Boilup ratio minimum

Seader and Henley (1998) considered the separation of a ternary mixture in a batch distillation column with B0 = 100 moles, xB0 = = <0.33, 0.33, 0.34> molefraction, relative volatility a= <2.0, 1.5, 1.0>, theoretical plates N = 3, reflux ratio R = 10 and vapour boilup ratio V = 110 kmol/hr. The column operation was simulated using the short-cut model of Sundaram and Evans (1993a). The results in terms of reboiler holdup (Bj), reboiler composition profile (xBI), accumulated distillate composition profile (xa), minimum number of plates (Nmin) and minimum... 

This formulation of the Newton-Raphson method for columns with infinitely many stages is analogous to the 2N Newton-Raphson method for a column with a finite number of stages. First the procedure is developed for a conventional distillation column with infinitely many stages for which the condenser duty Qc (or the reflux ratio Lx/D) and the reboiler duty QR (or the boilup ratio VN/B) are specified and it is required to find the product distribution. Then the procedure is modified as required to find the minimum reflux ratio required to effect the specified separation of two key components. 

A maximum L/V and hence a minimum boilup ratio V/B can also be defined. The pinch points will look the same as in Figure 4-26A or 4-26B. Problem 4.C12 looks at this situation further. 

This criterion is similar to using a reflux ratio of 1.2 times the minimum reflux ratio in a full distillation column. It provides a reasonable compromise between the number of trays and the vapor boilup required. The slope of the resulting operating line is the liquid-to-vapor ratio F/D ... 

Set the actual reflux ratio RR equal to 1.1 times the minimum and calculate the vapor boilup in the column ... 

Case Boilup Rate Optimal Reflux Ratio at Minimum... 

Reflux ratio or vapor boilup These should be above the minimum... 

It is common to design and operate reasonably close to the minimum reflux or minimum boilup (Sec. 3.1.4). A computer solution at such low reflux ratios can be unstable and fail. A solution may only be reached if very good initial values are available. The technique of "sneaking up on an answer" is powerful in these cases. Initially, the column is solved at a higher reflux ratio. This solution is used as the initial value for the subsequent calculation, in which the reflux ratio is slightly lowered. This process is continued until the desired reflux ratio is reached. Other examples of how to use the solution of one simulation to initialize another simulation are described by Brierley and Smith (106). The "sneaking-up technique is part of the basis of the homotopy methods (Sec. 4.2.12) and these and other forcing techniques may also be used. 

Scheme 3. This scheme is a common armngemenl in die chemical industry, used primarily on stripping columns. This arrangement indirectly edjusis die material balance in die same manner as scheme 2. It has the advantage of offering the minimum effective dead time to total lag time in the composition loup. It has the disadvantage of having the greatest interaction butween die material and energy belances. Because the reboiler is this interactiou, scheme 3 1b nor recommanded in columns wham the reboiler boilup to column fred ratio is 2,0 or greater.

The highest energy consumption is at total reflux, that is, when all of the boilup is returned as reflux, and there is no feed to the column and no distillate or bottoms. A computer simulation for this separation at total reflux required a minimum number of 27.8 theoretical stages. This was accomplished by setting the reflux/distillate ratio to 10 million in a computer simulation. This is the shortest column that can achieve the desired separation. 

Column operation at minimum pressure. Lower pressure usually means higher relative vdatility. Therefore, the necessary separations can be accomplished with lower boilup/feed and reflux/feed ratios. Condenser capacity may be limited, however, and the column may flood at lower boilup rates than it would when operating at higher pressures. 

As liquid level rises from 75 to 100 percent, override A5 closes the feed valve. At a level transmitter output of 15 psig, A5 has an output of 3 psig. With steam/feed ratio control, this would also cut off steam. But it is desirable to maintain enou boilup that the trays (sieve or valve) do not dump or weep. For this we provide an override (not shown) that is a minimum steam flow controUer. In most cases we have found it advisable to adjust override biases so that a high level pinches feed before steam. 

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