Gas separation – Means within gas stream for conducting concentrate to collector
Patent
1983-01-24
1984-10-09
Spitzer, Robert
Gas separation
Means within gas stream for conducting concentrate to collector
55 48, 55 50, 55 88, 55 89, 55217, 55228, B01D 5314
Patent
active
044759282
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for the recovery of petrol from a mixture of petrol vapour and air, in which the petrol vapour is absorbed in an absorption means by direct contact with a cooled petroleum distillate, and then the petrol dissolved in the petroleum distillate is stripped so that the petroleum distillate circulates in a substantially closed circuit, and a system for carrying out the method.
Ordinary motor petrol has a high vapour pressure at normal ambient temperatures both in summer and winter. Therefore, in normal circumstances, the concentration of petrol vapour above the liquid in a storage tank will amount to about 1.3 l (calculated as liquid petrol) per M.sup.3. This high petrol vapour concentration causes a great loss of petrol vapour particularly in the filling of storage tanks and tank trucks. Likewise, losses occur from storage tanks by breathing because of variations in the temperature. The total losses in these processes are about 0.2%, based on the amount of petrol, and in a country such as Denmark this corresponds to several millions Danish kroner.
Some methods are already known in the field which aim at reducing these losses. The purpose of some of them is to prevent the occurrence of petrol vapour, e.g. by the mounting of floating roofs on storage tanks. However, this principle is only useful on stationary tanks and involves considerable operating problems.
Moreover, in the pumping of petrol from one container to another it is known to recycle the petrol vapours from the second container to the first one. This method is effective, but has so many limits in practice that normally it is only used in combination with other methods.
The predominant methods among these are based on filtration with active carbon. However, these methods involve great investment and operating costs and are vitiated by the considerable weakness that they cannot handle the very large amounts of petrol vapour which occur at peak loads, e.g. in the filling of storage tanks from a tanker, where the amount of petrol may e.g. constitute 6000 m.sup.3 in ten hours, unless the system is unrealistically big and thus expensive.
The art moreover comprises methods based on cooling condensation or freezing of the hydrocarbons contained in petrol/air mixtures by intense refrigeration of the mixture, often to -70.degree. C. A method of this type is described in the SE Published Application No. 391 046. Since the petrol vapours are mainly composed of gases, such as butane, very low temperatures are required, which makes the process less attractive. As moreover the mixture usually contains water vapour which will condense together with the hydrocarbons, a special separation step is required to remove the water. To reduce the danger of explosions it is necessary to presaturate the mixture with crude petrol in order to safely increase the dew point of the mixture during compression and to thereby reduce the cooling requirement. This compression of the air, however, adds to the cost of the process.
An alternative principle relies upon washing of the petrol vapours with an absorbing liquid, preferably oil, from which the absorbed petrol is stripped, usually by distillation. This principle has several drawbacks. First, the vapour pressure of the mixture increases with increasing absorption of petrol, which reduces the absorption capacity, and secondly the solution of petrol in the oil is accompanied by generation of heat, which has an additional adverse effect on the absorption equilibrium This entails that the system becomes uneconomical, both in terms of size and operation, if it is to be dimensioned with a view to the peak loads occurring e.g. when a tanker is emptied.
In the DE Offenlegungsschrift No. 2 218 199 features from some of the above-mentioned methods are combined; for the recovery of an evaporated liquid, such as petrol, contained in a gas, a stream of the same liquid is continuously passed from a storage tank in a closed circuit through a cooling zone and a primary concentration zone back to the stora
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