Apparatus for reintroducing vapor into a volatile liquid

Abstract

The device for reintroducing steam into a volatile liquid comprises a tank (10) for the volatile liquid, a circulation pump (14) and connecting conduits (13, 113, 213) for pumping liquid from said tank (10) through a mixing zone or mixing device (12, 112, 212) for mixing the steam with the liquid, and a conduit (15, 115, 215) for conveying steam from the space above the level of the liquid to said mixing zone or mixing device (12, 112, 212), the mixing zone or mixing device (12, 112, 212) being arranged to ensure a pressure independent of the level of the liquid in the tank (10).

Description

[Technical field]

[0001] The present invention relates to an apparatus for the reintroduction / absorption of vapours from volatile liquids, in particular for the reintroduction of oil vapours into oil in the cargo tanks of a tank ship. [Background technology]

[0002] The transportation of volatile and flammable fluids such as oil in large tanks on tanker ships can result in losses to the cargo if vapours released from the oil are not captured and / or reintroduced into the oil, and further includes the risk of fire or explosion as such vapours are highly flammable.

[0003] These vapours will establish equilibrium with the corresponding components in the liquid phase under a certain overpressure in the tank. However, the pressure required to reach such equilibrium is higher than the current tank design pressures of oil tankers, making it neither practical nor economically viable to design a vessel capable of meeting this pressure requirement. Generally, vapours emitted from oil cargoes are referred to as "volatile organic compounds" (VOCs).

[0004] Attempts have been made in many different ways to overcome the above mentioned problems. Norwegian Patent No. 316 045, US Patent No. 6,786,063 and US Patent No. 3,003,325 describe systems and devices using equipment placed outside the storage tank, while Norwegian Patents Nos. 315 293 and 315 417 describe systems integrated into the tank. A more recent solution is described in WO 2007086751. Summary of the Invention [Problem to be solved by the invention]

[0005] Lack of efficiency at low or moderate oil levels in cargo tanks is a common drawback of conventional systems that do not function optimally unless the tanks are nearly full. Improvements are needed in this field to further reduce losses and improve safety in connection with the transportation of oil in oil tankers.

[0006] It is therefore an object of the present invention to provide an apparatus for absorbing vapor from a volatile liquid that is efficient, inexpensive, and eliminates the inherent drawbacks of existing solutions. The apparatus should be easy to manufacture, easy to maintain, easy to operate, and inexpensive.

[0007] In particular, it is an object to provide a method for reintroducing oil vapour evaporated from oil in a cargo tank of an oil tanker back into the oil. [Means for solving the problem]

[0008] This object is achieved by a device according to the invention as claimed in claim 1.

[0009] Preferred embodiments are disclosed in the dependent claims.

[0010] The main advantage of the present invention is unique in the fact that the reintroduction of vapor into the volatile liquid occurs at a pressure exerted by a column of volatile liquid that is maintained independent of the level of liquid in any tank to which the device is connected.

[0011] The vertical and horizontal columns included in some embodiments of the present invention may be oriented somewhat differently from strictly vertical and strictly horizontal, respectively.

[0012] In the further description, the terms "volatile liquid", "liquid" and "oil" are used interchangeably and, unless otherwise specified, refer to the liquid form of the volatile liquid described in the appended claims.

[0013] The invention will now be described in more detail in the form of non-limiting embodiments illustrated by the accompanying drawings, in which: FIG. [Brief description of the drawings]

[0014] [Figure 1A] FIG. 1A is a diagram of a first embodiment of the present invention at the early stages of cargo tank filling. [Figure 1B] FIG. 1B is a view similar to FIG. 1 at a later stage of cargo tank filling. [Diagram 2] FIG. 2 is a diagram of a second embodiment of the present invention. [Diagram 3] FIG. 3 is a cross-sectional side view of one component of FIG. [Figure 4] FIG. 4 is a cross-sectional side view of an assembly of the two components of FIG. [Diagram 5] FIG. 5 is a side cross-sectional view of an alternative component to one of the components of FIG. [Figure 6A] FIG. 6A illustrates an embodiment of the invention that includes variations on the components shown in FIG. [Figure 6B] FIG. 6B illustrates an embodiment of the invention that includes variations on the components shown in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present invention requires a system, such as the device according to WO2007086751, for reintroducing volatile vapors into the liquid used as the absorbing medium. Other embodiments of such a reintroduction system may include a fan, pump or compressor, preferably in combination with a microbubble generator, for the supply of gas and a pump for circulating the absorbing medium liquid through the column. For simplicity, an ejector type mixing device (similar to that in WO2007086751) is used as the main example for all embodiments of the invention described herein. Hereinafter, this device will be referred to as the "mixing device".

[0016] 1A shows a portion of a tank 10 having a bottom wall 101 and a top wall 102. A vertical liquid-tight tube or column 11 is disposed within the tank. A mixer 12 is disposed adjacent the bottom of said vertical column 11. The mixer 12 is arranged to be filled with oil provided by a pump 14 from a location adjacent the tank bottom 101, and the oil flow 13 is used to extract vapor 15 from the space above the liquid. The flow of vapor 15 may be assisted by an optional pump, fan or compressor 16.

[0017] The space above the liquid from which the volatile vapor is extracted is typically the space within the tank, but may be the space from any space or conduit to which the vapor is directed after evaporation.

[0018] The combined flow 17 of liquid and entrained vapor bubbles leaves the mixer 12 close to the bottom of the column 11, where the pressure of the liquid above causes the gas bubbles to condense into the liquid. This mixing unit (assuming an ejector-type device) should be positioned to direct a jet from its outlet vertically downwards towards the bottom of the column 11, with the maximum distance to the bottom being small enough to allow the jet to carry the entrained gas bubbles to that bottom.

[0019] The recommended distance from the outlet of the mixing unit to the bottom plate typically ranges from 2 to 5 m, depending on the flow rate. Other types of mixing units, e.g., microbubble generators, should be placed at or close to the bottom of the column 11 to ensure maximum hydrostatic pressure available for efficient condensation of the bubbles.

[0020] The function of the vertical column 11 is to quickly achieve a pressure in the mixer 12 at least similar to that of a full cargo tank, even if the tank is less than full or almost empty. The conduit 18 is arranged to lead the oil from the top of the column to the surrounding cargo tank without splashing, even if the tank is almost empty. In Figure 1A, the cargo tank is filled to less than 10% level, but the desired conditions of pressure in the mixer are still obtained.

[0021] For operational purposes, column 11 is provided with an outlet 19 near the bottom of the column. An isolation valve 20 at outlet 19 is closed when pump 14 is on and the system is running, but may be opened to gravity drain column 11 when tank 10 is emptied. To avoid a lack of pressure in column 11 during gravity drain, conduit 21 connects the top of the column to the top of tank 10 when valve 22 is open. Gas from tank 10 can then flow into the column and displace liquid. Valve 22 is closed when pump 14 is on and the system is running.

[0022] Figure 1B shows the same setup when the cargo tank is filled to a higher level of about 75%. There is no change in the conditions regarding the pressure in the mixer, and the level of oil in the cargo tank is not a condition, only the level of oil in the vertical column is a condition.

[0023] Although the vertical column 11 is shown physically located within the tank 10, the column may be located outside the tank but fluidly connected to the tank in the manner generally taught by the above description.

[0024] Generally speaking, by placing the mixer 12 within a liquid-tight vertical column 11 that is filled with liquid to a level independent of the level within the tank 10, a desired pressure is ensured within the mixer independent of the level of liquid within the tank 10.

[0025] Assuming an ejector-type mixing unit similar to that of WO2007086751, the outlet of the mixing device 12 is typically located 2-5 m above the bottom of the liquid-tight vertical column 11. In this case, the outlet of the mixing device 12 also typically has a central axis coincident with the central axis of the column 11, and this outlet faces vertically downwards towards the bottom of the column 11. For other methods of reintroducing volatile vapors into the liquid used as the absorption medium, it may be more advantageous to locate the mixing / gas introduction unit closer to the bottom of the vertical column 11 in order to optimize the effective absorption pressure.

[0026] Figure 2 shows a significantly different embodiment to that of Figures 1A and 1B, but the advantages of pressure at the outlet of a mixer 112 located adjacent the bottom wall 101 of the tank are substantially the same. The mixer 112 is arranged to be filled with oil 113 supplied by a pump 114 from a position adjacent the tank bottom 101, and the oil flow 113 is used to extract vapor 115 from the space above the liquid.

[0027] The combined flow of liquid and entrained steam bubbles from the mixer 112 enters a horizontal column 116 that extends substantially horizontally along the bottom wall 101. The column outlet is attached to a conduit 117 that extends to a position near or preferably above the top wall 102 of the tank. In operation, the conduit 117 is filled with oil, so that the pressure in the horizontal column is determined by the vertical extent of the conduit 117. In case, for some reason, not all the air bubbles are absorbed in the oil, an optional gas separation unit 119 can be placed at or near the top of the conduit 117 to remove the remaining air bubbles, which then continue to a lower level of the tank 10 through an optional drop line 111, which will not be described in detail here.

[0028] This proposed configuration allows the vapor bubbles to intimately mix with the liquid at higher pressure for a longer period of time compared to the first embodiment, thereby also enhancing mass transfer (condensation) from the vapor to the liquid. In general, the parameters for the efficiency of this second embodiment of the invention are the pressure arising from the conduit 117, the diameter and length of the horizontal column 116 following the mixer, the volumetric flow rate per cross-sectional area, and the residence time arising at higher pressure downstream of the mixer 112.

[0029] The arrangement described above with reference to FIG. 2 ensures a desired pressure in the mixing device 112, which is independent of the level of liquid in the tank 10 and which is typically at least equal to the pressure in a tank filled with oil.

[0030] The mixer 112 may be located separately from the horizontal column 116 or may be combined with the column 116 to form an integral unit.

[0031] For the configuration depicted in FIG. 1, it is recommended that both the outlet near the bottom of column 116 and the gas supply conduit from the top of tank 10 be equipped with isolation valves for gravity drain of the system.

[0032] FIG. 3 is a side cross-sectional view of a mixer 112 in the form of an ejector useful in the present apparatus. The mixer is similar to that described in WO2007086751. The mixer 112 has an inlet on the left side in the form of a through opening 31 through which liquid oil 113 is filled, providing the driving force for the ejector as it moves from left to right at a significant velocity. Steam enters the mixer laterally as a stream 115 into an annular space 32 surrounding the through opening 31, and is drawn into the liquid stream by the suction force created by the moving liquid. The annular opening between the space 32 and the tube section 33 can optionally include static vane blades to create a helical flow. The advantage of such a helical flow is improved dispersion of steam bubbles in the liquid stream.

[0033] A drawback of the second embodiment is that due to the density difference between the liquid and the vapor, vapor bubbles tend to accumulate at the top of the horizontal absorption column 116, where many fine bubbles merge into fewer larger bubbles, reducing the contact area between the vapor and the liquid.

[0034] This potential negative effect can be countered by introducing one or more sections that include curved or angled vane blades 41, 42, as shown in Figure 4. When multiple sections are introduced, the horizontal column 116 is divided into multiple segments 1161, 1162 as shown in Figure 4. Although the vane blades according to Figure 4 are located only at the inlet end of each segment 1161, 1162, respectively, they could be located anywhere in each segment or even have an extension across the entire section.

[0035] The configuration according to Figure 4 converts the linear flow motion into a helical flow motion, providing a velocity component transverse to the axis of the column that is greater than the effect of the upward force on the bubbles caused by the density difference between the liquid and the vapor. An additional feature of such a rotational flow is the outward gravitational effect, which brings the heavier liquid to the outside and the lighter vapor to the inside of the column center in the horizontal column 116. Note that the reference numeral 116 is used to commonly refer to a single horizontal column 116 and to columns consisting of multiple horizontal column sections such as column sections 1161, 1162. The horizontal column 116 shown in Figure 4 has a longitudinal axis z.

[0036] FIG. 5 is a cross-sectional side view of a mixer 212 using a different principle than the mixer 112 of FIG. 3. The liquid flow 113 enters the mixer from the left side, and the steam flow 115 enters the mixer through a pipe 51 with a perforated section 52 in the mixer, which causes the steam to be entrained in the liquid flow as small bubbles. Although not shown in FIG. 5, a horizontal column section 116 is arranged downstream of the mixer 212 in a similar manner to that shown in FIG. 4. FIG. 6A shows a variant of the device shown in FIG. 5, arranged inside a vertical column 11 of the type shown in FIGS. 1 and 2. In this case, no housing such as 212 in FIG. 5 is necessary, and the conduit 15 conveying the steam from the top of the tank is arranged with a perforated end or a perforated box 62 which serves the same purpose as the perforated section 52 shown in FIG. 5, to diffuse the steam bubbles in the oil, thereby entraining the steam in the oil and causing steam condensation. The pump 14 pumps the oil in the direction of the perforated section or box 62, enhancing the effect of the entrainment and condensation of the steam in the oil.

[0037] Figure 6B is a close-up view of the lower part of the vertical column 11 and the surroundings of Figure 5, showing the mixing zone 63 as a circle, where the steam bubbles from the perforated box 62 come into contact with the oil entrained therein. Of course, this zone has no sharp boundary.

[0038] All embodiments of the present invention may be advantageously combined with other devices which are not part of the present invention, such as a drop line device configured to reduce evaporation from volatile liquids during filling of the tank, or a gas-liquid separator to remove any remaining gas bubbles in the liquid before it is reintroduced into the cargo tank.

Claims

1. 1. An apparatus for reintroducing vapor into a volatile liquid, comprising: a tank (10) for a volatile liquid; a circulation pump (14) and a connecting conduit (13, 113, 213) for pumping liquid from said tank (10) through a mixing zone or device (12, 112, 212) for mixing the vapor and the liquid; a conduit (15, 115, 215) for conveying steam from the space above the level of said liquid to said mixing zone or mixer (12, 112, 212); Including, The apparatus, wherein the mixing zone or mixing device (12, 112, 212) is arranged to ensure pressure independent of the level of the liquid in the tank (10).

2. 2. Apparatus according to claim 1, wherein the mixing device (12) is arranged in a liquid-tight vertical column (11) filled with liquid to a level independent of the level in the tank (10), thereby ensuring a desired pressure.

3. 3. The apparatus of claim 2, wherein the mixing device (12) is located less than 5 meters and preferably more than 2 meters from the bottom of the vertical column (11), the column being located within the tank (10), the column having an upper end in fluid communication (18) with the body of the tank (10).

4. 4. Apparatus according to any one of claims 1 to 3, wherein the central axis of the outlet of the mixer (12) is aligned with the central axis of the column, the outlet facing vertically downwards towards the bottom of the vertical column (11).

5. 2. The apparatus of claim 1, wherein the mixer (112, 212) is located adjacent to a bottom (101) of the tank (10), the outlet of the mixer (112, 212) is connected to a horizontal column (116) located adjacent to the bottom (101) of the tank (10), the column (116) being connected to a conduit (117) extending upward to a vertical height higher than the vertical height of the mixer (112, 212), the vertical height being independent of the level of liquid in the tank (10) thereby ensuring the desired pressure.

6. The apparatus of claim 5, wherein the mixing device (112, 212) is located separate from the horizontal column (116).

7. The apparatus of claim 5, wherein the mixing device (112, 212) is combined with a horizontal column (116) to form an integral unit.

8. 8. Apparatus according to any one of claims 5 to 7, wherein the horizontal column (116) comprises a section including one or more static vane blades (41, 42) arranged to induce vortices in a flow entering the horizontal column (116).

9. 9. The device according to claim 8, wherein the vane blades (41, 42) are inclined with respect to a longitudinal axis (z) of the horizontal column (116) and / or curved in their longitudinal direction.

10. An apparatus according to any one of claims 5 to 9, wherein a gas separation unit (119) is disposed between said horizontal column (116) and said conduit (117).

11. Apparatus according to any one of claims 5 to 10, wherein the conduits (117) from the horizontal columns (116) terminate in drop lines (111).

12. 11. Apparatus according to any one of claims 5 to 10, wherein the conduit (15) is arranged to convey vapor to a perforated end or box (62) arranged at the bottom of the vertical column (11) and the conduit (13) is arranged to pump liquid to a zone (63) surrounding the end or box (62).

13. Apparatus according to any one of the preceding claims, configured to maintain an overpressure of at least 1 bar, more preferably at least 1.5 bar, most preferably at least 5 bar compared to ambient pressure.

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