Note: Descriptions are shown in the official language in which they were submitted.
' CA 02281067 2003-04-17
Title: METHOD AND APPARATUS FOR DEWATERING A GAS STREAM
AND RETAINING CONTAMINANTS
Field of the Invention
This invention relates to a gas desiccation
procedure and apparatus that separates water from other
contaminants removed from the gas stream in the
desiccation process. More particularly, the invention
provides a de-watering method for natural gas streams
that allows near contamination-free water to be released
into the environment, and allows the non-water
contaminants to be disposed of without being released
into the environment.
Backaround to the Invention
In the treatment of gas streams, de-watering
processes generate extracted water that may contain
traces of contaminants originating from the principal
stream. In the case of natural gas, these contaminants
include hydrogen sulphide, sulphur dioxide and
mercaptans. Disposal of water containing contaminants of
this type can be subject to environmental restraints.
Where natural gas is being used to power
consumer vehicles, it is desirable to provide a compact
gas compressor that can operate at a consumer' s residence
To refuel a vehicle. It is necessary in many such
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situations to remove the residual moisture in the natural
gas during the compression process. The extracted water
thereby produced cannot be released locally into the
environment because of the contaminants present therein.
Apart from issues relating to hazards, even trace smells
of organic or sulphurous components from a natural gas
stream would suggest to a consumer that a leak existed in
the compressor system.
If the contaminated extracted water were to
simply be accumulated, then frequent service calls would
be required to remove the accumulated liquid. This
invention provides a means for extending the servicing
intervals for such a system.
While described in respect of removing water
from natural gas, the invention can be applied to any
case where water is removed from a fluid and a disposal
problem exists due to the presence of contaminants in the
water so removed.
The technology for drying gas and liquid
streams is well established. It includes absorption and
condensation methods and the use of membrane separation
systems. Examples of these technologies, used separately
and in combination are United States patent Nos.
5,034,025; 5,071,451 and 5,240,472 as well as the prior
art referred to therein.
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Further examples of prior art in this field
include the published application PCT/US84/01372
(W085/01058) filed by Pall Corporation of Glen Cove, USA
and U.S. 2,747,681 t:o Schuftan et al. In this Pall
application water coni~ained in purge gas is condensed-out
leaving H2S gas which is adsorbed in iron sponge beds
with residual purge alas being reintroduced into the gas
flow. No provision is made, however, to separate-out
residual contaminants contained or mixed in with the
water condensate, and then to release contaminant-free
water into the environment.
In the case' of Schuft<~n the desiccant bed is
regenerated using gas from the main gas stream as a sweep
gas. Water condensate is released from the system
through a drain pipe without any provision for removing
residual contaminant; present in. such water.
None of the known prior art references address
the objective of treating extracted water to separate it
into purified water and residual components in order to
permit the removal of the contaminant-free water from the
desiccation apparatus for di;~posal as such. No
references address the issue of separating of
contaminants from the extracted. water and disposal of
such contaminants separately fr«m such water, with the
purified water being released into the environment in a
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contaminant-free form. The invention herein address
these concerns.
The invention in its general form will first be
described, and then its implementation in terms of
specific embodiments will be detailed with reference to
the drawings following hereafter. These embodiments are
intended to demonstrate the principle of the invention,
and the manner of its implementation. The invention in
its broadest and more specific forms will then be further
described, and defined, in each of the individual claims
which conclude this Specification.
Summary of the Invention
In accordance with the present invention a
drying process for a fluid stream containing water and
contaminants comprises:
(1) passing a fluid stream containing water and
contaminants through an adsorbent to remove
water which water includes contaminants;
(2) regenerating the adsorbent with a purge gas to
purge it of a substantial portion of the water
and a portion of the contaminants it has
adsorbed, recovering with the purge gas such
water in vapour form with contaminants mixed
therein;
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(3) exposing the purge gas carrying the water with
contaminants mixed therein, so recovered, to a
separation means for separating the
contaminated water into substantially
5 contaminant-free water and residual
contaminants wherein the separation means in
successive stages comprises:
(a) a condenser with an inlet and outlet
producing water with contaminants
contained therein as a condensate and
releasing the purge gas that contains
residual contaminants at its outlet; and
(b) a semi-permeable membrane that produces
from the water condensate contaminant-
free water as a permeate,
followed by the further steps of:
(4) releasing the contaminant-free water permeate
so separated into the environment by
evaporation; and
(5) introducing the purge gas with residual
contaminants carried therein into the fluid
stream
without releasing contaminants into the environment.
In one version of the invention the water permeate
is substantially purified and released into the
environment as by evaporation or other convenient means.
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In preferred applications of the invention, the
fluid is a gas, such as natural gas.
In a further variant o:E the invention the non
water contaminants are disposed of by returning them to
the original fluid si:ream. This procedure is suitable
when the object is to remove water from the fluid stream,
and the presence of contaminants in the fluid stream is
acceptable . An example is natural gas which is to be
consumed in an internal combustion engine.
In a further preferred variant of the
invention, the fluid stream is a line gas which is
exposed to the adsorbent at high pressure and the
regeneration of the adsorbent is~ carried-out at reduced
pressure. The regeneration of the adsorbent may be
enhanced by applying heat during the regeneration stage.
The dried line gas may be used to regenerate the
adsorbent, with such line gas, after at least partial
water removal, being returned to the fluid stream
upstream from the adsorbent.
As a further variant of the invention the
contaminants present in the purge gas may be adsorbed in
a filter which is itself subject to regeneration. The
regenerating sweep gas for this contaminants filter may
reintroduce the contaminants into the original fluid
stream in order to effect dispos;~l of such contaminants.
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Such re-introduction may occur either upstream or
downstream from the adsorbent.
The foregoing summarizes the principal features
of the invention and some of its optional aspects. The
invention may be further understood by the description of
the preferred embodiments, in conjunction with the
drawings, which now follow.
Summary of the Figures
Figure 1 is a schematic depiction of the
components used to separate moisture from a gas at full,
output line pressure, and the contaminants from the
moisture at near the pressure of the arriving gas and a
semi-permeable membrane for releasing contaminant-free
water into the environment.
In Figure 2 the schematic of Figure 1 is
modified to show the moisture removal process operated at
near output pressure.
Figure 3 shows the configuration of Figure 2
with provision to trap and purge contaminant components,
such as organics present in the extracted moisture.
Figure 4 shows a configuration with dual
regenerateable contaminant filters, with valves and
conduits to allow exiting line gas to serve the purge
gas.
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Figure 5 shows a variation on Figure 1 wherein
the semi-permeable membrane is in the form of a spiral
tube through which water condensate is drawn by venturi
suction.
Description of the Preferred Embodiment
In Figure 1 moisturized line gas 1 containing
contaminants 8 enters the upstream side of a compressor
2 from which it proceeds under pressure through one of
two alternate entrance valves 3, 4 to one of two
adsorbent chambers 5, 6. A bed of desiccant material 7,
such as alumina or zeolite respectively contained within
such chambers 5,6, adsorbs the moisture in the gas 1,
including at least some of the contaminants 8. The
entrance valves 3, 4 are alternately opened to divert the
gas flow 1 through the two absorbent chambers 5, 6.
While one chamber, for example chamber 5, is
being used to dry the gas flow, the desiccant 7 in the
other chamber 6 is treated to regenerate it . In this
case, purge valve 11 on the active chamber would be
closed, and purge valve 12 on the regenerated chamber 6
would be open. The exit end 9 of purge valve 11 connects
at the exit end 10 of purge valve 12 providing both
columns 5,6 with access to the water/contaminant
separation system described next.
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The desiccant 7 is regenerated by being exposed
to a sweep gas 13 preferably originating from the
desiccated gas stream 1a and provided through a
connecting conduit 14 joining thE~ two adsorbent chambers
5,6 at their exit ends. This conduit 14 includes a
pressure-drop orifice 15 that releases the sweep gas 13
into the chamber 6 at: near the ~~ressure of the arriving
gas 1. This pressure: is slightly in excess of the line
pressure of the incoming gas 1, conveniently 1/2 psi in
excess resulting in approximately 6% of the main gas
stream flow being recirculated. One-way check valves 15a
prevent exiting gas de-moisturized line la from by-
passing the pressure drop valve 15. The sweep gas 13 is
drawn through the adsorbent bed '7 by a return conduit 16
that leads to the lower pressure intake side of the
compressor 2. Moi:~ture in the adsorbent bed 7 is
encouraged to vaporize into the sweep gas 13 by the
reduced pressure condition in th~~ regenerating column 6.
As an alternate arrangement, check valves 15a
may be eliminated along with connecting conduit 14, and
the pressure drop or~_fice 15 moi~ed to and duplicated at
each of the lines between purge valves 11,12 and the
condenser 18, i.e., in the path of the moisturized sweep
gas 13a as it proceeds to the water/contaminant
separation system. Vhile saving the cost of two check
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valves, this arrangement causes regeneration to occur at
full line pressure, reducing its efficiency.
To speed the regeneration process and assist in
recovery of the water subsequently a thermostatically
5 controlled electrical element 17 warms the desiccant 7.
The warm, moisturized sweep gas 13a containing
contaminants 8, on passing through the open purge valve
12, enters a condenser 18. Liquified water 19 then
accumulates in the bottom of the condenser 18 below the
10 level of the return conduit 16.
As contaminants 8 may be more volatile than
water, this condensation stage represents a first
separation of the contaminants 8 from the moisture
removed from the gas stream 1. This separation is
usually not 100 efficient, however, and the condensed
water 19 will still contain some contaminants 8.
As arranged in Figure 1, the flow of
moisturized sweep gas 13a over the condensed water 19
will continue as long as the adsorbent bed 7 is being
regenerated. Towards the end of this process, the sweep
gas 13a will contain very little moisture and
contaminants. At this point more volatile contaminants
8a contained in the condensed water 19 may separate from
this water 19 and re-enter the flow of sweep gas 13a.
This further separates the contaminants 8 from the
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moisture and water 19 that has been removed from the gas
flow 1.
The accumulated, partially separated water 19,
including residual contaminants 8a present therein, may
be simply accumulated and collected or it may then be
passed to a separation chamber 20 where it is presented
to a semi-permeable membrane 21. On the other side of
the membrane 21, water diffusing therethrough evaporates
23. This process is accelerated by a fan 24 that
maintains a constant air flow on the exterior side of the
membrane 21.
A fan may also be used to cool the condenser
18. Alternately, a thermal-electric cooling element 22
may assist in chilling the condenser 18.
As water diffuses through the membrane 21, some
contaminants 8a may accumulate on the interior surface of
the membrane 21. Eventually, the rate of diffusion may
drop to a level where the membrane 21 must be cleaned or
replaced. However, this step may be delayed according to
the amount of contaminants 8 separated from the water 19
upon condensation and through re-vaporization. It may
also be greatly delayed by inclusion of a contaminants
filter in the flow of sweep gas 13a before it reaches the
condenser 18. This arrangement is described in greater
detail below.
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Membranes such as hydroscopic ion exchange
tubes, made of modified TEFLON have been found suitable
for this application, showing 7_ife-times of practical
duration.
Figure 1 depicts a configuration wherein
regeneration occurs at a pres;~ure that is near the
incoming gas pressure. In Figure 2, regeneration occurs
at near the outgoing gas pressure.
In Figure 2 the return line 16 connects to the
gas line 25 downstream from tree compressor 2. This
connection is effected through a venturi orifice 26 that
creates a suction making use of <~ pressure drop valve 15
and check valves 15a unnecessary.
In this c7.osed loop system the circulating
purge gas 13,13a is drawn through the condenser 18 to
reduce its humidity, and then fed-back through the return
line 16 to be delivered again t:o the desiccant bed 7.
The two drying columns 5,6 are again regenerated
alternately, permitting the dryying of line gas 1 to
proceed continuously through one onstream bed while the
other offstream bed is being rectenerated. Valves 3, 4,
11 and 12 again allow the onstream bed to be isolated
from the regeneration loop during regeneration.
A carbon filter 27 may optionally be used to
selectively remove organic and other contaminant
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compounds from the moisturized :stream of purge gas 13a.
This filter 27 is intended to extend the life-time of the
membrane 21, delaying it from becoming blocked by
contaminants. Such a~ filter 27, shown in Figure 2, may
serve simply as a trap for contaminants which is replaced
regularly.
A further variant of the invention is shown in
Figure 3 as a modification of th.e closed loop system of
Figure 2, although this additional variant may equally be
applied to the configuration of Figure 1 with appropriate
adjustments.
In Figure 3, the falter 27a for organic
contaminants contains activated carbon or such other
regenerable adsorbent material as will remove organic
and/or sulphurous compounds, etc. from the extracted
moisture and be regenerateable by the application of heat
in the presence of a sweep gas.
The regenerateable f_'~~ltration means 27a of
Figure 3 may be subj ect to a regeneration cycle that will
allow the entire system to operate on a continuous
closed-loop basis. Regeneration of the regenerateable
filter 27a is effected by providing an intermittent flow
of purge gas 13 during a filter purge cycle while the
regenerateable filter 27a is being heated by heater
element 28. Alternately, the purge gas 13 may be heated.
Valves 29, 30 contro=L diversion the flow of the
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contaminant-laden purge gas 13b directly to the gas line
downstream of the beds 5 and 6 where it joins existing
line gas la. This process may be carried-out using
diverted, dried line gas 1a as the purge gas 13 once the
adsorbent bed 7 has been regenerated, without fear or re-
moisturizing the line gas la.
The contaminant-laden purge gas 13b is shown in
Figure 3 as transporting the organic contaminants 8
through a secondary sweep gas return line 16a for
disposal in the main gas stream under the influence of
the pressure differential created by venturi valve 40.
The main return gas line 16 may also be similarly
connected through a venturi valve (not shown) to the
outgoing gas 1a line, downstream from the beds 5, 6 if the
condensation process in condenser 18 is sufficiently
efficient at removing moisture or other efficient
moisture removal means are employed. Otherwise, return
of the moisturized sweep gas 13a to re-enter the fluid
stream upstream from the columns 5,6 will re-expose such
sweep gas 13a to adsorbent 7, stripping out the residual
water present therein.
As another alternative to use of a single
regenerateable filter 27a, dual beds of regenerable
filter material similar in construction to beds 5,6 may
be employed as shown in Figure 4. Such filter beds
27b,27c may contain activated carbon or such other
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regenerable adsorbent: material. While one bed 27b is
adsorbing the organic contaminations 8, the other bed
will be regenerating. Dried line gas flow la is routed
by valves 31,33 through conduits 35,35a, 36,36a through
5 filter 27b to regenerate the filter 27b. Valves 39,39a
are closed to isola.t:e filter 27b from the flow of
moisturized sweep gas 13a. The process is then reversed
with valves 31,33,40,40a closed and valves 32,34 and
39, 39a opened to provide flow through conduits 37, 37a and
10 38,38a. The adsorbed contaminants from the regeneration
stream are thereby returned to the main line downstream
of beds 5,6 without exposure of the line gas flow la to
a significant amount: of moisture. The regeneration
process of both filters 27b, 27c cr.ay be improved by use of
15 heaters (not shown) ~~imilar to Figure 3.
When the :system of t:he invention is being
employed to remove moisture from natural gas destined for
combustion, the reintroduction o:E organic and sulphurous
compounds into the natural gas i:~ normally acceptable as
such components were already present in the natural gas
at levels which can be tolerated. If the purging of the
organic filter 27a would create an unacceptably high,
transient level of contaminants E. in the main exiting gas
flow la, then the contaminant-laden purge gas 13b may be
returned to the upstream gas line 25. This will result
in the drying bed 7 being used a.s an intermediate
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receiver for such compounds 8, spreading-out over time
the reintroduction of contaminants 8 into the gas stream
1 at reduced levels of concentration.
The method. and apparatus described is an
example of the separation of water and contaminants 8
from a fluid stream (liquid or gas) 1, followed by the
separation of the water 19 and the contaminants 8. This
latter process may be effected with less than 100%
efficiency through condensation.. A greatly improved
efficiency of separation can be achieved through the use
of semi-permeable membrane materials 21.
The disposal of both the water 19 and the
contaminants 8, once separated :is also addressed. The
water can be disposed-of in bulk by direct removal; or it
can be drained or evaporated into the environment. The
contaminants 8 can be accumulated in a filter 27 and
removed periodically. Alternately, contaminants 8 can be
disposed of by reintroducing them into the main fluid
stream l,la. Optionally, this m~~y be effected by use of
a regenerateable filter 27a which serves to protect the
water-removing components, a.g. :semi-permeable membranes
21, from exposure and degradation by the contaminants 8.
In the foregoing disclosure a semi-permeable
membrane 21 has been represented schematically as a plate
formed as part of the wall of thE~ separation chamber 20.
Figure 5 shows another variant i.n which the semi
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permeable membrane is shown as a tube 41, preferably
spiralled. This tube is preferably formed of semi-
permeable hydroscopic ion exchange membrane material
referenced above.
In Figure 5, the tube 41 is optionally exposed to
a draft of air by fan 24 to enhance the flow of
evaporated water 23 through the semi-permeable tube wall.
As preferred format, tubes have an inside
diameter of on the order of 2 millimeters . This provides
a convenient ratio of surface area to volume that
facilitates dissipation of water through the tube wall.
However, for small diameter bores in the tubing 41,
surface tension may tend to retard migration of water
along the interior length of the tube 41.
To facilitate entry of condensed water 19 into
the tube 41, the end of the tube 41 remote from the
separation chamber 20 may be connected to a venturi
nozzle 42 at one of the conduits carrying the high
velocity main fluids flow 1 upstream from the drying bed
7. This nozzle 42 develops a suction that draws
condensed water 19 through the tube 41. While present in
the tube, the water will migrate through the semi-
permeable tube wall, leaving residual contaminants behind
within the tube. Selection of a suitable length of tube
41 and a suitable size of nozzle 42 will allow all, or
almost all, water entering the tube 41 to pass through
the tube wall for evaporation into the environment,
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without any water being drawn back into the main fluid
stream through the nozzle 42.
Tests have shown that this format of semi-
permeable membrane performs effectively for extended
periods of time. Residual contaminants 8a may commence
to coat and partially block the interior of the tube 41.
But this occurs progressively along the length of tube 41
beginning at the separation chamber end. By providing a
suitable length of tubing 41 a section will always, for
the planned lifetime of this component, remain
sufficiently uncontaminated to permit passage of the
condensed water as permeate through the tubing wall.
Conclusion
The foregoing has constituted a description of
specific embodiments showing how the invention may be
applied and put into use. These embodiments are only
exemplary. The invention in its broadest, and more
specific aspects, is further described and defined in the
claims which now follow.
These claims, and the language used therein,
are to be understood in terms of the variants of the
invention which have been described. They are not to be
restricted to such variants, but are to be read as
covering the full scope of the invention as is implicit
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within the invention and the disclosure that has been
provided herein.
