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.

Large Surface Area is Key to a Valuable Adsorbent

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

 

 

   

8 Steps to Prepare Your Vessel for Unloading Molecular Sieve 

 

You have decided to replace your molecular sieve, and now it’s time to load in what may be thousands upon thousands of pounds of molecular sieve in your vessel (of course the amount of sieve you load in depends on the size of your vessel).  What can you do to prepare your vessel before unloading all of this sieve?

In order to help you with that question we have prepared 8 useful guidelines that could help prepare your vessel for sieve unloading.

Note: These guidelines are to be carried out before you load your sieve into your vessel. 

1)      Before unloading the sieve you should regenerate your vessel by heating and cooling with process gas. Use the same operating conditions that you would normally use when regenerating your bed.

2)       If process gas is not available use nitrogen or another non-toxic gas instead.  Do not use any gas that contains any toxic components at hazardous levels to regenerate your vessel.

3)      After heating the sieve beds, cool them with gas by de-pressurizing the bed to flare.

4)      After using process gas you can start purging the vessel with inert gas at ambient temperature to flare.  It is important that the gas flow rate be sufficient enough to have good distribution inside the bed.

5)      It’s recommended, if you want to be very thorough in the purging process, to pressure up the bed and de-pressure to flare 2 to 3 times.

6)      When outlet gas is 50% below the L. E. L. and free of toxic materials the purging process should be complete.  Once purged the bed is ready to have the molecular sieve dumped inside.

7)      Unloading the sieve is done from the bottom dump port (or manway) with the flow of gravity guiding the sieve to the bottom.

8)      If you decide not to unload the sieve through the bottom dump port then you can unload the sieve with a vacuum hose from the top port.  Bins containers or dumpsters can be used to aid you.

Here are some additional things to consider…

Never enter a vessel that contains used molecular sieve.

During the unloading process the molecular sieve may have adsorbed chemical compounds.  These adsorbed chemicals may be desorbed again when the molecular sieve is exposed to open air, especially if humidity is high or the air is very moist.  

These desorbed chemical compounds can create hazards if the desorbed chemical compounds are toxic.   The plant manager or operator has the responsibility to know what chemicals may have be desorbed in this manner and to know what precautions may be necessary to ensure everyone’s safety.

How to Figure Out the Efficiency of Your Molecular Sieve Beds

This article will explain how to figure out the working capacity in an ethanol plant.  Using the Measuring-Ethanol-Working-Capacity document I attached to my previous article Top 6 Signs It’s Time to Change out the Sieve in Your Ethanol Plant I will offer a more thorough explanation (or walk-through) on the attached document that describes how to figure out the working capacity of your ethanol plant. 

Ultimately the purpose of making these calculations is to let you know how efficient your sieve beds are running, a good working capacity for an ethanol plant is between 0.7% and 1.0%, the higher the percentage the better.  If your sieve bed is running at a lower working capacity than 0.7%, then it could be a sign that your sieve beds may need to be changed out. 

Looking at the example on the attached document here is how these numbers came to be.  (These numbers are used strictly as an example and are not representing an actual ethanol plant, however the mathematical process is still the same). 

In the example you start with following numbers:

Feed rate of 300 gallons a minute

Feed concentration of 188 proof, or 94% ethanol

Sieve bed load of 2000 pounds

Product concentration of 99.5% ethanol

I will explain this process in reverse order, which makes it easier to explain what/why we are making each of these calculations and then I will take you through the example in a straight forward order with the numbers given in the example. 

In order to find the working capacity you need to know how much water is getting adsorbed in the sieve bed and divide it by how much sieve is in the bed or…

Working Capacity= Water adsorbed in bed/lbs of sieve

Since we know how many pounds of sieve we are using, we need to figure out how much water is being adsorbed in the sieve beds.  That can be determined by subtracting the water that’s remains after running  ethanol/water based gas through the sieve beds (water out in product) from the water going in to the product (water in) or…

Water adsorbed in bed= Water In – Water Out in Product 

However, we need to figure out the water in and water out in product.  Before you can figure this out you need to know how much ethanol is being run through your sieve beds.  In order to figure out how much ethanol is going in you have to multiply the percentage of ethanol in your feed concentration by how many gallons of liquid you are putting through the sieve beds total, the equation looks like this. 

Ethanol In= Ethanol Feed Concentration % x Feed Rate

You can now figure out the water in which is figured by subtracting the number of gallons of ethanol from the feed rate or…

Water In=Feed Rate- Ethanol In 

Water out in product is figured by multiplying product concentration to ethanol in.  The resulting number from this equation is subtracted from ethanol in.  

Water Out in Product = Ethanol In- (Product Concentration x Ethanol In)

Now that we have all the parts you need to figure it out here is how it will look with the example.

1. Ethanol In= Ethanol Feed Concentration % x Feed Rate (0.94×300=282) Ethanol In=282 gallons/min.
2. Water In=Feed Rate- Ethanol In (300-282=18) Water In=18 gallons/min.
3. Water Out in Product = Ethanol In- (Product Concentration x Ethanol In) ((282-(0.995×282)) Water Out in Product= 1.41
4. Water Adsorbed in Bed= Water In – Water Out in Product (18-1.41=16.59) Water Adsorbed in Bed = 16.59 gallons/min.
5. Working Capacity= Water Adsorbed in Bed/lbs of sieve (16.59/2000=0.008) Working Capacity = 0.008lb of water/lb of sieve or 0.8% Working Capacity.

 We can conclude from this example that with a 0.8% working capacity the sieve beds are in good working order, and that a change out would not be necessary.

Activated Carbon/Charcoal is What Separates Tennessee Whiskey From Bourbon.

 

The Lincoln County Process is the process by which Tennessee Whiskey manufacturer Jack Daniels and George Dickell make their famous whiskey’s.  This process is what distinguishes Tennessee Whiskey’s from Bourbon Whiskey’s and key difference in their creation is the use of charcoal, or activated carbon in the Lincoln County Process.  The use of charcoal is an added step not included in manufacturing bourbon, which otherwise would be exactly the same.

The charcoal used by Jack Daniels is made on site from sugar maple trees.  These trees are cut down and burned, but their temperature is controlled so that the don’t burn to ash but rather turn into charcoal.  The newly created charcoal is placed in a vat along with unaged whiskey for ten days before it is stored in a barrel to age further.

The taste of Tennessee whiskey has a smoother and smokier taste to it, where as bourbon is known for having a harsher and stronger taste.  This extra step was added to improve the very harsh taste of whiskey’s in the 19th century.  The difference in taste is due to the added step of using charcoal in the Lincoln County Process, the only added step not found in making bourbon.

Charcoal has long been known for its filtration and purifying properties.  Hippocrates, the famous Greek physician and father of western medicine wrote about using charcoal in his practice to adsorb unpleasant odors.  In modern times it plays an important role in the distillation of liquids.  When distilling whiskey congeners are creating, they are responsible for giving whiskey its harsh taste, however this harshness was reduced when most of the congeners in aged whiskey were adsorbed by the activated carbon found in charcoal.