Digester Control in the Kraft Process - Emerson

1 BACKGROUNDThe Kraft paper Process is particularly efficientbecause it involves continuous recycling of raw mate-rials. Wood chips are delignified by harsh chemicals(white liquor) to produce pulp and spent liquor (blackliquor). The spent liquor is later regenerated to whiteliquor in the most important physical parameter of pulpproduction is the Kappa number, which essentiallyindicates how well the chips have been the value of the Kappa number is difficultbecause the original wood chips may differ greatly incomposition and moisture content. Measuring conduc-tivity in the upper (or sometimes lower) recirculationzone of a continuous Digester can provide controlfeedback and lower variability in the Kappa number ofthe product : CHEMISTRY OF PULP DIGESTIONA continuous digesterhas a constant charge ofwood chips and a given flow rate of white liquor(basically NaOH and Na2S).

2 The Digester fluid is heatedto approximately 180 C (356 F) to accelerate thedelignification reaction. The NaOH (caustic) isconsumed by the acetic and lignin components in thewood to yield sulfates and carbonates:Na2S + NaOH + wood chips >Na2SO4 + Na2CO3 + pulp(white liquor)(black liquor)As residence time in the Digester increases, the yieldof pulp increases, but overcooking can degrade thestrength of the pulp. The pulp is later separated fromthe black liquor and washed to reclaim the traditional method of controlling the Kappa numberis to monitor the residual caustic in the black liquor andinfer how much has reacted in the Digester . A conduc-tivity measurement at this location is thus correlatedwith an acceptable Kappa number range, and the feedrate of white liquor to the Digester is adjusted to holdthis conductivity value.

3 Any monitoring system used in this service must with-stand the hot, highly caustic liquid at pressures over1700 kPa (246 psig). The black liquor/pulp mixture is athick, viscous slurry that tends to coat and clog systempiping, especially if allowed to cool. Automatic titratorshave been designed for use under these conditionsbut are hard to maintain and require extractive ( grab )sampling. Grab samples may not reflect actualconditions because the time delay required foranalysisallows other reactions to occur. The toroidalconductivity method is ideal for this kind of CONDUCTIVITYR osemount Analytical recommends using toroidalconductivity to measure alkali concentration in blackliquor because the sensors have proven rugged andreliable with a minimum of maintenance.

4 Althoughconductivity is non-specific and all ionic species willinterfere somewhat, the active components NaOH andNa2S are the strongest contributors to the total. Othercomponents are considerably lower in conductivityand any sudden increase in unreacted alkali concen-tration will be easily noticed. Conduc-tivity can bemeasured either at the product outlet or in the upperrecirculation zone for rapid response to processchanges. Figure 1 shows both of these concentrations are usually measured by wetchemical tests called ABC titrations. One testmeasures the sum of NaOH and Na2S, which is calledActive Alkali. Another test measures NaOH and onehalf the Na2S, which is called Effective Alkali.

5 Thesetwo quantities are related by the sulfidity, thepercentage of alkali present in the sulfide form. Sincesulfidity typically changes little in most processes,conductivity can by correlated to both Active orEffective Alkali with the addition of a constant Control in the Kraft ProcessApplication Data SheetADS 2600-02 2008 Pulp and Paper IndustryINSTALLATION OPTIONSR osemount Analytical has three separate sensoroptions for this application. The Model 242 flow-through sensor is flanged and uses a PEEK or ceramicliner for maximum chemical resistance. The retractableModel 228 sensor has a unique lead screw design thatallows safe retraction through a ball valve at up to 2030kPa (295 psig).

6 The Model 226 Sensor, with its cm ( in.) bore,can be used when large solid particles are present andcan also be adapted for flange toroidal conductivity analyzers include theModel 5081-T and Xmt-T DC-powered HART andFOUNDATIONF ieldbus analyzers, the Model 1056-21analyzer, and the Model 54eC HART analyzer/con-troller. The Model 5081-T is housed in a robust explo-sion-proof enclosure that is especially suited to theharsh pulp mill environment. The Model 54eC is com-patible with both toroidal and contacting conductivitysensors for optimum versatility in the field. Consult theappropriate Product Data Sheet for complete 5081-T Two-wire ConductivityTransmitter Automatic/Manual temperature compensationto ensure accurate monitoring and Control .

7 Hand-held Infrared Remote Control (IRC) linkto activate all transmitter functions. AMS 54eC Conductivity Analyzer Measures Conductivity, Resistivity, or Percent(%) Concentration. Uses either contacting or inductive sensors tomeet most application requirements. Optional TPC and PID Control capability. Fully Descriptive Diagnostic Messages and easy-to-use inter-face spells out each operation in English, French, German,Italian, or Spanish. Two Independent Outputs for conductivity and temperature. Three alarms with Programmable Logic, plus one dedicatedfault 228 Retractable ToroidalConductivity Sensor Convenient isolation of sensor for cleaningor replacement.

8 Easy insertion/retraction of sensor. Flush ports on assembly chamber to allow sensor 242 Flow-Through ToroidalConductivity Sensor Now available in 4" pipe size. Toroidal measurement principle greatlyreduces sensor fouling. Automatic temperature compensation with inte-gral RTD. Externally-mounted toroids are protected from harsh Process -es. Modular design allows liner replacement without 226 Toroidal ConductivitySensor Toroidal (inductive) principle of measurementgreatly reduces sensor fouling problems. Chemically-resistant materials withstand the effect of highlycorrosive solutions. Large-bore design prevents plugging by solid Rosemount AnalyticalFIGURE 1.

9 Kraft Digester ControlCHIPSWHITE LIQUORBLACK LIQUOR AND PULP)))))))))Rosemount Analytical 3 Credit Cards for Purchases right people,the right answers,right ORDERING NOW AVAILABLE ON OUR WEB Process ManagementLiquid Division2400 Barranca ParkwayIrvine, CA 92606 USATel: (949) 757-8500 Fax: (949) 474-7250 Rosemount Analytical Inc. 2008