Distribution Condenser Reboiler

Kayihan, F., Optimum Distribution of Heat Load in Distillation Columns Using Intermediate Condensers and Reboilers, AfC/iS Symp. Ser., 192(76) 1, 1980. 

The batch distillation column consisted of 3 internal plates, reboiler and a total condenser. The reboiler was charged with a fresh feed of 5 kmol with Benzene molefraction 0.6. The total column holdup was 4 % of the charge. Half the holdup was in the condenser and the rest was distributed over the plates. The vapour load to the condenser was 3 kmol/hr. The required product purities were x oi = 0.90 and x B2 = 0.15. The solution of Equations 8.1-8.4 therefore gives DJ = 3.0 kmol and B2 = 2 kmol. This problem is same as case 3 shown in Table 8.1. Three reflux ratio (control) intervals were used to achieve (Dl, x Di) and one control interval to achieve (B2, x B2). 

The column compositions are initialised to the composition of the mixed reboiler charge and a total of 2% of the fresh feed is used as column holdup. Half of the column holdup is assumed to be in the condenser and the rest is distributed equally over the plates. Piecewise constant reflux ratio was used, with 3 time intervals for the main-cut separation and 1 interval for the off-cut. 

This example is taken from Mujtaba (1989) and Mujtaba and Macchietto (1992) where the same ternary mixture as in example 1 was considered for the whole multiperiod operation which includes 2 main-cuts and 2 intermediate off-cuts. The column consists of 5 (NT) intermediate plates, a total condenser and a reboiler. The column is charged with the same amount and composition of the fresh feed as was the case in example 1. Column initialisation, holdup distribution and condenser vapour load are also same as those in example 1. 

The results for the corresponding adiabatic column are also shown for comparison. The most important trays for distributed heating were the trays closest to the reboiler and condenser. 

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. 

Determination of the Product Distribution for a Conventional Distillation Column at Specified Condenser and Reboiler Duties Qc and QR or Specified Values of L, D and VN/B... 

Calculate the product distribution, the minimum theoretical stages, the minimum reflux, and the theoretical stages at l.S times minimum LID and locate the feed stage. The column is to have a partial condenser and a partial reboiler. 

Before we start searching for feasible designs and interpreting the effects of Xa s and 7 a s, we first have to define our column and break it into a series of CSs. Consider then a two product distillation column terminated at the top by a condenser and at the bottom by a reboiler, as shown in Figure 6.1. We shall define the total feed to the column as Ft, which is to be distributed at multiple points down the length of the column. For the general case where the feed is to be split into N — 1 substreams, at A 1 feed points, the result will be a column consisting of N CSs. 

D9. A distillation column is separating a feed that is 20 mol% methanol, 50 mol% n-propanol, and 30 mol% n-butanol. Feed is 750 kmol/h. We want a 92 mol% recovery of methanol in the distillate and 95% recovery of n-propanol in the bottoms. Use L/D = 7. Feed is a saturated liquid. The column has a total condenser and a partial reboiler. As a guess, assume that n-butanol does not distribute (it all exits in the bottoms). Do not do iterations to inprove this guess. If we choose n-propanol as the reference, the relative volatilities are methanol = 3.58, n-propanol = 1.0, and n-butanol = 0.412. Assume relative volatilities are constant. Do not use Aspen Plus. This calculation is single enough that it can be done by hand or with a spreadsheet (with or without Visual Basic for Applications [VBA]) or with MATLAB. 

CMO and constant relative volatilities. The column has a total condenser and a partial reboiler. L7D = 6. As a first guess, assume that n-propanol does not distribute (it all exits in the bottoms). Relative volatilities are methanol = 3.58, i-propanol = 1.86, and n-propanol = 1.0. 

Going one level up in aggregation we consider the structures of complete columns. Conventionally, RD columns are classified in hybrid and non-hybrid units, depending on the distribution of reactive trays inside the column. Thus, hybrid columns are distillation columns with a reactive core and nonreactive sections (rectifying and/or stripping sections). Non-hybrid RD columns denote columns where all trays including condenser and reboiler are reactive (Giittinger, 1998). Hybrid columns are commonly... 

We can turn from partially coupled sequences to completely coupled ones (to Petl5Tik columns). For this purpose, the reboilers and condensers connected with the sections, products of which are components with intermediate volatilities, should be excluded. It is possible if each excluded condenser or reboiler is replaced by flow of liquid or vapor from another section. To ensure this flow, it is necessary to supplement the network with an additional node (i.e., with one top and one bottom section). We note that inclusion of additional nodes is equivalent to inclusion of columns with distributed components into the sequence. For example, in the side rectifier 2,3 2 condenser can be replaced if network 1,23 1,133 2,3,... 

One can get from completely coupled sequences all the other feasible sequences, including columns with distributed components, if top sections are supplemented with condensers and bottom sections are supplemented with reboilers. For example, the sequence shown in Fig. 6.12d (with prefractionator and complex column) can be obtained in this way. 

The stabilizer contains 20 trays and a total condenser. Feed enters at tray 10. The normal column overhead pressure is 700 kPa and there is a 20 kPa pressure difference that is evenly distributed between the condenser and the reboiler. Each tray is 2.0 m in diameter with a 0.10 m weir, which is 1.6 m long. 

---