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Background of the invention

A purpose of the invention is to replace during the
transport of crude oil in tankers a part of the bunker oil required for the propulsion of the tanker with
gas from the cargo. The cost of the bunker oil required for the propulsion of a tanker of 300.000-400.000
tons during a voyage from the oil-producing countries at the Persian Gulf to the refinery plants in Europe amounts at present to 1,3-2,2 million SKr, at which
a bunker consumption of 3-900-5.600 tons is estimated, which corresponds to 1-2% of the quantitv of transported crude oil.

During the transport of crude oil the most low-boiling components in the cargo are evaporated. Bv keeping
a certain overpressure (0,1-0,2 kp/cm2) in the tanks this evaporation can be reduced to 100-300 tons depending on the gas content of the oil. Owing to the high combustion heat of the gas this amount of gas corresponds to
about 120-360 tons bunker oil to the value of about
40.000-120.000 SKr or about 2- 8% of the bunker costs.
Today the gas which is evaporated from the cargo is let out in the air, often through valves on the deck.
During unfavourable wind conditions this gas causes serious deterioration of the air quality for the crew.

When the crude oil is pumped up at the oil fields
it contains a considerable amount of gaseous components. At present a part of this gas is evaporated from the crude oil- before pumping it to the shipping places.
There is seldom means for taking care of the gas.

Despite the great energy value considerable quantities of the gas are burnt in open torches whereby the energv produced is not taken care of.

It also applies for certain refineries that more gas is obtained during storing, pumping and refining the crude oil than can be effectively utilized. Gas is used for producing so called compressed gas and for producing energy required for the process.

Thus there is an excess of gas at the producers and in certain cases also at the consumers of crude oil. If the gas can be taken care of without costly liquefaction processes its price will be low.

Crude oils normally contain 1-2 and in certain cases 2-4 percentage by weight of gaseous components (methane to butane). As was previously mentioned the bunker consumption during a transport voyage corresponds to 1-2% of the cargo. A considerable amount of the bunker oil required for the propulsion of the tanker could therefore be replaced by gas, which is taken aboard and is stored dissolved in the crude oil cargo.

The method according to the invention can also be utilized for energy production in tanker tonnage being laid up and being intended as a stock reserve of crude oil and where the tankers perform the function as an intermediate storage in the crude oil harbours of the refineries.

It is previously known to utilize evaporated gas for the propulsion of tankers transporting condensed gas (LNG-carriers). The technology for taking care of and combusting gas from the cargo is thus available. This is described e.g. in th.e Swedish patent specifications 7202509-1 and 7316716-5. Here an uncontrolled evaporation of the gas takes place, which gas is utilized for the propulsion of the tanker.

In the Swedish patent specification 168.140 is described a method for heating oil by leading a stream of hot air through it. Such a procedure will lead to an outgassing, but this was not the purpose. This is probably the main reason why the method has never been used in practice.

The four lightest hydrocarbon compounds of alkane type are methane CH4, ethane. C2H6, propane C3H8 and butane C4H10. They all have boiling points below 0ÂșC and therefore exist in gaseous state at normal room temperatures. They however are soluble in heavier liquid hydrocarbon compounds, and can therefore exist in a liquid state also at normal ambient temperatures.

In a system where a liquid and a gaseous phase are in a state of equilibrium the partial pressure Pi.
in the gas phase and the concentration Ci in the liquid phase of the compounds concerned are depending on each other. According to Henry's law this relation can be written as Pi/Ci = Ki, where Ki is the phase distribution constant of the compound i.

The net transport between the gas phase and the liquid phase can thus be effected e.g. by varying the composi tion of the gas phase. With a gas-rich oil is meant an oil being in a state of equilibrium with a gas phase with high partial pressures of low-boiling components. If a decrease of the amount of low-boiling components in a crude oil is desired, this can thus be achieved by keeping the concentration of these compounds in the ambient gas phase low.

When the crude oil is stored in verv big tanks (15.000-50.000 m 3 ) and the transport of low-boiling components in the liquid phase is slow the outgassing will however take place relatively slowly. In order to increase the outgassing speed it is required besides a ventilation of the gas phase that the liquid phase is agitated and that the interface, through which a transport of low-boiling components can take place, is increased.

Description of the invention

The method according to the invention is based on that inert gas or tank gas is pressed .into the tanks close to the bottom through nozzles, which atomize the gas stream, so that small bubbles are formed.
When these bubbles rise through the crude oil they cause an agitation of the oil and an increase of the phase interface gas-oil, whereby the outgassing process is accelerated. By the fact that the gas flow can be adjusted the outgassing process can be controlled.

During the first days of a voyage the need of increasing the outgassing is probably low. The gas flow through the oil 'is then increased with the time, so that the evaporation amounts to a certain part of the fuel consumption. Since the supply of liquid fuel can be adjusted more rapidly the part of the fuel supply being adjusted preferably is the bunker oil.

The inert gas is taken from so called inert gas plants, which often are provided in modern crude oil tankers. These plants comprise an arrangement of flue gas washer (scrubber), fan, conduits, valves and control means, by aid of which combustion gases from the boilers are scrubbed and is led into the -tanks mainly in connec- tion with the discharging of the cargo. The purpose of this procedure is to decrease the oxygen content of the tank-gas below the limit at which a comhustion can be maintained. The main components of the inert gas are nitrogen, oxygen (15%), carbon dioxide and water vapour. Other components are nitrogen oxides, carbon monoxide and sulphur dioxide.

The inert gas flow should not be allowed to be too high. The concentration of hydrocarbons in the gas will then be so low that the introduction of the gas in the boilers would involve a poor combustion economy. Besides that energy is required for heating the inert gas a too extensive ventilation of the crude oil also involves that certain amounts of heavier, valuable fractions are removed from the crude oil.

In order to make an estimation of the required inert gas flows the following assumptions have been made:

1. Fuel consumption about 6 tons/hour
2. 4 tons/hour of the fuel consumption being gas from the cargo
3. The mean molecular weight of the gas is 40

4 tons gas corresponds to 4000 x 24/40 = 2.500 m3.

The inert gas flow required for driving off this quantity of gaseous hydrocarbons from the crude oil depends on a number of factors. Some of them are:
a) The amount of gaseous hydrocarbons dissolved in the oil
b) The desired concentration of hydrocarbons in the gas phase
c) The temperature of the oil
d) The method used for introducing the inert gas into the tanks. The number and design of the blower nozzles.

If the gas leaving the tanks has a volume ratio hydrocarbons/inert gas between 10/1 to 1/1 an inert gas flow of between 250-2.500 m3/hour (about 4-40 m3/min) is required for driving off 2.500 m3 hydrocarbon gas per hour at NTP. The inert gas plants in tankers usually have a capacity of at least 10.000 m3/hour.

By the fact that the oil is agitated during the transport the sedimentation in the tanks will decrease. This would involve that the necessity of crude-washing
and other tank cleaning operations decreases.

When the oil is pumped aboard the gas flow leaving the tanks corresponds to a power supply which probably exceeds the power required. Excess gas, if any, should if it cannot be burnt be led away and be emitted on a high level e.g. by way of stump-masts. In order
to prevent that drops of oil during the final phase of the loading is brought with the gas flow up in
the air the conduits should be provided with drop

The advantages of utilizing gas evaporated from the cargo for the propulsion of tankers can be summarized in the following points:

1. Utilization of the energy in the gas, which at
present is allowed to leave the tanks without being utilized (2-8% of the bunker costs during the transport). 2. Decreased bunker costs by the fact that high quality excess fuel is utilized without expensive liquefaction processes and without the need of special tanks for gas being needed.
3. An improved air quality for the crew.
4. Conversion of gas-rich crude oils to degazified and easily processable crude oils.

5. A reduction of the amounts of sulphur dioxide, carbon disulphide and mercaptans dissolved in the crude oil. These compounds involve air troubles
around the refinery plants.

The evaporation of gases from the crude oil is most extensive during the first days of the transport.
During short transports of crude oil, e.g. from the fields in the North Sea to European refineries, a larger part of the bunker consumption than the calculated 2-8% could probably be replaced by the gas, which at present flows out in the air.

Description of the drawings

Figure 1 shows schematically an arrangement for controlling the outgassing from the crude oil in the tanks of a tanker, and
Figure 2 and 3 show schematically a system working accor ding to the counter flow principle.

Description of an embodiment

In the figures B denotes a boiler, S a scrubber, FR a flow regulator, K a compressor and D drop separators.

The outgassing is controlled by leading inert gas and/or tank gas into the tanks at the bottom level thereof. For this purpose is used a system of ejectors E mounted at the bottom of some tanks and being driven by crude oil flows O. These flows are taken from other tanks in the tanker and are forced with pumps P placed in the pump room of the tanker. The ejectors suck inert gas I from the inert gas conduit on deck or tank gas
T from the ullage space of the tanks to the bottom level thereof. By arranging the system according to the figures 2 and 3 a system working according to
the counter flow principle is obtained. This gives certain advantages.

a) Only a part of the cargo is processed at a time, whereby the outgassing can be controlled more easily, so that the production is constant during the voyage. b) The cargo is processed in counter flow to the gas phase, so that the outgassing of hydrocarbon gas can be driven effectively towards an equilibrium. (The gas being sucked off has in the final phase been in contact with the oil in the process tank, where the oil is least processed and therefore is richest of gas). The required inert gas volume is therefore smaller per produced unit of outgassed hydrocarbon gas.
c) The quantity of inert gas being introduced in the boiler together with the hydrocarbon gas is also smaller, which means an improved boiler efficiency.
d) The system has been so constructed that the required arrangements can be concentrated to the stern parts of the tanker thereby decreasing the initial expenses.