LECTURE: 24

COURSE INSTRUCTOR: PROF. ABU TALEB KHAN

GREEN CHEMISTRY AND TECHNOLOGY

COURSE CODE: CH 426

CREDIT: 6
 LECTURE 24: APPLICATION OF GREEN CHEMISTRY IN ENGINEERING

The learning objectives today’s lecture are as follows:

• Become familiar with the basics of engineering and its main branches.
• Learn about the 12 principles of green engineering.
• Compares principles of green chemistry and green engineering.
• Learn essentials of sustainability and how it relates to green engineering.
• Learn about types of thinking which are useful in green engineering.

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 IMPORTANT REMARKS ON SUSTAINABILITY

“An important point, often overlooked, is that the concept of waste is human.
Green engineering focuses on how to achieve sustainability through science and
technology”

Paul T. Anastas and Julie B. Zimmerman (2003), Design through the twelve
principles of green engineering, Environ. Sci. Technol., 37, 94A-101A.
Tang, S. et al. Green Chemistry, 10, 268-269, 2008

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 ENGINEERING AND ITS BRANCHES

A common definition of engineering, as used in everyday life and as found in the

dictionaries, is given broadly as follows:
Engineering is a branch of science which is concerned with practical applications
of the knowledge of pure sciences, such as physics or chemistry or mathematics,
which result in designing and constructing engines, bridges, buildings, ships,
chemical plants, mines, etc. Engineering sometimes includes art, such as in its
architectural applications. (Wikipedia)

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 ENGINEERING AND ITS BRANCHES

The word “engineering” comes from Latin ingenium, which means “cleverness,”
and ingeniare, which means “to devise.”
A broader definition of engineering includes application of scientific, economic,
social, and practical knowledge, with the objectives to invent, design, build, and
improve structures, machines, devices, systems, materials, and processes. This
particular definition resonates well with many of the green chemistry objectives in
general. For example, we notice the invention factor, which is at the core of the
green chemistry movement
This definition is still quite broad, because it encompasses all branches of
engineering. Typically, engineering is divided into four main branches: Chemical,
Civil, Electrical and Mechanical.

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 ENGINEERING AND ITS BRANCHES
 Chemical engineering is most relevant for the objectives of the green
chemistry. It covers chemical processes on a commercial scale.
 Civil engineering is concerned with design and construction of roads, railways,
bridges, dams, building, for example.
 Electrical engineering focused on electrical and electronics systems,
telecommunication, generators and motors, and similar.
 Mechanical engineering deals with mechanical systems, such as power, energy,
aerospace, and transportation, as some examples.

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 ENGINEERING AND ITS BRANCHES

There are numerous subspecialties of engineering, such as automotive, computer,

architectural, agricultural, biomedical, petroleum, textile, and nuclear, among
many others. Many of these fields and subfields are interconnected.
For our purposes, we shall focus on chemical engineering, and then we shall
emphasize organic chemistry within a broad class of chemical manufacturing
processes. Still, many organic chemical processes may cross boundaries with
other engineering branches.

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 A BRIEF DESCRIPTION OF THE 12 PRINCIPLES OF GREEN ENGINEERING

 1. Inherent rather than circumstantial: Designers need to strive to ensure that

all materials and energy inputs and out puts are as inherently nonhazardous as
possible.
 2. Prevention instead of treatment: It is better to prevent waste than to treat
or clean up waste after it is formed.
 3. Design for separation: Design separation and purification operations to
minimize energy consumption and materials use.
 4. Maximize efficiency: Design products, processes, and systems to maximize
efficiency of mass, energy, space, and time.

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 A BRIEF DESCRIPTION OF THE 12 PRINCIPLES OF GREEN ENGINEERING

 5.Output-pulled versus input-pushed: Design products, processes, and systems

to become “Output-pulled” rather than “input-pushed” by an appropriate use of
energy and materials.
 6. Conserve complexity: Consider conservation of embedded entropy and
complexity while making choices on recycling, reuse or beneficial disposition of
materials.
 7. Durability rather than immortality: Target durability rather than immortality
while designing materials and products.
 8. Meet need, minimize excess: Design only to meet need. Design for
unnecessary capacity or capability should be considered a design flaw. This
includes “one-size-fits-all” solutions.

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 A BRIEF DESCRIPTION OF THE 12 PRINCIPLES OF GREEN ENGINEERING

 9. Minimize material diversity: Design multicomponent products in such a way

to minimize material diversity, to promote should be minimized to promote
disassembly (for reuse or recycling), and to value retention.
 10. Integrate material and energy flows: Design of products, processes, and
systems in such a way to include integration and interconnectivity with
available energy and materials flows.
 11. Design for commercial afterlife: Design products, processes, and systems
for performance in a commercial “afterlife.”
 12. Renewable rather than depleting: Material and energy inputs should be
renewable rather than depleting.

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 PRINCIPLE 1: INHERENT RATHER THAN CIRCUMSTANTIAL

Engineers should first evaluate the inherent (intrinsic) characteristics of the

materials they intend to use. If the materials exhibit hazardous characteristics, this
will become a serious drawback of the design. Hazardous materials can cause
harm to life and the environment. In addition, their handling, removal, and
disposal may be extremely costly and risky, and it would require time, material,
special equipment, and energy resources. This is not an economically or
environmentally sustainable approach.

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 PRINCIPLE 1: INHERENT RATHER THAN CIRCUMSTANTIAL
Instead, a prudent approach would be to search for other materials that are
inherently as nonhazardous to life and environment as possible. In addition,
designers need to ensure also that all energy inputs and outputs are inherently as
nonhazardous as possible. This especially important for large-scale manufacturing
processes.

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 PRINCIPLE 1: INHERENT RATHER THAN CIRCUMSTANTIAL

The present day energy sources are typically electricity and steam. These energy
sources are based on primarily the fossil fuels, which are not renewable. The
choice of energy also matters in terms of placing hazardous materials into the
environment. One can think about gases that are generated by burning coal as
one example. To minimize the output of toxic materials, engineers need to look at
the type of energy that is used, in addition to the chemical reactions. Engineers
need to choose intentionally both materials and energy, so as to give as
nonhazardous an outcome as possible.

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 PRINCIPLES 2: PREVENTION INSTEAD OF TREATMENT
This principle states that it is better to prevent waste than to treat it or clean it up
after it is formed. We recognize this principle as principle 1 of green chemistry,
which we have discussed in previous chapter. Here, we show more goals and
applications that are characteristic for green engineering.
Dr. Marin Abraham brings up several specific points on how to prevent waste.
First, one should make only the amount that is needed, and not an excess. The
latter can be quite costly, because one pays first for the extra starting materials
that are not needed, and then pays later for their safe disposal.

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 PRINCIPLES 2: PREVENTION INSTEAD OF TREATMENT
In another example, Dr. Abraham emphasizes the need to achieve selectivity in
chemical reactions. If a reaction can give two products, where only one is desired,
it is often possible to achieve selectivity in favour of the desired product. This
often can be done by manipulating the temperature at which the reaction is
performed.
To enforce this important point, we briefly review one of the classical reactions
whose outcome can be modulated by the temperature, and which is covered in
every beginning organic chemistry textbook. The reaction is an addition reactions,
which gives the 1,2-addition product as the main product at a low temperature,
whereas at the higher temperature, the 1,4-addition product predominates.

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PRINCIPLES 2: PREVENTION INSTEAD OF TREATMENT

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 PRINCIPLES 2: PREVENTION INSTEAD OF TREATMENT
At a low temperature (-80°C), the major product, which is obtained in 80% yield, is
formed by a 1,2-addtion of HBr. The minor product, which is the result of 1,4-
addition of HBr, is obtained in a 20% yield. At the higher temperature (40°C), the
same products are obtained, but the yields are reversed. Further, upon warming
of the mixture of 1,2- and 1,4-products that are at -80°C to temperature of 40°C,
the products are equilibrated to give the same mixture as originally obtained at 40
°C. if the desired product is 1,2-adduct, one should run the reaction at low
temperature. Should one desire the 1,4-adduct, one should run the reaction at a
higher temperature.

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 PRINCIPLE 3: DESIGN FOR SEPARATION
This principle addresses the observation that separation, isolation, and
purification of products generally consume most of the materials and energy
requirements in manufacturing processes. Typically, separation methods require
the use of toxic solvents, which then necessitate large amounts of energy to
evaporate the solvents and to retrieve the products. One example on laboratory
scale that students are familiar with is the separation and purification of mixtures
by column chromatography. This method requires the collection of multiple
fractions as they are eluted from the column by the mobile phase, which is
typically composed of volatile organic solvents. The latter may be hazardous, toxic,
and/or flammable. Later, the solvents are removed from the individual fractions
by distillation, which consumes energy.

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 PRINCIPLE 3: DESIGN FOR SEPARATION
Design for separation avoids the above problem because it focuses on self
separation of products. One such example is the multicomponent Passerini
reaction in which three starting materials are placed in an aqueous medium, and a
single product separates as a precipitate, in a quantitative yield. This reaction is
one of the “on-water” reaction. Other on-water reactions, such as Diels-Alder
reaction, also lend themselves to an easy self-separation process, because they
give water-insoluble products. All that is required is to filter the solid products
from the aqueous medium.

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 PRINCIPLE 3: DESIGN FOR SEPARATION
 It should be pointed out that in the past, classical Passerini and Diels-Alder
reactions, as well as many of the on-water reactions, have traditionally been
performed in organic solvents, which then required separation of products
from them. Only after these reactions were greened by performing them on
water, the design for separation was realized. However, not all chemical
reactions can be run on-water, and for many reactions the use and distillation
of solvents are still common. For such cases, green engineering offers other
successful strategies.
 For example, we give here a couple of such strategies. Distillation can be made
more efficient by designing the fractional distillation towers such that the heat
of condensation that is released is reused. Distillation may be avoided by using
alternative separation techniques, such as the reverse osmosis membrane
separation for desalination of saltwater to freshwater.

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 PRINCIPLE 4: MAXIMIZE EFFICIENCY
This principle states that products, processes and systems should be designed to
maximize efficiency of use of mass, energy, space, and time. Specifics are
provided, for examples, by Dr. Michael A. Gonzalez.
To achieve mass efficiency, the reactions should be designed in such a way so as
to use as much of the reactants as possible. Desired properties of the reactions
include high conversions and selectivity to give the desired products and to give a
minimal amount of by-products.

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 PRINCIPLE 4: MAXIMIZE EFFICIENCY

Energy efficiency is achieved by employing room temperature and pressure, when

possible. This eliminates the need for expensive cooling or heating. Minimization
of movement of the materials also saves energy, for example, for pumping the
materials for transport. Further, if the design minimizes separation and
purification steps, this also saves energy that would be otherwise needed for the
distillation of solvents.

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 PRINCIPLE 4: MAXIMIZE EFFICIENCY

Saving space is important, because it saves on cost and reduces hazards. For
example, constructing large reaction vessels is expensive in terms of cost and
energy. Large volumes of the produced materials may pose unacceptable hazards.
The smaller the better, providing that it meets the need.
Time efficiency strives to perform chemical reactions as fast as possible, to
liberate the reactor for use of the other reactions. Thus, reactions should be
catalytic, if possible.
All these different aspects of efficiency---mass, energy, space, and time-are
interconnected to a degree.

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED

This is a concept related to chemical equilibrium and states that products,

processes, and systems should be “output-pulled” rather than “input-pushed”
through the use of energy and materials.
Anastas and Zimmerman (2003) provide a clear example of this principle by
invoking the well-known Le Chatelier’s principle. First, we review this important
principle in its general form. Then we apply it to an organic reaction, because the
emphasis of Green Chemistry is on the organic chemistry. Finally, we show its
applications to green engineering.

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED

A general equation for a reversible reaction is shown below, which would be a

subject to Le Chatelier’s principle.
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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED

An example of a generalized reversible reaction, which is the subject to the Le

Chatelier’s principle. K is the equilibrium constant. Q is the reaction quotient,
whether or not the system in at equilibrium.
Q = K when system is at equilibrium. a-d are the stoichiometric coefficients for the
reacting species A-D.

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED
Le Chatelier’s principle summarizes the ways in which a system at equilibrium
responds to changes. When stress is applied to such a system, the system responds
by reestablishing the equilibrium in order to relieve, reduce, or offset the applied
stress.
Common ways to introduce stress on the system at equilibrium are changes in
concentrations and temperature. For a gas-phase reactions, changes in
pressure/volume also cause stress, providing that the number of moles of gas
differs between the reactants and products.

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED
We summarizes the stress and the system’s response in Tables 1 through Table 3,
which are based mostly on material. We discuss the relevance to principle 5 for
each type of stress and the system’s response.
The reversible reaction from Figure 1 at equilibrium would give us a mixture of
starting materials and products. Typically, only one product is desired. How can we
achieve getting only the desired product, instead of a mixture, which is the
consequences of the equilibrium?

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED
Inspection of the system responses from Table 1 shows us the options on how to
achieve this goal and helps us make a green choice of output-pulled versus input-
pushed. One can shift the equilibrium towards the products by two means: (1) by
decreasing concentrations of the products, for example, by removing them from the
reaction mixture, or (2) by increasing concentration of reactants, for example, by
adding extra reactants.

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED

Table 1
Response of the System at Equilibrium from Figure 1 to Changes in Concentration and
By Addition of Specific Components
Concentration Change Resulting Changes Shift in Consequences
in Q and K Equilibrium
Increase in [products] Q>K To the left More reactants are
formed
Decrease in [products] Q<K To the right More products are
formed
Increase in [reactants] Q<K To the right More products are
formed
Decrease in [reactants] Q>K To the left More reactants are
formed

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED
Table 1
Response of the System at Equilibrium from Figure 1 to Changes in Concentration and
By Addition of Specific Components
Component Added Consequences as Equilibrium is Established
[A] [B] Decreases [C] Increases [D]
Increases
[B] [A] Decreases [C] Increases [D]
Increases
[C] [D] Decreases [A] Increases [B]
Increases
[D] [C] Decreases [A] Increases [B]
Increases

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED

The choice 1 is output-pulled and is green, whereas the choice 2 is input-

pushed and is not green. One can easily see that in choice 2 at the end of
the process, we would be stuck with extra unused reactants. The output-
pulled option can be used to select only one of the products. We show an
example in Figure 2 for a Fischer esterification reaction, a common
esterification process, in which a alcohol and a carboxylic acid react under
acidic catalysis to give an ester.

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED
In the reaction shown Figure 2, the desired product is isoamyl acetate. The
equilibrium can be shifted towards the product by removing water from the
reaction mixture. This can be done by using concentrated H2SO4 beyond the
catalytic amount, typically, in a stoichiometric quantity. Other ways exist to
remove water. Examples include use of molecular sieves or a specialized
distillation technique using a Dean-Stark trap.

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED

Molecular sieves are porous materials of specified pore sizes. These sieves absorb
small molecules, such as water that can fit into the pores, whereas large molecules
stay outside the pores. Thus, molecular sieves can act as desiccants. Examples of
molecular sieves include various zeolites, which are microporous aluminosilicate
minerals.
In distillation that uses Dean-Stark trap, water co-distills with organic material and
falls on the bottom of the trap, whereas a less dense organic layer collects on the
top of the trap, and then flows back to the distilling flask.

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED

These methods of shifting the equilibrium by removing water are output-

pulled green methods. In some esterification experiments, an excess of
carboxylic acid is used to shift the equilibrium towards the products. This
would present an input-pushed non-green method, because one would
have unreacted carboxylic acid left, which would need to be removed at
the end. Still, one has to evaluate the entire process of shifting the
equilibrium, because not all output-pulled methods are necessarily green.

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED
If one has to conduct additional steps to make the output-pulled method work, and
if such steps are not green, this has to be taken into account. The above-mentioned
extra distillation step to remove water is inherently not green, but may be made
more efficient, and thus, the overall process is greener, by reusing the heat of
condensation that is released. This would be an implementation of principle 3.

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED
Output-pulled design can also be achieved via temperature control of reversible
reactions. One can consider exothermic and endothermic reactions in a way in
which one imagines that heat itself behaves as a chemical reactant or product.
Chemical equations that include heat are termed thermochemical.
Such equations allow us to predict the response of a system with change in
temperature in the same way as we have done previously for the change in
concentration.

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED
Am important difference, however, is that the temperature change alters the value
of the equilibrium constant. Table 2 shows the response of a system in equilibrium
upon a change in temperature.
For the green output-pulled design, one would need to decrease the temperature of
exothermic reaction, but to increase it for it for endothermic reaction.

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED

Response of a system at equilibrium upon temperature change for reactions that are
exothermic (Reactants = Products + Heat) and endothermic (Reactants + Heat =
Products)
Type of reaction Temp. Change Shift in Consequences
Equilibrium
Exothermic (Heat is a product) Increase (adding heat) To the left More reactants are formed

Exothermic (Heat is a product) Decrease (removing heat) To the right More products are formed

Endothermic (Heat is a reactant) Increase To the right More products are formed

Endothermic (Heat is a reactant) Decrease To the left More reactants are formed

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED

Finally, changes in pressure can affect the equilibria of gas reactions. The effect is
observed only if number of the moles of gas differs between reactants and
products.

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED

If an inert gas is added to a system, it will not affect the equilibrium, because it does
not change the partial pressure of the gasses in the reaction. Figure 3 shows two
gas reactions whose equilibria are affected by change in pressure, whereas, Table 3
summarizes the effect of changing pressure on equilibria of these reactions.
For the output-pulled green design, one needs to decrease the pressure if the total
number of the mole of the products is larger than that of the reactants, such as in
reaction 1, and to increase the pressure when the total number of the moles of the
products is smaller than that of the reactants, as in reaction 2.

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 PRINCIPLE 5: OUTPUT-PULLED VERSUS INPUT-PUSHED

It should be noted that the equilibrium constant for a reaction is not affected by the
presence of a catalyst or an enzyme, which acts as a biological catalyst. Catalysts
increase the rate by which the equilibrium is established, but do not affect its
position.

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 PRINCIPLE 6: CONSERVE COMPLEXITY
This principle states that embedded entropy and complexity must be viewed as
investment while making choices on recycling, reuse, or beneficial disposition of
materials.
Complexity is related to the number of atoms and functional groups in a molecule
used to produce the molecule or item. It is also a function of expenditure of
materials, energy and time. Thus, chemicals and products in general have a certain
amount of complexity that is built into them.

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 PRINCIPLE 6: CONSERVE COMPLEXITY
Many complex materials have high economic value. It would make no business
sense to take a material with high economic value and convert it into with lower
value. Highly complex materials should be reused, rather than recycled. In contrast,
materials with minimal complexity should be recycled or disposed of.
A decision to reuse, recycle, or dispose of the products needs to be made based on
the evaluation of multiple factors that are involved in these processes. Anastas and
Zimmerman bring up the example of a brown paper bag, which is made of a highly
complexed natural material.

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 PRINCIPLE 6: CONSERVE COMPLEXITY
However, the complexity of this material does not warrant the time, energy, and
cost for collection, sorting, processing, remanufacturing, and redistribution of the
bag for its reuse. In another example, they discuss silicone computer chips. These
chips have a significant amount of complexity built into them. Such complexity
represents most of the economic value of this product. Recycling a silicon chip in
order to recover the value of the starting materials makes no economic sense, but
reusing the chip, which preserves its complexity, does. In conclusion, even though
we strive to preserve complexity, we must evaluate such preservation from an
economic point of view.

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 PRINCIPLE 7: DURABILITY RATHER THAN IMMORTALITY
This principle states that product should be designed to be durable rather than
immortal. “Immortal” products are those that last well beyond their useful
commercial life and that persist in the environment because they are not
biodegradable. Such products may even have a short or single use. Immortality of
product should be avoided. Instead, product need to be designed for useful
lifetime and a capability for biodegradation.

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 PRINCIPLE 7: DURABILITY RATHER THAN IMMORTALITY
A good example of a successful design for durability but not immortality is that of
a starch-based packing material. Such material is made of natural, nontoxic,
biodegradable, and sustainable sources. At the end of its use, the so-called
product’s end of life, this product can be placed in compost piles. It is water
soluble and is easily biodegradable. It is competitive with polystyrene packing
which is non-biodegradable and has been shown to be persistent in the
environment.

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 PRINCIPLE 8: MEET NEED, MINIMIZE EXCESS
Based on this principle, the design of materials, processes, and systems should be
optimized for the anticipated need. A design that includes added capacity based
on unrealistic assumptions or “worst-case” scenarios has negative and costly
consequences. Making extra products that are not needed incurs the cost of
unnecessary materials and energy, the cost in time and salaries for the workers,
the cost for shortage, the burden on the environment in terms of produced waste,
and the cost of disposal and recycling of waste, among others. To avoid the
problem of overproduction and the associated cost and other negative
consequences, the green alternative target specific needs and demands of the
end users should be implemented.

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 PRINCIPLE 9: MINIMIZE MATERIALS DIVERSITY
This principle seeks to minimize material diversity of products, namely, to make
products from as few types of materials as possible. Material diversity is taken into
account when considering options for the end of the useful life of the product,
such as reuse or recycling. A good description of this principles provided by
Anastas and Zimmerman.

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 PRINCIPLE 9: MINIMIZE MATERIALS DIVERSITY

Many products are built of multiple components, and such components may be
composed of a variety of materials. For example, cars have components made of
metal, glass, plastic, paint and so on. Individual plastics typically contain additional
materials such as thermal stabilizers, plasticizers and flame retardant. At the end
of the useful life of the product, decisions about disassembly for reuse of parts
and recycling need to be made. Such decisions ultimately depend on material
diversity, because different materials have different requirements and potential
for green reuse and recycling. Lowering the material diversity in the product and
choosing or designing material with good potential for reuse and/ or recyclability
enable the implementation of the end-of-useful life options.

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 PRINCIPLE 10: INTEGRATE LOCAL MATERIAL AND ENERGY FLOWS
This principle states that optimization of mass and energy requirements for a
given processes is best accomplished via integration of heat exchange networks
(HENs) and mass exchange networks (MENs).
As an example, let us consider existing framework of energy and material flow at a
local chemical production facility. The application of the principle 10 can be
understood as an integration of HENs and MENs. HEN uses hot streams to heat
cold streams, for example. If one production unit generates heat, and another one
requires heating, two should be integrated.

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 PRINCIPLE 10: INTEGRATE LOCAL MATERIAL AND ENERGY FLOWS

MENs are analogous to Hens, but instead of exchanging energy, they exchange
mass. For example, we may need to separate unreacted material from the
product, both to purify the product and to recover the unreacted material so that
we can use it again in the reaction. One option is to use additional material, such
as a solvent, to remove the unreacted material and to send it back to the reactor.
However, such an option is costly, and the solvent may be toxic or otherwise
hazardous.

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 PRINCIPLE 10: INTEGRATE LOCAL MATERIAL AND ENERGY FLOWS

A better option would be to implement MEN, namely, to use streams with low
concentrations of high of a given material (lean streams) to separate and recover
material from streams that have high concentrations of the material in question.
We should use as much of the lean stream as possible to recover the material,
before we resort to using additional materials, such as solvents.
Although, HENs and MENs appear to be quite simple and straightforward in
principle, it takes a lot of expertise and creativity to implement them.

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 PRINCIPLE 11: DESIGN FOR COMMERCIAL “AFTERLIFE”
This principle states that design of a product should include the commercial
“afterlife.” This means that the product at the end of its commercial life should
still has value, which can be extracted from it, if the product is properly designed.
For example, many products reach their commercial end of life because they
become technologically or stylistically obsolete. They may still have perfectly good
and valuable working components. The latter can be recovered for reuse, if it is
economically feasible. Valuable components can be refurbished and sold, or may
be disassembled to extract other valuable parts. Various examples are given in the
work of Anastas and Zimmerman. A good example is that approximately 90% of
Xerox equipment is designed for remanufacturing.

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 PRINCIPLE 12: RENEWABLE RATHER THAN DEPLETING

This principle states that the materials and energy inputs should be renewable
rather than depleting. The ultimate goal is sustainability. For example, fossil fuels,
such as petroleum, natural gas, and coal are not renewable and their continuous
use is not sustainable. In contrast, solar, wind, and hydro energy sources that
generates electricity are renewable.
In terms of materials, bio-based plastic is one example of use of a renewable
feedstock. It is important to go through all the steps of the process by which a
product is obtained, before one decides on sustainability of the product. One
should not automatically assume that bio-based products are always more
sustainable than identical products that are manmade. Sometimes, extracting the
product from natural sustainable resources, such as plants, may be extremely
costly in terms of energy and auxiliary materials, and a green synthetic organic
path may be preferred.

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 PRINCIPLE 12: RENEWABLE RATHER THAN DEPLETING

This principle states that the materials and energy inputs should be renewable
rather than depleting. The ultimate goal is sustainability. For example, fossil fuels,
such as petroleum, natural gas, and coal are not renewable and their continuous
use is not sustainable. In contrast, solar, wind, and hydro energy sources that
generates electricity are renewable.
In terms of materials, bio-based plastic is one example of use of a renewable
feedstock. It is important to go through all the steps of the process by which a
product is obtained, before one decides on sustainability of the product. One
should not automatically assume that bio-based products are always more
sustainable than identical products that are manmade. Sometimes, extracting the
product from natural sustainable resources, such as plants, may be extremely
costly in terms of energy and auxiliary materials, and a green synthetic organic
path may be preferred.

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 COMPARISON OF PRINCIPLES OF GREEN CHEMISTRY AND GREEN
ENGINEERING
A quick comparison between these principles and the 12 principles of green
engineering reveals a substantial overlap. Examples include green chemistry
principle 1 and green engineering principle 2, both of which focus on waste
prevention. Green chemistry principle 7 and green engineering 12 both cover a
need for renewable resources. Green chemistry principle 3 and green engineering
principle 1 both state a goal of non-hazardous processes.

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 COMPARISON OF PRINCIPLES OF GREEN CHEMISTRY AND GREEN
ENGINEERING
Green chemistry principle 10 calls for design for degradation so that products do
not persist in the environment. This goal is embedded also in green engineering
principle 7, which calls for durability rather than immortality. The green aspects,
such as sustainability and design for biodegradation are common themes for both
green chemistry and green engineering.
They differ in that green engineering emphasizes the practical requirements for
sustainability, renewability of resources, end of life and afterlife of products,
durability versus immortality, material diversity, efficiency, conservation of
complexity, and other individual factors which are described in principles.

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