GREEN CHEMISTRY
Industry segment ( examples ) Annual product tonnage ( per chemical ) E-factor ( kg waste / kg product ) Annual waste ( tonnes )
Petrochemicals ( solvents , detergents ) 1,000,000 – 100,000,000 ~ 0.1 10,000,000
Bulk chemicals ( plastics , polymers ) 10,000 – 1,000,000 < 1 – 5 5,000,000
Fine chemicals ( coatings , electronic parts , pharmaceutical raw materials )
Pharmaceuticals ( antibiotics , drugs , vaccines )
100 – 10,000 5 - > 50 500,000
10 – 1,000 25 - > 100 100,000
Table 1 – Waste production across chemical industries Source : F . Roschangar , R . A . Sheldon & C . H . Senanayake , Green Chem ., 2014 )
determination of the E-factor where the optimum value is zero . Data from the US EPA shows that the quantities of production-related waste managed by the chemical manufacturing sector increased by 65 % from 2007 to 2019 , while production levels remained fairly constant . 2 Although there has been a large increase in reported chemical recycling since 2014 compared with previous years , there is still much more to be done in terms of reducing preventable waste . A closer look into waste generation per chemical industry segment ( Table 1 ) shows that the E-factor is generally inversely proportional to the manufacturing scale . In other words , at larger production scales (> 10,000 tonnes / year ) it tends to be lower . At lower production scales , such as in the manufacture of fine chemicals and pharmaceuticals , it is much higher and can even be greater than 100 kg of waste / kg of product . The essential reason for this is that the larger production-scale molecules are generally simpler and require a lower number of steps for their synthesis . At smaller production scales , target molecules are more complex , require a larger number of steps and are generally produced in solvent , which can contribute up to 60 % of process waste by weight . 3
Switching to flow
Continuous flow reactors often offer significant improvements in green chemistry over traditional batch processes due to substantial improvements in mixing , heat management , energy efficiency , safety and reproducibility . 4 Many of the benefits of flow chemistry and the 12 Principles are intertwined . Higher heat transfer efficiency and lower solvent consumption due to increases in surface-to-volume ratios of flow reactors compared with batch means that chemistry can be performed much more safely and quickly in a continuous manner than in batch . Thanks to the smaller overall volumes afforded by flow reactors , lower quantities of potentially hazardous intermediates , feeds or products are needed at one time compared with a large tank , reducing workplace risks . These benefits mean that many sustainable aspects of process improvement can be achieved through moving towards continuous processing . For example , Löwe et al . demonstrated a decrease in process time from about half an hour in batch to a few seconds in flow for the addition of amines to α , β- unsaturated carbonyl compounds ( Michael additions ). 5 In the traditional batch process , the olefin must be added slowly to the diluted amine to control temperature rises and ensure safe operation . By using a microstructured reactor , exceptional heat transfer rates ( 4,000 W / m 2 K ) enabled the heat generated to be removed much quicker than in batch and even allowed the chemistry to be performed solvent-free . Life-cycle assessments ( LCAs ) are often used to analyse the environmental impacts and compare the greenness of chemical processes from a holistic point of view . Epoxidised soybean oil is the most common oleochemical used as an additive to PVC to increase its light and heat stability and is produced on a scale of 240,000 tonnes / year . Kralisch et al . performed a LCA for the epoxidation of soybean oil . The study compared the traditional batch process to a novel continuous flow process . In the industrial batch route , the oxidant is gradually added to the soybean oil to control the exotherm . Performing the same reaction in flow , the energy demand per mole of product was lowest when the reaction was carried out at > 100 ° C , due to decreased reaction times . However , by increasing the temperature to > 180 ° C , energy demand increases . In the best case scenario , the authors showed that switching the existing process to a high temperature reaction in flow could give approximately 11-12 % reduction in global warming and human toxicity potential . 6 Microreactors , although great in principle , are in practice still limited by scale-up challenges and are not yet suited to meet the volume scale productions required by the fine chemical and pharmaceutical industrial sectors . 7 , 8
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