2. DESARROLLOS EN REDES DE TELECOMUNICACIONES
2.1. REDES DE ACCESO Y DISPOSITIVOS TERMINALES
2.1.1. TECNOLOGÍA LOON
The composition of an azeotrope varies with absolute pressure. In water/solvent mixtures, where this effect is industrially important, the water con- tent of the azeotrope increases with increasing pres- sure. Thus, if two columns at different pressures are run in series (Fig. 7.6), a dry solvent can be made without the need for an entrainer. This can also be done on a batch still but for both continuous and batch operation the equipment is specialized and the hazard of handling flammable solvents at high pres- sure must be borne in mind.
A number of solvents are totally water miscible at ambient temperature but they are not very hydrophilic
which can be seen by their activity coefficients in water. Among these solvents are acetonitrile ( 32.5), THF ( 24.3) and MEK ( 27.2).
All of them can be dried using the fact that their aqueous azeotropes are very sensitive to temperature and can be processed by the pressure swing technique (Fig. 7.7).
ADSORPTION
A number of highly porous solids adsorb water pre- ferentially when contacted by wet solvent mixtures and can remove water to very low concentrations. While they can be used on a once-through basis they are capable of being regenerated for many cycles of reuse by heating and such regeneration is economi- cal for long-term operations.
Molecular sieves are available in a range of pore
sizes and this allows solvents with larger molecu- lar size to be excluded from the pores (Table 7.8). The larger the pore size, the greater is the water capacity of the molecular sieve, so it is desirable to
Table 7.8 Properties of molecular sieves of different pore sizes
Pore size (Å) Adsorbs Excludes 3 Water All other solvents
Methanol
4 Ethanol Butanol
5 n-Alkanes All iso compounds Benzene and all aromatics
11% water 19% water 19% water 100 psia 14.7 psia Feed 75% water Water Dry MEK Fig. 7.6 Using pressure distillation to dry MEK.
0.0 0.2 0.4 0.6 0.8 1.0 0 20 40 60 80 100 120 140 160 180 T emper ature ( C) x1
Fig. 7.7 Azeotropic composition of ACN/water vs.
use the largest pores that will not be taken over by solvent.
Thus all solvents except methanol can be dried, although the level of water content that can be achieved may vary. Subject to a laboratory trial, 50 ppm of water is a reasonable target.
Silica gel and alumina have similar properties to
molecular sieves but with larger pore sizes and therefore a higher loss of solvent, although this can be recovered during regeneration (Table 7.9). They also have less favourable characteristic curves.
The capacity of the molecular sieve is also fairly constant whatever the water content of the solvent whereas the capacity of silica gel is proportional to the water content of the solvent over the range 1–30%.
Regeneration in each case needs a hot, dry gas, preferably nitrogen. In most industrial applications molecular sieve regeneration needs electric or flue gas heating since no normal heating medium (steam, hot oil) will attain the regeneration tempera- ture. Nitrogen or some other inert gas must be used because of the necessary consideration of the solvent autoignition temperature. There is some evidence to suggest that autoignition temperatures are lowered when solvents are adsorbed on active surfaces so the risk of an explosion if oxygen is present may be more than would be estimated.
The lower regeneration temperatures for silica gel and, less so, for alumina help to make up for their poorer other properties.
In all cases some solvent will be present in the regeneration gases in the early stage of the heating process and it would be desirable in most cases to pass this gas through a carbon bed adsorber to reduce solvent losses and environmental pollution.
The inorganic adsorbents are resistant to almost all solvents although heating for regeneration may cause reactions leading to blockage of the pores.
Organic adsorbents of the ion-exchange resin type
are less inert and may be attacked by some solvents. They are, however, attractive for dehydrating:
•
Ion-exchange resins can be regenerated by heating to 120 °C and may be damaged if this temperature, easily achieved from industrial steam sources, is exceeded. Lower temperatures can be accepted if the regeneration takes place under vacuum. Air is an acceptable gas for drying in most cases.•
Non-polar solvents can be dried to less than 50 ppm. This can be particularly useful for drying chlorinated solvents.•
Capital cost of adsorbent per unit weight of water adsorbed is about half that of molecular sieves. The type of resin suitable for this application is Rohm and Haas Amberlite IR-120 and Dowex 50W-X8. Both are sulphonic-type exchange resins in their sodium and potassium form, respectively.Applications
Solvent drying by adsorption cannot be made into a continuous process easily and is usually a single bed batch process or a twin bed with one bed on stream while the other is being regenerated. Since the solv- ent wets the adsorbent a considerable amount of solvent vapour is generated while the water is de- sorbed and the effluent air or gas needs to be passed through an AC (or similar) bed.
Free water in the solvent to be dried may cause harm to a molecular sieve bed because of the heat generated when free water is adsorbed. This can be great enough to turn the adsorbed water into steam which can shatter the pore structure as it expands.
If a one-off drying operation has to be carried out, due perhaps to an accidental contamination of a storage tank or road tanker, molecular sieves can dry the tank at a cost of UK£5000–10 000 per tonne of water removed. A simple adsorption bed capable of removing, say, 200 litres of water can be moved by
Table 7.9 Comparison of properties of molecular sieve, silica gel and activated alumina
aThe diameter of a water molecule is 0.265 nm.
bThis is the water content when there is no competing solvent
present. Although silica gel adsorbs water preferentially it may well pick up less water in a given application than the more precisely ‘tailored’ molecular sieve.
crane or fork lift truck to a position at which a portable pump can recycle the tank contents through the bed. This avoids the need of road transport of the solvent to a drying facility and for ‘wet’ and ‘dry’ tanks.