Kraft Cooking H-Factor
Computes the H-factor of a kraft cook, the index that combines time and temperature into a single number to control the digester. The H-factor is the integral of the relative delignification rate over the cook, and in the isothermal form used on the shop floor it equals H = time × exp(43.20 − 16113 ÷ absolute temperature), with time in hours and temperature in kelvin; the two constants come from Vroom's (1957) fit and were chosen so that the relative rate equals 1 at 100 °C. The number lets you trade time for temperature without changing the outcome: two cooks with the same H-factor and the same alkali charge reach the same kappa number, so raising the temperature allows shortening the plateau, and this is how production is recovered from a late digester. The isothermal form was adopted, considering only the time at the temperature plateau; the full H-factor also integrates the heating ramp and comes out 10 to 20% larger, depending on the ramp rate. Enter the time at the plateau and the cooking temperature.
Result
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H-factor: trading time for temperature in the digester
Inside a kraft digester, whoever controls kappa number is juggling two variables that do the same job. More time at the plateau delignifies; more temperature does as well. The H-factor folds both into a single index, and that is what makes the shop-floor call possible when a digester runs late: rather than blowing the schedule, raise the temperature, shorten the plateau, keep H unchanged. Without the number, tuning becomes expensive guesswork — high kappa loads the bleach plant with chlorine dioxide, low kappa costs yield and fibre strength.
In the isothermal form, H = time × exp(43.20 − 16113 ÷ T), time in hours and T in kelvin. That exponent is Arrhenius in disguise: 16113 times the gas constant gives an activation energy near 134 kJ/mol, and Vroom fitted both constants in 1957 so the relative rate equals 1 at 100 °C. With the defaults on screen, 1.5 hours at 170 °C, the relative rate is 934.3 per hour and the H-factor lands at 1401.5 — a common band for eucalyptus pulped to kappa 16 through 18. Sensitivity is brutal: every 8 °C doubles the rate. The isothermal form covers the plateau alone; a full H-factor integrates the heating ramp and comes out 10 to 20% larger.
H-factor alone will not fix kappa. It accounts for the time-temperature pair, while effective alkali charge, sulfidity, liquor-to-wood ratio and the wood species itself weigh just as heavily — two cooks at equal H and unequal alkali finish at different kappa. The isothermal form assumes a steady plateau as well; a digester wandering 3 °C through the cook has left the assumption behind. Temperature goes in degrees Celsius, and slips come easy: typing 443 while thinking in kelvin triggers an error, since any value above 250 °C gets refused. Time, in turn, goes in decimal hours: 90 minutes is 1.5, never 90.
Frequently asked questions
Where does the default 1401.5 come from?
How long at 175 °C gives the same H-factor?
Can I type the temperature in kelvin?
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.