US3304918A

US3304918A - Chemical recovery unit

Info

Publication number: US3304918A
Application number: US517299A
Authority: US
Inventors: Everett C Lewis; Robert G Tallent
Original assignee: Combustion Engineering Inc
Current assignee: Combustion Engineering Inc
Priority date: 1965-12-29
Filing date: 1965-12-29
Publication date: 1967-02-21
Anticipated expiration: 1984-02-21

Description

Feb. 21, 1967 E. c. LEWIS ETAL 3,304,918

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E. C. LEWIS ETAL 3,304,9l8

CHEMICAL RECOVERY UNIT Filed Dec. 29, 1965 2 Sheets-Sheet 2 United States Patent OfiFiCe 3 304318 Patented Feb. 21, 1967 3,304,918 CHEMICAL RECOVERY UNIT Everett C. Lewis, Avon, and Robert G. Tallent, East Granby, Conn., assignors to Combustion Engineering, Inc., Windsor, Conn., a corporation of Delaware Filed Dec. 29, 1965, Ser. No. 517,299 2 Claims. (Cl. 122-7) are lined with steam generating tubes, and this invention is concerned with 'an improved recovery unit Operating at a relatively high steam pressure with specific precautions 'being taken to prevent tube deterioration in the lower-most region of the furnace.

There has, in recent years, been a 'trend in the design and use of Chemical recovery units to increase the pressure of the unit, -i.e., the pressure of the steam produced by the unit for use in various processes and apparatus within the pulping *and paper making plant. As a result of this increase in pressure, the metal temperature of the steam generating tubes that line the furnace of the Chemical recovery unit has, accordingly, increased. It =has been found that at steam pressures at approximately 900 pounds per square inch and above there develops a serious problem with regard to metal wastage of the steam generating tubes in the lower region of the furnace with this problem being particularly severe in the region below the primary air ports. The present invention overcomes this problem by providing, in a particular manner, a protective layer over the portion of the tubes in this region exposed to the furnace interior.

In the operation of these Chemical recovery units the black liquor is sprayed into the furnace at a location spaced well above the bottom of the furnace. This liquor has a substantial moisture content and most of this moisture is driven from the liquor spray upon its introduction into the furnace because 'of the high temperature in the furnace and the =hot gases passing upwardly through the furnace and the spray. The solids that remain after thus removing the moisture fall onto the bottom or hearth of the furnace and form a roughly truncate d pile. During the descent to the hearth some of the lighter volatiles are driven from these solid particles and the remaining volatiles are liberated and the combustible material in the solids burned in this pile with this combustion being supported by the introduction of primary air at locations spaced somewhat above the bottom of the furnace. This air is introduced through ports such that it 'is directed generally Over and upon the pile of material. Some burnables are carried up through the furnace and are consumed above the location of liquor introduction with the 'Secondary air being introduced for this purpose.

The most severe Operating conditions insofar as deterioration of the tube metal that lines the furnace walls is encountered occur at the location immediately below the primary air introduction ports. A high rate Ofheat absorption is present at this location and, accordingly, the

tube metal temperatures on the furnace walls tend to be higher at this location than at other locations. Furthermore, there is at this location an interface or juncture between the burning material disposed on the furnace floor and the side walls of the furnace. Tube metal deterioration is also experienced at a location on the walls of the chemical recovery unit intermediate the Secondary air ports and the primary air ports. Although the metal wastage at this location is not as severe as that below the primary air ports, it is :of concern and does require attention and necessitates preventive measures being taken to prevent or prohibit this metal wastage.

In order to provide a satisfactory and economic solution to this problem of tube metal wastage suitable metallic members :are 'welded to the steam generating tubes that line the side walls of the furnace for the purpose of securing to these tubes a layer of solidified molten chemicals. During the operation of the Chemical recovery unit molten chemicals are collected :at the 'bottom of the furnace and are periodically withdrawn through a suitable spout. Molten chemicals *also run down the furnace walls. The furnace wall tubes are at a temperature below the melting point of this molten Chemical and, accordingly, a layer of Chemical solidifies in engagement with the wall tubes. However, the tubes are normally rel-atively smooth and this layer of Chemical does not adhere very well so that an un-broken layer of solidified chemical is not in engagement with the tubes. In a preferred embodiment of the invention the metallic means that is welded to the tubes to maintain a solidified layer of Chemical against the tubes is in the form of expanded metal. This is advantageous in that it is very eifective to maintain the solidified layer of Chemical intact and provide a oontinuous layer about the furnace walls and, moreover, it is relatively economic. Whatever the metallic means employed to maintain this layer of solidified chemical in engagement with the tubes, it is necessary that it be fabricated of Stainless steel in order to withstand the severe Operating conditions to which it is subjected.

Accordingly, it is an object of the present invention t-o provide an improved Chemical recovery unit :Operating at a steam pressure of approximately 900 pounds per square inch or above.

A further :object of the invention is to provide such an improved Chemical recovery unit wherein specific precautions are taken to prevent tube metal wastage of the steam generat-ing tubes.

Other and further objects of the invention will become apparent to those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination of the elements of the inventive organization -in such a manner as to attain the results desired, as hereinafter more particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the accompanying drawings wherein:

FIG. 1 is a sectional view of a Chemical recovery unit embodying the present invention;

FIGS. 2 :and 3 are detailed sectional views of the lower region of the chemical recovery furnace indicating particularly the regions wherein metal wastage of steam generat-ing tubes is a problem at pressures of approximately 900 pounds per square inch steam pressure or above;

FIGS. 4 and 5 are detailed views showing a preferred construction whereby expanded metal in the form of stainless steel is welded to the surface of the tubes directed inwardly of the furnace at the lower region of the furnace in order to maintain a solidified layer of Chemical in engagement with the tubes; and

FIG. 6 is in the nature of a side elevational view of the expanded metal that forms a part of the inventon.

Referring now to the drawings, Wherein like reference characters are used throughout to designate like elements, the illustrative and preferred embodiment of the invention as depicted therein includes a Chemical recovery unit that has a furnace 10. The walls of this furnace are lined with steam generating tubes 12 that may 'be in tangent relation or may be in closely spaced relation with the space intermediate the tubes bridged by a fin. The tubes 12 that line the furnace form part of the heat exchange surface of the chemical recovery unit with there being additional heat exchange surface identified generally as 14 at the upper region of the unit. The tubes 12 carry a mixture of steam and water at saturation temperature for the particular pressure at which the unit is operated with this mixture passing upwardly through these tubes. The illustrative steam generator is operative to produce steam at 950 pounds per square inch pressure with this steam being Conveyed from the header 16 to a desired point of use and with this steam being superheated to a desired value such as 900 F.

Residual liquor obtained from the kraft pulping process is introduced into the furnace 10 through the noZZles 18. The liquor thus sprayed into the furnace descends downwardly toward the furnace bottom passing through an upwardly rising stream of combustion gases such that a majority of the moisture in the liquor is immediately evaporated with the solid particles falling downwardly through this rising combustion gas stream and forming a pile 20 on -the hearth or 'bottom 22. A portion of the =burnables are consumed during this descent through the furnace with additional 'burnables being consumed on the pile 20 and with the non-burnable chemicals being smelted and periodically withdrawn through the discharge spout 25. As a result of this operation molten chemicals flow down the tubes 12 which line the furnace with the body or bath of molten chemicals being collected at the lower region of the furnace for discharge through said spout 25.

combustion supporting air is introduced into the furnace at two locations. The primary air is introduced through nozzles 24 spaced relatively close to the bottom as, for example, 3 feet above the furnace bottom, while the Secondary air is introduced through the noZZles or ports 26 located above the liquor introduction noZZles 18.

As previously mentioned Chemical recovery units Operating at approximately 900 pound per square inch steam pressure or above have encountered a serious metal wastage problem of the tubes 12 particularly at the location below the primary air noZZles identified in FIG. 1 as 24. Significant wastage is also experienced below the Secondary air introduction ports 26. The particular location of these areas of wastage are identified in FIGS. 2 and 3 as the

general areas

28 and 29. The apparent reason for this is that With the higher pressures the saturation temperatu-re within the tubes 12 is elevated which, in turn, Causes the tube metal temperature to rise. The region immediately below these

ports

24 and 26 is a region of high heat absorption such that the tube metal temperature here is substantially elevated and, furtherrnore, with relation to area 28, it is at this location that the burning on the upper surface of the pile joins or Contacts the side walls of the furnace. In any event and for whatever reason, serious tube metal wastage problems of the steam generating tubes on the upright furnace walls in units Operating at the aforernentioned steam pressure and above have been experienced.

This problem is extremely serious for several reasons, one of which s that a tube rupture in a Chemical recovery unit creates a very hazardous condition in that mixing water with the molten smelt in the bottom of the unit Causes serious explosions which have created extensive property damage and been the cause of loss of life for many Operating personnel. Moreover, replacement of the tubes particularly in the lower region of the Chemical recovery units is difiicult since the lower region is liquidtight so as to contain the molten smelt and, therefore, adjacent tubes or abutting fins are welded together to form a fiuid-tight Wall section.

To overcome this problem of metal wastage of the steam generating tubes on the upright furnace walls at the aforernentioned locations the tubes in the lower region of the furnace are provided with means which is effective to grip and hold a layer of solidified Chemical in engagement with the tube as well as to produce a Cooling effect With regard to this solidified layer. In the preferred embodiment illustratively disclosed, this means takes the form of expanded metal secured to the portion of the tube directed inwardly of the furnace thus providing the tube with extended surface. This expanded metal is secured to this portion of the tubes throughout the lower region of the furnace so that a sheet of this expanded metal extends all the way around the furnace at the interior thereof in this lower region. The expanded metal is of stainless steel, such as Type 304, and it is comprised of a series of strands 28 united at bonds 30, and it is formed such that the bonds 30 and the strands are set at a Sharp angle to the plane of the sheet. To illustrate this in FIG. 6 the plane of the sheet may be identified as 32 and the angle of the bond may be identified as 34, and the angle of the strand identified as 36. The purpose of this is so that portions of the strands of the expanded metal will be spaced from the tubes so that the molten Chemical can flow intermediate the strand and the tube and thus when it solidifies, it will be securely bonded in place with the expanded metal being embedded in the layer of solidified Chemical.

With the lower region of the Chemical recovery unit provided with this extended surface throughout the entire lateral extent of the furnace walls, a layer of solidified Chemical is effectively secured in engagement with the inwardly directed face or region of the tubes at this location in the furnace. This layer of solidified chemicals prevents contact of the tubes by the molten Chemical and shields the tubes from the deteriorating atmosphere within the furnace and thereby alleviating to a Substantial extent the tube metal wastage problem heretofore mentioned.

The expanded metal is welded to the surface of the tubes 12 by welds 40 at each of the bonds of the expanded metal. This has two effects, One of which is to securely fasten the expanded metal in place and the other of which is to provide good heat exchange between the expanded metal and the tube Wall. This latter is necessary and desirable in order to keep the temperature of the expanded metal as low as possible which, in turn, tends to build up a thicker layer of solidified Chemical. Because of the nature of the expanded metal with the many strands and because of the many welds, Cooling of the expanded metal is very effective.

While we have illustrated and described a preferred embodiment of our invention it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and Scope of the inventon. We therefore do not wish to be limited to the precise details set forth but desire to avail ourselves of such changes as fall within the purview of our inventon.

What is clai-med is:

1. A Chemical recovery unit having an upright furnace into which black residual liquor produced from kraft pulping process is introduced for burning and smelting,

said furnace having the inner surface of its upright walls lined with steam generating tubes forming part of a steam generatng system operative to generate steam of at least 900 pofinds per square inch, these walls having primary air incluction ports therein spaced somewhat above the furnace bottom for directing air inwardly toward a pile or sme lt and burnables on the bottom of the furnace, the tubes at the lower region of the furnace having their surface that is directed inwa-rdly ofthe furnace bare but provided with extended surface constructed and arranged to holdi a solidified layer of molten Chemical produced in the filrnace in engagement Withthis tube surface, said extended surface comprises Stainless steel sheet-like means generally conforming to the tubes and welded to the tubes, said sheet-like means having numerous relatively closely spaced openings dispersed throughout its area.

2. The Chemical recovery unit of claim 1 wherein said 5 sheet-like means comprises expanded metal.

References Cited by the Examiner KENNETH W. SPRAGUE, Primary Examiner.

Claims

1. A CHEMICAL RECOVERY UNIT HAVING AN UPRIGHT FURNACE INTO WHICH BLACK RESIDUAL LIQUOR PRODUCED FROM KRAFT PULPING PROCESS IS INTRODUCED FOR BURNING AND SMELTING, SAID FURNACE HAVING THE INNER SURFACE OF ITS UPRIGHT WALLS LINED WITH STEAM GENERATING TUBES FORMING PART OF A STEAM GENERATING SYSTEM OPERATIVE TO GENERATE STEAM OF AT LEAST 900 POUNDS PER SQUARE INCH, THESE WALLS HAVING PRIMARY AIR INDUCTION PORTS THEREIN SPACED SOMEWHAT ABOVE THE FURNACE BOTTOM FOR DIRECTING AIR INWARDLY TOWARD A PILE OR SMELT AND BURNABLES ON THE BOTTOM OF THE FURNACE, THE TUBES AT THE LOWER REGION OF THE FURNANCE HAVING THEIR SURFACE THAT IS DIRECTED INWARDLY OF THE FURNACE BARE BUT PROVIDED WITH EXTENDED SURFACE CONSTRUCTED AND ARRANGED TO HOLD A SOLIDIFIED LAYER OF MOLTEN CHEMICAL PRODUCED IN THE FURNACE IN ENGAGEMENT WITH THIS TUBE SURFACE, SAID EXTENDED SURFACE COMPRISES STAINLESS STEEL SHEET-LIKE MEANS GENERALLY CONFORMING TO THE TUBES AND WELDED TO THE TUBES, SAID SHEET-LIKE MEANS HAVING NUMEROUS RELATIVELY CLOSELY SPACED OPENINGS DISPERSED THROUGHOUT ITS AREA.