Conditioning phosphorus-vanadium-oxygen catalyst
BACKGROUND OF THE INVENTION 
     A. Field of the Invention 
     The invention relates to a method for preparing catalysts useful in the manufacture of maleic anhydride by the oxidation of aliphatic hydrocarbons, and more particularly to low attrition catalysts suitable for producing maleic anhydride from saturated hydrocarbons in high yields. 
     B. Description of the Prior Art 
     Bergman et al., U.S. Pat. No. 3,293,268 (hereby incorporated by reference) teaches a process of oxidizing saturated aliphatic hydrocarbons to produce maleic anhydride under controlled temperature conditions in the presence of a phosphorus-vanadium-oxygen catalyst. 
     Typical phosphorus-vanadium-oxygen catalysts for use in tube-type reactors are formed as pills, pellets, slugs, tablets, or extrusions, collectively termed &#34;agglomerates&#34;. Charging of such catalyst agglomerates to a reactor is a problem because the agglomerates are dusty, that is, have very low attrition resistance and the phosphorus-vanadium-oxygen dust is moderately toxic. Catalyst structures are easily broken and such breakage can cause undesirable pressure drop difficulties during reactor operation. In attempts to alleviate such problems, high density forms have been employed using higher tabletting pressures. These high density forms, with corresponding lower porosity, are less active than low density forms. 
     If the attrition of low density phosphorus-vanadium-oxygen catalysts could be significantly reduced without detriment to catalyst performance such reduction would constitute a significant advance in the art and is an object of this invention. 
     SUMMARY OF THE INVENTION 
     These and other advantages are achieved in a process for preparing phosphorus-vanadium-oxygen complex catalysts by contacting vanadium and phosphorus under conditions to provide a catalyst precursor having greater than 50 atom % tetravalent vanadium, recovering the precursor, forming the catalyst into agglomerates and calcining the catalyst precursors by the improvement comprising wetting the catalyst precursor with an aqueous liquid in an amount sufficient to enhance surface adhesion prior to calcining and then drying the catalyst precursor. The wetting may take place before the formation of the catalyst precursor into agglomerates, after the formation of the catalyst precursor into agglomerates, or both before and after. 
     DEFINITIONS 
     As used herein the term &#34;agglomeration&#34; means the gathering together of particulates, particularly powdered materials which are brought together in a loose state of bonding to form larger particulates or structures. Such &#34;agglomerates&#34; are usually redispersible. 
     The terms &#34;granules&#34; and &#34;nodules&#34; describe forms of agglomerates by reference to the end-condition of the product. &#34;Granulation&#34; and &#34;granules&#34; are used to describe the formation of, and the resultant irregularly shaped clusters, of particulates. &#34;Nodulizing&#34; and &#34;nodules&#34; are used to describe the formation of, and the resultant generally regularly shaped clusters, of particulates, such as pills and pellets. 
     As used herein, the term &#34;attrition&#34; means degrading or deagglomerizing by wearing or grinding down through friction or breakage of the catalyst agglomerates or structures into dusts and/or fines. 
     By &#34;aqueous liquid&#34; is meant water or any liquid containing a substantial amount of water. Any other constituents should, of course, be relatively inert to the catalyst precurser. Such other constituents may include alcohols such as methanol and ketones such as acetone. 
     The term &#34;percent (or %) attrition&#34; means, according to the test described herein, the weight loss in grams by friction and breakage of the catalyst structures (initial weight, grams--subsequent weight, grams) divided by the initial weight in grams of the catalyst structures, the quotient having been multiplied by 100. 
     The term &#34;yield&#34; means the ratio of the moles of maleic anhydride obtained of the moles to feed material introduced into the reactor. 
     The term &#34;space velocity&#34; means the hourly volume of gaseous feed expressed in cubic centimeters (cc) at 15.5° C. at standard atmospheric pressure, divided by the catalyst bulk volume, expressed in cubic centimeters, the term expressed as cc/cc/hour. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Broadly described, the catalysts of this invention are prepared by contacting a phosphorus compound and a vanadium compound under conditions which will provide a catalyst precursor having a phosphorus to vanadium atom ratio between about 1:2 and about 2:1, and having greater than 50 atom percent of the vanadium in the tetravalent state. The catalyst precursors are recovered and formed into structures for use in a maleic anhydride reactor. Thereafter, these catalyst precursors are calcined at a temperature between about 300° C. and about 600° C. to form the catalyst. 
     The vanadium compounds useful as a source of vanadium in the catalyst precursors are well known in the art. Suitable vanadium compounds include but are not limited to: vanadium oxides, such as vanadium pentoxide, vanadium tetroxide, vanadium trioxide, and the like, vanadium oxyhalides, such as vanadyl chloride, vanadyl dichloride, vanadyl trichloride, vanadyl bromide, vanadyl dibromide, vanadyl tribromide and the like; vanadium-containing acids, such as metavanadic acid, pyrovanadic acid, and the like; vanadium salts, such as ammonium metavanadate, vanadium sulfate, vanadium phosphate, vanadyl formate, vanadyl oxylate, and the like. Of these, however, vanadium pentoxide is preferred. 
     The compounds useful as a source of phosphorus in the catalyst precursors are also well known to the art. Suitable phosphorus compounds include: phosphoric acids, such as orthophosphoric acid, metaphosphoric acid, and the like; phosphorus oxides, such as, phosphorus pentoxide and the like; phosphorus halides, such as phosphorus pentachloride, phosphorus oxybromide, phosphorus oxychloride, and the like; trivalent phosphorus compounds, such as phosphorus acid, phosphorus trihalides (for example, phosphorus trichloride), organic phosphites (for example, trimethyl phosphite), sometimes known as phosphonates, and the like. Of these, orthophosphoric acid and phosphorus pentoxide are preferred, with a mixture of orthophosphoric acid and phosphorus acid being most preferred. 
     Preparation of Catalyst Precursors 
     To prepare the catalyst precursors by the process of the present invention, a vanadium compound is brought together with a phosphorus compound in an acid solution and the mixture is heated to dissolve the starting materials. A reducing agent is used to reduce pentavalent vanadium to tetravalent vanadium and to maintain the vanadium in the tetravalent state. As is well known to those skilled in the art, hydrogen halide acid or oxalic acid solutions, which are mild reducing agents, can serve not only as the acid but also as the reducing agent for the pentavalent vanadium. On the other hand, a trivalent phosphorus compound can be used to provide tetravalent vanadium and also serve as a source of phosphorus to provide the catalyst precursor. It is preferred to use phosphorus acid as the trivalent phosphorus compound which serves as an acid medium to provide the tetravalent vanadium in the precursor. The acid solution containing the phosphorus compound and the vanadium compound is heated until a blue solution is obtained, indicating that at least 50 atom percent of the vanadium is in the tetravalent state. The amount of time required to dissolve the phosphorus compound and the vanadium compound and to provide a substantial amount of the vanadium in the tetravalent state and to provide the catalyst precursors varies from batch to batch, depending upon the compounds used as starting materials and the temperature at which the compounds are heated. In general, however, heating the solution to at least 100° C. for about 4 hours is sufficient. 
     The atom ratio of phosphorus to vanadium in the starting materials is important since it controls the phosphorus to vanadium atom ratio in the final catalyst. When phosphorus-vanadium-oxygen precursors contain a phosphorus to vanadium atom ratio below about 0.5:1 or above about 2:1, the yield of maleic anhydride using the catalyst prepared from these precursors is so low that it is not of commercial significance. It is preferred that phosphorus-vanadium-oxygen precursors have a phosphorus to vanadium atom ratio between about 1:1 and about 5:1. When the catalyst is used to convert a feed that is primarily butane to maleic anhydride, it is even more preferable that the precursor have a phosphorus to vanadium atom ratio between about 1:1 and about 1.2:1. 
     If desired, although not actually required, a nonionic surfactant may be added to the mixture to control particle size and prevent agglomeration of the catalyst precursor during the preparation step. Nonionic surfactants may be broadly defined as compounds, aliphatic or alkylaromatic in nature, which do not ionize in water solution. For example, a well known class of nonionic surfactants is made available on the market under the trade name of &#34;Pluronic&#34;. These compounds are formed by condensing ethylene oxide with a hydrophobic base formed by the condensation of propylene oxide with propylene glycol. The hydrophobic portion of the molecule which, of course, exhibits water insolubility, has a molecular weight of from about 1,500 to 1,800. The addition of polyoxyethylene radicals to this hydrophobic portion tends to increase the water solubility of the molecule as a whole and the liquid character of the product is retained up to the point where polyoxyethylene content is about 50% of the total weight of the condensation product. Other suitable surfactants are described in U.S. Pat. No. 4,149,992. 
     The amount of surfactant, when employed, suitable for use in the process of the present invention, can vary within wide limits. It has been found that the amount of surfactant should be at least about 0.01% by weight, based on the total weight of the mixture or dispersion, since at lower concentrations the effect of the surfactant is diminished considerably. On the other hand, there is no upper limit as to the amount of surfactant that can be used, although there does not seem to be any advantage in using more than about 1.0% by weight, and usually between about 0.05% and about 0.5% by weight, based on the total weight of the dispersion. 
     Recovery of the Catalyst Precursor 
     After the vanadium and phosphorus compounds are contacted and a substantial amount of the vanadium is in the tetravalent state, it is necessary to recover the phosphorus-vanadium-oxygen catalyst precursors. Techniques for recovering the catalyst precursors are well known to those skilled in the art. For example, the catalyst precursors can be deposited from aqueous solution on a carrier, such as alumina or titania, or alternatively, the catalyst precursors can be recovered by gentle heating to dryness to provide solid phosphorus-vanadium-oxygen catalyst precursors. This latter technique is preferred. 
     Forming Agglomerates 
     After the phosphorus-vanadium-oxygen catalyst precursors have been recovered as dry powders, they are formed into agglomerates or structures, prior to subjecting them to calcining conditions. 
     Techniques for forming appropriate agglomerates from precursors for use in a fluidized bed reactor or in a fixed-tube heat-exchanger type reactor are well known to those skilled in the art. For example, the precursor can be structured for use in a fluidized bed reactor by depositing the phosphorus-vanadium-oxygen precursor on a carrier. Alternately, precursors can become comminuted for use in a fluidized bed reactor. They can be structured for use in fixed-tube heat exchanger/type reactors by extruding a paste of the precursors through an orifice and then breaking or chopping into slugs or broken slugs. They can also be formed by pilling, tabletting, pelletizing or granulating the precursors by conventional means. 
     Wetting and Drying the Catalyst Precursor 
     The wetting may take place before or after the forming of the agglomerates, or both before and after the forming of the agglomerates. The aqueous liquid is applied to the catalyst precursor by spray, fog, sprinkle, or immersion. Spraying is preferred. The amount of aqueous liquid applied is not critical, because any amount will to some extent enhance surface adhesion. It is not unknown to use liquids in the formation or the holding of agglomerates, the theory being that the liquid acts as a binder between particles, minute portions of the outer perimeters of which are dissolved and then solidified on drying. What is unique about this invention is that such liquid application can be applied to a phosphorus-vanadium-oxygen catalyst precursor, without interferring in any way with subsequent catalyst activity, to achieve low attrition not only in handling of the catalyst, but in the use of the catalyst as well. After wetting and preferably before calcining the water should be removed in a conventional drying step at temperatures of 70°-250° C. (100°-150° C. preferred). 
     The precursors are then calcined at temperatures between about 300° and about 600° C. for at least two hours to provide the catalysts of the present invention. It is preferred to convert part of the tetravalent vanadium to pentavalent vanadium during the calcination step. The tetravalent vanadium can be converted to pentavalent vanadium by calcining the precursor in a free oxygen-containing gas, such as air, at temperatures of about 300° to about 600° C. until about 20 to about 90 atom percent of the vanadium has been converted to pentavalent vanadium. If more than about 90 atom percent of the vanadium is converted to pentavalent vanadium, usually caused by calcining too long or at too high a temperature, the selectivity of the resultant catalyst, and consequently, the yield of maleic anhydride decrease. On the other hand, conversion of less than about 20 atom percent of the vanadium during air calcination does not seem to be beneficial. As will occur to those skilled in the art, the exact calcination conditions will depend on the method of preparing the precursor, equipment configurations, or additives to the precursor; however, it has been found that calcination at about 500° C. for about 4 hours is generally sufficient. 
     After calcining the catalyst precursors can be charged to a suitable reactor without suffering the attrition and dusting difficulties usually associated with prior art phosphorus-vanadium-oxygen catalysts and can be used in accordance with prior art techniques to convert non-aromatic hydrocarbons to maleic anhydride. 
     Preparation of Maleic Anhydride 
     The catalyst of the present invention are useful in a variety of reactors to convert non-aromatic hydrocarbons to maleic anhydride. Both fluidized bed reactors and fixed tube, heat exchanger type reactors are satisfactory, and the details of the operation of such reactors are well known to those skilled in the art. The reaction to convert non-aromatic hydrocarbons to maleic anhydride requires only contacting the hydrocarbons admixed with a free-oxygen containing gas, such as air or oxygen enriched air, with the catalysts at elevated temperatures. The hydrocarbon/air mixture is contacted with the catalyst at a concentration of about 1 mole percent to about 10 mole percent hydrocarbon at a space velocity of about 100 cc/cc/hour to about 3,000 cc/cc/hour at temperatures between about 300° C. and about 600° C. to provide excellent yields of maleic anhydride. Maleic anhydride produced by using the conditioned catalysts of this invention can be recovered by any number of means well known to those skilled in the art. For example, maleic anhydride can be recovered by direct condensation or by absorprtion in suitable media with subsequent separation and purification of the anhydride. 
     A large number of non-aromatic hydrocarbons having from 4 to 10 carbon atoms can be converted to maleic anhydride using the catalysts prepared according to the present process. It is only necessary that the hydrocarbon contain not less than 4 carbon atoms in a straight chain. As an example, the saturated hydrocarbon n-butane is satisfactory, but isobutane (2-methylpropane) is not satisfactory for conversion to maleic anhydride although its presence is not harmful. In addition to n-butane, other suitable saturated hydrocarbons include the pentanes, the hexanes, the heptanes, the octanes, the nonanes, the decanes, and mixtures of any of these, with or without n-butane. 
     Unsaturated hydrocarbons are also suitable for conversion to maleic anhydride using the agglomerated catalysts of this invention. Suitable unsaturated hydrocarbons include the butenes (1-butene and 2-butene), 1,3-butadiene, the pentenes, the hexenes, the heptenes, the octenes, the nonenes, the decenes and mixtures of any of these, with or without the butenes. 
     Cyclic compounds such as cyclopentane, cyclopentene, oxygenated compounds such as furan, dihydrofuran, or even tetrahydrofurfural alcohol are also satisfactory. 
     Of the aforementioned feedstocks, n-butane is the preferred saturated hydrocarbon and the butenes are the preferred unsaturated hydrocarbons, with n-butane being most preferred of all feedstocks. 
     It will be noted that the aforementioned feedstocks need not necessarily be pure substances, but can be technical grade hydrocarbons. 
     The principal product from the oxidation of the above feed materials is maleic anhydride, although small amounts of citraconic anhydride (methylmaleic anhydride) may also be produced when the feedstock is a hydrocarbon containing more than 4 carbon atoms. 
    
    
     The following examples illustrate the invention, but should not be construed as delimiting. 
     CATALYST PRECURSOR PREPARATION &#34;A&#34; 
     To a mixture of 340.0 grams (1.87 moles) of vanadium pentoxide, 1150 milliters of water and 2.3 grams of Sterox® NJ nonionic surfactant were added 228.0 grams (1.98 moles) of 85% orthophosphoric acid and 173.0 grams (2.06 moles) of 97.6% phosphorous acid. The phosphorus to vanadium atom ratio was about 1.08:1. The aqueous mixture of vanadium and phosphorus compounds was charged to a 2-liter Parr autoclave, fitted with a thermowell, two 6-bladed stirrers, and a vent, and heated to about 100° C. and thereafter sealed. The mixture, while being stirred at 1,000 revolutions per minute (rpm), was heated to about 150° C. in about 50±10 minutes and held at this temperature for about 4 hours. After the hold period, the autoclave was cooled to about 80° C. in 50±10 minutes and opened. The aqueous phosphorus-vanadium-oxygen catalyst precursor slurry was placed in an open dish casserole or tray and evaporated to dryness in an oven at about 120° C. The resultant phosphorus-vanadium-oxygen catalyst precursor powder was ground to pass an 18 mesh sieve (U.S. Standard Sieve Size). 
     CATALYST PRECURSOR PREPARATION &#34;B&#34; 
     In a 3 l round flask with stirrer and reflux cooler, 595 g vanadium pentoxide, 285.6 g 96.8% phosphorus acid, 404.6 g 85% phosphoric acid and 2012.5 g deionized water are introduced. The atomic ratio of phosphorus to vanadium was about 1.05:1, and the solution contained about 3% more phosphoric acid than stoichiometrically necessary for the reduction of the pentavalent vanadium. The mixture was heated under reflux conditions until the solution became blue. Then the heating was continued at a temperature of about 97° to 99° C. After 24 h, a pale blue-green deposit formed, and this formed a dispersion with the water and the dissolved phosphorus and vanadium compounds. It was heated for approximately 28 h more. 
     About 1985 g of the above material was brought together with about 250 ml deionized water in a 2 l autoclave. The autoclave was closed and heated for B 4 h at about 150° C. After the cooling, the autoclave was opened and the contents were placed in an open porcelain dish until dry in a forced ventilation oven heated to 120° C. The dried powder was then triturated until it ran through an 18-mesh screen. 
     ATTRITION TEST 
     This procedure illustrates the attrition test used to determine percent attrition of the phosphorus-vanadium-oxygen catalyst precursors (or catalysts). 
     A 17.78 centimeter (7.0 inch) high×9.525 centimeter (3.75 inch) outside diameter 0.946 liter (1.0 quart) jar equipped with a screw-on cap and two 1.27 centimeter (0.5 inch) high×8.89 centimeter (3.5 inch) long stainless steel baffles cemented lengthwise to the inner sides at 180° opposed angles was employed. 
     The catalyst samples were separately screened, using a 10 mesh sieve (U.S. Standard Sieve Size) to remove any dust and fines. Approximately 50.00 grams of each of the screened samples were accurately weighed (initial weight in grams) and charged to the apparatus described above. The baffled jar containing the catalyst precursor was placed on a roller mill and rolled at 160±5 revolutions per minute (rpm) for 15 minutes. The sample was then removed from the jar, screened, and weighed (subsequent weight in grams) to determine the amount of attrited material which passed through the 10 mesh sieve. The percent attrition was calculated as follows: ##EQU1## The results are shown in Table 1 under the columns headed &#34;% Attrition&#34;. 
     CRUSH STRENGTH TEST 
     This procedure illustrates the crush strength test used to determine the average crush strength (measured as longitudinal, side crush strength for tablets) of the phosphorus-vanadium-oxygen catalyst precursor structures. 
     A John Chatillon and Sons Universal Test Stand, Model LTCM (motorized, variable speed unit equipped with Chatillon Dial Push/Pull Gauges, either Model DPP-10 (10 pounds maximum) or Model DPP-50 (50 pounds maximum), depending on the anticipated range of crush strength, was used as the test equipment. 
     The tablet was placed on its side under the center of the plunger and the instrument driven in the automatic mode at a setting of 2. The maximum force exerted during the test was taken as the crush strength for the tablet. Generally 10 or more determinations were made from a given sample of tablets and the crush strengh values averaged to give an average crush strength. (ACS). The results are reported in Table 1. 
     EXAMPLES 1-17 
     Catalyst precursor powder like that made according to Preparation &#34;A&#34; or &#34;B&#34; (as indicated) was formed into 0.48 centimeter (3/16 inch) diameter tablets using 1 weight percent graphite as a pelletizing lubricant. Alternatively, the powder was formed into larger tablets, called slugs, of about 1.27 centimeter (0.5 inch) diameter, followed by breaking up of the slugs and sizing with stainless steel screens to obtain irregular shapes, called broken slugs, in the size range of about 0.4 centimeter to about 0.8 centimeter. 
     The catalyst precursor tablets or broken slugs were placed in a tray and quantities of solvent atomized or misted over the structure while being gently rolled so that all sides of the structures would be exposed to the solvent. The solvent was removed by placing the structures in a forced draft oven at room temperature (23° C.). The temperature of the oven was increased over a period of about one hour to about 125° C. and maintained at about 125° C. for 2-4 hours. Thereafter, the hardened structures were calcined at 400°-500° C. for about four hours to convert the catalyst precursor to active catalyst. 
     Samples of both uncalcined (after solvent removed) and calcined structures were subjected to the attrition test and the crush strength test to determine the effect of type and amount of solvent on the hardening or strengthening of the structures as a result of the treatment. The results are shown in Table 1. 
     
                                           TABLE 1                                 
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     Catalyst                                                             
     Precursor                                                            
           Form of                                                        
                 Hardening                                                
                         % Solvent                                        
                               Uncalc.                                    
                                    Uncalc.                               
                                         Calc.                            
                                            Calc.                         
Example                                                                   
     Preparation                                                          
           Structure                                                      
                 Solvent Used  ACS  % Attrit.                             
                                         ACS                              
                                            % Attrit.                     
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1(comp)                                                                   
     A     Tablet                                                         
                 None    0     2.7  41.8  7.4                             
                                            23.5                          
2    A     Tablet                                                         
                 Water   10    12.0 1.1  17.6                             
                                            0.7                           
3    A     Tablet                                                         
                 Water   15    23.0 0.4  20.5                             
                                            0.3                           
4(comp)                                                                   
     A     Tablet                                                         
                 None    0     2.2  6.9  10.5                             
                                            2.2                           
5    A     Tablet                                                         
                 Water   1.4   2.5  --   11.8                             
                                            1.5                           
6    A     Tablet                                                         
                 Water   2.1   2.9  3.0  10.3                             
                                            1.4                           
7    A     Tablet                                                         
                 Water   3.6   3.5  --   14.1                             
                                            0.8                           
8    A     Tablet                                                         
                 Water   6.7   3.6  1.3  15.5                             
                                            0.7                           
9    A     Tablet                                                         
                 Water   9.7   5.3  --   17.0                             
                                            0.6                           
10   A     Tablet                                                         
                 50:50 Acetone:                                           
                         3.0   2.6  2.5  11.0                             
                                            1.3                           
                 Water                                                    
11(comp)                                                                  
     A     Broken                                                         
                 None    0     --   6.9  -- 3.8                           
           Slug                                                           
12   A     Broken                                                         
                 Water   5-10  --   2.0  -- 1.3                           
           Slug                                                           
13(comp)                                                                  
     B     Low Dens.                                                      
                 None    0     2.5  --    8.7                             
                                            9.9                           
           Tablet                                                         
14   B     Low Dens.                                                      
                 Water   5     4.5  5.3  13.0                             
                                            1.9                           
           Tablet                                                         
15(comp)                                                                  
     B     High Dens.                                                     
                 None    0     3.9  --    7.5                             
                                            10.7                          
           Tablet                                                         
16   B     High Den.                                                      
                 Water   2%    5.3  12.3 18.9                             
                                            4.9                           
           Tablet                                                         
17   B     High Den.                                                      
                 Water   9%    7.4  --   23.8                             
                                            1.6                           
           Tablet                                                         
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 (comp) = comparative                                                     
 
    
     EXAMPLES 18-22 
     Catalyst precursor powder, made according to &#34;Preparation B&#34;, were tableted, hardened with water, and calcined as described in the previous examples. The amounts of water used to harden the tablets are shown at Table 2. Performance is compared below with similar tablets not hardened with water. 
     The catalysts were tested by placing the tablets in a 2.54-centimeter (1-inch) inside diameter fixed tube reactors which give results comparable to those obtained in a production reactor. At a temperature of about 400° C., using a feed stream containing 1.5 mole percent n-butane-in-air at a space velocity of about 1,470 cc/cc/hour, the n-butane was converted to maleic anhydride. The yield of maleic anhydride shown at Table 2 after the catalysts had been conditioned for at least 16 hours. 
     
                       TABLE 2                                                     
______________________________________                                    
Amount                Test Conditions                                     
                                    Maleic                                
       Water              Feed          Anhy-                             
       Used in   Reactor  Concen-                                         
                                 Space  dride                             
       Hardening Length,  tration,                                        
                                 Velocity,                                
                                        Yield                             
Example                                                                   
       Treatment cm       Mole % cc/cc/hr                                 
                                        Mole %                            
______________________________________                                    
18(comp)                                                                  
       0         121.92   1.5    1470   51.6                              
19     5         60.96    1.5    1470   54.0                              
20(comp)                                                                  
       0         121.92   1.5    1470   52.4                              
21     2         60.96    1.5    1470   54.7                              
22     9         60.96    1.5    1470   53.6                              
______________________________________                                    
 (comp) = comparative