Method and apparatus for deaerating liquid

A method for deaerating liquids, especially water, where liquid is introduced into a vacuum zone containing liquid distributing means, and where said liquid after deaeration is conveyed out from said zone is improved in that said water in said vacuum zone is ejected toward at least one vertical impingement surface of a porous material, whereby foaming is counteracted, and is directed via said porous material toward an outlet from said vacuum zone. An apparatus for carrying out said method comprises a casing (1) connected with an evacuation system (16,17,18,19), means (8) for introducing a liquid, e.g. water, into said casing and an outlet (15) for discharging treated liquid, and is characterized in that at least one vertical impingement surface of a porous material (6,11) preventing foaming is provided within said casing (1) as well as means (9) being provided for applying a liquid on the porous surface (6,11).

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and an apparatus for deaerating 
liquids, especially water, in order to remove a substantial portion of the 
oxygen that has been absorbed by the liquid. 
2. The Prior Art 
It is known to inject water, e.g., seawater into subterranean 
hydrocarbon-bearing strata in order to displace the hydrocarbons present 
so as to achieve a better recovery of the hydrocarbon resource. 
Injection of seawater containing absorbed oxygen, however, can cause 
certain disadvantages. The included oxygen will, for example promote 
corrosion of the utilized piping and, in addition, will, result in an 
undesired growth of aerobes which are conveyed by the injected water. 
Growth of aerobionts in the hydrocarbon bearing formation can result in 
colonies of the microorganisms which will act to seal off the 
hydrocarbon-bearing stratum, such that the desired recovery thereof cannot 
be achieved. 
Oxygen and other gases can be desorbed or removed from water by boiling the 
water at atmospheric pressure, but this method is unsuitable for treatment 
of large quantities of water. Absorbed gases can also be removed by gas 
stripping, which can be carried out in a gas stripper degasser. Such a gas 
stripper degasser is usually built as a counter current stripping column 
containing a packing or perforated trays. Water is introduced at the top 
of the degasser and the stripping gas, e.g., purified natural gas, is 
introduced at the bottom of the column and bubbles up through the water in 
intimate contact therewith, the stripping gas displacing other absorbed 
gases, so that water at least partly freed from, e.g., oxygen is obtained. 
Gas-stripping, however, has certain disadvantages, i.e., the utilized 
equipment is expensive, heavy and bulky, which is most disadvantageous on 
an offshore rig where it occupies space and represents an undesired 
weight. 
a large amount of natural gas that is poor in sulphur is consumed and must 
be burned after use; and there is a gas hazard in the area around such a 
plant which results in restrictions as to the positioning of the plant as 
well as necessitating various safety measures. 
Because the amount of absorbed gas in a liquid is proportional to the 
partial pressure of the gas above the liquid, absorbed gases can also be 
removed by reducing the partial pressure of the gas above the liquid. For 
this purpose, vacuum deaerators of a conventional kind, being constructed 
similar to the gas stripping degassers, can be used, but instead of 
introducing gas at the bottom, the upper part of the deaerator is 
preferably connected to a suitable vacuum pump for reduction of the 
partial gas pressure. In connection with the conventional vacuum 
deaerators, the desired effect necessitates a multi-stage deaerator. Such 
a vacuum deaerator is, thus, as heavy and as bulky as a gas-stripper 
degasser. Also, in the final step it is often necessary to add 
oxygen-consuming chemicals to achieve acceptable low oxygen values. An 
addition of chemicals requires further expensive dosing and control 
equipment and the cost of chemicals will further increase overheads. 
The known technology of the art has been described by Dr. Charles C. Patton 
in "Oilfield Water Systems" (Petro Tech Ltd. 1977). The mentioned paper 
contains a description of today's equipment and methods as well as a 
copious list of references to further literature. 
The present invention relates to an improved method and apparatus for 
vacuum deaerating of liquids, especially water, permitting one-step 
deaeration which results in very low values of residual free oxygen in the 
treated water. The improved method also permits use of substantially less 
bulky equipment than the known methods. 
SUMMARY OF THE INVENTION 
According to the present method, water to be treated is injected into a 
vacuum zone to impinge on one or several surfaces covered by a "foam 
killing" porous material which the atomized water penetrates, to then flow 
down through the material into a second zone, where deaerated water is 
maintained at the relatively constant level. Within said vacuum zone a 
lowest possible vacuum, preferably approximately corresponding to the 
vapor pressure of water, is maintained. In order to prevent the introduced 
water from impinging on the water in the subjacent zone, a permeable 
partition is provided between the water and the upper zone. The permeable 
partition can be made from the same material as the material upon which 
the water impinges during injection. 
There are several known devices and methods for separating gas from a 
liquid. Thus, U.S. Pat. No. 3,631,654, although not disclosing a vacuum 
deaerator, discloses an apparatus for separating a mechanical mixture of 
gases and liquids. A filter is wetted by liquid and only the liquid is 
passed through the filter due to the pressure differential. Another 
portion of the same filter repels the liquid and lets the gas pass to a 
separate outlet. 
U.S. Pat. No. 3,523,408 discloses an apparatus capable of separating gas 
and liquid and is based on the same principle of separation as U.S. Pat. 
No. 3,631,654. 
U.S. Pat. No. 4,039,305 discloses a filter which separates gas bubbles from 
oil. The mixture is urged through a filter material where the oil and gas 
phases are separated. 
U.K. Pat. No. 1,298,920 discloses a vacuum deaerator (-degasser). An object 
of the device is to provide a large area of contact between the liquid 
inside the vacuum tank and the vacuum. This is achieved by urging the 
liquid out through filter elements within the tank. 
U.K. Patent Application No. 2,013,520 discloses an apparatus that may be, 
but is not necessarily, a vacuum degasser. It is an object of the 
invention disclosed in the application to separate gas from a foaming 
medium. Gas is drawn off the upper portion of a vessel through filter 
elements, which elements do not pass foam. 
BRD Patent Application No. 2,645,561 does not disclose a vacuum deaerator 
(-degasser) which removes dissolved gases from a liquid, but an apparatus 
for separating undissolved gases and foreign matter from a liquid. Such a 
device is often suitable in front of a pump. 
U.K. Pat. No. 1,323,957 discloses features that have certain points of 
resemblance with the object of the present invention. This U.K. 
Specification shows a two-stage apparatus where a first deaeration or 
degassing is carried out in a first zone, whereafter the preliminarily 
treated liquid is delivered to a further stage of treatment of a 
corresponding nature. It is obvious that the preliminary stage is not 
sufficient to provide the desired gas level, and it is necessary to heat 
the liquid before it is subjected to the second stage. This represents an 
expensive method involving the use of heat. 
According to the last mentioned U.K. Specification liquid is sprayed onto 
or toward a horizontally arranged porous material, e.g. Raschig rings. 
According to the present invention the liquid to be treated is injected by 
the aid of one or several nozzles toward vertically arranged surfaces that 
are coated with a porous material which counteracts the formation of foam. 
Some of the liquid will of course penetrate the porous material and 
migrate through it, but the bulk of the liquid will flow down along the 
surface of the porous material, so that a possible filter effect will be 
quite limited. 
As will appear from experimental data shown below, substantially improved 
deaeration is achieved by the present method as compared with those known 
in the art. It is assumed that when, e.g., water, is injected into a 
deaerator where the vacuum zone is maintained at a very low pressure, the 
absorbed gases can form "boiling nuclei", around which vapor or lager 
vapor bubbles are formed and are immediately liberated from the ejected 
water before or after impingement on the porous material. It is, 
furthermore, assumed that the poorer efficiency of conventional packed 
vacuum degassers is due to the formation of foam and gas bubbles on the 
surface of the present water. Due to the surface tension of water, the 
partial pressure of the oxygen within a bubble will be higher than the 
partial pressure outside the bubble, and as a result the oxygen content of 
water in contact with such a bubble will be higher than the total pressure 
within the column would imply. It is assumed that the poorer efficiency of 
a conventional vacuum degasser is due to bubble and foam formation during 
the movement of water downward through the packing of the column and that 
the foam formation sets bounds to the efficiency of degassers of such a 
kind. 
For carrying out the method according to the present invention an apparatus 
is provided which comprises at least one vacuum zone into which water is 
injected and at least one subjacent zone for collecting and drawing off 
deaerated water. Within the first zone at least one impingement surface of 
a "foam killing" porous material is provided against which water is 
injected by one or more suitable nozzles. 
The first and second zones are separated by a permeable partition 
comprising a porous material, e.g., the same material as that used on the 
impingement wall which the injected water strikes. 
The object of the partition between the water in the subjacent zone and the 
zone where water is introduced is partly to prevent foaming and partly to 
prevent direct contact between water from the nozzles and the treated 
water, which otherwise would reduce the efficiency of the apparatus. The 
level of the treated water is preferably adjusted in such a manner that 
the water surface is maintained inside the porous partition separating the 
two zones. 
The zones are provided in a suitable casing which can be put under the 
necessary vacuum. The casing is preferably insulated to prevent undesired 
condensation on the outside casing surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in the FIGURE, the apparatus includes a sealed exterior tank, 
constructed in such a manner that it can be put under a necessary vacuum. 
Inside said tank 1 an interior shell 2 is provided to prevent water from 
contact with the exterior tank wall, at least in the upper portion of the 
tank 1. The space 3 formed between the shell 2 and the tank 1 is vented at 
4 to the suction side of, e.g., a "ROOTS" blower 17 via the piping 16. As 
a consequence, the space 3 will be under a vacuum and, thus, has a heat 
insulating effect which renders further insulation unnecessary. 
Inside the shell 2 one or more concentric vertically-extending walls 5 and 
5a are provided. These walls are coated by a porous material 6, e.g., 
glass wool or a textile made from natural or artificial fibres or the 
like, having a thickness of, e.g., 50-100 mm. These walls 5 and 5a extend 
down to and through coating 11 of a horizontally extending porous material 
that covers the bath 7. Coating 11 and the concentric walls 5 and 5a rest 
on a perforated sheet 20 or the like. Water is introduced via control 
valve 13 and pipe 8 through nozzles 9 with sufficient pressure to ensure 
good distribution, approximately 0.5-0.7 bar for a 15 mm full-jet nozzle. 
From nozzles 9 water is ejected toward the porous material 6 on walls 5 
and 5a. Coating 11 will prevent water having a relatively high velocity 
from contact with the treated water in the bath 7. Thus, oxygen-containing 
water will not come into direct contact with treated water. Experiments 
have shown that unless there is a coating 11, it is difficult to achieve a 
residual oxygen content of less than 0.40 ppm in a one-stage vacuum 
deaerator at a water temperature of 10.degree.-20.degree. C. 
In coating 6 the water is decelerated and further distributed without any 
foaming and it flows through coating 6 and possibly along walls 5 and 5a 
down into bath 7. Due to the fact that water from nozzles 9 is prevented 
from direct contact with the water in bath 7 and that it flows down into 
the bath through the porous coating, desorbed gases cannot be carried 
along with the water to be introduced into the bath again. Because the 
water surface of bath 7 is in contact with the porous coating, foaming on 
the water surface is avoided. The surface of bath 7 is preferably 
maintained within coating 11 as indicated at 12. This is achieved by the 
aid of a control valve 13 operated by stay 21 which is connected with 
float 14. Water is sucked out from the deaerator through outlet 15. Sucked 
off gas and vapor is sucked off at the top of the deaerator through pipe 
16 by a suitable vacuum pump, e.g., a ROOTS blower 17. From the ROOTS 
blower the drawn off gas/vapor can be condensed in injection cooler 18 by 
the introduction of cold water. The volume that liquid ring pump 19 has to 
pump is, thus, much reduced. Water that is mixed into injection cooler 18 
and the obtained condensate can be utilized to replenish the liquid ring 
pump, which is, thus, replenished automatically. 
A comparison between a conventional seawater deaerator operating according 
to the vacuum principle and the above-described deaerator shows that the 
method and apparatus according to the present invention have substantial 
advantages. For a capacity of 340 m.sup.3 water/h, the following 
dimensions and efficiencies are typical. 
CONVENTIONAL APATUS 
Diameter 3 m 
Height 10 m 
Efficiency 0.05 ppm residual O.sub.2 at 30.degree. C. and 0.10 ppm residual 
O.sub.2 at 20.degree. C. 
APATUS ACCORDING TO THE INVENTION 
Diameter 1.5 m 
Height 3.9 m 
Efficiency 0.015 (0.015 was the lower limit of the measuring device used) 
ppm residual O.sub.2 at 13.degree. C. 
As apparent from the shown results, substantially improved removal of 
oxygen is achieved by the deaerator according to the invention as compared 
to conventional means. With the last-mentioned apparatus, the residual 
O.sub.2 content was 7 times higher at 20.degree. C. than the findings with 
the present deaerator in spite of the fact that the present deaerator was 
tested at a water temperature of 13.degree. C., which additionally 
emphasizes the advantages of the present method apparatus. 
The volume of the conventional apparatus is about 10 times larger than that 
of the present apparatus, which means considerable reductions as regards 
weight and bulk. 
Although the apparatus shown in the accompanying drawing is provided with 
one absorbing coating 6 on wall 5, it is feasible to cover the opposite 
surface of wall 5 with a corresponding coating and arrange the nozzles in 
such a manner that the last-mentioned coating is also sprayed by the 
introduced water.