Treatment of organics containing water

Organic solvents miscible with water and containing from 2 to 50% of water can be dried by passage over a molecular sieve capable of retaining water after it has been dehydrated, at a superficial velocity of less than 15 cm/min and using columns having specified lengths relative to the mass transfer zone.

This invention concerns the treatment of organics containing water, and 
more particularly concerns the removal of water from organic solvents. 
The treatment of organic solvents containing from a few ppm up to 1% of 
water by passage through a column containing a molecular sieve, is known 
and is industrially practised. Waste solvents are thus dried to very low 
water contents and upgraded. The organic solvents treated are those 
miscible with water and they contain substituents such as oxygen, 
halogens, nitrogen and include certain sulphur-containing compounds. 
The value of organic solvents containing more than 1% water very much 
reduced and, depending upon the water content, they are distilled to a low 
water content at which they can be treated with a molecular sieve (subject 
to azeotrope formation) or they are used as low grade solvents, as fuel or 
even discarded completely. 
It has been proposed, in U.K. Pat. No. 1,193,127 (equivalent to French Pat. 
No. 1,542,755) to dehydrate industrial solvents using powdered molecular 
sieves with certain specified particle size and attrition index 
properties. An example is given of the dehydration of ethanol containing 
4% water, the ethanol-water azeotrope. This patent does not concern itself 
with the superficial velocity of the solvent over the molecular sieve, and 
it recommends the use of a moving bed sieve in which a portion of the 
sieve is continuously removed and regenerated. Details are given of the 
velocity of the regenerating gases but not, as has been said, of the 
velocity of the solvent to be tested; it is only stated that the liquid 
contact time should be "sufficient" to provide the desired degree of 
drying. 
U.K. Pat. No. 1,111,943 (equivalent to French Pat. No. 1,442,418) also 
proposes that a certain activated naturally-occuring molecular sieve can 
be used to dehydrate organic solvents, and gives better results than other 
commercial sieves, including Linde (Registered Trade Mark) Type 4A, 
particularly with regard to exhaustion values. Examples are given of the 
dehydration of ethanol containing 0.5% water and toluene containing 0.052% 
water. 
It is an aim of the present invention to dry organic solvents containing 
relatively large amounts of water. In particular, it is an aim to provide 
a process using molecular sieves in which an adequate capacity of the 
sieve for water can be utilised and which hence does not require 
continuous regeneration or a very short time between regenerations, which 
we believe to be the case with any prior proposed process. 
The present inventors have discovered that the previously held beliefs in 
the molecular sieve industry that the capacity of any sieve for water is 
low (in the examples of U.K. Pat. No. 1,111,943 capacities of 4 to 6% are 
observed) and that the capacity of the sieve for water is not dependent 
upon the water content of the feed, are not accurate. That is, it has been 
found that, by adjusting the superficial velocity of high water-content 
solvents, a short mass transfer zone can be achieved, and also the usable 
capacity of the sieve for water surprisingly is substantially increased 
with several attendant advantages. 
Accordingly, the present invention provides a method of drying an organic 
solvent miscible with water and containing from 2 to 50% by weight, 
preferably 4 to 25% by weight, of water, comprising passing the solvent 
through a column containing a dehydrated molecular sieve, at a superficial 
velocity of less than 15 cm/min and wherein the ratio of the column length 
to the mass transfer zone defined as the volume of liquid at between 5 and 
30% by weight of the feed water content passing through the column divided 
by the cross-sectional area of the apparatus is at least 4 to 1. 
The solvent is advantageously an alcohol, a ketone, an aldehyde, an ester 
or glycol or chlorine-substituted derivative, or an ether such as 
tetrahydrofuran providing it is miscible with water. The present invention 
is especially useful for the treatment of solvents which form azeotropes 
with water. A particular example is the removal of water from a 4% ethanol 
binary azeotrope, for example in the production of synthetic fuels by the 
route termed "biomass". The solvent need not be pure but could be a blend 
of compounds. 
Previously, manufacturers of molecular sieves recommended superficial 
velocities for the drying or organics of a minimum of 30 cm/min up to 
about 300 cm/min, whereas particularly suitable superficial velocities in 
the present invention are from 0.25 to 7.5 cm/min. Experimental work has 
shown that not only does the invention provide a method of reducing the 
water content of solvents from what was previously regarded as very high 
levels to about 0.1% by weight, but also these very low superficial 
velocities at least double the useful capacity of the molecular sieve, and 
thus in the method of the invention the mass transfer zone for transfer of 
water from the solvent to the molecular sieve is found to be short; this 
enables optimisation of the design of columns which contain the molecular 
sieves, within quite wide variations in diameter to length ratios, leading 
to advantages in the construction of plant. 
The molecular sieves for use in the present invention are conveniently the 
zeolites having an A-type crystal structure, for example those zeolites 
marketed under the designations A3, A4 and A5. referably an A3 zeolite is 
used. 
The method of the invention is suitably carried out under ambient 
conditions of temperature and pressure, but may, if required, be carried 
out at elevated or reduced temperature or pressure. 
For example, certain materials can be degraded when passing through a 
molecular sieve. The heat of absorption of water produced in the narrow 
mass transfer zones found in the present invention can lead to quite high 
temperatures, sometimes close to the boiling temperature of the solvent 
being processed. Furthermore, molecular sieves may catalyse decomposition 
because of their large surface areas and other characteristics. 
Tetrahydrofuran, for example, suffers from degradation in the method of 
the invention, but it has been found that cooling the column to maintain 
the temperature in the mass transfer zone to below 50.degree. C., 
preferably below 30.degree. C., is effective to reduce this effect. 
Cooling may be achieved by having a cooling water or forced air jacket 
around the column, and good results have also been achieved by dilution of 
the molecular sieve with an inert material, for example sand, arranged in 
alternate layers within the sieve or dispersed throughout the sieve. 
If desired, the product of the method of the invention can be dried 
further, that is "polished", by passage at conventional rates through a 
further column, which is suitably a type A zeolite, especially a type A3 
zeolite. 
The molecular sieve may be regenerated in conventional manner, by passing a 
hot gas through the column. 
The present invention will now be illustrated by reference to the following 
Example.

EXAMPLE 
Isopropanol containing 12.1% by wt. water was passed upwards through a 
conventional column containing a 1175 g bed of a type A3 molecular sieve 
at a superficial velocity of 5.1 cm/min, at ambient temperature and 
pressure. The product was isopropanol containing 0.1% by wt. of water. The 
ratio of column length to mass transfer zone was approximately 13:1. 
If the feed was passed through the column at the minimum rate recommended 
by the manufacturers of the molecular sieves, namely 30 cm/min, it was 
found impossible to reduce the water content of the accumulated product 
below 1% by wt, and the usable capacity of the sieve was only half that 
when using the present invention (16%). In the example of the invention, 
2000 ml of solvent product were obtained before the water content of the 
product exceeded 0.1%. 
Columns of a variety of dimensions up to and including pilot plant size, 
but all having a column length to mass transfer zone lengths (for the 
various solvents treated) of greater than 10:1 and usually 13:1, were 
filled with commercial type A3 molecular sieves. The following solvents 
were each passed through a column, at a superficial velocity of 5 cm/sec: 
Industrial Methylated Spirit (IMS, having an approximately 95:5 
ethanol:methanol ratio) and containing various water contents from 4 to 
10% by wt. of water; 
Ethyl acetate containing 3.1% by wt. of water; 
Ethanol containing 4 to 15% by wt. of water; 
Methyl Isobutyl Ketone containing 3 to 5% water by wt; 
Acetone containing 15% water by wt; 
Pyridine containing 23% water by wt; 
Tetrahydrofuran containing 3.4 to 10% water by wt. 
All the solvents were dried to water contents of less than 0.1% by weight 
in a single pass; the product pyridine contained less than 0.01% by weight 
of water. 
The method of the invention has been found to give excellent results in a 
particularly cost-effective manner for the drying of a wide variety of 
hitherto low-value or difficult to handle solvents. The control by the 
invention of the size of the mass transfer zone gives an excellent 
capacity for water, frequently five or six times that shown by published 
prior art processes.