BACKGROUND OF THE INVENTION 
     The present invention relates to coke ovens generally and, more particularly, to the recovery of naphthalene from coke oven by-product gas. 
     In a conventional light oil recovery plant, the oil condensate from the vapor to oil heat exchanger (primary light oil) is usually returned to the benzolized wash oil stream of the wash oil still. This minimizes the loss of wash oil and does not normally cause a significant increase in the naphthalene concentration in the debenzolized wash oil. 
     Such practice is carried out in plants in which a naphthalene scrubber or oil type final cooler is used in conjunction with the light oil plant. Frequently, an additional rectifying section is added to the still which strips only the naphthalene rich oil from the naphthalene scrubber or oil type final cooler with the total vapors from the wash oil still. Such practice greatly improves the removal of naphthalene. 
     The method of operation normally recovers about three times the amount of naphthalene removed in a conventional light oil recovery plant. For this reason, the wash oil condensed in the vapor to oil heat exchanger, which is in equilibrium with the vapor leaving the heat exchanger, contains approximately three times the concentration of naphthalene which this condensate from conventional equipment would contain. Calculations indicate a concentration of 15 to 18% naphthalene in this condensed wash oil. Even though the amount of this condensed wash oil is small, calculations indicate that the concentration of naphthalene in the debenzolized oil is increased approximately 0.4%. This value can be even higher if there is a significant mechanical carry-over from the wash oil still. This higher concentration of naphthalene in the debenzolized wash oil means a higher concentration of naphthalene in the debenzolized gas, with attendant problems due to plugging of the subsequent piping and equipment. 
     How the method of the present invention minimizes the naphthalene content of the debenzolized gas will be evident to those skilled in the art from the following description and drawing. 
     SUMMARY OF THE INVENTION 
     Primary light oil along with oil bled from a naphthalene scrubber or oil type final cooler is introduced into the top portion of a wash oil still. 
     For a further understanding of the invention and for features and advantages thereof, reference may be made to the following description and the drawing which illustrates a flow diagram of a system in accordance with the invention which is suitable for practicing the method of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The drawing illustrates a flow diagram of an improved system for the removal of naphthalene from coke even gas in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing, oil from a naphthalene scrubber (not shown) flows in conduit 11 into a conventional wash oil still 13 which includes a group of twelve normal trays 15 for the stripping of light oil, and a group of five additional trays 17, above the normal trays 15, for the stripping of naphthalene. Steam enters the wash oil still 13 through conduit 19 about where shown in the drawing. 
     Oil vapors emerge from the top of the wash oil still 13 through conduit 21 and flow into a heat exchanger 23. Likewise, wash oil from conventional benzol washers flow in conduit 25 into a wash oil pump 27 which urges the wash oil through conduit 29 into the heat exchanger 23. The wash oil passes through the heat exchanger and flows therefrom in conduit 31 into a final heater 33 into which steam flows through conduit 35. From the final heater 33, the wash oil flows in conduit 37 into the wash oil still 13, at a location between the additional trays 17 and the normal trays 15. 
     Vapors are removed from the heat exchanger 23 and they flow in conduit 39 to a light oil rectifier (not shown). Also, primary light oil and condensed water flows from the heat exchanger 23 through conduit 41 into a primary light oil separator 43, from which water flows in conduit 45 to a suitable sump-type receptacle (not shown). 
     The primary light oil leaves the light oil separator 43 in conduit 47 and flows into a primary light oil tank 49. From the primary light oil tank 49, light oil is drawn by a primary light oil pump 51 through conduit 53 and is discharged from the pump 51 through a conduit 55 into either one of two conduits 29 or 57. Valve 59 in conduit 55 and valve 61 in conduit 57 control the flow of primary light oil from the primary light oil pump 51. 
     In accordance with the invention, the valve 59 is normally closed so that the light oil flows from the pump 51 through conduit 57 into the top portion of the wash oil still 13; entering at a level above the uppermost additional tray 17 in the still, as shown in the drawing. 
     Wash oil 63 collects in the bottom of the wash oil still 13 and is removed therefrom through conduit 65 connected to the suction of a wash oil pump 67. The wash oil pump 67 urges the wash oil through conduit 69 toward conventional wash oil coolers (not shown). 
     From the foregoing description of a system in accordance with the invention, which is suitable for practicing the method of the invention, those skilled in the art should recognize many important features and advantages thereof, among which the following are particularly significant: 
     That, by pumping primary light oil along with oil bled from a naphthalene scrubber or oil type final cooler to a level above the topmost additional tray of a wash oil still, the recycled naphthalene is kept out of the wash oil that flows to the benzol washers, whereby the naphthalene content of the debenzolized gas is a minimum; and 
     That, by so directing the primary light oil, with the resultant minimizing of the naphthalene content of the debenzolized gas, conventional problems of plugging of piping and equipment with naphthalene are practically eliminated. 
     Although the invention has been described herein with a certain degree of particularity, it is understood that the present disclosure has been made only as an example and that the scope of the invention is defined by what is hereinafter claimed.
A method for decreasing the naphthalene concentration in debenzolized light oil for greater naphthalene removal comprises pumping the primary light oil condensate and oil bled from a naphthalene scrubber or oil type final cooler to a level above the topmost additional tray in the top portion of a wash oil still.