Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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V OR RECOVERY ~IETHOD AND APP~RATUS
Technical Field
This invention relates to the recovery oE
condensable vapors from a vapor source by refrigeration
condensation. More particularly, this invention relates to
the use of a concentrating system, including an accumu-
lating device and a noncondensable carrier gas streamcirculated through the accumulatinc3 device, intermediate
the vapor source and the condensing means in order to
provide a higher vapor concentration for the condensing
means. The condensable vapor is removed from the carrier
gas by re~rigerating at least a portion of the gas stream
to cause condensation and separation o~ at least a portion
of the condensable vapor.
A particularly useful application of the present
invention relates to the recovery of solvent ~rom a drying
oven system.
sackground _
Various techniques to remove solvent and other
condensable vapors ~rom process gas streams have been
developed. One such technique involves the use of
activated carbon beds to adsorb the solvent ~rom a ~as
stream passed through the bed. Once the bed has adsorbed
all o~ the solvent it can hold, the bed is desorbed using
steam. In this desorption cycle, the steam is passed
through th~ bed causing the bed to heat up and causiny all
but a residual amount of the solven-t -to vaporize and be
carried out o~ the bed by the steam. The steam and solvent
vapors are then cooled and condensed. Once the bed has
been desorbed, the bed is cooled and dried and is ready for
another adsorption cycleO The solvent can be separated
from the condensed steam (water) using a variety
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of known separation techniques, e.g., distillation, phase
separation etc., depending on the particular solventts)
involved.
There are a number of disadvantages associated
with steam desorption. Primary among these is the
difficulty of separating the solvents, particularly water
miscible solvents, from the condensed steam~ Desorption
with steam ls also costly due to the amount of energy
consumed to desorb and separate the solvent. For example,
it can typically require 3,000 to 10,000 BTU's to desorb
and recover one pound of solvent depending on the solvent
and operating conditions. Thus, steam desorption can
consume a significant amount of energy relative to the
value of the solvent. Indeed, the cost of condensing and
separating the solvent may in some instances equal or
exceed the value of the solvent recoveredO It is also
expensive to treat the resulting waste water even
following separation.
An alternate technique for desorbing carbon beds
is by the use of an "inert" gas rather than steam. The
bed is hea~ed and inert gas is circulated through the
heated bed to carry away the vapor in the bed. The use of
inert gas allows the presence of relatively high vapor
concentrations without forming explosive mixtures. When
recovery of the vapor is desired the inert gas stream
containing the vapor is cooled with water to condense and
separate the liquid component.
The use of refrigera-tion condensation to recover
solvent directly from a gas stream is known. See for
example, U.S. patent 4,295,282 which discloses the use of
an open cycle heat pump for solvent recovery~ Refri-
geration condensation encounters some difficulties when
used with solvents having high vapor pressures (low boiling
points) or when the solvent is present in the gas stream in
low concentrations as is generally required in industrial
environments for safety reasons. This is due to the low
temperature, and accompanying frost problems~ associated
with solvent separation Imder such condltions. The use of closed cycle re-
frigeration systems for direct condensation of solvents is also known and is
fraught with similar diffieul~ies when working with such gas streams.
Disclosure of _h _Invention
The present invention seeks to overeome the diffieulties associated
with the various methods of direct vapor condensation and recovery known to
the prior art. An advantage has been accomplished in the present invention
by employing refrigeration condensation in eombination with a eoneentrating
system between the vapor souree and the eondensation system. This allows the
eondensation system to operate on a vapor-eontaining gas stream having a
higher concentration of vapor than in the vapor souree, e.g., drying oven pro-
eess gas stream, while avoiding the need to operate at the extremely low tem-
peratures, with attendant frost problems, whieh would be required to direetly
condense such a vapor souree.
The eoneentrating system of the present invention eomprises an
aceumulating device, such as a carbon bed, which ean temporarily hold vapor
from a vapor source for subsequent release. The coneentrating system also
eomprises earrier means, sueh as a noncondensable gas stream, which can be
eireulated through the aecumulating deviee to earry the aecumulated vapor to a
condensing system in a concentration greater than the concentration in the
vapor source. Thus, the vapor-containing gas presented to the condensing
system can have a higher concentration of condensable vapor than was present
in the vapor source. Moreover, such higher concentration is independent of
the ~apor concentration in the vapor source. In a typical case, the accumu-
lating device will be a bed of activated carbon. The bed will be heated to
desorb and release the accumulated vapor and to allow the carrier gas to remove
the vapor from the bed.
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Refrigeration condensation as used in the
present invention refers to the cooling of a gas stream
under substantlally non~pressurized condltions, e.g.,
nominally 3 atmospheres or less, to temperatures below the
normal cooling temperatures for water condensers, e.g.,
below about 50F. Preferably the gas is cooled at some
point in the condensation cycle to a temperature in the
range of +40F to -40F and is generally cooled to within
the range of +35F -to -20F for most common industrial
solvents~ Indeed, with common water miscible solvents it
may be advantageous to cool to temperatures of 0F or less
at some point during the recovery cycle in order to
deplete the vapor in the accumulator to a desirably low
level. Such temperatures can be achieved without frost
problems when using such solvents due to their ability to
depress the freezing point of water.
The use of refrigeration condensation in the
present invention offers several advantages over the use
of conventional water cooled condensers in combination
with gas desorption of carbon beds. Refrigeration conden-
sation allows more complete removal of condensable vapor
from the carrier gas stream which in turn allows more
complete removal of solvent from the accumulator. This
reduction in the residual amount of solvent (solvent
"heel"~ in the accumulator in effect increases the working
capacity of the accumulator. This would allow the use of
smaller accumulators and require less frequent accumu-
late/discharge cycling with a given size of accumulator.
This becomes important from an energy use standpoint since
during each discharge cycle the accumulator must generally
be heated. The heating of a large carbon bed can consume
a significant amount of the energy required in the
recovery process and, thus, minimizing the number of
heating cycles can be meaningful.
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The ability to reduce the solvent heel also
reduces cross contamination problems when the vapor in the
vapor source is changed. For example, when a dryiny oven
is used to dry different solvent-containing articles.
The present invention may also incorporate a
number of preferred features which improve performance
over the prior art systems. In one embodiment, the
recirculating carrier gas stream is used to heat (and
cool) a carbon bed. In another ernbodiment, only a portion
of the recirculating carrier gas stream is taken as a side
stream to be cooled Eor condensation and separation of the
solvent. This provides increased efficiency and economi-
cal sizing of cooling components. In yet another embodi-
ment, the side stream entering the condensing means is
directed into heat exchange relationship with the gas
stream leaving the condensing means. Thus, the gas
entering the condensing systeln is pre-cooled while the gas
leaving the condensin~ system is pre-heated before being
returned to the carrier gas stream recirculating through
the carbon bed. This recuperative heat exchange feature
is partic~larly useful to provide improved eEficiencies
and economical operation.
More specifically, the method of the present
invention may comprise passing a Eirst condensable
vapor-containing gas stream through a vapor accumulator,
such as a packed carbon bed, to accumulate the vapor in
the bed. Following acculnulation oE vapor in the bed, the
vapor is released and carried from the hed by circulating
a second carrier gas stream through the bed. The bed is
heated, preferably by the second gas stream, to cause
release, e.g., desorption, and admixture of the vapor in
the carrier gas passing through the bed~ Due to the use
oE the concentratincJ system, the average concentration o~
vapor in the desorption stream can be made much higher
than the average concentration generally found in the
Eirst gas strealn. ~rl,is concelitrated vapor-containirlg
carrier gas stream can then be more efficiently and
3L~9~5
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conveniently reEriyerated to condense and recover the
vapor therein.
As noted, the recovery of the vapor is accon-l-
plished in the present invention by directing at least a
portion of the vapor-containing carrier gas stream through
a refrigeration condensing system. Typically all or a
portion of the vapor-containing inert gas stream is
directed through a refrigeration condensing means, such as
an open cycle or a closed cycle refrigeration system, to
cool the side stream and cause the vapor to condense so
that it can be separated and removed from the strealn.
Preferably the condensate is recovered for reuse. The
cooled gas stream is then returned to the accumulating
device and the recirculation of the gas through the
accumulating device and condensing system continued until
the vapor in the accumulating device is depleted to the
desired level.
As can be appreciated by one skilled in the art,
additional accumulating devices may be employed in
parallel to the ~irst device so that while one device is
in the accumulating mode one or more of the others can be
in the discharge rnode to allow use in a continuous
process.
The term "condensable vapor" as used herein,
refers to materials which are normally liquid at room
temperature, that is those materials which can be
va~orized at temperatures normally encountered in
inc]ustrial dr~ing conditiorls, e~g. 20C to 200lC ~hether
at a standard or reduced pressure, ~ut which can exist as
a liquid at telnperatures at or near room ternperature and
at pressures a~ or near atmospheric press~re. The ~erm
"condensable vapor" thus includes the commonl~ used
industrial solvents which are used in coating resin
formulations, degreasing, painting, printing~ and -the like
and which can be flashed or vaporized in conventional
industrial drying ovens.
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The method and apparatus described herein have
particular utility in combination with a drying oven to
recover solvents removed by drying solvent-containing
articles.
S Brief Description Of Drawings
FIGURE 1 is a simplified block diagram of a vapor
recovery system.
FIGURE 2 is a schematic diagram of a basic vapor
recovery apparatus employing a carbon bed and refrigeration
condensing apparatus.
FIGURE 3 is a schematic diagram of an open cycle
refrigeration condensing apparatus.
FIGURE 4 is a schematic diagram of a condensing
apparatus comprising a closed cycle refrigeration system
for cooling.
FIGURE 5 is a schematic diagram of a vapor
recovery system employing dual carbon beds and an open
cycle refrigeration condensing system.
Detailed ~escription
The use of refrigeration condensing means to
separate and recover condensable vapor directly from a gas
has been described in U.S. Patent 4,295t282. However, due
to the principle of operation of this system, the
condensation of condensable vapor from gas streams
containing relatively low concentrations of vapor or
containing vapor having a high vapor pressure necessitates
cooling the gas to relatively low temperatures to achieve
condensation. Obtaining these low temperatures may be
difficult and expensive and is accompanied by a number of
problems including frost formation in the event of moisture
being present in the gas stream.
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_3
A means oE avoiding some oE the problems
associated witll low temperature operation is to ensure
that the vapor concentration in the gas stream is as high
as possible. ~nfortunately this is not always practical
in an industrial environment~ Due to the nature of many
industrial processes, e g~, solvent remova' in a drying
oven, the vapor concentration will inherently be low due
to the flow rate of the clrying gas, the minimal residence
time of the solvent-containing article in the oven, and
primarily, the need to maintain low solvent concentrations
in air streams in or~er to avoicl exceeding some saEe
fraction of the lower explosive limit (LEL) of the vapor/
gas combination.
The safety problem can in some few cases be
avoided by using non-flammable solvents. Where flammable
solvents must be used, an "inert" gas can be used so that
higher concentrations of solvent vapor can be safely
employed~ An inert gas is a cJas which will not form com-
bustible mixtures with the vapor, e.g., a gas containing
less than 11% oxygen, by volume, such a gas being pure
nitrogen or a mixture of nitrogen and oxygen. The use of
inert gas for clrying is shown in the literature, for
example in U~S. Patent No. 4,150,494. However, this
practice requires tightly sealed processing equipment,
e.g., drying ovens, and modification of existing ec~uiprnent
to use inert gas may entail significant expense.
It has now been found that the condensation
technique can be efEectively employed in combination with
industrial processes where rela-tively low vapor concentra-
tions are encounterecl if, instead of attempting tocondense the process gas directly, the process gas is
first operated on by a concentrating system to remove the
vapor from the process gas ancl store or accumulate the
vapor. This vapor can then be removed from -the
accumulator by a carrier gas stream which can be made to
have a higher concentration oE the vapor than the process
gas from which the vapor originally came. This concen-
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g
trated gas stream can then be advantageously condensed bythe refrigeration condensation techniques to be described
herein at acceptable levels of eEficiency and at tempera-
tures or under conditions which promote rapid and complete
condensation, but which avoid the frost problems which
would otherwise be encountered in a direct condensa-tion
process. Thus, by employing a unique variety and
arrangement of concentrating and condensing apparatus a
particularly efficient indirect condensation system and
process has been developed.
The indirect condensation technique as employed
in the present invention has been represented in FIG 1 of
the drawing where there is a block diagram showing a vapor
source 1 a concen-trator 3 and a refrigeration condensing
system 5.
The vapor source 1 can be any source of gas
containing a condensable vapor. For the sake of
simplicity and practicality specific reference will be
made throughout this specification to solvent vapors and
the vapor source discussed by way of illustration will be
an industrial drying oven. A typical industrial drying
oven consists of a tunnel of appropriate length through
which solvent-containing articles may pass. A gas strearn
is typically passed through the tunnel, in a co-current,
~5 cross-current or countercurrènt fashion, generally at
elevated temperaturè to mix with the solvent released from
the articles and carry the solvent from the oven.
Alt~-lough tilC (Ja5 dryincJ ov~n is discusse~l hercin
as a representative vapor source it will be appreciated
that the system and method described herein may be used to
advantage with other processes where solvents or conden-
sable vapors are encountered such as in connection with
dry cleaning processesl paint spraying booths, parts
degreasing and cleaning, polymer processing, printing or
other situations in which larc~e ~uantities of such
vapori~able liquids are transferred, transported or
stored.
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The concentrator sys-tem 3 represented in FIG 1
includes an accumulating device which can be any device
which will ef~ectively remove condensable vapor Erom the
condensable vapor-containing gas available Erom the vapor
source 1 and store or accumulate it for further
proeesslng. Representative accumulating devices whieh are
readily available are devices containing aetivated earbon
which will adsorb most of ~he common solvent and other
condensable vapors from a gas stream passed therethrough.
Fixed and mo~7ing bed devices are preferred over ~luidized
bed devices for use in the present invention. Fixed bed
deviees are generally less expensive, ean aeeommodate a
wider range of flow rates, ean be eyeled from the adsorb
to desorb mode more quickly, require less expensive
adsorbents and the like. Fluidized beds are less
preferred due to more restrictive flow rate ranges and the
expense of the adsorbent material employed. Although
activated carbon is the preferred adsorbent, other
adsorbent materials such as molecular sieves etc. may b~
use~ul in certain applications.
As noted above, the concentrator system 3 may
comprise a single fixed bed which is alternately adsorbed
and desorbed. The concentrator may also comprise plural
beds so that one bed is always availahle ~or adsorption to
allow continuous operation. Alternatively, moviny hed
devices, such as rotating or conveyor beds may be employed
whereby the adsorbent medium is continuously moved through
adsorbing and desorbiny areas which are sealecl froln one
another, again allowing continuous operation.
The refrigeration condenser system 5 represented
in ~'IG 1 can be any means for refrigerating and condensing
at least a portion of the vapor in the condensable vapor-
containing carrier gas stream employed to carry the vapor
from the aceumulator. Mechanical re~rigeration systems
3S such as open cycle heat pumps and elosed cycle re~rigera-
tion systems can be employed to advantage and will be
described in greater detail hereina~ter. Other means o~
refrigeration such as liquified gasses, freezing polnt
depressants etc. can be used, but are less pre~erred. The
condensed material can be recovered as a liquid from the
condenser.
The flow represented by the arrows in FIG 1 show
circulation of the gas from vapor source 1 to concentrator
3 and back again and also from concentrator 3 to condenser
5 and back again. In practice, the gas taken from vapor
source 1 may be partially or totally exhausted froln
concentrator 3 without being returned to the vapor source
1. Si~ilarly some or all of the gas entering condenser 5
may be exhausted after being processed or may be returned
to an adsorbing concentrator as will be described in
greater detail in connection with the other Figures.
~eferring to FIG 2 there is shown a processing
system 7 comprising a vapor source 9, such as a drying
oven. Vapor source 9 is connected to an accumulator sucn
as carbon bed 11 by lines 13 and 15. In the adsorb mode
the condensable vapor-containing gas enters bed 11 through
line 13 and is adsorbed on the carbon in bed 11. The
stripped gas exits bed 11 through line 15 and is either
recirculated to the vapor source 9 or exhausted through
line 17 by the selective opening and closing of valves 19
and 21.
When the amount o~ vapor has reached the desired
upper lilnit or capacity of carbon bed 11 vapor source 9 is
shut off or connected to a second accumulator (not shown)
by manipulation of valves 19 and 23.
The carbon bed 11 can then be desorbed by
circulating a noncondensable carrier gas through the
carbon bed 11. When ~lammable solvent is beiny recovered,
an inert carrier gas is initially supplied through valve
25 and line 27 to purge the s~stem and reduce the ox~gen
content to a non-combustible level. The carrier gas flows
through bed 11 exiting the carbon bed through line 29
whicll is a recirculation pa~h bdck to the bed 11. ~lower
31 provides the pressure to cause the flow through the
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lines. ~3eater 33 dnd cooler 35 are ernployed, respective-
ly, to heat the carrier gas stream during the desorb mode
and to cool the gas stream following completion of
desorption. Heater 33 Jnay be any conventional rneans used
to heat a gas s-tream such as a s-team coil, electrical
resistance hea-ter, gas fired heater or the like.
Similarly, cooler 35 may be a cold water heat exchanger,
refrigeration coil or the like.
In the desorption mode, the carbon bed 11 is
heated, such as by internal heating coils or by heating
the carrier yas circulating through lines 27 and 29 by
heater 33. Heating of the gas is generally preferred
since the carbon bed 11 is more uniformly heated and can
be more completely desorbed at generally lower maximum bed
temperaturesO That is, there are no concentrated hot
spots in the bed resulting in an inefficient use of
energyO The temperature to which the bed 11 must be
heated will, oE course, vary with the vapor adsorbed
therein.
As desorption of the bed occurs, the concentra-
tion oE vapor in the carrier gas stream in line 29
increases. By regulation of valve 37 some or all oE the
circulating gas is directed into the refrigeration con-
denser system 39 through line 40 where the gas is cooled,
optionally under pressure of up to about 3 atmospheres, to
cause con~ensatioll o~ at least a portion of the condens-
able vapor. The condensing system 39 can be an open cycle
reErigeration systelll or a closed cycle~ reErigeration
system capable of refrigerating the vapor-containing
carrier gas to the point where condensation of the vapor
will occur. These refrigera~ion systems will be described
in greater detail hereinafter in connec-tion with FIGURES 3
and 4.
The condensate can be removed through drain line
41 while the carrier gas whi¢h has been processed in the
condensiny s~item 39 is returned to the desorbing ga~
stream through lines 43 and 29 or alterna-tively exhausted
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through line 45 or returned to a carbon bed in the
adsorbing mode not shown.
As noted previously, the noncondensable gas used
to desorb the concentrator can be an inert gas to allow
lligh concentrations of flammable vapors to he fed to the
condensing system. Air may be used where the vapor does
not form an explosive mixture with the gas in the
concentrations to be employed.
FIGS 3 and 4 show two different refrigeration
condensing systems which can be employed in the present
invention. FIG 3 depicts an open cycle refrigeration
condensing system shown generally at 49 which could be used
as the condensing system 39 shown in ~IG 2. The open cycle
system is disclosed in U.S Patent 4,295,2~2. Condensing
system 49 comprises a compressor 51, heat exchanger 53,
turbine or expander 55 and drive means 57, such as an
electric motor, for the compressor 51. In the most
advantageous arrangement, the compressor 51, and the
turbine 55 are coupled together in such a manner that the
work produced by the turbine 55 is utilized to help drive
the compressor 51, thus reducing the load on the drive
motor and improving the overall efficiency of the system.
Condensate separator 59 completes the system along with
associated valves and lines.
In operation, the vapor-containing carrier gas
enters the condensing system 49 through inlet line 61 at
temperature Tl and pressure Pl and is compressed by
compressor 51 to T2 and P2 where the ratio of P2/Pl is
generally about 1~3:1 to 3:1.
Depending on the pressure ratio of the compres-
sor and the initial temperature, the gas may leave the
compressor at about 150C. After leaving compressor 51,
the gas passes through line 63, heat exchanger 53 and line
65, on its way to expander 55.
The precooled gas from heat exchanger 53 enters
expander 55 where it expands back to nominal atmospheric
pressure becominc~ cooled in the process. The work
produced during expansion is used to help drive compressor
51, thus reduciny the load on drive motor 57.
Depending upon the vapor component, the concen-
tration of vapors, the pressure and temperature, somevapors rnay condense in heat exchanger 53 enroute to
expander 55. A drain line 67 is provided to remove and
collect any liquid that may condense. Additional vapor
may con~ense on cooling during passage through expander 55
and e~it line 69. This condensate is captured and
collected by means of a condensate separator 59 and may be
drained through drain line 71. The cooled carrier gas
with much of the vapor condensed and removed, exits the
separator through line 73 and exits the condensing system
49 via heat exchanger 53 and line 75. Hea-t exchanger 53
thus performs the dual function of precooling the gas
enroute to expander 55 and reheating the cooled gas from
expander 55, returning through line 73, before exiting the
condensin~ system 49.
Thè colnpressor 51 and expander 55 used in the
present invention may be of any suitable type:
reciprocating, vane, rotary screw, centrifugal, axial
Elow, or other type. High efficiency, e.g., about 70~ or
greater, is desirable in order to minimize the net drive
power and to permit attainment of the low temperatures Eor
solvent vapor cond~nsa~iorl.
The pressure ratio of the co~pressor and
expander is a desigrl variable that can be selected to
optimize any given application. The greater the pressure
ratio the greater the temperature change through the
compressor and expander, but also the yreater the net
power re~uired to drive the system. Generally, a pressure
ratio of up to about 4:1, with pressure ratios in the
range of 1.3.1 to 3:1 being generall~ pre~erred 7 are the
most advantageous. However, for some applications, for
example, where very volatile vapors are to be condensed or
other low tempera-ture conditions Inust be obtained, a
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1,
higher pressure ratio may be desirable. The compressor
nay be driven by an electric motor, gas turbine engine,
steam turbine, or other suitable means.
The heat exchangers used in the open cycle
system can he any conventional type such as co-current,
countercurrent, crossflow, gàs-gas, gas-liquid, etc. It
is desirable that the heat exchanger have an efEiciency oE
about 70% or greater in order to enhance the economics oE
the process. Further, since the open cycle system
generally operates at relatively low pressure ra-tios, it
is desirable that the pressure drop across the heat
exchangers also be minimized to maintain efEiciency.
However, while the pressure drops must be minimized in a
low pressure open cycle system, certain advantages also
accrue in that the heat exchangers need not be hermetic-
ally sealed as with a closed cycle freon s~stem and, due
to the low pressures encountered, can be constructed of
light duty economical materials.
AlthoucJh a single heat exchanger is shown Eor
purpose of illustration in FIG 3, in practice, additional
heat exchanyers may be used beEore the cornpressor 51 or
between the compressor 51 or expander 55 for example, to
reject heat fron~ the gas entering the heat exchanger 53
through line 63. This is because a phase change (conden-
sation) may occur in the heat exchanger 53 with respect toat least some of the condensable vapor contained in the
gas in line 63, while no phase change will occur in the
cooliny gas in line 73 since a:l.l condensed vapor has been
removed therefrom in separator 59~ Accordingly, the
3~ capacity OL the gas in line 73 to accept heat in heat
exchanger 53 may be l:imited to the acceptance of sensible
heat up to a limit of te~nperature T2. Any excess heat
given oEE by the condensation oE vapor entering in line 63
can be advantageously rejected to another sink to i,nprove
efficiency of the sys-tem. Generally, this can be readily
accomplished by the use oE an auxillary heat exchanger -to
extract heat from the gas entering in line 63 at some
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pOillt interlnedi.lte hea~ exchanyer 53. This conEiguration
takes advantage oE the Inaximum temperature diEferences at
eaeh end of the exehanger in a counter eurrent mode in
order to obtain maximum heat transfer.
In certain applieations, it may be desirable to
utilize additional air or water eooled heat exchangers in
line ~5 prior to expan~er 55 to provide additional
preeooling oE the gas prior to expansion~
~hen operating eonditions in the system are such
that iee may form, e.g., when operatiny for long periods
below 0C with a solvent, sueh as heptane, which does not
depress the freezing point of water, some means or
teehnique inay be neeessary for preventing the formation oE
or for the removal of frost and iee from the separator 59,
heat exehanger 53 or other parts of the system. This may
be no more than a dual set of heat exehangers permitting
defrosting of one set while the other is operating, or
means, sueh as a moleeular sieve, may be used to remove
the moisture before it has a ehanee to colleet on the
component surfaces. Yet another teehnique is to in~ect a
small quantity of aleohol, or other freezing point
depressant, to depress the free~ing point suEEiciently to
prevent frost or ice formation
The separator, shown generally at 59 in FIG 3,
must perform the function of separating the eondensed
liquid droplets Erom the gas strealn in whieh it is
entrained. rrhis may require a sereen or paeked column
whicll provides ~l klrge surface .area on which tl~e corldcn~ec
droplets can coalesce and drain away~
Figure 4 is a simplified diagram of a condensing
system employing elosed eycle reErigeration. This
condensing system is shown generally at 79 and comprises
heat exchanger 81 dual condensers 83 ancl 85 eontaining
cooling eoils 87 and 89 through whieh the refrigerant
flows. These coils 87 and 89~;form part of a closed cycle
reErigeration systern represented by bloeks ~1 and 93.
These closed cycle refrigeration systems may be oE the
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conventional vapor compression type employing, for
example, "Freon" as the re~rigerant or they may be o~ some
other closed cycle type of re~rigeration systern. The
essential difference between the closed cycle condensiny
system 79 oE FIG. 4 and the open cycle system ~9 o~ FIG. 3
is in the method employed to cool the condensable vapor
containing carrier gas entering the system. In the open
cycle system ~9 the gas is directly compressed and cooled
without the use of a separate refrigerant. In the closed
cycle system 79 of FIG. 4 a separate re~rigerant is cooled
and heat withdrawn from the vapor containiny gas into the
refrigerant which is maintained in a closed system
separate from the vapor containing gas.
In operation the vapor-containing carrier gas
taken from the accumulator is taken into condensing system
79 though line 95, passes through heat exchanger 81 where
it is pre-cooled by the carrier gas stream which has been
cooled by cooling coils 87 or ~9. Some condensation may
occur as a result o~ the cooling and the condensate may be
drained from line 97 through drain line 99O The pre-
cooled gas is directed through line 97 into condenser 85
where it is cooled by cooling coil 89 causing condensation
of vapor which may be drained throuyh drain line 101. The
cooled gas exits the condenser 85 into line 103 to be
returned to heat exchanger 81 where it recover.s heat ~rolr
the gas entering in line 95 and exits through line 105.
Blower 107 supplies the means to circulate the gas in the
systeln in the event the pressurc differential between
lines 95 and 105 is not great enouyh.
A second condenser 83 is shown in parallel with
condenser 85. This condenser can be used to cool a
portion of the gas in line 97 to increase the capacity o~
the system by proper adjustment of valves 109, 111, and
113. This arrangement also allows the condensers 83 and
85 to be used consecutively. ~For example, in the event
the ~irst condenser rnust be defrosted the second condenser
t~ ~ c~
s
can be employed whi.le the firs~ is defrosted to allow
continuous operation.
Figure 5 is a schematic flow diagram oE a dry:ing
oven system ernploying an indirect condensation systern
accordiny to the present invention. The system comprises
a drying oven 115 which is essentially a tunnel l:hrough
which a solvent laden article (not shown) such as a coated
web, or other coated article is passed through so that the
solvent can be removed by contact with a gas stream which
will take up and carry away at least a portion of the
solvent. The drying process is generally continuous
wherein the solvent--laden articles move co-, cross- or
counter-current to the moving gas system.
The gas stream flowing through oven 115 exits
through line 117 and is direGted into either of carbon
beds 119 or 121 through lines 123 and 125 or lines 123 and
127 by appropriate manipulation of valves 129 and 131. It
is generally advantageous to have the gas stream enter the
top of a vertical bed so that if ~ouling of the bed
occurs, removal of the fouled carbon can be most readily
accomplishedO The vapors are adsorbed in the carbon beds
119, 121 and the process gas stream leaves the beds
through lines 133 and 135 or lines 137 and 135 (dependin~
on which bed 119 or 121 is in the adsorb mode) by manipula-
~5 tion of valves 139 and 141. The clean gas can then be
recirculated to oven 115 through line 143 or exhausted
through line 145.
~ s has been noted, two carbon beds 119 and 121are employed in the s~stem shown in Figure 5. This al.lows
one of the beds to be desorbed while one is adsorbing
which in turn allows drying oven 115 to operate
continuously. Appropriate manipulation of valves 129,
131, 139 and 141 allows switching ~rom one bed ~o the
other.
Once bed 119 has adsorbed the desired amount of
vapor or reached its capacit~, closing valve 129 and 139
while opening valves 131 and 1~11 will divert the gas
~2~ ~5
-19-
stream in line 117 to bed 121. Bed 119 can then be
desorbed by a separate carrier gas stream. This carrier
gas stream, pre~erably an inert gas s-tream, enters through
line 147 and flows into recirculation loop 1~9 and into
carbon bed 119 through line 125. During desorption, bed
119 is heated either by heating elements contained in the
bed (not shown) or by heating the noncondensable carrier
gas entering the bed. On heating the bed 119, the vapor
is desorbed and admixed with the carrier yas stream and
swept froln the bed through lines 133 and 149. The
desorption stream is moved through the bed by blower 151
and can be heated and cooled by heater 153 or cooler 155.
As the vapor-containing carrier gas stream
leaves bed 119 through line 1~9, at least a portion o~ the
stream is directed into the refrigeration condensing
system through line 157. The portion of the gas stream
entering the condensing system is controlled by valve 158.
The amount of gas diverted into the condensing system
through line 157 is determined by a number of factors, but
primarily by the size of the carbon beds 119 and 121 and
the desired flow rate through the beds and the sizing of
the condensing s~stem. The desorption time ~including
heating and cooling times) of the beds 119 and 121 is
determined in part by the flow rate of carrier gas through
the beds. The selected flow rate may be too large to be
accomodated by the available condensing system. Moreover,
a condensing system large enough to handle all of the
volulne Elowing through line 1~9 may not be cost justi~.ied
in order to achieve incremental decreases in desorption
time.
The vapor-containing gas entering the re~rigera~
tion condensing system is compressed b~ compressor 159,
cooled in heat exchanger 161, expanded in turbine 163 to
provide further cooling, and enters separator 165, where
the condensed vapor is removed through drain line 167 as
described in connection with i.~igure 3. The cooled gas is
exha~sted ~rom the separator t~lrough line 169 and is
s
-20-
directed through heat exchanger 161 where it is pre-he~ated
by the gas Erom compressor 159. The pre-heated gas is
then returned to the desorption loop through line 171.
As shown in Figure 5, heat exchanyer 161 is
adapted to allow additional cooling of the gas frorll
compressor 159 by means of a cooling coil 173, generally
cooled by water~ Since some condensation may occur as a
result of the cooling in heat exchanger 161, a drain line
174 is provided.
In the system shown in Figure 5 an optional ven-t
line 175 is shown which allows a reduction in the pressure
of bed 119 or 121 as it is being desorbed. This occurs as
a result of the pressure ratio across compressor 159 and
turbine 163. For example, if the line pressure entering
compressor 159 is 1 atmosphere and both the compressor 159
and turhine 163 are operating at 2:1 compression ratios,
then the pressure in line 177 leaving compressor 159 will
be 2 atmospheres. Similarly, the pressure in line 179
leavin~ turbine 163 will be 1 atmosphere. However, if
valve 181 is opened to atmospheric pressure then the
~ressure in line 179 due to expansion in turbine 163 will
be 1/2 atmosphere and the pressure in the carbon bed 119
will be brough~ below 1 atmosphere. This reduced pressure
will reduce the residual vapor concentration in the bed.
OE course~ venting in this manner draws ofE solne o~ the
vapor-containing gas and routes it back to line 117 w~-,ere
it is ~ed to bed 121 in the adsorb mode. Thus, in e~ect,
~some vapor is rnerely trans~erred from one bed to the other
and it may only be advantageous to employ the venting
procedure near the end of the desorption cycle.
Once bed 119 has been desorbed to the desired
level the hot bed must be cooled in order to be ready to
enter the adsorb mode again. This is accomplished in part
by the use of cooler 155 to cool the recirculating gas in
loop 149. As the cooling of the bed 119 continues and the
vapor level in the bed 119 has dropped below the residual
level no more solvent will appear in the carrier gas in
-21-
line 171 anci the output can be exhausted through lines 133
and 185 with relatively cool ambient air being brouyht in
throucJh line 187 during the final cooling stage if
desired. Routing the output in line 171 throuyh line 133
and back to an adsorbing carbon bed through line 117 may
also be desirable when the concentrating circuit is first
purged with gas prior t~ beginning desorption.
Typical operating conditions for a systerll
similar to that shown in Figure 5 can be predicted under
the following conditions:
Solvent Laden Air Stream from Oven
10,000 CFM - 30~ L.E.L. at 90F
Solvent - 70~ Heptane
30% Isopropanol
Approx. 500 pounds solvent per hour
Carbon Beds
9300 lbs. carbon each bed, 11%
working charge (by weiyht) o~ solvent
Condensing System
2000 CFM open cycle heat pump
90 hp drive
The carbon bed would re~quire about 120 min. to
adsorb the speciEied charge o~ solvent.
~rO de~joY~ e solve[~- ch~rged bed, nitroyen gas
circulated by a 50 hp blower (151) at the rate of 10,000
C.F.M. will heat the bed to 300 - 350F. 2000 Co F~M~
would be taken as a side stream and routed through the
reErigeration condensing system. During desorption the
average vapor concentration would be about 3O5% by volurne
with a peak concentration of about 7% by volume. The tur-
bine exhaust temperature in li,n~e 179 will vary from about
-~40~F at tlle beginniny oE desorption and falling to about
-40F at the end of the~ desorption cycle when ~he va~or
-22-
concentration is lowest. The working charge (approximate-
ly 1000 pouncls) oE solvent will be desorbed in about 76
minutes requiring about 1900 BTU of energy per pound of
solvent recovered.
The bed can be adequately cooled in about 28
minutes by cooling the top half of the bed with nitrogen
at 100F flowing co-current to the direction of adsorption
flow. The lower half of the bed will then be cooled by
the gas flowin~ through duriny the adsorption cycle.
The performance of the indirect condensa-tion
system of the present invention has been conEirmed in a
pilot system similar to that shown in FIG. 2. A carbon
bed containing 1135 grams of carbon with a bed depth of 18
inches was used to adsorb the vapor from an air stream
flowing at the rate of 225 scfh. The air stream was at
90F and 50% relative humidity and contained 3800 pprn
mixture of isopropanol (30 wt. %) and a heptane fraction
(70 wt. %)~ The adsorb cycle was continued for about 108
minutes at which time the concentration of vapor in the
air stream leaving the bed was 380 ppm (10~ of entering
concentration).
The charged bed was then desorbed using nitroyen
gas heated to 356F and circulated at the rate of 139
scfh. A refrigerated condensing system was used to cool
the desorption gas strealll to -29F. After circulating the
gas stream through the condenser Eor about 90 Ininutes 104
grams of heptane, 45 grams of isopropanol and 14 grarl~s of
water was condensed and recovered. This represented about
98% of the solvent initially charged to the bed. Cooling
the bed by recirculating the gas without heating required
about 30 minutes, suggesting that substantially cornplete
desorption and cooling could be accomplished in an amount
of time substantially equivalent to the adsorb tiMe.
