WO2001058813A1

WO2001058813A1 - Improved water cleaning process

Info

Publication number: WO2001058813A1
Application number: PCT/NO2001/000041
Authority: WO
Inventors: Eivind Aarebrot; Trygve HÅLAND
Original assignee: Den Norske Stats Oljeselskap AS
Current assignee: Equinor ASA
Priority date: 2000-02-08
Filing date: 2001-02-06
Publication date: 2001-08-16
Anticipated expiration: 2002-08-08
Also published as: NO20000617L; US20030127387A1; MXPA02007544A; US6875358B2; NO312506B1; EP1268349A1; AU2001232497A1; NO20000617D0; DE60116070D1; BR0108171B1; DK1268349T3; ATE313515T1; BR0108171A; NO312506B2; EP1268349B1

Abstract

Method and system for extraction of fat or oil-soluble components, e.g. PAH, NDP and/or BTX, from produced water being separated from an oil or gas plant. The method is characterized by injection of a liquid consisting of hydrocarbons from the oil or gas plant into the water flow, the injection liquid having a lower content of the oil soluble components than the original oil phase in equilibrium with the water, the injection liquid is atomized into the whole water phase and the water/hydrocarbon mixture is given a predetermined retention time thus to allow mass transferring of the oil soluble components from the water to the injection liquid, before the injection liquid is separated from the water in a separator, preferably a hydro cyclone, the injection liquid being adapted to the separators working range for pressure and temperature.

Description

IMPROVED WATER CLEANING PROCESS

The invention relates to an improved method for purifying water containing dissolved hydrocarbons and chemicals. The improvement implies that some of the most environment hostile, dissolvable components also can be separated. This may be achieved by minor modifications of the existing purification equipment, by using a condensate flow from the production system as extraction medium. There is a demand for cleaning processes which removes dissolved aromatics from produced water. Spillage to water has attracted increased attention, and new environmental requirements introduced may result in requirement for reinjection of produced water, if the methods for cleaning are not improved. For older oil fields with increasing water production, such requirements may ultimately result in close down. One single platform at the Statfjord field produces to-day 32,000 m³/d produced water.

Hydrocyclones are used for purifying water in a production plant for oil and gas. This technology meets the present cleaning requirements of 40 ppm for free oil in water, most often within good margins. Harmful effects of serious degree on the environment occur rarely at such low concentrations of spillage. Hydrocyclones do not remove dissolved hydrocarbons from the water. Produced water contains also dissolved components, including aromatics such as benzene, toluene and xylene (BTX) , naphthalene, phenantrene (NPD) , and polyarom- atic hydrocarbons (PAH) . Such compounds are destroyed slowly, and accumulates in the food chain. Certain chemicals employed in oil production may have corresponding properties .

International Patent Application No. PCT/NO97/00060 discloses a system for removal of water soluble hydrocarbons in a water flow. This process is based on injection and finely dispersion of natural gas in a water flow at pressure and temperature conditions permitting the gas to function as solvent for hydrocarbons dissolved in the water. The dispersed gas bobbles expand when absorbing the water

industry that cooling and changes in pressure require equipment which ma/ result in heavy deterioration of the purifying process for produced water. Hence, it is not a very desirably solution to change the pressure and temperature of the fluids to be separated.

PCT/NO97/00060 describes in principle application of "liquefied natural gas" for improving separation of dispersed oil in hydrocyclone based systems. A major feature in the Patent Specification is that increase of differences in density between the oil phase and the water phase is credited as providing a major effect. The Patent Specification does also specify that dissolved components are removed during the process, but the specification lacks a description on how this may be achieved. The present invention relates to a process for removal of dissolved components from produced water in hydrocyclone based systems, and where a suitable extraction fluid may be chosen/produced for the purpose. Emphasis is made with respect to the requirements which must be met in order to make the separation of the dissolved components function. A surprising and cost saving element is that extraction fluid may be produced from a condensate phase in the very same production plant, from which the water is produced.

Even though both solutions as such relates to improved cleaning of produced water in hydrocyclone based systems, they relates to different solutions having substantial environmental importance, as most of the existing hydrocyclone based systems will function satisfactory.

Hence, the present invention relates to an improved method and a system for extraction of fat or oil soluble components, for example PAH, NPD, and/or BTX, from produced water which is separated from an oil and/or gas plant, and where the method is characterised by injection of a liquid comprising hydrocarbons from an oil and/or gas plant in a water flow, said injection liquid having a lower content of the oil dissolved components than original oil phase in equilibrium with the water, where the injection liquid is finely dispersed in the entire water phase and mixture of water/hydrocarbon is given a predetermined retention, thereby allowing mass transfer of the oil soluble components from the water to the injection liquid, where after the injection liquid is separated from the water in a separator, preferably a hydrocyclone, where the injection liquid is adapted to the working range of the separator both with respect to pressure and temperature.

Correspondingly, the present invention relates to a system as described above, characterised in that it comprises means for injecting and distributing a liquid in the water, said liquid consisting of hydrocarbons which are separated out in the oil/gas treatment process of the plant and having a lower content of the oil soluble components than the equilibrium concentration against the water, means for giving the hydrocarbon/water mixture a predetermined time of retention parallel with the liquid being held dispersed in the mixture, and a separator for separation of hydrocarbons from the water subsequent to the period of retention.

According to a preferred embodiment, the invention comprises production of an extraction medium directly from the same plant as the water. Production of an extraction medium to a suitable composition, pressure and temperature, will otherwise represent a substantial cost or represent a direct limitation. In a production plant for oil and gas, condensate fractions which will be suitable for extraction of dissolved aromatics in water, may be extracted. It is demonstrated in experiments that extraction of dissolved aromatics will occur.

The present invention relates thus to an improved method for purifying water containing dissolved hydrocarbons and chemicals. In addition to the separation, the following improvements having substantial impact on costs related to such processes, exist:

It is possible to use an extraction fluid taken directly out from the production plant producing the water. There is no need for performing extra separation of extraction fluids.

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showing that the described extraction functions in practice.

The invention will be described in more details below, referring to the accompanying drawings, illustrating the invention by means of examples. Figure 1 illustrates schematically the extraction process. Figure 2 illustrates schematically one separator for production of the extraction medium. The extraction process is shown on Figure 1. The Figure is made based on an existing production plant for oil and associated gas on the Norwegian Continental shelf. Here, the oil contaminated water 2 is separated from oil 3 and gas 4 in a well stream 1 at the inlet separator CD01. Further, water contining oil is separated out in a second stage separator, CD2 , processed in a separate hydrocyclone unit. A typical oil content in the water from such separator is 300 - 2000 ppm. This oil is separated by means of hydrocyclones, till about 20 - 40 ppm, as shown on the Figure. In Figure 1 water fractions 3 are conducted from the first separator 1 to a second separator where the first fluid fraction is further separated into a second liquid and gas fraction 7, 8. Here, the liquid fraction 7 will consist of heavier hydrocarbons, which for example may be conducted into a process plant, while the lighter hydrocarbons in the gas fraction 8 are conducted through a cooler 9 and into a further separator CD7.

The produced water 2 which escapes out of the separator CD01 will at present let all the dissolved substances escape out with the water, including naphtas, polyaromatic hydrocarbons, benzene, toluene and xylene (BTX). Extraction of these components is achieved by means of the extraction process indicated within the dotted line.

The fluid fraction 10 is drawn from a fluid separator CD7, as shown on Figure 1 and the required amount is returned to the water flow. Selection of an suitable fluid fraction 10 is important, as described below. Components included in the improved water treatment system according to the invention is shown inside the dotted line while the remaining components 1-CD7 may be included in an existing process plant. The pressure of the water fraction may in practice be higher than for the water flow to be treated. An extra pressure surcharge of 5 bar or more is required in order to disperse condensate in the water flow. Liquid fraction 10 from the third separator CD7 in Figure 1 is therefore directed through a pump 11.

By regulating the separators CD01, CD2 and CD7 a condensate 10, 14 may be obtained, comprising a relatively low fraction of the water solubles to be removed from the water fraction 2. This condensate 10, 14 may consequently be mixed with the water fraction 2 for mass transfer of these components from the water flow to the condensate, so that the remaining water 19 has a reduced content of the water soluble hydrocarbons .

The water stream 2 is normally warmer than the condensate 14, and one cannot allow the condensate to evaporate subsequent to the mixing. In this case the condensate is heated up in a heat exchanger 12 to the temperature required to avoid evaporation when mixed with the water flow. Excess of gas is separated in a fourth separator CD10, where after the fluid fraction is mixed into the water phase 2, in the pipeline system 15 upstream of the water purifying hydrocyclone 16.

Extraction of the dissolved components from the water phase will mainly happen in the pipeline system, which will have a substantially longer time of retention for liquid than the hydrocyclone. The principal function of the water purifying hydrocyclone 16 will thus be to separate free hydrocarbons 17 from the water phase 2.

The pressure of the condensate 14 may be adjusted to the pressure at which the hydrocyclone 16 operates, by means of a valve or a pump, depending of the pressure at which the water fraction is taken out. The temperature of the liquid fraction should be adjusted by means of a heat exchanger, if it otherwise may be formed free gas when mixed with produced water. Excess of gas will then have to be separated within a separate separator. Both these modifications may be achieved by means of the referenced pump 11, separator CD10 and the heat exchanger 12. If gas is not formed when the condensate 10, 14 is mixed with the water phase, then it is unnecessary to install heat exchanger and separator.

In the embodiment shown on Figure 1, CD01 is a three phase separator which separates oil, water and gas as separate phases. As an alternative fluid separator the Figure also show the use of a gas scrubber CD8 which also separates out a liquid fraction which may be considered used as an extraction medium.

Mixing of the extraction medium 14 with the water phase 2 is of major importance to make the system function efficiently. A series of modifications are required in order to enable the extraction process to function as an optimum:

1. Mixing should preferably take place in the pipe line system upstream of the hydrocyclone 16 in order to achieve an increased time of contact between the phases. A time of retention upstream of the hydrocyclone of 1 - 2 seconds or more is preferably, but not always required. Preferably, the time is more than 0.3 seconds. Recent tests have in addition shown that in a certain number of cases the time of retention of 3 - 5 seconds may be favourable.

2. The extraction medium/condensate 14 may be finely dispersed in the water phase. This may be achieved in any known manner by the use of injection nozzles, at an operational pressure sufficient to produce a fine dispersion.

3. It is important that the extraction fluid 14 is dispersed over a substantial part of the cross sectional area of the flow. This may be achieved by design of the nozzle means. A static mixer may be installed downstream of the point of injections in order to distribute the condensate droplets in the entire water phase. In order to ensure that the liquid fraction is not separated out to early, it is important that the water fraction is sufficiently finely dispersed and that the time of retention upstream of the hydrocyclone is not becoming too long. The function of the hydrocyclone may be affected by inclusion of a liquid fraction. The following assessment or ) ω t to H H

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The process is suitable for extracting fat dissolvent chemicals from produced water. This is due to the fact that fat dissolvent chemcals also will be dosslved and extracted in a condensate fraction. Some of the bio accumulative compounds are characterised in that they are fat dissolvable and for this purpose, the described extraction process is well suited.

Choice and treatment of condensate fraction

The choice of liquid fractions of essential importance in order to make the extraction process function. Condensate fractions from a production plant are often suitable since they contain a small amount of PAH and NPD and moderate amounts of BTX. Separation occurs normally for compression and drying equipment at 30 - 40 °C, and pressure of 5 - 45 bar. Condensate which is separated out at a higher pressure has a tendency to evaporate when coming into contact with hot produced water. There is certain characteristics which have to be satisfied for a suitable liquid fraction:

1. Concentration of the extracted components (PAH, NPD, BTX) must be substantially lower in the liquid fraction, than in hydrocarbon liquid phase which originally was in contact with produced water.

2. The liquid fraction must be able to exist in liquid form at separation pressure in the hydrocyclone. 3. A sufficient quantity of liquid fraction has to be available .

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Low viscosity and high diffusibility reduces the required time of contact between the liquid phases. By sufficient area of contact between the liquid phases, the required time of contact will be very short. Tests which has been per- formed, show that it is possible to achieve good separation of dissolved components in a common hydrocyclone plant.

Diffusibility may be increased radically if one operates close to the critical point for the hydrocarbon phase. There exists no basis for supercritical conditions being required in order to succeed with this type of extraction process. Supercritical conditions of the hydrocarbon phase provide reduced requirements to area of contact and time of contact between the phases.

The most important critical factor is the exit concentration of the extracted components (BTX, NPD, PAH and/or chemicals) in the liquid fractions, used as extraction medium. Figure 1 shows a three phase separator CDOl which separates out oil, water and gas as separate phases. Table 1 shows concentration of xylene and a more heavy aromatics in the oil phase from three phase separator CDOl, in a liquid fraction from an oil scrubber CD8 and in a liquid fraction which is taken out from a liquid separator in the recompression system, respectively. Condensate from two other scrubbers CD5, CD6 (not shown) in the plant are also included for the purpose of comparison.

Table 1:

Mole fraction of aromatics in liquid fractions in process plant, oil field

Parameter CDOl CD8 CD7 CD6 CD5 BTX 0.0108 0.0045 0.0040 0.0070 0.0313

NPD 0.00863 0.00028 0.00023 0.00037 0.00261

PAH 0.00034 0.000009 0.000007 0.00012 0.000071

Volume, m³/h 947 35 46 35 0.58

Pressure, bar 68.7 64.5 22.6 5.4 1.1 Temp., °C 82 35 36 28 35

% degassing - 14 0 0 0 If the liquid shall be used as extraction medium the concentration of the aromatics must be substantially lower than in the oil phase from which produced water is separated from. It can be seen that CD7 generally has the lowest concentration of all the groups of aromatics. CD8 has nearly as good concentration, but produces degassing. This means that such condensate must be heated up and gas is separated for injection. This is fully possible, but results in a more expensive solution.

Table 2:

Theoretical obtainable separation at equilibrium, CD7 - condensate .

Parameter Water, Water, Condensate Deg. of prior to subseq. to cleaning %

BTX, kg/h 0.9319 0.3488 7.2 62, .6

NPD, kg/h 0.6252 0.0192 81.7 96, .9

PAH, kg/h 0.0352 0.0009 0.26 97, .3

Volume, m³ / 1331 1331 19

The table shows maximum degree of cleaning if equilibrium is achieved. It is achieved a degree of cleaning of 80 - 90% for PAH in laboratory. Substantially lower degree of cleaning is achieved for BTX since the starting level for BTX in condensate phase is too high. If condensate from CD6 is used, the degree of cleaning for BTX will decrease to 35.5%. In this specific case it can in fact be seen that condensate from CD7 and CD8 are the ones which best satisfies the criteria of selection. Condensate from CD5 and CD6 is not suitable, since separation of BTX is too low. The volume of condensate from CD5 is also too low to perform a satisfactory separation.

The calculations are by no means optimum. In practice, the volume of condensate may be lowered substantially without reducing the results substantially. The real degree of separation will be below the calculated values, since a full equilibrium is not achieved. In order to achieve separation close to equilibrium, the defined criteria should be met. Poor dispersion and/or distribution of condensate gives prolonged time of retention in order to achieve equilibrium. If is desired to further improve the degree of separation of BTX, it is necessary to perform fractionating or rectification of the flow of condensate.

Results from research show that produced water contains large amounts of droplets of oil/condensate. A substantial part of the dispersed hydrocarbons may have a characteristic droplet size of 10 microns or less. Hydrocyclones used in hydrocyclone plants shall according to suppliers specification not be able to remove so small droplets. A limit of approximately 20 microns for the range of use is commonly set.

In order to explain such excellent separation of oil from water, it must be assumed that a substantial part of the smaller droplets also is removed in those hydrocyclones which are used. The cause is assumed to be flotation effects: Dissolved gas is released due to pressure reduction through the hydrocyclone. Normally the amount of gas, which is dissolved in water, is sufficient, but it can be considered that addition of some gas may further increase the effect of flotation. This solution may of course be combined with the solution according to this invention.

On this basis it is not considered that addition of a lighter hydrocarbon fraction normally will have a substantial effect on the water cleaning in real hydrocyclone plant. One will not necessarily break emulsions by addition of a lighter fraction. The environmental gain by reducing free oil in water below present level, which in certain places already is as good as 15 ppm, is also uncertain.

Claims

C l a i m s

1. Method for extraction of fat or oil soluble components, for example PAH, NPD, and/or BTX, from produced water which is separated from an oil and/or gas plant, c h a r a c t e r i z e d i n t h a t it comprises the following steps: separation of a liquid consisting of hydrocarbons from the oil and/or gas plant, said liquid having a lower content of the above mentioned components than original oil phase in equilibrium with the water, adjustment of the temperature of the liquid to the temperature of the water and adjustment of the bobble point pressure to the pressure of the water injection of liquid into the water flow and atomising the liquid in the entire water phase and the water/hydrocarbon mixture, where the injected liquid is given a predetermined time of exposure in the mixture, thereby allowing mass transfer of the fat or oil soluble components from the water to the injection liquid, and separation of injection liquid from the water in a separator, preferably a hydrocyclone.

2. Method according to Claim 1, c h a r a c t e r i z e d i n t h a t the liquid is finely distributed by means of injection through a nozzle, and/or distributed in the entire liquid volume by means of a static mixer in order to maximise the area of contact between the water and the liquid and consequently mass transfer of the dissolved components in the water of the injection liquid.

3. Method according to Claim 1, c h a r a c t e r i z e d i n t h a t required time of exposure for water/hydrocarbon mixture with valid flow rate is achieved in the pipeline system between the point of injection and the separator, by placing the point of injection at a distance sufficiently upstream from the separator.

4. Method according to Claim 1, c h a r a c t e r i z e d i n t h a t the time of exposure from the liquid being injected until separation of the mixture of hydrocarbon/water is undertaken, is greater than 0.3 sec .

5. Method according to Claim 1, c h a r a c t e r i z e d i n t h a t the water is separated from an oil flow in a production plant for oil and/or gas, and that the injection liquid consists of condensate which is extracted from the oil/gas process in the production plant in a known manner.

6. Method according to Claim 1, c h a r a c t e r i z e d i n t h a t the injection liquid has a mole weight of 50 - 100, has a bobble point pressure at the temperature of the hydrocarbon/water mixture which is lower than the operation pressure of the separator, and where the content of the oil soluble components is less than 50% of the equilibrium concentration of the components in the liquid phase.

7. Method according to Claim 1, c h a r a c t e r i z e d i n t h a t a portion of the injection liquid is recirculated in order to achieve a higher grade of equilibrium and utilisation of the injection liquid and/or that the injection liquid is regenerated.

8. Method according to Claim 1, c h a r a c t e r i z e d i n t h a t a filter coalescer is used as separator suited for pressures above atmospheric operation.

9. System for cleaning of water in an oil/gas plant for cleaning of oil soluble components, such as PAH, NPD and/or BTX, from extracted water in a well flow, c h a r a c t e r i z e d i n t h a t it comprises : 0 μ- 1

Ω ø

Ω t_ι.

0 φ hi Ω cn rt 0

- Φ 0

P- Φ μ- cn ø O

0 hi

H rt

O μ- 3

0 o

LQ Hi φ cn Φ hi φ

TJ cn rt SD φ tr Hi TJ

9> 91 SD

0 rr hj μ- SD o 0 rt

. 0 0

CO hi

0 cn cn hh φ Hi ^

Ω tr O

. ^ Hi

Pi h{ cn

0 φ

Ω TJ

SD SD

K Hi tr ID

0 rt

0 μ-

O z ø

SD rt 0 φ Hi

Hi

SD g μ- H

X μ- rt Λ

0 ø hi μ- φ P.

13. System according to Claim 9, c h a r a c t e r i z e d i n t h a t the water is separated from a well flow in a production plant for oil and/or gas, and that the injection liquid consists of condensate extracted from the oil/gas process of the production plant in a known manner.

14. System according to Claim 9, c h a r a c t e r i z e d i n t h a t the injection liquid has a mole weight of 50 - 100, has a bobble point pressure at the temperature of the hydrocarbon/water mixture which is lower than the operation pressure in the separator, and where the content of the oil soluble components is lower than 50% of the equilibrium concentration with components in the water phase.

15. System according to Claim 9, c h a r a c t e r i z e d i n t h a t a portion of the injection liquid is recirculated in order to obtain a higher degree of equilibrium and utilisation of the injection liquid and/or regeneration of injection liquid.

16. System according to Claim 9, c h a r a c t e r i z e d i n t h a t. it comprises a filter coalescer used as separator for pressures exceeding atmospheric operations.