Abstract:
Ash composition measurements of calcium carbonate and gypsum in paper is accomplished with a dual X-ray sensor system with one X-ray source that is powered at about 5.9 KV and a second X-ray source that is powered at about 4.2 KV. Corresponding detectors measure radiation from the respective X-ray sources that is emitted from the paper. Data derived from the measurements yields the gypsum and crystal water content in the paper. The dual X-ray sensor system can be employed in conjunction with infrared total moisture measurements of paper products being manufactured on a papermaking making machine, which contain gypsum and calcium carbonate, in order to correct for the gypsum crystal water effect.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application 61/101,137 that was filed on Sep. 29, 2008. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to sensors and methods for measuring the moisture content in paper products and particularly to techniques for measuring the levels of gypsum, which contains crystal water, in order to determine the amount of “free” water that is present in the paper products especially paper products that also contain calcium carbonate. 
     BACKGROUND OF THE INVENTION 
     In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric and water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The paper machine is, in essence, a water removal, system. A typical forming section of a papermaking machine includes an endless traveling papermaking fabric or wire, which travels over a series of water removal elements such as table rolls, foils, vacuum foils, and suction boxes. The stock is carried on the top surface of the papermaking fabric and is de-watered as the stock travels over the successive de-watering elements to form a sheet of paper. Finally, the wet sheet is transferred to the press section of the papermaking machine where enough water is removed to form a sheet of paper. 
     Paper is generally made of three constituents: water, wood pulp fiber, and ash. “Ash” is defined as that portion of the paper which remains after complete combustion. In particular, ash may include various mineral components such as calcium carbonate (CaCO 3 ), titanium dioxide (TiO 2 ), and clay (a major component of clay is SiO 2 ). Paper manufacturers use fillers such clay, titanium dioxide and calcium carbonate to enhance printability, color and other physical characteristics of the paper. Because of its low cost, paper manufacturers are also adding gypsum (CaSO 4 2H 2 O) as filler, especially in combination with calcium carbonate. The dihydrated water is commonly referred to as “crystal” water. Gypsum loses its associated water molecules when it heated to a temperature of about 200° C. 
     It is conventional to measure the moisture content of sheet material upon its leaving the main dryer section or at the take up reel employing scanning sensors. Such measurements may be used to adjust the machine operation toward achieving desired parameters. One technique for measuring moisture content is to utilize the absorption spectrum of water in the infrared (IR) region. A monitoring or gauge apparatus for this purpose is commonly employed. Such an apparatus conventionally uses either a fixed gauge or a gauge mounted on a scanning head which is repetitively scanned transversely across the web at the exit from the dryer section and/or upon entry to the take up reel, as required by the individual machines. IR moisture measuring devices do not distinguish “free” water that is present in paper products from “crystal” water, in other words, IR moisture measurements yield a moisture content that is the sum of the free water and crystal water. It is desirable to obtain on-line measurements of the free water content. 
     The total amount of ash in paper and the composition of the ash are controlled by setting the rates of flow of gypsum and other ash components as well as the flow of wood pulp fiber and water to the papermaking system. The resulting sheet is periodically sampled and burned in the laboratory to determine the composition and amount of ash in the sheet. In the laboratory, the paper is burned under predetermined conditions and the resulting ash is accurately weighed and chemically analyzed. The papermaking parameters can then be altered based upon the resulting measurements. However, this procedure of manual control suffers from the main disadvantage that it is time consuming, even when the gypsum is the only ash component used. Thus, large quantities of paper which do not meet specifications may be manufactured while the laboratory tests are being conducted. The art is in search of improved on-line moisture sensing techniques for measuring the free water content of paper products that include gypsum. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to techniques for correcting for gypsum crystal water effect, on infrared moisture measurements, that can be obtained directly from analysis of X-ray absorption spectra of paper products that contain both calcium carbonate and gypsum (CaSO 4 2H 2 O). The invention is based in part on the discovery of a unique X-ray spectrum that enables the measurement and determination of the amount of gypsum that is present even in the presence of calcium carbonate. In particular, it has been demonstrated that an X-ray system that employs dual X-ray sensors operating at different known X-ray spectra, one spectra being sensitive to the total ash quantity that is present in the paper product and the second spectra being primarily sensitive to gypsum, yields accurate calculations of the amount of gypsum that is present. The level of crystal water can be readily derived from the gypsum content. 
     In one aspect, the invention is directed to dual X-ray sensors that include (i) a first X-ray source for directing first X-rays through a first portion of the paper product wherein the first X-rays source is powered by a first voltage power supply that powers the first X-ray source at a voltage of about 5.9 KV and corresponding means for detecting first X-rays that are transmitted through the first position on the paper product and generating a first signals indicative of the amount of first X-rays detected and (ii) a second X-ray source for directing second X-rays through a second portion of the paper product wherein the second X-rays source is powered by a second voltage power supply that powers the second X-ray source at a voltage of about 4.2 KV and corresponding means for detecting second X-rays that are transmitted through the second portion of the paper product and generating second signals indicative of the amount of second X-rays detected. 
     The dual X-ray sensor system can be employed to compute the amount of crystal (non-free) moisture in paper which contains both gypsum and calcium carbonate. In particular, in the manufacture of paper, the on-line infrared total moisture measurements of the paper products are corrected for the gypsum crystal water effect to yield free moisture measurements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a system that employs dual X-ray gauges (sensors) for on-line gypsum measurements; 
         FIG. 2  is a graph of calculated X-ray absorption curves showing the absorption coefficients for pure calcium carbonate and for gypsum vs. applied voltage to X-ray tube; and 
         FIG. 3  illustrates a sheetmaking system incorporating the dual X-ray gauges of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention is directed to a non-contact, on-line sensor system for measuring the free moisture content of paper products that contain gypsum. The sensor system is particularly suited for incorporation into industrial paper making machines.  FIG. 1  illustrates a sensor system for measuring the free water content in paper sheet  10  which contains gypsum. The system includes dual X-ray absorption measurement devices that operate at different wavelengths: (1) ash X-ray sensor includes X-ray tube  24 , that is powered by a fixed high voltage power supply  28 , and a corresponding X-ray detector  12 , and (2) the gypsum X-ray sensor includes X-ray tube  26 , that is powered by a fixed high voltage power supply  30 , and a corresponding X-ray detector  14 . A radiation shield  40  partially encloses the dual X-ray sensors. The system preferably includes a scanner device  18  that moves the sensors across sheet  10  with the X-ray tubes  24  and  26  positioned on one side of sheet  10  and the corresponding X-ray detectors (or receivers)  12  and  14  positioned on the opposite side. The space between the X-ray tubes and detectors defines a measurement gap. 
     The fixed high voltage power supplies  28  and  30  are used to generate X-rays at selected energies. The fixed high voltage supply powers total ash X-ray tube  24  at a voltage so that X-rays generated are sensitive to both gypsum and calcium carbonate. Preferably this voltage is maintained at about 5.9 KV. For the gypsum X-ray sensor, the Fixed high voltage supply powers X-ray tube  26  at a voltage so that the X-rays generated are sensitive to primarily gypsum. Preferably, this voltage is maintained at about 4.3 KV. X-ray filters  12  and  14 , in the form of aluminum plate, for example, can be employed to enhance the composition analysis. 
     In operation, X-rays from X-ray tubes  20  and  22  that are transmitted through sheet  10  are received by X-ray detectors  12  and  14 , respectively. Simultaneously, detector  12  generates analog signals that are transmitted through amplifier  32  and analog-to-digital converter  36  to computer processor  16 . Similarly, detector  14  generates analog signals that are transmitted through amplifier  34  and analog-to-digital converter  38  to processor  16 . 
     The effective absorption coefficient for calcium carbonate (curve  1 ) and gypsum (curve  2 ) were measured at a spectral region of about 4.2 to 6.2 KV and the results are shown in  FIG. 2 . An X-ray gauge consisting of a conventional X-ray tube and corresponding detector with no aluminum filter was used. As is apparent, there is a cross over point at about 5.9 KV and there is almost a factor 2 difference in absorption region below about 4.2 KV. Thus, in a preferred embodiment, the first (or ash) X-ray sensor operates at 5.9 KV and the second (or gypsum) X-ray sensor operates at 4.2 KV. Processor  16  correlates the weighted sum of transmittance measurements to the amount of gypsum wherein the sum-coefficients are given by a fit to laboratory data. 
     Specifically, processor  16  initially calculates the relative readings for each X-ray sensor, which is defined by the relationship: R=V/Vo, where V is the measured detector response when the sheet is in place and Vo is the measured detector response with no sheet in the measurement gap. This sensor ratio R is then employed to calculate the gypsum fraction using the following relationship:
 
CaSO 4 2H 2 O %= a  Ln(R h )/BW n   +b  Ln(R l )/BW n   +c  
 
where BWn is the total mass of the sheet and R h , R l  are the ratios for X-ray sensor set to the higher (h) and the lower (l) voltage and (a, b, c) are constants to be determined by comparing to chemical laboratory data.
 
     The dual X-ray sensor is particularly suited for use in a papermaking machine such as that illustrated in  FIG. 3 . The sheetmaking system for producing a continuous sheet of paper material  44  includes a headbox  62 , a steambox  58 , a calendaring stack  60 , a take-up reel  76  and scanner system  70  that includes the inventive dual X-ray sensors. In the headbox  62 , actuators are arranged to control discharge of wetstock onto supporting wire or web  66  along the cross direction. The sheet of fibrous material that forms on top of the wire  66  is trained to travel in the machine direction between rollers  64  and  68  and passes through a calendaring stack  60 . The calendaring stack  60  includes actuators that control the compressive pressure applied across the paper web. The sheetmaking system includes a press section (not shown) where water is mechanically removed from the sheet and where the web is consolidated. Thereafter, water is removed by evaporation in the dryer section (not shown). The finished sheet product  74  is collected on a reel  76 . In practice, the portion of the paper making process near a headbox is referred to as the “wet end”, while the portion of the process near a take-up reel is referred to as the “dry end”. 
     The scanner system  70  generally includes pairs of horizontally extending guide tracks  54  that span the width of the paper product  74 . The guide tracks are supported at their opposite ends by upstanding stanchions  52  and are spaced apart vertically by a distance sufficient to allow clearance for paper product  74  to travel between the tracks. The dual X-ray sensors are secured to a carriage  56  that moves back-and-forth over to paper product  74  as measurements are made. On-line scanning sensor systems for papermaking manufacture are disclosed in U.S. Pat. No. 4,879,471 to Dahlquist, U.S. Pat. No. 5,094,535 to Dahlquist et al., and U.S. Pat. No. 5,166,748 to Dahlquist, all of which are incorporated herein fully by reference. 
     The dual X-ray sensors can be employed to adjust water measurements to account for the presence of gypsum crystal water in order to determine the free water content of paper products. On-line moisture measurements are typically obtained by infrared detectors that are positioned at various locations in the papermaking process in the machine direction and/or cross direction. For example, moisture detector  50  ( FIG. 1 ) can also be secured to carriage  56  of the scanner system  70  ( FIG. 3 .). Suitable moisture detection devices are described, for example, U.S. Pat. No. 7,382,456 to Tixier et al., U.S. Pat. No. 7,321,425 to Haran, and U.S. Pat. No. 7,291,856. to Haran et al., which are incorporated herein by reference. Once the free water content is calculated, operating parameters of the papermaking machine can be adjusted, if necessary, should the water profile deviate from normal. Suitable control process is described in U.S. Pat. No. 6,092,003 to Hagart-Alexander which is incorporated herein by reference. Both dry end parameters, e.g., temperature of heating devices, and wet end parameters, e.g., wet stock water content and filler content, can be controlled to achieve the desired final product. Process control techniques for papermaking machines are further described, for instance, in U.S. Pat. No. 6,805,899 to MacHattie et al., U.S. Pat. No. 6,466,839 to Heaven et al., U.S. Pat. No. 6,149,770, to Hu et al., U.S. Pat. No. 6,092,003 to Hagart-Alexander et. al, U.S. Pat. No. 6,080,278 to Heaven et al., U.S. Pat. No. 6,059,931 to Hu et al. U.S. Pat. No. 5,853,543 to Hu et al., and U.S. Pat. No. 5,892,679 to He, which are all incorporated herein by reference. 
     The foregoing has described the principles, preferred embodiment and modes of operation of the present invention. However, the invention should not be construed as limited to the particular embodiments discussed. Instead, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of present invention as defined by the following claims.