CHEMICAL RECOVERY

INTRODUCTION On completion of pulping step, the residual pulping chemical and dissolved substance from an aqueous solution called Black liquor. The recovery system process the weak black liquor and regenerates the pulping chemical

Function of Kraft Recovery Recovery and reuse of the inorganic Pulping chemical. Removal and Sale of valuable organic by-product chemical. Remaining Organic Material and recovery of its energy value as process steam and electrical power

Process Steps Pulping 2.washing 3. Evaporation 4.Combustion 5.Causticizing 6. Calcining 7. Soap Skimming and by-products 8. Black liquor oxidation 9. Waste gas incineration

1. Pulping Wood and white liquor are reacted in the digester at about 170⁰C to yield Kraft pulp and weak black liquor. Turpentine may be recovered as aby-product.

2. Washing Weak black liquor is separated from the Kraft pulp at the washer. The black liquor is diluted by wash water. Modern pulp washing facilities normally recover 98% of chemical applied in the digester.

3. Evaporation Steam heated, multiple effect evaporator are used to concentrate the weak black liquor from 13-17% to up to 60-70%.

4. Combustion The objective is complete combustion of organic matter in black liquor and also recover the sodium and sulfur content in the form of suitable for regeneration the pulping chemical.

5. Causticizing The main component of green liquor is clarified and causticized with lime to produce Whit liquor. Na2CO3 + CaO +H2O → 2NaOH + CaCO3

6. Calcining The washed mud is then calcined in the lime Kiln or Fluid bed Calciner to regenerate Lime. Calcining is a high temperature, heat absorbing reaction. CaCO3 + heat → CaO + CO2

7. Soap Skimming and by- products Fatty and resin acid called tall oil, are a minor constituent in black liquor that precipitate as soap as the liquor is concentrated.

11. Black liquor Oxidation Residual sulfide in black liquor will be converted to H2S, a very odorous gas, in the direct contact evaporator.

12. Waste gas incineration Odorous sulfur gases are released at many point in the process. For odor control purpose these are often collected.

BLACK LIQUOR

1. Composition The substance in black liquor derived from two sources: wood and whit liquor. Wood consist of Ligneous material, Saccharine acid, Low molecular weight, Extractives

2. Nature Of Constituent Lignin Hydroxy and Low Molecular Weight Organic acid Extractives Inorganic compound

3. Physical Properties 3.1 Viscosity Black liquor is Newtonian at solid level below about 50% but become Non- Newtonian are at higher solid content. The apparent viscosity of black liquor can be strongly affected by the residual Alkali content.

3.2 Density and Specific gravity Density of black liquor measure by the Pycnometric method. Specific gravity is measured by hydrometer which is calibrated in Baume. The Baume gravity compare densities at 60⁰ C. Specific gravity 60/60°F = 145/(145 - °Be)

3.3 Specific heat It determined in term of calorimetrically. In absence of actual data used Cp = 1 – ( 1 – Cp,s)S Cp = Specific heat

3.4 Boiling Point Rise The difference b/w the temperature of black liquor and that of pure water at the same pressure. It is a strong function of liquor solid content and only weak function of pressure. BPR = K [S/(1 – S)]

Direct contact Evaporator

1. In this liquor is brought into direct contact with hot combustion gases without any intervening heat transfer surface. 2. Sensible heat is removed from the hot gas to supply the heat of vaporization for the water evaporation from the liquor.

It require about a 250-300°F drop in recovery boiler flue gases temperature to concentrate liquor from 50-65% solids when only black liquor is fired in the furnace. Two type of direct contact evaporator were commonly used: 1. Cascade Evaporator 2. Cyclone Evaporator

1. Cascade Evaporator It employs a rotating bundle of steel tube which serve to pick up the liquor and move it through the hot flues gases. The cascade evaporator consist of one or more wheel consist of two circular side plates connected by tube, and is mounted on the shaft which is rotated at about 6rpm by a constant speed drive.

Cascade Evaporator

2. Cyclone Evaporator It is cylindrical vessel with a conical bottom. Flue gases is admitted through a tangential inlet near the bottom and flow in helical path . Liquor to be evaporated is sprayed across the gas inlet. A distributing skirt arrangement is used to discharge the liquor evenly onto the walls.

Cyclone Evaporator