Efficiency and Process Design - 'Metrics' Flashcards
What are ‘Metrics’ in a chemsitry sense?
Metrics are a process of how we judge a chemical reaction to determine how sustainable it is
When questioning the sustainability of competing chemical reactions we tend to compare the nature of 5 indiviual components
What are they
- Raw materials
- Reaction types (e.g. addition, eliminations etc)
- Reagents (catalysts)
- Reaction conditions (including solvents)
- Toxicity of products (including waste by-products)
Why would we not use a life cycle analysis to evaluate an entire production process
The complete answer of greeness comes from a life cycle analysis which is expensive and requires data from an already operating process
And often there is insufficient data for this
(Hence we might use the principles of Green Chemistry to improve this)
What is the idea around the 3 spheres of sustainability
The three spheres are Environmental, Social and Economic
Where these factors conicide, you should get maxmium sustainability
There are drivers between two factors (e.g. subsidies for environmental/econmic)
What does Roger A Sheldon describe green and sustainable manufacture of chemicals as?
“An essential component of the transition from a linear economy that is devouring the planet’s natural resources and degrading the ecosphere to a circular economy”
What is the difference between traditional synthesis of dimethylcarbonate and modern synthesis
The modern way is far more preferred as it avoids the use of phospgene (which has been used as a chemical wepon) and it gives less harmful side-products
Also produces a purer product, so less energy intensive purification set
(Choice on which process to use is pretty straightforward)
There are two ways we can synthese lactic acid: chemically using hydrocyanation of acetaldehyde or biologically using fermentation of sugars/starch
What are the advantages of the chemical synthesis?
- Faster, and higher yielding reaction (allowing more product to be produced or smaller chemical plant = reducing costs)
- MeOH generated in the final step is recycled
- Produces high purity lactic acid (>99%)
There are two ways we can synthese lactic acic: chemically usin hydrocynation of acetaldehyde or biologically using fermentation of sugar/starch
What are the disadvantages of the chemical synthesis
- HCN is highly toxic; CH₃CHO and MeOH are also toxic
- Waste stream of (NH₄)₂SO₄ can also be used as a fertiliser (but not on acidic soils as NH₄⁺ will be produced)
- Several energy intensive distillations required
The method shown below is the biosynthesis of lactic acid
What are the advantages of this
- It has a renewable feedstock
- Uses non-hazardous material
- Many bacteria can be employed to produce a spectrospecific product (less cost of seperation)
The method shown below is the biosynthesis of lactic acid
What are the disadvantages of this?
- Slow (every cycle takes 4-6 days) = larger chemical plant
- Quantity of product per reactor volume is low
- Evaporation required = energy intensive
- Technical grade lactical acid is produced and if a higher purity is needed further reactions required
- Waste stream of CaSO₄ produced and very large quantities of waste water (with a lot of impurities)
Even though there are more disadvantages to the biological process it is the one used as the saftey precautions make it too expensive to operate the chemical process
Two of the most common metrics are Atom Economy and E-factor
Both methods focus on what?
Both methods concentrate on reducing waste
(other factors like toxity, risks, energy consumption are ignored)
What is the equation for Percentage yield
actual yield/theoretical yield x 100
What is a key issue with percentage yield?
You may have a high yielding reaction however produce a lot of waste
An example of this is the Wittig reaction with an average 86% yield but produces 2446g of waste per 1000g of starting material
What is Atom Efficiency in terms of a chemical reaction
Synthetic methods should be designed to maximise the incorporation of all materials used in the process into the final product
What is the equation for atom economy?
Work out the atom economy of the reaction show
There was a 86% percentage yield for this reaction, however only a 25.7% atom economy
Over 190 million tones of urea is produced each year
Why is so much produced
Over 80% of produced urea is used to fertilise crops
(however <15% of fixed N ends up in consumable food)
What types of reactions have a 100% atom economy
- Rearrangements (e.g. Claisen rearrangement)
- Hydrogenations
- Carbonylations
- Cycloadditions
The Diel-Alder reaction makes the pesticide ‘Aldrin’ with high efficiency
With around a overall Atom Economy of 82.2%
What is the issue however?
Aldrin is whats called a persisten pollutant as it stays in the soil for years and is now band worldwide
Exposure linked to health problems such as Parkinson’s, breast cancer and immune, reproductive and nervous system damage (it accumulated in fatty tissue)
The traditional way of synthesis of Phenol was through the sulfonation of Benzene with around a 20.4% atom economy, and industrially was used for years
The modern process uses the oxidation of Cumene and has a 62% atom economy
How can the atom economy of the modern process go up to 100%?
Because as well as phenol being produced, so is acetone, which can be sold on not making it a waste product
Hence the atom economy goes up to 100%
What is the formula for E-factors
What is the equation for Effective Mass Yield
Work out the effective mass yield for this reaction
Acetic acids is in dilute concentrations (vinegar) so should be considered benign
But Butanol is not
(have to work out what is and isn’t benign and value is >100% so not the greatest metric)
