For The Subject – CHEMICAL PROCESS INDUSTRIES-I For The Subject Code Topic Name- To study about black liquor recovery from kraft process GOVERNMENT ENGINEERING COLLEGE, VALSAD

ENROLLEMENT NO. NAME PRAJAPATI MITUL PRAJAPATI RAJINDAR LALMANBHAI PRAJAPATI SAGARKUMAR PANKAJBHAI

 The kraft process is a form of chemical pulping. This means that, in order to degrade and dissolve away the lignin and keep most of the cellulose and hemicellulose, the wood chips are cooked with appropriate chemicals in an aqueous solution at elevated temperature and pressure. The main challenge is to form fibers that are practically intact.  More specifically, the kraft process involves cooking of the chips using a solution of sodium hydroxide (NaOH) and sodium sulfide (Na 2 S)  One major characteristic of the kraft process is that the cooking chemicals are regenerated in a recovery process that will be discussed later.

Kraft Pulping and Recovery Chemical convert wood into cellulosic pulp High quality, strong pulp for papermaking Operate safely Minimize impacts on air and water Satisfy customer needs – Maximum product quality Minimize cost

Advantages:  High strength pulp  Utilizes proven technology for chemical recovery  Handles with a wide variety of wood species  Tolerates bark in the pulping process Disadvantages:  Unbleached pulp is characterized by a dark brown color  It is associated malodorous gases (organic sulfides)

White liquor: Liquor containing the active cooking chemicals (NaOH and Na 2 S) and used to cook chips. Black liquor: Residual liquor from cooking containing the reaction products of lignin solubilization. This liquor is concentrated and burnt into a recovery furnace and yields an inorganic smelt of Na 2 CO 3. Green liquor: Liquor obtained by dissolving the latter smelt. The green liquor is reacted with CaO in order to convert the Na 2 CO 3 in NaOH and regenerate the white liquor. Cooking & Washing Chips Pulp Evaporation & Burning Causticizing BLACK LIQUOR Alkali lignin Hydrolysis salts Sulphonation products WHITE LIQUOR NAOH NA 2 S GREEN LIQUOR Na 2 CO 3 Na 2 S The kraft sodium cycle (Source:Smook, 1994)

PSE 476: Lecture 6 7 * Median concentrations as g/l as Na 2 O

Black liquor is an important resource for steam generation in the pulp and paper industry. It is a substance unique to the pulping process and represents a readily available renewable energy source. It is also one of the few fuels to have been locally produced and used in the countries with a pulping industry. From the point of view of fluid mechanics, black liquor is a liquid. However, it has two states, fluid and solid. Its main components are inorganic cooking chemicals, lignin and other organic constituents removed from the wood during pulping in the digester, and water. These organic constituents are combined chemically with sodium hydroxide (NaOH) in the form of sodium salts such as Na2S, Na2CO3, Na2SO4. Some organics may be recovered in the chemical recovery process, such as the mixture of resin and fatty acids known as tall oil, or the turpentine recovered in a liquid separation sequence. The exact composition of black liquor depends on the wood species, the pulp yield, and the alkali charge used. Considerable differences exist between liquors from different species especially between those from hardwood and softwood. An important aspect of black Liquor combustion is elemental analysis of the liquor solids, that is, of the percentage by weight of each chemical element in the black liquor solids. Five elements are always present: sodium (Na), sulphur (S), carbon (C), hydrogen (H), and oxygen (O). In some cases, potassium (K) and chorine (Cl) are also present. The approximate composition of black liquor is given in Black liquor

Table 1: Typical elemental composition of black liquor solids and char Element Present (wt%) Na 19.17% S 4.76% C 35.93% H 3.56% O 35.20% K 1.02% Cl 0.12% Inerts 0.24% Total 100.0%

Functional role of various units (a) Multi effect evaporator  It is a series of single effect evaporators.  Multi-effect evaporator and disc evaporator concentrate the black liquor from 15-18% solid to 60-65% solid. (b) Mix tank  It is designed with two agitators to mix the product.  Make up chemicals (Na2SO4 + S) are added. (c) Boiler  It produces molten slag.  Here organic carbon present in black liquor is burned.  High pressure 28 atm to 30atm steam is produced.  The reaction that takes place inside furnace is Na2SO4 + 2Cà Na2S + 2CO2

(d) Dissolving tank  The molten chemical smelt dissolve immediately with cold water and yield green liquor (aqueous Na2CO3). (e) Clarifier  It is used for filtering.  It separates calcium carbonate sludge and white liquor.  Filtering medium is monel metal. (f) Washing tank  Impurities are washed away in this tank after clarifying. (g) Causticizing tank  It precipitates lime mud.  Carbonate is causticized by adding Ca(OH)2

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