Method and apparatus for purifying exhaust air of a dryer apparatus

Abstract

A method and apparatus for dryers for products containing volatile hydrocarbons such as impregnated hard board, electrical insulation having insulating varnish, aluminum sheets coated with varnish, etc.; the products being dried normally producing, during drying, gaseous hydrocarbons which, for ecological reasons, cannot be discharged directly into the atmosphere but which must be "treated;" the method and apparatus concerning a two-sectioned dryer in series in which the material being dried, passes from section to section; in the first section the more volatile hydrocarbons are driven off and are "after-burned" with the heat of combustion being recovered and used to heat the dryer sections; the second section having less volatile hydrocarbons driven off which are "washed" or condensed; the hydrocarbons precipitating out during "washing" being used as a fuel to assist in the "after-burning" of the more volatile hydrocarbons; the washed exhaust from the second section and exhaust gases from "after-burning" of the more volatile hydrocarbons being combined after washing and after-burning for dehumidification of the washed exhaust and discharged directly into the atmosphere.

Description

FIELD OF THE INVENTION

The instant invention concerns method and apparatus for purifying the exhaust gases and by-products from a dryer apparatus for products moving therethrough, wherein said products contain light and heavy hydrocarbons which become volatile during the drying phase and are incinerated before their exit into the atmosphere.

BACKGROUND OF THE INVENTION

In dryers for such type of products, the drying air must be retained at a predetermined temperature, for example at 160.degree.C, so that the light and heavy hydrocarbons become volatile and, mixed with the air, can be exhausted as spent or used air. The heated gases which are drawn from the dryer are replaced by a corresponding amount of fresh air, which must be heated to the predetermined temperature; this requires a continuous supplying of heat. The fresh air is drawn into a suspension dryer through the dryer inlet and outlet for the products moving therethrough and through non-sealed points of the suspension dryer by an extractor-fan or vacuum blower.

In the drying of the aforementioned products of the character involved, there exist certain regulations for the environmental protection under which the volatilized hydrocarbons must be rendered almost completely safe by utilizing an after-burning of the dryer exhaust air.

A first solution for partial recovery of the drying heat, and to render the volatilized hydrocarbons safe by means of "after-burning," consists of the system shown diagramatically in FIG. 1, the exhaust gases of the dryer are entirely subjected to after-burning and a part of the flue-gas which has been heated for example up to 800.degree.C, is directed back into the dryer for the purpose of heating the dryer-atmosphere, while the remaining part of the flue-gas is emitted into the open atmosphere.

This method is impractical for a multitude of reasons: firstly, a large part of heat is lost with the flue-gas which is being discharged into the free atmosphere; secondly, the installation required for such a process is costly since on one hand the large volumes, large-column, long flue-gas pipes and the large valves must be heat-resistant on account of the high temperatures of the flue-gas and must therefore be manufactured from expensive material, such as, for example, chrome-nickel steel, and on the other hand, it is difficult to retain the under-pressure in the dryer which is required for a satisfactory suspension of the products due to the flue-gas supplied in addition to the fresh-air and the additional sealing points at the connections of the flue-gas conduits which are distributed throughout the dryer.

Another system to render the polluted drying air safe for the environment, and to partially recover the heat with a less expensive installation, consists in the system shown in FIG. 2; wherein the flue-gas of the entire dryer is subjected to an after-burning phase and the thus produced flue-gas is directed into the open atmosphere through an economizer, whereby the flue-gas heats a heat-carrier in an economizer, for example, a heating-oil, which is supplied to the dryer, in order to retain its oven-temperature at the desired temperature level. This method, due to the low temperature of, for example 300.degree.C, does not require conduits of a high heat-resistant steel and ordinary steel products suffice. On the basis of this special heat-recovery, the difficulties in retaining the required under-pressure in the dryer do not exist since, in addition to the fresh air, no flue-gas is directed into the dryer; however, in this type of heat-recovery, the heat-loss is still too high. Additionally, in such type of heat-recovery there exists the danger that the heating-oil in the economizer may become overheated due to the flue-gas being heated to approximately 800.degree.C.

A further system to render the flue-gases safe for the environment and to more efficiently recover more heat than in the two above-described systems of FIGS. 1 and 2, whereby the costs of the installation can be minimized, and whereby difficulties with the retention of the reduced pressure in the dryer can be avoided, consists of the system of FIG. 3 in that all of the flue-gas from the dryer is directed to the apparatus for an after-burning process by means of a recuperator, whereby the flue-gases first are directed through the recuperator and then through an economizer for a heat-carrier (thermo-oil) then the flue-gases are exhuasted into the free atmosphere. As noted in the above example, there exists also in this method the danger that the heating-oil, which is being utilized as the heat-carrier, and which is supplied to the dryer for heating its atmosphere, may be overheated. In contrast to the earlier mentioned example, this method has, however, the advantage that the heat is more efficiently recouped, however, the higher costs for such an installation because of the recuperator must be taken into consideration.

SUMMARY OF THE INVENTION

It is the scope of the instant invention to provide a method and apparatus of the above-mentioned type in which, under low expenditures, for the installation, a reduced amount of heat compared to the amount of the above-described methods, is required in order to purify the exhuasted air.

This problem is solved by the instant invention in that only the flue-gas from a first section of the dryer in the path of the products moving therethrough are immediately "after-burned," while the flue-gas from the second section of the dryer is washed and the residue or precipitate from this washing phase together with the burnable materials is burned off with the flue-gas of the first section of the dryer.

In this method, surprisingly, it is possible to remove and burn approximately 95 percent of light hydrocarbons from the first section of the dryer, while approximately 95 percent of the heavy hydrocarbons from the second section of the dryer are removed and burned. The inventive method distinguishes over the prior art with regard to heat-balance (thermal balance) as well as with regard to the expenditure for the installation. Since the flue-gas is after-burned immediately from the first section of the dryer, it is possible to maintain the installation on a smaller scale for the after-burning process as well as for the heat-exchanger installed for reheating the dryer air, in contrast to expenditures of prior art methods in which all of the flue-gases of the dryer are after-burned. Even though the inventive method requires a purifyer or wash column for the flue-gas from the second section of the dryer, the costs are by far not as high as the additional costs for a device for the after-burning and the heat-exchanger, which must process all of the flue-gases emitted from the dryer apparatus. The total heat- or thermal-balance in the inventive method compared with the prior art methods is improved, first since the flue-gas from the first section contains more volatile (readily evaporated) concentrations up to the permissible limits so that in the after-burning phase only a small amount of supplemental fuel-gas is required; in general, the light and heavy hydrocarbons deliver the heat for the reheating of the dryer, and secondly since only a part of the column of the exhaust gases received from the dryer will have to pass as flue-gas, a drop in temperature (or heat gradient) twice the size in comparison to the prior art methods, since it has to deliver the heat which is required for the first and the second sections of the dryer.

Since the washed spent air has a high degree of humidity, it is advantageous to mix the after-burned exhaust from the economizer with the spent air which has been washed in the wash column before exhausting into the open atmosphere so that the entire flue-gas/used-air mixture is dehumidified.

Preferably, heating-oil is utilized as the heat-carrier.

An installation for performing the preferred method comprises a dryer which is divided into two adjacently-arranged chambers disposed in the direction the products to be dried therein are moved, whereby the first chamber of the dryer is connected, by means of an exhaust conduit, with a device for after-burning phase, the conduit being series-connected, for the purpose of removing the after-burned spent air, with a heat-exchanger for the purpose of reheating the product dryer, and the second chamber having a further exhaust conduit which is connected with a wash-column in which the washed-out heavy or less volatile hydrocarbons from the sump of which, via a conduit, the washed out heavy hydrocarbons are supplied to a burner device used for the purpose of after-burning the more volatile exhaust from the first section of the dryer.

Embodiments of such installations consist in that the heat-exchanger is an economizer using heating-oil as a heat-carrier, whereby the device for the after-burning process is provided with means for atomizing the heavier hydrocarbons which are extracted from the sump of the wash-column, wherein the device for the after-burning is provided with auxiliary burners, and wherein the outlet of the heat-exchanger for the after-burned spent air and the outlet gas of the wash column for the washed spent exhaust gases terminate into a common flue-gas conduit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of one proposed dryer installation in which flue-gas is directed back to the dryer section;

FIG. 2 is a schematic view of another dryer installation in which an economizer is provided between the after-burner and flue-gas outlet and in which a heating oil is used to recover the heat from after-burning of the dryer byproducts or exhaust for heating the dryer;

FIG. 3 is a schematic view showing still another proposed installation of an after-burning system utilizing a recuperator intermediate and after-burner and economizer for treating the spent air of the dryer; and

FIG. 4 is a schematic view of the preferred embodiment of the after-burning system of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The instant invention is explained hereinbelow by means of a schematic illustration shown in FIG. 4. A suspension dryer 1 is divided into a first section and a second section disposed in series in the direction of the path 2 for the products to be dried therein, for example, impregnated hard carboard, electrical insulating material comprising base-material and insulating varnish, varnished aluminum foil (sheets) etc. The dividing of the dryer 1 into a first and a second section is accomplished in such a way so that in the area of the first section, at temperatures of about 130.degree.C, approximately 95 percent of the light or more volatile hydrocarbons are driven off, while in the second section at temperatures of about 150.degree.C approximately 95 percent of the heavy or less volatile hydrocarbons become volatile and are driven off. A mixing of the dryer air from both sections of the dryer (which occurs with regard to the concentration of the hydrocarbons in both parts of the air of the dryer) generally does not occur since a trnsformation of the dryer air occurs zonally.

On one hand, the dryer must be provided with openings for the path-entry and path-exit for the purpose of the suspended guiding of the products, and on the other hand there should not exit from the opening the dryer-air which is mixed with volatiles, the dryer is maintained under low pressure. Fresh air is continuoursly supplied to the dryer by means of the suspended jets or nozzles (not shown) which are necessary for the suspended transport of the products. In order to enable the retention of the under-pressure, spent air is continuously removed or drawn from the first and the second section and is directed to an after-burning device 3. In addition to that, the heavy hydrocarbons are sprayed as a fuel into the after-burner device 3 by means of an atomizing nozzle 4. In order to permit after-burning, auxiliary burners 5 are provided through which supplemental fuel which is supplied to the device 3 for after-burning exhaust dryer gases. The portion of the extraneous fuels must be sufficient to ignite and retain the burning of the light and heavy hydrocarbons in the device 3. At an exotherm of about 400.degree.C of the spent air from the first section, it is necessary to supply enough supplemental fuel in order to heat the exhaust air to an average of about 400.degree.C. The light hydrocarbons and the heavy hydrocarbons which are contained in the exhaust air of the dryer are then heating the exhaust since they burn at about 800.degree.C. The flue-gases which are heated to this temperature level are taken from the device 3 and are directed through an economizer 6 to which they transfer a large part of their heat to a heat-carrier, for example heating oil, which is circulated via a conduit system 7 to the first and the second section of the dryer 1 to thus maintain the dryer-section atmosphere at predetermined drying temperature levels.

The exhaust gases sucked from the second section of the dryer 1, containing the heavy or less volatile hydrocarbons, are moved through a wash column 8 in which the heavy hydrocarbons are extracted or precipitated by means of water which is sprayed thereinto. From a sump 9 of the wash column 8, the liquified heavy hydrocarbons, as explained, are drained off in the form of a water slurry and supplied to the after-burner device 3 through nozzle 4 where they are burned. The cleaned humid exhaust gas of the wash column 8 passes into a mutual flue-gas canal 10, joining the dry flue-gases which exit from the economizer 6, whereafter they exit as dehumidified dry flue-gas mixture into the open atmosphere.

SUMMARY

The dryer utilizes the principal in FIg. 4 of the fact that certain hydrocarbons become volatile at about 130.degree.C and are drawn off in the first drying section; accordingly, less heat is required in this section to remove the more volatile byproducts of the drying process; in the second section, the product is subjected to about 160.degree.C at which the heavier hydrocarbons become volatile. The exhaust from the second section is "washed " where the heavier carbons precipitate to a sump and are pumped to a fuel nozzle to be used as a fuel supplementing the fuel at burner 5 in the after-burner 3. The heat produced at the after-burner 3 is recovered at the economizer 6 which includes a circulating heat-absorbing medium which is redirected back to the dryer for heating the respective first and second sections of the dryer; the washed exhaust from washer 8 is combined with the exhaust from the economizer 6 for dehumidification and passes from outlet 10 to the atmosphere and comprises afterburned exhaust (of the lighter hydrocarbons) and washed exhaust (of the heavier hydrocarbons) from respective dryer sections 1 and 2.

Claims

What is claimed is:

1. A method for purifying the exhaust products from drying apparatus for products containing light (low-boiling) and heavy (high-boiling) hydrocarbons which become volative during drying of the products and which are burned in a flame prior to being exhausted into the atmosphere, comprising:

moving the product to be dried through two consecutive sections of a dryer into contact with a heating medium at a temperature so that light (low-boiling) hydrocarbons are first volatilized mainly in the first section and in which heavy (high-boiling) hydrocarbons are volatilized mainly in the second section;

directly after-burning the volative exhaust products of the first section;

and washing the volatile exhaust products of the second section, producing a residue including heavy hydrocarbons and directing the residue together with burnable material of burned exhaust products of the first section and burning it with the residue.

2. The method according to claim 1 including the step of supplying sufficient supplemental fuel to the after-burning for igniting and maintaining continuous after-burning of both the light hydrocarbons and the residue including heavy hydrocarbons.

3. The method according to claim 1 including mixing the exhaust products from the second section, purified by washing, with afterburned exhaust products and emitting them as a mixture into the atmosphere.

4. The method according to claim 2 including heating in a heat-exchanger a heat-absorbing medium by the after-burner and directing the heated heat-absorbing medium to the respective dryer sections for treating the product being dried as it successively passes through the first and second sections of the dryer.

5. Apparatus for purifying the exhaust air from a dryer containing volatile hydrocarbons comprising:

a dryer divided in consecutive chambers through which a product being dried is moved;

means in said chambers for passing heated air over said product;

individual exhaust conduit means connected to the respective chambers for drawing off exhaust air containing light (low-boiling) hydrocarbons and heavy (high-boiling) hydrocarbons as they become volatile in the respective chambers;

after-burning means connected to the conduit receiving the light hydrocarbons for burning the same; and

washing means connected to the conduit receiving the volatile heavy hydrocarbons for precipitating a residue including heavy hydrocarbons.

6. The apparatus as claimed in claim 5 including heat-exchanger means connected in series with the after-burner means for recovering heat produced at the after-burner means; and

means for directing the recovered heat to the respective dryer chambers.

7. The apparatus as claimed in claim 5 including sump means operatively connected to said wash means for recovering the precipitated residue including heavy hydrocarbons; and means connected between said sump means and said after-burner means for burning the precipitated residue including heavy hydrocarbons and said light hydrocarbons together in the after-burner means.

8. The apparatus as claimed in claim 6 in which said heat-exchanger means includes a heat-carrier comprising a heat-absorbing fluid, and means for circulating the heat-absorbing fluid through the heat-exchanger means and about said dryer chambers.

9. The apparatus as claimed in claim 7 including nozzle means connected to the after-burner means and said sump means for directing the precipitated residue including hydrocarbons into the after-burner means.

10. The apparatus as claimed in claim 6 including auxiliary burners in said after-burner means for providing supplemental igniting fuel to maintain continuous operation of said after-burner means.

11. The apparatus as claimed in claim 6 including exhaust gas conduits each respectively connected to one of the washing means and said heat-exchanger means, and a common flue-gas conduit joining the respective exhaust conduits whereby the wet exhaust gas from the washing means and causes products passing through said heat exchanger means are mixed prior to discharge into the atmosphere.