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9.8 Operating Time

Operating time and batch size may be controlled by economics or other factors. For instance, it is not uncommon for the entire batch including startup and shutdown to be done in one 8-hour shift. If the same apparatus is used for several different chemicals, the batch sizes may vary. In addition, the time to change over from one chemical to another may be quite long, since a rigorous cleaning procedure may be required.

The total batch time, tbatch, is

The down time, tdown, includes dumping the bottoms, cleanup, loading the next batch, and heating the next batch until reflux starts to appear. This time can be estimated from experience. The operating time, top, is the actual period during which distillation occurs, so it must be equal to the total amount of distillate collected divided by the distillate flow rate.

Dtotal is calculated from the Rayleigh equation, with F set either by the size of the still pot or by the charge size. For an existing apparatus the distillate flow rate, D in kmol/h, cannot be set arbitrarily. The column was designed for a given maximum vapor velocity, uflood, which corresponds to a maximum molal flow rate, Vmax (see Chapter 10). Then from the mass balance around the condenser,

We usually operate at a fraction of the maximum flow rate, such as D = 0.75 Dmax. The operating time, top, can be estimated from Eqs. (9-29) and (9-30). If the resulting tbatch is not convenient, adjustments must be made.

The energy requirements in the reboiler or still pot, QR, and in the total condenser, Qc, can be estimated from energy balances. For a total condenser Eq. (3-13) is valid, but V1, hD, and H1 may all be functions of time (if xD varies, the enthalpies will vary). If the reflux is a saturated liquid reflux, then H1hD = λ, the latent heat of evaporation. In this case the total condenser just condenses the vapor to saturated liquid. Likewise, the still pot vaporizes a saturated liquid to a vapor. Thus,

If CMO is valid, then V1 = Vpot, λ1 = λpot, and

Since V = (1 + L/D)D, we obtain

For an existing batch distillation apparatus we must check that the condenser and the reboiler are large enough to handle the calculated values of |Qc| and QR. If |Qc| or QR is too large, then the rate of vaporization needs to be decreased. Either the operating time, top, will need to be increased or the charge to the still pot F will have to be decreased.

During operation (assume that the charge and the still pot have been heated and vapor is flowing throughout the column), the energy balance around the entire system is

If CMO is valid, Eq. (9-31c) holds, and saturated liquid hD = hpot, the result simplifies to

If the assumption of negligible holdup is not valid, then the holdup on each stage and in the accumulator acts like a flywheel and retards changes. A different calculation procedure is required for this case (Diwekar, 2012; Mujtaba, 2004). Batch distillation also has somewhat different design and process control requirements than continuous distillation. In addition, startup and troubleshooting are somewhat different (Sorensen, 2014).

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