Dehydrating Oil. When water is present in a free state in oil, it is easily separated by heating with steam. The latter is piped into a storage tank in 1 or 2-in. coils, the coils being placed horizontally from 4 to 6 in. from bottom. They should be kept covered with water in order to prevent the hot oil from adhering to them. A temperature of 100 to 150° F. is usually sufficient to cause the water to settle to the bottom, where it is drawn from the tank by a valve placed for the purpose. Should the oil be emulsified, the problem of separating the water is not so simple, additional equipment being necessary for the purpose. An emulsified oil is one in which the water portion carries a mineral salt in solution, the latter acting as a saponifying agent and surrounding the globule with a membrane or skin which sometimes cannot be broken by steaming, even at the boiling point. The emulsion is reddish brown in color, has a jelly-like appearance and is extremely viscous. The belief that it contains shale or other foreign matter is erroneous, although its appearance as a mass is deceiving. It often runs as high as 75% in oils, although the latter percentage undoubtedly contains a great deal of free water. A 35% emulsion, however, is common and quite as difficult to separate as are the higher percentages. The problem that confronts the operator is not only one of breaking up the globules by rupturing the encasing membrane, but in saving the volatile portions of the oil, which naturally tend to evaporate under the extreme heat conditions necessary. Four systems which have been successfully and economically used will be described.
r. Dehydrating by Electricity. This method, known as the Cottrell process,* has been successfully used on emulsions of vary ing proportions. The oil is first allowed to flow through the wetted septum water trap A (Fig. 163), and during its passage through this trap the free water is deposited on the wetted septum 2 and passes down it to the bottom of the trap and so away through outlet 3, which is so adjusted as to height as to make it self-regulating. The desired oil level in the trap is maintained by means of float valve 1, which controls the supply. From this trap the oil and water emulsion is discharged through outlet 4, whence it is taken by the rotary pump 5 and delivered to the treaters B. In cases where the contour of the ground permits, the wetted septum water trap may be placed at an elevation above the treaters, thus securing gravity feed and making rotary pump 5 unnecessary. The wetted septum 2 is merely a pervious canvas bag which has been thoroughly wetted with water, and is long enough to reach below the permanent water-level in the lower element of the trap. Under these conditions the canvas has an affinity for water, but not for oil. When the mixture of emulsion and free water, in its passage through the trap, reaches the canvas, the emulsion passes through, while the water, for which the canvas has an affinity, is deposited on and drawn down the canvas to join the main body of water. The treaters B consist of a sheet-metal tank 6, cylindrical for the major part of its height, but having an inverted conical top portion 7. The object of this increase
in diameter near the top is to lengthen the distance between the electrodes along the surface of the oil, and thus prevent surface leakage.
An outer electrode is formed by tightly stretching a number of wires 8 from a ring 9 at the base of the inverted cone to a circular plate 10 fastened to the bottom of the tank. Outside this electrode is a wetted septum 11. An inner electrode is formed by tightly stretching wires 12 between two circular plates 13 suspended in the tank by vertical shaft 14. The wires of the inner electrode are parallel to, and exactly concentric with, the wires of the outer electrode. The inner electrode is supported by a clamp 15 on the shaft 14, riding on a bearing saddle 16, which in turn is supported by the channel-iron frame 17 on insulators 18.
The vertical shaft 14 is rotated through insulating shaft 19, and universal joint 20 by the shaft and gearing 21, the latter being operated by a small electric motor.
The treater has a cover 22 with a large circular opening in the centre through which the inner electrode passes. The top ring of the treater is made of pipe which is perforated with a large number of holes pointing horizontally, and which is connected through valve 24 to a steam supply ; this valve is normally held closed by wire 25 and fusible link 26, but in the event of the oil in the treater catching fire, the fusible link will melt, releasing valve 24, and so filling the space below the cover with steam and choking the fire out. The oil enters the treater at inlet 27 (the flow being regulated by the size of the inlet orifice), and is maintained at a suitable temperature, depending on the viscosity, by means of a steam coil 28. After treatment the oil and water are discharged through outlet 29 and proceed to the separator C.
The inner electrode is connected through the saddle and frame with a source of electricity at a voltage between 10,000 and 15,000. The action of the electricity is to create a strong electrostatic field between the electrodes. As the emulsion under treatment comes between these electrodes the infinitely small particles of water, being conductors of electricity, will be formed into chains from electrode to electrode along the electrostatic lines of force, and, if the voltage be sufficiently high, the fine films of non conducting oil between the water particles will be punctured, bringing the entire chain together in the form of one comparatively large drop. This drop is now free water and is deposited on the septum 11 and conveyed to the bottom of the treater. It may happen, however, that so many chains of water particles are formed at the same instant, that they constitute a short circuit between the electrodes, thus lowering the voltage below that point at which it can puncture the oil films. In order to prevent such short-circuiting, the inner electrode is rotated, which gives the desired result, probably owing to the lengthening of the chain between corresponding wires in the outer and inner electrodes as the latter is revolved.