Boilup

If the produc ts from a column are especially pure, even this configuration may produce excessive interaction between the composition loops. Then the composition of the less pure product should oe con-troUed by manipulating its own flow the composition of the remaining product should be controlled by manipulating reflux ratio if it is the distillate or boilup ratio if it is the bottom product. 

Solution Because vapor rate changes are reflected up and down the column much faster than liquid rate changes, the temperature difference controller w as disconnected and the tower was controlled instead by boilup. A temperature 10 trays from the bottom set reboiler heating medium and the reflux W as put on flow control. 

Note that this equation holds for any component in a multi-component mixture. The integral on the right-hand side can only be evaluated if the vapor mole fraction y is known as a function of the mole fraction Xr in the still. Assuming phase equilibrium between liquid and vapor in the still, the vapor mole fraction y x ) is defined by the equilibrium curve. Agitation of the liquid in tire still and low boilup rates tend to improve the validity of this assumption. 

Assuming a specific boilup rate D , the compositions may now be calculated as a function of time ... 

The usual Raleigh Equation form [130] is for the conditions of a binary simple differential distillation (no trays or packing), no reflux, but with constant boilup. 

G = mol/hr boilup overhead L = mols reflux in the column D = overhead receiver contents, mols... 

Starting with an empty overhead receiver, the time Oj to condense D mols of vapor to fill the receiver, when the vapor boilup rate is G mols/hr. 

For a batch differential distillation where no reflux is used, there is only boilup of a mixture of the desired lighter component, which leaves the kettle, and a desired residual bottoms composition is left in the kettle. This type of distillation follows the Raleigh equation to express the material balance. However, while simple, not having tower packing or trays or reflux does not offer many industrial applications due to the low purities and low yields involved. Repeated charges of the distillate back to the kettle and redistilling w411 improve overhead purity. 

This is the boilup rate, which is approximately 3.3 ft vapor/sec. An approximately 1 ft 0 in. diameter column can handle this rate however, because it is in the usual size for a packed tower (or cartridge trays), the diameter must be checked using the packed tower calculations in Chapter 9 of this volume. 

P = Fugacity at reference standard condition f = Feed composition, i, or, = total mols of component, i, in distillate and bottoms G = Boilup rate, mols/hr... 

Calculate boilup and check against required value. 

Repeat calculations, adjusting flow loop geometry if necessary, until assumed x gives the proper boilup rate. 

Investigate the response of the column to changes in the boilup rate, V. 

COLUMN FEEDRATE SATURATED LIQUID FEED FEED COMPOSITIONS REFLUX RATIO RELATIVE VOLATILITIES LIQUID HOLDUPS VAPOUR BOILUP RATE... 

The simulation is based on a fixed vapour boilup rate. Component balance equations are represented for benzene and toluene . The xylene concentrations are determined by difference, based on the sum of the mole fractions being equal to one. 

Remember these V s are not necessarily constant with time. The vapor boilup can be manipulated dynamically. The mathematical effect of assuming equimolal overflow is that we do not need an energy equation for each tray. This- is quite a significant simplification. 

Theoretical trays, equimolal overflow, and constant relative volatilities are assumed. The total amount of material charged to the column is M q (moles). This material ean be fresh feed with composition Zj or a mixture of fresh feed and the slop cuts. The composition in the still pot at the begiiming of the batch is Xgoj. The composition in the still pot at any point in time is Xgj. The instantaneous holdup in the still pot is Mg. Tray liquid holdup and reflux drum holdup are assumed constant. The vapor boilup rate is constant at V (moles per hour). The reflux drum, eolumn trays, and still pot are all initially filled with material of eomposition Xg j. 

We will assume constant holdups in the reflux drum Aij> and in the column base Mg. Proportional-integral feedback controllers at both ends of the column will change the reflux flow rate and the vapor boilup V to control overhead composition and bottoms composition Xg at setpoint values of 0.98 and 0.02 respectively. 

To illustrate the disturbance rejection effect, consider the distillation column reboiler shown in Fig. 8.2a. Suppose the steam supply pressure increases. The pressure drop over the control valve will be larger, so the steam flow rale will increase. With the single-loop temperature controller, no correction will be made until the higher steam flow rate increases the vapor boilup and the higher vapor rate begins to raise the temperature on tray 5. Thus the whole system is disturbed by a supply-steam pressure change. 

The idea is best explained with an example. Suppose the base level in a distillation column is normally held by bottoms product withdrawal as shown in Fig. 8.4a. A temperature in the stripping section is held by steam to the rcboiler. Situations can arise where the base level continues to drop even with the bottoms flow at zero (vapor boilup is greater than the liquid rate from tray 1). if no corrective action is taken, the reboiler may boil dry (which could foul the tubes) and the bottoms pump could lose suction. 

If bottoms composition is to be controlled by vapor boilup, the control tray should be located as dose to the base of the column as possible in a binary system. In multicomponent systems with heavy components in the feed which have their highest concentration in the base of the column, the optimum control tray moves up in the column. 

Avoid saturation of a manipulated variable. A good example of saturation is the level control of a reflux drum in a distillation column that has a very high reflux ratio. Suppose the reflux ratio (R/D) is 20, as shown in Fig. 8.10. Scheme A uses distillate flow rate D to control reflux drum level. If the vapor boilup dropped ouly 5 percent, the distillate flow would go to zero. Any bigger drop in vapor boilup would cause the drum to run dry (unless a low-level override controller were used to pinch back on the reflux valve). Scheme B is preferable for this high reflux-ratio case. 

If the only disturbances were feed flow rate changes, we could simply ratio the reflux flow rate to the feed rate and control the composition of only one end of the column (or even one temperature in the column). However, changes in feed composition may require changes in reflux and vapor boilup for the same feed flow rate. 

There is a first-order dynamic lag of t minutes between a change in the signal to the steam valve and vapor boilup. The low base-level override controller pinches the reboiler steam valve over the lower 25 percent of the level transmitter span. 

Eventually, of course, the liquid rates will return to normal when the liquid inventory on the trays has dropped to the new steadystate levels. Then the eifect of the increase in vapor boilup will drive down. 

A new value of frequency is specified and the calculations repeated. Table 12.3 gives a FORTRAN program that performs alt these calculations, The initial part of the program solves for all the steadystate compositions and flow rates, given feed composition and feed flow rate and the desired bottoms and distillate compositions, by converging on the correct value of vapor boilup Vg. Next the coeflicients for the linearized equations arc calculated. Then the stepping technique is used to calculate the intermediate g s and the final P(j transfer functions in the frequency domain. 

Notice that the pairing assumes distillate composition jtp is controlled by reflux R and bottoms composition Xg is controlled by vapor boilup V. 

Example 16.5. Yu and Luyben (Ind. Eng. Chem. Process Des. Dev., 1986, Vol. 25, p. 498) give the following steadystate gain matrices for three alternative choices of manipulated variables reflux and vapor boilup (R — V), distillate and vapor boilup (D — V), and refiux ratio and vapor boilup (RR — V). 

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