Sustainable Design Through Process Integration. Fundamentals and Applications to Industrial Pollution Prevention, Resource Conservation, and Profitability Enhancement

416 Pages·2011·26.486 MB·English

by  Mahmoud M. El-Halwagi (Auth.)

C H A P T E R Introduction to Sustainability, Sustainable
1
Design, and Process Integration
Industrial processes exert some of the most profound l Dwindling natural resources (for example, fossil fuels)
impacts on the ecosystem. These impacts are attributed and increase in the consumptions of the nonrenewable
to several factors including the significant usage of natural resources
resources, the environmental discharges associated with l Global climatic changes
the processing, and the ecological effects of the products. l Risk to biodiversity and ecosystems
The process industries span a wide range of commodities
Sustainability is based on balancing three principal
including chemical, petroleum, gas, petrochemical, phar-
objectives: environmental protection, economic growth,
maceutical, biofuel, food, microelectronics, metal, textile,
and societal equity (see Fig. 1.1). These are sometimes
and forestry feedstocks and products. The objectives of
referred to as the “triple bottom line: people, planet, and profit”
preventing pollution, conserving resources, increasing pro-
(Elkington, 1994).
ductivity, and enhancing profitability are among the top
Metrics and indicators are used to assess the sustainability
priorities of the process industries. Process engineers and
performance of a process or a system, to evaluate the prog-
managers who are routinely charged with tasks of achieving
ress toward enhancing sustainability, and to assist decision
these objectives face the following primary challenges:
makers in evaluating alternatives. The terms metrics and
l How to systematically evolve solutions and innovative indicators are typically used interchangeably to provide a
designs measure of sustainability. However, a metric usually gives
l How to efficiently assess and screen process a quantitative characterization or an index value, whereas
alternatives indicators provide a narrative description in addition to
l How to navigate through the complexities of industrial the quantitative characterization (Tanzil and Beloff, 2005)
processes and develop an insightful understanding of and may include one or more metrics. There are numerous
the process, its limitations, and its opportunities sustainability metrics, indicators, and approaches published
l How to reconcile the different objectives of the proc- by researchers (for example, Sikdar, 2011, 2009a, 2009b;
ess (for example, economic, technical, environmental) Fan and Zhang, 2011; Jiménez-González and Constable,
l How to continue process development and improve- 2011; Powell, 2010; Uhlman and Saling, 2010; Ukidwe
ment in ways that can be sustained and Bakshi, 2008; Piluso et al., 2008; Cabezas et al., 2007;
Martins et al., 2007; Beloff et al., 2005; Tsoka et al., 2004;
The foregoing challenges raise the issues of what consti-
Schwartz et al., 2002; Pennington et al., 2001) and profes-
tutes a sustainable improvement of the process and how to
sional organizations such as the Institution of Chemical
methodically and efficiently address these challenges. The
Engineers (IChemE, 2002) and the American Institute of
following sections provide a brief discussion on sustain-
Chemical Engineers (AIChE) (for example, Cobb et al.,
ability and the role of process integration as a powerful and
2009). One way of categorizing sustainability indicators
effective framework for sustainable design and for address-
is to classify them based on the economic, environmental,
ing the aforementioned process-engineering challenges.
and social dimensions of sustainability as one-, two-, and
three-dimensional metrics (Sikdar, 2003) as follows:
WHAT IS SUSTAINABILITY?
l One-dimensional metrics are based on only one of
Although there are several definitions of sustainability, the the economic, environmental, and social dimensions.
most commonly quoted definition derives from the defini- Examples of one-dimensional economic metrics include
tion of sustainable development in the “Brundtland Report” capital investment, operating cost, return on invest-
of the 1987 World Commission on Environment and ment, and payback period. Examples of one-dimensional
Development (WCED, 1987), in which sustainable devel- environmental metrics include toxicity, biological
opment means “meeting the needs of the present without com-
promising the ability of future generations to meet their own
needs.” A group of professionals at the U.S. Environmental
Society
Protection Agency proposed the following definition: “sus-
tainability occurs when we maintain or improve the material
and social conditions for human health and the environment over
time without exceeding the ecological capabilities that support
Sustainability
them” (Sikdar, 2003). There is a growing interest in sustain- Economy
Environment
ability because of:
l Increasing population, industrialization, and standards
of living FIGURE 1-1 The three primary dimensions of sustainability.
Sustainable Design Through Process Integration. 1
© 22001122 Elsevier Inc. All rights reserved.
2 Sustainable Design Through Process Integration
l Material consumption: The use of feedstocks,
TABLE 1-1 Examples of GWP of Some GHGs over Two Time water, and material utilities has a major impact on
Horizons the depletion of nonrenewable resources and the
discharge of wastes. Inefficient material use nega-
GWP (20-Year GWP (100-Year
GHG Time Horizon) Time Horizon) tively affects the economic and the environmental
dimensions of sustainability. An example of mate-
Carbon dioxide (CO) 1 1
2 rial consumption metric is the mass intensity index
Methane (CH) 56 21
4 that may be defined as:
Nitrous oxide (NO) 280 310
2
Refrigerant HFC-23 (CHF) 9,100 11,700
3 [1.2] Mass of raw materials
Mass intensity Mass of products
index
Mass of products
oxygen demand (BOD) of wastewater, chemical oxygen
demand (COD) of wastewater, ozone depletion in the
In the case of water, the index may be defined as:
stratosphere, acidification of the atmosphere and aquatic
ecosystems (resulting from the emission of acidifying
[1.3]
chemicals such as sulfur and nitrogen oxides), and aquatic
eutrophication (which involves excessive growth of bio- Massoffreshwaterused
mass that can be exacerbated by the discharge of mineral Waterintensityindex (cid:31)
Massofproducts
nutrients such as nitrogen and phosphorus compounds
into water bodies). Another one-dimensional environ-
l Energy consumption: Energy is a major driving
mental metric is the global warming potential (GWP)
force for operating industrial processes. Excessive
introduced by the United Nations Intergovernmental
usage of energy leads to economic losses and nega-
Panel on Climate Change (UN IPCC) (for example,
tive environmental impact (for example, emission
Houghton et al., 1992, 1990). The GWP is intended to
of GHGs, contribution to ozone depletion and
account for the impact of emissions of greenhouse gases
atmospheric acidification). One way of measuring
(GHGs) on global warming. Specifically, GWP is a
energy efficiency is the energy intensity index that
measure of the relative radiative effects of the emissions
may be defined as:
of several GHGs. Each GHG is given a GWP relative to
CO (which is taken as the basis with a GWP being 1).
2 [1.4]
Therefore, the GWP is expressed in units of CO equiv-
2
alent (for example, tonne CO equivalent). Values of the
2 Netenergyusedintheprocess
GWP for different GHGs are estimated for a specific Energyintensityindex (cid:31)
time horizon over which the impact of such GHGs is Massofproduccts
tracked and integrated. Examples of the GWP values for
two time horizons are shown in Table 1.1. The global l Environmental discharges: The release of haz-
warming index (GWI) is defined as follows: ardous and toxic pollutants causes harmful (and
sometimes irreversible) effects on the environ-
ment. It also has negative economic consequences
[1.1] GWI (cid:31) ∑m *GWP
i i either because of the required cost of treatment or
i because of the financial liability to the industrial
sources of these discharges.
where m is the mass of GHG i emitted over a certain
i l Land use: When land is used for an industrial
period.
purpose (directly as in the case of installing facili-
l Two-dimensional metrics are based on the simulta- ties or indirectly as in the case of planting bio-
neous assessment of two out of the three sustainability mass for the production of biofuels), there are
dimensions. This category includes economic-environ- important ecological and societal consequences.
mental, socioeconomic, and socioenvironmental indica- For instance, substituting one type of a crop for
tors. In this context, a particularly useful philosophy is another (to provide a feedstock to biorefineries)
eco-efficiency proposed by the World Business Council affects the use of water resources, involves the use
for Sustainable Development (WBCSD, 2000) as the and discharge of different chemicals, changes the
following: “Eco-efficiency is achieved by the delivery sequestration of carbon dioxide during photosyn-
of competitively priced goods and services that satisfy thesis, and impacts the communities around the
human needs and bring quality of life, while progres- farmed areas.
sively reducing ecological impacts and resource inten- It is worth noting that metrics such as the
sity throughout the life cycle to a level at least in line mass, water, and energy intensity indices can be
with the earth’s estimated carrying capacity. In short, used to compare different projects and processes.
it is concerned with creating more value with less Furthermore, the sustainability impact of process
impact.” Specific application of eco-efficiency to the modifications can be assessed through the concept
process industries involves the assessment and enhance- of an incremental return on sustainability (IROS)
ment of metrics associated with the following aspects (Spriggs et al., 2009). For instance, consider an
(Uhlman and Saling, 2010; Tanzil and Beloff, 2005): additional project for a process to reduce GHG
CHAPTER 1 Introduction to Sustainability, Sustainable Design, and Process Integration 3
l Material consumption: The use of feedstocks, emissions. The project leads to an improvement in Again, the question is how to methodically and effectively
water, and material utilities has a major impact on the environmental impact but requires additional achieve the objectives of a sustainable design. The answer is
the depletion of nonrenewable resources and the energy consumption. In this case, process integration!
discharge of wastes. Inefficient material use nega- A chemical process is an integrated system of inter-
tively affects the economic and the environmental Changeinenvironmentalimpact connected units and streams. Proper understanding and
dimensions of sustainability. An example of mate- [1.5] IROS (cid:31) solution of process problems should not be limited to
Changeinnetenergyusage
rial consumption metric is the mass intensity index symptoms of the problems but should identify the root
that may be defined as: causes of these problems by treating the process as a whole.
l Therefore, for an energy-reduction project to be Effective improvement and synthesis of the process must
[1.2] acceptable, it must meet a minimum value of the account for this integrated nature. Therefore, integrat-
IROS, which guarantees a basic level of environ- ing process resources is a critical element in designing
mental performance. For instance, a minimum and operating cost-effective and sustainable processes.
limit may be the best in class (for example, kilo- Process integration is a holistic approach to process design,
gram [kg] CO equivalent emission per kilojoule retrofitting, and operation that emphasizes the unity of the pro-
2
In the case of water, the index may be defined as: [kJ]). cess (El-Halwagi, 1997). In light of the strong interaction
l Three-dimensional metrics assess sustainability by among process units, resources, streams, and objectives,
[1.3] integrating the economic, environmental, and social process integration offers a unique framework along with
aspects. an effective set of methodologies and enabling tools for
sustainable design. The strength and attractiveness of pro-
The foregoing discussion highlights the importance of
cess integration stem from its ability to systematically offer
enhancing productivity, conserving resources, and abating
the following:
l Energy consumption: Energy is a major driving pollution (“getting more for less”) in the process industries
force for operating industrial processes. Excessive and describes several methods for assessing the sustain- l Fundamental understanding of the global insights of a
usage of energy leads to economic losses and nega- ability of various industrial processes. A central question is process and the root causes of performance limitations
tive environmental impact (for example, emission not just how to assess sustainability of an industrial process l Ability to benchmark the performance of various
of GHGs, contribution to ozone depletion and but how to achieve a sustainable performance and enhance objectives for the process ahead of detailed design
atmospheric acidification). One way of measuring it. The next section introduces sustainable design through through targeting techniques
energy efficiency is the energy intensity index that process integration as an enabling tool to attain sustainabil- l Effective generation and screening of solution alterna-
may be defined as: ity in a methodical, effective, and generally applicable way. tives to achieve the best-in-class design and operation
strategies
[1.4] Process integration involves the following activities
WHAT IS SUSTAINABLE DESIGN (El-Halwagi, 2006):
THROUGH PROCESS INTEGRATION 1. Task identification: The first step in synthesis
is to explicitly express the goal we are aiming to
Sustainable design of industrial processes may be defined
achieve and describe it as an actionable task. The
as the design activities that lead to economic growth, envi-
l Environmental discharges: The release of haz- actionable task should be defined in such a way so
ronmental protection, and social progress for the current
ardous and toxic pollutants causes harmful (and as to capture the essence of the original goal. For
generation without compromising the potential of future
sometimes irreversible) effects on the environ- instance, pollution prevention may be described as
generations to have an ecosystem that meets their needs.
ment. It also has negative economic consequences a task of reducing certain discharges of the process
The following are the principal objectives of a sustainable
either because of the required cost of treatment or to a certain extent, while quality enhancement may
design:
because of the financial liability to the industrial be described as a task to reach a specific composi-
sources of these discharges. l Resource (mass and energy) conservation tion or certain properties of a product.
l Land use: When land is used for an industrial l Recycle/reuse 2. Targeting: The concept of targeting is one of the
purpose (directly as in the case of installing facili- l Pollution prevention most powerful contributions of process integration.
ties or indirectly as in the case of planting bio- l Profitability enhancement Targeting refers to the identification of perfor-
mass for the production of biofuels), there are l Yield improvement mance benchmarks ahead of detailed design. This
important ecological and societal consequences. l Capital–productivity increase and debottlenecking is critical in the process integration motto of “big
For instance, substituting one type of a crop for l Quality control, assurance, and enhancement picture first, details later.” In a way, you can find the
another (to provide a feedstock to biorefineries) l Process safety ultimate answer without having to specify how it
affects the use of water resources, involves the use may be reached. Targeting allows us to determine
These objectives are closely related to the seven themes
and discharge of different chemicals, changes the how far we can push the process performance and
Keller and Bryan (2000) identified as the key drivers for
sequestration of carbon dioxide during photosyn- sheds useful insights on the exact potential and
process-engineering research, development, and changes in
thesis, and impacts the communities around the realizable opportunities for the process. Even if
the primary chemical process industries. These themes are:
farmed areas. we elect not to reach the target, it is still useful to
It is worth noting that metrics such as the l Reduction in raw material cost benchmark current performance versus the true
mass, water, and energy intensity indices can be l Reduction in capital investment potential of the process. This is particularly use-
used to compare different projects and processes. l Reduction in energy use ful in comparing the sustainability metrics with the
Furthermore, the sustainability impact of process l Increase in process flexibility and reduction in ultimate performance of the process.
modifications can be assessed through the concept inventory 3. Generation of alternatives (synthesis): Given
of an incremental return on sustainability (IROS) l Ever-greater emphasis on process safety the enormous number of possible solutions to reach
(Spriggs et al., 2009). For instance, consider an l Increased attention to quality the target (or the defined task), it is necessary to
additional project for a process to reduce GHG l Better environmental performance use a framework that is rich enough to embed all
4 Sustainable Design Through Process Integration
configurations of interest and represent alternatives appropriate alternatives, it is necessary to extract
that aid in answering questions such as: How should the optimum solution from among the possible
streams be rerouted? What are the needed transfor- alternatives. This step is typically guided by some
mations (for example, separation, reaction, heating, performance metrics that assist in ranking and
and so on)? Should we use separations to clean up selecting the optimum alternative. Graphical, alge-
wastewater for reuse? To remove what? How much? braic, and mathematical optimization techniques
From which streams? What technologies should be may be used to select the optimum alternative(s). It
employed (for instance, should we use extraction, is worth noting that the generation and selection of
stripping, ion exchange, or a combination)? Where alternatives are process synthesis activities.
should they be used? Which solvents? What type of 5. Analysis of selected alternative(s): Although
columns? Should we change operating conditions of synthesis is aimed at combining the process ele-
some units? Which units and which operating con- ments into a coherent whole, analysis involves
ditions? The right level of representation for gener- the decomposition of the whole into its constitu-
ating alternatives is critically needed to capture the ent elements for individual study of performance.
appropriate design space. Westerberg (2004) under- Hence, process analysis can be contrasted (and
scores this point by stating that “It is crucial to get complemented) with process synthesis. Once an
the representation right. The right representation can alternative is generated or a process is synthesized,
enhance insights. It can aid innovation.” The genera- its detailed characteristics (for example, flow rates,
tion of such design alternatives and representations is compositions, temperature, and pressure) are pre-
effectively handled through process synthesis, which dicted using analysis techniques. These techniques
involves putting together separate elements into a include mathematical models, empirical correlations,
connected or a coherent whole. The term process syn- computer-aided process simulation tools, evaluation
thesis dates back to the early 1970s, and gained much of sustainability metrics, techno-economic analy-
attention with the seminal book of Rudd et al. (1973). sis, safety review, and environmental impact assess-
Process synthesis may be defined as “the discrete ment. In addition, process analysis may involve
decision-making activities of conjecturing (1) which of predicting and validating performance using
the many available component parts one should use, experiments at the lab and pilot-plant scales, and
and (2) how they should be interconnected to struc- even actual runs of existing facilities. Thus, in pro-
ture the optimal solution to a given design problem” cess analysis problems, we know the process inputs
(Westerberg, 1987). Process synthesis is concerned along with the process structure and parameters
with the activities in which the various process ele- while we seek to determine the process outputs (see
ments are combined and the flow sheet of the sys- Fig. 1.3).
tem is generated so as to meet certain objectives.
Therefore, process synthesis and analysis serve as the
Therefore, the aim of process synthesis is “to opti-
two primary pillars for sustainable design through pro-
mize the logical structure of a chemical process, spe-
cess integration with synthesis generating alternatives and
cifically the sequence of steps (reaction, distillation,
analysis evaluating the generated alternatives. Figure 1.4 is
extraction, etc.), the choice of chemical employed
a schematic representation of such interaction.
(including extraction agents), and the source and des-
Over the past three decades, numerous contributions
tination of recycle streams” (Johns, 2001). Hence, in
have been made in the field of process integration. These
process synthesis, we know process inputs and outputs
contributions may be classified in different ways. One
and are required to revise the structure and param-
method of classification is based on the three primary areas
eters of the flow sheet (for retrofitting design of an
of integration: mass, energy, and properties. Mass inte-
existing plant) or create a new flow sheet (for grass-
gration is a systematic methodology that provides a fun-
root design of a new plant). This is shown in Fig. 1.2.
damental understanding of the global flow of mass within
Reviews of process synthesis techniques are
the process and employs this understanding in identifying
available in literature (for example, Foo et al., 2011;
performance targets and optimizing the generation and
Diwekar and Shastri, 2011; Majozi, 2010; Foo,
routing of species throughout the process. On the other
2009; Turton et al., 2009; Kemp, 2009; Seider et
hand, energy integration is a systematic methodology
al., 2008; Towler and Sinnott, 2008; Smith, 2005;
that provides a fundamental understanding of energy uti-
Westerberg, 2004; Dunn and El-Halwagi, 2003;
lization within the process and employs this understanding
Furman and Sahinidis, 2002; Bagajewicz, 2000;
in identifying energy targets and optimizing heat-recovery
El-Halwagi and Spriggs, 1998; Biegler et al., 1997).
and energy-utility systems. Finally, property integration
4. Selection of alternative(s) (synthesis): Once
is a functionality-based, holistic approach to the allocation
the search space has been generated to embed the
Process Process
Process Process Process Process
Structure Structure
Inputs Outputs Inputs Outputs
& Parameters & Parameters
(Given) (Given) (Given) (Unknown)
(Unknown) (Given)
FIGURE 1-2 Process synthesis problems. FIGURE 1-3 Process analysis problems.
CHAPTER 1 Introduction to Sustainability, Sustainable Design, and Process Integration 5
involves the vapor phase catalytic reaction of propylene,
Society ammonia, and oxygen at 842ºF (450ºC) and 2 atmosphere
(atm. To produce acrylonitrile (C H N) and water, for
3 3
example:
Sustainable
Environ- Process Design
ment Economy C H + NH +1.5O catalyst→C H N +3H O.
3 6 3 2 3 3 2
Process Process The reaction products are quenched in an indirect-contact
Synthesis Analysis cooler/condenser, which condenses a portion of the reactor
off-gas. The remaining off-gas is scrubbed with water, and
then decanted into an aqueous layer and an organic layer.
The organic layer is fractionated in a distillation column
under slight vacuum, which is induced by a steam-jet ejec-
? tor. Steam is generated by heated boiler feed water (BFW).
? Wastewater is collected from four process streams: off-gas
condensate, aqueous layer of decanter, distillation bottoms,
FIGURE 1-4 Pillars of sustainable process design. and jet-ejector condensate. The wastewater stream is fed to
the biotreatment facility. At present, the biotreatment facil-
ity is operating at full hydraulic capacity and, consequently,
and manipulation of streams and processing units, which is
it constitutes a bottleneck for the plant. The plant has a sold-
based on the tracking, adjustment, assignment, and match-
out profitable product and wishes to expand. Our task is to
ing of functionalities throughout the process. The funda-
eliminate the bottleneck from the process.
mentals and applications of mass, energy, and property
The intuitive response to debottlenecking the process is to
integration have been reviewed in literature (for example,
construct an expansion to the biotreatment facility (or install
Foo et al., 2011; Majozi, 2010; Rossiter, 2010; Foo, 2009;
another one). This solution focuses on the symptom of the
Kemp, 2007; El-Halwagi, 2006; Smith, 2005; El-Halwagi
problem: The biotreatment is filling up; therefore, we must
et al., 2004; Dunn and El-Halwagi, 2003; Hallale, 2001;
expand its capacity. A legitimate question is whether other
El-Halwagi and Spriggs, 1998; El-Halwagi, 1997; and
solutions, probably superior ones, will address the problem
Shenoy, 1995).
by making in-plant process modifications as opposed to an
“end-of-pipe” solution. Invariably, the answer in this case
MOTIVATING EXAMPLES ON THE and most other process design problems is “yes.” If so, how
do we determine the root causes of the problem (not just the
GENERATION AND INTEGRATION OF
symptoms), and how can we generate superior solutions?
SUSTAINABLE DESIGN ALTERNATIVES
Where do we start and how do we address the problem?
Consider the acrylonitrile (AN) process shown in Fig. 1.5a For now, let us start with a conventional engineering
(El-Halwagi, 2006, 1997). The main reaction in the process approach involving a brainstorming session among a group
Ejector
Water SJE
BFW Boiler Condensate
Tail Gases
Scrubber
AN
to Sales
Distillation
Reactor Decanter
Oxygen
Off-gas Aqueous
Ammonia Distillation
Condensate Layer
Propylene Bottoms
Wastewater
(to Biotreatment) Bottleneck
FIGURE 1-5a Process for AN manufacturing. Source: El-Halwagi (1997).
6 Sustainable Design Through Process Integration
Ejector
Water SJE
BFW Boiler Condensate
Tail Gases
Scrubber
AN
to Sales
Distillation
Reactor Decanter
Oxygen
Off-gas Aqueous
Ammonia Distillation
CondensateLayer
Propylene Bottoms
Wastewater
(to Biotreatment) Bottleneck
FIGURE 1-5b Recycle to the distillation column. Source: El-Halwagi (2006).
Ejector
Water SJE
BFW Boiler Condensate
Tail Gases
Scrubber
AN
to Sales
Distillation
Reactor Decanter
Oxygen
Off-gas Aqueous
Ammonia Distillation
CondensateLayer
Propylene Bottoms
Wastewater
(to Biotreatment) Bottleneck
FIGURE 1-5c Recycle to replace scrubber water. Source: El-Halwagi (2006).
of process engineers who will generate a number of ideas the process is still the same, water generated by the main
and evaluate them. Because the objective is to debottle- AN-producing reaction is the same, and therefore the
neck the biotreatment facility, an effective approach may wastewater leaving the plant will remain the same. So, the
be based on reducing the influent wastewater flow rate into plant could employ a recycling strategy that replaces fresh
biotreatment. One way of reducing wastewater flow rate is water with wastewater. This way, the fresh water use in
to adopt a wastewater recycle strategy in which it is desired the process is reduced and, consequently, the wastewater
to recycle some (or all) of the wastewater. For instance, leaving the process will be reduced as well. One option is
the plant could recycle some of the wastewater to the dis- to recycle the wastewater to the scrubber (see Fig. 1.5c),
tillation column (see Fig. 1.5b). After analyzing this solu- assuming that it is feasible to process the wastewater in
tion, it does not seem to be effective. The fresh water to the scrubber without negatively impacting the process
CHAPTER 1 Introduction to Sustainability, Sustainable Design, and Process Integration 7
Ejector
Water BFW SJE
Boiler Condensate
Tail Gases
Scrubber
AN
to Sales
Distillation
Reactor Decanter
Oxygen
Off-gas Aqueous
Ammonia Distillation
CondensateLayer
Propylene Bottoms
Wastewater
(to Biotreatment) Bottleneck
FIGURE 1-5d Recycle to substitute boiler feed water. Source: El-Halwagi (2006).
Water BFW Ejector
SJE
Boiler Condensate
Tail Gases
Scrubber
AN
to Sales
Distillation
Reactor Decanter
Oxygen
Off-gas Aqueous
Ammonia Distillation
Condensate Layer
Propylene Bottoms
Wastewater
(to Biotreatment) Bottleneck
FIGURE 1-5e Recycle to both scrubber and boiler. Source: El-Halwagi (2006).
performance. In such cases, both fresh water and waste- wastewater streams from mixing with the more polluted
water will be reduced. Alternatively, it may be possible to streams, thereby enhancing their likelihood for recycling.
recycle the wastewater to the boiler (see Fig. 1.5d). Along For instance, the off-gas condensate and the decanter aque-
the same lines, the wastewater may be recycled to both ous layer may be segregated from the two other waste-
the scrubber and the boiler (see Fig. 1.5e). However, how water streams and recycled to the scrubber and the boiler
should the wastewater be distributed between the two (see Fig. 1.5f). Clearly, there are many alternatives for
units? One can foresee many possibilities for distribution segregation and recycle. To safeguard against the accumu-
(50/50, 51/49, 60/40, 99/1, and so on). Another alterna- lation of impurities or the detrimental effects of replac-
tive is to consider segregating (avoiding the mixing of) ing fresh water with waste streams, it may be necessary
the wastewater streams. Segregation would prevent some to consider the use of separation technologies to clean up
8 Sustainable Design Through Process Integration
Water BFW SJE
Boiler
Tail Gases
Scrubber
Distillation
Reactor Decanter
Oxygen
Off-gas Aqueous
Ammonia Distillation
CondensateLayer
Propylene Bottoms
Wastewater Bottleneck
(to Biotreatment)
FIGURE 1-5f Segregation of wastewater and recycle of two segregated streams. Source: El-Halwagi (2006).
Water BFW Ejector
SJE
Boiler Condensate
Tail Gases
Scrubber
AN
to Sales
Distillation
Reactor Decanter
Separator
Oxygen
Off-gas Aqueous
Ammonia Distillation
Condensate Layer
Propylene Bottoms
Wastewater
Bottleneck
(to Biotreatment)
FIGURE 1-5g Combined separation and recycle. Source: El-Halwagi (2006).
the streams and render them in a condition acceptable for of streams, the distribution of streams, the changes to be
recycling. For example, a separator may be installed to treat made in the process (including design and operating vari-
the decanter wastewater (see Fig. 1.5g). But, what separa- ables), the substitution of materials and reaction pathways,
tion technologies should be used? To remove what? From and the replacement or addition of units.
which streams? Figures 1.5h through 1.5j are just three Notwithstanding the numerous design alternatives,
possibilities (out of numerous alternatives) for the type and process integration can determine the performance tar-
allocation of separation technologies. And so on! Clearly, get and synthesize the optimal solution without enumera-
there are infinite numbers of alternatives to solve this prob- tion. As will be shown by the overall mass targeting tools
lem. So many decisions have to be made on the rerouting described in Chapter 3, the benchmarks for water usage
CHAPTER 1 Introduction to Sustainability, Sustainable Design, and Process Integration 9
Water BFW Ejector
SJE
Boiler Condensate
Tail Gases
Scrubber
AN
to Sales
Distillation
Reactor Decanter
Ion
Extraction
Exchange
Oxygen
Off-gas Aqueous
Ammonia Distillation
Condensate Layer
Propylene Bottoms
Wastewater
Bottleneck
(to Biotreatment)
FIGURE 1-5h Defining separation technologies. Source: El-Halwagi (2006).
Water BFW Ejector
SJE
Boiler Condensate
Tail Gases
Scrubber
AN
to Sales
Distillation
Reactor Decanter
Ion
Exchange
Oxygen
Ammonia Extraction Distillation
Propylene Bottoms
Wastewater
Bottleneck
(to Biotreatment)
FIGURE 1-5i Hybrid separation technologies for the decanter wastewater. Source: El-Halwagi (2006).
and discharge can be first determined before detailed
The following observations may be inferred from the
design and without the need to create alternative configu-
foregoing discussion:
rations (similar to the ones shown by Figs. 1.5b through
1.5j). Figure 1.6 shows the values of these targets. Next, the l There are typically numerous alternatives that can
optimal solution (shown by Fig. 1.7) is systematically syn- solve a typical sustainable design problem.
thesized using the mass-integration techniques described in l The optimum solution may not be intuitively
Chapters 4 through 6. obvious.
10 Sustainable Design Through Process Integration
Water BFW SJE Ejector
Boiler Condensate
Tail Gases
Scrubber
AN
to Sales
Distillation
Reactor Decanter
Extraction
Oxygen Ion
Ammonia Exchange Distillation
Propylene Bottoms
Wastewater
Bottleneck
(to Biotreatment)
FIGURE 1-5j Switching the order of separation technologies. Source: El-Halwagi (2006).
Vacuum
Pump
Tail Gases
Scrubber
AN
to Sales
Distillation
Reactor Decanter
Adsorption
Oxygen
Offgas Aqueous
Ammonia Distillation
CondensateLayer
Propylene Bottoms
Wastewater
(to Biotreatment)
FIGURE 1-6 Benchmarking water usage and discharge for the AN example before detailed design.
l One should not focus on the symptoms of the process with the process work together to suggest and synthe-
problems. Instead, one should identify the root causes size several conceptual design scenarios (typically three
of the process deficiencies. to five). For instance, the foregoing exercise of gen-
l It is necessary to understand and treat the process as erating alternatives for the AN case study falls under
an integrated system. this category. Each generated scenario is then assessed
l There is a critical need to systematically extract the (for example, through simulation, techno-economic
optimum solution from among the numerous alterna- analysis, and so on) to examine its feasibility and to
tives without enumeration. evaluate some performance metrics (for example, cost,
safety, reliability, flexibility, operability, environmen-
Until recently, there were three primary conventional
tal impact, and so on). These metrics are used to rank
engineering approaches to addressing sustainable design
the generated scenarios and to select a recommended
problems:
solution. This recommended solution may be inaccu-
l Brainstorming and solution through scenarios: A rately referred to as the “optimum solution” when in
select few of the engineers and scientists most familiar fact it is only optimum out of the few generated