Cómo funciona pilas por dentro
37.9 Sistema de eventos
The available equipment for doing a separation may be a batch or a continuous column and the choice may be made for reasons other than fractionating power (Table 6.3). While it is possible in a continous column to run with most of the separating power (trays or packed height) simultaneously enriching the tops and stripping the bottoms there is only one strip- ping stage, the kettle itself, in a conventional batch still. It is therefore very convenient that the non-ideal method should favour stripping. If the reverse were true, and in a few cases usually involving a chlorinated solvent it is, there would be little call for batch stills.
The mole fraction of the more volatile component in the still kettle will reduce as the batch proceeds
1.0 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 1.0 y1 mole fr action in v apour
x1 mole fraction in liquid
Fig. 6.10 Water (1)/pyridine (2). An azeotrope ( 1.0)
at 0.75 mole fraction water so cannot be separated by fractionation. 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1 2 3 4
x1 mole fraction in liquid
y1
mole fr
action in v
apour
Fig. 6.11 Ethyl acetate (1)/p-xylene (2). At total reflux four theoretical stages are needed to split p-xylene from ethyl acetate yielding a xylene bottoms product of about 0.96 mole fraction and a distillate of 0.98 ethyl acetate. Of these, one stage is the reboiler.
until the composition of the material in the kettle corresponds to the composition in the reboiler of a continuous column fractionating the same original feed to the same purity specifications. The number of theoretical trays needed to produce a required distil- late from a given feed is thus, at the start of a batch distillation, less than that needed in a continuous one. Many solvent recovery operations cannot benefit from the long steady-state runs typical of continuous operation, because the necessary quantity of consis- tent feed is not available. The possibility of achieving a separation with a smaller number of trays makes batch distillation attractive in these circumstances.
Against this advantage there must be set the disadvantages of batch distillation:
•
Because operation is not steady state, much more attention must be devoted to running the plant or on-line analysing equipment must be fitted and depended upon.•
Longer residence times for much of the batch at high temperature can lead to decomposition and polymerization of components of the feed. Apart from reducing yield and creating impurities not originally present, this may increase the risk of an exothermic reaction. Because of the larger hold-up in the equipment, the energy and pot- ential damage of such a reaction is liable to be greater in a batch than in a continuous plant.•
Batch distillation tends to produce intermediate fractions because of the unavoidable hold-up in the system. These have to be recycled in subse- quent operations reducing the net size of charges.•
The ‘housekeeping’ involved in batch operation, charging the still and removing quantities of hot residues reduce the available running hours and require operator’s time and attention.On the other hand, a continuous column provides the flexibility of splitting the available fractionating power into any ratio of stripping to enriching, subject only to the availability of a feed point at the correct position.
Because of the small amount of stripping power available, it will often be difficult to strip the last of a volatile component from the residue in a batch still. In solvent recovery practice, this shortcoming is not as serious a disadvantage as in the production of new solvents. When producing new solvents, one is frequently dealing with a mixture of homologues (e.g. benzene, toluene and xylene) with values of near 1.0. In solvent recovery it is much more com- mon to be dealing with mixtures of chemically dis- similar compounds which have comparatively high values of. The vapour pressure of volatile impur- ities in residues tends to be non-ideal and much higher than Raoult’s law would predict. Stripping is therefore much easier and the reliance on a single stripping stage not as restricting as conventional practice derived from solvent production would lead one to expect.
Once it has been established that a column, whether batch or continuous, is capable of making a
Table 6.3 Choice of batch or continuous fractionation Campaign size Short Batch
Long Continuous
Shift operation Day work Batch Round-the-clock Continuous Feedstock Binary Continuous
Multi-component Batch Separating power Ample Continuous
Barely enough Batch Heat economy Important Continuous
Trivial Batch Feedstock stability Unstable Continuous
Stable Both suitable Residue discharge Difficult Batch
Easy Continuous
Residue as % Small Batch
of feed Large Continuous
Stripping Easy Both suitable Difficult Continuous Enriching Easy Both suitable
Difficult Batch Vent discharge Large Batch
Small Continuous Maintenance Good Continuous
standards Only fair Batch Size of tank storage Ample Both suitable
Tight Batch
Number of tanks Many Batch
Few Continuous
Azeotropic Both suitable
capability
Extractive Continuous distillation
separation at total reflux and, therefore, that a simple fractionation is possible, considerations of economics and available capacity need to be made. This involves estimates of the second parameter in making a binary separation, the reflux ratio.
To achieve a separation one needs to use at least a minimum reflux ratio (Rmin). Using simplifying assumptions of constant volatility and equal molar latent heat for the components,
(6.7)
where xFis the mole fraction of the lighter compon- ent in the feed in the case of continuous operation, or in the still for a batch plant. When a high degree of purity is needed (e.g. xT 0.995), the equation can be reduced to
(6.8)
This would indicate that it is impractical to achieve the purity of product at the same yields by batch as it is for continuous distillation when the relative volatility for the system lies in the middle range. Substitution of a higher value for brings the value of Rminat the end of a batch separation down to a more practical level.
Common operating practice usually involves set- ting the reflux at about 1.25Rmin. Once the values of
Nminand Rminare known, the Gilliland correlation between reflux ratio and number of theoretical stages allows the reflux ratio to be worked out for a column with a known number of trays (N):
(6.9)
Example 6.2
For a continuous column of 20 theoretical stages, what reflux ratio will be required for the separation where Nmin 8.36
R 1.21
It is very important to note that the calculation of
Rminand R is only valid if the column feed in a con- tinuous column is put into the correct position in the column. All trays between the actual feed point and the correct feed point are ‘lost’. This is one rea- son why the nearly complete flexibility that a tray column provides on feed point choice is valuable when the column is to be used for a variety of feed- stocks, some of which cannot be specified when the column is designed.
A packed column, whether filled with random or ordered packing, can only have a feed point where there is redistribution and at most every eight or so theoretical stages. The average loss of fractionating power due to malposition of the feed is therefore on average four or more, which may be significant in a short column.
The optimum position of a liquid feed point is where the composition of the feed is the same as that of the liquid leaving the feed tray or, if the feed is a vapour, the vapour leaving the feed tray.
Solvent recovery poses some fractionation prob- lems that are not often encountered when process- ing unused solvents. It is not uncommon, in both batch and continuous operation (although less in the latter because of the shorter residence time at boiling point), to find that the distillate is contam- inated with breakdown products. These may be from hydrolysis or decomposition of the solvents them- selves, or from solutes derived from the process in which they have been used. Aldehydes, which impart an unacceptable odour to the recovered solvent, are also sometimes present. Traces of water in hydrocar- bons or chlorohydrocarbons may also be found at the column top.
Provided that the column has more than enough separating power for the main fractionation it needs to do, it is often of benefit to take the main product off as a liquid side stream four or five actual trays from the column top. The top trays can be operated at total reflux with occasional purging when the concentration of the light impurities begins to spread down the column or starts to interfere with the effective working of the condenser. In the case of water, a small phase separator will prevent the water returning to the column, although this may need to treat the water as a top or a bottom phase depending on the density of the organic distillate.
20 20 0 5668 8.36 1 0.75 1 1 1 R R . N N N R R R 0.75 1 1 min min 1 0 5668 . R x min F 1 ( 1) * R x x x x min T F T F ( 1) 1 1 * * * 1
The side stream will be in equilibrium with the vapour leaving the same tray. As a result, the pro- duct will contain impurities to the extent of the relative volatility and concentration of product and contaminant in the column vapour at the product take-off tray.
The technique may be extended on batch stills to avoid the major disadvantage of their operation. A batch still can, in theory, make a series of pure pro- ducts whereas a continuous column can only pro- duce at best two pure products, tops and bottoms, although side streams containing concentrates of components can be taken off both as liquids and vapours. However, the column top must have a liquid hold-up in the condenser, reflux drum, phase separator, vent condenser and other vessels, together with their interconnecting pipework. At the point in the batch distillation when one product has almost all been distilled off and the subsequent one is reaching the column top, there is inevitably a mixing of the two, leading, if the product specifications require nearly complete separation, to the produc- tion of intermediates which have to be recycled to the feed tank. Design of the column top to include a partial condenser (otherwise known as a dephleg- mator) to reduce the column top hold-up and elim- inate the reflux lines can reduce the volume of the top works at the same time as it adds an additional separation stage to the column. Such a design effect- ively prevents a phase separator being installed which reduces the column top volume, but also reduces the flexibility of the plant as a whole.
Provided that adequate fractionation power exists for separating a second product from the third, or from the residue if no third product is required, it is attractive to take the second product as a liquid side stream at, say, the column mid-point. The upper half of the column then concentrates any traces of the most volatile product at total reflux. Thus, no still time is wasted on taking an intermediate fraction, which usually requires much testing, tank changing and labour-intensive plant operation.
There is no theoretical reason why a third take-off even lower down the column should not be installed for a further distillate fraction, but the increased complication would seldom be justified.
The satisfactory operation of such a system depends on there being no failure of boil-up so
that the material held at the column top does not fall down the column and reach the side stream take-off and spoil the product being taken off there. A tem- perature control linking a point in the column with a stop valve on the side stream is a desirable safety feature (Fig. 6.12).
If a series of batches of the same feedstock is planned, the column top will be left at the end of a batch at a suitable composition for turning into a product tank very shortly after the commencement of the next batch, since no ‘heavy’ material ever reaches the column top.
The other major operational disadvantage of a batch still is its lack of stripping plates, although this, too, can be partially overcome with the use of a connection at or near the column mid-point. The conventional way of starting a batch charge is to fill the kettle with feedstock and to commence boiling. If the feed is pumped not to the kettle but to, say, the column mid-point and boiling is commenced in the kettle as soon as the coils are covered or the circulat- ing pump can be primed, the vapours meeting the
Reflux Tops product Side Feed stream product Residue L.C. T.C. 0
feed descending the column will strip out the most volatile components of the feed. Not only will this provide a stripping action for producing the first fraction, but it will increase the size of the batch, since accommodation for most of the first fraction need not be found in the kettle. Thus, if the first fraction is a large proportion of the feed, a batch may be doubled or enlarged even more in size. Here again, safe operation requires a level control on the kettle to cut off the feed when the kettle is full.
The combination of batch and continuous opera- tion that these two techniques provide is particularly applicable to a solvent recovery plant where the separation requirements may vary widely and be difficult to predict.