Abstract:
A method of desulphurization of a flue gas comprising passing a flue gas to a plurality of adsorption beds each having an activated carbon layer packed therein thereby separating a sulphur oxide contained in the flue gas by adsorption of the sulphur oxide in the activated carbon; continuing the adsorption until the concentration of sulphur oxide acid in the activated carbon reaches a predetermined value; providing a liquid for regenerating the activated carbon with at least one of the adsorption beds in order to wash down the activated carbon layer, wherein the introduction of the liquid into the adsorption bed to be regenerated is carried out while keeping all the passageways for introducing the flue gas into the adsorption beds open, thereby reducing a flow amount of flue gas in the adsorption bed to be regenerated so as to suppress adsorption of the sulphur oxide therein and to regenerate the adsorption bed. The supply of the regenerating liquid to each of the adsorption beds is repeatedly carried out in accordance with a predetermined time schedule.

Description:
This application is a continuation-in-part of Ser. No. 872,286, filed Oct. 29, 1969, and now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method of desulphurization of a flue gas exhausted from boilers, industrial furnaces and the like and more particularly to a method of desulphurization of a flue gas which enables the operation of a desulphurizing apparatus or plant to be simplified. The present invention also relates to an improved desulphurizing apparatus having a simplified construction and a lowered production cost. 
     With the development of industry, the quantity of harmful gases, especially, sulphurous acid (SO 2 ) and sulphuric acid anhydride (SO 3 ) contained in the flue gas from a furnace in which sulphur-containing coal or heavy oil is burnt or in the flue gas from a sulphuric acid production plant, exhausted into the atmosphere along with the flue gas from various industrial facilities, is increasing and hence the air pollution in the vicinity of the facilities is increasing. As a result, the harmful effect of such gases on the inhabitants in the area of the facility is becoming a great social problem in recent years, and development of various types of flue gas treating apparatus is urgently desired. 
     As one such exhaust gas treating apparatus, an apparatus has been developed in which the sulphur oxides (SO 2  and SO 3 ) are separated and removed by passing the exhaust gas through an adsorption bed packed with activated carbon. The general techniques used in apparatus are known from U.S. Pat. No. 2,992,065, U.S. Pat. No. 2,992,895 and the technical treatise of Mr. F. Johswich, published on page 18 of Combustion, Oct. 1965, under the title of &#34;The Present Status of Flue Gas Desulphurization&#34;. 
     A flue gas treating apparatus of adsorption bed type generally comprises a plurality of adsorption beds each being packed with activated carbon. After the activated carbon has become inactive to adsorb the sulphur oxide, the adsorption beds must be regenerated or recovered by a suitable manner such as washing down the activated carbon. 
     In the conventional regeneration methods a regenerating liquid such as water or a dilute sulphuric acid solution is supplied to an adsorption bed to be regenerated after the flue gas inlets thereof are closed. After the regeneration or washing down of the activated carbon has finished, the flue gas inlets are opened again. 
     Since the operation of the flue gas inlets has inevitably to be performed in a cycle at a certain time interval and in an actual apparatus the valve of the flue gas to be treated is extremely large, the switching operation of the valve means provided in the flue gas inlets requires considerable labor or elaborate control devices. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method of desulphurization of a flue gas containing sulphur oxide wherein operation of the method is simplified and a simplified apparatus is employed. 
     It is another object of the present invention to provide a method of desulphurization of a flue gas which enables the construction of a simplified desulphurizing apparatus and saves production costs. 
     It is still another object of the present invention to provide a method of desulphurization of a flue gas which has an increased desulphurization rate. 
     It is a still further object of the present invention to provide apparatus for desulphurization of a flue gas which has a simplified construction and an improved operability and a low cost. The desulphurization apparatus according to the present invention has an improved maintenance due to its simplified construction. 
     The present invention is based upon the fact that a flow of flue gas in an adsorption bed is effectively regulated by providing a proper amount of a liquid for regenerating activated carbon, whereby, closing and opening of flue gas inlets can be accomplished without operation of a valve means in passageways connected to the inlets. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and other objects and features of the present invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a graph showing the relationship between the velocity of the flue gas passing through an adsorption bed and the velocity of the regenerating liquid flowing down through the adsorption bed; 
     FIG. 2 is a diagram of a desulphurizing apparatus according to the present invention; 
     FIG. 3 is a graph showing the relationship between the flue gas flow rate, the adsorbing quantity of sulphur oxide and sulphuric acid content in the activated carbon layer, and the time with respect to a single adsorption bed in a desulphurizing apparatus of the present invention; 
     FIG. 4 is a table of a time schedule for operating a desulphurizing apparatus of another embodiment; 
     FIG. 5 is a graph showing the relationship between temperatures of flue gases and time with respect to the desulphurizing apparatus shown in FIG. 2; and 
     FIG. 6 is a logarithmic graph showing the relationship between the adsorption rate of sulphurous acid and the relative humidity of the system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As heretofore stated, the present invention provides a method of regulating the flue gas flow in an adsorption bed to be regenerated such that the flue gas flow is intercepted or suppressed by supplying a regenerating liquid such as water or dilute sulphuric acid solution to the adsorption bed. According to this regulation method, the operation of the desulphurizing apparatus will be remarkably simplified since there is no need to operate valve means provided in flue gas inlets for opening and closing the passageways communicated with the inlets. Further, it is a significant advantage of the apparatus according to the present invention that valve means for flue gas inlets, which are made of expensive sulphur oxide-resisting materials would be completely omitted thereby lowering the production cost of the apparatus because the switching operation of the supplying of the flue gas to each adsorption bed can be performed by controlling the amount of regenerating liquid. 
     Referring to FIG. 1, which explains the relationships between velocities of water and flue gas in an adsorption bed, the abscissa is the velocity of water flowing down in an adsorption bed, and the coordinate is the velocity of the flue gas passing upwardly through the bed. In this experiment, the thickness of activated carbon layer in the adsorption bed was 2000 mm and the activated carbon was in the form of pellets of 5 to 8 mm in diameter. 
     As apparent from FIG. 1, when the supply of water has begun, the flow rate of the flue gas passing through the adsorption bed decreases sharply, even with a slight increase in the flow rate of water. When the flow rate of water is 0.1 × 10 -   2  m/s for instance, the flow rate of the flue gas is 0.1 m/s which is about one fifth of that when water is not supplied, and when the flow rate of water is 0.2 × 10 -   2  m/s or higher, the flow rate of the flue gas is only one tenth or less of that when water is not supplied. The flow rate of the flue gas is effectively controlled in accordance with the flow rate of the regenerating liquid. 
     Since the flue gas untreated should be subjected to desulphurization without interruption, the plural adsorption beds are arranged in parallel with respect to the flow of the flue gas, wherein some of the adsorption beds or a single adsorption bed is always subjected to regeneration while the predominant number of adsorption beds are in an adsorption step. 
     As apparent from FIG. 1, the regulation of the flue gas flow can effectively be done by providing at least 0.2 × 10 -   2  m/s of a regenerating liquid. With an increase in an amount of regenerating liquid, the velocity of the flue gas can be made very small. In case of 0.4 × 10 -   2  m/s of the velocity of the regenerating liquid, the velocity of the flue gas is only about 0.02 m/s which is about one thirtieth (1/30) of that when the regenerating liquid is not supplied. Therefore, considering the small amount of the flow of the flue gas passing through an adsorption bed in the regeneration step, it can be said that substantially all of the flue gas to be treated is evenly distributed among the remainder of the adsorption beds. 
     Referring to FIG. 2 illustrating an example of a desulphurizing apparatus according to the present invention, a flue gas containing sulphur oxide (SO 2  and SO 3 ) from a furnace 1 is led through a duct 2 and a dust arrestor 3 for removing dust therefrom to a blower 4 thereby pressurizing the flue gas. The pressurized flue gas is provided to adsorption beds 7a, 7b, 7c through gas passageways 5a, 5b, 5c, respectively. Since water is being fed to the adsorption bed 7a by a distributor 17a, the flow of the flue gas in the bed 7a is intercepted or extremely reduced. Therefore, substantially all the pressurized flue gas is distributed between the beds 7b, 7c. 
     Each adsorption bed is provided with an activated carbon layer 8a, 8b, 8c which is formed on a support such as a grid or lattice (not shown). With reference to one adsorption bed, the activated carbon layer therein is alternatively subjected to adsorption and regeneration in a cycle in a predetermined order and a predetermined time schedule. 
     After the activated carbon layer has been washed down sufficiently to be regenerated, a valve for feeding the regenerating liquid is closed and, subsequently, the drying of the wetted activated carbon starts. Since the drying of the activated carbon, which takes place by vaporization of the regenerating liquid, is easily completed or an amount of the liquid held up by the activated carbon is sufficiently reduced to adsorb the sulphur oxide, in a short period of time, the adsorption of sulphur oxide takes place in all adsorption beds other than that undergoing the regeneration step. 
     The regenerating liquid used is discharged into a drain trap 13 through a drain valve 10a and a drain conduit 10. When ends of the inlets 5a, 5b, 5c are opened opposite to the direction of the flow of a regenerating liquid as shown in FIG. 2, flowing of the regenerating liquid into the inlets is avoided since the inlets and conduit 5 are always pressurized. This means that the apparatus is more simplified in its construction than the conventional apparatus which are provided with valve means for switching the flue gas flow. If the switching is performed by valve means it is not avoidable that a considerable amount of the regenerating liquid enters into the inlets. Therefore, in the conventional apparatus it is necessary to provide a suitable water seal means for the inlets. 
     The flue gas streams leaving the respective adsorption beds are gathered and the flue gas having been desulphurized is exhausted through a mist removing means 18 and a conduit 12 from a chimney 16 at a desired temperature such as 100°C. and sufficiently dispersed into the atmosphere. A part of the flue gas is by-passed by a conduit 6 and it is mixed into the treated gas in a conduit 12 through a valve 11 so as to adjust the final temperature of the flue gas exhausted from chimney 16. 
     A regenerating liquid is supplied into each adsorption bed by a distributor 17a through valves 9a, 9b, 9c and a conduit 9. The liquid is pressurized by a pump 19. In case of desulphurization of a flue gas a dilute sulphuric acid solution produced in the desulphurizing apparatus is more preferable than water. In FIG. 2, drain trap 13 and washing water source 20 are interconnected so that the drainage drawn from the bottoms of the beds through valves 10a, 10b, and 10c is gathered and recycled as a regenerating liquid and additional water is added to the recycle at desired intervals. In view of recycling of the resulting dilute sulphuric acid, the total amount of water used and the capacities of the flow control devices such as pump 19 and other parts of apparatus can be made small. In this method, a suitable facility known per se for recycling the drainage liquid may be used when necessary. It will be understood that recycling is not always necessary. 
     For the sake of better understanding of the present invention, the following examples are presented. 
     EXAMPLE 1 
     In this example experimental data resulting from the operating of the desulphurizing apparatus shown in FIG. 2 are tabulated below: 
     A. Conditions of Operation 
     1. Composition of a flue gas: CO 2  12%, H 2  O 10%, O 2  5%, SO 2  1500 ppm, N 2  remainder, the percentages are all percent by volume. 
     2. Temperature of the flue gas at an inlet: 130°C. 
     3. Flow rate of flue gas: 10,000 m 3  /h. (i.e. 3380 Nm 3  /h per one bed. 
     4. Activated carbon: pellets of 5 to 8 mm in diameter. 
     5. Height of activated carbon layer: 2,000 mm which includes 10 cm of a thickness of a lattice and coaks layer supporting the layer. 
     6. Amount of activated carbon: 5 m 3  /one bed 
     7. Column volume: 1.8 m. in diameter × 4 m. in height. 
     8. Regenerating liquid: Water (20°C.) 
     9. Amount of water supplied to one bed: enough to reduce the flow rate of flue gas to one tenth or less of that of adsorption beds in adsorption step. 
     B. Results Obtained 
     1. Content of sulphur oxide at an outlet of beds 8b and 8c: less than 150 ppm (i.e. 90% of desulphurization rate). 
     2. Content of sulphur oxide at an outlet of a bed in a drying step for about 5-20 minutes just after the supplying of water has been stopped: less than 300 ppm. 
     3. Content of sulphur oxide in the flue gas to be exhausted from the chimney: less than 150 ppm. 
     4. Content of H 2  O in the flue gas at the position in front of the chimney: 12%. 
     5. Pressure drop of the flue gas in each bed in adsorption step: 300 mm Aq. 
     6. Temperature of flue gas at an outlet of each bed: 135°C. 
     Although the temperature of flue gas from the furnace is 130°C., the temperature of flue gas at the outlet of each bed is increased to 135°C. because the adsorption heat is generated in the bed. 
     7. Temperature of flue gas at the position in front of the chimney: 95°C. 
     Since in the apparatus of FIG. 2 there are three beds one of which is always subjected to regeneration and therefore the temperature of flue gas from the bed in drying step is 55°C. (the saturated temperature), the average temperature of flue gas in the conduit 12 is 95°C., as apparent from FIG. 5. 
     EXAMPLE 2 
     FIGS. 3 and 4 illustrate the operation and features of another embodiment of the desulphurizing apparatus according to the present invention, wherein the arrangement of apparatus shown in FIG. 2 is modified by increasing the number of carbon-containing beds and the conditions set forth in Example 1 are employed. In this example, five (5) beds are employed in order to carry out more smoothly the switching operation of flue gas and to avoid an excess decreasing of the temperature of flue gas at the chimney. That is, the above requirements are satisfied by decreasing the volume of flue gas cut off by regeneration from the flue gas passageway. 
     In the operation of this apparatus, as shown in FIG. 4, the regeneration cycle (R) is continued for 12 hours and the drying (D) of a wetted activated carbon layer is accomplished within 12 hours. The adsorption cycle (A) is carried out for 36 hours. But, as can be seen from FIG. 4, the adsorption of sulphur oxide takes place even in the drying step or cycle, since each drying step is accomplished within 12 hours and an amount of activated carbon dried or effective to adsorb the sulphur oxide increases with time. Actually, it is practically impossible to clearly show the critical point between the drying step and adsorption step. Thus it will be recognized that these steps are parts of a continuous process unlike the regeneration step. 
     In any event, the regeneration, drying and adsorption steps in one bed are repeated in accordance with a predetermined time schedule, and it is practical to carry out the operation in such a manner that the regeneration step of each adsorption bed does not superpose that of other adsorption beds. 
     Referring to FIG. 3, curve x represents a change of an amount of flue gas flowing through an adsorption bed, curve y a change of an adsorbing quantity of sulphur oxide in the flue gas exhausted from the adsorption bed, and curve z a change of a sulphuric acid concentration in the activated layer of the adsorption bed. Note that the percentage of curve z is a percentage of an actual sulphuric acid concentration with respect to a predetermined concentration at which the activated carbon should be regenerated. Sulphuric acid is formed in the actual carbon layer by the reaction between SO 2  or SO 3  and O 2  contained in the flue gas and H 2  O as follows: 
     
         SO.sub.2 + 1/2O.sub.2 → SO.sub.3 
    
     
         SO.sub.3 + H.sub.2 O → H.sub.2 SO.sub.4 
    
     From FIG. 3, which shows the changes of three components in one of adsorption beds whose operation is disclosed above, taken in conjunction with FIG. 4, the flow rate of the flue gas is sharply lowered as an increase in an amount of a regenerating liquid. Also, at the same time, the content of sulphuric acid is rapidly reduced as shown by curve z. During this regeneration step, the desulphurization is negligibly small; but as soon as the supplying of liquid is stopped, the flue gas passes through upwardly the adsorption bed so that the drying of the activated carbon takes place. 
     After a valve provided in an inlet of the regenerating liquid supply conduit has been closed; the amount of flue gas flowing out the adsorption bed rapidly increases and it reaches 100% in a short time. This occurs even when the activated carbon has not been dried completely, as shown by curve x. On the other hand, a desulphurization rate is very small during the regeneration step and at the beginning of the drying step as shown by curve y. But, since the adsorption bed can be rapidly recovered to adsorb as soon as the flue gas is fed to the bed, the desulphurization rate reaches 90% or more in a short time. 
     EXAMPLE 3 
     Referring to FIG. 6 which shows the effect of water contained in a flue gas on the adsorption rate in an apparatus of the type shown in FIG. 2, T is a temperature of activated carbon or adsorption temperature, P an amount of water, PS a saturated humidity. A flue gas used in this experiment comprises 2% by volume of SO 2  ; 6% of O 2  and N 2  remainder. According to FIG. 6, it will be understood that the higher the humidity in the flue gas, the higher the adsorption rate becomes. The inventor does not fully understand why the presence of water increases the adsorption. 
     It was found that the proper temperature of flue gas fed to adsorption beds ranges between about 55° and 150°C., more particularly, between about 70° and 130°C. If the temperature is higher than about 150°C., the activated carbon might be deteriorated because the temperature is increased by adsorption-heat; while the activated carbon itself can withstand a temperature of 150°C. On the other hand, it is not practical to use a flue gas of a temperature lower than 55°C., because the adsorption efficiency is not high at such low temperatures as can be seen from FIG. 6.