Note: Descriptions are shown in the official language in which they were submitted.
CA 02277449 1999-07-07
TITLE OF THE INVENTION
VAPOUR MANAGEMENT SYSTEM
BACKGROUND OF THE INVENTION
Volatile solvents are used in many industrial processes in
which the volatile solvent is used for cleaning purposes. As a
result of such use the volatile solvent becomes contaminated with
foreign matter. Due to the cost of such volatile solvents,
environmental concerns, and the cost of disposing of such
contaminated volatile solvents, it is desirable to maximize the
use that can be made of the volatile solvent by removing the
contamination from it by recycling it into the purified solvent
form for further use in the industrial process.
In addition to industrial processes requiring purification
and recovery of volatile solvents, there are also many industrial
processes in which it is important to control the escape of
vapour from volatile liquids, The escape of vapour from such
volatile liquids represents an environmental and occupational
health concern, and a financial cost in that the vapour is lost.
The current procedure for preventing or reducing vapour loss is
to provide a scrubbing system on tanks, containers or stacks in
2
CA 02277449 1999-07-07
which the volatile liquid, or the vapour is stored. While the
scrubbing system may reduce or even eliminate the environmental
concerns of vapour loss, it does not overcome the cost of the
loss of such vapour. Further, the used scrubbing material may in
itself present an environmental disposal problem. Examples of
such containers include oil storage tanks and gasometers. Other
examples include the containment of vapours from stacks or from
recycled solvents in processes such as gun-washing in automotive
paint shops.
Currently the common practice for purifying and recovering
contaminated volatile solvents is distillation and condensation.
Typically the solvent is boiled such that a vapour is formed. The
vapour is then allowed to pass through a spiral or serpentine
tube where it is cooled by heat exchange with air blown across
the tube or with another liquid which flows around the outside of
the tube. The heat exchange leads to condensation into the liquid
phase inside the tube. This liquid phase then runs off into an
open collection vessel. This method has a disadvantage in that as
the vapour condenses in the tube, it has the potential for the
build-up of back pressure within the distillation vessel which
gives rise to a "champagne effect", i.e. vigorous boiling and
cavitation, rather than controlled evaporation. This has the
3
CA 02277449 1999-07-07
consequence of a potential safety hazard, reduced efficiency, and
increased operating costs.
Further, the run off into an open collection vessel used in
conventional systems leads to a loss of volatile solvent to the
environment. This loss of solvent from an open collection vessel
to the environment reduces the recovery of the solvent and causes
an environmental hazard for operators around such tanks.
A further problem with conventional processes is the presence
of uncondensed vapour in the collection vessel. Further, sealing
the collection vessel, or the connection to the collection vessel
from the distillation vessel in conventional systems causes
pressure buildup unless vented. Such a pressure buildup or back
pressure caused by uncondensed vapour in the collection vessel,
or the conduit leading thereto can result in "foaming" in the
distillation vessel. In order to prevent buildup of back pressure
in the collection vessel and consequently the distillation
vessel, the collection vessel must be vented. Similarly, any
vessel in which, or into which, a volatile solvent is placed
3
requires a vent in order to avoid vapour lock. Conventional
methods of venting lead to loss of volatile solvent in the form
of vapour into the atmosphere, thereby further reducing the
4
CA 02277449 1999-07-07
efficiency of conventional methods of distillation and
condensation and creating an environmental hazard.
Hence, there is a need for a method of condensing a volatile
vapour which will ideally prevent any significant loss of
volatile solvent from the system, and enhance recovery and
condensation of the vapour to liquid. The consequence of this
increased recovery of volatile vapour is a decrease in the
overall cost of the vapour recovery, improved safety of the
system, and a reduction in potential environmental harm.
SU1~1ARY OF THE INVENTION
Accordingly, the present invention provides a vapour recovery
system for efficient and safe recovery of a vapour from a solvent
comprising:
a distillation module comprising a distillation chamber for the
solvent and heating means for heating the chamber to vaporize the
solvent;
a direct condensation module comprising a container for condensing
the vapour and collecting the sblvent in the liquid phase;
conduit means for directing the vapour substantially without
condensation from the distillation chamber to the direct condensation
module, the conduit means sloping downwardly towards the distillation
CA 02277449 1999-07-07
chamber to allow any condensate formed within the conduit to drain
into the distillation chamber;
a vapour management module for condensing vapour remaining
uncondensed by the direct condensation module; and
a vapour outlet located above the surface of the liquid in the
direct condensation module, the vapour outlet communicating with the
vapour management module to allow for passage of vapour from the
direct condensation module to the vapour management module.
In a further aspect of the invention, a vapour management system
comprises a container containing heat absorbing material, a vent, a
vapour inlet and means for guiding vapour from the vapour inlet
through the heat absorbing material to the vent. The vapour may be
guided through the container by means of a conduit extending between
the vapour inlet and the vent, the conduit passing through the heat
absorbing material. Alternatively, the container may contain solid
heat absorbing material which is permeable to vapour and condensation
and through which the vapour passes from the vapour inlet to the vent.
a
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic view of a preferred embodiment of the
vapour recovery system comprising a distillation unit, a direct
condensation module and a vapour management module.
6
CA 02277449 1999-07-07
Figure 2 is a sectional view of a preferred embodiment of the
distillation unit of the vapour recovery system illustrated in
Figure 1.
Figure 3 is a sectional view of a preferred embodiment of the
distillation unit of the vapour recovery system illustrated in
Figure 1, in which the heating means is located at the upper end
of the distillation chamber.
Figure 4 is a sectional view of a preferred embodiment of a
lined distillation chamber of the vapour recovery system
illustrated in Figure 1.
Figure 5 is a sectional view of a preferred embodiment of the
direct condensation module of the vapour recovery system
illustrated in Figure 1.
Figure 6 is a sectional view of a preferred embodiment of the
vapour management module of the vapour recovery system
illustrated in Figure 1.
a
Figure 7 is a partial sectional view of an alternative
embodiment of the vapour management module in which embodiment
the heat absorbing material is in a solid form.
7
CA 02277449 1999-07-07
Figure 8 is a schematic representation of a heating control
system that is employed in the distillation module to control the
rate of vaporization of solvents.
DETAILED DESCRIPTION OF PREFERRED EI~ODIMENTS
The preferred embodiment of a vapour recovery system for
solvents according with the invention is shown in Figure 1. In
this embodiment the vapour recovery system [10] comprises a
distillation module [1], a direct condensation module [11] and a
vapour management module [31]. In this embodiment of the
invention the solvent or mixture of solvents to be recovered is
heated in the distillation module [1] to generate solvent vapour.
This solvent vapour is then directed through a downwardly
inclined conduit [12] to a vapour inlet [13] to the direct
condensation module [11]. This downwardly inclined conduit [12]
in a preferred embodiment of the invention extends within and
toward the bottom of the condensation module [11]. There is
substantially no condensation of the vapour in the conduit [12]
but any condensate which does form runs down into the
condensation module [11] and, therefore, no back-pressure is
created by the condensate formation. The condensation module [11]
is charged with a coolant liquid [14] which is the same solvent
as that being distilled. Thus, as will be described in more
8
CA 02277449 1999-07-07
detail hereinafter, in the condensation module [11], the vapour
is subjected to a first round of cooling in which the heat
absorbing material is the solvent to be recovered in the liquid
phase [14]. Some vapour may pass through and exit from this
liquid [14]. This vapour, plus air, then passes from the
condensation module [11] through the vapour outlet [32] into the
vapour management module [31]. In the vapour management module
[31] the vapour air mixture is required to pass through a heat-
absorbing material [44] and this heat exchange process leads to
further condensation of vapour from the mixture. The remaining
gases then leave the vapour management module [31] through a vent
[42] and are substantially free of any solvent vapour.
Preferably, any condensate formed in the vapour management module
[31] runs back into the condensation module [11]. However the
condensate formed in the vapour management module [31] can also
be run into a secondary container, if desired.
In a preferred embodiment illustrated in Figure 2, the
distillation module [1] comprises a distillation chamber [2] in
which the contaminated solvent or solvent mixture [S] to be
recovered is collected. The distillation chamber [2] is closed to
prevent the escape of any vapour generated therein other than
through the conduit [12]. The size of the conduit [12] and the
opening of the vapour inlet [13] to the direct condensation
module [11] should be adequate to allow free passage of vapour
9
CA 02277449 1999-07-07
from the distillation chamber [2] without resulting in a pressure
build up in the distillation chamber [2]. Further, the conduit
[12] is ideally positioned toward the upper end of the
distillation chamber [2] since the hot vapour will rise. This
distillation chamber [2] sits within a larger heating vessel [3]
containing an oil bath [5]. The heating vessel [3] is provided
with one or more heating elements [4] immersed in the oil and, in
operation, each heating element [4] heats the oil [5], which in
turn heats the distillation chamber [2] at least until the
solvent [S) within the distillation chamber reaches its boiling
point and vapour is generated. Once the boiling point of the
solvent is reached, the power supplied to the heating element is
controlled to regulate the rate of vaporization of the solvent
until the solvent is substantially all evaporated.
It is essential that the oil [5] have a boiling point higher
than that of the solvent to be recovered or, in the case of a
solvent mixture, the boiling point of the highest boiling
component of the mixture. In addition, the oil [5] should not be
flammable within the temperature ranges in which the distillation
a
module [1] will operate. In the preferred embodiment, the oil [5]
in the heating vessel [3] will surround a substantial portion of
the distillation chamber [2] (for example, to the level 5a in
Figure 2) to ensure that there is sufficient heat to maintain the
CA 02277449 1999-07-07
evaporated solvent in the vapour phase at least as far as the
conduit [12].
While the preferred embodiment describes the means for
heating the distillation chamber [2] as comprising a heating
vessel [3] containing one or more heating elements [4] immersed
in oil [5], alternative means of heating the distillation chamber
[2) are possible. In one alternative embodiment, the means for
heating the distillation chamber [2] is an infrared lamp [L]
located toward the upper end of the distillation chamber [2] as
illustrated in Figure 3. The heating provided by such a lamp can
be regulated by rheostatic control of the intensity of the lamp.
In this embodiment of the invention, the solvent [S] in the
distillation chamber [2] is heated from the top down. This top
heating provides the advantage that only the top layer of the
liquid to be distilled needs to be heated to initiate
distillation. Further, as the distillation progresses, it is only
the energy of vaporization for the top layer of the liquid that
needs to be provided to continue the distillation.
The distillation chamber [2] is preferably provided with an
anti-vacuum valve [V] (Figure 2) which prevents vapour from
escaping from the distillation chamber [2], but is triggered to
allow entry of outside air when the pressure within the
distillation chamber [2] falls below atmospheric. As the
11
CA 02277449 1999-07-07
distillation chamber [2] cools down following distillation, the
pressure in the distillation chamber [2] falls. Since the conduit
[12] provides a passage from the distillation chamber [2] to the
vapour inlet [13] which communicates with the liquid condensed in
the condensation module [11], without the anti-vacuum valve [V]
in the distillation chamber [2], the reduction in pressure would
lead to a back-flow situation with condensed solvent being drawn
from the condensation module [11] into the conduit [12] through
the vapour inlet [13], and thence into the distillation chamber
[2] .
In an alternative embodiment of the distillation module [1]
there is a line feeding contaminated solvent directly into the
distillation module [1] through source inlet [7]. In this
embodiment, a means for preventing flow-back from the
condensation module [11] to the distillation chamber [2] is
provided. In addition, the anti-vacuum valve can be shut off such
that any negative pressure created in the distillation chamber
[2] as it cools can be used to draw more contaminated solvent for
recycling into the distillation chamber [2] through source inlet
[7]. This acts as a natural pump which can be used as part of the
process since it allows continuous flow, without the need for
separate pumps.
12
CA 02277449 1999-07-07
One aspect of the current invention is a heating control
system, as illustrated schematically in Figure 8, that is
employed in the distillation module to control the rate of
vaporization of the solvent or solvents. In the distillation
module, the solvents are separated from the impurities in the
solution and may also be separated from each other. The control
system is designed to do this in a time efficient manner while
also minimizing operator intervention.
The heating control system has three key components, a
variable heating means [61], for example, a plurality of the
heating elements [4] of Figure 2, a temperature sensing means
[62], for example, a temperature probe [6] (see Figure 2) which
may be either a platinum thermistor or a thermocouple, and a
control computer [63], for example, a microprocessor with
required software. The control computer [63], by use of a
control law, selectively triggers relays [64] which are in series
between a power supply and heating means [61] and which energize
the heating means [61] in an ordered manner, in response to
temperature reference signals input to the control computer [63]
3
from the temperature sensing means [62]. In a preferred
embodiment, which is especially useful where a mixture of
solvents is to be distilled, an important element of this control
system is a control law that allows the system to operate in 2
modes. The first mode is a solvent mode where the system begins
13
CA 02277449 1999-07-07
to heat the mixture of solvents until the boiling point of the
solvent with the lowest boiling point is reached. At the
temperature of that lowest boiling point the power delivered to
the heating means is maintained at a constant level to provide
the energy of vaporization at a desired rate, until the first
solvent has been substantially distilled from the solution. Once
the first solvent has substantially distilled, the power provided
to the system is no longer used to provide the required energy of
vaporization and no further heat is being lost as a result of
heated vapor, and therefore the temperature of the liquid in the
distillation chamber [2] begins to rise. The power provided to
the heating means can again be increased until the boiling point
of the next solvent is reached. This control cycle is repeated
until the solvents have all been vaporized.
The second mode is the water mode. In the water mode of the
control system there is no advantage to stepped regulation of the
temperature. Water has a relatively high specific heat capacity
and requires significant energy input in order to boil.
Therefore, in the water mode, it is not necessary to regulate the
a
temperature as closely. In other words, in the water mode the
distillation chamber and its contents can be heated continuously
until the evaporation is complete, at which time a timed heating
cycle can then be brought into effect. The two modes of the
control system allow the distillation chamber to be used for
14
CA 02277449 1999-07-07
vaporization of water and organic solvents. Distillation can take
place with a mixture of water and organic solvents, or either
alone, with no significant foaming.
The power applied to the heating means [61] during the
vaporization phase for any particular solvent provides the energy
of vaporization for that solvent. Therefore, controlling the
power applied to the heating means [61] also controls the rate of
vaporization, and is used to maintain an equilibrium with the
condensation rate of the vapour in the system.
In the embodiment shown in Figure 2, the temperature sensing
means is a probe [6] that is used to monitor the temperature of
the distillation chamber [2]. The probe [6] detects an increase
in temperature of the distillation chamber [2] as the solvent is
heated. The rate of evaporation of solvent from the distillation
chamber [2] is regulated by means of the control system of Figure
8.
Controlling the rate of vaporization of the solvent in the
.;i
distillation chamber [2] means that the vapour pressure in the
distillation chamber [2] is closely regulated. Since the
pressure differential between the distillation module [1] and the
condensation module [11] is important, this strict regulation of
vapour pressure will enhance the efficiency of vapour recovery.
CA 02277449 1999-07-07
In addition, by closely regulating the rate of vaporization of
the solvent, it is possible to reduce the required mass for
solvent condensation in the condensation module [11].
After the liquid in the distillation chamber [2] has
evaporated, a timed heating cycle is brought into effect by means
of a timer system (not shown) which maintains the temperature in
the distillation chamber [2] for a predetermined period of time.
This post-evaporation heating ensures that essentially all of the
solvent to be recovered is distilled. Further, the continued
heating drives off any residual solvents and bakes any
contaminants in the distillation chamber [2) so that the
resulting solids can be disposed of more conveniently at a lower
cost and with reduced environmental problems as compared to
unbaked contaminants. In the embodiment of the invention as
illustrated in Figure 4, the distillation chamber [2] is lined
with a bag [8] such that, following baking, the entire bag [8]
containing the baked contaminants can be disposed of. The bag [8]
is stable within the temperature range of the distillation
chamber [2]. In addition, it?is important that the bag [8] be
inert with respect to the solvents to be distilled. It is clear
that the bag [8] can be made of any material provided it is heat
stable, does not react with the solvents to be distilled, and is
non-permeable, such as Teflon (trade-mark for
polytetrafluoroethylene).
16
CA 02277449 1999-07-07
As can more clearly be seen by reference to the preferred
embodiment illustrated in Figure 5, the condensation module [11]
is comprised of a container [20] into which the vapour to be
recovered enters via vapour inlet [13]. The condensation module
is primed using a predetermined volume of the solvent to be
recovered in the liquid phase [14]. This liquid [14] is free from
contamination. The vapour entering the condensation module [11]
is directed by the inlet [13] to pass beneath the surface [15] of
the liquid [14] such that the vapour bubbles through the liquid.
Ideally, the vapour entering the condensation module [11] is
forced to the bottom of the container [20] by the inlet [13].
Some of the vapour will be cooled by the liquid [14] such that it
condenses and combines with the liquid. As a result of such
condensation the level of liquid in the container [20] will rise.
The condensation may lead to an increase in the temperature of
the liquid [14] if there is no external source for cooling the
liquid. However, the corollary increase in the volume of liquid
[14] in the container [20] means that the vapour which
subsequently enters the con~~iner [20] must travel a greater
distance through the liquid [14]. In addition, the coolest liquid
will fall to the bottom of the container [20] where the vapour to
be condensed is entering the condensation module [11].
17
CA 02277449 1999-07-07
Some vapour may pass through the liquid [14] and accumulate
above the surface [15] in the upper volume [21] of the container
[20]. The only way this accumulated vapour can escape from the
container [20] is by passing through a vapour outlet [32]
positioned above the upper volume [21]. The vapour, having
entered the vapour outlet [32] from the condensation module [11],
then passes from the outlet [32] into the vapour management
module [31].
In a preferred embodiment of the invention, as illustrated by
Figure 6, the vapour management module [31] comprises a sealed
chamber [45] containing a serpentine tube [43] communicating with
the vapour outlet [32] at its entrance end and a vent [42] at its
exit end. The mixture of air and solvent vapour exiting the
condensation module [11] is directed via the outlet [32] through
the tube [43], which is surrounded by a cooling medium [44]
inside the sealed chamber [45]. The cooling medium [44] absorbs
heat and therefore causes further condensation of the vapour so
that substantially all of the vapour escaping from the
condensation module [11] is3condensed and prevented from reaching
the atmosphere through vent [42]. The medium [44] can be any
suitable coolant medium. Ideally, the medium [44] will provide
sufficient heat absorption for condensation of all vapours, such
that only air exits the system through vent [42].
18
CA 02277449 1999-07-07
In one preferred embodiment of the invention, the medium [44] is
water mixed with a salt to form a crystallized mass which has a
low expansion rate and can be contained safely in the sealed
chamber [45]. The tube [43] allows condensed solvent to flow back
by gravity through the outlet [32] into the condensation module
[11], while allowing the remaining uncondensed vapour (which is a
very small amount, if any) and air to exit the vapour management
module [31] to the atmosphere through the vent [42]. The
condensate can continue to flow counter to the air/vapour flow,
through the outlet [32] and be collected in the container [20] of
the condensation module [11], or in a separate container.
The container [20] may be provided with a tap [T], which can
be used to drain the liquid [14]. Ideally, this tap is situated
below the surface [15] of the liquid, and the closer to the base
of the container [20], the more liquid [14] can easily be drained
off. The advantage of this arrangement is that liquid [14] being
collected can be accessed at any time without losing vapour from
the system. Further, condensed liquid [14] can be removed from
the container [20] without ~;nterruption of the condensation.
If desired, a downwardly sloping overflow pipe [D] may be
provided in the container [20] at a predetermined overflow level.
As the level [15] of liquid [14] rises to that overflow level, a
fixed volume of liquid is maintained in the container [20]. Any
19
CA 02277449 1999-07-07
liquid [14] in excess of that volume enters the overflow pipe and
runs by gravity into a collection vessel. This provides an
advantage in that a large volume of material can be used as the
coolant.
In another embodiment of the vapour management module [31],
as illustrated in Figure 7, the air/solvent vapour mixture is
allowed to pass from the outlet [32] to the vent [42] without the
use of a tube [43). In this embodiment, the material [44] is a
solid material for example, ball bearings or glass chips, through
which the vapour and condensate are able to pass. The material
may be prevented from falling back through the outlet [32] into
the container [20] either by selecting the size of the material
particles to be sufficiently large, or by the insertion of a
support member [46] which is permeable to both the liquid [14]
and the air/vapour mixture, but through which the material [44]
cannot pass. Maximizing the ratio of the heat exchange surface to
the volume of vapour passing into the vapour management module
[31] increases the efficiency of the condensation in the vapour
management module [31].
In accordance with the present invention, the vapour
management module [31] can be designed such that essentially no
vapour exits the vent [42]. This is achieved by ensuring that the
rate of heat input does not exceed the rate of heat absorption in
CA 02277449 1999-07-07
the vapour management module. It is clear that factors such as
the rate of vapour flow, the vapour temperature, the heat
absorption properties of the material and the path length through
the vapour management module are important. In one embodiment of
the invention, the heat absorbing material is a crystalline salt,
the pipe [43] is of 12mm bore and 75 cm in length, made of heat
conductive metal such as copper, and the heat input to the vapour
management module in the form of vapour is 1500W.
In selecting the appropriate solid material [44] in the above
embodiments, ideally the material should allow the vapour to flow
through at approximately atmospheric pressure, provide the
required absorption of heat, and allow the condensed liquid [14]
to flow by gravity back to a reservoir.
Those skilled in the art will appreciate that various
combinations of condensation modules [11] and vapour management
modules [31] are possible. Further, it is possible to use either
the condensation module or the vapour management module in
isolation from the system, provided there is an input of vapour
to such modules. It is possible to use the vapour management
module [31] to recover vapour from any vent or exhaust. Examples
of situations where the vapour management module [31] could be
used are on a vent to an oil storage tank, a vent on a gasometer,
or a vapour stack of a spray booth. In these situations, the
21
CA 02277449 1999-07-07
vapour would arise as a consequence of evaporation of the liquid
to be recovered. Condensate formed in such vapour management
modules can then run back by gravity into the vent and ultimately
back to the container of the liquid to be recovered, or to
another container. It is clear to those skilled in the art that
in these situations the heat absorbing material must be cooler
than the vapour to be recovered.
Further, while the preferred embodiment utilizes both a
condensation module [11] and a vapour management module [31] and
has been found to have excellent results, each solvent to be
recovered will have different requirements. It is certainly
possible to subject the vapour to sequential steps of direct
condensation and/or to sequential vapour management modules [31].
The efficiency of vapour recovery will determine the most
suitable module arrangement.
It should be clear that while the preferred embodiments
described above describe specific arrangements of heat absorbing
material, variations are possible. For example, it is not
essential to have any liquid in the condensation module [11)
prior to initiation of condensation. Alternatively, any material
that absorbs heat from the vapour but does not chemically react
with it would be suitable to use within the condensation module,
including a solid mass or air. A solid inert mass such as rocks
22
CA 02277449 1999-07-07
or ball bearings are suitable to absorb heat in the condensation
module [11]. Alternatively, air in the condensation module [11]
can rapidly absorb heat from the vapour entering the condensation
module [11] since heat exchange is rapid between gases. Any
liquid [14] accumulating as a result of condensation in the
condensation module [11] also acts to absorb heat from the
vapour.
While only specific embodiments of the invention have been
described, it is apparent that various additions and
modifications can be made thereto, and various alternatives can
be selected. It is, therefore, the intention in the appended
claims to cover all such additions, modifications and
alternatives as may fall within the true scope of the invention.
23
