Reboiled absorbers

It should be notea that the design procedures discussed in this section are not apphcable to reboiled absorbers, which should be designed according to the methods described in Sec. 13. 

This section is a companion to the section titled Fractionators-Optimization Techniques. In that section the Smith-Brinkley method is recommended for optimization calculations and its use is detailed. This section gives similar equations for simple and reboiled absorbers. 

A more quantitative and lengthy method, but still very useful for checking of the type required here is the Smith-Brinkley method (Reference 5). It uses two sets of separation factors for the top and bottom parts of the column for a fractionator or reboiled absorber and one overall separation factor for a simple absorber. The method is tailor-made for analysis of a column design or a field installed column. The Smith-Brinkley method starts with the column parameters and calculates the resulting product compositions unlike other methods that require knowing the compositions to determine the required reflux. 

This author developed a reboiled absorber alternate from the Smith-Brinkley generalized equation, and friends have developed plant fractionator control algorithms using Smith-Brinkley. 

Rose et al. (1958) and Hanson and Sommerville (1963) have applied relaxation methods to the solution of the unsteady-state equations to obtain the steady-state values. The application of this method to the design of multistage columns is described by Hanson and Sommerville (1963). They give a program listing and worked examples for a distillation column with side-streams, and for a reboiled absorber. 

For the first of these illustrations. Fig. 2(a) shows a reboiled absorber. 

The column is a nine-stage reboiled absorber, fed with a vapor of seven components, the feed entering stage 3 of the column. The feed consists of ... 

For packed towers, the continuous differential nature of the contact between gas and liquid leads to a design procedure involving the solution of differential equations, as described in the next subsection. Note that the design procedures discussed in this section are not applicable to reboiled absorbers, which should be designed according to the procedures described in Sec. 13. 

Multiple equilibrium stage processes simulated in this program are distillation, absorption, and stripping. Both simple and reboiled absorbers are included, and multiple feed plus side-stream products are possible from the fractionators. Matrix- and short-cut-type solution methods are provided in separate subroutines. 

Another multistage method included in the program is the absorption and stripping factor method of Edmister (25). ASFPH, as it is called, can simulate simple and reboiled absorbers and also fractionators. The method used does not have very good convergence characteristics however, it is of value in studying plant performance data. 

The SR method is suitable for modeling absorbers and strippers. For some extremely wide boiling systems, especially those with noncondensables, it is the best method. It has been found to work very well for the side strippers of a refinery fractionator. Absorbers typically have a rich gas bottom stage feed and a lean oil top stage feed. The equations of the SR method do not allow its direct use for reboiled absorbers, absorbers with condensers, or distillation columns. For these columns, other methods like that of Tomich (Sec. 4.2.8) or Russell Sec. 4.2.10) can be used. 

The best-known presentations are by Tomich (32), Holland (8), and Orbach et al. (33). These vary in their choice of Newton-Raphson equations and independent variables and each may solve a different range of columns, These methods have been shown to work well for wide-boiling mixtures including refinery fractionators, absorber-stripper columns, and reboiled absorbers. 

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... 

Absorption and stripping RadFrac Absorber, Refluxed Absorber, Reboiled Absorber... 

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