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Nonisothermal case: adiabatic approach. Prof. Dr. Marco Mazzotti - Institut für Verfahrenstechnik. 1. Approach. In the adiabatic approach energy balances are calculated under following assumptions: There is no heat exchange through the walls of the column
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Nonisothermal case: adiabatic approach Prof. Dr. Marco Mazzotti - Institut für Verfahrenstechnik
1. Approach • In the adiabatic approach energy balances are calculated under following assumptions: • There is no heat exchange through the walls of the column • The gas and liquid stream leaving a stage are at the same temperature • Therefore two new concepts become important: • Energy balances • Changes of L and G within the column Calculating changes of L and G within the column are important when dealing with large solute fractions. This is often the case when calculating non-isothermal columns.
TG1 TL0 Data and set values Specifications Unknowns G1 L0 x0 y1 Pressure, p Gas final composition, y1 Solvent flow rate at inlet, L2 Gas flow rate, Gin Number of stages, n Gas initial composition, yn+1 Solvent final composition, xn Equilibrium data, y=f(x,T) Solvent flow rate outlet, L1 Solvent initial composition, x0 Gas Temp. at outlet TL1 p n Gas Temp. at inlet TG1 Gas Temp. at outlet TG2 Liquid Temp. at inlet, TL2 y = f(x,T) yn+1 xn Gn+1 Ln TGn+1 TLn Once, pressure and initial composition and temperature of the solvent are set we can consider them as data. This is a list of known variables:
Material Balance of column: H Enthalpy J/mol Energy Balance of column H0 Enthalpy at reference state J/mol Hs Change of solute enthalpy between liquid and gas phase J/mol cp Heat capacity J/(mol K) HS represents the molar enthalpy of mixing or the integral heat of solution, at the prevailing concentration and at the base temperature T0 2. Balances The amount of liquid increases from the bottom to the top of the column. It is calculated by adding up the amount of solvent and the amount of absorbed solute. The same way the amount of gas is calculated. It deacreases from the top to the bottom of the column as the solute absorbs to the liquid.
From the bottom of the column a new set of mass and material balances can be made including the first stage. (green canvas) From these balances all unknowns of the first stage can be calculated. This is repeated including one more stage every time until the required specification is reached. 3. Solution of the system To solve the problem the gas outlet temperature and the amount of liquid solvent have to be estimated or calculated by a nother method, e.g. the isothermal approach Now all necessary values of the bottom of the column can be calculated by the energy and mass balances.
