water pinch technology for designing wastewater reduction...
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WATER PINCH TECHNOLOGY FOR DESIGNING WASTEWATER REDUCTION AND WATER CONSERVATION SYSTEMS
P.RAŠKOVIĆ
Faculty of Technology Engineering, Leskovac, Serbia, [email protected]
ABSTRACT
During the past decade, wastewater reduction and water conservation are becoming increasingly more important issues in process industries. Process integration technology has been viewed as one of the most important methodology for overcoming these problems. Two key branches of process integration can be recognized as: Energy integration and Mass integration. Mass integration is a systematic methodology that provides a fundamental understanding of the global flow of mass within the process and employs this understanding in identifying performance targets and optimizing the allocation, separation, and generation of streams and species.. In the context of wastewater minimization, a mass integration problem involves transferring mass from rich process streams to lean process streams in order to achieve their target outlet concentration, while minimizing waste production and utility consumption (including freshwater and external mass separating agents). In this paper water-pinch technology, a type of mass integration involving water-using operations is presented. Water pinch is a systematic technique for analyzing water networks and reducing water costs for processes. It uses a graphical design method to identify and optimize the best water re-use, regeneration and effluent treatment opportunities. Theoretical background of water pinch technology is illustrated by dephenolization problem in oil-recycling plant.
Keywords: System Engineering, Process Integration, Water Pinch Analysis, Industrial Wastewater, Environmental Process Design.
Introduction Water is the most widely occurring substance in the Earth.By modern estimates the Earth’s hydrosphere1 (Fig. 1) contains a huge amount of water of about 1386 million km3. However, 97.5% of this amount are saline water and only 2.5% fresh water. The greater portion of the fresh water (68.9%) is in the shape of ice and permanent snow cover in the Antarctic, the Arctic, and mountainous regions. Next 30.8% are fresh ground waters. Only 0.3% of the total amount of fresh waters on the Earth is concentrated in lakes, reservoirs, and river systems. They are most accessible for economic needs and very important for water ecosystems. (Fig.2)
1 A hydrosphere (Greek hydro means "water") in physical geography describes the collective mass of water being in state in liquid, solid or gaseous state in the atmosphere, on the Earth’s surface and in crust down to the depth of 2000 meters.
Fig.1. The water cycle on the Earth
Fig 2. Global Freshwater Availability Today, according to United Nations estimates, one third of the world's population lives in areas with water shortages and 1.1 billion people lack access to safe drinking water. Climate change has also provoked more frequent and intense droughts in sub-tropical areas of Asia and Africa, exacerbating shortages in some of the world's poorest countries. While the world's population tripled in the 20th century, the use of renewable water resources has grown six-fold (Fig.3). Agriculture is by far the largest consumer of water, mostly because of the spread of irrigation. Two-thirds of all the water consumed in the world goes to farming, a share expected to shrink only slightly by 2025 (Fig. 4).
Fig. 3. Dynamic of renewable water use for different continents and world Within the next fifty years, the world population will increase by another 40 to 50 %. This population growth - coupled with industrialization and urbanization - will result in an increasing demand for water and will have serious consequences on the environment. In present time, most countries in the Middle East and North Africa can be classified as having absolute water scarcity. In many of these
areas, water supply is actually less than 1,000 m3 per capita ( the limit for “water scarcity”) which causes serious problems for food production and economic development. By 2025, these countries will be joined by Pakistan, South Africa, and large parts of India and China and as a result 1.8 billion people will live in countries or regions with absolute water scarcity (Fig. 5a).
Fig. 4. Dynamic of water use by sector When per capita water supply is less than 1,700 m3 per year, an area suffers from “water stress” (today, 2.3 billion people live in water-stressed areas). According to Population Action International, more than 2.8 billion people in 48 countries will face water stress or scarcity conditions by 2025. Of these countries, 40 are in West Asia, North Africa or Sub-Saharan Africa. By 2050, the number of countries facing water stress or scarcity could rise to 54, with their combined population being 4 billion people - about 40% of the projected global population of 9.4 billion (Gardner-Outlaw and Engleman, 1997; UNFPA, 1997) (Fig. 5c).
Fig. 5.
Table 1. Freshwater Resources in Europe 2005
Sources: Food and Agriculture Organization of the United Nations (FAO),Population Division of the Department of Economic and Social Affairs of the United Nations Secretariat, World Bank.
Today, there is about 6,800 m3 of water available per person on a yearly basis. If current trends continue, only 4,800 m3 will be available in 2025. This is an optimistic calculation because it is based upon estimates of all the water flowing in rivers after evaporation and infiltration into the ground. The primary consumers of water include industry as well as agriculture sectors 2(Fig. 6). It stated that industrial wastes (contaminant with heavy metals and persistent organic compounds) are significant sources of water pollution. Some 300-500 million tons of heavy metals, solvent, toxic sludge and other wastes accumulate each year from industry. Fig. 6. shows the global estimates of emissions of organic water pollutants by different industry sector (World Bank, 2001).
Fig. 6: Contributions of main industrial sectors to the production of organic water pollutants3 Water and wastewater reduction drivers Water scarcity has been identified as a driver of implementing new technologies for water use as well as reuse, recycling and regeneration options. However there are other drivers for initiating water and wastewater saving initiatives like: • Regional water shortages/resource limitations –there may not be enough capacity in the piping
and distribution system, or local treatment works may limit effluent loading. • Regulation and compliance – regulations can stipulate the quantity and sometimes quality of
wastewater discharged so the companies have to ensure that their releases comply with regulatory requirements.
• Site infrastructure barrier – plant water systems are generally already complex due to years of plant modifications. There may be opportunities to make better use of the existing equipment and even improve the water quality.
• Economics – The scarcity of good quality industrial water and the stricter discharge regulations has resulted in higher costs for fresh water and the treatment of wastewater respectively. The increasing fresh water and effluent discharge costs are prompting companies to look for means to save on water use and effluent discharge thereby saving costs. ;
2 With one m3 of wastes sufficient to pollute about eight m3 of water 3 "High income countries" are defined by the World Bank as countries with a Gross National Income per capita of $11,116 or more
The amount of water used in manufacturing varies significantly from industry to industry and process to process. In chemical manufacturing, total process and cooling water use 0.0045-0.045 m3 per kg of product produced are fairly typical. In most countries, industrial water tariff has been increasing from time to time. One of the main reasons that cause this is the current inflation level, which resulted in higher chemical cost, labour cost and construction cost. At a cost of $1-3 per m3
and with use rates of
order 0.02 m3 per kg of product, the costs of water is becoming a significant factor in commodity
material manufacturing. As water supply and treatment costs increase, there will be increasing pressure in the chemical process industries to reduce water consumption. Over the past four decades, significant efforts have been directed toward reducing industrial wastewater. In the 1970s, the main wastewater treatment activity of the process industries was end-of-the-pipe treatment. This approach is based on installing pollution control units that can reduce the load or toxicity of wastes to acceptable levels. In the 1980s, the chemical process industries have shown a strong interest in implementing recycle/reuse policies in which pollutants are recovered from wastewater streams (typically using separation processes) and reused or sold. This approach has gained significant momentum from the realization that waste streams can be valuable process resources when tackled in a cost effective manner. At present, there is a substantial industrial interest in the more comprehensive concept of pollution prevention Pollution prevention Pollution prevention is a newly developing field, thus there is a lot of terminology being used by different groups and individuals, not all of which is yet well defined or consistently used. According to the EPA's official definition, pollution prevention means "source reduction", but also includes Toxic Chemical Use Substitution (replacement of toxic chemicals with less harmful chemicals). Source reduction comprises the actions for reducing the quantity of the waste at the source. This term includes: Equipment or technology modifications, process or procedure modifications, Reformulation or Redesign of products, Substitution of raw materials, improvements in housekeeping, maintenance, training or inventory control. Pollution prevention does not include out-of-process recycling, waste treatment, or combustion of wastes for energy recovery. On the other side Waste Minimization refers to source reduction and recycling activities, but does not include treatment and energy recovery activities. Recycle/reuse involves the return of a potential waste material to the originating process (or to another process) as a substitute for an input material. Recycle/regeneration(reclamanation) involved recover of fresh water as well as valuable materials such as solvents, metals, inorganic species which are typically realized by separation technologies. The hierarchical representation of EPA’s definitions is presented on Fig. 7.
Fig. 7: A Hierarchical Representation of Pollution Prevention Definitions as Used by the EPA
In the absence of feasible prevention or recycling opportunities, two new stages are added to the proposed hierarchy. End-of-pipe treatment refers to the application of chemical, biological, and physical processes to reduce the toxicity or volume of downstream waste. Treatment options include biological systems, chemical precipitation, flocculation, coagulation. Disposal involves the use of postprocess activities that can handle waste, such as off-site shipment of hazardous materials to wastemanagement facilities. Similar hierarchical approach for fresh water conservation (and wastewater minimization) called ZM water management hierarchy (Fig.8) is proposed by, Manan et al., (2004). The hierarchy consists of five levels and each level represents various water management options. The levels are arranged in order of preference, from the most preferred option at the top of the hierarchy to the least preferred at the bottom. Source elimination and source reduction at the top of the hierarchy is concerned with the complete avoidance of fresh water usage. When it is not possible to eliminate or reduce fresh water at source, wastewater recycling and regeneration should be considered. Discharge after treatment should only be considered when wastewater cannot be recycled. Through the ZM water management hierarchy, the end-of-pipe treatment may not be eliminated, but it will become economically legitimate.
Fig. 8: A holistic approach for water minimisation through the ZM Water Management Hierarchy
Water uses and general approaches to water minimization within industry Most common water uses within a manufacturing facility in the process industries is presented in Fig.9. The figure also illustrates common sources of wastewater, including process uses, condensate losses, boiler blowdown, and cooling-tower blowdown, wastewater from other uses such as housekeeping and storm-water runoff. Following preliminary water treatment, water is directed to • process uses, • utility uses or • other uses.
There are four general approaches to water minimization:
Process changes (Fig.10a)-where a single source of fresh water is used to supply a variety of processes, P. Once used, the process waters are mixed and sent to a series of treatment operations, T, before discharge. Replacing the technology employed in a process can reduce the inherent demand for water. Sometimes it is possible to reduce water demand by changing the way existing equipment is operated, rather than replacing or modifying it.
Fig.9: Typical water use on a process site (Smith, 1995) Water reuse- the effluent from some process water uses can be used as the feed material for other process uses (Fig. 10b).Wastewater from one operation can be directly used in another operation, provided the level of contamination from the previous process does not interfere with the subsequent process. This will reduce overall fresh water and wastewater volumes, but not affect contaminant loads in the overall effluent from the system. Generally, reuse excludes returning, either directly or indirectly, to operations through which it has already passed, in order to avoid build-up of minor contaminants which have not been considered in the analysis.
Fig. 10. Conceptual models for water minimization in industry
Regenaration reuse-partial treatment of wastewater can remove contaminants which would otherwise prevent reuse. The regeneration process might be filtration, stream-stripping, carbon adsorption or other such processes. In this case both volumes and contaminant loads will be reduced.
Regeneration recycling-refers to the situation where water is reused in an operation through which it has already passed. In this case, the regeneration step must be capable of removing all contaminants which build up in the system
Water-using operations in a process plant can generally be classified into: • mass-transfer– based operation, characterized by the transfer of species from a rich stream which
contain a high concentration of the undesirable species, to lean stream (or a mass separating agent –MSA) which contain a low concentration of the undesirable species. The input and output flow rates of a mass-transfer process are assumed to be equal.
• non-mass-transfer– based operations, in which the water is used for operation other than as a mass separating agent. a non-mass-transfer– based process can have different inlet and outlet flowrates process uses.
The examples of water using operation are presented in Table 2. Table 2. Water-using operations in a process plant
Mass transfer-based operations Non-mass-transfer–based operations Vessel washing Reactor processes
WaterWastewater from washing
C H NO3 5 2
Fe
H O2
C H NH+
Fe O
6 5 2
3 4
O2An+H O2NH5
C H3 6
a) b) Cleaning involves the preferential transfer of species (contaminant) from a “rich stream” (in this case, the vessel) to a lean stream or an MSA (in this case, water).
Water being fed as a raw material, or being withdrawn as a product or a by-product in a chemical reaction (a) A reactor that consumes water in aniline production; (b) a reactor that produces water as a by-product in acrylonitrile (AN).
Sour gas absorption Water utility usses Water as absortion solvent
Sour gas
Sweetened gas
Water to regeneration unit
Cooling towermake-up water
Boilerblowdown
a) b) Absorption process where water is the MSA used to remove contaminants such as H2S and SO2 from a sour gas stream.
Typical of non–mass transfer operations because these operations are not designed to preferentially transfer species between streams. For such operations, sometimes, only water demands or water sources exist.
Process integration methods Process Integration (PI) is a fairly new term that emerged in the 80's and has been extensively used in the 90's to describe certain systems oriented activities related primarily to process design. Process integration can be described as “a holistic approach to process design, retrofitting, and operation which emphasizes the unity of the process”, differently to a design approach that optimizes at the unit operation level. Process integration enables the designer to see “the big picture first, and the details later”. Based on this approach, it is not only possible to identify the optimal process development
strategy for a given task, but also to uniquely identify the most cost-effective way to accomplish that task. The implementation of PI methods can lead to significant energy savings and waste reduction (primary wastewater minimization). Some of research centers [2] reported that “PI is probably the best approach that can be used to obtain significant energy and water savings as well as pollution reductions for different kind of industries”. Their experience, summarized for the wide variety of industrial processes (Fig. 11), point out the great potential for improving the efficiency of large and complex industrial facilities. This potential exceeded the results obtained by than traditional audits, based on separate optimization of individual process units.
Fig. 11. Potential for reduction of energy and water consumption through PI Today Process integration can be broadly categorized into mass integration and energy integration. Energy integration deals with the global allocation, generation, and exchange of energy throughout the process. Mass integration create the picture of the global flow of mass within the process and optimizing the allocation, separation, and generation of streams and species. It has been developed and applied to the environmental and mass processing problems of the processes. A review of some process integration design tools for addressing energy conservation and waste reduction is provided in the Table 3. Mass integration and mass exchanger network synthesis From historical point of view the single most important concept and the one that originally gave birth to the field of Process Integration is the Heat Recovery Pinch4. Pinch Technology is originally developed as a tool for the design of energy-efficient heat-exchange networks during petroleum crises in late 1970s and early 1980s, in response to the sharp increase in the price of energy5.From that time, pinch technology based techniques have found application in a wide range of system design, including: distillation column profiling, low-temperature process design, batch process integration, emissions targeting and water and wastewater minimization. The concept has later been expanded into new areas, like Mass Pinch, Water Pinch, Hydrogen pinch, by using various analogies. The most obvious analogy is between heat transfer and mass transfer, first recognized by El-Halwagi and Manousiouthakis (1989). The similarities of these mechanisms suggested that a design method for heat-exchange networks might be possible to use for the design of mass-exchange networks.
4 Discovered independently by Hohmann (1971.), Umeda et al. (1978-79) and Linnhoff et al. (1978-79). 5 Before that time energy costs usually represented around 5% of the total cost, but after that it rose to around 20%
Table 3. Summary of process integration design tools
PI methodology Schematic representation Short description Technology
targeted
Heat integration systems or Heat Exchanger Networks (HENS)
Heat exchangenetwork
Hot stream in
Hot stream out
Cold
stre
am in
Cold
stre
am o
ut
Heat exchangerc c unitounter- urrent
Cold stream outCold stream in
Hot stream out Hot stream in
The identification of heat recovery options, devices that minimize environmental emissions resulting from utility generation systems
• Heat exchangers
• Heat pumps • Boilers/cooling
towers
Mass exchange network (MENS) and reactive mass exchange network (REAMENS)
Mass exchangenetwork
Lean Streams (MSA’s) in
Lean Streams (MSA’s) outRegenerated/Recycled Streams
Rich
(Was
te) s
tream
s in
Envi
ronm
enta
lly A
ccep
tabl
eEm
issi
ons
Mass exchangerunit
Mass separating agent Out
Mass separating Agent in
Waste stream out Waste stream in
A network of process units that removes pollutant(s) from end-of-pipe streams via the use of physical or chemical, direct-contact, mass separating agents (MSAs).
• Adsorption • Absorption • Liquid–liquid
extraction • Ion exchange
Heat-induced separation network (HISENS) and energy-induced separation network (EISENS)
Heat-inducedseparation
network
Energy separating agent in
Was
te s
trea
ms
in
Environmentally
Acceptable
Emissions Heat-induced separator unit
ESA in
Waste stream out Waste stream in
Energy separating agent out Specie
s sep
arated
from was
te str
eam
ESA out
Indirect-contact,c c unitounter- urrent
A network of process units that removes pollutant(s) from end-of-pipe streams via the use of indirect-contact energy separating agents (ESAs), including stream pressurization and/or depressurization.
• Condensation • Evaporation • Drying • Crystallization • Compressors • Vacuum pumps
In-plant separation design via waste interception and allocation networks (WINS)
Waste interceptionand allocation network
Mass separating agent in
Raw
mat
eria
ls
Environmentally Acceptable Gaseous Emissions
Products&
By-Products
Mass separating agent out Environmentally Acceptable
Aqueous Emissions
A network of process units that removes pollutant(s) from in-plant streams via the use of physical or reactive direct-contact mass separating agents (MSAs) and/or rerouting of in-plant process streams.
• Direct recycle opportunities
• Adsorption • Absorption • Liquid–liquid
extraction • Ion exchange
In-plant separation design via heat-induced waste minimization networks (HIWAMINS) and energy-induced waste minimization networks (EIWAMINs)
Heat and energy-induced waste minimization networks
Energy separating agent in
Raw
mat
eria
ls
Environmentally Acceptable Gaseous Emissions
Products&
By-Products
Energy separating agent out Environmentally Acceptable
Aqueous Emissions
A network of process units that removes pollutant(s) from in-plant streams via the use of indirect-contact energy separating agents (ESAs) with stream pressurization and/or depressurization and/or rerouting of in-plant process streams. Full site heat integration is simultaneously addressed by this technique.
• Direct recycle opportunities
• Heat exchange/heat integration
• Condensation • Evaporation • Drying • Crystallization • Compressors • Vacuum pumps
• Heat pumps
Wastewater minimisation systems
Water use process network
Fre
sh w
ater
Was
te w
ater
Water-using process
Waste water outWaste water in
Process water out Process water in
A design strategy for reuse, regeneration reuse, and regeneration recycling of wastewater streams that minimizes water usage and minimizes wastewater discharge.
• Direct recycle opportunities
• Regeneration reuse and recycling opportunities
Membrane networks separation
Retentate Feed in
Permetate
Membraneseparation unit.
Membrane separation network
Feed
Reten
tate
Permetate
A network of process units that removes pollutant(s) from end-of-pipe streams via the use of membranes and stream pressurization and/or depressurization.
• Reverse osmosis
• Pervaporation
The differences between heat transfer and mass transfer mechanisms however indicated that the design of mass-exchange networks involves a problem of higher dimensionality due to the differing criteria for thermal equilibrium and phase equilibrium. Thermal equilibrium between a pair of hot and cold streams is established when the temperatures of both streams and cold stream are the same and does not depend on the nature the streams. Only energy is transferred, and thus temperature represents the sole equilibrium criterion for the system. For mass-exchange, the distribution of species between two phases is determined by the phase equilibrium characteristics of the solute-solvent system. Thus, in addition to the temperature of the different phases being the same, phase equilibrium is dependent on the nature of the solute-solvent system and the distribution of species between the phases. Since that the number of conserved quantities which are exchanged in mass transfer can be large. The analogy between these mechanisms can be expanded on the case of heat exchanger and mass exchanger network. Descriptive definition of MEN Synthesis can be:
the systematic generation of a cost-effective network of mass-exchangers with the purpose of preferentially transferring certain species from a set of rich streams to a set of lean streams
(El-Halwagi and Manousiouthakis, 1989).
The mass-exchangers in this definition include almost any mass transfer operation that uses a Mass-Separating Agent (MSA) for the selective transfer of certain solutes. The MSA should be partially or totally immiscible in the rich phase. When the two phases are in intimate contact, the solute are redistributed between the two phases and causes depletion in the rich phase and enrichment in the lean phase. The problem, in its basic form, may be stated as follows (El-Halwagi, 1997):
Given a set of : rich streams (number iR , flowrate kg/s)(Gi , supply s
iy and target tiy composition)
process lean streams-MSAs (number PjS , supply s
jx and target tjx composition)
external lean streams-MSAs (number EjS , supply s
jx and target tjx composition).
The flowrate of each MSA kg/s)(L j is unknown and: limited according to their availability on the plant in the case of process lean streams, unlimited in the case of external lean streams since they can be can be purchased from the market.
The flowrate of each MSA is to be determined in order to minimize the MEN operating cost, by maximizing the use of the Process MSAs and minimizing the amount of External MSAs
Fig.12. Mass exchanger
Fig.13. Mass exchange network MEN synthesis task attempts to provide cost-effective solutions to the following design questions: Which mass-exchange operations should be used (e.g., absorption, adsorption)6? Which MSAs should be selected (e.g., which solvents, adsorbents)?
6 A brief review of mass exchange operation is presented in Apendix
What is the optimal flowrate of each MSA? How should these MSAs be matched with the waste streams (i.e., stream pairings)? What is the optimal system configuration (e.g., how should these mass exchangers be arranged? Is there any stream splitting and mixing
Conceptual approaches for solving this task are usually classified in the two branches: Pinch design methods - This method is directly analogous to that proposed by Linnhoff and co-workers for the design of heat-exchange networks,
Mathematical optimization techniques.
Reviews and overviews of the field of MEN Synthesis are provided by El-Halwagi (1997; 1999), El-Halwagi and Spriggs (1998), Spriggs and El-Halwagi (2000). An overview of capital cost targeting in MEN Synthesis is provided by Fraser and Hallale (2000) while Grossmann et al. (1999) provide an overview of the application of mathematical programming to MEN Synthesis7. The complete MENS design by Pinch Technology can be summarized in the four phases:
1. Data Extraction phase, the main goal of this phase is to identify the process streams (reach and lean) inside the flowsheet of plant and externan MSAs, which could be used for building the new, or retrofitting the existing, MEN. This is a primal and maybe the most important step in pinch design. 2. Targeting phase, where is possible to quantify targets for minimum amount of external MSA and minimum number of units, ahead of the actual design stage.. 3. Design phase, where an initial mass exchanger network, that satisfies the previously defined performance target, is established. The design starts where the process is most constrained, at the pinch, and is carried out separately above and below pinch. The duty of each mass exchanger is made as large as possible in order to minimize the number of units in the design. External utility exchangers are placed on streams which do not meet the target concentrations, when using only process mass exchangers. 4. Optimization phase, the MEN from the initial design is simplified and improved economically. The strict decomposition at the Pinch normally results in networks with at least one unit more than the minimum number, as well as a few units of inappropriate small duty. By manipulating with Heat Load Loops, Heat Load Paths, stream splitting and restoring minimum approach concentration the final solution is improved in order to achieve a more cost optimal MEN. Targeting phase-Minimum amount of external MSA Targeting approach is based on the identification of economic targets ahead of design and without prior commitment to the final network configuration. Targeting approach comprises the calculation of: The minimum amount of external MSA required to achieve the overall mass-exchange balance. This task is accomplished by using the mass-exchange pinch diagram.
The minimum number of mass-exchangers required to achieve the targeted flow of external MSA.
The first task of targeting phase start by defining the quantitative relationships between the concentrations of all streams (process streams and MSAs). This tool established a one-to-one correspondence among the compositions of all streams for which mass transfer is thermodynamically feasible. By that way several composition scales (one for all the rich streams and one for each process MSA) are established and thermodynamic and other constraints are incorporate into the stream data. If the relation between the process-MSA scales, jx , and the rich stream concentration scale, y is linear this can be expressed as :
jj xay = (1)
In order to ensure feasible mass transfer throughout the networks (similar to ΔTmin used in HENS) a minimum approach concentration (MAC) for mass transfer, jε , have to be defined.
7 In practise not all MEN Synthesis problems involve Process MSAs and in such cases only External MSAs are used to achieve the MEN task. Water minimization, in which water is used as the sole MSA is an example of this kind of problem (Hallale and Fraser, 1998), and the starting point of the work on the design of optimal water-use systems by Wang and Smith (1994a). These approaches will not be reviewed in this paper.
On that way mass transfer from a process stream, at concentration y , to MSA j, which concentration of the contaminant in the is jx is possible only if the scale relations are
( )jjj xay ε+= (2)
or
jj
j ayx ε−= (3)
By using MAC a “practical- feasibility line”, that is parallel to the equilibrium line (but offset to its left by a distance jε ).can be drawn .Fig. 14 illustrates the feasibility of mass transfer between a
process stream (at concentration y ) and MSA j (at concentration jx ). The solid line corresponds to the case defined by Equation (A1.1) without a MAC. At point A, no driving force for mass transfer exists between a rich process stream at concentration Ay and a MSA at concentration A
jx .The dashed line corresponds to feasible mass transfer defined by Equations (A1.2 or A1.3) where the MAC shifts the line jε to the right along the jx axis. In this case, the driving force for mass transfer between a rich
process stream at concentration Ay and MSAj at concentration Bjx is at its minimum due to
thermodynamic or other constraints.
A
εj
xj
y
Practical Feasibility Region
Practical F
easibility
Line
Equilib
rium Line
Ay
Ajx B
jx
Fig.14. Feasible mass transfer and the minimum approach concentration (MAC) A more general form of the linear equilibrium expression given in Equation (1) including the equilibrium constant jb :
( ) jjjj bxay ++= ε (4)
Rearrange of Eq. (4) , can produce the linear equilibrium relationships into the analysis as: ( )jjjjj baxay ε++= (5)
In order to simplify the calculations it is useful to introduce a new term called the capacity flowrate. To explained this term we can start with expressions of the mass load of contaminant transferred from process streams to MSAs.
iii xGm ΔΔ = , for rich streams (6)
jjj xLm ΔΔ = , for MSAs (7)
By substituting ixΔ in Equation (7) we can obtained mass load of contaminant transferred to MSAj in
terms of the modified flowrate, ja
L⎟⎠⎞
⎜⎝⎛ , and the process stream concentration scale, y :
yaLm
jj ΔΔ ⎟
⎠⎞
⎜⎝⎛= (8)
The new formed modified flowrate, ja
L⎟⎠⎞
⎜⎝⎛ become the capacity flowrate of MSAj. It has to be noted
that capacity flowrate of process stream i is simply the available flowrate, Gi . The initial step in constructing the mass pinch diagram is to create a global representation for all the streams which exist in the task. The mass load (mass exchanged) by each stream is plotted versus its composition scale. Each stream is represented as an arrow whose tail corresponds to its supply composition and its head corresponds to its target composition. The slope of each arrow is equal to the capacity flowrate (Apendix 2).
slope1
yym
ymGi out
iini
=−
==Δ
ΔΔ (9)
slope1
yym
ym
aL
outj
injj
=−
==⎟⎠⎞
⎜⎝⎛ Δ
ΔΔ (10)
Having represented the individual streams, we can construct the composite curves (CC). A composite rich (lean) curve represents the cumulative mass of the targeted species lost (gained) by all the rich (lean) streams. It can be readily obtained by using the “diagonal rule” for superposition to add up mass in the overlapped regions of streams. Hence, the rich composite curve is obtained by applying superposition to all the rich streams. Similarly, the lean composite curve is constructed by employing linear superposition to all the process MSAs. The construction of CC (either for reach or lean streams not both) is presented on Fig. 15.
Fig. 15. Construction of the composite curve
Next, both composite curves are plotted on the same diagram. On this diagram, thermodynamic feasibility of mass exchange is guaranteed when the lean composite curve is always below the rich composite curve. This is equivalent to insuring that at any mass-exchange level (which corresponds to a vertical line), the composition of the reach composite curve is located to the left of the lean composite curve, thereby asserting thermodynamic feasibility. Therefore, the lean composite curve can be slid until it touches the rich composite curve. The point where the two composite curves touch is called the “mass exchange pinch point” and, hence, the name “mass pinch diagram” (Fig. 16).
External MSA Duty
Excess Capacity of Process MSAs
Process-Stream Composite Curve
Mass exchange Pinch point
Process-MSA Composite Curve
X
X
ypinch Tpinch
a) b)
y yExcess Capacity of Process MSAs
External MSA Duty
Δm[kg/s] Δm[kg/s]
Reach Stream
Lean Stream
Fig. 16. Pinch diagram(a) and grand composite curve (b) The horizontal overlap between the two composite curves represents the maximum feasible amount of the species that can be transferred from the rich curves to the process MSAs. It is referred to as the “integrated mass exchange.” The horizontal distance at the bottom left represents the excess mass load of contaminant in the process streams that is not removed by process MSAs. This excess must be transferred to external MSAs. The horizontal distance at the top right represents the excess capacity of process MSAs to remove mass load from process streams. Such excess can be eliminated by lowering the flowrate and/or the outlet composition of one or more of the process MSAs. It is not necessary to replot the process MSA composite curve, as minor adjustments above the pinch concentration will not affect the utility target below the pinch concentration. By the use of mass load (horizontal) differences between the composite curves, it is possible to create another graphical approach, known as the Grand Composite Curve-GCC (Linnhoff et al., 1982). The GCC provides the same information as Pinch diagram, but in a slightly different fashion. The mass exchange pinch decomposes the synthesis problem into two regions: above the pinch (containing all streams or parts of streams richer that the pinch composition) and below the pinch (containing all streams or parts of streams leaner that the pinch composition). Above the pinch, only process MSAs are required. Using an external MSA in this region will incur a penalty of eliminating an equivalent amount of process lean streams from service. On the other hand, below the pinch, both the process and the external lean streams should be used. Furthermore, if any mass is transferred across the pinch, the composite lean stream will move to the right and, consequently, external MSAs in excess of the minimum requirement will be used. Therefore, to minimize the cost of external MSAs, mass should not be transferred across the pinch.
Targeting phase-Minimum Number of Mass-Exchange Units Another useful target in MENs synthesis is the theoretical minimum number of mass-exchange units. This is done by the use of Euler’s graph theory, which identifies the number of contaminant-rich process streams and MSAs. For systems where the pinch divides the design in to two separate regions the minimum number of units is:
pinch belowSRpinch aboveSRunits 1)-N (N1)-N (N N + + += (11) According to El-Halwagi and Manousiouthakis (1989), this target attempts to minimise indirectly the capital cost of the network, since the cost of each mass exchanger is usually a function of the unit size. It is also desirable from a practical point of view. Mass exchanger Network Design After establishing the performance target in the next phase is dedicated to the design oh initial mass exchanger network. The most common network design is carried out using a grid diagram (Fig. 17), in which: Horizontal lines at the top of the diagram represent process streams. These streams flow from the left (rich side) to the right (lean side) of the diagram.
Horizontal lines at the bottom of the diagram represent process and external MSAs. These streams flow from the right (lean side) to the left (rich side) of the diagram.
Vertical lines represent mass-exchange units. Each line connects a rich process stream and a process or external MSA. We indicate the mass load of the unit (kg/s) within the circles connecting the lines, and also show the inlet and outlet concentrations of the rich process stream and the MSA.
R1
G (kg/s)1
R2
S1
S2
S3
Pinchy
G (kg/s)2
)s/kg(aL
2⎟⎠⎞⎜
⎝⎛
)s/kg(aL
3⎟⎠⎞
⎜⎝⎛
S2
)s/kg(aL
2⎟⎠⎞
⎜⎝⎛
R1
G (kg/s)1
R2
S1
G (kg/s)2
)s/kg(aL
1⎟⎠⎞
⎜⎝⎛
1R1Sm →Δ
1R1Sm →Δt
1Sy s1Sy
t2Sy s
2Sy
t3Sy s
3Sy
s1Ry
s2Ry t
2Ry
t1Ry
Fig. 17. A mass exchange match show on a grid diagram Designing mass-exchange networks is a two-step process. First, we generate a preliminary network guaranteed to meet the minimum flowrates of external MSAs determined by targeting approach. In order to meet the minimum MSA target, the region above and below the pinch are designed separately with no mass being transferred across the pinch. (mass pinch technology phase 3)
Second, we improve the network by relaxing restrictions on pinch regions and minimum approach concentrations (mass pinch technology phase 4)
Design of preliminary mass-exchange networks starts from the pinch point and move away from it. A general rule for matching a process stream to a MSA is that the capacity flowrates of streams leaving the pinch concentration (process MSAs above the pinch concentration or process streams below the pinch concentration) must be greater than or equal to the capacity flowrate of streams approaching the pinch concentration (process streams above the pinch concentration or MSAs below the pinch
concentration). A simple technique for identify matches with respect to the capacity flowrates of streams entering and leaving the pinch is the tick-off table. In the table, we match streams above the pinch by drawing lines from a MSA to a process stream (right to left) such that the line always points to a process stream with a lower capacity flowrate. Conversely, below the pinch, we draw lines to identify matches from a process stream to a MSA (from left to right), such that the line always points to a MSA with a lower capacity flowrate. We do so until each stream entering the pinch (process streams above the pinch concentration and MSAs below the pinch concentration) has been matched with a stream leaving the pinch. For some problems, we may not be able to strictly follow the capacity-flowrate rule for stream matching without segmenting streams. In that case we can split the available MSA and use a portion of its capacity flowrate to remove contaminant from each process stream. After design a preliminary mass-exchange network, starts the network improvement by identifinig the loops and shifting mass loads away from small, inefficient units in order to create fewer, larger, more cost-effective units. Improving enable the relaxing of the restrictions on preliminary networks and allowing individual exchangers to operate below minimum approach concentration and/or transfer mass across the pinch. Design and improve of mass-exchange networks are presented in case study. Case study – MENS in oil-recycling plant For the case study the oil-recycling plant shown in Fig. 18. is used. In this plant, two types of waste oil are handled: gas oil and lube oil. The two streams are first deashed and demetallized. Next, atmospheric distillation is used to obtain light gases, gas oil, and a heavy product. The heavy product is distilled under vacuum to yield lube oil. Both the gas oil and the lube oil should be further processed to attain desired properties. The gas oil is steam stripped to remove light and sulfur impurities, then hydrotreated. The lube oil is dewaxed/deasphalted using solvent extraction followed by steam stripping.
Fig. 18. Flowsheet of oil-recycling plant
The process has two main sources of waste water. These are the condensate streams from the steam strippers. Table 4. Data of Waste Streams for the Dephenolization Example
Stream Description iG [kg/s] siy t
iy
R1 Condensate from
first stripper I 2.0 0.050 0.010
R2 Condensate from
first stripperII 1.0 0.060 0.010
The principal pollutant in both wastewater streams is phenol that can be separated using several techniques. Solvent extraction using gas oil (S1) or lube oil (S2) as process Mass Separation Agents (MSA) is a potential option. Besides the purification of wastewater, the transfer of phenol to gas oil and lube oil is a useful process for the oils. Phenol tends to act as an oxidation inhibitor and serves to improve color stability and reduce sediment formation. The data for the process MSAs are given in Table 5. Table 5. Data of Process MSAs for the Dephenolization Example
Stream Description jL [kg/s] six t
ix
S1 Gas oil 5.0 0.01 0.015 S2 Gas oil 5.0 0.005 0.015
Three external technologies are also considered for the removal of phenol. These processes include adsorption using activated carbon, S3, ion exchange using a polymeric resin, S4, and stripping using air, S5. In this paper we will consider only a S3 as an external MSA, which is available for purchase at an unlimited flowrate. The equilibrium data for the transfer of phenol to the jth lean stream is given by jj xay = where the values of ja are given in followed tables. Also, listed are the supply and target compositions each MSA. Throughout this example, a minimum allowable composition difference, jε of 0.001 (kg phenol)/(kg MSA) will be used. Corresponding Concentration Scales
Table 6. Process MSA stream data for Dephenolization Example. Concentrations shifted to the corresponding process-stream scale (y) with a minimum approach concentration of 0.001
Stream ja jε jL [kg/s] siy t
iy
S1 2.00 0.001 5.0 0.0120 0.0320 S2 1.53 0.001 3.0 0.0168 0.0474
Table 7. Corresponding concentration scales for the external MSA, S3.
Stream ja jε six t
ix siy t
iy
S3 0.04 0.001 0.100 0.200 0.00404 0.00808
Fig. 19. Corresponding concentration scales for process streams (y), process MSAs (xS1 and xS2) and external MSA (xS3 )
Capacity Flowrates
Table8. Shifted stream data for the MSAs with capacity flowrates. Concentrations shifted to the corresponding process-stream scale (y) with a minimum approach concentration of 0.001..
Stream ja ja
L⎟⎠⎞
⎜⎝⎛ [kg/s] s
iy tiy
S1 2,00 2.5 0,0120 0,0320 S2 1,53 1.961 0,0168 0,0474 S3 0.04 - 0.00404 0.00808
Minimum External MSA Duty without Mass Integration Table Minimum flowrates of the external MSA S3 without integrating process MSAs (aS3=0.04)
Stream Description 3Sa
L⎟⎠⎞
⎜⎝⎛ [kg/s] 3SL
R1 Condensate from first stripper 19.802 0.80 R2 Condensate from second stripper 5.941 0.24 Sum 25.743 1.04
Elimination of excess capacity of the process MSAs equal to 0.01842 kg/s. by the reducing of capacity flowrate of process MSA S2 according to:
( ) ( ) 359.101683.004743.0
01842.0961.1yy
01842.0aL
aL
s2S
t2S2S
pod
2S
=−
−=−
−⎟⎠⎞
⎜⎝⎛=⎟
⎠⎞
⎜⎝⎛
s/kg0793.2359.153.1aLaL
pod
2S2S
pod2S =⋅=⎟
⎠⎞
⎜⎝⎛−=
Fig. 19. Composite curves and mass pinch diagram
Flowrate of process MSA S2 from 3.0 kg/s is deacreased to 2.0793 kg/s.
( ) ( ) )s/kg(105.300404.000808.0
01242.0yy
01242.0aL
s3S
t3S3S
=−
=−
=⎟⎠⎞
⎜⎝⎛
The capacity flowrate required, 3Sa
L⎟⎠⎞
⎜⎝⎛ , of external MSA S3 :
( ) ( ) )s/kg(105.300404.000808.0
01242.0yy
01242.0aL
s3S
t3S3S
=−
=−
=⎟⎠⎞
⎜⎝⎛
Fig. 20. Starting grid diagram
Fig. 21.Ttick-off table.
Stream spliting
skg6667.1
0.10.20.2
skg105.3
GGG
aL
aL
2R1R
2R
3S1R1S=⎟
⎠⎞
⎜⎝⎛
+⎟⎠⎞
⎜⎝⎛=⎟⎟
⎠
⎞⎜⎜⎝
⎛+
⎟⎠⎞
⎜⎝⎛=⎟
⎠⎞
⎜⎝⎛
→
skg8333.0
0.10.20.1
skg5.2
GGG
aL
aL
2R1R
2R
1S2R1S=⎟
⎠⎞
⎜⎝⎛
+⎟⎠⎞
⎜⎝⎛=⎟⎟
⎠
⎞⎜⎜⎝
⎛+
⎟⎠⎞
⎜⎝⎛=⎟
⎠⎞
⎜⎝⎛
→
( ) ( )s
kg00805.00120.001683.0s
kg6667.1yyaLm s
1Spinch1S
1R1S1R1S =−⎟
⎠⎞
⎜⎝⎛=−⎟
⎠⎞
⎜⎝⎛=
→→Δ
( ) ( )s
kg004025.00120.001683.0s
kg8333.0yyaLm s
1Spinch1S
2R1S2R1S =−⎟
⎠⎞
⎜⎝⎛=−⎟
⎠⎞
⎜⎝⎛=
→→Δ
( ) ( )s
kg00561.000805.001.001683.0s
kg0.2myyGm 1R1Ss
1Rpinch1R1R1R3S =−−⎟
⎠⎞
⎜⎝⎛=−−= →→ ΔΔ
( ) ( )s
kg0068.0004025.0006.001683.0s
kg0.1myyGm 2R1Ss
2Rpinch
2R2R2R3S =−−⎟⎠⎞
⎜⎝⎛=−−= →→ ΔΔ
skg4025.1
00404.000804.000561.0
yym
aL
s3S
t3S
1R3S
1R3S
=−
=−
=⎟⎠⎞
⎜⎝⎛ →
→
Δ
skg72.1
00404.000804.00068.0
yym
aL
s3S
t3S
2R3S
2R3S=
−=
−=⎟
⎠⎞
⎜⎝⎛ →
→
Δ
Fig. 22. Starting grid diagram above the pinch with stream spliting
y=0.01683
R12.0kg/s
R2
1.0kg/s
S1
2.5kg/s
S2
1.359kg/sS2
1.359kg/s
y=0.01683
y=0.01683
y=0.01683
y=0.01683
y=0.01280 y=0.01
y=0.006y=0.01280
y=0.012
y=0.012
y=0.00404
y=0.00404y=0.00804
y=0.00804
S3
3.1038kg/s
0.8333kg/s
1.6667kg/s
1.4025kg/s
1.7012kg/sS3
3.1038kg/s
R12.0kg/s
R21.0kg/s
0.00805
0.004025
0.00561
0.0068
0.00805
0.004025
0.00561
0.0068
Pinch
S12.5kg/s
1.7012kg/s
1.4025kg/s
1.6667kg/s
0.8333kg/s
y=0.03579y=0.05
y=0.03 y=0.01683
y=0.032
y=0.01683y=0.02652y=0.04743
0.01317
0.013170.028742
0.028742 0.037925
0.037925
Fig. 23. Final MEN for case study
References [1] El-Halwagi, M. M. (1997). Pollution Prevention through Process Integration: Systematic Design
Tools. San Diego: Academic Press. [2] El-Halwagi, M. M. and Manousiouthakis, V. (1989). Synthesis of Mass Exchange Networks.
AIChE Journal. 35 (8). 1233-1244. [3] Rašković P. “Industrial energy system optimization based on heat exchanger network synthesis”,
Faculty of Mechanical Engineering, University of Nis, Serbia and Montenegro, 2002. [4] Gundersen T., ”Process Integration PRIMER”, SINTEF Energy Research, May 2000. [5] Gundersen T., ”A worldwide Catalogue on Process Integration”, SINTEF , June 2001. [6] Alva-Argáez, A., Kokossis, A. C. and Smith, R. (1998). Wastewater Minimisation of Industrial
Systems Using An Integrated Approach. Computers and Chemical Engineering, 22: S741-744. [7] Hallale, N. (2002). A New Graphical Targeting Method for Water Minimisation. Advances in
Environmental Research. 6 (3): 377-390. [8] Hallale, N. and Fraser, D.M. (1998). Capital Cost Targets for Mass Exchange Networks A Special
Case: Water Minimisation. Chemical Engineering Science. 53 (2): 293-313 [9] Shenoy, U. V. (1995) Heat Exchanger Network Synthesis: Process Optimization by Energy and
Resource Analysis. Houston Gulf: Publishing Co. [10] Smith, R. (1995). Chemical Process Design. New York: McGraw Hill.
Apendix -VOC SEPARATION SYSTEMS FOR AQUEOUS WASTES (WASTEWATER)
Technologies Description Advantages Disadvantages Air /Steam Stripping Air Stripping
Wastewater flows into the top of a packed (or try) column and is distributed throughout the packing. Air flows into the bottom of the column and the VOC is transferred from the wastewater to the air via direct contact. “VOC-free” wastewater exits the bottom of the column. The VOC-laden air exits the top of the column and the VOC is subsequently condensed from this gas stream prior to emitting the stream to the environment.
Operating temperatures are lower than steam stripping
More effective for VOC’s with higher volatility (lower boiling point).
Preventive maintenance costs are typically low.
System can usually be easily upgraded to strip greater amounts of VOC’s with relatively small increases in capital costs.
Maximum allowable concentration of VOC in the air exiting the stripper is limited by the Lower Flammability Limit (LFL) of the VOC in exit air stream.
Exceeding the LFL in the exit air stream may lead to fires and/or explosions in the stripper or the subsequent air handling equipment/condenser.
Steam Stripping Wastewater flows into the top, steam flows into the bottom of the column. The VOC-laden steam exits the top of the column and the VOC is subsequently condensed with the steam. If the VOC is immiscible in water, the condensate will form an aqueous layer and a solvent layer that can be separated using a decanter. If the VOC is miscible in water, additional distillation can be used to further separate the VOC and water.
A widely used technology with well known operating characteristics.
Operating temperatures are higher than air stripping.
More effective for VOC’s with lower volatility (higher boiling point).
Can remove a wider range of VOC’s. Allows a wider range of removal levels.
Higher operating temperatures than air stripping may accelerate equipment and/or compound degradation.
Some steam will condense and add to the hydraulic load of the system.
Activated Carbon Adsorption Wastewater flows into the top or bottom of an adsorption column, filled with porous activated carbon, and is distributed throughout the carbon bed. The VOC is adsorbed onto the surface of the activated carbon and onto the surface of the pores. In the moment when carbon becomes saturated with VOC (loses its capacity for additional adsorption), the carbon must be regenerated or replaced with virgin carbon. Regenerating a bed of activated carbon typically involves the direct injection of steam, hot nitrogen or hot air to the bed which causes the VOC to release from the carbon and exit the bed via a vapor or steam condensate stream. The regenerated stream, containing a higher concentration of the VOC than the original wastewater stream, is subsequently condensed. If the VOC is immiscible in water, the condensate will form an aqueous layer and a solvent layer that can be separated using a decanter. If the VOC is miscible in water, additional distillation can be used to further separate the VOC and water.“VOC-free” wastewater exits the adsorber after the contact with the activated carbon.
A widely used technology with well established performance levels.
Can be used for low concentration inlet streams.
Can achieve high removal efficiencies. Can efficiently handle fluctuations in
wastewater flow rates and VOC cencentrations.
Higher complexity of operation than other technologies.
Larger space requirements than other technologies.
Water adosrption lowers the VOC adsorption capacity.
Exceeding the LFL in the exit regeneration stream (when using air regeneration) may lead to fires and/or explosions in the adsorber or the subsequent air handling equipme-nt/condenser.
Description Advantages Disadvantages
Liquid-Liquid Extraction
Liquid-liquid extraction involves the separation of VOC’s by contact with another liquid (solvent) in which the VOC’s are more soluble. Extraction solvent selection is based on: selectivity, ease of regeneration, low miscibility with the feed solution, significant density difference between the extraction solvent and the wastewater feed, low flammability and toxicity,low cost and ready availability. Separation of the solvent-VOC waste can be handled via air stripping, steam stripping, distillation, or additional liquid-liquid extraction. Separation of the exiting wastewater stream can occur via air stripping, steam stripping, activated carbon adsorption or biological treatment. Process efficiency can be increased by increasing the flowrate of solvent to wastewater or by increasing the number of extraction stages.
Generally, liquid-liquid extraction is easy to operate.
Capital costs are relatively low; however, if additional separation (distillation) of exiting streams is required capital cost generally increases by a factor of 8-10 and operating costs increase by a factor of 20.
Can be used for heat sensitive materials. Can be used to separate close-boiling
mixtures, such as isomers.
VOC gaseous emissions may occur from the extraction unit.
Energy costs are high. Additional treatment (distillation) of
streams leaving the extraction unit is generally required.
Pervaporation
Pervaporation (a technology that combines permeation and evaporation) is a membrane-based process that operates on the principle of selective permeation of a VOC through the membrane. For VOC removal from wastewater, a hydrophobic (organophilic) membrane (typically a rubbery polymer) is required. Feed streams to pervaporation membranes are typically preheated. The warm wastewater feed enters the membrane housing at a high pressure and the VOC (and some water) preferentially permeates to the low pressure side of the membrane. As the VOC (referred to as the permeate) goes from high pressure to low pressure it flashes to the vapor state and a condensation process (typically using chilled water) is used to condense the VOC.“VOC-free” wastewater (referred to as the retentate) exits the high pressure side of the membrane.
Organics can be concentrated 10 to 50 times higher than in the original wastewater stream.
Systems are modular and compact. Efficient at low VOC inlet concentrations
(100-5000ppm). Less energy intensive than reverse osmosis. Can be used to separate close-boiling or
azeotropic compounds.
Restricted to smaller flowrates (<15 gpm). Membrane fouling may occur. Although a proven technology, no large
scale industrial uses currently exist.
Reverse Osmosis
Reverse Osmosis is a membrane-based process that operates on the principle of selective permeation of a VOC through the membrane. No phase change occurs across the membrane. Wastewater streams enter the membrane housing at a high pressure and the water (and some VOC) preferentially permeates to the low pressure side of the membrane. “VOC-free” wastewater exits the low pressure side of the membrane.The “reject” stream exits the high pressure side of the module and contains a higher concentration of VOC than the initial wastewater stream. This stream can be disposed of or distilled for further VOC recovery.
Generally compact in size; therefore, a reverse osmosis system can be easily added to existing plant operations.
Flexibility of adding modules or bypassing modules can accommodate surges in wastewater loads and/or concentrations.
Fouling of membranes may occur. Can not be used on streams with a high
osmotic pressure. May require additional treatment of the
reject stream to purify the VOC to levels acceptable for reuse in the plant.