Naphtali-Sandholm method

FIG. 13-53 Specifications for the calculation of a rebelled stripper by the Naphtali-Sandholm method. 

The functions and variables are salved together using a large -Jacobian of size N(2C + 1) x N(2C + 1). When originally presented, The Naphtali-Sandholm method used derivatives of IT-values and enthalpies with respect to composition and temperature, but it was not stated whether these are analytical or numerical derivatives. 

Christiansen et al. (54) applied the Naphtali-Sandholm method to natural gas mixtures. They replaced the equilibrium relationships and component vapor rates with the bubble-point equation and total liquid rate to get practically half the number of functions and variables [to iV(C + 2)]. By exclusively using the Soave-Redlich-Kwong equation of state, they were able to use analytical derivatives of revalues and enthalpies with respect to composition and temperature. To improve stability in the calculation, they limited the changes in the independent variables between trials to where each change did not exceed a preset maximum. There is a Naphtali-Sandholm method in the FraChem program of OLI Systems, Florham Park, New Jersey CHEMCAD of Coade Inc, of Houston, Texas PRO/II of Simulation Sciences of Fullerton, California and Distil-R of TECS Software, Houston, Texas. Variations of the Naphtali-Sandholm method are used in other methods such as the homotopy methods (Sec. 4,2.12) and the nonequilibrium methods (Sec. 4.2.13). 

Any of the global Newton methods can be converted to a relaxation form in Ketchum s method by making both the temperatures and the liquid compositions time dependent and by having the time step increase as the solution is approached. The relaxation technique should be applied to difflcult-to-solve systems and the method of Naphtali and Sandholm (42) is best-suited for nonideal mixtures since both the liquid and vapor compositions are included in the independent variables. Drew and Franks (65) presented a Naphtali-Sandholm method for the dynamic simulation of a reactive distillation column but also stated that this method could be used for finding a steady-state solution. 

The equations will still form the same block-banded sparse matrix as in the Naphtali-Sandholm method. No matter what size the time step, the same matrix solution technique can be used to calculate the next set of independent variables. 

Vickery and Taylor (81) used a Naphtali-Sandholm method containing all of the MESH equations and variables [M2C + 3) equations] with the variables represented by x. H is the Jacobian from the Naphtali-Sandholm method solution of the known problem, G(x) = 0, This is numerically integrated from t = 0 to t - 1, finding a H, at each Step and updating H when the solution is reached at each step, With Hj. and H, known, dxjdt is solved, and with step size t, a new set of values for the independent variables x is found by Euler s rule... 

With Emvo = 0.1, solve the Naphtali-Sandholm method for the column, The solution criteria for intermediate valueB of MVy (0.1,... 

The global Newton methods, such as the Naphtali-Sandholm method (Sec. 4.2.9), are often used to solve highly nonideal systems. These are frequently prone to failure. Good explanations of the theory of homotopy methods are provided by Seader (86) and Wayburn (83). A homotopy method can greatly expand the global Newton method ability to solve difficult nonideal systems. Homotopy methods have been associated with the Naphtali-Sandholm method, where the derivatives of the if-values and enthalpies with respect to all compositions directly appear within the Jacobian. Using a thermodynamic homotopy for another method such as a Tomich has not been presented in the literature. 

Standard specifications for the Naphtali-Sandholm method are Q-(including zero values) at each stage at which heat transfer occurs and sidestream flow ratio Sj or Sj (including zero values) at each stage at which a sidestream is withdrawn. However, the desirable block tridiagonal structure of the Jacobian matrix can still be preserved when substitute specifications are made if they are associated with the same stage or an adjacent stage. For example, suppose that for a reboiled absorber, as in Fig. 13- it is desired to specify a boil-up ratio rather than reboiler duty. Equation (13-95) for function is removed from the N(2C + 1) set of equations and is replaced by the equation... 

The Newton-Raphson technique is also modified in this method. A damping factor a, between zero and one, is applied to the corrections as above. The way a is calculated, however, is different from the Naphtali-Sandholm method. 

Explain the difference between the bubble-point and Naphtali-Sandholm methods... 

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