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Title: Program for North American Mobility in Higher Education Introducing Process Integration for Environmental Control in Engineering Curricula


1
Program for North American Mobility in Higher
EducationIntroducing Process Integration for
Environmental Control in Engineering Curricula
  • Module 3 Environmental Challenges Pulp Paper
    Industry
  • Created at
  • École Polytechnique de Montréal
  • Texas AM University, 2003

2
LEGEND
  • Go to the web site
  • Go to next subject
  • More information on the same subject
  • Look for the answer to the question

3
Tier IIIOpen-Ended Problem
4
Tier III Statement of Intent
  • Tier III Statement of Intent
  • The purpose of this is to provide students with
    an open-ended problem which assimilates the
    concepts of minimum impact manufacturing
    including process integration and LCA .

5
Problem Statement for Q-1 2
  • You are an environmental engineer in a pulp and
    paper mill. The head office wants to enhance its
    competiveness by putting together a technology
    roadmap with the ultimate goal to be a minimum
    impact manufacturing mill.
  • Some information about the mill is given at the
    following page.

6
Mill Description
  • Conventional pulping technology, ECF bleaching,
    drying, activated sludge plant
  • Debarking dry
  • Lime kiln normal
  • Lime kiln fuel heavy fuel oil
  • Lime kiln flue gas high eff. ESP
  • Bark boiler (HW bark)
  • Total efficiency 0.87
  • Fluidized bed boiler
  • Electric power generation from excess heat in
    mill condensation turbine
  • Since no information is available concerning the
    effluent treatment plant, its efficiency will be
    consider constant. As a consequence of that,
    from a relative point of view, the effluent ion
    loads can be considered proportional to the ones
    before the effluent treatment.

7
Question 1
  • A few months ago the company ordered a partial
    LCA study in order to have an idea about its life
    cycle environmental impacts. As a first step,
    your boss had asked you to look at this study as
    well as at the mill simulation and give him your
    recommendations for environmental improvement.
    To do this look at unit process contribution to
    each impacts and perform sensibility analysis.
    Do not use any normalization or weighting.
    Without doing calculations, you can also use cost
    arguments. Also determine, by mass balances by
    how much fresh water can be theoretically reduced
    (by recycle).
  • System boundaries are defined in the LCA study
    and the main hypothesis are presented next pages.

8
Functional Unit
  • All LCA results are presented relative to the
    functional unit. The functional unit has been
    defined as follow
  • The production of 1 admt of pulp.

9
Chemical Production
  • Chemical production as been included into the
    system boundaries. Chemicals are considered to
    be transported an average distance of 100 km
    using 40 ton diesel trucks and empty trucks
    return to the supplier. For calculation purpose
    a weight of 1/10 of the transported chemicals has
    been assumed for the return of the truck.
  • No data was available for talc manufacturing.
    Therefore it has been excluded from the system
    boundaries. However, its transportation has been
    considered.

10
Birch Growth and Harvesting
  • Birch growth and harvesting as been included in
    the boundaries. The wood is transported an
    average of 100 km. The same assumptions as for
    chemicals apply.

11
Others
  • By product have been located.
  • A credit has been considered for the generated
    energy (but only on the energy).
  • Pulp is transported an average distance of 200 km
    to the customer (same assumptions as chemicals).
  • Industrial landfill is located 5 km from the
    mill. 16 ton diesel trucks are used to transport
    the solid wastes, the return of the trucks is
    considered negligible.

12
Necessary documents
  • LCA Base Case
  • Process Simulation

13
Question 2
  • Your boss is convinced that most of the
    competitive advantages that can be gained with
    environmental improvements are related with fresh
    water reduction.
  • In this case, recycling the effluent water is the
    most obvious way to reduce fresh water
    consumption, but this can result in the build-up
    of non-process elements and so reduce process
    performance.
  • For this reason, he has also mandated a
    consulting company to perform a water pinch study
    subject to process constraints.

14
Question 2 (Contd)
  • The consultant has first evaluated possibility of
    direct recycle because it does not implicate
    major capital costs. Major results are presented
    in the following table.

Water Consumption 23 reduction
Liquid Effluent Reduction of ion content of 2.3
Gas Effluent Cl, K 0.2 increase Na 6.8 increase
Energy produced 5 reduction (need more energy to pump)
Dust 13.4 increase
Solid wastes Neglictible difference
15
Question 2 (Contd)
  • Using the LCA model, discuss if this represents a
    real environmental improvement. To compare
    results, normalize against the base case.
  • A panel of experts has determined that the
    importance of each impact category can be
    described by the weights in the following tables.
    Resources and emissions are weighted separatly.
  • What is the influence of the weights on the final
    decision.

16
Question 2 (Contd)
  • Resource depletion
  • Emissions

Impact Weight
Raw water consumption 0.83
Energy consumption 0.08
Virgin Fiber consumption 0.01
Other resources 0.08
Impact Weight
Carcinogenic substances 0.70
Heavy metals 0.07
Acidification 0.01
Eutrophication 0.01
Summer smog 0.07
Winter smog 0.07
Solid Wastes 0.005
Global Warming 0.065
17
Solution Q1
  • The process simulation does not give a lot of
    insights on the environmental impacts of the
    process. However it is obvious that the
    bleaching plant consumes a lot on fresh water and
    rejects a lot in the environment. The following
    is the solution for potential water reduction

18
Solution Q1 (Contd)
  • Water balances can be summarized by this picture.
  • The total fresh water consumption is
    9.330.973.8320.6634.79 ton/ton of dry pulp.
  • Only liquid water can be directly recycle
    0.9675.920.68120.66 28.23 ton/ton of dry
    pulp.
  • For mass conservation reasons, only the min of
    fresh water or liquid effluent can be recycle ie.
    28.23 ton.
  • So the minimum water consumption is
    34.79-28.236.56 ton (ie a reduction of 81).

19
Solution Q1 (Contd)
  • The following graph show the contribution of each
    process unit to resource consumption.

20
Solution Q1 (Contd)
  • The last figure show that the manufacturing
    activities consumes a lot of resources water,
    virgin fiber and other natural resources.
  • It also shows that chemical production is
    particularly energy-consuming.
  • From a first look, reducing chemical and water
    consumption will result in a significant
    environmental benefit.

21
Solution Q1 (Contd)
  • The following graph show the contribution of each
    process unit to emission-related environmental
    impacts.

22
Solution Q1 (Contd)
  • From this graph it is possible to note that
  • Manufacturing activities are a large contributor
    to acidification, eutrophication, winter smog and
    solid wastes
  • Chemical production is a large contributor to all
    impact categories but more specifically
    eutrophication, heavy metals and summer smog.
  • Transportation seems also to be a large
    contributor to several impact categories global
    warming, carcinogenic substances and summer smog.
  • Global warming is due to almost all unit
    processes.

23
Solution Q1 (Contd)
  • Even if it is impossible to talk about the
    relative importance of each impacts since no
    weighting has been performed, it is clear from
    the last two graphs that manufacturing
    activities, including chemical consumption must
    be targeted in order to reduce the overall
    environmental impacts. Transport is also a
    significant contributor.
  • The following results show how much a 5
    reduction in transportation and chemical
    consumption will affect the environmental
    impacts. Manufacturing is more difficult to
    assess but the impact of an increase of 5 of the
    yield (from 50 to 52.5) is also presented. It
    as been assumed that an increased yield will only
    impact the quantity of wood required and not the
    chemical consumption in order to keep both effect
    separate.

24
Solution Q1 (Contd)
  • It is important to note that here only easily
    manipulable variable have been modified in order
    to determine which changes will influence the
    more the environmental impacts.
  • The most important results are the following
  • A 5 increase in the yield will result in a
  • 5.64 reduction in fresh water consumption
  • 4.70 reduction in virgin fiber consumption
  • 4.39 reduction in natural resources consumption.
  • A 5 reduction in transportation will result in
    a
  • 4.86 reduction in energy consumption
  • 4.26 reduction in carcinogenic substances.
  • A 5 reduction in chemical will not affect
    significantly the environmental impacts.

25
Solution Q1 (Contd)
  • As an environmental engineer, you will propose
    the followings
  • Increase the process performance, which will also
    reduce costs.
  • Since reducing transportation distance is not
    easily realizable, you suggest to find a mode of
    transportation less pollutant.
  • Even if a reduction of chemical consumption will
    necessarily reduce the cost, it is not an
    environmental priority.
  • The mass balances have shown that there is a lot
    of potential for fresh water reduction.

26
Solution - Q2
27
Solution - Q2 (Contd)
  • The last graph shows the LCA results (resources)
    for the direct water recycle option. The results
    have been normalized against the reference case.
    From this graph, it is possible to say that
  • Raw water consumption from the manufacturing
    process unit has been reduced to 70 of the
    reference case.
  • Energy consumption by the manufacturing has been
    increase by 5.
  • Everything else is constant.

28
Solution - Q2 (Contd)
29
Solution - Q2 (Contd)
  • The preceding graph shows a reduction in the
    following impact categories
  • Acidification from the manufacturing process
    unit.
  • It also shows an increase in
  • Winter smog from the manufacturing process unit.
  • All the remaining impact categories are almost
    constant.

30
Solution - Q2 (Contd)
  • The aggregated indicators are
  • Resources 0.76
  • Emissions 1.00
  • From this it is possible to conclude that the
    direct water recycle solution has a positive
    impact on the resource impact categories (almost
    25 improvement) and almost no impact on the
    emissions.

31
Solution - Q2 (Contd)
  • A lot of importance has been given to the raw
    water consumption. A sensitivity analysis on the
    weights has been conducted. First, weight of raw
    water has been decreased while maintaining the
    other relative weights constant.
  • The results are presented in the table. It can
    be seen than even if the raw water importance
    passes from 83 to 10. There is still an
    environmental benefit.

Weight of the raw water consumption Aggregated Indicator
0.83 0.76
0.50 0.85
0.30 0.91
0.10 0.97
32
Solution - Q2 (Contd)
  • The impact category the most influenced by the
    direct recycle other than raw water is the
    energy.
  • By increasing the weight of energy while
    maintaining the other ratios constant we obtain
    the results presented in the table.
  • The conclusion of the 2 tables is that the
    environmental improvement is robust to the
    weights.

Weight of the Energy Aggregated Indicator
0.08 0.76
0.16 0.78
0.32 0.82
0.64 0.90
0.83 0.96
33
Solution - Q2 (Contd)
  • The same strategy has been applied to the
    emission impact categories. Sensitivity analysis
    have been conducted on the acidification and
    winter smog weights.
  • Acidification has been reduced so the sensitivity
    analysis try to determine if more weight on this
    impact category will reduce significantly the
    aggregated indicator.
  • The table shows that even if acidification weight
    passes from 1 to 80 this will results in only
    2 improvement.

Weight of the Acidification Aggregated Indicator
0.01 1.00
0.10 1.00
0.20 1.00
0.40 0.99
0.80 0.98
34
Solution - Q2 (Contd)
  • Winter smog has been increased so the sensitivity
    analysis try to determine if more weight on this
    impact category will increase significantly the
    aggregated indicator.
  • The table shows that even if winter smog weight
    passes from 7 to 80 this will results in only
    1 degradation.
  • The 2 previous tables show that the emissions
    indicator is robust to the weights.

Weight of the Winter Smog Aggregated Indicator
0.07 1.00
0.14 1.00
0.28 1.00
0.56 1.00
0.80 1.01
35
Solution - Q2 (Contd)
  • Overall conclusion
  • Direct water recycle results in a positive
    resource saving (24) without compromising the
    other impact categories.
  • Furthermore, it is a low cost solution.
  • In consequence, its implementation is highly
    recommended.

36
Problem Statement Q3-7
  • Consider the following Kraft pulp mill depicted
    below


wash


pulp
water
chips






D
E
D
E
D

screening


Brown Stock Washing
To papermaking
Digester


Flue


Recovery Boiler
Gas
concentrator

cond.

cond.

SBL

weak

ESP

smelt

black liquor

salt

Multiple Effect Evaporators


dust recycle
cake

wash
white liquor



fluegas
lime

water


weak white liquor
mud

dissolving

wash

tank



lime kiln
mud
water

dregs


white liquor
filter

dregs

mud

clarifier

washer

washer

filter

green liquor clarifier



slaker
causticizer
grits
37
Problem Definition
  • Chips 6000 tons (wet basis)
  • Moisture 50 0.56000 t 3000 t
  • Pulp Yield 50 of Dry 0.5 3000 t
  • Consistency (CY) 0.12
  • Dilution Factor (DF) 2
  • Wash Water for Pulp (1-CY)/CY DF
  • Ion Content of Process Water
  • Cl 3.7 K 1.1 Na 3.6 (values in ppm)

38
Problem Definition
  • Given this Kraft pulping process, it is desired
    to develop cost-effective strategies for the
    reduction of water discharge from the mill. It
    should be noted that any water reduction
    objectives will entail the use of recycle
    consequently, various species will build up in
    the process, leading to operation problems.

39
Problem Definition
  • To alleviate the detrimental effect of build-up,
    comprehensive mass integration strategies are
    required to provide answers to the following
    questions
  • What are the rigorous targets for reduction in
    water usage and discharge?
  • Which streams need to be recycled? To which
    units?
  • Should these streams be mixed or segregated?
  • What interception devices should be added to the
    process? To remove what load?
  • What new research needs to be developed to attain
    the optimum solutions?
  • Q3 7 will address some of these questions

40
Question 3
  • What are the rigorous targets in water discharge
    and reduction?

41
Species Tracking Model
  • Before one can begin to tackle the water
    targeting problem, it is crucial to develop a
    species tracking model of the system with the
    right balance in details.
  • A too-simplified model will not adequately
    describe the process nor will it capture critical
    aspects of the process.
  • A too-detailed model cannot be readily
    incorporated into the process integration and
    optimization framework and will negatively impact
    the effectiveness of the optimization
    computations.

42
Species Tracking Model
  • In order to develop the species tracking module,
    we will make use of path diagram equations,
    perform degrees of freedom analysis, and use the
    mixer splitter models. These topics were covered
    in Module II, though they are included here as a
    quick reference
  • Path Diagram Equation
  • Degrees of Freedom
  • Mixer-Splitter Model

43
Mathematical Modeling
  • The modeling techniques covered in module II
    allow one to describe unit performance without
    requiring detailed models while still
    capitalizing on nominal plant data and knowledge
    about the process. With this information, one
    can begin to make choices for the selected model
    and streams/species.
  • Consider the following unit

44
Pollutant/Water Load Balance Representation
W1 (kg water/s) P1 (kg pollutant/s
W2 (kg water/s) P2 (kg pollutant/s
W3 (kg water/s) P3 (kg pollutant/s
W4 (kg water/s) P4 (kg pollutant/s
45
Mathematical Modeling
  • W and P refer to the loads of water and a
    pollutant, respectively.
  • Suppose that the load of the water were to change
    as a result of process improvement (e.g. mass
    integration). The load of the pollutant will be
    affected as well thus, it will be necessary to
    determine the new load of the pollutant.
  • Furthermore, suppose that there exists a
    proportional relationship between the pollutant
    loads in streams 2 and 3 (much more so than
    between streams 1 3, 1 4, etc).

46

Mathematical Modeling
  • With this knowledge, the ratio model can be used
    to relate the pollutant loads in streams 2 and 3
  • P3new (P3old/ P2old) P2new
  • The pollutant load in stream 4 can then be
    determined by a simple component material
    balance
  • P4new P1new P1new P3new

47
Nominal Balance Model
  • By using these modeling techniques, path
    equations can be developed for tracking water and
    targeted NPEs throughout the process, resulting
    in a mathematical model for the nominal case
    study. The nominal case study can then be
    revised to reflect the impact of mass integration
    on the process.

48
Nominal Balance Model
  • For this case study, the nominal balance model
    will be developed with the purpose of tracking
    water and three nonprocess elements, chloride,
    potassium, and sodium. These ions were selected
    because they are among the most important species
    that cause buildup problems and limit the extent
    of mass integration

49
Nominal Balance Model
  • Using process knowledge, nominal plant data,
    modeling techniques, initial assumptions, etc.,
    one can begin to develop the nominal balance
    model unit by unit.
  • The overall result for the nominal balance model
    will be provided at this time. However, the full
    development of the nominal balance is provided at
    the end of this module for the readers
    understanding.

Nominal Balance
50
Material Balance
W2 13995
W6 1450
Chemicals

W4 10995
W33 30990
C2 0.052
C6 0.005
33
C4 0.492
W7 10995
2
6
K2 0.015
K6 0.002
K4 0.819
C7 0.394
N2 0.050
N6 0.005
Bleach Plant
N4 4.347
K7 0.655
7
Screening
N7 3.478
W35 10995
Washer
Pulp
Bleached pulp
8
W8 1450
4
C8 0.104
to papermaking
Wood chips
K8 0.165
N8 0.875
37
W37 30990
W1 3000
10
C37 15.495
W10 8901
C1 1.000
W12 1024
W15 1202
K37 0.155
C10 0.000
K1 2.500
C1 2 0.000
C15 0.197
N37 15.495
K10 0.000
Digester
N1 0.973
W16 0
K12 0.000
K15 0.327
12
N10 0.000
C16 4.230
N12 0.000
N15 0.386
1
15
K16 9.904
W5 11127
N16 73.033
C5 9.838
W14 0
W9 2225
ESP
K5 39.230
14
C14 0.472
C9 9.838
16
N5 483.020
W3 5127
K14 1.146
K9 40.927
W11 1202
C3 9.278
N14 0.966
N9 483.020
5
MEE
Concent
.
C11 9.838
W13 1202
K3 39.230
13
K11 40.927
C13 4.899
N3 486.344
3
9
N11 483.020
K13 11.378
N13 74.385
11
Recovery
W32 1016
White
C32 2.173
W18 0
W26 423
Furnace
K32 0.631
18
C18 0.182
C26 0.042
Liq Clar
N32 89.882
K18 0.026
K26 0.042
26
N18 18.225
N26 12.033
Na
SO
Lime
2
4
W17 0
smelt
32
17
27
C17 9.351
W31 6143
Kiln
K17 39.479
31
C31 11.451
N17 499.893
K31 39.862
W27 0
N31 576.226
W25 423
C27 2.297
W19 6402
Dissol
.
C25 2.339
K27 0.604
C19 1.272
K25 0.647
N27 0.633
K19 5.369
25
Causticizer
N25 12.666
Tank
N19 95.323
19
Washers/
W23 3.84
W29 32
24
W30 6143
W20 6402
C23 0.038
C29 0.000
Filters
20
C30 11.451
C20 10.623
K23 0.004
K29 0.000
30
23
K30 39.862
K20 44.848
N23 0.960
N29 0.000
W24 5762
N30 576.226
N20 595.216
C24 0.021
W22 51
K24 0.006
29
C22 1.455
N24 0.021
W21 6351
Green
Liq
K22 5.382
Slaker
C21 9.167
N22 19.047
K21 39.466
W28 8
Clarifier
21
N21 576.169
C28 0.014
28
K28 0.209
22
N28 0.576
51
Back to Question 3
  • What are the rigorous targets in water discharge
    and reduction?
  • The objective here is to minimize the amount of
    fresh water used in the process and the amount of
    wastewater discharged from the system.

52
Solution Q3
  • Beginning with the nominal balance model (figure
    ), the first step is to identify all possible
    sources of water entering, leaving or being
    consumed in the process in order to obtain the
    overall water balance for the process, as
    depicted in next figure

53
OVERALL WATER BALANCE
Screening 1450
MEE 8901
Chips 3000
OVERALL Water Balance
Stripper 1024
Washer 13995
ESP 1202
Screening 1450
Washer/Filter dregs 4
Lime Kiln 423
Washers/Filters 5762
Slaker grits 8
Bleach Plant 30990
Slaker 32
Water consumed By reaction 168
BP water 30990
Pulp leaving Bleach Plant 10995
Total Water In 55197 168 55029 tpd
Total Water Out 55029 tpd
54
Solution Q3
  • Next, all streams that use fresh water and all
    streams that contain potentially recyclable water
    are identified.
  • There are four fresh water streams (S2, S6,
    S24 and S34) giving a total fresh water use of
    52,197tpd.
  • There are also four potentially recyclable
    streams, S8, S10, S12 and S37 giving a total of
    42,365 tpd.
  • The overall water balance diagram has been
    modified to reflect this information (see figure )

55
FRESH AND RECYCLABLE WATER BALANCE
Screening 1450
MEE 8901
Water Balance
Stripper 1024
Washer 13995
Screening 1450
Washers/Filters 5762
Water consumed By reaction 168
Bleach Plant 30990
BP water 30990
Total Fresh Water in 52197 tpd
Total recyclable water out 42365 tpd
56
Solution Q3
  • If the recyclable water can be intercepted and
    cleaned up to the point where it is acceptable
    for use in place of fresh water and if
    self-recycle is allowed, then one can determine
    the target for fresh water usage
  • Minimum water consumption
  • 52197 42365 9832 tpd

57
Solution Q3
  • By adding up the flowrates of the water streams
    leaving the process except the recyclable streams
    (S8, s10, s12, s37) and water in the produced
    pulp, we get a target for wastewater discharge of
    1,669 tpd

58
OVERALL WATER TARGETING FOR CASE STUDY
Wastewater
Chips
W1 3000
Target 1669
tpd
W2 13995
Water Consumed
168
Fresh Water
W6 1450
Target 9832
Water going out with pulp
10995
tpd
W24 5762
WtoBP
30990
Target for minimum water consumption 52,197
42,365 9,832 tons per day
59
Solution Q3
  • Therefore, the rigorous targets are
  • Minimum Fresh water target 9832 tpd Wastewater
    target 1669 tpd

60
Limitations on Self-Recycle
  • Previously, it was permitted to consider
    recycling the effluent back to the same unit.
    However, self-recycle may sometimes be forbidden
    due to numerous reasons such as
  • To prevent the build-up of impurities in a flow
    loop
  • To avoid dynamic instabilities that may arise due
    to the high interconnectivity between the input
    and output
  • To enhance process reliability by disengaging the
    dependence of the input from the output.

61
Limitations on Self-Recycle
  • If self-recycle is not allowed, then it is
    possible that the targets identified earlier may
    not be reached even if interception technologies
    are used to clean up the recyclable water
    streams. As a result, new targets will need to
    be determined, which leads to the next question

62
Question 4a
  • In the case of no self-recycle with one
    interceptor, which streams can be intercepted?

63
Solution Q4a
  • There are four recyclable streams for
    consideration
  • W8 MEE
  • W10 Concentrator
  • W12 Screen
  • W37 Bleach plant effluent
  • In the development of the nominal balance model,
    it was assumed that there were no ions in the
    water leaving the MEE and Concentrator (i.e. it
    has the same quality as demineralized water)
    therefore, the only interception candidates are
    the screen and bleach plant effluents.

64
Question 4b
  • Choosing the bleach plant effluent for
    interception and assuming that the quality of the
    screen effluent is acceptable for direct recycle
    to the pulping process, what are the new water
    targets (remember, no self-recycle)?

65
Solution Q4b
  • The flow of the intercepted bleach plant
    effluent, along with the screen effluent is more
    than enough to replace all of the fresh water
    used in the pulping process. Therefore, the
    fresh water target for the pulping process is
    zero.
  • For the bleach plant, only water meeting
    dimineralized quality can be used. Thus, the
    effluents from the multiple effect evaporator and
    the concentrators can be used, replacing a total
    of 9925 tpd of BPE.

66
Solution Q4b
Wastewater To bio 30990 19757 11233 tpd
Fresh water 21065 tpd
Bleach
interception
W2 13995
PULPING
W8 1450
W6 1450
Consumption by Chemical Reaction and other
losses 9832 tpd
W10 8901
W24 5762
W12 1024
13995 5762 1450 21207
67
Solution Q4b
  • The new targets are now
  • Pulping process fresh water 0 tpd
  • Bleach Plant Effluent fresh water target
  • 30990 (8901 1024)
  • 21065 tpd
  • Wastewater target
  • 30990 19757 11233 tpd

68
Process Integration Strategies
  • The overall targeting has identified that fresh
    water consumption can be significantly reduced
    from 52197 tpd to 9832 tpd.
  • The next step, then, is to determine how this can
    be accomplished. What is the optimal strategy
    for water reduction? How are the streams to be
    allocated? This cannot be easily perceived
    simply by looking at the process flowsheet.
  • Process integration strategies will be employed
    to determine the optimal ways of reaching the
    target

69
Why Process Integration?
  • Process Integration is a holistic approach to the
    design and operation of complex systems. It is a
    sound framework that utilizes well-developed and
    proven mass and energy integration techniques for
    optimizing the design and operation of a process.

70
Process Integration
  • It is important to coordinate both process
    integration and process simulation. The
    application of process integration provides
    performance targets, solution strategies, and
    proposed changes to the process. Process
    simulation reassesses the process performance as
    a result of theses changes.

71
Coordination of Process Integration and Simulation
Process Objectives, Data and constraints
Process Modifications Structural changes
Process Simulation
Process Integration
Input/Output relations New Processes
Closing the information loop of integration and
simulation ensures that the developed insights
and solution strategies are refined and
validated.
72
Mass Integration Strategies
  • Now that the rigorous targets have been developed
    for the minimum feasible water usage and
    discharge, various cost-effective mass
    integration strategies should be used to attain
    the targets. These strategies include
  • Segregation,
  • Low cost/no cost modifications,
  • Direct recycle,
  • Interception
  • high cost process modifications.
  • The above strategies can be represented as a
    pyramid (see next slide), where it is desired to
    begin at the bottom of the pyramid, which
    represents the lowest cost and perhaps more
    easily implemented strategies, and work up until
    the target is achieved.

73
Mass Integration Strategies
Target
Chemical Process
HCPM
Interception
Mixing Recycling
Low Cost Process Modifications
(LCPM)
Segregation
74
Segregation
  • Segregation refers to avoiding the mixing of
    streams. In some industrial applications, dilute
    streams have been mixed with concentrated streams
    and even different phases have been mixed
    together unnecessarily. Segregation of streams
    at the sources can provide several opportunities
    for cost reduction such as
  • Generate environmentally benign streams
  • Enhance the opportunities for direct recycle
    since dilute streams are easier to recycle.
  • The separate concentrated streams are now more
    thermodynamically favorable for interception

75
Low-cost process modifications
  • In some cases, a change in process conditions
    (such as temperature, pressure, compositions,
    etc) may be all this is needed to decrease or
    eliminate the waste produced in a unit.
  • Provided that the cost is low, a unit can be
    replaced with a more environmentally benign one.

76
Recycle
  • Discharged waste can be reduced by recycling
    pollutant-laden streams back to the process to be
    utilized in process or non-process requirements.
    In some instances, several streams need to be
    mixed with each other to achieve the desired
    level of flowrate and composition.

77
Interception
  • Interception refers to the utilization of
    separation techniques to selectively remove
    targeted species from targeted streams. In most
    industrial applications, inteception is needed to
    enhance the opportunities of recycling and to
    generate environmentally benign streams.

78
High-Cost Process Modifications
  • After all other strategies have been exhausted,
    one may need to employ high cost process
    modifications. This may include completely new
    chemistry (such as new solvent or new reaction
    path), new technology (new plant), etc.

79
Question 5
  • What is the optimal water allocation using direct
    recycle?

80
Solution Q5
  • To answer this question, a mass allocation
    representation of the process from the species
    viewpoint needs to be developed.
  • For each species, there are sources, those
    streams that contain the desired species, and
    sinks, those streams units which can accept the
    species.
  • Each sources can be segregated, intercepted to
    adjust species content, mixed, etc and allocated
    to the different units or sinks, as depicted in
    the following figure.

81
SOURCE-INTERCEPTION-SINK REPRESENTATION
Mass Energy Separating Agents In
Sinks
Segregated Sources
j 1
Source i 1
Species Interception Network (SPIN)
j 2
Source i Nsources
Fresh Source
j Nsinks
(e.g., El-Halwagi et al., 1996, Spriggs and
El-Halwagi, 1998, Dunn and El-Halwagi, 2003)
Mass Energy Separating Agents Out
82
Process Sinks
  • There are a number of process units, or
    Nsinks,that employ fresh water and are designated
    by the index j (j ranges from 1 to Nsinks).
  • Each jth sink has two sets of contraints on
    flowrates and composition
  • Flowrate to each sink
  • Wjmin ? Wj ? Wjmax j 1, 2,.,Nsinks
  • Wj is the water flowrate entering the jth sink
  • Ion content to each sink
  • Yion,jmin ? Yion,j, ? Yion,jmax j 1,
    2,.,Nsinks
  • Yion,j is the compostion of a certain NPE
    entering the jth sink

83
Process Sinks
  • Each source, represented by I, is split into
    Nsink fractions that can be assigned to various
    sinks. The flowrate of each split is denoted by
    li,j (see figure)
  • Each split fraction then has the opportunity to
    be mixed (or not) and assigned to sinks (see
    figure)

84
SPLITTING OF SOURCES TO SINKS
Source Li Yion,i
li,j
  • Splitting of the ith source
  • where i 1,2, , Nsources

85
MIXING OF SOURCES BEFORE SINKS
Wj Yion,j
li,j yion,j
j
  • Mixing for the jth sink
  • where j
    1,2, , Nsinks

86
Direct Recycle Strategy
  • For this case study, four sources have been
    identified Bleach Plant effluent, Screen
    effluent, Multiple Effect Evaporator effluent and
    Concentrator effluent. Fresh Water is included
    since it is the objective function of the
    optimization problem (where the objective
    function is to minimize the flowrate of fresh
    water via direct recycle).
  • Four sinks have been identified Screening,
    Brown Stock Washer, Washer/Filters, and the
    Bleach Plant. Waste Treatment is also be
    included since it is possible that the best
    allocation for a source may be biotreatment.
  • The following figure shows the assignment
    representation for the Direct Recycle/Reuse
    problem

87
Direct Recycle/Reuse Representation
Sinks
Sources
S
Wastewater
8
Screening
from Screening

S

Condensate
10
Brown Stock
from MEE

Washer
S

Condensate
12
from
Concenrator

Washers/Filters
S
Bleach Plant
37
Effluent

Bleach Plant
Fresh water
Biotreatment
88
Direct Recycle Optimization Formulation for
Source/Sink Analysis w/Path Connection
  • The problem can now be formulated as an
    optimization problem, where the objective
    function is the minimization of the flowrate of
    fresh water. This objective funtion can be
    represented as
  • Min. flowrate of fresh water
  • Subject to the following constraints

89
Direct Recycle Optimization Formulation for
Source/Sink Analysis w/Path Connection
Flowrate to each sink
j 1, 2, , Nsinks
NPE content in feed to each sink
j 1, 2, , Nsinks and k 1, 2, , Nk
Splitting for the ith source
i 1, 2, , Nsources
Mixing for the jth sink
j 1, 2, , Nsinks
Component material balances for the pollutants
j 1, 2, , Nsinks and k 1, 2, , Nk
Non-negativity of each fraction of split sources
i 1, 2, , Nsources and j 1, 2, , Nsinks
90
Direct Recyce/Reuse Optimization formulation
  • It should be remember that no self-recycle is
    permitted and that the bleach plant c,an only
    accept dimineralized water.
  • Furthermore, there is an additional issue with
    respect to the build-up of NPEs in the recovery
    furnace which is affected by sticky
    temperature. It is related to Cl, K, and Na
    through the following constraints where Ci, Ni,
    and Ki, are the ionic loads of Cl, Na and K,
    respectively, in the ith source

91
Optimization Solution for Direct Recycle/Reuse
  • LINGO programming was used to develop and solve
    the mathematical formulation. The optimal water
    allocation is depicted in the following slide.
  • The fresh water to screening has been replaced
    with 751 tpd of concentrator effluent and 699 tpd
    of bleach plant effluent
  • The fresh water to the washers/filters has been
    replaced with 273 tpd of concentrator effluent
    and 5489 tpd of MEE effluent.
  • A portion of the fresh water to the Brown Stock
    Washers has been replaced with 3412 tpd of MEE
    effluent and 1450 tpd of screening effluent.

92
Optimum Solution for Direct Recycle/Ruse
30990
9133
Bleach
Screening
BSW
1450
30291
Wood Chips
751
699
3412
273
5489
Digester
ESP
Stripper
Stripper
Recovery Furnace
MEE
Concent.
White Liq. Clarif.
Lime Kiln
Dissolv Tank
Causticizer
Washers/ Filters
Green Liq. Clarif.
Slaker
93
Results of Direct Material Exchange
  • The fresh water consumption has been reduced to
    40,123 tons per day, a 23 reduction from the
    nominal fresh water usage of 52,197 tons per day.
  • This solution is a direct recycle/reuse which
    requires piping and pumping but involves no
    capital investment for new processing units.
  • It should be noted that the mathematical solution
    can generate alternate solutions that yield the
    same fresh water consumption but require
    different piping and allocation alternatives.

94
Question 6
  • Through direct recycle, fresh water usage went
    down from 52197 to 40123. However, from water
    targeting, we know that interception can get the
    fresh water usage down to 21065 tpd.
  • Consider the interception of the bleach plant
    effluent. How much Cl must be removed in order
    to meet meet the fresh water target of 21065 tpd?

95
Solution Q6
  • In this problem, the objective function has
    changed from one of minimizing the fresh water
    consumption to one of minimizing the load of the
    Cl to be removed from the bleach plant effluent
    subject to
  • Desired water target
  • Path equations for tracking water and Cl
  • Recycle Model
  • Interception equations
  • Unit constraints

96
Solution Q6
  • Basically, this problem is just like the recycle
    problem except that the objective function has
    changed. We know that the fresh water target is
    now 21065 tpd and that approximately 19750 tpd of
    intercepted bleach plant effluent is being
    recycle back to the process. Thus, in order to
    minimize the load to be intercepted from the BPE,
    a target has to be set for the maximum recyclable
    flowrate of the bleach plant effluent (the 19750
    tpd).

97
Solution Q6
  • Again, LINGO programming was used to solve the
    mathematical formulation
  • A total of 8.99 tpd of Cl must be removed from
    the bleach plant effluent.
  • The fresh water consumption has been reduced to
    21072 tons per day, a 60 reduction from the
    nominal fresh water usage of 52,197 tons per day.

98
Exploring other interception opportunities
  • So far, only terminal streams (those streams
    going directly to waste treatment) have been
    considered. However, it is possible that other
    inter-process streams may be intercepted, perhaps
    providing greater economical and environmental
    benefits.
  • A literature search reveals that salt removal
    technologies exist for other kraft units, among
    those
  • White Liquor Interception
  • Green Liquor Interception
  • Of course, this leads to the next question

99
Question 7
  • How much chloride needs to be removed from
  • Case 1 Green Liquor
  • Case 2 White Liquor
  • in order to meet the fresh water target?

100
Interception Alternatives
  • This is quick and easy to determine. The
    objective function will remain the same (minimize
    chloride removal) as in Q6 but rather than
    minimizing the Cl removal from the bleach plant
    effluent, it will be minimized from the white
    liquor or green liquor streams. Thus, the
    optimization program only needs to be slightly
    altered to reflect the stream in question.
  • Interestingly enough, though it should come as no
    surprise, the load removal for Green Liquor and
    White Liquor interception is the same as the case
    for Bleach Plant interception (approx. 9 tpd of
    Cl). However, the three solutions are not
    identical. Each one has a different
    configuration of optimal water allocation (see
    figures)

101
Optimum Solution for Bleach Plant Interception
30990
9133
Bleach
Screening
BSW
1450
30291
Wood Chips
751
699
3412
273
5489
Digester
ESP
Stripper
Stripper
Recovery Furnace
MEE
Concent.
White Liq. Clarif.
Lime Kiln
Dissolv Tank
Causticizer
Washers/ Filters
Green Liq. Clarif.
Slaker
102
Optimum Solution for Green Liquor Interception
30990
9133
Bleach
Screening
BSW
1450
30291
Wood Chips
751
699
3412
273
5489
Digester
ESP
Stripper
Stripper
Recovery Furnace
MEE
Concent.
White Liq. Clarif.
Lime Kiln
Dissolv Tank
Causticizer
Washers/ Filters
Green Liq. Clarif.
Slaker
103
Optimum Solution for White Liquor Interception
30990
9133
Bleach
Screening
BSW
1450
30291
Wood Chips
751
699
3412
273
5489
Digester
ESP
Stripper
Stripper
Recovery Furnace
MEE
Concent.
White Liq. Clarif.
Lime Kiln
Dissolv Tank
Causticizer
Washers/ Filters
Green Liq. Clarif.
Slaker
104
Life Cycle Analysis
  • But which of the three technologies is the better
    solution?

105
END
106
Path Diagram Equation
  • Typically, the Path Diagram Equation defines
    outlet flows and compositions from key units as
    functions of inlet flows, inlet compositions and
    process design and operating conditions
  • This mass integration tool tracks the targeted
    species as they propagate through the system and
    provide the right level of details that will be
    incorporated into the mass integration analysis

Return to the flowsheet
107
Degrees of Freedom
  • Assumptions
  • All inlets to a unit are known and it is desired
    to determine the outputs of the unit.
  • F must provided as additional modeling equations,
    assumptions, measurements, or data in order to
    have an appropriately specified (determined) set
    of equations that is solvable.
  • NV NS x NC
  • F NV - NE NC (NS - 1)
  • F degrees of freedom
  • NV number of variables
  • NE number of equations
  • NC number of targeted species
  • NS number of outlet streams

Return to the flowsheet
108
Mixer-Splitter Model
  • The mixer-splitter model is a modeling technique
    which relies on nominal data .
  • The nominal data are those for the plant prior to
    any changes and can be obtained via simulation,
    fundamental modeling, direct measurements, or
    literature data.
  • There are various of the mixer splitter model
  • Fixed split model
  • Flow ratio model
  • Species ratio model.
  • Based on the knowledge of the process, choices
    can be made for the selected model and
    streams/species.
  • Path equations can be developed for water and
    targeted NPEs throughout the process.

Return to the flowsheet
109
Fixed Split Flow Model
Fixed Split Model
a F
F
(1 a) F
  • The Fixed Split model takes a certain split, a,
    for the flows of streams leaving the unit

Return to the flowsheet
110
Flow Ratio Model
Flow Ratio Model
G
F
Gnew Gold ( Fnew / Fold )
  • The Flow Ratio model assumes that streams or
    components maintain a certain fixed ratio. Thus,
    if the flow rate of a certain stream increases or
    decreases, all other related streams adjust
    according to the same ratio.

Return to the flowsheet
111
Species Ratio Model
Species Ratio Model
G
F
I species 1 II species 2
IInew Inew (IIold/ Iold )
  • Similar to the Flow Ratio Model, the Species
    Ratio Model maintains a fixed relationship
    between species in related streams. Thus, if one
    species changes, the other one adjusts by the
    fixed ratio. This model is especially useful if
    one species can be accurately tracked whereas the
    other one cannot.

Return to the flowsheet
112
Initial Data - Digester
  • Assumption all inlet streams are known.
  • Flowrate of wood chips, Chips 6000 tpd
  • Moisture content of wood chips 50
  • Pulp Yield 50
  • Pulp Dry Chips Yield
  • Mass fraction ions with incoming wood chips
  • C1 1 Chips/6000
  • K1 2.50 Chips/6000
  • N1 0.973 Chips/6000

Return to the flowsheet
113
Initial Data Brown Stock Washer
  • Composition of ions in incoming wash water
  • Cl 3.7 ppm
  • K 1.1 ppm
  • Na 3.6 ppm
  • Consistency of pulp leaving Brown Stock Washer,
    CY 0.12
  • Dilution Factor, DF 2.0
  • Ratio of ions in slurry leaving the BSW to the
    chloride in the pulp stream leaving the digester
  • Cl 0.050
  • K 0.020
  • Na 0.009

Return to the flowsheet
114
Digester
W2 (from consistency)
S2
C2 (from comp of Cl in wash water)
K2 (from comp of K in wash water)
N2 (from comp of N in wash water)
Brown Stock Washer
S4
W4 (from dilution factor
C4 (from ratio to C5) 0.05C5
K4 (from ratio to K5) 0.02K5
S1
N4 (from ratio to N5) 0.009N5
Digester
W1 (from moisture content
C1 (from comp of Cl in chips)
S5
K1 (from comp of K in chips
N1 (from comp of N in chips
W5
C5
S3
K5
N5
All species data will be calculated as an output
stream from white liquor clarifier
115
Digester
  • W1 MoistureChips0.560003000
  • W4 (1-CY)/CYPulp(1-.82)/(0.82)3000
  • DF (W2 - W4 )Pulp DF is given as 2
  • W2 can be determined from after W4 has been
    calculated.
  • Ion Content in streams 2 and 4
  • C2 (3.710-6) W2 C4 0.05C5
  • K2 (1.110-6) W2 K4 0.02K5
  • N2 (3.610-6) W2 N4 0.009N5
  • Recalling the assumption that all inlet streams
    are known, then
  • stream 5 will need to be determined. The number
    of unknowns is
  • our (flowrate of water and the three ions in S5)
    these can be obtained
  • Via the four material balances for the 4 species
  • W5 W1 W2 W3 W4
  • C5 C1 C2 C3 C4
  • K5 K1 C2 K3 K4
  • N5 N1 C2 N3 N4

116
Multiple Effect Evaporator
  • 80 of the water in the weak black liquor is
    evaporated (water recovery ratio is 0.8).
  • It is assumed that no ions are in the condensate
    of the multiple effect evaporators
  • The material balances can be used to calculated
    the concentrated stream leaving the multiple
    effect evaporators
  • W10 Water recovery in evaporator W5
  • W9 W5 - W10
  • C9 C5 - C10
  • K9 K5 - K10
  • N9 N5 - N10

117
Multiple Effect Evaporators
W10 Water Recovery W5
S10
C10 0
Evaporator Condensate
K10 0
N10 0
S5
Black Liquor
S9
Multiple Effect Evaporators
Evaporator Concentrate
W5
C5
K5
N5
W9
C9
K9
N9
118
Multiple Effect Evaporator
  • 46 of the water in the black liquor entering the
    concentrators is evaporated (water recovery ratio
    is 0.46).
  • Again, it is assumed that no ions are in the
    condensate of the multiple effect evaporators
  • The material balances can be used to calculated
    the concentrated stream leaving the multiple
    effect evaporators
  • W12 Water recovery in concentrator W9
  • W11 W9 - W12
  • C11 C9 - C12
  • K11 K9 - K12
  • N11 N9 - N12

119
Concentrator
S10
W12 Water Recovery W9
C12 0
Concentrator Condensate
K12 0
N12 0
S9
S11
Evaporator Concentrate
Concentrator
Strong Black Liquor
W9
C9
K9
N9
W11
C11
K11
N11
120
Recovery Furnace and Electrostatic Precipitator
(ESP)
  • It is assumed that all the water in the strong
    black liquor leaves with the ESP off-gas so W15
    W11.
  • The ions in the solids return, ESP dust and
    off-gass are related to the ions in the strong
    black liquor stream
  • C13 0.278C11 C14 0.048C11 C15 0.02C11
  • K13 0.498K2 K14 0.028K11 K15
    0.008K11
  • N13 0.154N2 N14 0.002N11 N15
    0.0008N11

121
Recovery Furnace and ESP
  • The component material balance around the ESP is
  • W13 - W14 - W15 - W16 0.0
  • C13 - C14 - C15 - C16 0.0
  • K13 - K14 - K15 - K16 0.0
  • N13 - N14 - N15 - N16 0.0
  • It is assumed that the saltcake has a makeup flow
    of 0.0375 Pulp. Knowing this and the molecular
    formula for saltcake,
  • N18 223/142 Saltcake
  • The content of Cl and K in the saltcake is
    obtained by assuming ratios to Na in the
    saltcake. In addition, there is virtually no
    water in saltcake.
  • W17 0.0
  • W18 0.0
  • C18 0.01N18
  • K18 0.0014N18

122
Recovery Furnace and ESP
  • The ion content in the smelt is determined via
    component material balance around the Recovery
    Furnace and ESP
  • C11 C18 - C15 - C14 - C17 0.0
  • K11 K18 - K15 - K14 - K17 0.0
  • N11 N18 - N15 - N14 - N17 0.0

123
Smelt
  • The smelt flowrate consists of the saltcake
    solids in strong black liquor (SBL) solids
    lost with the purge streams (S14 and S15)
    solids volatilized in the furnace. Assuming that
    5 of the solids in the SBL leave the ESP in the
    flue gas and that 47 of the SBL solids are
    volatized in the furnace
  • Smelt Saltcake SBL 0.05SBL 0.47SBL
  • Or
  • Smelt Saltcake 0.48SBL

124
Recovery Furnace and Electrostatic Precipitator
(ESP
W15 W11
S15
C15 (from ratio to C11)
S14
Off-gas
K15 (from ratio to K11)
N15 (from ratio to N11)
Dust Purge
W14 0
S11
C14 (from ratio to C11)
ESP
K14 (from ratio to K11)
Strong Black Liquor
N14 (from ratio to N11)
W11
C11
K11
N11
Recovery Furnace
S17
Smelt
S18
Salt Cake 0.0375Pulp
W17
C17
W18 0
K17
C18 (from ratio to N18)
N17
K18 (from ratio to N18)
N18 0.324 Salt Cake
125
Dissolving Tank
  • The dissolving water-to-smelt ratio used in the
    dissolving tank is typically 85/15
  • W19 (85/15)Smelt
  • The ionic content of S19 is determined by
    assuming ratios of Cl and K to those in the smelt
    and Na to the white liquor
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