Molecular Sieve Dehydration Units work on the principle of adsorption. Molecular Sieve Dehydration units typically have higher initial capital investments than comparable glycol units but also are able to achieve very low dew points which are required for cryogenic plants. Additionally, Molecular Sieve Units can also handle large flow variations as well as higher inlet gas temperatures.
Where very low water dew points are required molecular sieve dehydration is the preferred technology over glycol dehydration. Certain applications, such as LNG processing, are suited to the use of solid desiccant adsorbents like molecular sieves and silica gels.
Custom designs and builds each system as a complete turn-key package with particular emphasis given to:
Discharge gas moisture content and Optimized absorption cycles
Reduced maintenance via the use of specialty equipment, such as Rising Stem Switching Valves Minimum power consumption,
Optimum plant efficiency and design integrity
Compliance with SHEQ requirements
Environmentally conscientious design
Drying Liquids and Gases
Alcohols (Methanol, Ethanol, Isopropyl, etc.) and other solvents (Toluene, etc.)
Most alcohol dehydration for production is based on distillation with packed or trayed columns which include reflux condensers and re-boilers. Depending upon the starting concentrations and final requirements, the column design is determined by the number of theoretical plates (column height), versus the amount of reflux and reboiling (energy input). Although there are limits with azeotropic mixtures (such as the ethanol water azeotrope), distillation has been a standard in the processing industry for many years. However, because this is a thermal separation process, the energy consumption is a major factor in the operation.
When the water content is below 15% and there is a very dry requirement, utilizing molecular sieves (or mole sieves) are an attractive alternative to distillation. Molecular Sieve adsorption has proven to provide a very dry (even anhydrous) product, while drastically reducing energy cost with minimal operator supervision. Molecular Sieves can offer significant energy savings (up to 40%) compared to distillation, especially when a very dry result is required.
Azeotropes leave the equation
For alcohol azeotropes such as the ethanol-water azetrope or IPA-water, or solvent azeotropes such as MEK-water, simple distillation is no longer an option and more complicated 'azeotropic distillation' methods are required to dehydrate past the azeotrope. Carcinogenic entrainers have been a common but risky solution.
Because certain molecular sieves preferentially adsorb water, water is adsorbed in to the sieve as vapor or liquid passes thru a column. As the vapor or liquid continues to move through the column, more water is removed, resulting in a dryer alcohol until it is fully dehydrated. Over time, the column will reach a saturation limit, at which point the flow is diverted to a second (dry) column, and the first column is then regenerated.
When properly designed, a molecular sieve system can selectively separate water from other molecules such as alcohols (methanol, ethanol, IPA, etc) and other solvents (Ethyl Acetate, Toluene, etc). The water holding capacity of the molecular sieve is dependent upon several variables, but typically has a maximum capacity of 22 wt% water per unit weight of mole sieve.
(MSDU) Gas Phase Molecular Sieve Dehydration wit pressure adsorption and vacuum (PSA/VSA) swing desorption (for 2-10% water)
This design passes the wet alcohol (or solvent) stream in vapor state at a controlled pressure through a column filled with molecular sieve beads. As the vapors pass through the bed of molecular sieve beads, water is drawn into the internals of the beads by polar attraction, leaving the remaining vapors dehydrated as they exit the column. After a period of time, the gas flow is diverted to a second column to allow the process vapors to be continuously dehydrated, while the first column is regenerated.
Regeneration in this case is accomplished by reducing the pressure and/or by pulling a vacuum on the column, while using a purge gas to help strip the water out of the molecular sieve beads. Some designs utilize a portion of the dehydrated alcohol as the purge gas, which can be as high as 30% with some competitors, which is then required to be reprocessed through distillation or simply wasted.
KASRAVAND MSDUs are high efficiency designs providing:
- Recovery rates of 90-95%
- Low Energy Use
- Minimal Labor/operator intervention
- Designed to handle a wide variation in feed concentrations (0-10% water, 15% possible).
- High Purity:
- Standard Dryness: 99.5% dehydration
- Extra Dry: 99.9%
- Ultra Dry: 99.95%
- Process from ½ gpm to over 25 gpm of wet alcohols/solvents
- Packaged Units, fully piped, wired
- Basic process is very simple, reducing labor and training.
(MSDUL) Liquid Phase Molecular Sieve Adsorption with thermal regeneration (for < 2% water)
This design passes a liquid stream of wet alcohol or solvent through a column filled with molecular sieve beads. As the liquid passes through the bed of sieve beads, the water molecules are sucked into the beads due to polar attraction. As the bulk liquid continues through the column, more and more of the water is adsorbed onto the internal surfaces of the molecular sieve, until essentially all of the water is removed and the dried alcohol (or solvent) exits the column. This process continues until the sieve becomes saturated with water, at which time the sieve must be changed, or regenerated. Regenerating the sieve typically involves 1) draining the liquid, 2) heating the sieve beads, 3) gas purging either at pressure or vacuum, and lastly, 4) cooling the sieve.
Heating the mole sieve beads is many times done by purging hot dry gas (Nitrogen or other) thru the bed, or by circulating a hot gas with a partial purge. Both are variations of an “externally” heated design.
- Externally heated designs suffer from long regeneration times due to long heat up and cool down requirements. The BTU required to heat up bed including the vessel, drive off any wetted solvent, and then regenerate the mole sieve beads is only coming from the heat content of the gas each time it circulates thru the bed and back to the external heat exchanger.
- This design also suffers from higher emissions, even though condensers are used, due to relatively high purge gas rates which are exhausted with saturated vapors.
KASRAVAND unique MSDUL Liquid Phase Molecular Sieve design utilizes internal heating and cooling which offers many advantages:
- Very Dry Product to less than 50 ppm
- Very low energy
- Very high product recovery is possible due to very low purge gas requirements: >99% recovery
- Very high heating efficiency with short gas path for heat transfer: reduces heat up time
- Very high cooling efficiency with short gas path for heat transfer: reduces cool down time
- High processing rates due to short regeneration times
- Short heat up time + Short Cooling time = higher processing rates
- Packaged Units, fully piped, wiredPackaged Units, fully piped, wired
- Fully automated, reducing labor and training.