US3447895A - Method of preventing smelt-water explosions - Google Patents

Description

United States Patent Ofice 3,447,895 Patented June 3, 1969 Int. 'Cl. D21c 11/12 US. Cl. 23-48 6 Claims ABSTRACT OF THE DISCLOSURE A method of preventing explosions in kraft chemical recovery furnaces caused by water leaking from the furnace or boiler or other source onto the molten smelt in the furnace. Involves introducing into the furnace and onto the smelt and ash bed an aqueous quenching solution which will rapidly cool the smelt bed to temperatures below the explosive range without the solution itself causing an explosion. The solutes in the aqueous solution inhibit explosions and allow rapid cooling of the smelt. The materials which may be employed for this purpose are ammonium sulfate and certain high molecular weight, water-soluble organic compounds such as polyethylene glycols.

Background Many serious and even fatal explosions have occurred in kraft chemical recovery furnaces and there have been a few prior studies conducted to determine the causes of such explosions and the possible preventive measures which can be taken to avoid them. Although it appeared from these studies that the explosions sometimes occur when sufiicient quantities of water accidentally contact the ash bed containing molten kraft smelt in the furnace bottom, the exact nature or mechanism of the explosions remained uncertain. Many theories were advanced ranging from the formation of explosive combustible materials by chemical reactions to purely physical explanations for the explosions. The studies leading up to the present invention have indicated that this latter theory appears to be correct and that the explosions are purely physical in nature, i.e., they are due to extremely rapid formation of steam.

Theory Although the present invention is not to be limited by any particular theory for the causes of the smelt water explosions, the following explanation is offered. Violent noncombustible explosions can be caused when liquid water or water solutions of most chemicals becomes submerged beneath the surface of molten smelt. Such explosions can even be caused by feeding black liquor to the furnace which is substantially below the normal firing concentration, i.e., below about 51% solids. The water can become submerged in thesmelt by injection, such as from a tube leak; by violent spontaneous thermal agitation occurring at the interfacial contact between the smelt and water; or by molten smelt flowing into a pool of water.

It has been theorized that when liquid water becomes submerged beneath the smelt, the smelt surrounding a small amount of entrapped water begins to freeze or solidify thus forming a shell which encapsulates this small portion of the water as a liquid. Heat transfer between the smelt and encapsulated water continues rapidly, and tremendous water pressures can be developed within the solidified smelt shells in a matter of a few milliseconds. When this pressure increases to the point to which the tensile strength of the encapsulated shell is exceeded,

the shell ruptures causing a minor explosion; this however causes mixing of bulk (unencapsulated) water with bulk smelt. Although the initial explosion may be small, it can cause further mixing of smelt and water, which results in one or more major damaging explosions.

Description of the invention The experiments leading to the present invention revealed that smelt-water explosions occur when the smelt temperatures were between about 1425 and 1725 F. At higher temperatures, explosions normally did not occur; but as the smelt is gradually cooled by the water leaking onto the bed, this upper temperature would eventually be reached at which time an explosion might occur. This fact accounts for the often noted time delay between the initiation of a water leak and an explosion. The present invention therefore proposes to use this period of grace to rapidly cool the smelt bed below the low temperature limit of 1425 'F. which is approximately the freezing point of kraft smelt before water from the tube leak can cause an explosion. This cooling can most rapidly be attained with a water quench since water has a high specific heat and heat of vaporization. Along with the cooling effect of the water, the steam formed will tend to inert any flammable gas mixtures in the furnace cavity against a combustible explosion. It would, of course, be disastrous to quench with pure water since this would only serve to hasten an explosion. The quenching water must, therefore, have materials added to it which prevent the quench water from causing or hastening an explosion.

During the course of the experiments leading to the present invention, a number of materials were found which would prevent explosions under a specific set of conditions. But for an eifective explosion-preventing system, it is essential that the technique be operative under all conditions that might exist when an emergency arises. Therefore, many materials which worked only under certain conditions were eliminated. It is also evident that an effective quenching solution must be kept in storage for considerable periods of time and be ready for immediate use. The quenching solution must therefore be able to stand up during such storage and not deteriorate, decompose, or otherwise be adversely effected. Also, the solution must not freeze, precipitate, oxidize or be attacked by bacteria during storage. Various quenching solutions were eliminated on this basis.

Two basic types of materials have been found to be practical and elfective over the required range of conditions. These materials are ammonium sulfate and polymeric glycols of relatively high molecular weight.

The preferable material for use in the invention is ammonium sulfate. It is stable in solution, noncorrosive, nontoxic, easy to store, thermally stable, low cost, and has a low freezing point. It is theorized that the ammonium sulfate in the quench solution would undergo the following reaction with sodium hydroxide which has been formed from the sodium sulfide in the smelt through hydrolization by contact with water solution:

It can be seen that the reaction products as well as any decomposition products are not flammable and thus would not present a chemical reaction explosion problem. Also the reaction and decomposition products would not contaminate the chemical recovery system since the ammonia would be released as a gas while sodium sulfate is a natural constituent of the smelt.

The experiments revealed that even a 5 or 10% by weight solution of ammonium sulfate will prevent smelt water explosions except with a high sulfidity smelt (high percentage of sulfides) contaiminated with sodium chloride. The high sulfidity and the presence of chlorides makes the smelt-water mixtures more susceptible to explosions. On the other hand, a quenching solution containing 40% by weight ammonium sulfate inhibited explosions under test conditions representing all possible extremes of smelt compositions. It must be realized that when the quenching solution is added to the furnace, it will be diluted with the water leaking into the furnace before solidification of the smelt occurs. The degree of dilution will, of course, depend upon the size of the leak and the quantity of quenching solution added. Therefore, the safest solution to use is one capable of safely quenching after dilution with the leaking water. This would suggest the use of a high concentration of solute in the quenching solution. The above-mentioned 40% ammonium sulfate solution represents the approximate limit of solubility of this salt in water at 32 -F. and thus represents a practical concentration to employ as a quenching solution. Since ammonium sulfate has been found to be effective in preventing explosions in concentrations as low a 5%, there could be as much as an 8-fold dilution of the 40% solution with the leaking water depending upon the smelt composition and other relevant factors.

Although the present invention is not to be limited in any way by the theory by which the ammonium sulfate operates, the following explanation is believed to be correct. As globules of ammonium sulfate solution are trapped beneath the smelt surface, the above-noted reaction takes place to release ammonia gas. This ammonia forms a gas blanket between the water and the surrounding smelt which tends to reduce the rate of heat transfer from the smelt to the water. This lower rate of heat transfer will tend to reduce the interior pressure and also limit the thickness of any frozen smelt shell surrounding the quenching solution. These results will either tend to prevent rupture of the shells and thus prevent explosions or will tend to reduce the explosiveness of any shell ruptures. The gas blanket will also provide space into which the liquid water may expand upon heating which will, of course, greatly reduce the pressures involved when shell rupture occurs.

As an optional measure, the ammonium sulfate solution can be buffered with ammonia so as to maintain a pH of about 9. This will protect the quenching solution storage tank and piping from corrosion. The small amount of ammonia added for this purpose may also tend to increase the explosion-inhibiting characteristics of the solution.

The second group of compounds suitable for forming a quenching solution is the water soluble, polymeric glycols of moderate molecular weight. The compounds of thi group found to be effective include polyethylene glycol and polypropylene glycol. Although the mechanism by which these compounds prevent explosions is not clear, it is thought that they in some manner decrease the heat transfer rate between the smelt and water as does the ammonia sulfate solution.

The molecular weight of the glycols employed has a direct bearing on the effectiveness of the quenching solution. For instance, a 10% solution in water of a polyethylene glycol mixture having an average molecular Weight of 1540 inhibited explosions of chloride-free kraft smelt with a relatively high sulfide content but did not work on smelts containing 5% sodium chloride. A concentration, however, inhibited expolsions even with 10% sodium chloride in the high-sulfide smelt. Solutions containing 10% polyethylene glycols of 200-400 molecular weight were effective with the most highly explosive smelts. Polypropylene glycol with a molecular weight of about 400 in a 10% solution also proved effective with highly explosive smelt compositions. It is thus evident that although the higher molecular weight glycols will work in higher concentrations, molecular weights of about 200-400 are effective in lower concentrations and thus are more suitable for the present invention.

It is realized that the polymeric glycols can produce flammable pyrolysis gases on application to molten smelt but these are not produced in quantity until most 'of the water content of the solution is driven off. The production of dangerous proportions of flammable gases from application of these quenching solutions to the smelt may be prevented by continued introduction of fresh quenching solution to replace the water distilled off by contact with the hot smelt and ash. The large volumes of steam produced Would tend to inert the furnace cavity against the chance of a combustible explosion due to the pyrolysis gases. The glycols must also be kept from reaching freezing temperatures. 1

The operating procedure to be followed when a hazardous condition is discovered, such as a water leak or low solids in the black liquor feed,'is to immediately cut off any auxiliary fuel being fired and to stop the forced draft fans. The black liquor feed should'also be immediately cut off. Immediately upon the detection of a hazardous condition the quenching solution should be added onto the smelt in the form of a fine spray rather than in a heavy stream. A heavy stream of the quenching solution would tend to force Water down into the smelt and promote the dangerous conditions which the quench is intended to prevent. The fine spray, on the other hand, operates to form a uniform and continuous layer of quench solution over the smelt bed. It is, of course, obvious that steps should be taken when possible to reduce or cut off the water leaking into the furnace.

The amount of quenching solution which must be added to cool the smelt bed to temperatures below the explosive range will depend up on many factors. Some factors to be considered when determining the proper amount of solution to be stored are the size of the furnace, the amount and depth of the smelt bed, and the initial temperature of the smelt bed. As an example of the amounts which may be required, about 260 galions of solution would be required for a furnace having a hearth measuring 20 feet by 16 feet and having an average smelt depth of 3.82 inches. Upon quenching there would be about 60,000 cubic feet of steam produced which would inert the furnace.

An additional factor to be considered in the selection of the proper quenching solution and concentration thereof is the temperature of the stored solution. The higher the temperature upon introduction into the furnace and onto the smelt, the more effective is the quenching solution. Thus if the solution is stored in a heated location, the concentration of a particular solution could be lower than if stored where temperatures might drop to low levels.

While preferred embodiments of the invention have been described with reference to specific materials,,it will be understood that the inventionis t-o be limited only by the scope of the following claims.

We claim:

1. A method of preventing smelt-water explosions due to a Water leak in a chemical recovery furnace containing molten kraft smelt comprising the steps of detecting said water leak and introducing onto said molten kraft smelt an aqueous quenching solution containing a solute selected from the group consisting of ammonium sulfate and polymeric glycols, said solute being present in said solution in such concentration as to be effective in preventing said explosions.

2. A method as recited in claim 1 wherein said aqueous quenching solution is introduced in the form of a fine spray onto said molten melt.

3. A method of preventing smelt-water explosions due. to a water leak in a chemical recovery furnace containing molten kraft smelt which is substantially chloride-free comprising the steps of detecting said water leak and introducing onto said molten kraft smelt an aqueous quenching solution containing amounts of a solute efiec- 5 tive to prevent explosions selected from the group consisting of ammonium sulfate and polymeric glycols.

4. A method of preventing smelt-Water explosions due to a water leak in a chemical recovery furnace containing molten kraft smelt comprising the steps of detecting said water leak and introducing onto said molten kraft smelt an aqueous quenching solution containing about 40 percent by weight ammonium sulfate.

5. A method of preventing smelt-water explosions due to a water leak in a chemical recovery furnace containing molten kraft smelt comprising the steps of detecting said water leak and introducing onto said molten kraft smelt an aqueous quenching solution containing about 20 percent by weight a polymeric glycol, said polymeric glycol having a molecular weight of from 200 to 400.

6. The method as recited in claim 5 wherein said polymerie glycol is selected from the group consisting of polyethylene glycol and polypropylene glycol.

References Cited UNITED STATES PATENTS EDWARD STERN, Primary Examiner.

US. Cl. X.R. 23-1, 121, 193, 230; 162-30