Total Reflux Operation

Columns are frequently operated on total reflux during short (from a few minutes to a few days) feed interruptions. Columns are also frequently started up on total reflux. A total reflvix startup is one of the most powerful, and often one of the least troublesome, startup techniques. 

A total reflux startup is best performed with a mixture whose composition resembles the feed composition. Alternatively, total reflux operation can be carried out using one or more of the feed components. The column, reboiler, condenser, and reflux system are stabilized at 

A total reflux startup is one of the least troublesome startup techniques because 

Additional benefits of total reflux startups are saving in plant startup time (by having the column off the critical path), and a reduced amount of off-spec material. 

Total reflux startup is most attractive in large superfractionators which use high reflux ratios (e.g., isobutane-normal butane ethylene-ethane separation) and/or in heat-pumped columns. Such columns take from a few hours to a couple of days to start up and stabilize. Due to the high reflux ratio, they are relatively insensitive to feed variations. These features make them ideal candidates for total reflux startups. Total reflux startup is least attractive when the ratio of reflux to feed is low (in such cases, most of the stabilizing can only be performed after feed is introduced), and when the column is easy and trouble-free to start up. 

---

Operation can he continuous or hatchwise at total reflux. Total reflux operation is not feasible. 

The dominant mechanism of purification for column ciystallization of sohd-solution systems is reciystallization. The rate of mass transfer resulting from reciystallization is related to the concentrations of the solid phase and free hquid which are in intimate contac t. A model based on height-of-transfer-unit (HTU) concepts representing the composition profQe in the purification sec tion for the high-melting component of a binaiy solid-solution system has been reported by Powers et al. (in Zief and Wilcox, op. cit., p. 363) for total-reflux operation. Typical data for the purification of a solid-solution system, azobenzene-stilbene, are shown in Fig. 22-10. The column ciystallizer was operated... 

FIG. 22-10 Steady-state separation of azobenzene and stilbene in a center-fed column crystallizer with total-reflux operation. To convert centimeters to inches, multiply by 0.3937. (Zief and Wilcox, Fractional Solidification, vol. 1, Marcel Dekker, New York, 1967, p. 356.)... 

If no product was withdrawn in step 3, the column is now run under total reflux operation until the unit is taken to a steady state or to a state when the distillate composition reaches the desired product purity. 

Mujtaba and Macchietto (1988) used the measure, q, the degree of difficulty of separation (section 3.5.1) to correlate the extent of benefit of production and recycling of off-cuts for binary mixtures. It is reported that for some separations more than 70% batch time savings were possible when compared to no off-cut production and no recycle cases. Using the measure, q, the authors were also able to explain whether and when an initial total reflux operation was required in batch distillation. Further details are presented in Chapter 8. 

Step 5 (section 3.3.1) now simply requires to run the column (i.e. simulate the column operation using a model) at total reflux until the unit reaches the steady state (Holland and Liapis, 1983 Nad and Spiegel, 1987) or until the instant distillate composition reaches the product composition so that the product can be collected at constant distillate composition (Coward, 1967 Kerkhof and Vissers, 1978 Logsdon and Biegler, 1993). A variation of this total reflux operation concept can also be found in the literature and in practice. In this variation only a part of the condensed liquid is returned to the column and the rest is taken out as product (product period starts from step 3 of section 3.3.1) (Converse and Huber, 1965 Mayur and Jackson, 1971 Mujtaba, 1989 Mujtaba and Macchietto, 1992). Mujtaba and Macchietto (1988) and Mujtaba (1989) clearly explain whether and when an initial total reflux operation is required. 

Nad and Spiegel (1987) carried out experimentation in a conventional packed batch distillation column using a cyclohexane-heptane-toluene mixture. The column consists of 20 theoretical stages (equivalent) including the condenser and reboiler. The feed to the column was 2.93 kmol of which 1.9% was total column holdup and 1.2% was condenser holdup. The column underwent an initial total reflux operation for about 2.54 hr before any product was collected. After then the mixture was separated into 3 main-cuts with 2 off-cuts in between, leaving a final product in the reboiler. 

Skogestad et al. (1997) simulated a MultiBD column with a similar to type III model using a total reflux operation. The input data for the column is presented in Table 4.15. 

Mujtaba (1989) considered the minimum time problem with a separation task (D = 1.16 kmol, x D= 0.906). The task is same as those reported for ideal case in Table 4.8 (Chapter 4). The simulation used 4 reflux ratio levels including an initial total reflux operation. Mujtaba also used 4 reflux ratio levels to compare the simulation results. The lower and upper bounds on the reflux ratio are (0.3 and 1.0). 

The simulation results reported in Table 4.8 used 2.54 hr of initial total reflux operation (also used in the experimental column by Nad and Spiegel, 1987) before any product was withdrawn from the column. Here, the aim was also to find out-... 

The optimisation results are summarized in Table 5.8. Figure 5.4 shows the reflux ratio profiles used in the simulation and those obtained by optimisation. Figure 5.5 shows the optimal accumulated and instant distillate composition profiles. The reflux ratio profile is increasing with time as expected. The results clearly show the benefit of optimising the reflux ratio. As can be seen from Table 5.8 the operation time is reduced by at least 50% compared to that in the simulation and experiment. The results also show that for the given separation task an initial total reflux operation is not at all required and the product can be collected from the very beginning of the process (see notes in section 3.3.1). 

An interesting part of the results mentioned so far is that in all cases, an initial total reflux operation was found to be necessary. This was to eliminate the third component from the overheads. The length of that period again depends on the ease of separation. For no recycle cases, the times at total reflux are slightly larger than those for recycle cases. This is due to the fact that the recycle of off-cut eases the separation of the main-cut. After the total reflux period of operation an increasing reflux ratio profile was obtained for the main-cut in all cases. The off-cut was always obtained at low reflux ratio as were the case with binary mixtures (Rose,... 

The maximum conversion, the corresponding amount of product, optimal constant reflux ratio and heat load profiles for different batch times are shown in Figures 9.3-9.6. The maximum conversion profile achieved under total reflux operation (where no product is withdrawn) is also shown in Figure 9.3. The latter approximates the conversion which would be achieved in the absence of distillation. Note that if there is a large column holdup, the conversion under total reflux will not approximate the conversion achieved in the absence of distillation. 

The percent improvement in conversion (compared to the conversion achieved under total reflux operation) achieved for different fixed batch times is shown in Figure 9.7. The results show that about 40% more conversion is possible when the column is operated optimally compared to total reflux operation. 

For different values of F, Rmax, optimum reflux ratio (/ 2)> minimum operation time, productivity are shown in Table 10.4. In all cases the total amount of distillate is 3.95 kmol with 95% purity in Heptane. The productivity (Prod) is calculated using total operation time (rwfo/) which includes 2 hrs of total reflux operation time in STEP 1. 

In Table 10.8, R decreases with the increase in F according to Equation 10.2. The operation time decreases and the productivity increases with the increase in F . These are as expected. Initial total reflux operation was not needed for any case. Significant improvement in productivity (about 77%) is noted for case 4 compared to the base case. This clearly reveals the importance of extractive distillation for... 

Other methods. A cycling procedure can also be used for the column operation. The unit operates at total reflux until a steady state is establisned. The distillate is then taken as total drawoff for a short time, after which the column is returned to total reflux operation. This cycle is repeated throughout the course of distillation. Another possibility is to optimize the reflux ratio to achieve the desired separation in a minimum time. More complex operations may involve withdrawal of sidestreams, provision for intercondensers, addition of feeds to trays, and periodic feed additions to the pot. 

Arwickar (1981) reported some results for distillation under total reflux conditions of the system acetone-methyl acetate-methanol. The experiments were carried out in a laboratory scale column of 7.62 cm diameter packed with 0.635 cm Raschig rings. The simulation of total reflux operations using the nonequilibrium model is discussed by Krishnamurthy and Taylor (1985a). In simulations of Arwickar s experiments Taylor et al. used the correlations of Onda et al. (1968) to estimate the mass transfer coefficients in each phase and the effective interfacial area. The average absolute discrepancy between predicted and measured mole fractions was less than 2 mol% for acetone and methyl acetate and less than 4 mol% for methanol. 

---