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F E A T U R E by W. B. Hart, Pantech, Inc.
Flocculation as a Treatment for Petroleum Refinery Waste Water With the addition of more and more chemicals into oil refining procedures, separators can no longer efficiently handle oil-water separation. One answer is the installation of flocculating units
I N PAST years, the typical equipment for the removal of oil from waste waters to be discharged from petroleum refineries was the oil-water separator. This piece of equipment was regarded as a settling basin consisting of a n u m b e r of so-called "pockets," the waste water flowing from the first pocket to the second, to the third, and so on to the outlet. Some separators were constructed so that the flow moved from the first pocket laterally to the second, then forward to the third, with again a lateral movement to the fourth, with this same general pattern to the outlet end of the equipment. As a rule, however, the c o m m u n i cation between the pockets was a boxlike structure, with the sides of the box extending well below the surface of the flow so as to retain the oil which had separated in the pocket. This permitted its collection by so-called "skimmers," t h r o u g h which the oil was p u m p e d from the surface layer to a collection tank. Here the excessive amounts of water could settle to the bottom and be d r a w n off to the first pocket of the separator. T h e dried oil then was returned to the usual processing. During the earlier history of refining this arrangement served its purpose, which usually was to prevent loss of uneconomical amounts of oil to the waterway into which the separator discharged. Little thought was given to the extent of
oil pollution, for it would be too expensive to attempt to retain the last few parts of oil per million parts of waste water flow, and consequently no saving would result. Nor was petroleum refining as widespread as it has become, since the advent of automotive transportation, oil heating, industrial heating, and petrochemicals. But all these applications of petroleum products came into existence in a relatively short time and the refining of petroleum, and the refineries in which this was done, increased at a tremendous rate. Consequently the seemingly small losses of oil from the refinery separators in the aggregate became a serious menace to the water supply in all areas where petroleum products were manufactured. Largely this was d u e to the fact t h a t the supply of crude oil seemed to be almost unlimited a n d did not require too m u c h conservation, and there often was not too great interest, by the industry, in how m u c h injury was being caused by careless handling of waste materials. Laws Regulating Oil Pollution T h e whole situation was topped off by World W a r I, and in 1924 we had the first federal law aimed at oil pollution. T r u e , this concerned only navigable waters, but before m a n y years the urge devel-
oped to consider every stream as a potential navigable stream, if it could ever be m a d e so, from the springhouse to the sea. As a result of the degree to which " w a t e r pollution by oil" had developed, by a few years after the end of World W a r I I , very nearly every state in the union had a pollution law on its books and a second federal act was passed in 1948. T h e r e is today pressure to a m e n d this act to gain greater control by the Federal Government. Now, w h a t were the refiners and their associates doing? O n e of the first things they did was to obtain permission from the Army Engineer Office to conduct a nationwide survey, which had been ordered by Congress to be made by that office. This was done in 1927, by the American Petroleum Institute. Instead of the " w h i t e w a s h " which practically everybody expected, the survey report stated that oil pollution was serious and widespread and something should be done promptly. A committee of refinery people was formed to do it. As a part of the survey, the design of separators in general use was studied and it soon was seen that the communicating boxes between pockets, through which the entire flow must pass, were large contributors to the ineffectiveness of the device. W h e n the general equation for resistance experienced by a particle VOL. 49, NO. 5
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falling through a m e d i u m is a p plied—i.e., the general equation for resistance developed by N e w t o n — it soon becomes apparent that great turbulence, due to velocity in the passage through the box that carries the flow to the next pocket, is developed. T h e turbulence is so great that its influence is felt not only in the pocket the flow is entering but also in the pocket the flow is leaving. I n passing through this connecting structure, oil that was separated on the surface is d r a w n back into admixture and carried on into the next pocket. T h e effect of this turbulence was noticed promptly a n d just as promptly steps were taken to correct it. Interpocket communications were redesigned and the greatest possible opening in the walls between pockets was recommended. Emulsions
O t h e r conditions of the waste were becoming more prominent as a result of introduction of more chemicals into refining procedures. These introductions frequently resulted in the formation of emulsions, and if any kind of a gravity-type oil-water separator is to operate successfully, the oil must be in the free state. T h a t oil-in-water emulsions would not separate was definitely proved by the so-called "milk water," or "white water," problems that a p peared all over the country. Electrical charges on the particles also were involved. T h e net result was that the emulsion, usually arising from the water washing and neutralizing of acidtreated lubricating stocks, passed on through the separator practically unchanged, and was discharged into the receiving stream or lake, where it could be seen for miles, until separation occurred and the oil broke out. Serious oil pollution became apparent immediately. It was clear that separators, of the design known a n u m b e r of years ago, were not capable of preventing oil pollution arising from emulsions. T h e petroleum refining industry had come to the closing years of a n era, in so far as protection of neighboring streams was concerned. A new era was being opened by the advent of the extensive use of chemicals 78 A
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in refining operations and the onset of extensive production of petrochemicals. Separator design was studied extensively by the Refinery Waste Committee of the American Petroleum Institute, but the designs evolved were to no avail where oilin-water emulsions were involved. T h e water-treating industry for some time had been practicing water treatment with alum as a flocculating agent, and flocculation as a treatment for clarifying water for potable supply. Great success was experienced. Finally, several small petroleum refineries installed flocculating units for treating waste water prior to discharge. T h e success of these units was outstanding, although there were some disastrous experiences, mainly due to an effort to reduce costs by using iron salts as flocculating agents. Somewhere in the refining process from which the waste water came, soluble sulfides entered the waste flow. T h e soluble sulfides react with the soluble iron salts and the outflow from the flocculator was black. T h e situation, however, soon was corrected simply by going back to alum as a n agent. T h e combinations of a l u m i n u m and sulfur do not contribute color. M u c h time a n d effort h a d been given to the development of flocculating agents for clarification of water for water supply, so that not too great effort was needed to develop o p t i m u m conditions for the treatment of refinery wastes. It was well known that certain p H conditions were necessary for best results. These conditions readily were brought about by pH-controlled feeders for lime and sulfuric acid or for caustic soda solution in place of lime. Final polishing was accomplished by sand filters in plants where the water o u t p u t h a d to be potable. Actually, the waste treatment procedures developed, using theyflocculation processes as the major step, practically duplicated the water-treatment plant for producing a potable water supply. T h e product of such a treatment plant was really never completely cleaned u p to the degree that was brought about by the municipal treating plant. But whereas the municipal plant operates on raw water, usually from a carefully pro-
INDUSTRIAL AND ENGINEERING CHEMISTRY
tected source, the input to the waste treatment plant is a waste water which has been used to remove chemicals used for treating petroleum products and also the compounds formed when the chemicals arc applied. T h e principal purposes for which flocculation is applied can be carried out only after the major portion of the oil usually present in the waste flow has been removed. Otherwise the floe formed will pick u p so much oil that it will not settle, and the excess oil will rise and settle out as a layer on the water in the equipment. This layer then becomes a mixture of oil a n d floe a n d passes out of the equipment, through the normal release channels, to form a type of pollution in and on the receiving waters that is worse than oil pollution alone. Preliminary separation of the free oil is carried out in a separator very similar in design to that of the separators used as a final step in refinery waste treatment only a few years ago, m a n y of which are in use today. Such separators will remove the oil from waste waters when it is not emulsified and exists in globules free to rise or sink under the influence of its density as related to the density of the water in which the globules are suspended. T h e rate of flow through the separator will largely determine the percentage of the total oil present that will rise to the surface where it can be retained. If the rate of flow is too great, or the separating section is too short, the smaller globules will not reach the surface before they are caught by the retention baffle a n d will pass out with the discharge waste w a t e r ; the rate of rise of the oil globule is proportional to the square of the globule diameter. For this reason, related to the requirements of the various state laws, flocculation is becoming the prevalent and probably the most desirable procedure for treatment of refinery wastes, for flocculation picks out and retains these small globules in much smaller equipment, producing a n effluent that is almost oil-free. I n the next article a flocculation plant that is operating successfully, together with the principle of its design, will be discussed.
