GB2454723A

GB2454723A - Reducing Vapour Loss in a Tank

Info

Publication number: GB2454723A
Application number: GB0722555A
Authority: GB
Inventors: Robert Seymour Bridges
Original assignee: MassTech International Ltd
Current assignee: MassTech International Ltd
Priority date: 2007-11-16
Filing date: 2007-11-16
Publication date: 2009-05-20
Also published as: GB0722555D0

Abstract

A closed vessel 1 contains apparatus for reducing the evaporation of a liquid 10 contained, in use, within the vessel. The apparatus comprises a plurality of flotation members 40, which are arranged to float on the surface of the liquid such that they form a carpet 41 of at least one flotation member in depth over substantially the entire surface of the liquid. The floatation members preferably have a polarised static charge and may be electrically conductive. They may be spherical, may comprise a number of different shapes and sizes or may be tessellating polyhedrons. The floatation members may also have different densities. They are preferably hollow, and filled with a thermally insulating material which insulates the liquid from the vapour space. The members 40 may have an abrasive surface to clean the inside of the tank. They may be suitable for introduction to/removal from the tank via an inlet/outlet 24, 22 pipe and may be strung together.

Description

Method and Apparatus for Reducing Vapour Losses The present invention relates to a method and apparatus for reducing vapour losses from liquids such as hydrocarbon fuels or other liquid chemicals stored in a closed vessel.

Liquid hydrocarbon fuels are a valuable commodity both in terms of monetary value and availability. Fuels such as petrol and diesel are produced by refining crude oil into various hydrocarbon fractions depending on the particular properties required. This is an expensive and time consuming process which relies upon ever decreasing supplies of crude oil. The fuels produced are often highly volatile and therefore losses due to evaporation can be substantial.

It is therefore important to reduce evaporation losses as far as possible, in particular, during transportation, storage and transfer of the fuel between storage tanks.

Significant vapour losses also occur in the storage and transportation of other types of liquids commonly used or stored in, for example, chemical plants, refineries, ocean going bulk carriers and costal depots. The reduction of vapour losses is therefore of great importance in these and other industries.

Fuel storage tanks may be located above or below ground level. Often, for example in areas having very low ambient ground temperatures, fuel storage tanks are located above ground level in order to protect their structural integrity.

In some instances, for example in the storage of #6 fuel oil in Antarctica, stored fuel is heated in order to prevent freezing. In such cases fuel losses through evaporation can be much increased.

The contents of a fuel storage/transportation tank can be thought of as comprising three layers, a liquid layer, a heavy vapour layer (HVL) and a light vapour layer (LVL).

The LVL is located above the HVL and the HVL is located immediately above the liquid layer. The HVL is an area of highly concentrated vapour which undergoes continuous evaporation and condensation. Therefore, ideally only vapour from the LVL is able to escape to atmosphere through the ventilation pipe.

There are two main types of vapour losses associated with fuel storage tanks, breathing' losses and displacement' losses. Breathing' losses occur when vapour pressure in the tank increases due to, for example, changes in temperature and/or pressure/altitude. Breathing' losses may also occur in the presence of a positive pressure gradient across the ventilation pipe due to the existence of a negative chemical gradient. Breathing' losses are worsened by turbulence (for example during transportation or as a result of vibrations caused by passing traffic) since turbulence increases the energy and exposed surface area of the liquid and thus increases the rate of evaporation.

Turbulence also causes increased mixing between the HVL and the LVL leading to increased vapour concentration in the LVL and consequential elevated vapour losses.

Displacement' losses occur during filling or replenishing of the tank. When the tank is replenished the fuel entering the tank displaces the LVL through the ventilation pipe. As mentioned above, ideally only the IVL will be displaced.

However, often, due to over filling or turbulence caused by the entry of the replenishing fuel, the HVL will also be wholly or partially displaced.

Methods for reducing vapour losses using controls, valves, membranes, restrictors etc., to physically limit the tendency of the vapour to escape into the atmosphere, are well known. Known methods largely aim to minimise vapour losses by containment and/or minimising stirring and/or turbulence. Whilst these methods have met with some success they have the inherent disadvantage that they require expensive components and have high installation costs.

The present invention provides a closed vessel containing apparatus for reducing the evaporation of a liquid contained, in use, within the vessel. The apparatus comprises a plurality of flotation members, wherein the plurality of flotation members are arranged to float on the surface of the liquid such that they form a carpet of at least one flotation member in depth over substantially the entire surface of the liquid.

Providing a plurality of flotation members on the surface of a liquid within a closed vessel reduces the exposed surface area of the liquid and thus reduces the levels of evaporation. The flotation members are advantageous as they may be of a simple and inexpensive construction and may be easily introduced into new or existing vessels without the need for expensive and complex fitting operations. This is particularly advantageous in the case of underground storage vessels. A similar approach has been used in a laboratory environment with some success. For example, a number of floating balls are used to insulate a chemical mixture in an open vessel during heating.

In the case of vessels containing flammable liquids, the provision of flotation members is beneficial as the decreased level of vapour emission reduces the explosiveness of the vapour space within the tank. The flotation members also provide a physical barrier between the vapour and liquid phases, thus helping to prevent a disastrous ignition of the liquid in the event of a vapour explosion.

In a preferred example the flotation members are statically charged. This is advantageous as the flotation members may be mutually attractive or repulsive. In a particularly preferred example the flotation members are shaped to provide a polarised static charge. This, in turn, encourages the flotation members to mutually attract and align to form an ordered carpet within the closed vessel and maximise the liquid surface coverage.

In an alternative example the flotation members may be electrically conductive to facilitate the dissipation of static charge build up from within the closed vessel. This is of particular benefit for flammable liquids as static build up may cause sparks which can ignite the liquid or vapour (for example, static build up generated when the vessel is replenished) The flotation members are preferably spherical. Spherical flotation members are beneficial as they are able to roll along without becoming locked against one another and thus installation of the apparatus into the closed vessel is made easier.

The flotation members may be of a plurality of different shapes and/or sizes. One advantage of this is that it helps to reduce fabrication costs as no stringent dimension controls need to be employed in the fabrication process.

In another preferred example the flotation members are tessellating polyhedrons. This is beneficial as it allows the plurality of flotation members to pack very closely together to rnaximise the coverage of the liquid surface and thus reduce the level of evaporation.

The flotation members preferably have a plurality of different densities. The ability to vary density provides the system designer with a further variable which may be used alone, or in conjunction with shape and/or size variations, to design flotation members which arrange themselves in a particular way when floating on the surface of the liquid.

In a preferred example the flotation members are hollow.

One advantage of hollow flotation members is that they are better able to insulate the liquid from the vapour space within the vessel. Thermal insulation is desirable as liquids often have high thermal expansion coefficients.

Thus, a reduction in thermal expansion/contraction is beneficial for stock control and gauging purposes. The hollows of the flotation members may contain a filler material which is preferably a thermally insulating material and the flotation members are preferably arranged to thermally insulate the liquid from the vapour space within the closed vessel.

In another preferred example the flotation members have an abrasive surface to clean the inner surface of the vessel.

Cleaning the inside of the closed tank is beneficial since it inhibits microbial growth which can lead to microbial corrosion of the vessel.

The flotation members are preferably arranged to be introduced into and/or removed from the vessel via the inlet pipe, the outlet pipe or the ventilation pipe. This allows the flotation members to be simply fitted to and removed from new and existing vessels. The flotation members may be strung together to enable easier retrieval from the vessel.

The horizontal cross-section of the vessel preferably decreases towards its lowermost end so that the surface area of the liquid decreases, and the thickness of the carpet of flotation members increases, as the liquid is drawn from the vessel. This is advantageous as it maximises the weight exerted on the surface of the liquid by the flotation members when the vessel is nearly empty. Therefore, the effect of turbulence caused by entry of replenishing liquid into the vessel is minimised at the point where the turbulence is greatest.

In a second aspect, the present invention provides a method for reducing the evaporation of a liquid contained within a closed vessel, the method comprising floating a plurality of flotation members on the surface of the liquid such that they form a carpet of at least one flotation member in depth over substantially the entire surface of the liquid.

In a preferred example the method further comprises cleaning the inner surface of the vessel by abrasion of the flotation members.

Examples of the present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic isometric view of a fuel storage tank containing liquid hydrocarbon fuel.

Figure 2 is a schematic isometric view of the fuel storage tank of Figure 1 further comprising a carpet of flotation members.

Figure 3 is a schematic view of a section of a layer of flotation members having a tear-drop shape.

Figures 4a to 4c show schematic example shapes for the flotation members.

Figure 5 is a schematic isometric view of the fuel storage tank of Figure 2 showing the tank when it is almost empty.

Figures 6a and 6b are schematic views of a section of a carpet of flotation members having different densities.

Figures 7a and 7b are schematic views of a section of a carpet of flotation members having different sizes.

Figures 8a and 8b show a schematic views of an alternative shape of flotation member and carpet of tessellating flotation members.

Figure 1 shows a schematic view of a typical steel fuel storage tank 1 containing a liquid hydrocarbon fuel 3 such as petrol or diesel. The tank 1 is an underground storage tank of a generally cylindrical shape orientated in the horizontal plane. However, the tank 1 may be located above ground and may be made of any suitable material and have any suitable shape or cross-section, for example an ellipse, or may be orientated in the vertical plane.

The fuel tank 1 comprises an inlet pipe 24 through which the fuel 3 is fed when the tank is being filled or replenished, an outlet pipe 22 through which the fuel 3 is drawn-down by the customer and a ventilation pipe 20. The ventilation pipe 20 is connected to the tank 1 via a low pressure vacuum valve (not shown) whose settings are typically +35mbar and - 2rnbar. Alternatively, the ventilation pipe 20 may vent to atmosphere via a flame proof grill. The fuel tank 1 also comprises a fuel level gauge (not shown) and an overfill valve (not shown) which detect and control the fill level of the tank.

The contents of the tank 1 consists of three layers, a liquid layer 10 having a surface 11, a heavy vapour layer (HVL) 15 having a depth (t) of approximately 100mm and a light vapour layer (L,VL) 16. The HVL and LVL together comprise a vapour space 14. As shown, the ventilation pipe is in communication with region of the vapour space 14 consisting of the LIVL 16.

In this condition, the fuel 3 may evaporate freely into the vapour space 14. The rate of evaporation will depend upon many factors such as vapour pressure and may increase or decrease due to changes in temperature and/or pressure/altitude. The rate of evaporation may also increase due to turbulence of the fuel 3. When the pressure in the vapour space 14 reaches a level which is sufficient to activate the low-pressure vacuum valve, vapour will vent to atmosphere through the ventilation pipe 20 and be lost.

Figure 2 shows the fuel tank 1 of Figure 1 further comprising a plurality of floatation members 40. The flotation members 40 may, for example, consist of 2mm diameter hollow nylon spheres. The flotation members 40 float on the surface 11 of the fuel 3 to form a carpet 41 of height (h1) . The height (h1) of the carpet 41 is approximately 100mm (i.e. equivalent to the depth (t) of the HVL). However, the height (h1) may greater or less than 0mm.

The carpet 41 forms a physical barrier between the surface 11 of the fuel and the vapour space 14 thereby obstructing evaporation of the fuel 3 into the vapour space 14. The plurality of flotation members 40 also provide nucleation sites which encourage the vapour to condense back into the liquid phase as it passes through the carpet 41.

In addition to the reduction in evaporation, the carpet of flotation members 40 has the benefit that the inside of the tank will be cleaned by the abrasive action of the flotation members 40 against the inner surface of the tank 1. This -10 -effect may be enhanced by providing the flotation members 40 with an abrasive outer surface.

If desired the flotation members 40 may activate tank overfill gauge mechanisms before the liquid level reaches a critical level, thus reducing the risk of overfill. The fuel level gauge will generally have to be re-calibrated to account for the presence of the carpet of flotation members and any instruments which the flotation members might interfere with, such as the overfill valve, may be shielded by a wire mesh of sufficiently small aperture size (for example 1.5mm) to prevent passage the flotation members 40.

The re-calibration of instruments and provision of protective wire cages are procedures which are well known in the art and no further discussion of this is given here.

The flotation members 40 may be introduced to and removed from the tank 1 via any one of the inlet pipe 24, outlet pipe 22 or ventilation pipe 20. One exemplary retrieval method is to suck out the flotation members 40, alternatively the flotation members may be displaced out of the tank 1 by overfilling with water. The introduction/retrieval of the flotation members 40 may be ameliorated by connecting the flotation members 40 to one another in the form of a daisy chain.

If desired, the flotation members 40 may be statically charged. If the flotation members 40 are hollow nylon spheres, as suggested above, they will generally be statically charged anyway. However, they may be physically charged by passing through a fur lined orifice or tunnel (or silk lined if the opposite polarity is desired). The -11 -flotation members 40 may be shaped so that the charge is equally distributed over the surface of the flotation member (for example a sphere), in this case the flotation members will mutually repel one another and thus fill the available area evenly. Alternatively, if mutual attraction is desired, the flotation members 40 may shaped so as to provide a polarised static charge.

An example of a suitable shape for providing a polarised static charge is shown in Figure 3 which depicts a small section of a layer of flotation members 50 having a tear-drop shape. Since static charge will tend to accumulate at the most pointed end of a charged body, the tear-drop shaped flotation members 50 will be polarised. Thus, as shown, the flotation members 50 will mutually attract and align.

Figures 4a to 4c show examples of other shapes of flotation member which will be polarised when statically charged.

Figure 4a depicts a generally cylindrical flotation member 51 which has a convex first end 61 and a concave second end 62. Figure 4b shows a generally spherical flotation member 52 which has a pointed projection 63 and Figure 4c shows an alternative generally spherical flotation member 53 which has a pointed dimple 64. Of course, other shapes of flotation member may also be envisaged which will be polarised when statically charged and the above examples are not intended to be limiting.

The mutual alignment of polarised flotation members may be enhanced in the case of a steel (or other metal having a low magnetic reluctance) fuel tank 1 since the Earth's magnetic field will be locally strengthened due to the low reluctance -12 -of the steel. This effect may be capitalised upon by aligning the tank 1 with the Earth's magnetic field.

Many different materials may be statically charged as is well known in the art. The flotation members 50 may therefore be fabricated from other suitable materials.

There are three main non-mutually exclusive categories of material which are particularly suitable: anti-static, static dissipative and conductive. They may comprise a whole flotation member or may just comprise a coating on the flotation member. Suitable anti-static materials, or materials that can be produced in an anti-static form, include: polystyrene, polypropylene random copolymer, PET (polybutylene terephthalene), POM acetal copolymer and ABS.

Static dissipative materials, or materials that can be produced in a static dissipative form, include: polyetherimide, PEEK, PEI, PPS, ABS, HDPE, Nylon 6/6 glass or carbon fibre.

It is often desirable to dissipate static charge from within fuel storage tanks as static build up may cause sparks which can ignite the vapour or liquid fuel. The flotation members may be designed to assist with the dissipation of static charge from within the tank 1 by either providing the flotation members 40 with an electrically conductive coating, for example by vacuum metallising, or by impregnating the flotation members 40 with a conductive element as is well known in the art of plastics fabrication.

Such impregnation may be at the molecular level or may comprise the impregnation of metallic particles -13 -If the tank 1 is made of metal the dissipation of charge to ground will be a simple matter of contact of the flotation members 40 with each other and the inner surface of the tank 1. However, even in the case of a non-conductive tank 1, a grounding strip (not shown) may be provided inside the tank and connected to ground for the purposes of dissipating static build-up.

Figure 5 shpws the fuel tank 1 of Figure 2 almost empty of fuel 3. In this condition, the surface of the fuel 11 has decreased in area due to the decreasing cross-section of the fuel tank 1 towards its lowermost end 2. The reduction in area of the surface of the fuel 11 results in a thicker carpet 41 of height h2. There is therefore an increase in the weight per unit area that the carpet of flotation members 40 applies to the surface of the fuel 11. As discussed above, applying weight to the surface of the fuel 11 reduces the turbulence of the fuel 3 and therefore reduces the rate of evaporation that would otherwise be elevated by, for example, the introduction of replenishing fuel 3 in to the tank 1.

The thickness of the carpet of flotation members 40 may also be increased through design of the flotation members 40 in combination with a decreasing cross-sectional area. Figures 6a, 6b, 7a and 7c show examples of such designs. For ease of illustration each of Figures 6a, 6b, 7a and 7b show a small section of a carpet of flotation members.

Figure Ga shows a small section of a carpet of flotation members 41 comprising flotation members 70 of a first density Pi and flotation members 71 of a second density P2, -14 -where Pi < P2. Each of the flotation members 70, 71 have the same diameter. Therefore, since Pi < P2, the displacement of the flotation members 71 is greater than the displacement of the flotation members 70 9as illustrated by the liquid surface level 11) . Thus the overall height d1 of the carpet of flotation members 41 is measured from the bottom of the dense flotation members 71 to the top of the less dense flotation members 70.

Referring now to figure Gb, as described above, as the tank empties the surface area of the liquid 10 decreases and the carpet of flotation members 41 increases in depth. This increase in depth is amplified in the case of the flotation members 70, 71 since the flotation members 70, 71 will rise and fall respectively relative to one another. The carpet of flotation members 41 will therefore increase in height from d1 to d2, where (d2-d1) > (h2-h1).

As shown in Figures 7a and 7b, a similar effect can be achieved by employing flotation members 80, 81 which have the same density but which differ in size. The smaller flotation members 81 may advantageously be sufficiently small to plug the gaps between the layer of larger flotation members 80.

It is clear that the flotation characteristics of the flotation members 40 can be adapted as desired by varying their size, shape, density etc. In addition, the flotation members 40 may be hollow or may contain a filling in order to vary the density and/or thermal insulation properties of the flotation members. The weight or overall density of the flotation members can be varied by the addition of dense -15 -material such as lead or brass. Examples of thermally insulating materials include foam fillings with an insulating material such as polystyrene. In any event, hollow flotation members containing air will have thermally insulating properties.

Figure Ba illustrates an alternative shape of flotation member 90 having a hexagonal cross-section. Since the cross-section is hexagonal the flotation members may tessellate and therefore form a very closely packed carpet of flotation members 91 on the surface of the fuel 11 as illustrated in Figure 8b. The depth 92 of the flotation members 90 may be small in comparison with the width to enable a thin carpet to be formed on the surface 11. Of course, other tessellating shapes may also be employed to similar effect.

Claims

-16 -Claims 1. A closed vessel containing apparatus for reducing the evaporation of a liquid contained, in use, within the vessel, the apparatus comprising a plurality of flotation members, wherein the plurality of flotation members are arranged to float on the surface of the liquid such that they form a carpet of at least one flotation member in depth over substantially the entire surface of the liquid.
2. The vessel of claim 1 wherein the flotation members are statically charged.
3. The vessel of claim 2 wherein the flotation members are shaped to provide a polarised static charge.
4. The vessel of claim 1 wherein the flotation members are electrically conductive.
5. The vessel of any of claims 1, 2 or 4 wherein the flotation members are spherical.
6. The vessel of any one of claims 1, 2, 3 or 4 wherein the flotation members are a plurality of different shapes.
7. The vessel of any preceding claim wherein the flotation members are a plurality of different sizes.
8. The vessel of any one of claims 1, 2, 3 or 4 wherein the flotation members are tessellating polyhedrons.

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9. The vessel of any preceding claim wherein the flotation members have a plurality of different densities.
10. The vessel of any preceding claim wherein the flotation members are hollow.
11. The vessel of claim 10 wherein the hollows of the flotation members contain a filler material.
12. The vessel of claim 11 wherein the filler material is a thermally insulating material.
13. The vessel of any preceding claim wherein the flotation members are arranged to thermally insulate the liquid from a vapour space within the vessel.
14. The vessel of any preceding claim wherein the flotation members have an abrasive surface to clean the inner surface of the vessel.
15. The vessel of any preceding claim wherein the flotation members are arranged to be introduced into and/or removed from the vessel via an inlet pipe, outlet pipe or ventilation pipe of the vessel.
16. The vessel of any preceding claim wherein the flotation members are strung together.
17. The vessel of any preceding claim wherein the horizontal cross-section of the vessel decreases towards its lowermost end.

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18 - 18. A method for reducing the evaporation of a liquid contained within a closed vessel, the method comprising floating a plurality of flotation members on the surface of the liquid such that they form a carpet of at least one flotation member in depth over substantially the entire surface of the liquid.
19. The method of claim 18 further comprising cleaning the inner surface of the vessel by abrasion of the flotation members.