Abstract:
A method for removing contaminants from thermally cracked waste oil is disclosed. The thermally cracked oil is mixed with methanol at an elevated temperature and pressure to accelerate the whole extraction process and to obtain better productivity and better quality of end product oil. The contaminants in methanol liquid phase are separated by flash evaporation and the resulting methanol solvent is recycled.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to the removal of contaminants from thermally cracked waste oils, and more particularly, to the application of an accelerated solvent extraction process for removing contaminants from thermally cracked waste oils such as waste engine oil, metal cutting oil and hydraulic oils.  
         [0003]     2. Related Art  
         [0004]     A variety of processes have been developed for the production of diesel fuel from waste oils such as used engine oils and hydraulic oils by thermal cracking process. However, regardless of the processes of the technologies developed so far, the thermal cracking process itself gives rise to unavoidable problems of the quality of the end product oil. The end product oil has a strong odor mostly generated from sulfur compounds, dark tar precipitates originated from oxidation and the oil becomes darkened as it ages. These problems give rise to questions on the process itself and marketability of the end product oil.  
         [0005]     U.S. Pat. No. 5,855,768 issued in January 1995 to Stanciulescu et al. discloses that 95% of tar is removed by methanol extraction method at the same time with removal rate of 60% sulfur, 90% nitrogen and 60% chlorine. The temperature employed was between room temperature and 60° C. which is below the boiling point of methanol. It is well described by Stanciulescu et al. that the thermal cracking process produces olefin-rich oil which rapidly changes color and composition due to oxidation and polymerization reactions. This is also pointed out in “A literature review on fuel stability studies with particular emphasis on diesel fuel”, Energy &amp; Fuels, vol. 5, 2, 1991 written by Balts, B. D. and Fathoni, A. Z. This paper gives full explanation on the tendency of deterioration of thermally cracked distillates more rapidly than straight run distillates during storage and that cracked product oils induce gum and sediment formation in fuels. It was claimed from the review that major causes of gum or sediment formation include oxidation, auto-oxidation and chemical reactions such as polymerization involving unsaturated hydrocarbons and/or reactive organic compounds of sulfur, nitrogen and oxygen present in the fuel.  
         [0006]     The article “Influence of methanol extraction on the stability of middle distillate fuels” in Fuel, 73(2), 269-271 (1994) written by Sharma, Y. K, and Agrawal, K. M. also describes about methanol extraction. Its research data proves that middle distillate extracted with methanol shows much less insolubles, which proves the effective removal of insoluble precursors.  
         [0007]     U.S. Pat. No. 5,446,231 issued to Arganbright et al. on Aug. 29, 1995 also shows a method for the removal of nitrile contaminants from C5 streams in which methanol may be used as an extraction solvent. U.S. Pat. No. 4,746,420 issued to Daren et al. on May 24, 1988, have also found that methanol may be used as co-solvent in the removal of nitrogen compounds from diesel oils.  
         [0008]     Wecher, M. A. and Hardy, D. R. investigated methanol extraction in “The isolation of precursors responsible for insolubles formation in mid-distillate diesel fuels” in Fuel Science and Technology International, 7(4), 423-441 (1989). This paper claims that when mid-distillate diesel fuel is subject to solvent extraction using methanol, insolubles in the fuel are reduced dramatically, with more than 95% of the fuel insolubles being soluble in methanol.  
         [0009]     Accelerated Solvent Extraction (ASE) method has been developed and taken as an analytical method by U.S. Environmental Protection Agency (EPA) where elevated temperature and pressure are applied. However, the solvent extraction process has not been applied at elevated temperature and pressure to the removal of contaminants from thermally cracked waste oils and the optimum range of the high temperature and pressure has not been investigated. Perfect or full contact of methanol with contaminants in the cracked oil is not made by conventional batch type extraction processes running at room temperature and atmospheric pressure where mixing is totally dependent on mixing blade.  
       SUMMARY OF THE INVENTION  
       [0010]     Accordingly, it is an object of the invention to provide a method for removing contaminants from thermally cracked waste oils such as engine oil with respect to obtaining better productivity using high temperature and pressure than any other processes developed by now.  
         [0011]     It has been found from the present invention that the contaminants containing sulfur and nitrogen from the thermally cracked distillates may readily be removed at much faster rate at high temperature and pressure in solvent extraction than by conventional batch type extraction process which runs at room temperature and atmospheric pressure. In the invention, the substantial amount of contaminants containing sulfur and nitrogen is separated into the solvent and the contaminants can be separated from the solvent by simple flash evaporation and then the solvent can be recycled.  
         [0012]     As confirmed in the prior art of U.S. Pat. No. 5,855,768, the solvent extraction with methanol must be carried out within short period of time after thermal cracking process, preferably within 24 hrs. However, it is confirmed from the present invention that leaving the thermally cracked oils more than 6 hrs at room temperature makes the solvent extraction process more difficult or ineffective at all. By employment of methanol extraction within short period of time, preferably within 0˜20 min, substantial amount of contaminants could be removed after a series of repetitive methanol extractions.  
         [0013]     Preferably, the first extraction is carried out for a mixture of methanol and the thermally cracked oil at a ratio of 50:50 by volume and the second extraction is carried out for a mixture of methanol and oil (treated by the first extraction) at a volume ratio of 40:60. Then, the oil treated by the second extraction is mixed with 20% (by vol.) methanol for the storage of the final end product oil. The first and second extractions are carried out for 3˜15 min at 50˜70° C. and 30˜55 psi for the completion of the entire extraction process.  
         [0014]     The extraction process between methanol and the oils is carried out in a pressure vessel equipped with a thermal electric jacket and a mixer. Pumping methanol by a high-pressure pump mainly generates pressure.  
         [0015]     The result of using methanol extraction process at high temperature and pressure shows much faster reaction rate and produces better quality of end product oil. Each methanol solvent extraction takes 3˜5min before completion of the reaction compared to 40˜70 min of conventional continuous batch type extraction process at room temperature and atmospheric pressure. The sulfur contents drops to 69% and the nitrogen contents to 96%. And the tar contents decreases by 97˜98%, which is 2˜3 times faster rate than that of batch type extraction process. When continuous process is employed then the difference of reaction time and quality of end product oil would be greater.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The above object and other advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawing in which:  
         [0017]      FIG. 1  shows a flow diagram of the contaminant removal process which employs methanol extraction according to the embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]     Preferred embodiments will be described with reference to the accompanying drawing. Referring to  FIG. 1 , waste oils such as engine oil, metal cutting oil and hydraulic oils are thermally cracked in a thermal cracking unit. The thermally cracked oil is collected using a distillation tower and a condenser, and then is transferred into a first pressure vessel  5 . Recycled 99.8% methanol is also pumped into the first pressure vessel  5  using a high-pressure pump  41  to increase the pressure within vessel  5 . The temperature of the thermally cracked oil at the end of the condenser is about 30° C. The temperature of vessel  5  increases using an electrical thermal jacket  48  and is kept constant at a temperature of 50 to 70° C. The temperature of vessel  5  usually drops as the recycled methanol is pumped into vessel  5  by high-pressure pump  41 . Therefore, vessel  5  usually keeps its temperature higher than desired to prevent excessive temperature drop. Another option could be using a preheating heating coil wound around a methanol supply line when methanol is pumped into vessel  5 . Within vessel  5 , the first extraction process is performed. The desirable pressure within vessel  5  is 30˜55 psi and the volume ratio of methanol and cracked oil is 50:50. To keep the amount of methanol within vessel  5  constant, fresh methanol is supplied to vessel  5  from a fresh makeup MeOH.  
         [0019]     After the first extraction process, the resulting mixture is transferred to a first decanter  6  using a pump  24 . Then, the methanol phase containing contaminants floats to the top of decanter  6  and is pumped to a solvent flash tank  11  by a pump  51 . In solvent flash tank  11 , contaminants are separated from methanol liquid phase by simple flash evaporation and then discharged as a sludge cake. The operating temperature range of methanol solvent flash tank  11  is 70˜80° C. and the condenser  42  operates at below 45° C. The methanol treated in solvent flash tank  11  is transferred through a condenser  42  to a methanol storage tank  12  for recycling.  
         [0020]     For the second extraction process, the primarily cleaned oil is pumped to a second pressure vessel  7  using a pump  25 . The recycled methanol is transferred to vessel  7  using a high-pressure pump  31  in the same manner as with the first vessel  5 . The temperature within vessel  7  is also kept constant at a temperature of 50 to 70° C. as described above. The desirable pressure is 30˜55 psi and the volume ratio of methanol and cracked oil is 40:60.  
         [0021]     After the second extraction process, the resulting mixture is transferred to a second decanter  8  using a pump  26 . Then, the methanol phase containing contaminants floats to the top of decanter  8  and is pumped to solvent flash tank  11  by a pump  53 .  
         [0022]     The secondarily cleaned oil is sent through a pump  27  into a storage tank  10  and mixed with recycled methanol at the volume ratio of 80:20. The resulting mixture within storage tank  10  is stirred with a mixer at slow speed. Some of the methanol floating to the top of storage tank  10  is sent to a vessel  9  and then transferred to solvent flash tank  11  by a pump  52 . The mixture is subjected to batch type phase separation or centrifugal separation before shipping the end product oil.  
         [0023]     In the method of the present invention, condensers  45 ,  46  and  47  are used to prevent any evaporation of methanol into atmosphere resulting in loss of the methanol solvent and possible coagulation of tar materials. The pressure gauges  43  and  44  can be equipped with pressure valve to control over pressure and to collect pressure information.  
         [0024]     Table 1 shows typical chemical analysis of an example of the waste oil and product oil to which the method of the present invention is applied.  
                                             TABLE 1                                   Waste Oil   Product Oil                                        Water (%)   6.00   0.05           Ash (%)   2.47   &lt;0.01           Sulfur (%)   0.213   0.069           Carbon (%)   83.02   82.65           Hydrogen (%)   14.26   13.23           Nitrogen (ppm)   489   98.5           Specific Gravity,   0.905   0.807           24/25° C.           Specific gravity,   0.909   0.819           15/15° C.           Ignition point (° C.)   42   38           Density-API   24.23   40.39           Heating Value (BTU/lb)   18963   19631           Color-ASTM   NA   2.5                      
 
         [0025]     Hereinafter, the resulting example of each process step will be described.  
       EXAMPLE OF THE FIRST EXTRACT STEP  
       [0026]     A mixture of 500 ml cracked oil and 500 ml methanol was used as a sample. The sample from vessel  5  has been taken at given time intervals and the results are shown in Table 2.  
                                                                         TABLE 2                                   0 min   1 min   2 min   3 min   4 min   7 min                                    Sulfur (%)   0.213   0.187   0.101   0.085   0.086   0.085       Nitrogen (ppm)   489   193.9   78.5   47.8   46.3   46.5       Oil yield (%)   100   96.6   94.3   93.4   93.3   93.5       Residue yield   0   3.4   5.7   6.6   6.7   6.5       (%)       Color (ASTM)   7.5   6.0   4.0   3.5   3.5   3.5                 *specific gravity of sulfur - 0.87            **specific gravity of methanol - 0.79             
 
         [0027]     The resulting oil obtained at 3 min is not very stable even though sulfur and nitrogen contents substantially decreased since 15 small amounts of tar forms as it ages. The sample obtained from vessel  5  at 3 min has been used for the second extraction process.  
       EXAMPLE OF THE SECOND EXTRACTION STEP  
       [0028]     A mixture of 600 ml cracked oil and 400 ml methanol was used as a sample. The sample from vessel  7  has been taken at given time 20 intervals and the results are shown in Table 3.  
                                                                         TABLE 3                                   0 min   1 min   2 min   3 min   4 min   7 min                                    Sulfur (%)   0.085   0.074   0.070   0.069   0.070   0.068       Nitrogen (ppm)   47.8   35.3   23.2   19.7   19.8   19.7       Oil yield (%)   100*   99   98.5   98.5   98.5   98.5       Residue yield   0   1   1.5   1.5   1.5   1.5       (%)       Color(ASTM)   3.5   3.5   3.0   2.5   2.5   2.5                 *The value is corrected back to 100 for comparison.             
 
         [0029]     The resulting oil obtained after the second extraction step remains very stable with respect to tar formation, color change and smell. Mixing the final product oil with 20% (vol.) methanol in storage tank  10  completes the entire extraction process and produces stable oil with much less odor. Methanol extraction at high temperature and pressure is proven to be very quick process, which completes whole extraction process within 10˜15 min.  
                           TABLE 4                                   Sulfur reduction   Nitrogen reduction                           68%   96%                      
 
         [0030]     As described in Table 4 the reductions of sulfur and nitrogen are 68% and 96%, respectively, after the second extraction process. Final conditioning extraction process in storage tank  10  for the mixture of secondarily cleaned oil and methanol at the volume ratio of 80:20 lowers further oxidation problem and increase the sulfur and nitrogen reduction rate up to 1.0%.  
         [0031]     Although the preferred embodiment of the present invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions can be made without departing from the scope and spirit of the invention as defined in the accompanying claims.