was found that, among the processed active carbon catalysts, those with PS added by about 1 wt % showed the highest activity regardless of the average diameter of PS particles and that those carrying PS w
Asaoka Sachio
Kawamura Kazushige
Nishijima Hiroaki
Sonehara Naonori
Takeda Dai
Armstrong Westerman & Hattori, LLP.
Bos Steven
Chiyoda Corporation
Vanoy Timothy C.
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