Until the early twentieth century, when recovery boilers first appeared, black liquor was a disposable by-product in paper production. In modern times, pulp mills use chemical recovery technologies to recapture spent cooking chemicals from the pulping process for economic and environmental reasons. An example is Black Liquor Recovery Boilers that enable papermakers to recover and reuse inorganic compounds while also extracting energy from pulping leftovers.
The Kraft process is used in paper production, in which wood is transformed into wood pulp and ultimately into paper. However, the method generates a byproduct known as black liquor, a hazardous mixture of pulping wastes (such as lignin and hemicellulose), and inorganic Kraft process chemicals (such as sodium hydroxide and sodium sulfide). The weak black liquor is directed through evaporators to increase the solids content to form strong black liquor. Injection of the concentrated black liquor into the recovery furnace burns organic molecules and reduces Na2SO4 to Na2S. The high energy content of the black liquor burned in the recovery furnace recovers as steam for process requirements such as boiling wood chips, heating, evaporating black liquor, preheating combustion air, and drying the pulp or paper products.
The pulp industry places a high value on accurate, continuous measurement of black liquor solids concentration. Black liquor solid is an essential measurement operation influencing the economics of the pulping process. The mill's environmental load and the solids content of liquor injected into a recovery furnace can significantly impact the firing behavior and thus efficiency. Current trends toward better recovery furnace control for safety and lower air emissions necessitate more robust control over incoming solids content and proper modification of operating parameters to accommodate changes in solids content. Typical analyses of black liquor solids are conducted periodically throughout the mill, from the washers through the evaporator to the ring header. These analyses consist of taking many samples at critical locations and analyzed in the lab. Alternatively, as pulp and paper mills are modernized and automated, inline process refractometers are installed, providing dependable, accurate, and reliable solids measurement and contributing to the safe functioning of recovery boilers. Inline process refractometers are time-tested and proven to provide reliable and precise automatic monitoring of black liquor solids concentration and providing considerable time and labor savings.
For more information about inline process refractometers for black liquor processes visit this link or contact Electron Machine at 352-669-3101.
