Method and system for measuring and controlling digester or impregnation vessel chip level by measuring chip pressure

Abstract

A method and system for determining chip pressure in a vessel containing a slurry of lignocellulosic material including: sensing a total static pressure of the slurry in the vessel; sensing a hydrostatic pressure of the liquor in the vessel, and determining the chip pressure based on a difference of the sensed total static pressure of the slurry and the sensed hydrostatic pressure of the liquor.

1 . A pressure sensing element for a vessel containing a slurry of lignocellulosic chip material, the element comprising:
 a first pressure sensing surface in contact with the slurry, wherein the first pressure sensing surface generates a signal representative of a pressure of the slurry, which is sum of a hydrostatic pressure of liquor and chip pressure;   a second pressure sensing surface generating a signal representative of the hydrostatic pressure of the liquor, and   a pressure difference transmitter receiving the signals generated by the first and second pressure sensing surfaces and outputting a signal indicative of the chip pressure.   
     
     
         2 . The pressure sensing element in  claim 1  wherein the output signal from the signal generator is representative of a difference between the signals generated by the first and second pressure sensing surfaces. 
     
     
         3 . The pressure sensing element in  claim 1  further comprising a filter between the slurry and the second pressure sensing surface, wherein the filter passes liquor to the second pressure sensing surface and blocks chip material from reaching the second pressure sensing surface. 
     
     
         4 . The pressure sensing element in  claim 1  wherein at least the first pressure sensing surface has an orientation in a range between parallel to a flow direction of the slurry and inclined at twenty degrees to the flow direction. 
     
     
         5 . The pressure sensing element in  claim 1  wherein the pressure sensing element is positioned in the vessel above an uppermost extraction screen in the vessel and below an inlet separator for the vessel. 
     
     
         6 . The pressure sensing element in  claim 1  wherein the pressure sensing element is mounted on an inside surface of an outer wall of the vessel. 
     
     
         7 . The pressure sensing element in  claim 1  wherein at least the first pressure sensing surface has a surface area of approximately 0.1 meters squared. 
     
     
         8 . A method for determining a pressure attributable to lignocellulosic chip material in a vessel containing a slurry of the lignocellulosic material and liquor, the method comprising:
 sensing a pressure of the slurry in the vessel;   sensing a pressure of the liquor in the vessel, and   determining the pressure attributable to the chips based on a difference of the sensed pressure of the slurry and the sensed pressure of the liquor.   
     
     
         9 . The method of  claim 8  wherein the pressure of the slurry is a static pressure sensed with a first pressure sensing surface in contact with the slurry and generating a signal representative of the static pressure of the slurry. 
     
     
         10 . The method of  claim 8  wherein the pressure of the liquor in the slurry is sensed by a second pressure sensing surface generating a signal representative of a hydrostatic pressure of the liquor in the slurry. 
     
     
         11 . The method of  claim 8  wherein a filter is positioned between the slurry and the second pressure sensing surfaces and wherein chips are blocked by the filter and the filter passes liquor which contacts the second pressure sensing surface. 
     
     
         12 . The method of  claim 8  including determining a chip level in the vessel based on the determined pressure of the chips. 
     
     
         13 . The method of  claim 8  wherein the sensing of the pressures of the slurry and liquor occurs above an uppermost extraction screen for the vessel. 
     
     
         14 . The method of  claim 8  wherein a chip level (L 1 ) in the vessel is determined by applying at least one of the following algorithms:
     L 1 =k 1 *PC /[( R 1− R 2)* G ] if  L 1 is greater than  L 2, and       L 1 =[k 1* PC+k 2*( R 2* G*L 2)]/( R 1* G ) if  L 2 is greater than  L 1,   where the PC is the determined pressure of the chips; the k 1  and the k 2  are calibration constants, the L 1  is an average chip level in the vessel; the L 2  is a liquor level in the vessel; the R 1  is a density of the entrained chips in the slurry; the R 2  is a density of liquor in the slurry, and the G is a gravitational force constant.   
     
     
         15 . A method for calibrating an algorithm to determine chip level comprising:
 measuring the liquor level in a vessel;   measuring a chip pressure in the vessel;   determining a calculated chip level using an algorithm to calculate the chip level based on the chip pressure;   while varying the liquor level in the vessel, detecting a transition between a steady state chip pressure and a varying chip pressure;   determining a difference existing at the transition between the liquor level and the calculated chip level, and   adjusting the algorithm to compensate for the difference.   
     
     
         16 . The method of  claim 15  wherein the chip level in the vessel is maintained substantially constant during the calibration method. 
     
     
         17 . The method of  claim 15  wherein the liquor level is raised and lowered above and below the chip level during the calibration method. 
     
     
         18 . The method of  claim 15  wherein the chip pressure is measured at an elevation in the vessel below the chip level and the liquor level, and above a first extraction screen in the vessel. 
     
     
         19 . A method for processing lignocellulosic chip material in a continuous flow vessel, the method comprising:
 determining a chip pressure of the chip material based on a difference of a sensed total pressure of the slurry and a sensed pressure of the liquor;   comparing the determined chip pressure of the chip material to a predetermined desired chip pressure, and   adjusting a flow rate of the chip material through the vessel based on the comparison of the determined chip pressure to the predetermined desired chip pressure.   
     
     
         20 . The method of  claim 19  wherein the predetermined desired chip pressure is a constant chip pressure.