CA1118705A - Selective condensation process and condenser apparatus - Google Patents

Abstract

SELECTIVE CONDENSATION PROCESS AND CONDENSER APPARATUS

ABSTRACT OF THE DISCLOSURE

Contaminant substances having high biochemical oxygen demand, foul odors, etc., are often carried by vapors to be condensed in industrial processes. Certain of these contaminants are more volatile than the primary constituent of the vapor to be condensed. The invention concerns a process and apparatus for separating most of the more volatile contaminants so that separate streams of condensate are produced, one of which is relatively clean while the other stream, though considerably smaller in volume, carries most of the contaminants. A plate type surface condenser having a barrier on the condensing side which prevents the passage of contaminants into a stream of relatively clean condensate is disclosed. In case several different streams of condensate are to be separated, the condenser apparatus has several sections arranged in series.

Description

o 8~7~;)5 BACKGROUND OF TH~'INYENTION
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Field o the Invention ... . . .... .

~ he invention relates to the condensation of vapor in industrial processes, by means of a plate type condenser.

-Descript-ion of the Prior Art .. . . . . ..

In many industrial applicatisns, the vapor or steam produced in an evaporator is subsequently condensed for removal from the system, re-use o~ water or for some other reason. For example, sur~ce condensers used in evaporator systems in the pulp and paper industry allow re-use of warm condenser water reco~ered from steam.
When steam or other vapor to be condensed carries components in the vapor phase that are more volatile than the water or other substances comprising the principal constitu-ent to be recovered by condensation, one way to treat the vapor is to fully condense everything, including the more highly volatile materials, often, by subcooling to a con-siderable degree.
Evaporator and condenser systems of several ki~ds have been illustrated in Fig. 11-17 at page 11-25 of Perry's Chemical En~ineer's Handbook, Fourth Edition, 1963.
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"~ 7~5 U.S. Patent No. 3,788,954 to Cantrell relates to a distillation process and shows a condensation section having upper and lo~er condensation chaI~ers separated by a horizontal wall or baffle intended to separate less volatile from more volatile components of vapor being condensed. U.S. Patent No. 3,261,392 shows an evaporator having a vertically disposed ~affle dividing a heating space; and plate type heat exchang-ers have been desc~ibed in C. F. Rosenblad's U.S. Patent No.
3,332,469.
There is considerable experience and other published information relating to industrial heat exchange technology in general, and more particularly to surface condensers of various kinds. Yet no fully satisfactory system for the effective separation of condensate in a plate type heat exchanger to concentrate the more volatile components of steam or other vapor being condensed from those that are less volatile has achieved wide industrial acceptance. The present invention overcomes difficulties of prior art systems and provides a highly effective method and apparatus for selective condensa-tion.

SUMWARY OF THE INVENTION
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The condenser of the invention comprises a housing enclosing a plurality of heat exchange elements outside of which elements a coolant fluid passes to condense vapor within the elements. Preferably the heat exchange elements are formed o~ pairs of broad plates secured together around their peripheries with an opening to the intarior of each element at the top and bottom of each element. A header communicat~s 87~5 ("~ ~

with all of the elements at theix upper ends so that vapor can pass freely from one element to another for condensation within the elements. A bottom header opens on to each elemen~, but there is a barrier closing off one end portion of the bottom header ~rom the other end thereof.
Vapor to be condensed is fed to the interiors of all those heat exchange elements at one side of the header barrier. The vapo~r rises within those elements and is partially condensed therein. The condensate formed comprises the more easily condensed constituents of the vapor.
Thé more volatile components o the vapor are not so readily condensed and pass on through the upper header to the heat exchange elements whose bottoms open to the lower header on the other side of the barrier from the vapor entry area. Further cooling condenses the more volatile components and the co~densate containing these contaminant substances collects at the bottom of the conde~ser at the other side of ~he barrier from the cleaner condensate, and the contam~na~ed condensate is withd~awn as a sQparate stream from the clean liquid. Noncondensible and vent gas~s exit from the same sïde of the barrier as the contaminated condensate.
Most of the heat exchange elements communicate with the vapor entry side of the lower header, and most of the condensate is withdrawn from that side. ~he vapor passes upwards through this majority of the heat exchange elements.
The smaller number of heat exchange elements where~
in the more volatile suhstances are condensed carry the vapor downward so thclt vent and noncondensible gases can exit at these elements 1l lower ends. The vent gases can subse~uently ~e condensed ~ld their heat recovered in subsequent treatment~

7~5 The foregoinq description has followed the vapor being condensed, but the coolant flow should also be consider-ed. The cooling medium can be a continuous supply of cooling water which is heated while condensing the vapor; or cooling water that is recycled ~y means o~ a circulation pump and cooled by means of evaporative cooling outsid~ the system shown before return to the condenser as coolan~; or liquid which is to be evaporated. In the latter case, where water or other liquid to be evaporated is employed as the coolant medium for condensing the ~apor inside the heat exchange elements, the flow of liquid as a thin film down the outside surfaces of the heat exchange elements results in evaporation of a considerable amount o~ the coolant liquid. Thus, while the interior spaces within the heat exchange elements are working as a condenser, the exterior space outside the heat exchange elements and within the housing, functions a~ an evaporator.
Pilot plant trials have been made by condensing steam contaminated by substances that are malodorous and ha~e high biochemical oxygen demand ~BOD~. About 90% of the cond~nsate is formed during the upward pass of steam through the heat exchange elements on the steam entry side of the barrier, but the condensate produced in this pass and discharg- -ed from th~ bottom header carries less than 20~ of the contam-inants. The remaining 80~ af the contaminant substances go over to the downward flowing stream in ~he elements whose bottoms communicate with the lower header on the other side of the barriex. The 20~ of the total condensate produced and collected on that side of the barrier and the vent gases dis-charged on the dirty condensate side together carry over 80%

r 7~5 of the total contamlnants.
T~e relatiYely clear condensate stream from the majority of the elements i5 esseIltially odor free and can be recycled to the mill or plant without further treatment.
The separated stream o contaminated water can be passed on for further treatment in a stripping column or the like.
These and othex objects and advantages of the selective condensation system of the in~ention will be more fully understood from the following detailed d~scription of tha invention, especially when- that description is rsad with reference to the ~ccompanying drawing.

Brief Description of ~he Drawing In the drawing, in which like reference characters-indicate like parts throughout: -~ Fig. 1 is a sectional view through apparatus according to the invention;
- Fig. 2 is a view in section taken along lines 2-2 of Fig. 1, looking perpendicular to the view of Fig. 1 in the direction of the arrows and showing the clean condensate-side of the apparatus;
Fig. 3 is a view similar to that of Fig. 2, but showing the foul condensate side of the apparatus;
Fig. 4 is a ~iew in section taken along lines 4-4 of Fig. 1 and looking downward, with the path of flow through the upper part of the apparatus-shown;
Fig. 5 is a view in section taken along lines 5-S
of Fig. 1 showing the flow at the Iower part of the apparatus;

~8~ 0 Fig. 6 is a view in perspectiYe of apparatus o~
Figs. 1-5 with some parts broken away and some parts illus-trated by dashed lines, and Fig. 7 is a view in section of apparatus according -~ to the invention for producing several separate condensate streams.

DETAILE~ DESCRIPTION OF A PRE~ERRED EMBODIMæNT
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In the drawings the condenser of the invention, generally designated 10, has a housing 11 with generally vertically extending ront and back walls 12 and 13 respect-ively and a pair of side walls 14. Within the housing 11 there is an array of spaced, parallel falling film heat -exchanger elements 15. The heat exchange elements 15 are o~
the type formed-by pairs of spaced parallel broad flat plates secured together around their peripheries to provide enclosed - spaces within the elements 15. A preerred method of manu-facturing plate-heat exchange elements is disclosed in my prior U.S. Patent ~o. 3,512,239 granted ~ay 19, 1970. As -those familiar with the art will understand, the elements 15 ~-can be employed to condense steam or other vapors passing within the elements 15 by indirect heat exchange with a coolant medium, such as water ~lowing as a thin film down the outer surfaces of th~e elements 15.
Mea.ns for introducing coolant liquid into the housing 11 and distributing the liquid evenly over the sur-faces of heat e~change elements 11 are shown in Figs. 1, 2 and 3. A perforated, generally horizontally disposed tray 16 is mounted across the interior of the housing 11, abo~e and spaced from the heat exchange elements 15. Water or other coolant liquid flows through the per~orations of the tray 16 to run down the outer surfaces of the heat exchange element~
15 as illustrated in the several drawing figures. Prefexably the cooling liquid is not poured directly onto the tray 16, ~ut is fed to an upwardly open box 17 spaced above the tray 16 to overflow the box 17 and thus distribute liquid more evenly. When very large amounts of liquid coolant are used, the box 17 is not ~eeded. Pipe 18 is shown in Eigs. 2 and 3 leading to the box 17 for supplying cooling liquid thereto.
` Water or oth~r cooling liquid that has traversed the vertical length of the heat exchange elements 15 is shown in Figs. 1, 2, 3, and 6 to be collected at the bottom of the housing 11, where inwardly converging bottom portions 22 and 23 of the front and back walls 12 and 13 form a trough 24.
Liquid is discharged from the trough 24 through an outlet conduit 25. The inlet pipe 18 can supply additional resh coolant liquid as needed to the tray 16. If racycling of the coolant liquid is desired, a pump and means for cooling the liquid before recycling can be used.
- The preferred structure related to flow of liquid externally of the heat exchange elements has as one result the provision of a uniform and effective flow of cooIant along the outer surfaces of the heat exchange elements 15 to condense steam or other vapor within the elements 15.
Ste,~m or other vapor to be condensed entexs through the front wall 12 of the housing 11 by way of a conduit 26 as best shown in Figs. 2, 5 and 6 of the drawing. A baf1e (not shown) can be provided to promote better distribution of the vapor. The conduit 26 is located at the lower part of the condenser 10 adjacent the lower ends of the heat exchange -- 8 ~

r, 37~5 elements 15. It will be seen that the lower front corner of each heat exchange element 15 has a cutout area as shown at 27 in Figs. 2, 5 and 6; that is, the peripheries of the plates rorming the plate type heat exchange elements 15 are not sealed togather at the elements' lower front corners. Alter-nately inlet and outlet boxes could be welded on the elements 15, or some other method of fabrication could be employed.
The cutouts or openings 27 all communicate with a bottom head-er B that extends across the front of the housing 11 as shown in Fig. 1. It will be seen that the bottom header B has a top wall 28 and a bottom wall 29, that the front wall 12 of the housing ~orms a front wall o~ the header B, and that except for the openings 27 into the interiors of the heat exchange elements 15, the header B is closed at its rear by a back wall 30. Thus, nothing can pass to or from the bottom header B to the space within the housing ll where cooling liquid circulates, and the header B communicates with the interior spaces of the heat exchange elements 15 through the openings 27, Steam or other vapor to be condensed enters the bottom header B through the conduit 26 and thence passes to the heat exchange elements 15 via openings 27 to be condensed as it passes upward as shown in Fig. 2.
The header B is not open and continuous along the entire length c~f the ront of the housing 11, but is inter-rupted by a bar.rier 31, as shown in Figs. 1, 5 and 6. Vapor entering the header B through the conduit 26 can only pass directly to some of the heat exchange elements 1~, the openings 27 o~-the other elements 15 being separated from the conduit 26 by the barrier 31.

`~J ~1~8~7~5 ~) It has been found that the barrier 31 should pre-ferably be positioned to divide the bottom header B into a relati~ely longer section 32 and a relatively shorter section 33. This- permits direct communication of most o~ the heat exch nge elements lS with the vapor entering through the conduit 26 by way oi the longer header section 32 as seen in Figs. 5 and 6.
Attentibn is now directed to the upper front area of the heat exchange elements where an upper header ~ i5 shown to extend within the housing 11 to interconnect the upper front ends of all of the heat exchange elements 15.
The upper header H extends u~obstructed across the entire array of heat exchange elements 15, which elements 15 all have cutout or unjoined areas at 37 opening on to the upper header ~. Except for the openings 37 communicating with the i~teriors of all of the heat exchange elements 15, the upper header is enclosed by a top wall 38, bottom wall 39, rear wall ~0 and by the housing front wall 1~.
The structure of the headers B and ~, and the ~ .
arrangement of heat exchange elements 15 which have openings only at 27 and 37, thus constrains vapor to ~low upward-through those elements 15 which communicate with the header B at the portion 32 and to flow downward through those elements 15 which communicate with the area 33 of the bottom header B on the other side of the barrier 31. These flow paths will be :more fully apparent from Figs. 4, 5, and 6.
Conde~sate formed within the heat exchange elements 15 is discharged by way of two condensate outlets 41 and 42, shown in Figs. 2, 3 and 6. The condensate outlet 41 drains the portion 32 of the bottom header B and the condensate out-let 42 ser~es to drain the shorter portion 33 of the bottom O ~ 37~5 ~

header B~ Thus the outlet 41 ls located below the vapor inlet conduit 26 and the outlet 42 is located below the vent and noncondensible outlet conduit 36. As shown in Fig. 6, the lower wall of the bottom header B can be formed to facilitate condensate discharge.
The barrier 31 splits the bottom header B into two une~ual sections 32 and 33 as already indicated. Thus ~apor such as steam to be condensed is passing upward ~hrough most of the heat exchange elements 15. The relationship between the amounts o heating surface provided by the sections 32 and 33 depends upon the fluid to be condensed. As an example, for a pre-evapoxator for spent liquor from the kraft pulping process, it is presently preferred that about 90% o~ the heat exchange elements 1~ be in commun.ication with the steam entry portion 32 of the bottom header B, the remaining elements 15 communicating with the vent outlet area 33 of the header B.
For other services, a ratio other than 9 to 1 can be efect-ively employed.
- In the typical case, of the pre-evaporator for spent kraft pulping liquor where steam is to be condensed, about g0%~ -of the steam is condensed duxing the upward pass through the majority of the heat exchange elements 15, lea~ing only about -10~ of the steam to travel along the upper header H for the downward passage through heat exchange elements 15 that commu-nicate with the outlet 36. How2ver this 10~ of the steam is very rich in the lower boiling point or volatile conta~inant substances. The condensate formed du.ring the downward p~ss and discharged through the outlet 42 is much richer in malo-dorous compounds and BOD-producing components than the con-densate discharged on the steam entry side through the outlet L8t7 :35 pipe 41. Pilot plant trials have produced a yield of less than 20% of the BOD and malodorous condensate in the 90% of the condensate formed during the upward pass of the steam, and over 80~ of the BOD and foul~smelling components ha~e appeared in the condensate and vent gases exiting at the vent 36 and condensate outlet 42.
The foregoing discussion has treated the flow of vapor through the interiors of the heat exchange elemen~s 15 and has treated the li~uid coolant flowing down the exterior ~urfaces o~ ~he heat exchange elements only as a coolant for the vapor to be condensed. However, it is also important to consider evaporation of this coolant liquid by heat transer from the condensing vapor. As the water or other coolant liquid flows down the heat exchange elements lS as a film, a considerable amount o~ the liquid will evaporate. This result can be ad~antageously used by`employing as the coolant a liquid which is to ba evaporated. Thus, while the interior spaces o the heat exchange elements lS are working as a condenser, the ext~rior space outslde the elements 15 and within the housing 11 works as an evaporator.
For example, liquor re-circulated to the tray 16 by the pump P is mixed with liquor to be evaporated whi~h is introduced through the pipe 18 and ~oiled off vapors pass -outward from ~nong the heat exchange elements 15 as the liquor-is heated by the hot vapor within the elements 15. The vapor boiled of rises to thP upper part of the apparatus, above the tray 16. Fig. 6 of the drawing illustrates how the housing wall- 13 can be spaced from the nearest heat exchan~e element -15 to permit outward and upward flow of boiled-off vapor with-in the housing 11. ~his vapor can then be allowed to pass upward either through space provided alongside th~ tray 16 or 7~S

through a conduit to the top o~ the housing 11, where an entrainment separator or the like (not shown in the drawing) can be provided ~or trea~ment of the vapor generated by evaporation o the liquid coolant. Figs. 1-3 show the housing 11 as vented centrally at its top at ~0, but it will be under- -stood by those familiar with the art that an entrainment separator can be provided at the location shown by the refer-ence numeral 50 to capture droplets of li~uid carried by the flow of vapor.
The system for selective condensate separation according to the invention is not limited to the separation of two condensate streams, but can be extended to the separa-tion of three or more streams of condensate with di~ferent degrees of purity. Fig. 7 shows an arr~ngement for selec~ive condensation of four condansate streams, indicatad as con-densates I through IV, of which condensate stream I i~ made up of the most raadily condensed portion of the feed and - -condensate stream IV contains the most dificult to condense of the substances that are condensed within the apparatus~
It will be seen that the appar tus of Fig. 7 has a housing like that of the previously discussed embodiment,-with side walls 114 corresponding generally to the walls 14 in Fig. 1, a liquid distribution tray 116 similar to the tray 16, heat exchange elements 115 like the elements 15, and so on. The apparatus of Fig. 7 difers from the embodiment shown in the other drawing features in that it is sectionalized to separate the several condensates. The spaced parallel heat exchange elements 115 have their lower openings 127 and upper openings 137 axranged in four groups. Vapor to be condensed - - is fed to a chamber or compartment 100 communicating with a 7~5 (~

number (7 shown) of the heat exchange elements 115 at their lower openings 127 for the condensation of some o~ the vapor within ~hose elements 115 as the vapor rises up~ardly within the interior spaces of the elemenks. Vapor not condensed in passage upward through the elements 115 of this first group emerges into a chamber 101 at the upper end of ths elements 115 of the first group. Condensate formed in the elements of the first group~is relatively free of hard-to-condense substances, and is drawn off from the bottoms o~ the elements 115 as condensate I.
The chamber lCl is closed except for the openings 137 of the firs~ group of heat exchange elem,ents and a conduit C 1 shown in dashed lines as C 1 which leads uncondensed vapor from the chamber 101 to a chamber 10~ that opens on to a second group of elements 115 at the lower openings 127. The vapor passes upwards through the interior spaces o this second ~roup of elements 115 wherein some is condensed to be drawn off as condensate II while the uncondensed remainder em2rges into a chamber 103 at the uppex ends of the elements. The process is repeated as vapor is passed downward through conduit C 2 to,a chamber 102 and thence upwards through the interior spaces of a third group o heat exchange elements 115 wherein further condensation oc:curs. Vapor emerging into the upper chamber, 105 is passed c[own to the final lower chamber 10~ by way of conduit C 3 ancl the final upward passage through several heat exchange elements 115 (three shown) condenses volatile compon-ents to produce condensate stream IV which contains the most dif~icult to condense of the YaporS that are condensed in the system. Vent gas and remaining uncondensed ~apors exit from chamber 107 at the top of the last group of heat exchange ``~ 3~31 1~7~S
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elements 115 as shown at the ~pper right in Fig. 7.
- Embodiments separating two and four di~ferent - - condensates have been illustrated, but it will be clear that three condensates, or more than four could also be differen-tiated in apparatus according to the invention . ~arious modifications, applications, substitutions of parts and structural variations of the method and apparatus - described in terms o~ a presently preferred embodiment will suggest themselves to those familiar with heat exchange techniques and are considered to be wlthin the spiri~ and scope of the invention.

B ~L~T I~ CL~IM~D IS~ - _ . ~ ,

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A falling film heat exchanger comprising: a housing, a plurality of plate-type heat exchange elements within said housing, means for distributing coolant liquid to outer surfaces of said heat exchange elements, an upper transverse header communicating with upper ends of all of said heat exchange elements, a lower transverse header communicating with lower ends of all of said heat exchange elements, a barrier across said lower header for prohibiting passage of vapor between a vapor entry section of said lower header and a vent discharge section of said lower header, means for introducing vapor to be condensed into said vapor entry section of said lower header, and means for discharging uncondensed substances from the vent discharge sec-tion of said lower header, said vapor entry section of said lower header communicating directly only with a first group of said heat exchange elements and said vent discharge section of said lower header communicating directly only with a second group of said heat exchange elements, means for discharging condensate from said first group of heat exchange elements comprising an outlet from said vapor entry section of the lower header, and means for dis-charging condensate from said second group of heat exchange elements compris-ing an outlet from said vent discharge section of the lower header.

2. The apparatus of claim 1 and including means for collecting coolant liquid below said heat exchange elements, means for recirculating said coolant liquid, and means for discharging vapor evaporated from said coolant liquid by heat exchange with said vapor to be condensed.

3. Apparatus for selective condensation of vapors contaminated by volatile substances to produce a first, relatively clean stream of condensate and a second condensate stream carrying more of contaminants than said first stream, comprising a falling film heat exchanger comprising: a housing, a plurality of plate-type heat exchange elements within said housing, means for distributing coolant liquid to outer surfaces of said heat exchange ele-ments, an upper transverse header commmunicating with upper ends of all of said heat exchange elements, a lower transverse header communicating with lower ends of all of said heat exchange elements, a barrier across said lower header for prohibiting passage of vapor between a vapor entry section of said lower header and a vent discharge section of said lower header, means for introducing vapor to be condensed into said vapor entry section of said lower header, and means for discharging uncondensed substances from the vent discharge section of said lower header, said vapor entry section of said lower header communicating directly only with a first group of said heat exchange elements and said vent discharge section of said lower header com-municating directly only with a second group of said heat exchange elements, means for discharging condensate from said first group of heat exchange ele-ments comprising an outlet from said vapor entry section of the lower header, and means for discharging condensate from said second group of heat exchange elements comprising an outlet from said vent discharge section of the lower header.

4. The apparatus of claim 3 wherein each of said heat exchange elements comprises a pair of spaced generally parallel plates sealed together around substantially their entire peripheries.

5. The apparatus of claim 3 wherein said second condensate stream is of lesser volume than the first stream of condensate.

6. The apparatus of claim 3 and including bottom header means com-municating with all of said heat exchange elements near bottoms of said ele-ments, said bottom header means being blocked by a transverse barrier prevent-ing the passage of vapor through the bottom header means past the barrier, said means for introducing vapor to be condensed leading only to heat exchange elements on one side of said barrier, and means for discharging said second condensate stream and vent gases leading from heat exchange elements on the other side of said barrier.

7. The apparatus of claim 3 wherein liquid coolant is distributed to said element surfaces by means above said heat exchange elements, and including means for collecting liquid after flow over said elements and means for discharging vapor evaporated from said liquid.

8. The apparatus of claim 7 and including means for recirculating liquid collected to the liquid distributing means above said heat exchange elements.

9. A method for selective condensation of vapor contaminated by volatile substances comprising introducing vapor to be condensed into lower portions of an interior space of each of a first group of heat exchange ele-ments of a plate-type falling film heat exchanger for upward passage of the vapor through the heat exchange elements of said first group while less volatile constituents of said vapor are substantially condensed during said upward passage, and discharging condensate produced during said upward passage as a first condensate stream; leading uncondensed vapor from upper portions of the interior spaces of the heat exchange elements of said first group to the interior spaces of upper portions of heat exchange elements of a second group for downward passage through the heat exchange elements of said second group, withdrawing condensate produced during said downward passage as a second condensate stream which is more highly contaminated by said volatile constituents than said first condensate stream, keeping said first and second condensate streams separate, and removing uncondensed vapors from the elements of said second group.

10. The method of claim 9 wherein most of the vapor is condensed during said upward passage so that said first condensate stream is consider-ably greater in volume than said second condensate stream.

11. The method of claim 9 wherein coolant liquid is distributed to all of said heat exchange elements from above said elements to flow downward over outer surfaces of said heat exchange elements, and including collecting coolant liquid which has passed over the heat exchange elements and recircu-lating collected coolant liquid.

12. The method of claim 9 wherein the vapor to be condensed is primarily steam from an industrial process.

13. The method of claim 9 wherein said uncondensed vapor is removed from the elements of said second group near lower ends of said elements.