Process Flowsheeting

Process Flowsheeting

1. Mass Balance

   • The Phase Rule
     • DOF = n - π + 2
       • Specify variables
         • Simulation problem
           • feed stream and unit parameters
         • Design problems
           • Other specifications
   • Calculation Procedure
     • Sequential Modular
       • uses matrix representation to solve the flowsheet by solving each set of equations for each unit in order of which they appear in the flowsheet.
       • 1. Guess values for stream 4
         • 2. Using these values, solve for streams 2, 3, 5 and new set of values for stream 4
           • 3. Modify the guessed values for stream 4
             • 4. Redo the flowsheet calculations until calculated values are equal to the guessed values for stream 4
               • 5. When the two values are essentially the same, the procedure is said to have converged
                 • What is essentially equal? When comparing the guessed value and calculated value for stream 4, a ‘tolerance’ or ‘ԑ’ value may be set as an acceptable accuracy (10-3 , 10-7 etc.)
               • Successive substitution
                 • ADV: simple, easy to implement DIS: may diverge (unlikely), very slow rate of convergence
               • Convergence acceleration techniques
                 • ADV: faster convergence DIS: possible divergence, more complex implementation
     • Equation Oriented
       • uses matrix representation of material and energy balance problems to solve the flowsheet. It solves the equations simultaneously.
       • occurrence matrix
         • When the matrix is square, the degree of freedom is zero – this means the number of equations is equal to the number of variables, hence a solution can be sought.
         • Jacobian matrix (refer to slides on equation solving methods) to find a solution.

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