Hood 's laboratory at the California Institute of Technology announced the first semi-automated DNA sequencing machine in By , the U. Meanwhile, sequencing of human cDNA sequences called expressed sequence tags began in Craig Venter 's lab, an attempt to capture the coding fraction of the human genome. The circular chromosome contains 1,, bases and its publication in the journal Science  marked the first published use of whole-genome shotgun sequencing, eliminating the need for initial mapping efforts.
By , shotgun sequencing methods had been used to produce a draft sequence of the human genome. Several new methods for DNA sequencing were developed in the mid to late s and were implemented in commercial DNA sequencers by the year Together these were called the "next-generation" sequencing methods. In , Life Sciences marketed a parallelized version of pyrosequencing. The large quantities of data produced by DNA sequencing have also required development of new methods and programs for sequence analysis.
Phil Green and Brent Ewing of the University of Washington described their phred quality score for sequencer data analysis in This method's use of radioactive labeling and its technical complexity discouraged extensive use after refinements in the Sanger methods had been made.
The concentration of the modifying chemicals is controlled to introduce on average one modification per DNA molecule. Thus a series of labeled fragments is generated, from the radiolabeled end to the first "cut" site in each molecule. The fragments in the four reactions are electrophoresed side by side in denaturing acrylamide gels for size separation. To visualize the fragments, the gel is exposed to X-ray film for autoradiography, yielding a series of dark bands each corresponding to a radiolabeled DNA fragment, from which the sequence may be inferred.
The chain-termination method developed by Frederick Sanger and coworkers in soon became the method of choice, owing to its relative ease and reliability. Because of its comparative ease, the Sanger method was soon automated and was the method used in the first generation of DNA sequencers. Sanger sequencing is the method which prevailed from the s until the mids. Over that period, great advances were made in the technique, such as fluorescent labelling, capillary electrophoresis, and general automation.
These developments allowed much more efficient sequencing, leading to lower costs. The Sanger method, in mass production form, is the technology which produced the first human genome in , ushering in the age of genomics. Large-scale sequencing often aims at sequencing very long DNA pieces, such as whole chromosomes , although large-scale sequencing can also be used to generate very large numbers of short sequences, such as found in phage display.
For longer targets such as chromosomes, common approaches consist of cutting with restriction enzymes or shearing with mechanical forces large DNA fragments into shorter DNA fragments. Short DNA fragments purified from individual bacterial colonies are individually sequenced and assembled electronically into one long, contiguous sequence. Studies have shown that adding a size selection step to collect DNA fragments of uniform size can improve sequencing efficiency and accuracy of the genome assembly.
In these studies, automated sizing has proven to be more reproducible and precise than manual gel sizing. The term " de novo sequencing" specifically refers to methods used to determine the sequence of DNA with no previously known sequence. De novo translates from Latin as "from the beginning". Gaps in the assembled sequence may be filled by primer walking.
The different strategies have different tradeoffs in speed and accuracy; shotgun methods are often used for sequencing large genomes, but its assembly is complex and difficult, particularly with sequence repeats often causing gaps in genome assembly. Most sequencing approaches use an in vitro cloning step to amplify individual DNA molecules, because their molecular detection methods are not sensitive enough for single molecule sequencing.
Emulsion PCR  isolates individual DNA molecules along with primer-coated beads in aqueous droplets within an oil phase. A polymerase chain reaction PCR then coats each bead with clonal copies of the DNA molecule followed by immobilization for later sequencing. Emulsion PCR is used in the methods developed by Marguilis et al. Shotgun sequencing is a sequencing method designed for analysis of DNA sequences longer than base pairs, up to and including entire chromosomes.
This method requires the target DNA to be broken into random fragments. After sequencing individual fragments, the sequences can be reassembled on the basis of their overlapping regions. Another method for in vitro clonal amplification is bridge PCR, in which fragments are amplified upon primers attached to a solid surface    and form " DNA colonies " or "DNA clusters".
This method is used in the Illumina Genome Analyzer sequencers. Single-molecule methods, such as that developed by Stephen Quake 's laboratory later commercialized by Helicos are an exception: High-throughput formerly "next-generation" sequencing applies to genome sequencing, genome resequencing, transcriptome profiling RNA-Seq , DNA-protein interactions ChIP-sequencing , and epigenome characterization.
The high demand for low-cost sequencing has driven the development of high-throughput sequencing technologies that parallelize the sequencing process, producing thousands or millions of sequences concurrently. The first of the high-throughput sequencing technologies, massively parallel signature sequencing or MPSS , was developed in the s at Lynx Therapeutics, a company founded in by Sydney Brenner and Sam Eletr.
MPSS was a bead-based method that used a complex approach of adapter ligation followed by adapter decoding, reading the sequence in increments of four nucleotides. This method made it susceptible to sequence-specific bias or loss of specific sequences. Lynx Therapeutics merged with Solexa later acquired by Illumina in , leading to the development of sequencing-by-synthesis, a simpler approach acquired from Manteia Predictive Medicine , which rendered MPSS obsolete. However, the essential properties of the MPSS output were typical of later high-throughput data types, including hundreds of thousands of short DNA sequences.
The Polony sequencing method, developed in the laboratory of George M. Church at Harvard, was among the first high-throughput sequencing systems and was used to sequence a full E. A parallelized version of pyrosequencing was developed by Life Sciences , which has since been acquired by Roche Diagnostics. The method amplifies DNA inside water droplets in an oil solution emulsion PCR , with each droplet containing a single DNA template attached to a single primer-coated bead that then forms a clonal colony.
The sequencing machine contains many picoliter -volume wells each containing a single bead and sequencing enzymes. Pyrosequencing uses luciferase to generate light for detection of the individual nucleotides added to the nascent DNA, and the combined data are used to generate sequence read-outs. Solexa , now part of Illumina , was founded by Shankar Balasubramanian and David Klenerman in , and developed a sequencing method based on reversible dye-terminators technology, and engineered polymerases.
In , Solexa acquired the company Manteia Predictive Medicine in order to gain a massivelly parallel sequencing technology invented in by Pascal Mayer and Laurent Farinelli. The cluster technology was co-acquired with Lynx Therapeutics of California. In this method, DNA molecules and primers are first attached on a slide or flow cell and amplified with polymerase so that local clonal DNA colonies, later coined "DNA clusters", are formed.
To determine the sequence, four types of reversible terminator bases RT-bases are added and non-incorporated nucleotides are washed away. A camera takes images of the fluorescently labeled nucleotides. Then the dye, along with the terminal 3' blocker, is chemically removed from the DNA, allowing for the next cycle to begin. Unlike pyrosequencing, the DNA chains are extended one nucleotide at a time and image acquisition can be performed at a delayed moment, allowing for very large arrays of DNA colonies to be captured by sequential images taken from a single camera.
Decoupling the enzymatic reaction and the image capture allows for optimal throughput and theoretically unlimited sequencing capacity. Here, a pool of all possible oligonucleotides of a fixed length are labeled according to the sequenced position. Oligonucleotides are annealed and ligated; the preferential ligation by DNA ligase for matching sequences results in a signal informative of the nucleotide at that position. The resulting beads, each containing single copies of the same DNA molecule, are deposited on a glass slide. Ion Torrent Systems Inc. This method of sequencing is based on the detection of hydrogen ions that are released during the polymerisation of DNA , as opposed to the optical methods used in other sequencing systems.
A microwell containing a template DNA strand to be sequenced is flooded with a single type of nucleotide. If the introduced nucleotide is complementary to the leading template nucleotide it is incorporated into the growing complementary strand. This causes the release of a hydrogen ion that triggers a hypersensitive ion sensor, which indicates that a reaction has occurred. If homopolymer repeats are present in the template sequence multiple nucleotides will be incorporated in a single cycle.
This leads to a corresponding number of released hydrogens and a proportionally higher electronic signal. DNA nanoball sequencing is a type of high throughput sequencing technology used to determine the entire genomic sequence of an organism. The company Complete Genomics uses this technology to sequence samples submitted by independent researchers.
Unchained sequencing by ligation is then used to determine the nucleotide sequence. Heliscope sequencing is a method of single-molecule sequencing developed by Helicos Biosciences. It uses DNA fragments with added poly-A tail adapters which are attached to the flow cell surface. The next steps involve extension-based sequencing with cyclic washes of the flow cell with fluorescently labeled nucleotides one nucleotide type at a time, as with the Sanger method.
The reads are performed by the Heliscope sequencer. SMRT sequencing is based on the sequencing by synthesis approach. The sequencing is performed with use of unmodified polymerase attached to the ZMW bottom and fluorescently labelled nucleotides flowing freely in the solution. The wells are constructed in a way that only the fluorescence occurring by the bottom of the well is detected. The fluorescent label is detached from the nucleotide upon its incorporation into the DNA strand, leaving an unmodified DNA strand.
According to Pacific Biosciences PacBio , the SMRT technology developer, this methodology allows detection of nucleotide modifications such as cytosine methylation. This happens through the observation of polymerase kinetics. This approach allows reads of 20, nucleotides or more, with average read lengths of 5 kilobases. The DNA passing through the nanopore changes its ion current.
This change is dependent on the shape, size and length of the DNA sequence. Each type of the nucleotide blocks the ion flow through the pore for a different period of time. The method does not require modified nucleotides and is performed in real time. Early industrial research into this method was based on a technique called 'Exonuclease sequencing', where the readout of electrical signals occurring at nucleotides passing by alpha- hemolysin pores covalently bound with cyclodextrin.
Two main areas of nanopore sequencing in development are solid state nanopore sequencing, and protein based nanopore sequencing.
Green Chemistry and Engineering
The concept originated from the idea that single stranded DNA or RNA molecules can be electrophoretically driven in a strict linear sequence through a biological pore that can be less than eight nanometers, and can be detected given that the molecules release an ionic current while moving through the pore. The pore contains a detection region capable of recognizing different bases, with each base generating various time specific signals corresponding to the sequence of bases as they cross the pore which are then evaluated.
Another approach uses measurements of the electrical tunnelling currents across single-strand DNA as it moves through a channel. Depending on its electronic structure, each base affects the tunnelling current differently, allowing differentiation between different bases. The use of tunnelling currents has the potential to sequence orders of magnitude faster than ionic current methods and the sequencing of several DNA oligomers and micro-RNA has already been achieved.
Sequencing by hybridization is a non-enzymatic method that uses a DNA microarray. A single pool of DNA whose sequence is to be determined is fluorescently labeled and hybridized to an array containing known sequences.
- Laravel Application Development Blueprints.
Strong hybridization signals from a given spot on the array identifies its sequence in the DNA being sequenced. This method of sequencing utilizes binding characteristics of a library of short single stranded DNA molecules oligonucleotides , also called DNA probes, to reconstruct a target DNA sequence. Non-specific hybrids are removed by washing and the target DNA is eluted.
The benefit of this sequencing type is its ability to capture a large number of targets with a homogenous coverage. However, with the advent of solution-based hybridization, much less equipment and chemicals are necessary. Mass spectrometry may be used to determine DNA sequences. With this method, DNA fragments generated by chain-termination sequencing reactions are compared by mass rather than by size. The mass of each nucleotide is different from the others and this difference is detectable by mass spectrometry.
Single-nucleotide mutations in a fragment can be more easily detected with MS than by gel electrophoresis alone. The higher resolution of DNA fragments permitted by MS-based methods is of special interest to researchers in forensic science, as they may wish to find single-nucleotide polymorphisms in human DNA samples to identify individuals. These samples may be highly degraded so forensic researchers often prefer mitochondrial DNA for its higher stability and applications for lineage studies.
MS-based sequencing methods have been used to compare the sequences of human mitochondrial DNA from samples in a Federal Bureau of Investigation database  and from bones found in mass graves of World War I soldiers. Even so, a recent study did use the short sequence reads and mass spectroscopy to compare single-nucleotide polymorphisms in pathogenic Streptococcus strains. This approach directly visualizes the sequence of DNA molecules using electron microscopy. The first identification of DNA base pairs within intact DNA molecules by enzymatically incorporating modified bases, which contain atoms of increased atomic number, direct visualization and identification of individually labeled bases within a synthetic 3, base-pair DNA molecule and a 7, base-pair viral genome has been demonstrated.
One end of DNA to be sequenced is attached to another bead, with both beads being placed in optical traps. RNAP motion during transcription brings the beads in closer and their relative distance changes, which can then be recorded at a single nucleotide resolution.
In other words, newer methods of kinetic activation, which minimize the energy input by optimizing reaction conditions, will be discussed along with the need for an elegant synthetic design. In most reactions, the reaction vessel provides three components as shown in Fig. Components of chemical reaction.
The role of alternate reagents, solvents, and catalysts in greening chemical reactions is discussed in other chapters. In this chapter we shall see newer methods of kinetic activation of molecules in chemical reactions. Pressure and temperature are important parameters in reaction processes in chemical systems. However, it is a less well-known fact that other than thermally initiated reactions can also lead to sustainable results. The basic requirement is to capture the energy required by a reaction. The energy required for synthesis as well as that required for cooling are of interest here.
Approaches are being taken and possibilities investigated to use until now scarcely used forms of energy, so-called nonclassical energy forms, in order to optimize the duration and product yield and avoid undesired side products. Teams working in this area are also interested in the energetic aspects of the preparation of starting substances and Newer Synthetic Methods 29 products and the conditioning of reaction systems e. We now have six well-documented methods of activating molecules in chemical reactions, which can be grouped as follows: Each of these methods has its advantages and niche areas of applications, alongside its inherent limitations.
A comparative study of these techniques is given in Table 2. What do we mean by classical and nonclassical energy forms? In classical processes, energy is added to the system by heat transfer; by electromagnetic radiation in the ultraviolet UV , visible, or infrared IR range; or in the form of electrical energy. On the other hand, microwave radiation, ultrasound, and the direct application of mechanical energy are among the nonclassical forms.
Not only can this high-energy input enhance mechanical effects in heterogeneous processes, but it is also known to induce new reactions, leading to the formation of unexpected chemical species. What makes sonochemistry unique is the remarkable phenomenon of cavitation, currently the subject of intense research, which has already yielded thought-provoking results. With highfrequency ultrasound, the chemistry produced displays characteristics similar to high-energy radiation more radicals are created.
One of the most striking features in sonochemistry is that there is often an optimum value for the reaction temperature. In contrast to classical chemistry, most of the time it is not necessary to go to higher temperatures to accelerate a process. In fact, heterogeneous reactions are those in which ultrasound is likely to play the most important role by selective accelerations between potentially competitive pathways. It was reported that the sonochemical decomposition of volatile organometallic precursors was shown to produce nanostructured materials in various forms with high catalytic activities.
Ultrasound is known to enhance the reaction rate, thus minimizing the duration of a reaction. A large number of published examples, which highlight this observation, are shown in Appendix 2. Use of Microwaves for Synthesis In synthetic chemistry, was an important year for the use of microwave devices.
Since that year, countless syntheses initiated by microwaves have been carried out on a laboratory scale. The result is often a drastic reduction in the reaction time with comparable product yields, if microwaves are used instead of classical methods of energy input. Unwanted side reactions can often be suppressed and solvents dispensed with. Reactions listed in Appendix 2.
Apart from the obvious advantages of the use of microwaves in chemical syntheses, microwave technologies are being tested as energy- and cost-saving alternatives. Newer Synthetic Methods 33 Electro-Organic Methods Over the past 25 to 30 years, the use of electrochemistry as a synthetic tool in organic chemistry has increased remarkably. According to Pletcher and Walsh , more than electro-organic synthetic processes have been piloted at levels ranging from a few tons up to tons.
Many excellent reviews and publications highlight the synthetic utility of electro-organic methods Lund and Baizer, These cover a broad spectrum of applications of electrochemical methods in organic synthesis, including their use in the pharmaceutical industry. Mild reaction conditions, ease of control of solvent and counter-ions, high yields, high selectivities, as well as the use of readily available equipment, simply designed cells, and regular organic glassware make the electrochemical syntheses very competitive to the conventional methods in organic synthesis. This approach was very successful for synthesis of the organosilicon compounds Fry and Touster, Elegant and Cost-Effective Synthetic Design The heart of synthesis is in the design of the synthetic scheme for the given target molecule.
All the technological advances discussed above can only supplement the synthetic scheme. Structures of atropine, tropinone, and cocaine. Thus, synthesis was often a matter of utilitarian necessity rather than the creative, elegant art form illustrated by the work of many of the great synthetic chemists such as Woodward and Corey. In , Robinson approached the synthesis in a totally radical way. Tropinone was obtained by condensation of succinaldehyde with acetone and methylamine in aqueous solution see Fig.
In fact, we can view this synthesis of tropinone as one of the earliest examples of multicomponent reactions MCR. MCRs are convergent reactions in which three or more starting materials react to form a product, where basically all or most of the atoms contribute to the newly formed product. Carbonyl compounds played a crucial role in the early discovery of multicomponent reactions. One example is the Mannich reaction see Fig. Thus, the chemistry development time, which can typically take up to 6 months for a linear six-step synthesis, is considerably shortened.
With only a limited number of chemists and technicians, more scaffold synthesis programs can be achieved within a shorter time. Conclusions The various reaction types most commonly used in synthesis can have different degrees of impact on human health and the environment. Substitution reactions, on the other hand, necessarily generate stoichiometric quantities of substances as byproducts and 38 Green Chemistry and Processes waste.
As such, elimination reactions are among the least atom-economical transformations. For any synthetic transformation, it is important to evaluate the hazardous properties of all substances necessarily being generated from the transformation, just as it is important to evaluate the hazardous properties of all starting materials and reagents that are added in a synthetic transformation. The atom-economy of various reaction types is shown in Fig.
The most atom-economy—suited reactions are condensations, multicomponent reactions, and rearrangements. Atom-economy of various reaction types. Maximize yield per step. Maximize atom-economy per step. In multistep syntheses, perform the following: Minimize frequency of substitutions protecting group strategies and redox reactions.
If forced to use oxidations, opt for hydrogen peroxide as oxidant. If forced to use reductions, opt for hydrogen as reductant. Devise catalytic methods where catalysts are recycled and reused. Opt for solventless reactions, recycle solvents, or use benign solvents ionic liquids.
EtOH, ZnBr2, , r. Lie Ken Jie, M. Newer Synthetic Methods 43 NaOH, MW, 25sec a: Newer Synthetic Methods 51 In the last decade, green chemistry has been widely recognized and accepted as a new means for sustainable development. Industries are often forced to pay heavy prices to meet with the standards set by the pollution regulatory boards while using the traditional methods of treating or recycling waste. Also, with growing environmental problems, law-making boards are now looking more critically at the possible hazardous effects of a larger number of chemical substances. This can be quite obviously attributed to three general characteristics of catalysts: Catalytic reagents reduce the energy of the transition state, thereby reducing the energy input required for a process.
Catalysts are required in small quantities. In the case of biocatalysts, the number of catalysts generally enzymes needed compared to the quantity of reactants is very low. The regeneration and reversibility of catalysts are good for green processes. As much as it is a key in achieving economic objectives, catalysis is also a powerful tool in realizing the goals of green chemistry. It is calculated by dividing the molecular weight of the desired product by the sum total of the molecular weight of all substances produced in the stoichiometric equation for the reactions involved.
Some authors also describe it as the number of atoms of all the reactants that are converted into atoms of the desired product in a reaction. For instance, when replaced with cleaner catalyzed oxidation, traditional oxidations using oxidants such as permanganate or chromium reagent as shown in Fig. Jones oxidation of secondary alcohol. Atom-economical oxidation of secondary alcohol.
The oxidation contained in Fig. Trost and co-workers Trost, used a variety of palladium catalysts to effect allylic alkylation reaction. The reaction, as it occurs at room temperature, is also an example of catalysis reducing energy usage. Though the usage of HF, a toxic substance, is a drawback of the process, the recovery of HF is effected with The process shown in Fig. This leaves a need for truly catalytic procedures: Use of zeolites in an acid-catalyzed rearrangement of epoxides to carbonyl compounds Elings et al. Traditionally, Lewis acids such as ZnCl2 were used in stoichiometric amounts for the type of reaction displayed in Fig.
The following examples are two commercially relevant processes. The products are precursors of chemicals used for their fragrance see Fig. Zeolites and clay-catalyzed, high-AE reactions. The use of zeolites in the manufacture of cumene is of immense importance. About 7 million metric tons of cumene are produced annually worldwide.
The earlier-used process involved alkylation of benzene over a solid phosphoric acid or an aluminum chloride catalyst. Both catalysts are toxic in nature. In addition, it also generates less waste and requires less energy than the earlier catalysts, thus simultaneously satisfying various conditions of green chemistry. The use of zeolites in making industrial processes ecocompatible is growing with the widespread research on using these as catalysts. One such example of zeolite being used to better the existing process is that of the Meerwin—Ponndorf—Verly MPV reduction.
The MPV reduction process is an extensively used technology for reducing aldehydes and ketones to their corresponding alcohols. MPV reduction using zeolite. The stoichiometric requirement of aluminum alkoxide, due to the slow exchange of the alkoxy group, was an inherent drawback in the method. The example in Fig. In this reaction, the trans-alcohol was the preferred product in the traditional MPV reduction. The zeolitecatalyzed reaction forms the thermally less stable cis-isomer, which is an important fragrance chemical intermediate.
Catalysis offers an edge over stoichiometric reactions in achieving selectivity in production, when mono substitution is preferred over disubstitution, when one stereo-isomer is preferred over another or one regioisomer over another. Hence, by driving the reaction to a preferred product, catalyzed reactions decrease the amount of waste generated while reducing the energy requirements, as mentioned earlier.
The contribution of Spiney and Gogate Spivey and Gogate, in developing heterogeneous catalysts for the condensation of acetone to methyl isobutyl ketone MIBK is commendable. The reaction typically requires stoichiometric amounts of base and could also result in considerably overcondensed products.
In the production of biologically active molecules pharmaceuticals and pesticides , there is often a need to produce chiral molecules as the pure enantiomer. CO , electrolysis 2 2. This need has directed the focus onto asymmetric catalysis using chiral metal complexes and enzymes. The Novartis process for the synthesis of the optically active herbicide s -metachlor Blaser and Spindler, involves a chiral metal complex as a catalyst see Fig.
An iridium I complex of a chiral ferrocenyldiphosphine catalyzes the asymmetric hydrogenation of a prochiral imine, a key step in the process. Novartis process for the synthesis of the optically active herbicide. Production of phenol from benzene. Solutia USA , in joint work with the Boreskov Institute of Catalysis, Russia, developed a one-step process to manufacture phenol from benzene using nitrous oxide as the oxidant see Fig. Production of cumene from benzene. Production of p-methoxyacetophenone from methoxybenzene. Manufacture of methylethyl ketone MEK from ethylene and butylenes.
The Rhodia process for the production of p-hydroxyacetophenone from methoxybenzene using clay as the catalyst eliminates the use of toxic chemicals such as AlCl3 and BF3 and also eliminates toxic waste see Fig. The Catalytic process for the manufacture of methylethyl ketone MEK from ethylene and butylenes uses a mixture of palladium, vanadium, and molybdenum oxides as catalyst see Fig.
The original process used chlorinated chemicals, which led to a large amount of chlorinated waste that posed several problems during disposal. The Enichem process for the preparation of propene oxide from propylene involves using H2O2 as the oxidizing agent using titanium silicate catalyst see Fig. The Avetis process for preparing halo benzaldehyde is to oxidize corresponding halo toluene using air and a mixture of iron, vanadium, and molybdenum oxide catalyst see Fig.
The catalytic process eliminates the formation of chlorinated byproducts. Preparation of propene oxide from propylene. Microbial mediated aromatic ring hydroxylation. Biocatalysis is the other option when selectivity sterio or regio is a priority in a reaction. The various aspects of biocatalysis are discussed elsewhere in the book; the following are some examples of biocatalysts that have been used in important synthesis. Kirner conducted microbial ring hydroxylation and side chain oxidation of hetero-aromatics see Fig.
As the example in Fig. The classical method calls for the protection of the carboxy group of Penicillin-G, making it a four-step process. Enzymatically, this conversion can be achieved in a single step Sheldon, Genetic engineering also comes in handy when dealing with chemical reactants that are not biological substrates. It involves the conversion of a ketone into a lactone commonly using the reagent m-chloroperoxybenzoic acid m-CPBA. This reagent is both sensitive to shocks and explosive. This is a classic example of biocatalysis making a reaction eco-compatible.
Synthesis of cephalexin through the use of CLECs. The reaction is also run in an aqueous medium. The industrial scope of the reaction is under study. Enzymes do have their disadvantages. Their solvent incompatibility and instability restrict their industrial use.
Altus Biologics has developed cross-linked enzyme crystals CLECs to increase the versatility of enzymes in organic reactions. CLECs exhibit a high level of stability in extreme conditions of temperature and pH and in exposure to both aqueous and organic solvents. The N-protection step of methyl phenyl glycinate in the classical synthesis was eliminated.
Genetically engineered microbes have been used by Draths and Frost a, b to synthesize common but important chemicals such as adipic acid and catechol see Fig. The noteworthy aspect of this work is that the starting materials were renewable feedstock. This reaction addresses this principle and more, as it can be seen. Classical catechol synthesis beginning with benzene obtained from petroleum, a nonrenewable feedstock involves a multistep process see Fig.
Classical synthesis of catechol. Biocatalysis for the synthesis of catechol from a renewable source. The biocatalyzed reaction is a far better process than the classical one, as it replaces the hazardous starting chemical, benzene, with D-glucose and tremendously decreases the energy demands apart from replacing a nonrenewable feedstock with a renewable one. In a similar effort, Ho and colleagues have succeeded in creating recombinant Saccharomyces yeast that can ferment glucose and xylose simultaneously to ethanol see Fig. Cellulose biomass made of materials such as grasses, woody plants, etc.
Recombinant yeast for fermentation of both glucose and xylose. The central role these catalysts play in directing the course of a reaction, thereby minimizing or eliminating the formation of side products, cannot be disputed. Hence, catalysis—or rather, designed catalysis—is the mainstay of green chemical practices. References Altus Biologics, Inc. Catalysis and Green Chemistry 67 Draths, K. Oxford University Press, New York, Biotransformations have been known since the early stages of human civilization and have been used since then to make fermented foods and beverages.
These numbers suggest the important role of biocatalysis to the chemical industry. Chemical reactions performed by microorganisms or catalyzed by enzymes are essentially the same as those carried out in 69 70 Green Chemistry and Processes TABLE 4. The most striking differences between enzymes and chemical catalysts are summarized in Table 4.
Advantages Within Industrial Applications Microorganisms are undoubtedly the most superior enzyme sources among living organisms. They show high adaptability to new environments and high growth rates. These characteristics are especially useful for easy handling and large-scale cultivation without a high cost. At present, several new techniques such as extractive biocatalysis, immobilization, biocatalysis in organic solvents, and recombinant DNA technology for enzyme engineering are rapidly being developed in order to make biocatalysis industrially viable.
Furthermore, protein engineering and cell technology, such as cell fusion, will become useful techniques for microbial transfor- Biocatalysis: A large number of biologically and chemically useful compounds are prepared through microbial transformations. Challenges to Make Biocatalysis Industrially Viable Many of the unique features of the enzymatic reactions prove to be limitations for their commercial use. Approaches to overcome the limitations of biocatalysis. Many of these problems have been addressed by a large variety of approaches, all of which can be summarized, as shown both in Fig.
Enzyme engineering helps in designing the enzyme for a given transformation. Green Chemistry 73 concentration , use of organic co-solvent or micelles and carrying out the catalysis in organic solvent have been attempted with considerable success. Numerous reviews on conventional approaches, such as immobilization techniques, genetic engineering, and extractive biocatalysis, have appeared in the literature at regular intervals.
The subsequent isolation of DNA polymerases that can function at high temperatures has revolutionized the biotechnology industry. Bioremediation is often the most cost-effective means of cleaning up contaminated soil and water. However, bioremediation may not always be viewed as an appropriate treatment option due to the chemical nature of the contaminant or a mixture of contaminants present at a site. Yet microbial biodiversity is so immense that it is usually possible to either isolate from nature, or evolve in the laboratory, a microbial culture capable of treating almost any type or mixture of environmental contaminants.
Applications for Enrichment Cultures One example of the use of enrichment culture techniques was the isolation of a microbial culture that could degrade a chemical warfare agent. Resolution of CPA by soil-enriched Pseudomonas putida. Still, researchers were able to use enrichment culture techniques to isolate a bacterial culture that cannot only survive exposure to this deadly compound and its derivatives but can also use the chemical as a food source for growth.
Besides their role in degrading unwanted chemicals and pollutants, enrichment culture techniques can also be used to isolate microbial cultures that possess biochemical pathways that are useful for making chemicals by biocatalysis. For example, consider the conversion of racemic 2-chloropropanoic acid CPA to L-CPA, by the dehalogenase from Pseudomonas putida—the necessary strain AJ1 was isolated from the environment with high-chlorine-containing compounds—the road tanker off-loading point see Fig. Enrichment culture techniques can also be used for bioremediation to detoxify xenobiotic pollutants such as polycyclic aromatic hydrocarbons PAHs , heterocyclic polyaromatics, and halogenated aromatics in soils and sediments through microbial degradation.
An effective way to do this is by isolating microbes through enrichment cultures with the substrate one wants to detoxify as a limiting compound. Approach Enrichment culture techniques rely on creating a condition in which the survival and growth of bacterial cultures, with whatever Biocatalysis: Green Chemistry 75 traits are desired, are favored. The nutritional composition of the microbial growth media can be adjusted so that an environmental contaminant serves as the only available source of food and energy or the growth conditions favor the growth of only those bacteria that can grow at a certain temperature or in the presence of other chemicals.
In these ways, the conditions can be controlled in the laboratory to allow for the selection of those bacteria that can provide solutions to various problems. In addition to selecting naturally occurring microbial cultures that possess a desired metabolic trait, it is also possible to use enrichment culture techniques to develop microbial cultures with unique biochemical traits. The substrate range of enzymes catalyzing a certain reaction can be expanded through the use of enrichment culture techniques.
This process of evolving new biochemical traits in the laboratory can also be accelerated by the use of directed evolution. The similar shape and polarity within a series of substrates of different reactivity bioisosteres eliminate effects due to differences between enzyme-substrate binding ES , which is hence a good method of extending the range of substrates that can be chosen for the transformation.
A number of instances can be cited from the literature wherein the isosteres had similar transformations. Bacterial dioxygenase-catalyzed cis-dihydroxylation of the tetracyclic arene benzo[c]phenanthrene was found to occur exclusively at fjord region cavity region bonds. The isosteric compounds benzo[b]naphthol [1,2-d]furan and benzo[b]naphthol[1,2-d]thiophene were also similarly cis-dihydroxylated at the fjord region bonds by bacterial dioxygenases Boyd et al. The isosteres 1,2dihydronaphthalene, 2,3-dihydrobenzothiophene, and 2,3-dihydrobenzofuran gave similar corresponding diol products on incubation with Pseudomonas putida UV4.
Often times in nonaqueous media enzymes exhibit properties drastically different from those displayed in aqueous buffers. These novel properties are given in Table 4. In addition to those mentioned in Table 4. Once organic solvent becomes a reaction medium, there cannot be contamination, which thus precludes release of proteolytic enzymes by microbes and favors the direct application of the process in an industrial setting.
Most proteins enzymes inherently function in an aqueous environment, and hence their behavior in nonaqueous solvents is completely different due to the loss in the three-dimensional structure. Thus, only polar solvents Biocatalysis: Effects 1 Enhances the reaction rates. In many cases maximal rate of the reaction in water—organic mixture is higher than the rate of the same reaction in aqueous buffers Khmelnitsky et al. Changes the reaction pathway by promoting change in substrate cleavage and product synthesis Pal and Gertler, ; Blankeney and Stone, There are cases when stability of enzymes drastically improved in water—organic solvent mixtures as compared to aqueous media Guagliardi et al.
Shift in the direction of the biocatalyzed reaction Deschrevel et al. Homogenous biocatalysis in organic solvents requires the solubility of enzymes in nonaqueous media. Since proteins inherently function in aqueous environments, initial efforts were to study biocatalysis in water—organic mixtures. Biocatalysis in nonaqueous systems using water—miscible organic solvents was studied in detail and has been reviewed previously Butler, ; Blinkovsky et al.
In general, enzyme activity in a homogenous mixture of water—organic solvent is extremely sensitive to the nature and amount of organic solvent Budde and Khmelnitsky, It is interesting to note that in many cases the maximal rate of the reaction in a water—organic mixture is higher than the rate of the same reaction in aqueous buffers Khmelnitsky et al. The most obvious reason for shifting to water—organic mixtures as a reaction medium is to enable bioconversion of substrates poorly soluble in water.
Application of water—organic mixtures often enables a shift in the direction of a biocatalyzed reaction due to a decrease in the content of water, a reaction substrate. For example, synthesis of dipeptides using chymotrypsin and procine pancreatic lipase present good examples of reverse reaction becoming predominant while moving from aqueous media to water—organic mixtures Deschrevel et al.
The authors reported an increase in the dipeptide concentration in reaction medium concurrent with the decrease in the water content. It is worth mentioning that bioconversions in water—organic solvent mixtures are not limited to monomeric enzymes. Aspartate transcarbamylase ATCase from E.
Ionic liquids in mixtures with water display a potential to modify properties of biocatalyst. Green Chemistry 79 Malhorta, Conclusions Biocatalysis in nonaqueous systems has proven itself as a powerful tool.
Preliminary characterization of some natural dyes
A combination of directed evolution and rational enzyme design is likely to result in many more exciting developments in the near future. Use of Cyclodextrins The use of enzymes as valuable catalysts in organic solvents has been well documented. However, some of their features limit their application in organic synthesis, especially the frequently lowerenzyme activity under nonaqueous conditions, which constitutes a major drawback in the application of enzymes in organic solvents. In addition, many enzymatic reactions are subject to substrate or product inhibition, leading to a decrease in the reaction rate and enantioselectivity.
To overcome these drawbacks and to make enzymes more appealing to synthesis, cyclodextrins are used. The effects of the cyclodextrins range from increasing the availability of insoluble substrates to reducing substrate inhibition to limiting product inhibition. In each case, the effects of the cyclodextrins are interpreted in terms of the formation of inclusion complexes.
It is thus demonstrated that cyclodextrins can be used rationally to increase the utility of enzymes in organic synthesis. In an interesting study, cyclodextrins were used as regulators for the Pseudomonas cepacia lipase PSL and macrocyclic additives to enhance the reaction rate and enantioselectivity E in TABLE 4. This maintains activity, but is not suited for pure organic solvents.
Very helpful in resolution of racemic compounds with high enantioselectivity. HIP results in highly active and stable preparations. Sodium bis 2Activity depends on water ethylhexyl content and organic solvent. Molecular biology Involves techniques of techniques molecular biology, and success in this area depends on screening methods. An innovative method of forming an inclusion complex with the product was reported by Easton et al. Use of Crown Ethers Today it is well established that enzymes can be catalytically active in organic solvents.
Compared to aqueous solutions, the use of an organic reaction medium can have some interesting advantages, such as the enhanced thermal stability of the enzyme, the easy separation of the suspended enzyme from the reaction medium, the increased solubility of substrates, the favorable equilibrium shift to synthesis over hydrolysis, the suppression of water-dependent side reactions, and possibly the new stereo- selectivity properties of the enzyme.
An important drawback of the use of organic solvents for enzyme reactions is that the activity of the enzyme is generally several orders of magnitude lower than in aqueous solution. Prior lyophilization of the enzyme from an aqueous solution, which is buffered at the pH of optimal aqueous enzyme activity, if necessary in the presence of an inhibitor, improves the activity in organic solvent.
In recent years the effects of crown ethers on enzyme reactions in organic solvents have been investigated. Depending on their ring size and structure, crown ethers can form complexes with metal ions, ammonium groups, guanidinium groups, and water, species that are all common in enzymatic reactions. Therefore, these results show that pretreatment of enzymes by lyophilization with crown ethers or by simply adding C-6 to the organic solution can enhance the enzyme activity to a level where they are suitable for practical applications.
Moreover, it was reported that for relatively reactive substrates the enantioselectivity of proteases in organic solvent is very sensitive for small changes in solvent composition. This offers the possibility to tune the enantioselectivity and to apply these enzymes as catalysts for conversions of both the L- and the D-enantiomers Johan et al. The acceleration of the initial rate, V 0 , ranged from less than fold to more than fold. This suggests that molecular imprinting is likely the primary cause of subtilisin activation by crown ethers, as recently suggested Santos et al.
The PCL co-lyophilized with each additive showed simultaneous enhanced enzyme activity and enantioselectivity when compared to the native lipase lyophilized from buffer alone; in contrast, such enhancement was not observed for the colyophilized CRL. Hence, this method seems to be of practical use for the large-scale production of optically active compounds Mine et al. Process Design Now that the various methodologies to overcome the limitations of biocatalysis have been discussed, a brief account of process design will give the necessary information to make these processes Biocatalysis: Green Chemistry 83 industrially viable.
Three categories of questions need to be answered for any process design: How much material do you need to supply to customer— g, 1 kg, 10 kg? How long does one batch take to run in process per unit of reaction volume? How long do you have to make your delivery? How large is the reaction vessel? How long can the vessel be practically and safely operated? Establish a baseline process. Determine how to run the process to meet a target delivery. Details of these four steps are given in Table 4. Future Trends Organic synthesis—chemistry as it is done in the laboratory or manufacturing plant—traditionally uses a step-by-step approach.
They require extra energy to overcome the thermodynamic hurdles to produce and isolate intermediates B and C if they lie in high-energy states.
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Select a catalyst screening: Choose 10 to 20 enzymes, based on commercial availability. Set up identical reactions and keep enzyme loading mass constant. Choose enzyme with good conversion and high selectivity. Choose to strains from pre-existing library, more from environmental library. Add same amount of substrate and then normalize to cell mass. Determine amount of enzyme needed to achieve target conversion in target time. Take initial read on kinetics. Optimize growth phase pH, temperature, medium for target activity.
Optimize conversion phase pH, temperature and use of cyclodextrins, crown ethers, etc. Determine operational limits max achievable substrate charge and product titer. Determine total processing time required for delivery. Comparison of classical chemical synthesis versus biosynthesis. Such multistep, combined syntheses are common in everyday life. They are carried out in a fully catalytic way by using enzymes with relatively limited amounts of reagents cofactors and thus produce much less waste.
The four canonical bases
The mutual compatibility and high selectivity of the enzymatic conversions make it possible to proceed without intermediate recovery steps. They save energy by avoiding the separation and isolation of intermediates B and C.
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The four consecutive enzymatic conversion steps in one reactor without any separation of intermediates consist of 1. Glycerol is phosphorylated with pyrophosphate by phytase at pH 4. By raising the pH to 7. Four-step, one-pot synthesis of carbohydrates from glycerol. Catalase is added to suppress the buildup of hydrogen peroxide. The D-isomer is converted back to glycerol and phosphate in the last step. More than 20 aldehydes are known to be substrates for the aldolases from Staphylococcus carnosus and S.
Stereo-selectivity of the aldolases must be looked at for each acceptor substrate, because isomers are formed in different proportions. The oxidation and aldol reaction can be carried out simultaneously. Economic and environmental goals will cause chemical and biotechnological conversion methods to merge into integrated biology—chemistry routes based on optimum feedstock. Conservation of matter and energy from starting material to end product is required to achieve sustainable conversion processes. Neither chemistry nor biotechnology, neither fossil fuels nor renewable feedstock, will be the ultimate winner—only the combination of these disciplines and resources.
Green Chemistry 89 References Arnold, F. Stabilization of T and R conformations of Escherichia coli aspartate transcarbamylase by organic solvents, Biochem. Enhanced reaction rate and enantioselectivity, Org. A new quantitative criterion for the selection of organic solvents as reaction media in biocatalysis, Eur. Green Chemistry 91 Pardkar, V. In most reactions, the reaction vessel provides the following three components see Fig. Just as in a chaotic system, every component—minor or major—affects the outcome of the change or transformation, every component affects the outcome of the reaction.
At the heart of green chemistry are alternative reaction media. They are the basis of many of the cleaner chemical technologies that have reached commercial development. Basic components of a reaction. Safer Solvents A major concern with regard to sustainability is the release of hazardous substances into the environment. Solvents, for example, are ubiquitous in academic, industrial, and government laboratories.
Applications of solvent in chemical processes. Various types of solvent applications in all types of industries and academic laboratories are illustrated in Fig. They also pose serious environmental, health, and safety EHS concerns, including human and eco-toxicity issues, process safety hazards, and waste management issues. Many of the commonly used solvents benzene, chlorinated organic solvents, etc. Alternatives to organic solvents are needed to decrease the negative environmental impact of these substances. One consequence is the need to avoid the use of organic solvents where possible.
When elimination is not feasible, efforts should be made to optimize, minimize, and recycle solvents. Solvents that are stable, inexpensive, and readily available, with an acceptable environment impact, are the most suitable. The selection of the solvent should consider the following: Solvents having an average to high degree of safety are listed in Table 5. As water is immiscible with most organic substrates, most reactions involving water are done with liquid—liquid biphasic systems. The use of biphasic organometallic catalysts to catalyze aqueous-phase reactions is a novel method to address this issue.
The catalyst in such reactions is a water-soluble transition metal complex with substrates that are partially water-soluble. The catalyst employed is a water-soluble Rhodium I complex of trisulfonated triphenylphosphine tppts see Fig. Chemo-selective hydrogenation in biphasic system. The same concept was used in the chemo-selective hydrogenation of unsaturated aldehydes see Fig.
Green Solvents Green solvents are environmentally friendly solvents, or biosolvents, which are derived from the processing of agricultural crops. The use of petrochemical solvents is the key to the majority of chemical processes but not without severe implications on the environment. Green solvents were developed as a more environmentally friendly alternative to petrochemical solvents. Ethyl lactate, for example, is a green solvent derived from processing corn. Ethyl lactate is the ester of lactic acid.
Ethyl lactate is a particularly attractive solvent for the coatings industry as a result of its high solvency power, high boiling point, low vapor pressure, and low surface tension. Ethyl lactate has replaced solvents such as toluene, acetone, and xylene, resulting in a much safer workplace. Other 98 Green Chemistry and Processes applications of ethyl lactate include being an excellent cleaner for the polyurethane industry. Ethyl lactate has a high solvency power, which means it is able to dissolve a wide range of polyurethane resins. The use of ethyl lactate is highly valuable, as it has eliminated the use of chlorinated solvents.
The chemistry in natural systems biochemical reactions is based on water. The use of water as solvent for synthetic chemistry holds great promise for the future in terms of the cheaper and less hazardous production of chemicals. Researchers in this area are discovering that reactions in water may be predisposed to favor transition states that optimize hydrophobic interactions, thereby achieving unusual, unique selectivity in organic reactions Sijbren and Engberts, Water as a solvent favored a more compact endotransition state in Dies—Alder reactions.
The accelerating effect of water has been ascribed to a number of factors, including the hydrophobic effect as well as hydrogen bonding between water molecules and reactants Breslow, Breslow and co-workers showed that hydrophobic interactions might determine the ratio of O versus C alkylations of phenoxide ions in water Breslow et al. They also used hydrophobic borohydrides to control the regio-selective reduction of the sulfated, naturally occurring steroid.
Use of hydrophobic oxaziridinium salts for epoxidation of cinnamic and crotonic acid derivatives showed Alternate Solvents 99 a very large increase in selectivity for the hydrophobic cinnamic acid derivatives. Solvent-Free Conditions Several advantages are associated with the use of a solvent-free system over the use of organic solvent.
There is no reaction media to collect, dispose of, or purify and recycle. Greater selectivity is often observed. Reaction times can be rapid, often with increased yields and lower energy usage. Economic considerations are more advantageous, since cost savings can be associated with the lack of solvents requiring disposal or recycling. Not surprisingly, solvent-free synthesis has recently drawn attention from the wider synthetic community. Reactions epitomizing the simplicity, versatility, high-yielding, and selective nature of solvent-free systems include aldol condensations, sequential aldol and Michael additions, Stobbe condensations, O-silylation of alcohols with silyl chlorides, and clay-catalyzed syntheses of transchalcones.
Thus, measurement of heat of reaction in solvent-free systems is important, as is effective heat dissipation. Ionic Liquids An ionic liquid generally consists of a large nitrogen-containing organic cation and a smaller, inorganic anion. The asymmetry reduces the lattice energy of the crystalline structure and results Green Chemistry and Processes in a low-melting-point salt. These simple liquid salts single anion and cation can be mixed with other salts including inorganic salts to form multicomponent ionic liquids. There are estimated to be hundreds of thousands of simple ion combinations to make ionic liquids and a near endless number of potential ionic liquid mixtures.
This implies that it should be possible to design an ionic liquid with the desired properties to suit a particular application by selecting anions, cations, and mixture concentrations. The components of ionic liquids ions are constrained by high coulombic forces and thus exert practically no vapor pressure above the liquid surface.
Importantly, the near-zero vapor pressure nonvolatile property of ionic liquids means they do not emit the potentially hazardous volatile organic compounds VOCs associated with many industrial solvents during their transportation, handling, and use. It should be noted, however, that the decomposition products of ionic liquids from excessive temperatures can have measurable vapor pressures. Another use for ionic liquids is as a medium for separation of biologically produced feedstock from a fermentation broth, such as acetone, ethanol, or butanol. Ionic liquids can act as both catalyst and solvent.
In many systems, the reaction products can be separated by simple liquid—liquid extraction, avoiding energy-intensive and costly distillation. Structural similarities among certain ionic liquids, herbicides, and plant growth regulators have been noted. New health and safety concerns could also result from ionic liquid residuals in polymers, particularly those used for packaging food and personal care products.
Broad commercialization of ionic liquids will require a sound, science-based understanding of their environmental, safety, and health impacts. The development of exposure and handling guidelines for ionic liquid production, transportation, storage, use, and disposal are required. CO2-based processes can also be used for dry cleaning, metal cleaning, and textile processing. Liquid CO2 is also used in the microelectronics industry to spin-coat photoresists instead of using traditional organic solvents. Biphasic catalytic oxidation of alcohols using PEG-stabilized palladium nanoparticles in scCO2 was studied.
This catalytic system shows high activity, selectivity, and stability in the conversion of structurally diverse primary and secondary alcohols to their corresponding aldehydes and ketones. Supercritical Water Water has obvious attractions as a solvent for clean chemistry. Both near-critical and supercritical water scH2O have increased acidity, reduced density, and lower polarity, greatly extending the possible range of chemistry that could be carried out in water. In an interesting report for the synthesis of terephthalic acid, scH2O was used as a solvent. The process is highly selective, but it can be energyintensive.
By contrast, p-xylene, oxygen, and terephthalic acid are all soluble in supercritical water. Thus, in the new method, oxygen from hydrogen peroxide was used as the oxidant with ppm of manganese dibromide as the catalyst. A key advantage of chemistry with supercritical water SCW is the possibility of varying the properties of the reaction medium over a wide range solely by changing the pressure and temperature and of optimizing the reaction in this way without changing solvent.
Furthermore, the reaction kinetics can be strongly affected in the supercritical region by varying the pressure kinetic pressure effect. In addition, many nonpolar organic substances e. In summary, SCW has great potential with regard to the optimization of chemical syntheses. There are, however, drawbacks from working at high pressures high investment costs , the problem of corrosion expensive materials , and the lack of kinetic and thermodynamic data.
But an approach that is instead intrinsic for instance, employing molecular design can be not only more reliable but also less costly. Some of the challenges facing chemists in this regard could be summarized as the following: The aim of all research must be to study a piece of reality so as to completely realize and experience all aspects of it. In the normal secular mode, we do not experience much at all. We read about a subject, we study it, and at the most we discover a few underlying and hidden laws and principles, but we have not experienced the joy of it.
But once we make the proper kind of study, seeing them all as perspectives, it will take us to this principle of delight and give us the total experience of reality. And once we have that, life will change all its colors, and research will enrich our sensibility. It will have made us know life as it ought to be known and studied. In other words, many new insights will be generated.
Hence, every step in this direction will go a long way in environmental protection. Originally PI was developed for the bulk chemical industry, but it has been extended to value-added chemicals and pharmaceutical active ingredient manufacture Wegeng et al. This energy savings could be achieved through the design of processes using compact heat exchangers using PI or using innovative heat-exchanger designs.
Micro reactor and heat exchanger heat transfer coefficient: So, compact heat exchangers will perform better and are less expensive than conventional shell and tube heat exchangers. These heat exchangers are made of ceramics or polymers or use new designs such as printed circuit heat exchangers or multistream heat exchangers.
Fine chemicals accounted for U. Multidisciplinary approach, which considers opportunities to improve the process technology and underlying chemistry simultaneously. High selectivity and rate and reduced batch cycle time. Process intensification—its features, key principles, and benefits. Delphi is a forecasting technique based on answers to questionnaires. The technique is widely used to facilitate formation of a group judgment without permitting interaction and biasing that normally happen during a group discussion.
It is a method for achieving a structured anonymous interaction between selected experts by means of a questionnaire and feedback. Delphi is commonly used Process and Operations 1. The questionnaire was sent to a preselected group of experts in order to obtain individual responses to the problems posed. Lower energy use in the shorter term. Improved safety, inventory reduction, and plant physical size reduction. Next to reducing energy, achieving safer plants is seen as the most important goal.
PI would lead to reduction in inventory, which would once again lead to safer plants. Switching from batch to continuous processes will help the chemical industries improve their public image. Sixty-two percent of the experts surveyed expressed interest in transforming chemical processes from batch to continuous processes in the short, medium, or long term. A move away from batch would improve safety, since controlling the continuous process would be easier. Except during startup and shutdown, the reactor operating conditions would be constant.
PI technology based on continuous processes provides an improvement in heat and mass transfer, lower residence time, lower 4. Green Chemistry and Processes inventories, better control, and better-quality products. Further reductions may not be practically achievable. In the s and s, the chemical industries were able to compete due to their larger scales of operation. A small-sized plant was not as economical as a large-scale plant examples of large-scale plants include petrochemicals, polymers, fertilizers, and sulfuric acid.
Seventy-eight percent of the respondents considered environmental improvements as a priority in the short term but not a top priority in the long run. The compact heat exchangers CHE are a good example of PI technology, which now forms the basis of very small reactors, as well as being routinely used for intense heat transfer, in many demanding applications. Such plants are expected to be feasible by Fifty-seven percent of the respondents believed in the feasibility of the heat pipe technology by the year Total heat transferred will also be high during phase changes. Such technological and business related barriers include 1.
Conservatism in the user industries. Lack of industrial and academic awareness. Academics are not taking the initiative to drive the advantages of PI to the industrial community. Loss of the buffering effect of large volumes. Lack of codes of practice, unlike the conventional systems, which have the complete data. Concern about fouling of the hardware and handling of solids. Lack of supporting tools. Many industries move their manufacturing operations to other countries to achieve lower production costs.
The experts believe that all the technologies studied such as heat pipes, rotating equipment, and enhancement devices may approach the growth stage in the medium term around to Reactions PI in reactions has led to several new designs and techniques. Miniaturization can be developed based on the categorization of reaction kinetics. Type A reactions are very fast, reaction times are of the order of less than 1 s, and the overall reaction is mixing controlled. Process and Operations 2. Type B reactions are rapid, where the reaction times are of the order of 1 s to 10 min.
These reactions are kinetically controlled, but temperature control is very essential. Type C reactions are slow, where the reaction times are of the order of greater than 10 min. These reactions are well suited for batch processing, but safety and quality are issues that need to be handled. They found that nearly half the reactions that are practiced in industries bear microreactor potential. It was also concluded that Type A reactions could be handled with micromixers possibly with integrated or subsequent heat exchangers.
Also, a microstructured component could be inserted into a conventional batch plant for Type B and C reactions, for instance, tube reactors. Reactor Designs PI has led to the design of a variety of new and innovative reactor designs to overcome mass and heat transfer limitations that are normally encountered in large-scale vessels Semel, With these designs it is possible to carry out highly exothermic reactions, speed up the rate of reaction by several orders of magnitudes, totally eliminate side reactions and hence waste formation, combine reactions with unit operations, and telescope several steps into a single step.
Microreactor The ubiquitous batch reactor can be used to carry out polymerization reactions in the laboratory, but the recipe to be used on the plant scale has to be changed to match the relatively poor heat transfer and mixing performance of a larger-diameter vessel. Reactor on chip multichannel. During scale-up instead of tuning the hardware to match the process, many times the process is matched to a particular hardware.
Related Chromium -VI Reagents: Synthetic Applications (SpringerBriefs in Molecular Science)
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