GB2532249A

GB2532249A - Apparatus for generating an antifoam spray curtain in a liquid and gas separating vessel

Info

Publication number: GB2532249A
Application number: GB1420168.5A
Authority: GB
Inventors: Kaasa Oyvind
Original assignee: Statoil Petroleum ASA
Current assignee: Equinor Energy AS
Priority date: 2014-11-13
Filing date: 2014-11-13
Publication date: 2016-05-18
Also published as: GB201420168D0

Abstract

A method and apparatus for generating an antifoam spray curtain in a liquid and gas separating vessel comprises a plurality of spray nozzles 24 configured to generate a substantially horizontal curtain of liquid droplets 14 at a predetermined level so as to suppress and/or contain a foam layer 20 below the curtain. The apparatus may be incorporated in a liquid and gas separating vessel such as an oil and gas separator 10. The separator may have a gas space 16 above a liquid (oil) level 18 and the foam layer 20 forms on top of the liquid. Preferably two parallel liquid manifolds 22 include a number of the horizontally directed spray nozzles 24. The manifolds 22 may be arranged above the liquid level 18 in the gas space 16 and project across a width of the separator 10 such that liquid droplets emerging from the spray nozzles 24 generate a horizontal curtain 14 substantially across the entire width of the separator. Advantageously the apparatus is more effective at suppressing foam than a system in which liquid droplets are sprayed vertically downward from a height.

Description

Apparatus for Generating an Antifoam Spray Curtain in a Liquid and Gas Separating Vessel

Field of the Invention

This invention relates to apparatus for generating an antifoam spray curtain in a liquid and gas separating vessel. The invention may be employed in any vessel in which liquid and gas are separated. However, the invention may be particularly useful in an oil and gas separating vessel.

Background to the Invention

In a continuous process plant foaming in vessels where liquid and gas separates may cause liquid carry-over in the gas stream depending on the process conditions if the foam reduces the gas cross-sectional area to increase the gas velocity above a maximum allowable velocity.

Foaming in oil and gas separators is a typical example of a field where the invention applies. Foaming in the separator reduces the gas flow capacity because the foam occupies parts of the gas cross-sectional area. The foam may vary from large bubbles to high density foam. Excessive foaming may also result in foam accumulating at the gas outlet section from the separator. Normally, the gas outlet section has a demister to prevent liquid carry-over. However, in the case of liquid carry-over, the gas flowing out of the separator will carry liquid through the gas pipe as a liquid film on the pipe wall and/or as droplets in the gas stream. In either case, the liquid will end up in the gas scrubber downstream of the separator via the gas cooler. The gas scrubber functions as a liquid knock-out drum to protect the gas compressor downstream of the scrubber. In the case that the gas stream carries too much liquid from the separator, the liquid level in the gas scrubber will increase and will set off a high liquid level alarm. Because a liquid carry-over incident is not a steady state situation, the automatic control system is sometimes not able to control a sudden increase in the liquid level.

Hence, the level in the scrubber may increase further and trigger the emergency shut-down (ESD) system causing a production shutdown.

When designing liquid and gas separators the cross-sectional area for gas has to take into account foaming to ensure that gas velocity stays below the maximum allowed gas velocity in the separator normally calculated by this formula: V_max= constant' square root ((liquid density -gas density)/gas density). In the middle of the separator there are one or two cross-sections with so called TP-vanes or similar installed to knock out liquid droplets from the gas stream. These sections knock down foam in the gas stream as well. Foaming means a reduced gas x-section, hence causing increased gas velocity in the separator and increased risk for liquid carry-over. To minimize the risk for production shutdown due to liquid carry-over, the separator design has to include an extra volume to account for foaming. This means that foaming has a negative volume and weight effect on offshore platforms and in particular Floating Production Storage and Offloading (FPSO) vessels. The manufacturers of oil and gas separators offer special internal structures to prevent foaming. This includes a momentum breaker at the separator inlet which reduces turbulence at the inlet section of the separator. This has a preventive effect on foam tendencies because of reduced turbulence.

To reduce foaming in oil and gas separators and to prevent process disturbances, chemicals for foam reduction are normally injected into the piping upstream of the separator. However, the retention time in the piping is often too short to achieve the full effect of the chemicals. Injection of antifoam chemicals adds substantially to the budget for production chemicals and extends the time taken by operators for optimizing the injection system.

It is an aim of the present invention to provide apparatus for generating an antifoam spray curtain in a liquid and gas separating vessel which addresses the aforementioned problems and/or provides an alternative to the above.

Summary of the Invention

According to a first aspect of the present invention there is provided apparatus for generating an antifoam spray curtain in a liquid and gas separating vessel comprising a plurality of spray nozzles configured to generate a substantially horizontal curtain of liquid droplets at a predetermined level so as to supress and/or contain a foam layer below the curtain.

Embodiments of the present invention provide apparatus which is more effective at suppressing foam than in a system in which droplets are sprayed vertically downwardly from a height some distance above a foam layer. In embodiments of the present invention, the droplets themselves are less likely to be carried out via the gas outlet (e.g. presenting a potential production shutdown risk) and more likely to remain in the top of the foam layer. Furthermore, the spray generated droplets are less likely to be entrained by the gas flow and carried to the gas outlet by the gas phase in the vessel.

As a horizontally directed curtain is believed to be effective at reducing foaming, the flow and/or separation capacity of the vessel in relation to liquid and gas content will be increased. In addition, the risk of production shutdown due to liquid carry-over (which would typically last for 3-4 hours) will be reduced. Furthermore, operating costs (including logistics) will be reduced because less antifoam chemicals are required.

It will be understood that the horizontal curtain will be generated by ejecting liquid horizontally from the spray nozzles. Accordingly, the spray nozzles may be orientated substantially horizontally.

The spray nozzles may be provided on one or more liquid manifolds arranged around and/or across a transverse cross-section of the vessel. Each manifold may be constituted by a longitudinal horizontal pipe having a typical diameter of 1-2 inches (2.5 -5.1 cm).

It will be understood that the momentum (mass x velocity) of the liquid droplets from the nozzles needs to be sufficient to create the horizontal curtain between the vessel walls before gravity pulls the droplets significantly downwardly and to penetrate the foam in such a way that the foam collapses. The design of the spray nozzles (i.e. size, shape, location and spacing) will therefore be optimised taking into account the differential pressure across the nozzles, the density of the foam and the desired momentum of the droplets.

Depending on the diameter of the vessel several manifolds may be required to be mounted in parallel to reduce the horizontal distance the droplets have to travel. For example, a 2m diameter vessel may require 2 or 3 manifolds (with spray nozzles along both sides of each manifold) such that each nozzle is configured to provide a minimum spray length of approximately 60cm.

The size of the droplets and the droplet density forming the "curtain" may also be varied depending on the physical properties of the foam.

The predetermined level will be located in a gas space above an expected liquid level and may take into account a desired maximum foam level on top of the liquid level and a "high level" alarm for the separator.

The spray nozzles may be arranged so that the horizontal curtain of liquid droplets substantially covers an area where foam is expected to form on the liquid phase. Spray nozzles may not be provided in an area adjacent an inlet of the vessel since turbulence in this area may make a spray curtain ineffective. In certain embodiments, the spray nozzles may be configured to generate a horizontal curtain of liquid droplets across at least 70% of the diameter of the vessel, at least 80% of the diameter of the vessel, or at least 90% of the diameter of the vessel. In other embodiments, the spray nozzles may be configured to generate a horizontal curtain of liquid droplets across up to 100% of the diameter of the vessel.

The liquid droplets may be formed from any suitable liquid. Preferably, the liquid will have the same physical properties as the liquid (e.g. oil) in the vessel to ensure smooth mixing of the droplets into the liquid phase. Thus, the liquid droplets may be formed from liquid discharged from the vessel. A pump may be required to provide sufficient pressure of the liquid that is recirculated through the spray nozzles.

Accordingly to a second aspect of the invention there is provided a liquid and gas separating vessel (configured to extract gas from a liquid phase) comprising the apparatus according to the first aspect of the invention.

In a particular embodiment, the vessel may be configured as an oil and gas separator. Alternatively, the vessel may be configured as a pressure relief tank, a flare knock-out drum, a stabilizer/fractionator/flash/distillation/absorption/stripper or amine tower or another similar type of vessel where gas is separated from a liquid phase.

Accordingly to a third aspect of the invention there is provided a method of reducing foaming in a liquid and gas separating vessel, the method comprising: arranging a plurality of spray nozzles at a predetermined level in said vessel; and generating a substantially horizontal curtain of liquid droplets from said spray nozzles so as to supress and/or contain a foam layer below the curtain.

Brief Description of the Drawings

Specific embodiments of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows a side cross-sectional view of a liquid and gas separating vessel incorporating apparatus for generating an antifoam spray curtain according to an embodiment of the invention; Figure 2 shows top view of a manifold including a number of spray nozzles in accordance with an embodiment of the invention; Figure 3 shows a side cross-sectional view of a stabilizer/fractionator/flash/distillation/absorption/stripper or amine tower (e.g. containing trays for refining crude oil, or an amine tower to remove CO2/H2S from a hydrocarbon gas stream) incorporating apparatus for generating an antifoam spray curtain according to an embodiment of the invention; and Figure 4 shows a schematic diagram of a separating system including a stabilizer/fractionator/flash/distillation/absorption/stripper or amine tower (e.g. containing trays for refining crude oil, or an amine tower for removal of CO2/H2S in a hydrocarbon gas stream) in accordance with an embodiment of the invention.

Detailed Description of Certain Embodiments

With reference to Figure 1, there is illustrated a liquid and gas separating vessel in the form of an oil and gas separator 10 incorporating apparatus 12 for generating an antifoam spray curtain 14 according to an embodiment of the invention. The separator has a gas space 16 above a liquid (oil) level 18 and a foam layer 20 is formed on top of the liquid.

The apparatus 12 comprises two parallel liquid manifolds 22, each of which includes a number of horizontally directed spray nozzles 24. The manifolds 22 are arranged above the liquid level 18 in the gas space 16. The manifolds 22 project across the width of the separator 10 such that liquid droplets emerging from the spray nozzles 24 generate a horizontal curtain 14 substantially across the entire width of the separator 10 to supress and/or contain the foam layer 20 below the curtain 14.

As shown in Figure 2, each manifold 22 includes a number of spray nozzles 24 configured to eject liquid 26 flowing through the manifold 22, into the vessel 10 to form of the curtain 14. The nozzles 24 are provided along opposite sides of the manifold 22 and are spaced along each side at a distance which takes into account the spray angle a in the horizontal plane so that the droplets from each nozzle 24 will overlap in the horizontal plane in order to form the curtain 14 without any substantial gaps. Furthermore, the nozzles 24 on one side of the manifold 22 are staggered with respect to the nozzles on the other side of the manifold 22 -again, to minimise the possibility for gaps in the spray curtain 14.

Figure 3 shows a liquid and gas separating vessel in the form of a stabilizer/fractionator/flash/distillation/absorption/stripper or amine tower 50 (e.g. containing trays for refining crude oil, or an amine tower to remove CO2/H2S from a hydrocarbon gas stream) incorporating apparatus 12 for generating an antifoam spray curtain 14 according to a further embodiment of the invention. Although, the tower 50 is different to the separator 10 (e.g. since it includes trays 52 between the gas space 16 and liquid level (not shown)), it will be understood that, in terms of the present invention, the apparatus 12 is essentially as described above and operates in a similar way to create a horizontal spray curtain 14 to contain and/or suppress a foam layer 20.

In such towers 50 foaming on top of the trays is often a problem due to liquid carry-over and so chemicals are typically used to prevent foaming.

Figure 4 shows a schematic diagram of a separating system including a stabilizer/fractionator/flash/distillation/absorption/stripper or typically an amine tower 50 like the one illustrated in Figure 4 and so like reference numerals will be employed where appropriate. The tower 50 has a feed 54 for fluid to be separated, a lower outlet 56 for liquid and an upper outlet 58 for gas.

In this case, two recirculation pumps 60, 61 (one pump 60 for distillation/absorption reflux and one pump 61 for forming an antifoam spray curtain) are provided which utilise liquid 62 which has been obtained from gas from the gas outlet 56 after it has passed through a cooler 64. The liquid 62 is then injected back into the tower 50 by pump 60 via an overhead reflux stream 66, as is known in the art. A lower recycle stream 67 is further generated by pump 61 and used for generation of an antifoam spray curtain in accordance with an embodiment of the present invention. The recycle stream 67 is ejected through spray nozzles (not shown), as per Figure 3.

It will be appreciated by persons skilled in the art that various modifications may be made to the above embodiments without departing from the scope of the present invention, as defined by the claims. Furthermore, features described in relation to one embodiment may be incorporated into other embodiments.

Claims

CLAIMS: 1. Apparatus for generating an antifoam spray curtain in a liquid and gas separating vessel comprising a plurality of spray nozzles configured to generate a substantially horizontal curtain of liquid droplets at a predetermined level so as to supress and/or contain a foam layer below the curtain.
2. The apparatus according to claim 1 wherein the spray nozzles are provided on one or more liquid manifolds arranged around and/or across a transverse cross-section of the vessel.
3. The apparatus according to claim 2 wherein the spray nozzles are designed taking into account a differential pressure across the spray nozzles, the density of the foam and a desired momentum of the droplets.
4. The apparatus according to claim 2 or 3 wherein several manifolds are mounted in parallel to reduce the horizontal distance the droplets have to travel.
5. The apparatus according to any preceding claim wherein the predetermined level is located in a gas space above an expected liquid level and takes into account a desired maximum foam level on top of the liquid level.
6. The apparatus according to any preceding claim wherein the spray nozzles are arranged so that the horizontal curtain of liquid droplets substantially covers an area where foam is expected to form on the liquid phase.
7. The apparatus according to any preceding claim wherein spray nozzles are not provided in an area adjacent an inlet of the vessel since turbulence in this area may make a spray curtain ineffective.
8. The apparatus according to any preceding claim wherein the liquid droplets have approximately the same physical properties as the liquid in the vessel to ensure smooth mixing of the droplets into the liquid phase.
9. The apparatus according to claim 8 wherein the liquid droplets are formed from liquid discharged from the vessel.
10. The apparatus according to claim 9 wherein a pump is provided to increase the pressure of the liquid that is recirculated through the spray nozzles.
11. A liquid and gas separating vessel comprising the apparatus according to any preceding claim.
12. The vessel according to claim 11 configured as an oil and gas separator.
13. The vessel according to claim 11 configured as a pressure relief tank. 10
14. The vessel according to claim 11 configured as a flare knock-out drum.
15. The vessel according to claim 11 configured as a stabilizer/fractionator/flash/distillation/absorption/stripper or amine tower. 15
16. A method of reducing foaming in a liquid and gas vessel, the method comprising: arranging a plurality of spray nozzles at a predetermined level in said vessel; and generating a substantially horizontal curtain of liquid droplets from said spray nozzles so as to supress and/or contain a foam layer below the curtain.
17. Apparatus substantially as described herein with reference to the accompanying figures.
18. A liquid and gas separating vessel substantially as described herein with reference to the accompanying figures.
19. A method of reducing foaming in a liquid and gas separating vessel substantially as described herein with reference to the accompanying figures.