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Starting Up New Molecular Sieve Beds in an Ethanol Unit

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Posted on : 25-07-2013 | By : Mr. Ethanol | In : 3A, Ethanol Industry, Industry Issues, Molecular-Sieve-Mavens
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13 Steps to Set Up Ethanol Plant Molecular Sieve Beds

 

Model scale of 3A molecular sieve in a vessel

Model scale of 3A molecular sieve in a vessel

 

3A molecular sieve is typically used in the final stage of ethanol dehydration in order to get ethanol to over 99% pure.  Getting your molecular sieve beds to run optimally can be a challenging process, so this list of steps has been compiled in order to help with this challenge.  Below are 13 steps that will help ethanol plant managers and operators set up their molecular sieve beds in their ethanol units.

 

1)      Different plants have different bed configurations and the procedure will vary slightly depending on the number of beds and the layout. Some plants have two beds, some three beds some six and some nine. In general the following principles hold for the conditioning of all layouts but the sequencing of conditioning may vary. The important concept is to maintain control of the heat generated during conditioning, this can be as much as 1800 btu/lb of water adsorbed. The temperature should never be allowed to rise above 500 degrees Fahrenheit.

2)      For maximum efficiency, the pressure in the sieve bed should be as high as the vessel construction allows with 10-15 degrees Celsius (50-60 degrees Fahrenheit) of super heat.

3)      Set the 1st bed to accept the feed with the product valve open. Set the next bed in line to pull vacuum down through the first bed to be conditioned. This helps pull heat and water vapor down through the first bed to speed and it balances the conditioning while beginning the conditioning of the second bed from the bottom.

4)      Feed to the beds should be set at 50-75% of normal operating flow rate. Usually a centrifugal pump is used to move the vapor to the beds. Sometimes there are limitations to the minimal amount of flow capacity from the pump. The initial feed should be set as low as possible, if the 50-75% rate is unachievable.

5)      Temperature monitoring is critical. Ideally thermal transmitters are available to allow monitoring in the control room.  Realistically, thermal readouts should be available for reading externally at the top 1/3 and the bottom 1/3 of the bed. Personnel should be stationed with communications equipment allowing information to be fed to the operator in the control room.

6)      Ideally the feed should begin with 200 proof ethanol to wet the sieve bed and the surface of the beads. This will help reduce the evolution of heat from the water adsorption on the external surface of the bead while creating a heat sink within the bed. The heat capacity of the bed itself and the beads is low. The pre-wetting of the beads will allow some of the heat to be adsorbed by the ethanol. Ethanol when adsorbed onto the sieve media will generate about 700 btu/lb alcohol adsorbed, which is 29% less of the heat generated when exposed to water. If 200 proof is not available feed can begin with 190 proof.

7)      Begin vapor feed at the recommended temperatures and pressures while pulling vacuum down through the bed into the second bed. Monitor the temperature at the top of the bed closely. When the temperature reaches 380 degrees Fahrenheit, cut feed to the first bed and begin feed to the second bed while pulling the vacuum on the first bed. Begin pulling vacuum on the third bed while pulling heat and vapor down through the second bed. If only two beds are available then open the first bed, while pulling the vacuum on the first bed, to the second bed. The vapor from the second bed will help cool the first bed. If this is a two bed system then you may get to a point where feed is cut while you are pulling vacuum on both beds waiting for them to cool.

8)      The temperature in the first bed should begin to drop while the temperature in the second bed rises. When the temperature in the second bed reaches 380 degrees Fahrenheit, cut feed to the second bed and pull the vacuum to remove heat and adsorbed moisture. If this is a two bed system then you may get to a point where the feed is cut while you are pulling the vacuum on both beds waiting for them to cool.

9)      Pull the vacuum on the beds until the temperature in the beds is below the vapor feed temperature. This will vary from plant to plant depending on the layout.

10)   When the temperature in the bed is low enough feed may resume to that bed.   In regards to temperature at the center of the bed, cut the feed to the bed when the center temperature reaches 380 degrees Fahrenheit. If no center bed temperature reading is available, then monitor the temperature at the bottom of the bed and cut the feed when the temperature at the bottom reaches 350 degrees Fahrenheit.

11)   Resume feed to the next bed to be conditioned while pulling the vacuum to the bed being cooled. if the temperature of the next bed to be conditioned is not close to the feed temperature continue to cool down with vacuum and wait.

12)   Repeat this sequence gradually increasing feed rate until the temperature throughout the bed has stabilized to the temperature of the feed.  At this point the beds are commissioned and ready to resume normal operation.

13)   The sieve beads can endure much higher temperatures than what is typically experienced in an ethanol plant, but this regimen is designed to protect not only the equipment, but the plant personnel from potentially dangerous temperature excursions.

 

Not every situation is the same – for assistance with your particular adsorbent/ethanol issue please contact Mark BinnsHengye USA Technical Business Director. 502-232-5256 and/or email at mbinns@hengyeusa.com

Mark has a degree in Chemical Engineering from the University of Louisville, and has specialized in working with industrial adsorbents and the ethanol industry for over a decade.

Molecular Sieve Questions: Finding Molecule Mass Sizes

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Posted on : 29-05-2013 | By : Mr. Green | In : 13X, 3A, 4A, Molecular-Sieve-Mavens
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Molecule Masses and Molecular Sieve Pore Sizes

 

Molecular Sieve

 

Awhile back we received a question on one of our blog articles that asked the following:

Can you provide sizes of molecules of Isopropyl Alcohol (IPA), Methylene Chloride (DCM) and Water for comparison with 3A and 4A sieves? Thank you.

We forwarded this to our onsite engineer, got the following answer, and decided to post it as a blog article for anyone who had similar questions about finding the volume of a molecule based off of density and mass.

The answer to this question is not as straight forward as it sounds.  There are a couple of different ways to look at it. You have to remember it is not only the size of the molecule but the shape. Isopropyl alcohol is a three dimensional structure while n-propyl alcohol is linear. Linear molecules can go into a zeolite pore “end first” so the only thing presented to the pore is the –OH group or the methyl group at the other end. There is also the  “radius of gyration” phenomenon to consider. Isomers are often separated from linear molecules using molecular sieve because the isomers are too big to fit while the linear molecules are not even though they have the same molecular weight and density.

 

1)      The average volume of a molecule based on density and mass.

Critical Properties for calculations

IPA

DCM

H2O

Molecular Mass, g/mol

60.1

84.93

18.02

Density, g/cm

0.786

1.33

1

Volume of a mol of a molecule

IPA

DCM

H2O

Volume: cm3/mol

76.46

63.86

18.02

Avagadros Number

6.02E+23

Size of a molecule, Avagadros number

IPA

DCM

H2O

Volume: cm3/molecule

1.27E-22

1.06E-22

2.99E-23

 

2)      Calculating the size from the size of the atoms and the average bond length.  The following was considered for these calculations “presented size”, formation, gyration, atomic radius, and bond length.  It’s important to remember that molecules change size and shape depending on temperature, pressure, electrical environment and gyrate or flex continuously.  Thus the end results can only be approximations due to these extraneous factors

 

Species

Approximate Angstrom

IPA

16.05

DCM

9.21

Water

2.63

 

With a 2.63 approximate angstrom size water can be adsorbed by any standard molecular sieve, while DCM can only fit in the pores of  a 13X molecular sieve.  IPA can’t fit in any standard molecular sieve pore sizes, but there are specially designed zeolites and molecular sieves that can be made that do.

 

How Industrial Adsorption Works and The Most Common Processes

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Posted on : 09-04-2013 | By : Mr. Ethanol | In : Molecular-Sieve-Mavens
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Understanding the Adsorption Process

 

 

Adsorption is the process of adhesion used by atoms and molecules to attach themselves to a surface.  It is a process that has been capitalized on by many industries and has become essential to producing many of the different everyday products people use.  Below is an, “Industrial Adsorption Course 101,” for people looking to learn more about adsorption and some of the common types of adsorption processes.

Below are some helpful definitions in bold:

Capacity – is defined by temperature and pressure.

Working capacity – in a regenerative process is defined by the difference in the adsorption state (Ta, Pa) and the desorption state (Tr, Pr).

The adsorption and desorption of any material in liquid of vapor phase is a state function.  Adsorption/desorption works the following way, while imaging a Carnot cycle.  The higher the temperature is the lower the adsorption capacity will be; the lower the temperature is, the higher the adsorption capacity will be. The higher the pressure is, the higher the adsorption capacity will be; the lower the pressure is, the lower the adsorption capacity will be.

There must be a change between two states for adsorption and regeneration to work – State 1. The states can be in the vapor or liquid,  but there will be no regeneration of the sieve without a state difference. The working capacity of the sieve is defined as the difference in partial vapor pressure of the two states of the material being adsorbed. Ethanol/water combinations (Daltons Law Pt = p ethanol + p water) are fairly well defined, but other systems with multiple constituents can get a bit more messy.

Liquid phase adsorption can be done, but the vapor pressure variations of most liquids over narrow temperature and pressure ranges are small.  The narrow temperature and pressure ranges decrease the working capacity, rendering the system inefficient.  Most  liquid phase adsorption processes will fit the description of  sacrificial adsorption process (listed below) because it is cheaper to replace the sieve, compared to the cost of energy that will be needed to regenerate it.

 

There are three basic adsorption processes as well as hybrid systems used for adsorption.  These are listed below:

1)      Sacrificial – you simply dispose of the sieve material after one adsorption cycle: Ta=Tr and Pa = Pr. Expensive solvent recovery would be one example of this.

2)      Pressure Swing Application “PSA” – Isothermal process where Ta=Tr and Pa >>>>>>Pr –Isotherms for the particular adsorbent and process material is used to define the process. Ethanol would be one example of this.

3)      Temperature Swing Application “TSA” – Isobaric process where Ta<<<<<<<<<<<<<<<<Tr and Pa=Pr – Isotherms for the particular adsorbent and process material is used to define the process. Natural gas would be one example of this.

 

Note One: There do exist hybrid systems which utilizes changes in temperature and pressure with or without vacuum.

Note Two: There are proprietary computer programs written by myself and others to calculate partial pressures for SOME multiple component systems. As you probably remember from thermodynamics fugacities, Van der Waal interactions, viscositys, density changes in multi component systems can get very tricky.

Molecular Sieve and Zeolites: Their Roles in the Fukishima Nuclear Disaster

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Posted on : 28-06-2012 | By : Mr. Green | In : Industry Issues, Molecular-Sieve-Mavens, Zeolites
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Fukushima Nuclear Disaster

Fukushima Nuclear Disaster Overview

On March 11, 2011 a 9.0 Earthquake, the fourth largest on record since 1900, occurred in the Pacific Ocean causing a series of large tsunamis to strike the eastern coast of Japan.  Now called the Tohoku earthquake, it (and the subsequent tsunamis) caused one of the worst natural disasters on record with over 15,361 people killed, a million buildings damaged or destroyed, and a financial cost of $235 billion dollars (the most expensive natural disaster on record).

In addition to the large amount of damage caused by this natural disaster, it also caused one of the greatest man made disasters on record at Fukushima.  Japan, a country that heavily relies on nuclear power, had a number of nuclear plants that were built near the eastern coast when the tsunamis struck.  Fortunately Japan, a country that sits near two fault lines and has a history with experiencing earthquakes, had containment measures set in place for just such an occurrence.  Once powerful enough earthquake tremors are recorded near Japan’s nuclear plants, they begin to shutdown and cool off.  For the most part this worked, except at Fukushima.

At Fukushima the plants power failed during the earthquake, so the emergency power system along with the emergency cooling condensers had to be used.  Less than half an hour after the emergency systems turned on, tsunamis began to strike the coast near the Fukushima power plant.  A seawall 19 feet high was put in place around the plant to protect it against tsunamis.  The waves from the tsunamis that hit Fukushima were over 46 feet tall, rendering the seawall worthless.  After crashing over the seawall, this wave not only destroyed backup power to the plant but also wiped out key equipment that was part of the emergency core cooling system.

The destruction of key cooling equipment by the tsunami triggered full meltdowns of reactors 1, 2, and 3, thus beginning what we now call the Fukushima Nuclear Disaster.  After the earthquakes subsided, cleanup began and radioactive damage was assessed.  High levels of  Caesium-134, Caesium-137, and some other radioactive isotopes were detected around the nuclear plant and in the ocean.   Currently efforts are on-going to clean up the radioactive waste as a result from the meltdowns.

Molecular Sieve and Zeolites Role in Clean Up

The Fukushima disaster is only the second nuclear disaster (the first was Chernobyl)  to receive a 7 on the International Nuclear Event Scale (INES), the highest disaster rating a nuclear event can be rated.  The clean up process will see new technological developments as well tried and true methods during the clean up of radioactive waste.

One method that’s currently being used, and was used in the past, is using zeolites.  Shortly after the disaster, the Japanese government began to order the dropping of zeolites in the oceans surrounding the disaster site.  The Japanese government is hoping that zeolites (the one’s that the Japanese government are using have specialized in nuclear waste processing), will help to slow down radioactive contamination of the ocean.  Zeolites had previously been used in the clean up of the 1979 Three Mile Island Nuclear Disaster in the United States.

Although dropping zeolite in the ocean seems like a desperate attempt to contain the disaster, Japan is also utilizing  molecular sieve in the clean up process, too.  The molecular sieve in use was specifically designed to capture Caesium, and is being used to treat radioactive wastewater that is on the disaster site. Since the disaster began over 43 million gallons of wastewater have been treated with this molecular sieve at Fukushima.

Experts expect the Fukushima disaster clean up to last decades.  As the clean up continues adsorption technology will continue to play an important role in cleaning up the oceans, environment, and reducing the amount of damage that will be done to our atmosphere.

 

Sources:

https://share.sandia.gov/news/resources/news_releases/fukushima_cleanup/

http://www.japannewstoday.com/?tag=fukushima-zeolite-absorbs-radiation

http://www.emfnews.org/Fukushima-Decontamination-and-Zeolite.html

http://www.world-nuclear.org/info/fukushima_accident_inf129.html

http://earthquake.usgs.gov/earthquakes/eqarchives/poster/2011/20110311.php

 

The Importance of Surface Area in Adsorbents

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Posted on : 10-05-2012 | By : Mr. Green | In : Industry Issues, Molecular-Sieve-Mavens
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Large Surface Area is Key to a Valuable Adsorbent

Electron-Microscope-Zeolite-Picture

Why is surface area key to a quality adsorbent?

Before we talk about surface area it’s helpful  to understand how adsorption works.

Adsorbents work by adsorbing liquids or vapors into pores on their surface.  The adsorption process doesn’t truly absorb the vapor or liquid that’s running through it (meaning the the liquid or vapor isn’t turned into a solid with the adsorbent),  rather molecules from the vapor or liquid are adsorbed and thus they get stuck on to the adsorbent.  In short an adsorbent acts like a magnet.

The pores on an adsorbent are where adsorbed molecules are kept.  The pores can have diameters between a couple of nanometers to hundreds of nanometers.  The purpose of the pores is to not only store molecules but sometimes to separate certain molecules by size.  The pore sizes can differ by nanometers or Angstroms (1 Angstrom = 1/10,000,000,000th of a meter) so you can separate liquids and gases at a molecular level.

For example if you wanted to separated methane from water you would use a 3A molecular sieve because the pore size on 3A is 3 Angstrom.  Water molecules have diameters up to 2.9 Angstrom and methane molecules have diameters up to 3.8 Angstrom.   The molecular sieve adsorbs the water and doesn’t adsorb the methanol thus separating the two molecules from one another.

Surface area measures how much exposed area there is on solid objects.  It’s important to distinguish that surface area and volume are not the same.  As long as the width, length, and height of an object remain the same the volume will never change.  Surface area, on the other had, can change if you break the object into smaller pieces.  See the example with the cube below.

Surface Area

Surface Area of a Cube = l*w*6

Volume of a Cube = l*w*h

 

 

Cube Length: 10mm

 Cube Width: 10mm

 Cube Height: 10mm

 


 

Cube Volume = 10*10*10=1,000mm3

 Cube Surface Area = 10*10*6=600mm2

The volume of an object will remain the same, but surface area can expand.  For example if you break the cube above into 5 parts you would find the following.

 


 

 Length: 10mm

  Height: 10mm

  Width: 2mm

Number of Cube Shaped Boxes: 5

 

Cube Surface Area:

 (2*10*10) + ( 4*2*10)*5=1,400mm2

 Cube Volume: (2*5)*10*10=1000mm3

 

By breaking the cube up into smaller sections, the surface area of the cube increases while the volume remains constant.

Surface area in adsorbents can be large.  1 gram of activated carbon for example has a surface that’s usually around 500m2

The pores on most adsorbents go only a few molecules deep so what you need is a lot of pores if you want to adsorb a lot of material.  Since pores are on the surface that is why you need a lot of surface area.  More surface area means more pores which means more liquid/gas is adsorbed.

 

Sources:

Size of methane molecule,  Slide 16 http://www.epa.gov/lmop/documents/pdfs/conf/12th/gladstone.pdf

Size of water molecule http://www.mc3cb.com/pdf_chemistry/What%20is%20the%20diameter%20of%20a%20water%20molecule.pdf