WO1992020571A1

WO1992020571A1 - Method for reduction of gas emission

Info

Publication number: WO1992020571A1
Application number: PCT/NO1992/000088
Authority: WO
Inventors: Otto Mejlænder MARTENS
Original assignee: Sinvent As
Priority date: 1991-05-10
Filing date: 1992-05-11
Publication date: 1992-11-26
Also published as: NO911834D0

Abstract

Method for reducing emission of volatile components from cargo being loaded, shipped and unloaded from a series of tanks of a ship. When unloading, the first tanks being unloaded are supplied with inert gas in a manner known per se, or supplied with gas substantially comprising volatile components, until all tanks are empty. When unloading subsequent tanks, the atmosphere in the preceding tanks that were unloaded is displaced by supplying inert gas, so that the atmosphere comprising volatile components is transferred to the tanks being unloaded. These steps are carried out until all tanks have been unloaded, whereupon the last unloaded tank(s) contain an atmosphere comprising volatile components, whereas the remaining tanks contain substantially pure inert gas. When loading, the tanks containing most volatile components are loaded first, and the atmosphere thereof is displaced to the following tanks to be loaded, at the same time as the atmosphere in these tanks is vented to the surrounding atmosphere. When loading the last tank(s), the gas comprising volatile components may be displaced or recovered, or optionally vented to the surrounding air.

Description

Method for reduction of gas emission.

The present invention concerns a method for the reduction of emission of gaseous components from liquid bulk cargo as stated in the introductory of claim 1. More particularly, the invention concerns a method for decreasing emission of gaseous hydrocarbons and other gases from oil tankers and tankers carrying chemicals, such emission ordinarily occuring during the loading procedure.

Marine transportation of crude oil, oil products and liquid chemicals is normally carried out in bulk, i.e. in several vessels comprising an integral part of the ship itself. During loading, travel and unloading, volatile components evaporate from such cargos, and when employing known handling procedures, part of these components will escape to the atmosphere, resulting in both air pollution and loss of cargo.

The emission of volatile components depends on the vapour pressure of the cargo at the cargo temperature in question, on the procedures practiced and on the condition and construction of the loading/unloading system. Typical vapour pressures of cargo being transported in this way varies e.g. in the range between 0.05 and 1.0 bar. Typical emission volumes for crude oil tankers comprise 0.15 to 0.2 percent of the total cargo weight per voyage. These values correspond to a loss of 250 to 300 tons per voyage for a medium sized crude oil tanker of 200 000 m³. The majority of emission normally occurs during the loading procedure since the atmosphere present in the tanks during travel in ballast, normally comprising an inert gas mixed with volatile components of the cargo, is displaced by the liquidous cargo during the loading of same. The tank atmosphere vented to the environmetal air is completely or partly saturated with volatile components. These components have their origin partly from the former cargo and partly from the cargo being loaded into the tanks. Because of the risk of explosion the use of inert gas is prescribed. Normally, the inert gas comprises nitrogen and carbon dioxide, produced from combustion gases purified with respect to particles and sulphur. The maximum allowable amount of oxygen is a few percent. Pure nitrogen gas may be utilized instead of combustion gases. Emissions also occur during the unloading and transportation of the cargo. However, these emissions can be avoided by known arrangements. Solutions to avoid emissions during loading are known, but these solutions have as far as the inventor is aware not been practiced. Emissions during loading can be avoided by means of a pipeline for simultaneously with loading transferring the tank atmosphere from the ship vessels to an onshore unit. US Coastguard has issued regulations concerning such procedures. Another method comprises recovery of a substantial part of the volatile components by pressurizing and cooling the displaced tank atmosphere onboard, thereby condensing the volatile components to a liquidous state. Hydrocarbon gases can also be absorbed in sub-saturated oil. The most serious drawback to the known methods for avoiding or decreasing emissions during loading is that the methods are expensive and to a certain degree techncally complex. The main object of the present invention is to provide a method to decrease emission of volatile components from the tank cargo to the environmental air without having to make substantial changes to the loading/unloading system of a ship, changes that would involve additional investments.

The above object is achieved by a method as stated in the characterizing part of claim 1. Further features appear from the independent claims 2-4.

It should be noted that the term "saturated gas" used in connection with inert gas saturated with a relatively high portion of hydrocarbon gas is not meant to constitute a complete saturation, that is a hydrocarbon gas partial pressure equal to the bulk medium vapour pressure; i.e. saturation may not necessarily be complete.

The present invention can be implemented on new as well as on existing ships, provided that the cargo is of a type which permits the tank atmosphere in one vessel to be moved to another vessel, and from one cargo trip to another. This condition is fulfilled with product tankers and, in some instances, with tankers carrying chemicals.

The invention is broadly characterized by the fact that evaporation from the cargo is avoided because the method allows the atmosphere above the liquid at any time to be kept as close to saturation as possible, i.e. the partial pressure of the evaporated gas in the atmosphere above the liquid is as close to the liquid vapour pressure as possible. Moreover, emission is minimized since the total amount of evaporated gas (hereinafter referred to as HC gas), after unloading the ship, is as small and concentrated as possible. Finally, the present invention is further characterized in that most of the evaporated gas, at the end of the loading procedure, is concentrated in a small part of the total tank volume, a feature making the method suitable for expanding the system with a relatively simple on board recovery unit for HC gas or alternatively gas recycling to a recovery unit at the on shore terminal.

In accordance with the present invention, a tank atmosphere is transferred sequentially from one tank to another, optionally from several tanks to other tanks both during loading and unloading. When unloading, inert gas is added in conventional manner to the first tanks to be unloaded in parallel as the liquid level in said tanks decrease. Some gas will evaporate during this unloading sequence. At the end of this sequence, and when a following sequence starts for unloading a number of tanks in parallel, inert gas is supplied constantly to the tanks initially unloaded, whereas the gas in said tanks is displaced by the inert gas being supplied and transferred through a separate pipeline system to the tanks being unloaded. This procedure is continued until all tanks are unloaded. The HC gas content is decreased substantially in the tanks which in this manner are flushed with inert gas after unloading, at the same time as the substantial part of the HC gas is collected in those tanks loaded previously. The evaporation here is prevented by the increasing portion HC gas in the tank atmosphere. Enrichment with HC gas is particularly high in tanks being flushed with cargo oil in accordance with the well known "Crude Oil Washing method" (COW). This gas is also collected in the last tanks that were unloaded.

When the ship is loaded again, loading starts with the tanks comprising an atmosphere enriched with HC gas. The atmosphere is displaced, by the in-flowing cargo in a manner similar to the unloading procedure, to the tanks to be loaded thereafter in such a way that the HC poor gas in these tanks is displaced to the environmental air and simultaneously evaporation of HC enriched atmosphere from the tanks being loaded is prevented. In this manner, the loading is continued sequentially. HC enriched atmosphere from the last tanks to be loaded is allowed to escape into the surrounding air, or transferred to a HC gas recovery unit onboard or on shore. Such an installation may be based upon condensation, absorption, adsorption or other methods.

In cases where no additional recovery of volatile compoents is utilized, the decreased emission of volatile components is most favorable when loading/unloading only one tank at a time, but the invention is not dependent upon this. The efficiency of the present method is most favorable when the transfer of saturated tank atmosphere from one tank to another occurs with as little interference (mixing) with the inert gas as possible, i.e. pure displacement. However, even at complete mixing of the gases, the present invention will result in decreased emission compared with known methods.

Ventilation by displacement, i.e. with minimum gas inter-mix, is achieved at low flow rates and with a suitable arrangement of an inert gas supply unit at the tank deck at the time of unloading, and a similar supply unit for saturated gas at the bottom of the tank during loading. The saturated gas is normally considerably heavier than pure inert gas. Concerning inert gas saturated with crude oil gas, the relative density is ca. 1:1.4-1.5, respectively, and for pure crude oil gas to inert gas, the ratio is about 1.7:1, respectively. A certain mixing of the gases occur by convection flow established at temperature differences between the gas and parts of the tank structure, e.g. as a result of a cooling action from sea water outside the tank. This can partly be prevented by having this possibility in mind when constructing the ship, and through ballast routines. A ship having double bottom and double sides can for example be unloaded without having to load ballast into the lateral tanks prior to replacement of the atmosphere in the cargo tanks located beyond.

Best efficiency is achieved when tanks being loaded/unloaded have the same size so that the gas volume being transferred equals one tank volume. If the tank volumes are different, the tanks are established in two or three groups, all tanks in one group having substantially the same volume. Each such tank group is loaded and unloaded in parallel. The same principle is valid if the ship carries more than one type of cargo simultaneously. If the respective cargos exhibit different vapour pressures and/or an intermix of the different gases from the cargo is impossible, the tanks are divided into groups to be loaded/unloaded in parallel. The method for loading/unloading the respective parallel units is similar to the method for loading/unloading one tank at a time; a procedure described in further detail below. In the following, the invention will be described in further details with reference to figures, in which:

Figure 1 illustrates schematically a loading/unloading cycle for a series of tanks, of which only one tank is loaded/unloaded at a time, and Figures 2 and 3 illustrate an example of a simplified pipeline diagram for three tanks, to realize the method according to the present invention.

Referring to Figure 1, a chematic longitudinal section of six tanks in a tanker is illustrated. Each tank is indicated by a reference numeral 1 through 6. Some sequences of the unloading procedure are shown at a, b, c and d and loading procedures at e, f and g. a) Unloading is started from tank 1. Inert gas (IG) is supplied to the tank. Alternatively,

HC gas, recovered from the former cargo and stored separately on board during the travel in ballast can be used. Optionally, the tank is flushed with the bulk medium (COW). In this way, the tank atmosphere is partly or completely saturated with gas from the bulk medium, if such gas has not already been supplied. Thus, further evaporation is avoided. b) Tank 1 has been unloaded, and unloading is continued from tank 5. Inert gas is supplied to tank 1, whereupon the tank atmosphere saturated with gas from the bulk medium from step a) above is displaced and transferred to tank 5. c) Tanks 1, 3 and 5 have been unloaded, and tank 4 is being unloaded. Tanks 1 and 5 are filled with pure inert gas. Inert gas is supplied to tank 3 at the same time as atmosphere saturated with gas from the bulk medium is displaced and transferred from tank 3 to tank 4. d) This illustrates the tanker in ballast, where all tanks, except for tank 6, which was the last tank to be unloaded, are filled with substantially pure inert gas. Tank 6 is filled with an atmosphere saturated with gas from the bulk medium. e) Loading of tank 6 is started and the saturated tank atmosphere is displaced and transferred to tank 2 which is the tank to next be loaded. Inert gas is vented from tank 2. f) Tanks 6 and 2 have been loaded, and loading of tank 4 is continued, whereupon the tank atmosphere saturated with gas from the bulk medium is displaced and transferred to tank 3, which is the tank to next be loaded. Simultaneously, inert gas is vented from tank 3. g) All tanks except for tank 1 have now been loaded, and the tank atmosphere in this tank, saturated with gas from the bulk medium, is displaced during the loading of tank 1 and vented to the environment or transferred to a purifying or recovery unit.

Figure 2 illustrates an example of a simplified piping diagram comprising three tanks 1, enabling transfer of saturated gas between the tanks through the piping system 2, gas venting from the tank system through pipe 3 to the surrounding atmosphere or to a purifying or recovery unit. Inert gas, supplied from the storage or the production unit 4, is guided through the piping system 5 to the tanks in question. Loading and unloading of the liquidous cargo occur through the piping system 6 by means of a pump 8.

Figure 3 illustrates the same arrangement as Figure 2, but here, gas comprising volatile components from the former cargo is supplied to the first tank being loaded through pipe 2 from an external storage unit 7.

Example

The following example illustrates the magnitude of the decreased emission from a typical crude oil cargo achieved in accordance with the present invention.

A typical crude oil cargo having a vapour pressure of 0.7 bar at 30°C is unloaded at this temperature. The density of evaporated hydrocarbon gas at this temperature is 1.96 kg/m³. In a saturated mixture with inert gas, the hydrocarbon gas will have a density of 1.37 kg/m³. Provided that the cargo is distributed among 15 equally sized tanks, and 5 of them are flushed with the cargo oil, 5 tanks will have a saturated tank atmosphere, whereas the remaining 10 tanks will have an atmosphere at approximately 50% saturation after unloading. Thus the hycrocarbon gas content in the tanks after unloading will be: (1.37 x 1/3) + (1.37 x 0.5 x 2/3) = 0.913 kg/m³.

By practising the method in accordance with the present invention, the hydrocarbon gas in the majority of the tanks is replaced with inert gas. At ideal conditions with displacement ventilation, the degree of purifying flushing will be 100%, whereby the tank only contains gas evaporating from the crude oil remaining in the tank. In the most unfavourable situation, the gases immediately and completely intermix. Thus the degree of purifying flushing will be 63%. Provided that 90% purifying flusing is achievable, the hydrocarbon gas in these tanks will consitute 7% of the volume, corresponding to 0.137 kg/m³. When loading, this gas is vented together with saturated gas from one or a few tanks. The resulting emission is:

a) 1 of 15 tanks are loaded/unloaded at a time: (0.137xl4/15)+(1.37xl/15) = 0.22 kg/m³. b) 2 of 15 tanks are loaded/unloaded at a time: (0.137xl3/15)+(1.37x2/15) = 0.30 kg/m³. c) 3 of 15 tanks are loaded/unloaded at a time: (0.137xl2/15)+(1.37x3/15) = 0.38 kg/m³.

These values correspond to emission of 24, 33 and 42%, respectively, of the emission which occurs during the loading period with ordinary procedures and at the same conditions. Thus, in accordance with the example above, the emission may easily be reduced by 76%. However, this reduction may vary with the cargo type, the tank structure as well as the method utilized for loading/unloading.

To achieve optimal performance of the method in accordance with the present invention, the method can beneficially be carried out by means of automatic systems and computer based control systems in accordance with common practice in process technology to ensure a fast and accurate transfer of the respective gas volumes, but the invention is not dependent on this.

Claims

Claims.

1. Method for the reduction of emission of volatile components from ship tanks, comprising a series of separate tanks, during loading of the tanks sequentially or partly in parallel, whereby the tanks are supplied with inert gas during the unloading of the same, c h a r a c t e r i z e d in that i) a first tank or tanks during unloading of liquidous bulk cargo therefrom, is/are supplied with inert gas or gas comprising volatile components from e.g. the former cargo, to replace the volume of the liquidous bulk cargo until these tanks are free from bulk medium and containing inert gas comprising volatile components from the bulk medium, or gas substantially comprising volatile components, ii) the atmosphere in preceding tank or tanks that were unloaded, is/are displaced to following tank(s) being unloaded by supplying pure inert gas thereto, so that the preceding tanks after unloading of same have an atmosphere comprising substantially pure inert gas, and the last tank(s) unloaded has/have the original atmosphere from the preceding tanks that were unloaded, iϊi) step ii) is repeated until all tanks have been unloaded, whereupon all tanks contain an atmosphere of practically pure inert gas, except for the tank(s) being the last unloaded loaded which contain inert gas comprising or being saturated with volatile components,

(iv) the tank(s) that after unloading contain an atmosphere comprising volatile components are loaded first, so that this atmosphere is displaced by the cargo and is transferred via a closed piping system sequentially to the next tank(s) to be loaded, whereupon the practically pure inert gas in the next tank(s) is displaced and vented to the surrounding air, and v) repating step iv) above until, optionally, all tanks have been loaded, whereupon the atmosphere in the last tank(s) being loaded, comprising volatile components, is either vented into the surrounding atmosphere or transferred to a purifying or recovery unit.

2. Method in accordance with claim 1, characterized in that the gas supplied to the first tank being unloaded, optionally the first tanks being unloaded in parallel, is a gas comprising substantially the volatile components generated from the cargo, optionally from a former cargo, or supplied from a terminal onshore, whereby the volume of this gas at least partly corresponds to the tank volume of the first tank(s) being unloaded.

3. Method in accordance with claim 1 or 2, characterized in that the first tank(s) to be loaded prior to the loading, and optionally after conclusion of the preceding unloading, is/are filled partly or completely with gas enriched with components collected on board from the preceding cargo, either in those tanks unloaded as the last, or supplied from a gas recovery unit with a storage utility on board, optionally supplied from a terminal onshore, in such a way to the bottom of the tanks that the original inert gas in the tanks is displaced and leaves the tank at tank deck level with as little as possible inter-mixing with in-flowing gas.

4. Method in accordance with claim 1-3, characterized in that gas supply to the tanks and gas transfer between the tanks is effected by means of a piping system arranged to avoid mixing of inert gas and gas comprising volatile components, by transferring inert gas in and out of the tanks at tank deck level, and by transferring gas enriched with volatile components in and out of the tanks at a lower level, arranged with deflecting plates, diffusers and the like, so that flow rate is limited and flow direction is favourable with respect to limiting inter-mixing.