Method for determining volatile phosphorus in carbon
BACKGROUND OF THE INVENTION 
     The sweetener industry employs wood-based carbon as an adsorber and filter medium for processes such as deashing and decolorization. When wood-based carbon is regenerated in the presence of steam, volatile phosphorus is released. This phosphorus, in elemental form or in the form of phosphates, is known to cause severe deterioration of the insulation in regeneration furnaces, the tubes of waste heat boilers, and parts of other regeneration equipment. 
     It would therefore be beneficial to have an analytical procedure to monitor volatile phosphorus released from wood-based carbon at regeneration conditions and such method should not require total phosphorus analysis. The desired method for determining the amount of volatile phosphorus in a carbon sample should be easy to perform and highly accurate. 
     Various methods for determining phosphorus content are known in the prior art. U.S. Pat. No. 4,599,316 to Hahn et al discloses a spectrophotometric method for determining the concentration of inorganic phosphate in a sample fluid, such as bodily fluids. An ammonium molybdate reagent is employed as a complexing agent. 
     In U.S. Pat. No. 3,796,543 to Kamphake, a continuous automatic system is employed for measuring the amount of phosphates in aqueous fluids, e.g., sewage effluent. This analytical technique is based on what is known as the molybdenum blue reaction. 
     U.S. Pat. No. 4,533,639 to Kojima et al discloses a colorimetric analytical method for determining the amount of phosphate in a phosphate-containing coating stripped from a ferrous metal surface. 
     U.S. Patent No. 3,425,805 to Grobin, Jr. discloses a colorimetric method for determining the phosphite content of electroless plating baths. A molybdate ion complex is formed in solution with the phosphite ions. 
     None of the above-mentioned analytical methods, however, are directed to problems associated with carbon used as a sorbent generally, and in particular, determining the volatile phosphorus content of solids such as carbon. Thus, there is still a present need for an accurate and efficient analytical technique for such phosphorus determinations. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a simple and accurate analytical technique for determining the volatile phosphorus content of various substances. 
     Another object of the present invention is to provide an analytical technique for measuring volatile phosphorus released from wood-based carbons at regeneration conditions. 
     Still another object of the present invention is a method for analyzing phosphorus compounds in an off-gas stream from a plant operation. 
     Briefly stated, volatile phosphorus contained on wood-based carbon is first removed by reaction with steam at elevated temperatures. Nitrogen or helium by themselves do not strip phosphorus from the carbon at the elevated regeneration temperature. The phosphorus released in the off-gas is then captured by a layer of granulated zinc oxide as zinc phosphate. It appears that all forms of phosphorus compounds (hypo-, pyro-, and polyphosphates) are converted to the orthophosphate form by zinc oxide and retained as zinc orthophosphate. The orthophosphate radical of zinc orthophosphate is then determined by a spectrophotometric method based on vanado-molybdo reagent. The optical intensity of the resulting yellow color is directly proportional to the phosphate concentration in the solution. 
     According to the present invention, there is provided a combustion reactor comprising an open-ended elongated quartz tube having arranged therein a plurality of beds (or layers) which are separated by quartz wool or the like. Preferably, individual layers formed of the phosphorus-containing carbon sample, granulated quartz, and zinc oxide are provided. The combustion reactor is heated and operatively connected to a source of steam at one end and to a water collection system comprising a condenser and a collector at the other end. The combustion reactor is heated while steam is passed through the beds for a time sufficient to strip the phosphorus from the carbon sample. If desired, an inert gas such as nitrogen is passed through the combustion reactor prior to the passage of steam to purge air from the system. 
     The combustion reactor is removed from the heating apparatus and the reacted zinc oxide is collected and contacted with a standard solution comprising molybdate and vanadate ions to obtain a colored soluble phosphate complex solution. By measuring the intensity of the colored solution and comparing the measured color intensity to a known standard, the volatile phosphorus content in the solid carbon sample is determined. 
     Thus, the orthophosphate radical of zinc orthophosphate can be quantitatively analyzed by a spectrophotometric method after formation of a colored solution by reaction with a vanadate-molybdate reagent. The optical intensity of the yellow color is directly proportional to the phosphate concentration in the solution. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a combustion reactor tube employed in the method of the present invention. 
     FIG. 2 is a calibration curve used for phosphorus determinations according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a combustion reactor 1 for performing the analytical method of the present invention is shown. Reactor 1 comprises a quartz tube housing 2 packed with, from top to bottom, a quartz wool layer 4, a carbon bed 5 weighing from about 2 g to about 10 g, a quartz wool layer 6, a fritted disc 7, a granulated quartz layer 8 measuring from about 0.5 in. to about 2 in. thick, a quartz wool layer 9, a zinc oxide layer 10 weighing from about 10 g to about 40 g, and a quartz wool layer 11. Inert gas, such as, nitrogen, and steam pass through the reactor 1 via inlet 12 and outlet 13. 
     A heating furnace 3 surrounds the reactor 1 and maintains the temperature of the beds at about 1600° F. or above. Thus, the carbon particles in bed 5 are heated to a temperature of about 1600° F. while steam is passed through the carbon for at least one hour. 
     In the reactor 1, the zinc oxide layer is preferably granulated or powdered zinc oxide. When the phosphorus-containing steam contacts the zinc oxide particles in bed 10, zinc phosphate, or more specifically zinc orthophosphate, is formed and retained therein until a spectrophotometric analysis is made, as will be explained in greater detail hereinafter. 
     It is known that wood-based carbon sorbents which are loaded with volatile phosphorus are conventionally regenerated with steam at elevated temperatures. Phosphorus which is stripped from carbon particles may contain all forms of phosphorus compounds, such as, pyrophosphates (P 2  O 7   -4 ), hypophosphates (P 2  O 6   -4 ), orthophosphates (PO 4   -3 ), and metaphosphates (PO 3   - ). In the present invention, volatile phosphorus compounds carried by the steam are passed through a layer of granulated zinc oxide where they are retained. Although not wishing to be bound by any particular theory, it appears that the phosphorus compounds which contact zinc oxide are converted to the orthophosphate form and retained in the zinc oxide bed as zinc orthophosphate. 
     The following non-limiting examples are included to more fully set forth the present invention. 
     EXAMPLE 1 
     Preparation of Vanadate-Molybdate Reagent 
     An ammonium molybdate tetrahydrate, i.e., ((NH 4 ) 6  Mo 7  O 24 .4H 2  O)), sample weighing 25 g was dissolved in 250 ml of distilled and deionized water to form Solution A. Then, 1.25 g of ammonium metavanadate were dissolved in 200 ml of distilled and deionized water to prepare a solution to which is added 330 ml of concentrated hydrochloric acid to form Solution B. Solution A and Solution B were mixed together and diluted to a 1 liter volume, thereby forming a standard vanadate-molybdate reagent solution. 
     EXAMPLE 2 
     Preparation of Spectrophotometer Calibration Curve 
     A calibration curve was prepared using a Spectronic 210 UV spectrophotometer (Bausch and Lomb) by adding the following volumes of standard phosphoric acid solution to 50 ml volumetric flasks: 0.0 (blank), 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0 and 8.0 ml. The standard phosphoric acid solution is prepared by adding one gram of concentrated reagent grade phosphoric acid to one liter of distilled-deionized water. For proper color development the pH of the solution must be adjusted to a level between 2 and 10. Reagent grade ammonium hydroxide was used to bring the pH to the required range. After adjusting the pH to 7, the volume was increased to 40 ml with distilled-deionized water. Ten ml of vanadate-molybdate reagent was added to each flask. After 20 minutes, the absorbance of the blank, versus distilled-deionized water was read on the Spectronic 210 spectrophotometer at 400 nm in 10 mm cells. The absorbance of each of the eight concentrations prepared, versus the blank, was then measured. Absorbance (abscissa) versus milligrams phosphoric acid (ordinate) data were plotted on 2 by 3 cycle logarithmic paper to yield a calibration curve which is useful for the life of the vanadate-molybdate reagent. The calibration curve obtained by the foregoing procedure is shown in FIG. 2. 
     EXAMPLE 3 
     Experimental Method 
     The reactor 1, made of quartz, was prepared by initially placing a plug of quartz wool 6 onto the fritted disc 7. A layer 5 of granular or powdered carbon (5 g) was then added, followed by an additional quartz wool layer 4 to secure the carbon bed. The tube was inverted to load the remaining sections of the reactor. A 1&#34; thick layer 8 of sized granular quartz was added to the tube 1 followed by another layer 9 of quartz wool to support the granular quartz layer 8. The zinc oxide layer 10 was formed by adding 20 g of 4×20 mesh granulated zinc oxide. A quartz wool layer 11 was placed over the zinc oxide layer. 
     Appropriate inlet and outlet fittings were added and connected, and the reactor tube was positioned inside the furnace 3. Nitrogen was initially passed through the reactor tube 1 to flush air out of the system. The reactor furnace 3 was turned on and the temperature raised to 1600° F. After the desired temperature was reached and stabilized for 10 minutes, steam from a steam generator (not shown) and nitrogen were introduced for one hour. The resulting condensate was collected. At the end of the run the reactor was cooled to ambient temperature. The carbon was removed from the tube and weighed. The steam recovery was calculated based on the following formula: ##EQU1## 
     The zinc oxide particles, wet with condensate from this run, were removed, diluted in volume to 100 ml, and digested with concentrated hydrochloric acid in a 250 ml Erlenmeyer flask. The digested zinc oxide was cooled and saved. The remaining quartz wool and quartz granules were removed from the reactor and washed with HCl. The quartz tube was rinsed with HCl followed by distilled water. Filtered washings were added to the 250 ml Erlenmeyer flask and saved. The flask was brought to volume with distilled-deionized water. The saved solutions was analyzed for phosphorus using the procedure described below. 
     A 1 ml aliquot was taken from the sample solution to be analyzed and added to 30 ml of distilled-deionized water in a 50 ml volumetric flask. One drop of concentrated ammonium hydroxide was then added, followed by the addition of 10 ml of the standard vanadate-molybdate reagent. The solution volume in the flask was increased to 50 ml by the addition of distilled-deionized water. Color was allowed to develop for 10 minutes. A sample of the colored solution was placed in a 10 mm light path cell and the cell inserted in the Spectronic 210 spectrophotometer. The absorbance reading was taken and compared against the previously prepared calibration curve of FIG. 2 to determine the phosphorus content in the aliquot. The amount of phosphorus in the entire sample solution was then calculated. The percent of volatile phosphorus in the carbon sample was calculated as follows: ##EQU2## where: W=initial weight of carbon sample (g); and w=weight of phosphorus (mg) (in 1 ml aliquot). 
     EXAMPLE 4 
     Results of Zinc Oxide Test Method 
     Test results show that the method of the present invention, described as the zinc oxide method, accurately accounts (within 1.5%) for all of the phosphorus volatilized during steam treatment at regeneration conditions. Carbon volatile phosphorus concentrations determined by the zinc oxide method were verified using data obtained from Element Analysis Corporation in Tallahassee, Fla. Samples of loaded and regenerated carbon from various trials were sent to Element Analysis Corp. for phosphorus analysis using the Proton Induced X-ray Emission (PIXE) technique. Typical results in Table 1 show that volatile phosphorus concentrations determined using the zinc oxide method were within 1.5% of results obtained using PIXE analysis. 
     For example, in Trial 1, the reactor was prepared as described above containing 4.85 g of dry carbon. Steam was passed through the reactor for one hour at a carbon bed temperature of 1600° F. Volatile phosphorus concentration in the carbon determined using the zinc oxide method was 62.0 mg compared to 61.1 mg determined by the PIXE method. Thus, volatile phosphorus measured by the zinc oxide method was within 1.5%, i.e., ((62.0-61.1)÷61.1)×100, of that measured by PIXE analysis. 
     
                       TABLE 1                                                     
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Zinc Oxide Method Provides Good Determination                             
of Volatile Phosphorus in Carbon                                          
                           Volatile                                       
                                   Difference                             
               Volatile    Phosphorus                                     
                                   Between                                
     Carbon    Phosphorus  Content by                                     
                                   ZnO Method                             
     Charge    Content by  PIXE*   and PIXE                               
Trial                                                                     
     (dry basis)                                                          
               ZnO Method, Analysis,                                      
                                   Results                                
No.  (g)       (mg)        (mg)    (%)                                    
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1    4.85      62.0        61.1    1.5                                    
2    4.75      62.0        61.1    1.5                                    
3    4.51      67.0        66.3    1.1                                    
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 *PIXE = Proton Induced Xray Emission                                     
 
    
     Although only preferred embodiments are specifically illustrated and described herein, it will be appreciated that many modifications and variations of the present invention are possible in light of the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.