Where heating alone is used, steam coils are placed in the bot tom of the gauge tanks. The steam heats the oil as high as 120° and causes its separation from the water. The danger in heating is the loss of volatile constituents, and for that reason care must be exercised in not raising the oil to too high a temperature.
Electrical System of electrical dehydrating system is quite simple. The electrical process of dehydration is based upon the principle that oil is practically a non-conductor of electricity while water is a good conductor.
Mechanical processes have not met with much success in treating with emulsions. Where the oil comes from a well and is whipped by the gas it may form a liver colored mixture that can not be separated by standing in a tank, by heating, or by centri fugal methods.
An emulsion of oil and water is an intimate mixture in which minute globules of water are coated by a film of oil, producing a mixture unfit for use.
An electric current can be used successfully to break up an emulsion and thus dehydrate the oil.
Temperature plays an important part in successful dehydration. The temperature may range from 140° to 180°F. In practice the thicker the emulsion the higher the temperature needed.
The oil treated in the above mentioned process ranges from 15 to 65 per cent water and the average amount of oil net after treating is approximately 18 bbls. per kilowatt hour or one-ninth of one cent per barrel for electricity at the rate of 2˘ per kilo watt hour. Electrical dehydration causes practically no loss of gasoline and the records show that after treatment the gravity of the oil has been raised from 1° to 2° and has in consequence an increased market value. This increase in market value in some cases is enough to pay the cost of dehydrating. The opposite is true in the heating process because crude oil con taining any appreciable gasoline will suffer evaporation under the temperature necessary to break down the emulsion and naturally the loss of gasoline means less dehydrates, less gravity and less market value.
The heating process necessitates close watching; the elec tric practically none. The heating process discolors the oil, impairing its market value; the electric dehydrator clarifies the oil leaving its natural color. The low fire hazard with electricity is important.
A record run of 7,000 bbls. of the same grade of crude oil was made, first by the heating process, then by the electrical. Eighteen hours was required with heat and only 7.5 hours with electricity, the net amount of oil being 5150 bbls. with the former process
and 5160 bbls. with the latter. The total cost by the heat process for this run was $387 or 740 per barrel, while the entire expense with electricity, even including a royalty of MO per barrel, was $102 or slight by less than 2˘ per barrel.
The electric dehydrator effectively treats oils of different grades at the same time without in any way impairing their efficiency. On a test, 28 gravity crude oil containing 25 per cent emulsion at a temperature of 70° was cleaned simultaneously and separately with 13 gravity oil containing 30 per cent emul sion at a temperature of 180°, by the same electric dehydrator, and the dehydrates showed only 1.3 per cent water and foreign matter in suspension, a limit of 2 per cent being permissible.
The electric dehydrating plant is made up of units called treaters and the usual size is a four-treater plant. The cost installed is about $2000 per treater. The cost of installation is generally borne by the oil companies and the ownership of the dehydrate is retained by the manufacturers, who also exact a royalty from the oil companies on each barrel of dehydrate produced.
In general, the electric dehydrator operates on a single phase alternating current at a pressure of 11,000 volts, the voltage being stepped up from the regular service of 440 or 2200 volts. The emulsion- is passed between high charged electrodes and in this electrostatic field the small globules and particles of oil, by static attraction for each other, form in chains which in turn coalesce into free water which readily settles to the bottom of the treater and is drawn off. In certain leases where water is very scarce, the water electrically removed from the oil is of considerable value.
Due to the condenser effect caused by the highly charged electrodes, the electric dehydrator operates at about 98 per cent loading power factor. The average maximum demand is 4 kw., the average load factor 50 per cent and the average gross income approximately $25 per month.
Centrifugal lastest development in separating oil and water in storage tanks is the centrifugal cream separator adapted to separating emulsified oil. The oil is heated and then run into centrifugal machines. (Fig. 59.) The whirling of the centrifuges breaks up the molecular cohesion. The water drops to the bottom of the centrifuge, the oil rises to the top. These centrifugal machines revolve at speeds of 6,000 r.p.m.