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Absolute alcohol is an important product required by industry.  As per IS Specification it is nearly 100% pure / water free alcohol.  Alcohol as manufactured is rectified spirit, which is 94.68% alcohol, and rest is water.  It is not possible to remove remaining water from rectified spirit by straight distillation as ethyl alcohol forms a constant boiling mixture with water at this concentration and is known as azeotrope.  Therefore, special process for removal of water is required for manufacture of absolute alcohol.
In order to extract water from alcohol it is necessary to use some dehydrate, which is capable of separating, water from alcohol.
Simple dehydrate is unslacked lime, Industrial alcohol is taken in a reactor and quick lime is added to that and the mixture is left over night for complete reaction.  It is then distilled in fractionating column to get absolute alcohol.  Water is retained by quick lime.  This process is used for small-scale production of absolute alcohol by batch process.

The process used for dehydration of alcohol using molecular sieves is as follows:
The salient features of the process are given herewith:

I)  Dehydration with Molecular Sieve Process
The rectified spirit from the rectifier is superheated with steam in feed super-heater.  Super-heated rectified spirit from feed super-heater is passed to one of the pair of molecular sieve beds for several minutes.  On a timed basis, the flow of superheated rectified spirit vapor is switched to the alternate bed of the pair.  A portion of the anhydrous ethanol vapor leaving the fresh adsorption bed is used to regenerate the loaded bed.  A moderate vacuum is applied by vacuum pump operating after condensation of the regenerated ethanol water mixture.  This condensate is transferred from recycle drum to the Rectified Column in the hydrous distillation plant Via Recycle pump.  The net make of anhydrous Absolute alcohol draw is condensed in product condenser and passed to product storage.
The life of molecular sieve may be around five to seven years.  However, the operating cost is considerably less than azeotropic distillation.


Most of the ethanol dehydration plants for production of absolute alcohol are based on Azeotropic distillation.  It is a mature and reliable technology capable of producing a very dry product.   However, its high capital cost, energy consumption, reliance on toxic chemicals like benzene and sensitivity to feedstock impurities, has virtually eliminated the use of azeotropic distillation in modern ethanol plants.  Benzene has been used as entrainer of choice of ethanol dehydration but it is now known to be a powerful carcinogen.

Advantages of MOLECULAR SIEVE technology for ethanol dehydration are as follows:-

1.        The basic process is very simple, making it easy to automate which reduces Labour and training requirements.

2.        The process is inert.   Since no chemicals are used, there are no material handling or liability problems, which might endanger workers.

3.        Molecular sieves can easily process ethanol-containing contaminants, which would cause immediate upset in an azeotropic distillation system.  In addition to ethanol, a properly designed sieve can dehydrate a wide variety of other chemicals, thereby providing added flexibility in future operating options.

4.        The molecular sieve desiccant material has a very long potential service life, with failure occurring only due to fouling of the media or by mechanical destruction.  A properly designed system should exhibit a desiccant service life in excess of 5 years.

5.        It can be configured to function as a stand-alone system or to be integrated with the distillation system.  This lets the customer make the trade-off between maximum operating flexibility versus maximum energy efficiency.

6.         If fully integrated with the distillation system, steam consumption rate only slightly above the absolute theoretical minimum for the separation can be achieved.

7.         A properly designed molecular sieve can reliably dehydrate 160-proof ethanol to 190 + proof, making strict control of rectifier overhead product quality unnecessary.

From Feed Tank, rectified spirit is pumped to the Stripper / Rectifier Column.  A partial steam of vapors from the Column are condensed in Condenser and sent back to the column as reflux.  Rest of the vapors are passed through a super-heater and taken to the Molecular Sieve units for dehydration.  The vapor passes through a bed of molecular sieve beads and water in the incoming vapor stream is adsorbed on the molecular sieve material and anhydrous ethanol vapor exists from the Mol. Sieve Unit .
Hot anhydrous ethanol vapor from the Mol. Sieve Units is condensed in the Mol. Sieve Condenser.  The anhydrous ethanol product is then further cooled down in the product cooler, to bring it close to the ambient temperature.
The two Mol. Sieve units operate sequentially and are cycled so that one is under regeneration while the other is under operation, adsorbing water from the vapor stream.  The regeneration is accomplished by applying vacuum to the bed undergoing regeneration.  The adsorbed from the molecular sieves material desorbs and evaporates into the ethanol vapor stream.  This mixture of ethanol and water is condensed and cooled against cooling tower water in the Mol. Sieves Regenerant Condenser.  Any uncondensed vapor and entrained liquid leaving the Mol. Sieve Regenerant Condenser enters the Mol. Sieve Regenerant Drum, where it is contacted with cooled regenerant liquid. 

The cooled regenerant liquid is weak in ethanol concentration, as it contains all the water desorbed from the Molecular Sieve Beds.  This low strength liquid is recycled back to the Stripper Column for recovering the ethanol.  The water leaves from the bottom of the column and contains only traces of alcohol. 

ADVANTAGES OF THE SYSTEM ( molecular sieves )

                     Minimal Labor

                     Stable operation

                     Near theoretical recovery

                    Steam consumption minimized by multi-stage preheating to permit substantial heat recovery and reuse.

                     An advanced control system, developed through years of experience, to provide sustained, stable, automatic operation.


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