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How green chemistry will change chemical engineering
C H A P T E R
2 How green chemistry will change chemical engineering Gaetano Iaquanielloa,b,*, Agnese Ciccic a
KT e Kinetics Technology S.p.A., Rome, Italy; bNextChem srl, L’Aquila, Italy; cBio-P srl, via di Vannina Roma, RM, Italy * Corresponding author. e-mail address: [email protected]
1. Introduction
1.1 What makes a chemical process green or greener?
Since the word “green” started being placed in front of “chemistry,” many people have argued about what the “right” definition of “green chemistry” is or is not [1e4]. And more importantly, what should actually be done to make chemistry green or greener; in other words, how will chemical engineering implement the green chemistry principles? Although quite a few people see green chemistry as separate from engineering, in this chapter we are going to see that they are not two separate disciplines, and we will explore what chemical engineering needs to face such new requirements. Starting from the green chemistry principles, we will discuss the need for new feedstocks, new plant architectures, and novel catalysts and new issues, like scale-up, that engineering is going to face.
Catalysis, Green Chemistry and Sustainable Energy https://doi.org/10.1016/B978-0-444-64337-7.00002-1
An early conception of green chemistry was developed in 1990 by P. Anastas and J. Warner [5] through 12 principles ranging from prevention and atom economy to real-time control for pollution prevention to inherently safer chemistry and spillover prevention. These principles, described below, offer a protocol to adhere in developing novel chemical processes. U Waste prevention: Prevent waste production, rather than cleaning up and treating wastes after having been produced. Plan to minimize waste at every process stage. U Atom economy: Reduce waste by recycling the number of atoms from all reagents that are incorporated into the final product. Use the atom recycling concept to evaluate reaction efficiency.
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Copyright © 2019 Elsevier B.V. All rights reserved.
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2. How green chemistry will change chemical engineering
U Less hazardous chemical synthesis: Design chemical reaction paths to be as safe as possible. Consider the hazards of all substances handled during each step of the reaction, including waste. U Designing safer chemicals: Minimize toxicity directly by proper design. Predict and analyze factors such as physical properties, toxicity, and environmental impact of each designed process step. U Safer solvents and auxiliaries: Look for the safest solvent available for any given step. Optimize the total amount of solvents and auxiliary substances used to minimize the waste produced. U Design for energy efficiency: Find the least energy-intensive chemical route, thus reducing heating and cooling, as well as pressurized and vacuum conditions (i.e., try to stay as close as possible to ambient temperature and pressure). U Use of renewable feedstocks: Use feeds that are made from renewable (i.e., plant-based) sources, rather than other chemicals made from petrochemical products. U Reduce derivatives: Minimize the use of temporary derivatives such as protecting groups. Avoid derivatives to reduce reaction steps, resources required, and waste produced. U Catalysis: Look for catalysts that help to increase selectivity, minimize waste, reduce reaction times, and increase energy efficiency. U Design for degradation: Design products that can degrade themselves easily into the environment. Ensure that both original and degraded products are not toxic, bioaccumulative, or environmentally persistent. U Real-time pollution prevention: Control closely chemical reactions in real time and prevent the formation and release of any potentially hazardous and polluting products into the environment.
U Safer chemistry for accident prevention: Develop chemical processes and procedures that are safer and inherently minimize the risk of accidents. Evaluate all the potential risks and assess them beforehand. Today, more than 98% of all products and materials essential to the modern economy are still derived from petroleum and/or natural gas, generating substantial quantities of waste and emissions. A typical example of such production is represented by plastic materials, which are also a typical example of a linear economy: nonrenewable resources, oil or ethane in this case, are used to produce plastic materials, which at the end of life become waste and dispersed into the environment. Today some 8 million metric tons escape into the world’s oceans each year [6], most of it from countries in Southeast Asia, where plastics use has outpaced waste management infrastructure and the situation is approaching catastrophic proportions. Fig. 2.1 depicts the interconnections between society, environment, energy, and the green chemistry processing industries. Such interconnections are shaped by the aforementioned 12 principles, and the massive implementation of them may bring about a real new industrial revolution [7], although a better understanding of the implementation time of such a revolution is required. This chapter reweaves some key aspects and tries to evaluate their impact on the future of chemical engineering. It will explore the use of renewable feedstocks and of atom economy, in particular regarding waste materials in smarter manufacturing processes in which new catalysts are used and less-intensive process conditions are selected, according to the previous green chemistry concepts.
I. Introduction
2. Renewable and waste materials as new feedstocks
FIGURE 2.1
23
New paradigm for society, environment, energy, and the green chemistry/processing industries.
2. Renewable and waste materials as new feedstocks While waste prevention is far from realization, the use of renewable feedstocks is a reality in modern society; the implementation of technology based on the use of wastes as feedstocks can indirectly contribute to limiting the total waste increment and, at the same time, will maximize the number of atoms incorporated into the final products. In the following sections we focus your attention on specific feedstocks that have relevant positions in the current technology. The seventh principle proposes indeed the use of waste materials as new feeds for new processes.
2.1 Principal solid wastes The shift from hydrocarbon-based feedstocks to renewable ones within conventional or novel processes represents one of the major changes to move to a greener and sustainable processing industry. Biorenewable resources must support the
growing demand for chemical commodities. At the same time, they will relax concerns regarding climate change and energy security. However, as far as costs go, competing against conventional products derived from low-priced oil and natural gas will be not easy, and at today prices almost an impossible task. Biobased materials are all more expensive than natural gas, for instance. According to Bomgardner [8], the raw material prices show a consistent variation, starting from natural gas ($1.80 per million British thermal units [BTU]), followed by biomass ($4.00 per million BTU), crude oil ($7.92 per million BTU), and corn ($8.00 per million BTU) and arriving to sugar at $22.60 per million BTU. Looking at such prices in comparison with that of natural gas, a clear picture of evident difficulties appears. Process developers need, then, to choose lower cost biobased feedstocks, including those traditionally classified as waste streams, such as fuel derived from municipal solid waste (MSW). The MSW amount is going to double in the coming years, with a steady increase in the costs of collection and disposal. From this
I. Introduction
24
2. How green chemistry will change chemical engineering
point of view, residue-derived fuel (RDF) may represent a very interesting negative-cost feedstock for chemical production, mainly for methanol, urea and even ethanol [9e11]. RDF is a sort of waste, basically what is left from MSW after recovering wastepaper, glass, metals, and the organic fraction, with a calorific value (lower heating value) ranging from 12,000 to 16,000 and even to 18,000 kJ/kg. Today, RDF is disposed of through either landfilling or thermochemical treatment processes aiming to produce electricity, the so-called waste-to-energy plants.
2.2 Plastic wastes A similar approach is suggested by the Ellen MacArthur Foundation [11] for a new plastic economy; renewably sourced virgin feedstock is strongly reduced by creating an effective after-use of plastic wastes by mechanical and chemical recycling (e.g., carbon recycling). A more specific chemical recycling for polyethylene terephthalate (PET) waste is proposed by the DEMETO project [12], whereby PET wastes are transformed into their original monomers, terephthalic acid and ethylene glycol, ready to make brand new PET and closing the loop of PET production, use, and recycling.
2.3 Biomass Biomass as such is a potential source that could be converted into energy. For large portions of the rural populations of developing countries, and for the poorest fractions of urban populations, biomass is often the only available and affordable source of energy for basic needs such as cooking and heating. This phenomenon is described by the concept “feedefoodefuel conflict.” This has driven the bioenergy sector to choose other types of biomass that do not occur in the nutrition chain, such as agricultural residues and food crop wastes. The second biofuel generation was born.
A quite different approach is the use of unconventional feedstocks such as microalgae for the production of biofuels and biochemicals. Several oleaginous microalgae are capable of accumulating lipids up to 70% in dry biomass, but even in these cases the lipid productivity is actually low. Together with neutral lipids, algae can also store carbohydrates (polysaccharides and starch), carotenoids (as lutein, astaxanthin, b-carotene), proteins, and other molecules [13]. Production of lipids and carbohydrates has received growing interest for biofuel production, while carotenoids and other minor products are usable as feed additives and nutraceutical compounds. Today biofuel production from microalgae does not seem economically feasible, while there are different manufacturing plants in the world to produce high-value chemicals such as carotenoids. Microalgae cultivation may solve social and ethical issues related to the use of fertile lands and edible plants. As a matter of fact, they can grow on infertile lands and reach a productivity per hectare higher than most plants. More recently, a two-stage process has been developed to improve process efficiency, to reduce operative and investment costs [13]. In the first stage, microalgae are cultivated under phototrophic conditions, and then, when biomass concentration rises and the light becomes the limiting factor for growth, the microalgae are transferred to a heterotrophic reactor, without illumination, where they are cultivated by using wastewater as a source of nutrients, mainly organic carbon. This will result also in less hazardous chemical synthesis.
2.4 Natural-based fatty acids Another unconventional feedstock is represented by natural-based fatty acids, which have been a workhorse of the chemical industry for many years. Fueled by the green chemistry agenda and growing demand for more
I. Introduction
3. Loweenergy-intensity processes
environmentally friendly and natural ingredients, such a market has received a considerable boost. An almost 6% per year compound annual growth rate is expected from 2017 through 2022. Animal and vegetable oils, and their derivates, are one of the most important biobased feedstocks that are receiving ever more increasing interest from industries. Vegetable fats, which are more unsaturated, are considered healthier and more functional than animal fats. Their abundance and low price make them an attractive staple for the chemistry industry. Furthermore, the oils of vegetable origin, composed of mono-, bi-, or triglycerides, are a significant source of polymer precursors. The double bonds and the allylic site of the triglycerides are highly reactive, being a suitable starting point for the production of several polymers with different structures and functionalities. Fatty acids are commonly used for the synthesis of renewable chemicals, such as the crossmetathesis of oil-derived olefins with ethylene; this is a valid alternative pathway for shortchain intermediate synthesis with several applications (from the polymer to the surfactant industries). Ethenolysis of methyl oleate produces 1-decene and methyl-9-decenoate. The first has numerous industrial applications (i.e., as an intermediate in epoxide production, amines, synthetic lubricants, and as a monomer in copolymers). The second is an intermediate in the synthesis of several products, such as nylon 10 or lubricants and plasticizers. Green metathesis with an ionic liquid (IL) may represent a very interesting process in olefin metathesis. We are going to discover why in the following sections.
3. Loweenergy-intensity processes The sixth principle highlights the importance of choosing the least energy intensive route; indeed, the level of energy requirements in a
25
chemical process determines its environmental and economic impact; from this point of view, carrying out a conversion at room pressure and temperature may represent an ideal approach in scaling low-intensity processes. A few examples will be given in the following, covering fermentation, membrane reactors, and the use of IL as solvent/catalyst.
3.1 Fermentation processes Biotechnological products are very attractive solutions because of their usage of renewable sources (sunlight, water, etc.) for the production of several molecules with low-energy and efficient processes that have been improved to give high-quality products with low toxicity. In some cases, the fermentation of microbial species is the only way to produce specific molecules in substantial quantities. In particular, biocatalysis and metabolic engineering are the most evolved biotechnologies that are transforming the conventional chemical industry more and more, always with the aim of increasing the efficiency and productivity of numerous bioprocesses. As an example of these considerations, Table 2.1 lists the annual production (in tons) by fermentation of each considered product [14]. 3.1.1 Energy consumption in fermentation processes Some fermentation processes have been used on a large scale to make ethanol, acetic acid, soy sauce, lysine, astaxanthin, and many other substances. These aerobic processes, however, need large quantities of air/oxygen. Despite the different requirements of individual reactions’ synthesis (or degradation), they require cooling at temperatures near to ambient to remove metabolic heat. This aspect, coupled with the aeration rate, the mixing system, and all the downstream processes for the separation of the desired product, leads to high electricity consumption. Focusing only on the fermentation
I. Introduction
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2. How green chemistry will change chemical engineering
TABLE 2.1
Bioproduct quantities obtained by fermentation.
Product
Annual production (tons)
Bioethanol
26,000,000
L-Glutamic
acid (MSG)
Citric acid
1,000,000 1,000,000
L-Lysine
350,000
Lactic acid
250,000
Food-processing enzymes
100,000
Vitamin C
80,000
Gluconic acid
50,000
Antibiotics
35,000
Feed enzymes
20,000
Xanthan
30,000
L-Threonine
10,000
L-Hydroxyphenylalanine
10,000
6-Aminopenicillanic acid
7,000
Nicotinamide
3,000
D-p-Hydroxyphenylglycine
3,000
Vitamin F
1,000
7-Aminocephalosporinic acid
1,000
Aspartame
600
L-Methionine
200
Dextran
200
Vitamin B12 Provitamin D2
known example is succinic acid. The most used process (with a petrochemical basis) for the production of succinic acid is the hydrogenation of maleic anhydride to succinic anhydride and it successive hydration to succinic acid. The latter step begins immediately when succinic anhydride is dissolved in hot water. The succinic acid obtained is finally separated by crystallization, filtration, and drying. Moreover, because of its competitive cost compared with the petrochemical route, despite its lower material efficiency, fermentative succinic acid is catching on more and more (see Table 2.2). These optimizations must be focused on both biological field (yeast optimization) and the overall process (i.e., reduction of water consumption, generation of valuable sub- and coproducts, optimization of downstream steps). As for total production cost, the reasons behind the lower values for the foresight case (Reverdia process, DSM patent) were the deletion of sulfuric acid and anhydrous ammonia production, thus reducing by 71% the bio-succinic acid variable costs. Another key point was the constant cost reduction of V743/MT SA achieved by means of ethanol coproduction in the foresight case, increasing also the energy efficiency value [16].
3.2 Development of membrane reactors
12 5
From M. Gavrilescu, Y. Chisti, Biotechnologyda sustainable alternative for chemical industry, Biotechnology Advances 23 (2005) 471e499.
reaction, the energy balances performed are based on the consideration that the heat of reaction per electron transferred to oxygen is relatively constant for the oxidation of a wide variety of organic molecules. Several molecules can be produced via both petrochemical and biological routes; a well-
The use of selective membranes in chemical reactors is a quite innovative step, allowing important benefits in terms of reactor performance, compactness, and cost, as result of the product’s removal or reactant’s feed [17]. Many papers have presented this concept related to various processes such as natural gas and methanol/ethanol steam reforming, water gas shift reaction, autothermal reforming (ATR), conversion of hydrogen sulfide into sulfur and hydrogen, and alkane dehydrogenation and validating it for both endothermic and exothermic reactions.
I. Introduction
27
3. Loweenergy-intensity processes
TABLE 2.2
Compared results for succinic acid production. Fermentative base case
Raw material
Sorghum, first generation
Sorghum, third generation
Fermentative foresight case
Sugar beet, first generation
Sorghum, first generation
Sorghum, third generation
Sugar beet, first generation
Petrochemical case Maleic anhydride
E factor
6.7
6.8
6.8
7.8
7.8
7.8
0.3
Material efficiency (%)
13
13
13
11
11
11
76
Total energy input (GJ/MT SA)
53.0
53.8
50.3
97.6
99.5
91.6
55.1
30
30
32
51
50
54
23
Land use (ha/ MTSA)
0.44
0.05
0.14
0.97
0.12
0.30
d
Total fossil energy substituted per unit land area (GJ/ha)
98.1
816.5
320.0
49.5
412.3
161.6
d
Total production costs (V/MTSA)
1045
629
649
1040
85
141
2554
Energy efficiency (%)
SA, succinic acid. H. Yukawa, T. Nambu, Y. Matsumoto, Design of group 5 metal-based alloy membranes with high hydrogen permeability and strong resistance to hydrogen embrittlement, in: A. Basile, A. Iulianelli (Eds.), Advances in Hydrogen Production, Storage and Distribution, Woodhead Pub. Series in Energy, 2015, pp. 341e367 (Chapter 13). Copyright: Elsevier, 2015.
-
In general:
-
Or, the membrane needs to be selective to a reactant. Then it is possible to manage the feedstock rate in the reactor or to distribute the feed on catalyst beds uniformly to better control the reaction thermal effects.
A membrane needs to be selective to a product. In such a case, the thermodynamic threshold related to conventional equilibrium can be overcome, since, by removing a product, the equilibrium is never achieved. The reaction conversion will not be limited by thermodynamics but will be regulated by the performance of the membranes in terms of permeation and reaction rates. This will result in a reduction in the operating temperature if the reactions in the membrane reactor are endothermic, thus increasing the overall efficiency, and conversely increasing the temperature when the reactions are exothermic, thus facilitating the kinetics.
In both cases, membrane reactors allow the chemical processes to overcome the barriers of thermodynamic equilibrium. The operating conditions and membrane reactor size must be properly selected to maximize the benefits. Temperature and pressure gradient limitations must be imposed for making the membrane operation safe and reliable. A membrane reactor is then a sort of chemical reactor in which a membrane selective for one of the reaction components is integrated by being placed internally or externally by the reactor
I. Introduction
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2. How green chemistry will change chemical engineering
itself. The example presented below is related to the selective feeding of a reactant. The hydrocarbon ATR reaction is considered: Cm Hn þ xH2 O þ yO2 4aCO þ ðm aÞCO2 þ ðx þ 2y 2m þ aÞH2 O n 2y þ 2m a H2 þ 2
(1)
ATR combines partial oxidation and steam reforming in a single reactor. The hydrocarbon feedstock is reacted with a mixture of oxygen or air and steam under substoichiometric conditions. The composition of the product gas is determined by the thermodynamic equilibrium. The exit pressure and temperature are determined by an adiabatic heat balance based on the composition and flow of the feed, and steam and oxygen are added to the reactor, which in turn determines the composition of the product gas at the thermodynamic equilibrium. A conventional ATR reactor consists of two zones: a thermal or combustion zone followed by a catalytic zone. In the first zone, a portion of the feed is oxidized. This generates the amount of heat required to drive the successive reaction of steam reforming in the catalytic zone. By proper control of oxygen-to-carbon and steam-to-carbon ratios, the partial combustion in the first zone supplies the heat for the subsequent endothermic steam reforming reaction. The main issue of the ATR reactor is that the partial combustion, drastically exothermic, is happening in a small zone at the upper part of the reactor, thus leading to the formation of hot spots. Moreover, since the steam reforming is strongly endothermic, the operating temperature is dropping fast along the catalytic zone. Supplying the oxygen stream all along the reactor, through a properly selected membrane, allows the elimination of the separation between the combustion zone (too high temperature) and the catalytic zone (too low temperature) and then the uniform distribution of the ATR reaction and the temperature profile along the
reactor. In this example, however, using a breakable and expensive element as the selective membrane inside a catalytic reactor may become quite complicated, particularly from the maintenance point of view. The reaction’s product removal is represented in Fig. 2.2, where the hydrogen produced via methane steam reforming is separated from the mixture through a metallic Pd membrane. The removal of H2 will allow a reduction in the operating temperature for the same methane conversion and at the same time reduce the heat requirement for product unity [17]. The design of such a membrane is quite a complex task, because of the need to have high hydrogen permeability, a strong resistance to embrittlement, and stability.
3.3 Room-temperature ionic liquids The demand for low-environmental-impact solvents led to the development of new green products and processes; among them, ILs are gaining growing interest from the scientific community for their features [18]. Indeed, the ILs have a very low volatility and flammability, and, on the other hand, they have high thermal and chemical stability. Furthermore, in accordance with the cations and anions used in their formulation, they can change their physical and chemical properties (acidity, basicity, water miscibility and immiscibility, hydrophilicity and hydrophobicity) [18]. Due to these properties, ILs have been used since 1970 for several applications, from electrolytes for batteries [19] to catalysis, biological reactions, and metal ion removal [20].
4. Green chemistry and catalysis (principle No. 9) Catalysis, which has played a vital role in the processing industries of the 20th century, will also be a key enabling technology to achieve the objectives of sustainable and green catalysis. It is
I. Introduction
4. Green chemistry and catalysis (principle No. 9)
FIGURE 2.2
29
Permeation of hydrogen through a metallic Pd membrane.
quite evident that to ensure cleaner, greener, and more efficient processes we need to develop new and longer lasting catalysts, with higher conversion and selectivity, to increase the product’s purity and minimize by-product formation, a potential source of pollution. The relationship between chemical yield and waste minimization needs to be better understood, because it is a key concept of “atom economy”: the conversion process should be developed in order to maximize the recycling of all materials used in the feed into the final product. Heterogeneous catalysis, in particular, is going to be more important by providing the easy separation of product from catalyst, thereby eliminating the need for a further separation through distillation and extraction. From an environmental point of view, catalysts such as clays or zeolites may replace conventional catalysts currently in use, which are more hazardous. In this section, we are going to discuss two new catalyst families, nanocatalysts and the perovskite type, which are promising candidates, and enzyme biocatalysis.
4.1 Perovskites Catalysts used in the modern chemical industry are mainly based on mixed metal oxides. Perovskite-type oxides are attracting much
scientific application interest due to their low price, adaptability, and thermal stability, which depends on bulk and surface characteristics. Perovskites are generally described in the literature by a set of letters: ABX3. They have a cubic structure, centered around the large cation B; cations A occupy the cube's vertices whilst anions X are positioned in the center of the cubic cell's sides. Although the most interesting perovskite structures are oxides, some carbides, nitrides, and hydrides may also crystallize in this structure. In particular, perovskites with transition metal ions in the B site show an enormous variety of electronic and magnetic properties and behaviors. Such a variety is related to the complex character of transition metal ions and their flexibility in coordination with oxygen.
4.2 Nanocatalysts Nanomaterials may play an important role in green chemistry catalysis due to the increased ability in the nanostate and the high surface area. Materials having structural components with at least one dimension lower than 100 nm are considered nanomaterials. Materials with area reduced to the microscale can show different properties compared to what they exhibit on the macroscale, enabling unique
I. Introduction
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2. How green chemistry will change chemical engineering
behaviors and applications. For instance, a material such as aluminum becomes combustible; solid gold turns to liquid at room temperature; silicon insulators become conductors. Even a chemically inert material at normal scale, such as gold, can serve as a potential chemical catalyst at nanoscale. The nanosized particles are such that the exposed surface area of the active components of the catalyst is much higher than that of a conventional catalyst, increasing the contact points between reactants and catalyst. They are acting as a heterogeneous catalyst with the advantages of a homogeneous environment. This makes the product separation from the catalyst easier than with a homogeneous catalyst. Where the transport of the reactants to the catalyst is limiting the specific process, such a catalyst may play an important role in increasing the rate of reactions. Nanotechnology can lower the cost of production (COP)/cost of removal and hence be more effective for the removal of pollutants in water, particularly in advanced oxidation processes, for instance.
4.3 Enzyme biocatalysis Since 2000, the usage of enzymes and other kinds of biocatalysts has greatly simplified the reactions needed to obtain a determined product, compared with conventional synthesis processes, increasing thus the manufacturing operations’ efficiency. These features had a large impact on the pharmaceutical and biotechnological fields, widening the portfolio of products made by this technology. The following points summarize the principal desired features of biocatalysts: 1. Biocatalysts should be better, faster, less expensive, and more versatile than comparable chemical catalysts. 2. Biocatalysts should be able to catalyze an increased range of reactions, have higher temperature stability, and have improved solvent compatibility.
3. Molecular modeling and other tools should be developed to permit the rapid design of new enzyme catalysts. Progress is under way in all the above areas to provide the chemical industry with diverse new useful biocatalysts. Furthermore, biocatalysts can produce or transform a single enantiomer of chiral compounds. Since in most cases only one enantiomer has the desired activity and the other may be harmful (e.g., thalidomide, where only one enantiomer prevents morning sickness during pregnancy, while the other causes deformities in the fetus), the pharmaceutical industries are pushing the regulatory authorities to ensure that the final products will be constituted only from the desired enantiomer and from a racemic mixture. This ability to make only one enantiomer of a racemic mixture in larger amounts lowers the general process expenses, such as all the costs related to further purification of a product contaminated with an unwanted enantiomer. An example of this technology can be represented by the production processes of semisynthetic penicillin and cephalosporin antibiotics derived from 6-aminopenicillanic acid and 7-aminocephalosporanic acid, respectively. Several other pharmaceuticals use enzymatic catalysis, such as the semisynthetic antibiotic cephalexin, which is derived from cephalosporin C. This compound is produced mainly by the DSM Company (www.dsm.com), using a 4-step enzymatic process to overcome the conventional one with up to 10 steps that generated up to 50 kg of waste per kilogram of antibiotic. The various processes for the production of cephalexin are compared in Table 2.3, to which a recently developed process with direct fermentation that seems to have less waste than the enzymatic route is added [14,21].
I. Introduction
31
5. The scale-up issue
TABLE 2.3
Comparison of chemical and biotechnological processes for producing cephalexin [14]. Process type
Production category
Conventional chemical
Enzyme biocatalysis
Direct fermentation
Waste (kg/kg cephalexin)
50 (1970) to 15 (1995)
10 (1995) to 5 (2000)
2e5
Inorganics (kg/kg)
0.5
0.5
Organics (nonhalogenated) (kg/kg)
1.0
0.2
Solvents (nonhalogenated) (kg/kg)
1.7
0.3
Solvents (halogenated) (kg/kg)
0.9
0
Electricity (%)
100
150
Steam (%)
100
40
Water (%)
100
300
Liquid nitrogen (%)
100
0
5. The scale-up issue Scale-up refers to those activities that enable a change in the production scale, from the laboratory scale, where the production is achieved, to full manufacturing, where it is common to have production levels of tons or hundreds of tons per day. From this point of view, the conventional chemical industry is dominated by one key economical aspect: it looks more attractive to build one big apparatus than several parallel ones. The investment costs (capital expenditures, or CAPEX) go up with production capacity , following roughly the 0.6 rule for scaling of process equipments' cost. If the digression coefficient is smaller than unity, there is an economic incentive to increase the size of equipment rather than to multiply the number of pieces of equipment. As a result of this paradigm, the plant capacity of ethylene production was pushed from 40,000 tons/year in the 1950s to 1,500,000 tons/year today. Green chemistry is dominated by batch or semibatch processes, with reactions taking place in a reactor under stirring, and eventually, heating and cooling. In such a discontinuous reactor,
the feed is introduced into the reactor and is voided after the reaction has taken place. The reaction step is followed by processing, purification, and/or isolation steps to obtain the required production. Will green chemistry follow the same path? Or will the presence of unit operations such as fermentation, introduced to lower the energy intensity of the processes, shift it to other directions Will the COP of these novel processes then be able to compete against the high plant capacity of the conventional chemical industry? In this section, we will try to give some answers related to on fermentation and to relevant catalysts.
5.1 Fermentation scale-up It is a fact that in quite few applications a reduced yield is obtained at large scale compared with the small scale. Such poor results are related to stirring energy, mass, and heat transfer coefficients, which become more problematic to control, limiting the outcoming yield; the microorganism will “see” a different microenvironment, which in some cases may be far away from the optimal behavior, which maximizes the yield.
I. Introduction
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2. How green chemistry will change chemical engineering
When working with larger reactors, micromixing becomes more difficult, and this mechanism is important, not only in fermentation, such as crystal formations, or chemical reactions sensitive to stoichiometry or heat. The most important requirement for experiments at lab scale is that they have to be representative of the conditions prevailing at the large scale. This determines the possibilities and limits of small-scale experiments, and it will allow a change of scale based on a dimensional analysis or a regime analysis. Optimization of a process at laboratory scale and modeling the predominant phenomena are fundamental steps. During optimization, one has to keep in mind, however, that the optimized solution has to be translated back to the production scale. Consequently, not all possible optimization results can be used in a quite complex situation such as the fermentation unit. The simplest technique for scale-up is, of course, “multiplication of elements,” but it can be very expensive relative to the COP. If you can do a fermentation at 20 L, you can do it at 20 m3 by using 1000 fermenters in a row, although it does not look like a cheap solution. By doing this you need to define a different scale-up strategy: first size enlargement until constraints are reached, then scale up by number. In such a case, you may be able to scale up to 1000 L and install 20 fermenters. What it is important to stress is that the introduction of fermentation into your process is going to create a scale-up issue, limiting the possibility of increasing the volume of the reactor and not going from the 0.6 rule of process equipment. The increase in CAPEX is going to negatively affect the COP and make it more difficult to compete.
5.2 Catalyst scale-up Catalysts are essential in most reactions, particularly in green chemistry, where a low temperature is considered. In such conditions
a fixed-bed catalytic reactor is probably reaction rate limited because the chemical reaction rate constant dominates the global rate constant. A change of catalyst makes sense in most cases in which the conversion rate is limited by the reaction rate. By changing the physical properties of a solid catalyst (i.e., shape, pore-size distribution, pore size, surface area) it is possible to increase the conversion rate but only if the process is pore-diffusion-rate limited. Conversely, if the process is limited by the film diffusion rate, increasing the linear fluid velocity through the bed solids will raise the conversion. From a green chemistry/engineering point of view, ineffective mass transfer may lead to a reduction in reaction mass efficiency and loss of yield due to side reactions with potential increase in by-products, which in turn may have a negative impact on downstream separation.
6. Conclusion and future trends Green chemistry will change the chemical engineering business radically, but to do this, first of all it needs to become competitive against the conventional process industry based on hydrocarbon feedstocks. Selection of low-priced renewable materials is a key aspect in such competition, and then waste materials are going to play a major role, being negative-cost feedstocks for chemical production. Recycling wastes related to MSW such as RDF, algae, and fats and oils of animal and vegetable origin may provide a platform to build a resilient green chemistry production of commodities. This will not be enough, however. New lowintensity processes together with more specialized catalysts are required to win such competition. Last, but not least, the scale-up issue has to be well addressed and developed to effectively compete against conventional processes.
I. Introduction
References
List of abbreviations and acronyms 6-APA 7-ACA ATR BioSA BTU CAGR CAPEX COP EG IL LHV MR MSG MSW O/C OPEX PAC PET RDF S/C SA SAN SR TMI TPA WTE
6-Aminopenicillanic acid 7-Aminocephalosporanic acid Autothermal reforming Bio-succinic acid British thermal unit Compound annual growth rate Capital expenditure Cost of production Ethylene glycol Ionic liquid Lower heating value Membrane reactor Monosodium glutamate Municipal solid waste Oxygen to carbon Operative expenditure Production capacity Polyethylene terephthalate Residue-derived fuel Steam to carbon Succinic acid Succinic anhydride Steam reforming Transition metal ion Terephthalic acid Waste to energy
List of Symbols a
Moles of produced carbon monoxide in the auto-thermal reforming reaction m Number of carbon’s atoms of the generic reactant hydrocarbon in the auto-thermal reforming reaction m-a Moles of produced carbon dioxide in the auto-thermal reforming reaction n Number of hydrogen’s atoms of the generic reactant hydrocarbon in the auto-thermal reforming reaction n/2-2y + 2m-a Moles of produced hydrogen in the autothermal reforming reaction x Moles of reactant water in the auto-thermal reforming reaction x + 2y-2m + a Moles of produced water in the auto-thermal reforming reaction y Moles of reactant oxygen in the auto-thermal reforming reaction
33
References [1] G. Centi, S. Perathoner, From green to sustainable industrial chemistry, in: F. Cavani, G. Centi, S. Perathoner, F. Trifir o (Eds.), Sustainable Industrial Chemistry, Wiley-VCH, Weinheim, Germany, 2009. [2] M. Lancaster, Green Chemistry: An Introductory Text, RSC, Cambridge, UK, 2016. [3] M. Poux, P. Cognet, C. Gourdon (Eds.), Green Process Engineering: From Concepts to Industrial Applications, CRC Press, Boca Raton, FL (US), 2015. [4] J.H. Clark, Green chemistry: challenges and opportunities, Green Chemistry 1 (1999) 1e8. [5] P.T. Anastas, J.C. Warner, The Twelve Principles of Green Chemistry, Oxford Univ. Press, Oxford, UK, 1998. [6] A.H. Tullo, Fighting ocean plastics at the source, Chemical and Engineering News 96 (16) (2018) 29e34. [7] P.S. Gosh, How Chemical Engineering will Drive the 21st Century? The Mega Possibilities Ahead, in: AIChE Leadership Forum October 2006. Accessed at: http:// web.mit.edu/psgleadership/pdf/1_required_ reading/How%20Chemical%20Engineering%20will% 20Drive%20the21st%20Century%20Woburn.pdf. [8] H.M. Bomgardner, Renewables firms focus on cost squeeze, Chemical and Engineering News 94 (18) (2016) 26. [9] G. Iaquaniello, G. Centi, A. Salladini, E. Palo, S. Perathoner, L. Spadaccini, Waste-to-methanol: process and economics assessment, Bioresource Technology 243 (2017) 611e619. [10] E. Antonetti, G. Iaquaniello, A. Salladini, L. Spadaccini, S. Perathoner, G. Centi, Waste-toChemicals for a circular economy: the case of urea production (Waste-to-Urea), ChemSusChem 10 (5) (2017) 912e920. [11] Ellen Mac Arthur Foundation, The new plastics Economy. Accessed at: https://www.ellenmacarthurfoundation. org/assets/downloads/publications/NPEC-Hybrid_English_22-11-17_Digital.pdf, 2017. [12] Demeto EU Project 768573, 2017. Accessed at: https:// www.demeto.eu/mission. [13] F. Di Caprio, A. Visca, P. Altimari, L. Toro, B. Masciocchi, G. Iaquaniello, F. Pagnanelli, Two-stage process of microalgae cultivation for starch and carotenoid production, Chemical Engineering Transactions 49 (2016) 415e420. [14] M. Gavrilescu, Y. Chisti, Biotechnologyda sustainable alternative for chemical industry, Biotechnology Advances 23 (2005) 471e499. [15] H. Yukawa, T. Nambu, Y. Matsumoto, Design of group 5 metal-based alloy membranes with high hydrogen permeability and strong resistance to hydrogen embrittlement, in: A. Basile, A. Iulianelli (Eds.),
I. Introduction
34
2. How green chemistry will change chemical engineering
Advances in Hydrogen Production, Storage and Distribution, Woodhead Pub. Series in Energy, 2015, pp. 341e367 (Chapter 13). [16] J.M. Pinazo, M.E. Domine, V. Parvulescu, F. Petru, Sustainability metrics for succinic acid production: a comparison between biomass-based and petrochemical routes, Catalysis Today 239 (2015) 17e24. [17] A. Basile, M. De Falco, G. Centi, G. Iaquaniello (Eds.), Membrane Reactor Engineering: Applications for a Greener Process Industry, Wiley, Chichester, UK, 2016.
[18]
[19]
[20] [21]
I. Introduction
T. Welton, Room-temperature ionic liquids. Solvents for synthesis and catalysis, Chemical Reviews 99 (8) (1999) 2071e2084. A. Lewandowski, A. Swiderska-Mocek, Ionic liquids as electrolytes for Li-ion batteries -An overview of electrochemical studies, Journal of Power Sources 194 (2) (2009) 601e609. H. Ohno, Electrochemical Aspects of Ionic Liquids, Wiley-Interscience, Hoboken, 2005. A.L. Demain, Small bugs, big business: the economic power of the microbe, Biotechnology Advances 18 (2000) 459e546.
2 How green chemistry will change chemical engineering Gaetano Iaquanielloa,b,*, Agnese Ciccic a
KT e Kinetics Technology S.p.A., Rome, Italy; bNextChem srl, L’Aquila, Italy; cBio-P srl, via di Vannina Roma, RM, Italy * Corresponding author. e-mail address: [email protected]
1. Introduction
1.1 What makes a chemical process green or greener?
Since the word “green” started being placed in front of “chemistry,” many people have argued about what the “right” definition of “green chemistry” is or is not [1e4]. And more importantly, what should actually be done to make chemistry green or greener; in other words, how will chemical engineering implement the green chemistry principles? Although quite a few people see green chemistry as separate from engineering, in this chapter we are going to see that they are not two separate disciplines, and we will explore what chemical engineering needs to face such new requirements. Starting from the green chemistry principles, we will discuss the need for new feedstocks, new plant architectures, and novel catalysts and new issues, like scale-up, that engineering is going to face.
Catalysis, Green Chemistry and Sustainable Energy https://doi.org/10.1016/B978-0-444-64337-7.00002-1
An early conception of green chemistry was developed in 1990 by P. Anastas and J. Warner [5] through 12 principles ranging from prevention and atom economy to real-time control for pollution prevention to inherently safer chemistry and spillover prevention. These principles, described below, offer a protocol to adhere in developing novel chemical processes. U Waste prevention: Prevent waste production, rather than cleaning up and treating wastes after having been produced. Plan to minimize waste at every process stage. U Atom economy: Reduce waste by recycling the number of atoms from all reagents that are incorporated into the final product. Use the atom recycling concept to evaluate reaction efficiency.
21
Copyright © 2019 Elsevier B.V. All rights reserved.
22
2. How green chemistry will change chemical engineering
U Less hazardous chemical synthesis: Design chemical reaction paths to be as safe as possible. Consider the hazards of all substances handled during each step of the reaction, including waste. U Designing safer chemicals: Minimize toxicity directly by proper design. Predict and analyze factors such as physical properties, toxicity, and environmental impact of each designed process step. U Safer solvents and auxiliaries: Look for the safest solvent available for any given step. Optimize the total amount of solvents and auxiliary substances used to minimize the waste produced. U Design for energy efficiency: Find the least energy-intensive chemical route, thus reducing heating and cooling, as well as pressurized and vacuum conditions (i.e., try to stay as close as possible to ambient temperature and pressure). U Use of renewable feedstocks: Use feeds that are made from renewable (i.e., plant-based) sources, rather than other chemicals made from petrochemical products. U Reduce derivatives: Minimize the use of temporary derivatives such as protecting groups. Avoid derivatives to reduce reaction steps, resources required, and waste produced. U Catalysis: Look for catalysts that help to increase selectivity, minimize waste, reduce reaction times, and increase energy efficiency. U Design for degradation: Design products that can degrade themselves easily into the environment. Ensure that both original and degraded products are not toxic, bioaccumulative, or environmentally persistent. U Real-time pollution prevention: Control closely chemical reactions in real time and prevent the formation and release of any potentially hazardous and polluting products into the environment.
U Safer chemistry for accident prevention: Develop chemical processes and procedures that are safer and inherently minimize the risk of accidents. Evaluate all the potential risks and assess them beforehand. Today, more than 98% of all products and materials essential to the modern economy are still derived from petroleum and/or natural gas, generating substantial quantities of waste and emissions. A typical example of such production is represented by plastic materials, which are also a typical example of a linear economy: nonrenewable resources, oil or ethane in this case, are used to produce plastic materials, which at the end of life become waste and dispersed into the environment. Today some 8 million metric tons escape into the world’s oceans each year [6], most of it from countries in Southeast Asia, where plastics use has outpaced waste management infrastructure and the situation is approaching catastrophic proportions. Fig. 2.1 depicts the interconnections between society, environment, energy, and the green chemistry processing industries. Such interconnections are shaped by the aforementioned 12 principles, and the massive implementation of them may bring about a real new industrial revolution [7], although a better understanding of the implementation time of such a revolution is required. This chapter reweaves some key aspects and tries to evaluate their impact on the future of chemical engineering. It will explore the use of renewable feedstocks and of atom economy, in particular regarding waste materials in smarter manufacturing processes in which new catalysts are used and less-intensive process conditions are selected, according to the previous green chemistry concepts.
I. Introduction
2. Renewable and waste materials as new feedstocks
FIGURE 2.1
23
New paradigm for society, environment, energy, and the green chemistry/processing industries.
2. Renewable and waste materials as new feedstocks While waste prevention is far from realization, the use of renewable feedstocks is a reality in modern society; the implementation of technology based on the use of wastes as feedstocks can indirectly contribute to limiting the total waste increment and, at the same time, will maximize the number of atoms incorporated into the final products. In the following sections we focus your attention on specific feedstocks that have relevant positions in the current technology. The seventh principle proposes indeed the use of waste materials as new feeds for new processes.
2.1 Principal solid wastes The shift from hydrocarbon-based feedstocks to renewable ones within conventional or novel processes represents one of the major changes to move to a greener and sustainable processing industry. Biorenewable resources must support the
growing demand for chemical commodities. At the same time, they will relax concerns regarding climate change and energy security. However, as far as costs go, competing against conventional products derived from low-priced oil and natural gas will be not easy, and at today prices almost an impossible task. Biobased materials are all more expensive than natural gas, for instance. According to Bomgardner [8], the raw material prices show a consistent variation, starting from natural gas ($1.80 per million British thermal units [BTU]), followed by biomass ($4.00 per million BTU), crude oil ($7.92 per million BTU), and corn ($8.00 per million BTU) and arriving to sugar at $22.60 per million BTU. Looking at such prices in comparison with that of natural gas, a clear picture of evident difficulties appears. Process developers need, then, to choose lower cost biobased feedstocks, including those traditionally classified as waste streams, such as fuel derived from municipal solid waste (MSW). The MSW amount is going to double in the coming years, with a steady increase in the costs of collection and disposal. From this
I. Introduction
24
2. How green chemistry will change chemical engineering
point of view, residue-derived fuel (RDF) may represent a very interesting negative-cost feedstock for chemical production, mainly for methanol, urea and even ethanol [9e11]. RDF is a sort of waste, basically what is left from MSW after recovering wastepaper, glass, metals, and the organic fraction, with a calorific value (lower heating value) ranging from 12,000 to 16,000 and even to 18,000 kJ/kg. Today, RDF is disposed of through either landfilling or thermochemical treatment processes aiming to produce electricity, the so-called waste-to-energy plants.
2.2 Plastic wastes A similar approach is suggested by the Ellen MacArthur Foundation [11] for a new plastic economy; renewably sourced virgin feedstock is strongly reduced by creating an effective after-use of plastic wastes by mechanical and chemical recycling (e.g., carbon recycling). A more specific chemical recycling for polyethylene terephthalate (PET) waste is proposed by the DEMETO project [12], whereby PET wastes are transformed into their original monomers, terephthalic acid and ethylene glycol, ready to make brand new PET and closing the loop of PET production, use, and recycling.
2.3 Biomass Biomass as such is a potential source that could be converted into energy. For large portions of the rural populations of developing countries, and for the poorest fractions of urban populations, biomass is often the only available and affordable source of energy for basic needs such as cooking and heating. This phenomenon is described by the concept “feedefoodefuel conflict.” This has driven the bioenergy sector to choose other types of biomass that do not occur in the nutrition chain, such as agricultural residues and food crop wastes. The second biofuel generation was born.
A quite different approach is the use of unconventional feedstocks such as microalgae for the production of biofuels and biochemicals. Several oleaginous microalgae are capable of accumulating lipids up to 70% in dry biomass, but even in these cases the lipid productivity is actually low. Together with neutral lipids, algae can also store carbohydrates (polysaccharides and starch), carotenoids (as lutein, astaxanthin, b-carotene), proteins, and other molecules [13]. Production of lipids and carbohydrates has received growing interest for biofuel production, while carotenoids and other minor products are usable as feed additives and nutraceutical compounds. Today biofuel production from microalgae does not seem economically feasible, while there are different manufacturing plants in the world to produce high-value chemicals such as carotenoids. Microalgae cultivation may solve social and ethical issues related to the use of fertile lands and edible plants. As a matter of fact, they can grow on infertile lands and reach a productivity per hectare higher than most plants. More recently, a two-stage process has been developed to improve process efficiency, to reduce operative and investment costs [13]. In the first stage, microalgae are cultivated under phototrophic conditions, and then, when biomass concentration rises and the light becomes the limiting factor for growth, the microalgae are transferred to a heterotrophic reactor, without illumination, where they are cultivated by using wastewater as a source of nutrients, mainly organic carbon. This will result also in less hazardous chemical synthesis.
2.4 Natural-based fatty acids Another unconventional feedstock is represented by natural-based fatty acids, which have been a workhorse of the chemical industry for many years. Fueled by the green chemistry agenda and growing demand for more
I. Introduction
3. Loweenergy-intensity processes
environmentally friendly and natural ingredients, such a market has received a considerable boost. An almost 6% per year compound annual growth rate is expected from 2017 through 2022. Animal and vegetable oils, and their derivates, are one of the most important biobased feedstocks that are receiving ever more increasing interest from industries. Vegetable fats, which are more unsaturated, are considered healthier and more functional than animal fats. Their abundance and low price make them an attractive staple for the chemistry industry. Furthermore, the oils of vegetable origin, composed of mono-, bi-, or triglycerides, are a significant source of polymer precursors. The double bonds and the allylic site of the triglycerides are highly reactive, being a suitable starting point for the production of several polymers with different structures and functionalities. Fatty acids are commonly used for the synthesis of renewable chemicals, such as the crossmetathesis of oil-derived olefins with ethylene; this is a valid alternative pathway for shortchain intermediate synthesis with several applications (from the polymer to the surfactant industries). Ethenolysis of methyl oleate produces 1-decene and methyl-9-decenoate. The first has numerous industrial applications (i.e., as an intermediate in epoxide production, amines, synthetic lubricants, and as a monomer in copolymers). The second is an intermediate in the synthesis of several products, such as nylon 10 or lubricants and plasticizers. Green metathesis with an ionic liquid (IL) may represent a very interesting process in olefin metathesis. We are going to discover why in the following sections.
3. Loweenergy-intensity processes The sixth principle highlights the importance of choosing the least energy intensive route; indeed, the level of energy requirements in a
25
chemical process determines its environmental and economic impact; from this point of view, carrying out a conversion at room pressure and temperature may represent an ideal approach in scaling low-intensity processes. A few examples will be given in the following, covering fermentation, membrane reactors, and the use of IL as solvent/catalyst.
3.1 Fermentation processes Biotechnological products are very attractive solutions because of their usage of renewable sources (sunlight, water, etc.) for the production of several molecules with low-energy and efficient processes that have been improved to give high-quality products with low toxicity. In some cases, the fermentation of microbial species is the only way to produce specific molecules in substantial quantities. In particular, biocatalysis and metabolic engineering are the most evolved biotechnologies that are transforming the conventional chemical industry more and more, always with the aim of increasing the efficiency and productivity of numerous bioprocesses. As an example of these considerations, Table 2.1 lists the annual production (in tons) by fermentation of each considered product [14]. 3.1.1 Energy consumption in fermentation processes Some fermentation processes have been used on a large scale to make ethanol, acetic acid, soy sauce, lysine, astaxanthin, and many other substances. These aerobic processes, however, need large quantities of air/oxygen. Despite the different requirements of individual reactions’ synthesis (or degradation), they require cooling at temperatures near to ambient to remove metabolic heat. This aspect, coupled with the aeration rate, the mixing system, and all the downstream processes for the separation of the desired product, leads to high electricity consumption. Focusing only on the fermentation
I. Introduction
26
2. How green chemistry will change chemical engineering
TABLE 2.1
Bioproduct quantities obtained by fermentation.
Product
Annual production (tons)
Bioethanol
26,000,000
L-Glutamic
acid (MSG)
Citric acid
1,000,000 1,000,000
L-Lysine
350,000
Lactic acid
250,000
Food-processing enzymes
100,000
Vitamin C
80,000
Gluconic acid
50,000
Antibiotics
35,000
Feed enzymes
20,000
Xanthan
30,000
L-Threonine
10,000
L-Hydroxyphenylalanine
10,000
6-Aminopenicillanic acid
7,000
Nicotinamide
3,000
D-p-Hydroxyphenylglycine
3,000
Vitamin F
1,000
7-Aminocephalosporinic acid
1,000
Aspartame
600
L-Methionine
200
Dextran
200
Vitamin B12 Provitamin D2
known example is succinic acid. The most used process (with a petrochemical basis) for the production of succinic acid is the hydrogenation of maleic anhydride to succinic anhydride and it successive hydration to succinic acid. The latter step begins immediately when succinic anhydride is dissolved in hot water. The succinic acid obtained is finally separated by crystallization, filtration, and drying. Moreover, because of its competitive cost compared with the petrochemical route, despite its lower material efficiency, fermentative succinic acid is catching on more and more (see Table 2.2). These optimizations must be focused on both biological field (yeast optimization) and the overall process (i.e., reduction of water consumption, generation of valuable sub- and coproducts, optimization of downstream steps). As for total production cost, the reasons behind the lower values for the foresight case (Reverdia process, DSM patent) were the deletion of sulfuric acid and anhydrous ammonia production, thus reducing by 71% the bio-succinic acid variable costs. Another key point was the constant cost reduction of V743/MT SA achieved by means of ethanol coproduction in the foresight case, increasing also the energy efficiency value [16].
3.2 Development of membrane reactors
12 5
From M. Gavrilescu, Y. Chisti, Biotechnologyda sustainable alternative for chemical industry, Biotechnology Advances 23 (2005) 471e499.
reaction, the energy balances performed are based on the consideration that the heat of reaction per electron transferred to oxygen is relatively constant for the oxidation of a wide variety of organic molecules. Several molecules can be produced via both petrochemical and biological routes; a well-
The use of selective membranes in chemical reactors is a quite innovative step, allowing important benefits in terms of reactor performance, compactness, and cost, as result of the product’s removal or reactant’s feed [17]. Many papers have presented this concept related to various processes such as natural gas and methanol/ethanol steam reforming, water gas shift reaction, autothermal reforming (ATR), conversion of hydrogen sulfide into sulfur and hydrogen, and alkane dehydrogenation and validating it for both endothermic and exothermic reactions.
I. Introduction
27
3. Loweenergy-intensity processes
TABLE 2.2
Compared results for succinic acid production. Fermentative base case
Raw material
Sorghum, first generation
Sorghum, third generation
Fermentative foresight case
Sugar beet, first generation
Sorghum, first generation
Sorghum, third generation
Sugar beet, first generation
Petrochemical case Maleic anhydride
E factor
6.7
6.8
6.8
7.8
7.8
7.8
0.3
Material efficiency (%)
13
13
13
11
11
11
76
Total energy input (GJ/MT SA)
53.0
53.8
50.3
97.6
99.5
91.6
55.1
30
30
32
51
50
54
23
Land use (ha/ MTSA)
0.44
0.05
0.14
0.97
0.12
0.30
d
Total fossil energy substituted per unit land area (GJ/ha)
98.1
816.5
320.0
49.5
412.3
161.6
d
Total production costs (V/MTSA)
1045
629
649
1040
85
141
2554
Energy efficiency (%)
SA, succinic acid. H. Yukawa, T. Nambu, Y. Matsumoto, Design of group 5 metal-based alloy membranes with high hydrogen permeability and strong resistance to hydrogen embrittlement, in: A. Basile, A. Iulianelli (Eds.), Advances in Hydrogen Production, Storage and Distribution, Woodhead Pub. Series in Energy, 2015, pp. 341e367 (Chapter 13). Copyright: Elsevier, 2015.
-
In general:
-
Or, the membrane needs to be selective to a reactant. Then it is possible to manage the feedstock rate in the reactor or to distribute the feed on catalyst beds uniformly to better control the reaction thermal effects.
A membrane needs to be selective to a product. In such a case, the thermodynamic threshold related to conventional equilibrium can be overcome, since, by removing a product, the equilibrium is never achieved. The reaction conversion will not be limited by thermodynamics but will be regulated by the performance of the membranes in terms of permeation and reaction rates. This will result in a reduction in the operating temperature if the reactions in the membrane reactor are endothermic, thus increasing the overall efficiency, and conversely increasing the temperature when the reactions are exothermic, thus facilitating the kinetics.
In both cases, membrane reactors allow the chemical processes to overcome the barriers of thermodynamic equilibrium. The operating conditions and membrane reactor size must be properly selected to maximize the benefits. Temperature and pressure gradient limitations must be imposed for making the membrane operation safe and reliable. A membrane reactor is then a sort of chemical reactor in which a membrane selective for one of the reaction components is integrated by being placed internally or externally by the reactor
I. Introduction
28
2. How green chemistry will change chemical engineering
itself. The example presented below is related to the selective feeding of a reactant. The hydrocarbon ATR reaction is considered: Cm Hn þ xH2 O þ yO2 4aCO þ ðm aÞCO2 þ ðx þ 2y 2m þ aÞH2 O n 2y þ 2m a H2 þ 2
(1)
ATR combines partial oxidation and steam reforming in a single reactor. The hydrocarbon feedstock is reacted with a mixture of oxygen or air and steam under substoichiometric conditions. The composition of the product gas is determined by the thermodynamic equilibrium. The exit pressure and temperature are determined by an adiabatic heat balance based on the composition and flow of the feed, and steam and oxygen are added to the reactor, which in turn determines the composition of the product gas at the thermodynamic equilibrium. A conventional ATR reactor consists of two zones: a thermal or combustion zone followed by a catalytic zone. In the first zone, a portion of the feed is oxidized. This generates the amount of heat required to drive the successive reaction of steam reforming in the catalytic zone. By proper control of oxygen-to-carbon and steam-to-carbon ratios, the partial combustion in the first zone supplies the heat for the subsequent endothermic steam reforming reaction. The main issue of the ATR reactor is that the partial combustion, drastically exothermic, is happening in a small zone at the upper part of the reactor, thus leading to the formation of hot spots. Moreover, since the steam reforming is strongly endothermic, the operating temperature is dropping fast along the catalytic zone. Supplying the oxygen stream all along the reactor, through a properly selected membrane, allows the elimination of the separation between the combustion zone (too high temperature) and the catalytic zone (too low temperature) and then the uniform distribution of the ATR reaction and the temperature profile along the
reactor. In this example, however, using a breakable and expensive element as the selective membrane inside a catalytic reactor may become quite complicated, particularly from the maintenance point of view. The reaction’s product removal is represented in Fig. 2.2, where the hydrogen produced via methane steam reforming is separated from the mixture through a metallic Pd membrane. The removal of H2 will allow a reduction in the operating temperature for the same methane conversion and at the same time reduce the heat requirement for product unity [17]. The design of such a membrane is quite a complex task, because of the need to have high hydrogen permeability, a strong resistance to embrittlement, and stability.
3.3 Room-temperature ionic liquids The demand for low-environmental-impact solvents led to the development of new green products and processes; among them, ILs are gaining growing interest from the scientific community for their features [18]. Indeed, the ILs have a very low volatility and flammability, and, on the other hand, they have high thermal and chemical stability. Furthermore, in accordance with the cations and anions used in their formulation, they can change their physical and chemical properties (acidity, basicity, water miscibility and immiscibility, hydrophilicity and hydrophobicity) [18]. Due to these properties, ILs have been used since 1970 for several applications, from electrolytes for batteries [19] to catalysis, biological reactions, and metal ion removal [20].
4. Green chemistry and catalysis (principle No. 9) Catalysis, which has played a vital role in the processing industries of the 20th century, will also be a key enabling technology to achieve the objectives of sustainable and green catalysis. It is
I. Introduction
4. Green chemistry and catalysis (principle No. 9)
FIGURE 2.2
29
Permeation of hydrogen through a metallic Pd membrane.
quite evident that to ensure cleaner, greener, and more efficient processes we need to develop new and longer lasting catalysts, with higher conversion and selectivity, to increase the product’s purity and minimize by-product formation, a potential source of pollution. The relationship between chemical yield and waste minimization needs to be better understood, because it is a key concept of “atom economy”: the conversion process should be developed in order to maximize the recycling of all materials used in the feed into the final product. Heterogeneous catalysis, in particular, is going to be more important by providing the easy separation of product from catalyst, thereby eliminating the need for a further separation through distillation and extraction. From an environmental point of view, catalysts such as clays or zeolites may replace conventional catalysts currently in use, which are more hazardous. In this section, we are going to discuss two new catalyst families, nanocatalysts and the perovskite type, which are promising candidates, and enzyme biocatalysis.
4.1 Perovskites Catalysts used in the modern chemical industry are mainly based on mixed metal oxides. Perovskite-type oxides are attracting much
scientific application interest due to their low price, adaptability, and thermal stability, which depends on bulk and surface characteristics. Perovskites are generally described in the literature by a set of letters: ABX3. They have a cubic structure, centered around the large cation B; cations A occupy the cube's vertices whilst anions X are positioned in the center of the cubic cell's sides. Although the most interesting perovskite structures are oxides, some carbides, nitrides, and hydrides may also crystallize in this structure. In particular, perovskites with transition metal ions in the B site show an enormous variety of electronic and magnetic properties and behaviors. Such a variety is related to the complex character of transition metal ions and their flexibility in coordination with oxygen.
4.2 Nanocatalysts Nanomaterials may play an important role in green chemistry catalysis due to the increased ability in the nanostate and the high surface area. Materials having structural components with at least one dimension lower than 100 nm are considered nanomaterials. Materials with area reduced to the microscale can show different properties compared to what they exhibit on the macroscale, enabling unique
I. Introduction
30
2. How green chemistry will change chemical engineering
behaviors and applications. For instance, a material such as aluminum becomes combustible; solid gold turns to liquid at room temperature; silicon insulators become conductors. Even a chemically inert material at normal scale, such as gold, can serve as a potential chemical catalyst at nanoscale. The nanosized particles are such that the exposed surface area of the active components of the catalyst is much higher than that of a conventional catalyst, increasing the contact points between reactants and catalyst. They are acting as a heterogeneous catalyst with the advantages of a homogeneous environment. This makes the product separation from the catalyst easier than with a homogeneous catalyst. Where the transport of the reactants to the catalyst is limiting the specific process, such a catalyst may play an important role in increasing the rate of reactions. Nanotechnology can lower the cost of production (COP)/cost of removal and hence be more effective for the removal of pollutants in water, particularly in advanced oxidation processes, for instance.
4.3 Enzyme biocatalysis Since 2000, the usage of enzymes and other kinds of biocatalysts has greatly simplified the reactions needed to obtain a determined product, compared with conventional synthesis processes, increasing thus the manufacturing operations’ efficiency. These features had a large impact on the pharmaceutical and biotechnological fields, widening the portfolio of products made by this technology. The following points summarize the principal desired features of biocatalysts: 1. Biocatalysts should be better, faster, less expensive, and more versatile than comparable chemical catalysts. 2. Biocatalysts should be able to catalyze an increased range of reactions, have higher temperature stability, and have improved solvent compatibility.
3. Molecular modeling and other tools should be developed to permit the rapid design of new enzyme catalysts. Progress is under way in all the above areas to provide the chemical industry with diverse new useful biocatalysts. Furthermore, biocatalysts can produce or transform a single enantiomer of chiral compounds. Since in most cases only one enantiomer has the desired activity and the other may be harmful (e.g., thalidomide, where only one enantiomer prevents morning sickness during pregnancy, while the other causes deformities in the fetus), the pharmaceutical industries are pushing the regulatory authorities to ensure that the final products will be constituted only from the desired enantiomer and from a racemic mixture. This ability to make only one enantiomer of a racemic mixture in larger amounts lowers the general process expenses, such as all the costs related to further purification of a product contaminated with an unwanted enantiomer. An example of this technology can be represented by the production processes of semisynthetic penicillin and cephalosporin antibiotics derived from 6-aminopenicillanic acid and 7-aminocephalosporanic acid, respectively. Several other pharmaceuticals use enzymatic catalysis, such as the semisynthetic antibiotic cephalexin, which is derived from cephalosporin C. This compound is produced mainly by the DSM Company (www.dsm.com), using a 4-step enzymatic process to overcome the conventional one with up to 10 steps that generated up to 50 kg of waste per kilogram of antibiotic. The various processes for the production of cephalexin are compared in Table 2.3, to which a recently developed process with direct fermentation that seems to have less waste than the enzymatic route is added [14,21].
I. Introduction
31
5. The scale-up issue
TABLE 2.3
Comparison of chemical and biotechnological processes for producing cephalexin [14]. Process type
Production category
Conventional chemical
Enzyme biocatalysis
Direct fermentation
Waste (kg/kg cephalexin)
50 (1970) to 15 (1995)
10 (1995) to 5 (2000)
2e5
Inorganics (kg/kg)
0.5
0.5
Organics (nonhalogenated) (kg/kg)
1.0
0.2
Solvents (nonhalogenated) (kg/kg)
1.7
0.3
Solvents (halogenated) (kg/kg)
0.9
0
Electricity (%)
100
150
Steam (%)
100
40
Water (%)
100
300
Liquid nitrogen (%)
100
0
5. The scale-up issue Scale-up refers to those activities that enable a change in the production scale, from the laboratory scale, where the production is achieved, to full manufacturing, where it is common to have production levels of tons or hundreds of tons per day. From this point of view, the conventional chemical industry is dominated by one key economical aspect: it looks more attractive to build one big apparatus than several parallel ones. The investment costs (capital expenditures, or CAPEX) go up with production capacity , following roughly the 0.6 rule for scaling of process equipments' cost. If the digression coefficient is smaller than unity, there is an economic incentive to increase the size of equipment rather than to multiply the number of pieces of equipment. As a result of this paradigm, the plant capacity of ethylene production was pushed from 40,000 tons/year in the 1950s to 1,500,000 tons/year today. Green chemistry is dominated by batch or semibatch processes, with reactions taking place in a reactor under stirring, and eventually, heating and cooling. In such a discontinuous reactor,
the feed is introduced into the reactor and is voided after the reaction has taken place. The reaction step is followed by processing, purification, and/or isolation steps to obtain the required production. Will green chemistry follow the same path? Or will the presence of unit operations such as fermentation, introduced to lower the energy intensity of the processes, shift it to other directions Will the COP of these novel processes then be able to compete against the high plant capacity of the conventional chemical industry? In this section, we will try to give some answers related to on fermentation and to relevant catalysts.
5.1 Fermentation scale-up It is a fact that in quite few applications a reduced yield is obtained at large scale compared with the small scale. Such poor results are related to stirring energy, mass, and heat transfer coefficients, which become more problematic to control, limiting the outcoming yield; the microorganism will “see” a different microenvironment, which in some cases may be far away from the optimal behavior, which maximizes the yield.
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When working with larger reactors, micromixing becomes more difficult, and this mechanism is important, not only in fermentation, such as crystal formations, or chemical reactions sensitive to stoichiometry or heat. The most important requirement for experiments at lab scale is that they have to be representative of the conditions prevailing at the large scale. This determines the possibilities and limits of small-scale experiments, and it will allow a change of scale based on a dimensional analysis or a regime analysis. Optimization of a process at laboratory scale and modeling the predominant phenomena are fundamental steps. During optimization, one has to keep in mind, however, that the optimized solution has to be translated back to the production scale. Consequently, not all possible optimization results can be used in a quite complex situation such as the fermentation unit. The simplest technique for scale-up is, of course, “multiplication of elements,” but it can be very expensive relative to the COP. If you can do a fermentation at 20 L, you can do it at 20 m3 by using 1000 fermenters in a row, although it does not look like a cheap solution. By doing this you need to define a different scale-up strategy: first size enlargement until constraints are reached, then scale up by number. In such a case, you may be able to scale up to 1000 L and install 20 fermenters. What it is important to stress is that the introduction of fermentation into your process is going to create a scale-up issue, limiting the possibility of increasing the volume of the reactor and not going from the 0.6 rule of process equipment. The increase in CAPEX is going to negatively affect the COP and make it more difficult to compete.
5.2 Catalyst scale-up Catalysts are essential in most reactions, particularly in green chemistry, where a low temperature is considered. In such conditions
a fixed-bed catalytic reactor is probably reaction rate limited because the chemical reaction rate constant dominates the global rate constant. A change of catalyst makes sense in most cases in which the conversion rate is limited by the reaction rate. By changing the physical properties of a solid catalyst (i.e., shape, pore-size distribution, pore size, surface area) it is possible to increase the conversion rate but only if the process is pore-diffusion-rate limited. Conversely, if the process is limited by the film diffusion rate, increasing the linear fluid velocity through the bed solids will raise the conversion. From a green chemistry/engineering point of view, ineffective mass transfer may lead to a reduction in reaction mass efficiency and loss of yield due to side reactions with potential increase in by-products, which in turn may have a negative impact on downstream separation.
6. Conclusion and future trends Green chemistry will change the chemical engineering business radically, but to do this, first of all it needs to become competitive against the conventional process industry based on hydrocarbon feedstocks. Selection of low-priced renewable materials is a key aspect in such competition, and then waste materials are going to play a major role, being negative-cost feedstocks for chemical production. Recycling wastes related to MSW such as RDF, algae, and fats and oils of animal and vegetable origin may provide a platform to build a resilient green chemistry production of commodities. This will not be enough, however. New lowintensity processes together with more specialized catalysts are required to win such competition. Last, but not least, the scale-up issue has to be well addressed and developed to effectively compete against conventional processes.
I. Introduction
References
List of abbreviations and acronyms 6-APA 7-ACA ATR BioSA BTU CAGR CAPEX COP EG IL LHV MR MSG MSW O/C OPEX PAC PET RDF S/C SA SAN SR TMI TPA WTE
6-Aminopenicillanic acid 7-Aminocephalosporanic acid Autothermal reforming Bio-succinic acid British thermal unit Compound annual growth rate Capital expenditure Cost of production Ethylene glycol Ionic liquid Lower heating value Membrane reactor Monosodium glutamate Municipal solid waste Oxygen to carbon Operative expenditure Production capacity Polyethylene terephthalate Residue-derived fuel Steam to carbon Succinic acid Succinic anhydride Steam reforming Transition metal ion Terephthalic acid Waste to energy
List of Symbols a
Moles of produced carbon monoxide in the auto-thermal reforming reaction m Number of carbon’s atoms of the generic reactant hydrocarbon in the auto-thermal reforming reaction m-a Moles of produced carbon dioxide in the auto-thermal reforming reaction n Number of hydrogen’s atoms of the generic reactant hydrocarbon in the auto-thermal reforming reaction n/2-2y + 2m-a Moles of produced hydrogen in the autothermal reforming reaction x Moles of reactant water in the auto-thermal reforming reaction x + 2y-2m + a Moles of produced water in the auto-thermal reforming reaction y Moles of reactant oxygen in the auto-thermal reforming reaction
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I. Introduction
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