Molecular Sieve Dehydration

Most efficient and easy to operate system.

Ethanol drying with molecular sieves

Water and ethanol form an azeotrope that limits how much water can be extracted by conventional distillation.

The Vogelbusch molecular sieve system allows the dehydration of ethanol past 95 % purity. It removes water from the ethanol/water vapor mixture that exits the rectification column to gain a dehydrated product. The dryness of this product can be tailored to meet specifications - anywhere from bioethanol with a water content of 0.5 % to super dry ethanol for pharmaceutical or industrial applications with a water content of 0.01 % or less.

Design options

Depending on the condition of hydrous ethanol feedstock and the presence of an alcohol distillation plant, there are two different design options for the dehydration unit: integrated or stand-alone.  

2 dehydration integrated2
Integrated drying units for vaporous feed

are linked to a distillation and receive hydrous ethanol vapors directly from the rectification column. The regeneration, or purge, stream is returned to the distillation for recovery of ethanol.

The greatest advantage of the integrated system is a considerable reduction in energy consumption when compared to uncoupled systems. The energy-efficient heat integration of dehydration with distillation/rectification/evaporation - a proprietary system introduced by Vogelbusch - also minimizes capital costs.

Feed requires a minimum pressure of 0.5 barg.

2 dehydration standalone2
Stand-alone drying units for liquid feed

are used for hydrous ethanol liquid from storage. The hydrous ethanol is vaporized in a small recycle column. The regeneration, or purge, stream is returned to the recycle column for recovery of ethanol.

The energy consumption of the ethanol drying unit is minimized by an optimal design of heat recovery under consideration of feedstock and utility conditions.

Process principle

Molecular sieve dehydration employs an adsorption process using synthetic zeolite, a crystalline, highly porous material. The process is based on the principle that zeolite's affinity for water changes at different pressures. The water loading of the zeolite depends on the partial pressure of the water in the feed which can be influenced by altering the pressure.

2 molsieve beds
PSA - Pressure swing adsorption process

The molecular sieve bed is fed with superheated vapor so that no condensation will occur. Water vapor is adsorbed into the pores of the zeolite, and ethanol vapor passes through the bed.

Once the molecular sieve bed is saturated with water and a breakthrough is imminent, it has to be regenerated: the water is desorbed from the zeolite by reducing the pressure applied to it.

Continuous production is realized by utilizing a pressure swing adsorption system with two molecular sieve beds. One bed is in dehydration service, while the other bed is regenerated under vacuum. During regeneration, bed pressure is lowered and the desorbed water is swept out of the bed with product vapors from the other bed in dehydration service. This regeneration, or purge, stream is then condensed and pumped to distillation for recovery of ethanol.

The process is fully automated.

2 zeolites
Zeolite testing

For best quality selection we examine molecular sieve materials of various manufacturers. In a standardized quick test developed by Vogelbusch, the key performance parameters are analyzed and compared. This quick test is done on every batch of molecular sieve material before it is loaded into a commercial molecular sieve unit.

2 Skid Cepsa
Skid-mounted installation

For ease of installation, and reduced field erection time and costs, Vogelbusch can supply prefabricated molecular sieve units. They are compact to ship and easily reassembled on site.

Read more about  » Skid-mounted solutions

2 molsieve tharaldson

Double set of molecular sieve beds with a daily dehydration capacity of 1.3 million liters ethanol.

Showcase projects including dehydration units


Frequently asked questions on the installation of a molecular sieve unit:

The economic minimum capacity of a dehydration unit is around 10,000 liters per day.

Capital expenditure always depends on plant capacity and configuration as well as local conditions. Costs for the process plant (excluding building, auxiliaries, infrastructure) are in the range of euro 500,000 to several millions.

This figure is for general reference only since each project has its own particularities that need consideration. 

Hydrous alcohol with a water content up to 15% can be economically processed in the molecular sieve. 

Feedstocks with higher water contents can be processed if a rectification column is added to the system.

Integrating a molecular sieve dehydration unit with the ethanol plant saves both energy and investment cost. Firstly, the heat of condensation of dehydrated ethanol can be used in the process, e.g. for pre-heating of mash or operation of an evaporation effect. Secondly, the regeneration, or purge, stream can be reprocessed in the existing rectification column and thus it is not necessary to add a recycle column. 

Standalone units are the choice for dehydrating purchased ethanol or for central processing of hydrous ethanol from several sources. They may also be an option if the available space within an ethanol plant is limited or if dehydration is carried out infrequent.

For super dry ethanol for pharmaceutical or industrial applications a standalone solution is recommended.

For the production of bioethanol, the potable alcohol plant can be adapted by adding an integrated dehydration unit. Concentrated ethanol vapor with sufficient pressure will be fed to the dehydration from a rectification or repurification column. The product specification of bioethanol allows to reduce the efforts for the removal of by-products compared to potable alcohol production, thus the rectification system will have some spare capacity to recycle the regeneration stream of the dehydration unit. The heat of product condensation can be used in an additional column reboiler for reduction of steam consumption.

Our compact skid-mounted designs include all process equipment within a structural steel framing.

They are easy to install and require minimum field connections thus reducing field erection time and costs.

We have supplied complete dehydration plants in skid modules with capacities up to 400,000 liters ethanol per day.

For larger capacities it is possible to have selected process components mounted on skids, including vessels, pumps and smaller heat exchangers, plus the interconnecting piping.

Zeolites are crystalline, highly porous materials made up of alumino silicates. These synthetic crystals are characterized by a three-dimensional pore system. The pores can be adjusted to precisely determined uniform openings, allowing for molecules smaller than its pore diameter to be adsorbed while excluding larger molecules - therefore they are referred to as molecular sieves.

Synthetic zeolites are available with openings in a range from 3 to 10 Angstroms (Å). In ethanol dehydration, 3Å zeolites (= with  a pore diameter of 3Å) are used to separate water from ethanol. The small water molecules with a diameter of 2.5Å are retained by adsorption within the crystals, but the larger ethanol molecules (4Å) cannot enter and, therefore, pass through the bed.

With well selected zeolite quality and considerate material handling - our process design keeps mechanical abrasion at a minimum - our customers report life spans of 15 years and longer.

Zeolites need to be mechanically stable and have to show a characteristic property profile, which is tested in our laboratories before loading of the vessels.

Our most important selection criteria for zeolites are high adsorption capacity of water and low co-adsorption of ethanol respectively high selectivity for water in the separation water/ethanol. 


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