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Combining Mass-Integration Strategies
C H A P T E R
6 Combining Mass-Integration Strategies 6.1 INTRODUCTION The previous three chapters set the stage for developing mass-integration strategies. Chapter 3, Benchmarking Process Performance Through Overall Mass Targeting, presented several approaches to the determination of overall mass targets depending on the type and extent of available data. These targets are identified ahead of detailed design. As mentioned in Chapter 3, Benchmarking Process Performance Through Overall Mass Targeting, these targets can be attained by a combination of strategies, where impact is a measure of the effectiveness of the proposed solution in partially reaching the target. The cumulative impact of all the strategies yields the desired target. Acceptability is a measure of the likelihood of a proposed strategy to be accepted and implemented by the plant. Chapters 4, Direct-Recycle Networks: Graphical and Algebraic Targeting Approaches, and 5, Synthesis of Mass-Exchange Networks, gave important classes of systematic mass-integration tools associated with identifying such strategies. Direct recycle was covered by Chapter 4, Synthesis of Mass-Exchange Networks, as a key element in no/low-cost strategies involving segregation, mixing, and rerouting of streams without the addition of new equipment. The synthesis of massexchange networks was described in Chapter 5, Synthesis of Mass-Exchange Networks, as an illustration of the addition of new units (interception) and the selection or substitution of solvents to achieve a separation task. This chapter combines the tools presented in the previous three chapters. First, the process representation from a species viewpoint is presented. Then, the combination of mass-integration tools is demonstrated through the applicability to a case study on the production of acrylonitrile.
Sustainable Design Through Process Integration DOI: http://dx.doi.org/10.1016/B978-0-12-809823-3.00006-0
6.2 PROCESS REPRESENTATION FROM A MASS-INTEGRATION SPECIES PERSPECTIVE Once an overall mass target is determined, it is necessary to develop cost-effective strategies to reach the target. For a given target, there are numerous design decisions that must be judiciously made. These include addressing the following challenging questions (Fig. 6.1): • What are the optimum stream-rerouting strategies? • Should any streams be segregated, mixed, or rerouted? Which ones? • Which streams should be recycled/reused? To which units? • Should design variables and/or operating conditions of existing units be altered? Which ones? To what extent? • Is there a need to add/replace units? Which ones? Where to add/replace? • Should interception (e.g., separation) devices be added? Which streams should be intercepted? To remove what? To what extent? • Which separating agents should be selected for interception? • What is the optimal flowrate of each separating agent? • How should these separating agents be matched with the rich streams (i.e., stream pairings)? Because of the prohibitively large number of alternatives involved in answering these questions, a systematic procedure is needed to extract the optimum solution(s) without enumerating them. This is the role provided by mass integration. Mass integration is a holistic and systematic methodology that provides a
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6. COMBINING MASS-INTEGRATION STRATEGIES
Cost/Impact
Target
Technology Changes (New Chemistry, New Processing Technology, etc.) Equipment Addition/Replacement (Interception, Separation, etc.)
Material Substitution (e.g., Solvent, Catalyst) Acceptability
Modest Manipulation of Units (e.g., Moderate Changes in Operating Conditions)
Minor Structural Modifications (Segregation, Mixing, Recycle, etc.)
FIGURE 6.1 Hierarchy of mass-integration strategies (El-Halwagi, 1999).
Sources
Segregated Sources
Separating Agents
Sinks/ Generators (Units)
Sources (Back to Process)
#1
#2 . . .
SPecies Interception Network (SPIN)
. . .
Nsinks
Separating Agents (to Regeneration and Recycle)
FIGURE 6.2 Process from a species perspective (El-Halwagi et al., 1996).
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. Mass integration is based on fundamental principles of chemical engineering combined with system analysis using graphical and optimization-based tools. In order to develop detailed mass-integration strategies, let us represent the process flowsheet from a species viewpoint (e.g., El-Halwagi and Spriggs, 1998; El-Halwagi et al., 1996) as shown in Fig. 6.2. For each targeted species, there are sources (streams that carry the species) and process sinks (units that can accept the species). Process sinks include reactors, separators, heaters/coolers, biotreatment facilities, and discharge media. Streams leaving the sinks become, in
turn, sources. Therefore, sinks are also generators of the targeted species. Each sink/generator may be manipulated via design and/or operating changes to affect the flowrate and composition of what each sink/ generator accepts and discharges. Stream characteristics (e.g., flowrate, composition, pressure, temperature, etc.) can be modified by adding new units that intercept the streams prior to being fed to the process sinks and condition their properties to the desired values. This is performed a speciesinterception network (SPIN) that may use mass- and energy-separating agents. Interception denotes the utilization of new unit operations to adjust the composition, flowrate, and other properties of certain process streams to make them acceptable for existing process sinks. A particularly important class of interception
6.2 PROCESS REPRESENTATION FROM A MASS-INTEGRATION SPECIES PERSPECTIVE
Sources
Segregated Sources
Mass Separating Agents in
Sinks/ Generators
201
Sources (Back to Process or Terminal Discharge)
1
2
Species Interception Network (Special Case: Mass-Exchange Network)
Nsinks
Mass Separating Agents out (to Regeneration and Recycle)
FIGURE 6.3 A process from a species viewpoint when MSAs are used for interception.
devices is separation systems. These separations may be induced by the use of mass-separating agents (MSAs) and/or energy-separating agents (ESAs). A systematic technique is needed to screen the multitude of separating agents and separation technologies to find the optimal separation system. The synthesis of MSA-induced physical-separation systems has been
covered in Chapter 5, Synthesis of Mass-Exchange Networks, through the synthesis of mass-exchange networks (MENs). Other interception systems are covered throughout the book. The MEN can be used to prepare sources for recycle. When MSAs are used for interception, Fig. 6.2 is revised as shown in Fig. 6.3 by placing a MEN instead of the SPIN.
E X A M P L E 6 . 1 A P P L I C AT I O N O F M A S S I N T E G R AT I O N T O D E B O T T L E N E C K A N A C RY L O N I T R I L E P R O C E S S A N D R E D U C E WAT E R U S A G E A N D D I S C H A R G E Acrylonitrile ðAN; C3 H3 NÞ is manufactured via the vapor-phase ammoxidation of propylene: catalyst
C3 H6 1 NH3 1 1:5O2 ! C3 H3 N 1 3H2 O: The reaction takes place in a fluidized-bed reactor in which propylene, ammonia, and oxygen are catalytically reacted at 450 C and 2 atm. The reaction is a single pass with almost complete conversion of propylene. The reaction products are cooled using an indirect-contact heat exchanger that condenses a fraction of the reactor off-gas. The remaining off-gas is scrubbed with water, then decanted into an aqueous layer and an organic layer. The organic layer is fractionated in a distillation column under slight vacuum induced by a steam-jet ejector. Fig. 6.4 shows the process flowsheet along with pertinent material balance data.
The wastewater stream of the plant is composed of the off-gas condensate, the aqueous layer of the decanter, the bottom product of the distillation column, and the condensate from the steam-jet ejector. This wastewater stream is fed to the biotreatment facility. Since the biotreatment facility is currently operating at full hydraulic capacity, it constitutes a bottleneck for the plant. Plans for expanding production of AN are contingent upon debottlenecking of the biotreatment facility by reducing its influent or installing an additional treatment unit. The new biotreatment facility will cost about $12 MM in capital investment and $0.7 MM/year in annual operating cost, leading to a TAC of $1.9 MM/ year using a 10-year linear depreciation. The objective of this case study is to use mass-integration techniques to devise cost-effective strategies to debottleneck the biotreatment facility.
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EXAMPLE 6.1
5.0 kg AN/s 5.1 kg H2O/s + Gases
O2 NH3
Water 6.0 kg H2O/s
(cont’d) Steam-Jet Ejector
B FW 1.2 kg H2O/s
Condensate
Steam Boiler
34 ppm NH3 0.2 kg AN/s 1.2 kg H2O/s
Tail Gases to Disposal
Reactor
Scrubber
C3H6
AN to Sales
Off-Gas Condensate 14 ppm NH3 0.4 kg AN/s 4.6 kg H2O/s
18 ppm NH3 4.6 kg AN/s 6.5 kg H2O/s
10 ppm NH3 4.2 kg AN/s 1.0 kg H2O/s
Distillation Column
1 ppm NH3 3.9 kg AN/s 0.3 kg H2O/s
Decanter
Aqueous Layer 25 ppm NH3 0.4 kg AN/s 5.5 kg H2O/s
Bottoms 0 ppm NH3 0.1 kg AN/s 0.7 kg H2O/s
20 ppm NH3 1.1 kg AN/s 12.0 kg H2O/s Wastewater to Biotreatment
FIGURE 6.4 Flowsheet of AN production. The following technical constraints should be observed in any proposed solution: Scrubber 5:8 # flowrate of wash feed ðkg=sÞ # 6:2
ð6:1Þ
0:0 # ammonia content of wash feed ðppm NH3 Þ # 10:0 ð6:2Þ
Boiler Feed Water (BFW) Ammonia content of BFW ðppm NH3 Þ 5 0:0
ð6:3Þ
AN content of BFW ðppm ANÞ 5 0:0
ð6:4Þ
10:6 # flowrate of feed ðkg=sÞ # 11:1
ð6:5Þ
Decanter
Distillation Column 5:2 # flowrate of feed ðkg=sÞ # 5:7
ð6:6Þ
0:0 # Ammonia content of feed ðppm NH3 Þ # 30:0 ð6:7Þ 80:0 # AN content of feed ðwt:% ANÞ # 100:0
ð6:8Þ
Furthermore, for quality and operability objectives the plant does not wish to recycle the AN product stream (top of distillation column), the feed to the distillation column, or the feed to the decanter. Three external MSAs are considered for removing ammonia from water: air (S1), activated carbon (S2), and an adsorbing resin (S3). The data for the candidate MSAs are given in Table 6.1. The equilibrium data for the transfer of the pollutant from the waste stream to the jth MSA is given by, y1 5 mj xj ; ð6:9Þ where y1 and xj are weight-based parts per million of ammonia in the wastewater and the jth MSA, respectively.
Solution The first step in the analysis is to identify the target for debottlenecking the biotreatment facility. An overall water balance for the plant (Fig. 6.5A) can be written as follows: Water in 1 Water generated by chemical reaction 5 Wastewater out 1 Water losses Since the wastewater discharge is larger than the freshwater flowrate, it is possible, in principle, to bring
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EXAMPLE 6.1 TABLE 6.1
(cont’d)
Data for MSAs of the AN Problem
Stream
Upper bound on flowrate LCj
Supply composition (ppmw) xsj
Target composition (ppmw) xtj
S1
N
0
S2
N
S3
N
mj
εj ppmw
Cj $/kg MSA
Crj $/kg NH3 removed
6
1.4
2
0.004
667
10
400
0.02
5
0.070
180
3
1100
0.01
5
0.100
91
(A) ACRYLONITRILE PLANT Scrubber Water 6.0 kg/s
Wastewater 12.0 kg H2O/s Water Generation 5.1 kg/s Water Loss (with AN Product) 0.3 kg H2O/s
BFW 1.2 kg/s
(B)
ACRYLONITRILE PLANT Scrubber Water 6.0 kg/s
No Fresh Water B FW 1.2 kg/s
Wastewater 4.8 kg H2O/s
Water Generation 5.1 kg/s
Water Loss (with AN Product) 0.3 kg H2O/s
FIGURE 6.5 Establishing targets for biotreatment influent: overall water balance (A) Before and (B) After Mass Integration.
wastewater to a quality that can substitute for fresh water using segregation, mixing, recycle, and interception. Furthermore, sink/generator manipulation can be employed to reduce flowrate of fresh water. Hence, freshwater usage in this example can in principle be completely eliminated, and for the same reaction conditions and water losses, the target for wastewater discharge can be calculated from the overall water balance as follows (Fig. 6.5B): Target of minimum discharge to biotreatment 5 5:1 2 0:3 5 4:8 kg=water=s:
ð6:10Þ
As can be seen from Fig. 6.5B, the corresponding target for minimum usage of fresh water is potentially zero. Fig. 6.6 illustrates the gap between current process performance and benchmarked mass targets. Having identified this target, let us now determine how to best attain the target. It is also necessary to sequence and integrate the solution strategies.
First, we develop no/low-cost strategies starting with minor process modifications. An operating parameter that can be altered to reduce freshwater usage (and consequently wastewater discharge) is the flowrate of the feed to the scrubber. As given by constraint (6.1), the flowrate of fresh water fed to the first scrubber may be reduced to 5.8 kg/s. This is a minor process modification that involves setting the flow control valve. Compared to the current usage of 6.0 kg/s, the net result is a reduction of 0.2 kg/s in water usage (and discharge). These results are shown in Fig. 6.7. To track water, the wastewater discharge is described in terms of kg H2O/s (not as total flowrate of wastewater including other species). Next, we consider other no/low-cost strategies including segregation, mixing, and direct-recycle opportunities. First, we identify the relevant sources and sinks. Once the streams composing the terminal wastewater are segregated, we get four sources that can be potentially recycled. Fresh water used in the scrubber
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EXAMPLE 6.1
(cont’d)
Current Discharge 12.0 kg/s Current Fresh 7.2 kg/s How? How? 4.8 kg/s Target Discharge 0.0 kg/s Target Fresh
FIGURE 6.6 Benchmarking water usage and discharge.
Current Discharge 12.0 kg/s 11.8 kg/s
Minor process modification
Current Fresh 7.2 kg/s
How?
Minor process modification
7.0 kg/s
How?
4.8 kg/s Target Discharge
0.0 kg/s Target Fresh
FIGURE 6.7 Reduction in water usage and discharge (expressed as kg H2O/s) after minor process modification. Air
Carbon Resin Aqueous Layer
Off-Gas Condensate Scrubber Aqueous Layer Feed to Biotreatment
Fresh Water to Scrubber
Distillation Bottoms
Ejector Condensate
SPecies Interception Network (SPIN)
Boiler/ Ejector
Ejector Condensate
Fresh Water to Boiler Air to AN Condensation
Carbon Resin to Regeneration and Recycle
FIGURE 6.8 Segregation, mixing, interception and recycle representation for the AN case study. and the boiler provides two more sources. In order to reduce wastewater discharge to biotreatment, fresh water must be reduced. Hence, we should focus our attention on recycling opportunities to sinks that employ fresh water, namely, the scrubber and the boiler. Fig. 6.8 illustrates the sources and sinks involved in the analysis.
Direct-recycle strategies are based on segregating, rerouting, and mixing of sources without the use of new equipment. Hence, there is no SPIN involved in direct recycle. Therefore, for direct recycle Fig. 6.8 is revised by eliminating the SPIN as shown in Fig. 6.9. Because of the stringent limitation on the BFW (no ammonia or AN),
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EXAMPLE 6.1
(cont’d) Aqueous Layer
Off-Gas Condensate Scrubber Aqueous Layer Feed to Biotreatment
Distillation Bottoms Ejector Condensate
Fresh Water to Scrubber
Boiler/ Ejector
Fresh Water to Boiler
Ejector Condensate
FIGURE 6.9 Schematic representation of segregation, direct recycle, and mixing for the AN example (El-Halwagi, 1997). Sink Data for the AN Example
300.0
Maximum inlet load, 1026 kg NH3/s
BFW
1.2
0.0
0.0
10.0
58.0
TABLE 6.3
150.0 100.0 Boiler 58.0 50.0 Scrubber
Source Data for the AN Example
Source
Inlet Inlet load, Flowrate composition 1026 kg (kg/s) of NH3 (ppm) NH3/s
Distillation bottoms
0.8
0.0
0.0
Off-gas condensate
5.0
14
70.0
5.7
25
142.5
Jet-ejector condensate 1.4
34
47.6
Aqueous layer
200.0
Load (10
Scrubber 5.8
250.0 kg NH3/s)
Maximum inlet composition of NH3 ppm
–6
Sink
Flowrate (kg/s)
a
a
Since the flowrate of the feed to the scrubber has been reduced by 0.2 kg/s as a result of minor process modification, the flowrate of the aqueous layer is assumed to decrease from 5.9 to 5.7 kg/s.
no recycled stream can be used in lieu of fresh water (segregation, mixing, recycle, and interception can reduce, but not eliminate, ammonia/AN content). Consequently, in Fig. 6.9 none of the process sources are allocated to the boiler. Hence, the boiler should not be considered as a sink for recycle (with or without interception). Instead, it should be handled at the stage of sink/generator manipulation. This leaves us with the five segregated sources and two sinks (boiler and scrubber). The data for the sources and sinks are summarized in Tables 6.2 and 6.3, respectively. Using these data, the material-recycle pinch diagram is constructed by first developing the sink composite curve (Fig. 6.10), then the source composite curve (Fig. 6.11), and the materialrecycle pinch diagram (Fig. 6.12). As can be seen from
0.0 0.0
2.0 1.2
4.0
6.0
8.0 7.0
10.0
12.0
14.0
16.0
18.0
Flowrate (kg/s)
FIGURE 6.10 Sink composite diagram for the AN example. 300.0 260.1 250.0 Load (10–6 kg NH3/s)
TABLE 6.2
Jet-Ejector Condensate
212.5 200.0 Aqueous Layer
150.0 100.0 70.0 50.0
Distillation Bottoms
0.0 0.0 0.8
2.0
Off-Gas Condensate 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 5.8
11.5 12.9 Flowrate (kg/s)
FIGURE 6.11 Source composite diagram for the AN example.
Fig. 6.12, when direct recycle is used the following targets can be obtained: Minimum fresh water 5 2:1 kg=s
ð6:11aÞ
Maximum direct recycle 5 4:9 kg=s Minimum waste discharge 5 8:0 kg=s
ð6:11bÞ ð6:11cÞ
As mentioned earlier, to track water it is useful to express the flowrate of waste discharge as kg H2O/s
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6. COMBINING MASS-INTEGRATION STRATEGIES
EXAMPLE 6.1 (not as total flow including other species). This can be readily calculated by noting that the reduction in freshwater usage is equal to the reduction in wastewater discharge (expressed as kg H2O/s). Since, Reduction in fresh water usage 5 7:2 2:1 5 5:1 kg=s ð6:11dÞ We have Minimum waste discharge ðexpressed as kg H2 OÞ=sÞ 5 12:0 2 5:1 5 6:9 kg H2 O=s
ð6:11eÞ
300.0 Wastewater = 8.0
Load (10–6 kg NH3/s)
250.0 Direct Recycle = 4.9
200.0 150.0
Source Composite Diagram
100.0 Sink Composite Diagram
50.0 0.0 0.0
Pinch Point
the boiler sink on the source-sink mapping diagram. Following the lever arm rules including sink-feed conditions and source-prioritization rules described in Chapter 3, Benchmarking Process Performance Through Overall Mass Targeting, the following statements can be made: • The composition of the feed to the boiler should be set to its maximum value (10 ppm). • The use of the sources should be prioritized as follows: distillation bottoms, off-gas condensate, aqueous layer, then jet-ejector condensate. Hence, we start by considering the use of the first two sources (shortest fresh arms): distillation bottoms and off-gas condensate. The flowrate resulting from combining these two sources (5.8 kg/s) is sufficient to run the scrubber. However, its ammonia composition2 as determined by the lever-arm principle is 12.1 ppm, which lies outside the zone of permissible recycle to the scrubber. As shown by Fig. 6.15, the maximum flowrate 7.0 Flowrate of a Source/Feed to a Sink (kg/s)
These results are shown by Fig. 6.13. It is beneficial to detail the direct-recycle strategies to attain the identified targets. Towards that end, the source-sink mapping diagram can be used to identify which sources should be recycled to which sinks. Fig. 6.14 is a source-sink mapping representation of the problem. As mentioned earlier, because of the stringent requirements on the feed to the boiler sink (0.0 composition of any pollutant), it is not possible to replace any of the feed to the boiler through recycle (even with interception). Consequently, we will not represent
(cont’d)
7.0
6.2 Scrubber
5.8 5.0
4.0
6.0
8.0
5.0
Off-Gas Condensate
4.0
4.0
3.0
3.0
2.0 1.0 0.8
10.0 12.0 14.0 16.0 18.0
1.0
Distillation Bottoms
0.0 0
Flowrate (kg/s)
2.0 1.4
Ejector Condensate
0.0 2.0
6.0 Aqueous Layer
5
10
15
20
25
30
35
Composition (ppm NH3)
Fresh = 2.1
FIGURE 6.12 Material recycle pinch diagram for the
FIGURE 6.14 Sourcesink mapping diagram for the AN
AN example.
example.
Current Discharge Minor process modification
Current Fresh
Direct Recycle
7.2 kg/s 7.0 kg/s
How?
2.1 kg/s
6.9 kg/s
Minor process modification Direct Recycle How?
Target Discharge
0.0 kg/s Target Fresh
FIGURE 6.13 Reduction in water usage and discharge (expressed as kg H2O/s) after minor process modification and direct recycle.
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6.2 PROCESS REPRESENTATION FROM A MASS-INTEGRATION SPECIES PERSPECTIVE
EXAMPLE 6.1 Flowrate of a Source/Feed to a Sink (kg/s)
New Feed to Scrubber
6.2 6.0
Scrubber 5.8
Aqueous Layer
5.0
Off-Gas Condensate
4.0
Fraction of Off-Gas Condensate to be Recycled
7.0
300.0
6.0
250.0
5.0 4.1 4.0
3.0
3.0
2.0 1.7
2.0
Fresh Water Distillation Bottoms
0.9 0.8 0.0 0
5
10
Ejector Condensate Fraction of Off-Gas Condensate to be Discharged
14 15
Load (10–6 kg NH3/s)
7.0
(cont’d)
25
30
Off-Gas Condensate
Intercepted Load = 12*10–6kg NH3/s Intercepted Off-Gas Condensate
2.0
4.0
6.0
8.0
10.0 12.0 14.0 16.0 18.0
Fresh = 1.2
35
FIGURE 6.15 Direct-recycle strategies for the AN example. of the off-gas condensate to be recycled to the scrubber3 is determined as follows: Arm of Gas Condensate Total Arm Flowrate of Recycled Gas Condensate Flowrate of Scrubber Feed
100.0 Boiler 70.0 58.0 50.0
Flowrate (kg/s)
FIGURE 6.16
y (ppm NH3)
5
150.0
0.0 0.0
1.0 0.0
20
200.0
ð6:12Þ
content has to be reduced. As can be seen from Fig. 6.16, in order to fully recycle the off-gas condensate the ammonia load to be removed is 12 1026 kg/s. The composition of the intercepted off-gas condensate is the slope of the intercepted stream, which is 11.6 ppm. Therefore, the interception task is to reduce the composition of ammonia in the off-gas condensate from 14.0 ppm to 11.6 ppm. The same result can be obtained algebraically as follows:
i:e:; 10 2 0 Flowrate of Recycled Gas Condensate 5 14 2 0 5:8 Hence, flowrate of recycled gas condensate 5 4.1 kg/s and the flowrate of fresh water is 0.9 kg/s (5.80.84.1). Therefore, direct recycle can reduce the freshwater consumption (and consequently the influent to biotreatment) by 5.1 kg/s. This is exactly the same target identified from the material-recycle pinch analysis as described by Eq. (6.11d). With the details of the direct-recycle strategies determined, it is possible to determine the implementation cost. The primary cost of direct recycling is pumping and piping. Assuming that the TAC for pumping and piping $80 is mUyear and assuming that the total length of piping is 600 m, the TAC for pumping and piping is $48,000/year. Since not all the off-gas condensate has been recycled, there is no need to consider recycle from any other source (since they have longer fresh arms). Therefore, there are no more direct-recycle opportunities and we have exhausted the no/low-cost strategies. Next, we move to adding new units and we consider interception. Before screening interception devices, it is necessary to determine the interception task. For all of the off-gas condensate to be recycled to the boiler, its ammonia
Intercepting the off-gas condensate.
Load removed from off-gas condensate 5 flowrate of off-gas condensate ðsupply composition 2 target compositionÞ
ð6:13aÞ
i.e., 12 1026 5 5:0 ð14:0 target compositionÞ Therefore, Target ðinterceptedÞ composition of ammonia in off-gas condensate 5 11:6 ppm
ð6:13bÞ
The same result may also be obtained through the sourcesink mapping diagram as shown in Fig. 6.17. Alternatively, it may be calculated as follows: ð5:0 kg=sÞyt ppm NH3 1 0 5 10:0 ppm NH3 5:8 kg=s i.e., yt 5 11:6 ppm:
ð6:13Þ
In order to synthesize an optimal MEN for intercepting the off-gas condensate, we construct the pinch diagram as shown in Fig. 6.18. Since the three MSAs lie completely to the left of the rich stream, they are all thermodynamically feasible. Hence, we choose the one with the least
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6. COMBINING MASS-INTEGRATION STRATEGIES
EXAMPLE 6.1 7.0
7.0
Flowrate of a Source/Feed to a Sink (kg/s)
Off-Gas Condensate
6.2
Scrubber
6.0
cost ($/kg NH3 removed), namely, the resin. The annual operating cost for removing ammonia using the resin is:
6.0
5.8
5 New Feed to Scrubber
5.0
5.0 4.1
4.0
kg Liquid kg NH3 ð14:0 1026 2 11:6 1026 Þ kg Liquid s
91
4.0 Intercepted Off-Gas Condensate
3.0
3.0
2.0
2.0
1.0
1.0
0.8
Distillation Bottoms 0.0
0.0 0
5
11.6
10
14
15
y (ppm NH3)
FIGURE 6.17
Determination of interception task for the off-
gas condensate. 12.0 Mass Exchanged (10–6 kg NH3/s)
(cont’d)
10.0 8.0 6.0
Off-Gas Condensate
4.0 2.0 0.0 0
3
6S1
9
12
0.1S2
2.3
4.4
6.6
145S3
295
445
595
295
595
895
1415 y 8.7 x1 745
x2
1195 1495
x3
FIGURE 6.18 The mass-exchange pinch diagram for the AN case study.
$ s 5 $34;437=year: T3600 8760 kg NH3 year
ð6:14Þ
The annualized fixed cost of the adsorption column along with its ancillary equipment (e.g., regeneration, materials handling, etc.) is estimated to be about $85,000/year. Therefore, the TAC for the interception system is $119,437/year. As a result of minor process modification, segregation, interception, and recycle, we have eliminated the use of fresh water in the scrubber, leading to a reduction in freshwater consumption (and influent to biotreatment) by 6.0 kg/s. Therefore, the target for segregation, interception, and recycle has been realized (Fig. 6.19). Next, we focus our attention on sink/generator manipulation to remove freshwater consumption in the steam-jet ejector. The challenge here is to alter the design and/or operation of the boiler, the ejector, or the distillation column to reduce or eliminate the use of steam. Several solutions may be proposed including: • Replacing of the steam-jet ejector with a vacuum pump. The distillation operation will not be affected. The operating cost of the ejector and the vacuum pump are comparable. However, a capital investment of $75,000 is needed to purchase the pump. For a five-year linear depreciation with negligible salvage value, the annualized fixed cost of the pump is $15,000/year. • Operating the column under atmospheric pressure, thereby eliminating the need for the vacuum pump. Here a simulation study is needed to examine the effect of pressure change.
Current Discharge 12.0 kg/s
Minor process modification
11.8 kg/s
Current Fresh Direct Recycle
7.2 kg/s 7.0 kg/s
6.9 kg/s 6.0 kg/s
Minor process modification Direct Recycle
Interception How?
4.8 kg/s
2.1 kg/s Interception 1.2 kg/s
Target Discharge
How?
0.0 kg/s Target Fresh
FIGURE 6.19
Reduction in water usage and discharge (expressed as kg H2O/s) after minor process modification, direct recycle,
and interception.
SUSTAINABLE DESIGN THROUGH PROCESS INTEGRATION
209
6.2 PROCESS REPRESENTATION FROM A MASS-INTEGRATION SPECIES PERSPECTIVE
EXAMPLE 6.1
Tail Gases to Disposal
Reactor
Scrubber
C3H6
AN to Sales 23 ppm NH3 5.1 kg AN/s 5.8 kg H2O/s
Off-Gas Condensate 14 ppm NH3 0.4 kg AN/s 4.6 kg H2O/s
To Regeneration and Recycle 10 ppm NH3 0.5 kg AN/s 5.3 kg H2O/s
Vacuum Pump
21 ppm NH3 4.7 kg AN/s 1.0 kg H2O/s
1 ppm NH3 4.6 kg AN/s 0.3 kg H2O/s
Distillation Column
Decanter
Adsorption Column
Resin
Aqueous Layer 25 ppm NH3 0.4 kg AN/s 4.8 kg H2O/s
11.6 ppm NH3 0.4 kg AN/s 4.6 kg H2O/s
Wastewater to Biotreatment
Bottoms 0 ppm NH3 0.1 kg AN/s 0.7 kg H2O/s
25 ppm NH3 0.4 kg AN/s 4.8 kg H2O/s
• Relaxing the requirement on BFW quality to a few parts per million of ammonia and AN. In this case, recycle and interception techniques can be used to significantly reduce the freshwater feed to the boiler and, consequently, the net wastewater generated. Fig. 6.20 illustrates the revised flowsheet with segregation, interception, recycle, and sink/generator manipulation. As can be seen from the figure, the flowrate of the terminal wastewater stream has been reduced to 4.8 kg H2O/s. This is exactly the same target predicted in Fig. 6.5B. In order to refine the material balance throughout the plant, a simulation study is needed. This is an effective use of simulation in coordination with process synthesis and integration. Fig. 6.20 is a tradeoff diagram (commonly referred to as the Pareto chart) for the cumulative reduction in wastewater and the associated TAC. The decision makers may decide to adopt all the strategies leading to the ultimate target for reducing wastewater flowrate or may decide to implement some of the strategies depending on the economic, technical, environmental, and safety factors (Fig. 6.21). We are now in a position to discuss the merits of the identified solutions. As can be inferred from Fig. 6.20, the following benefits can be achieved: • Acrylonitrile production has increased from 3.9 kg/s to 4.6 kg/s, which corresponds to an 18% yield enhancement for the plant. This production increase
240 Total Annualized Cost, $1000 (year)
NH3
FIGURE 6.20 Optimal solution to the AN case study.
Tail Gases to Disposal
5.0 kg AN/s 5.1 kg H2O/s + Gases
O2
(cont’d)
Unit Replacement (Vacuum Pump)
200
182 167
160 Interception (Addition of Adsorption Column)
120 80
Segregation and Direct Recycle
40
48
0 0
5.1 2 4 6 Reduction in Wastewater Flowrate (kg/s)
7.2
8
FIGURE 6.21 The Pareto diagram for total annualized cost of mass integration strategies versus reduction in wastewater flowrate entering the biotreatment facility. is a result of better allocation of process streams; the essence of mass integration. For a selling value of $0.6/kg of AN, the additional production of 0.7 kg AN/s can provide an annual revenue of $13.3 million/year! • Freshwater usage and influent to biotreatment facility are decreased by 7.2 kg/s. The value of fresh water and the avoidance of treatment cost are additional benefits. • Influent to biotreatment is reduced to 40% of current level. Therefore, the plant production can be expanded 2.5 times the current capacity before the biotreatment facility is debottlenecked again.
SUSTAINABLE DESIGN THROUGH PROCESS INTEGRATION
210
6. COMBINING MASS-INTEGRATION STRATEGIES
EXAMPLE 6.1 Clearly, this is a superior solution to the installation of an additional biotreatment facility. It is instructive to draw some conclusions from this case study and emphasize the design philosophy of mass integration. First, the target for debottlenecking the biotreatment facility was determined ahead of design. Then, systematic tools were used to generate optimal solutions that realized the target. Next, an analysis study was performed to validate or refine the results. This is an efficient approach to understanding the global insights of the process, setting performance targets, realizing these targets, and saving time and effort by focusing on the big picture first and dealing with the details later. This is a fundamentally different approach than using the designer’s subjective decisions to alter the process and check the consequences using detailed analysis. It is also different from using simple end-of-pipe treatment solutions. Instead, the various species are
(cont’d)
optimally allocated throughout the process. Therefore, objectives such as yield enhancement, pollution prevention, and cost savings can be simultaneously addressed. Indeed, pollution prevention (when undertaken with the proper techniques) can be a source of profit for the company, not an economic burden.
1
Algebraically, this composition can be calculated as follows:
ð5:0 kg=sÞð14 ppm NH3 Þ 1 0 5 12:1 ppm NH3 5:8 kg=s 2 Again, algebraically this flowrate can be calculated as follows: Flowrate of recycled off-Gas condensate314 ppm NH3 1010 5:8 kg=s 510 ppm NH3 :
where the numerator represents the ammonia in recycled off-gas condensate, distillation bottoms (none) and fresh water (none). Hence, flowrate of recycled off-gas condensate 5 4.14 kg/s.
6.3 HOMEWORK PROBLEMS
compensate for water losses with the wet cake (0.08 kg water/s) and the shredded tires (0.12 kg water/s). The mixture of filtrate and water makeup is fed to a high-pressure compression station for recycle to the shredding unit. Due to the pyrolysis reactions, 0.08 kg water/s is generated. The plant has two primary sources for wastewater: the decanter (0.20 kg water/s) and the seal pot (0.15 kg/s). The plant has been
6.1. Let us revisit the tire-to-fuel process described in Problem 5.1. Fig. 6.22 is a more detailed flowsheet. Tire shredding is achieved by using high-pressure water jets. The shredded tires are fed to the process while the spent water is filtered. The wet cake collected from the filtration system is forwarded to solid waste handling. The filtrate is mixed with 0.20 kg/s of freshwater makeup to
To Atmosphere
Gaseous Fuel Condenser
0.20 kg/s
Reactor Off-Gases Light Oil
Tires Shredded Tires Water-Jet Shredding
Filtration
Pyrolysis Reactor
Flare
Decanter
Wastewater to Treatment
Separation
Seal Fresh Pot Water 0.15 kg/s Flare Gas
Finishing
Wet Cake (to Solid Waste Handling)
Compression
Fresh Water 0.20 kg/s
FIGURE 6.22 Schematic flowsheet of tire-to-fuel process. SUSTAINABLE DESIGN THROUGH PROCESS INTEGRATION
Wastewater to Treatment
Liquid Fuels
211
6.3 HOMEWORK PROBLEMS
shipping the wastewater for off-site treatment. The cost of wastewater transportation and treatment is $0.01/kg leading to a wastewater treatment cost of approximately $110,000/year. The plant wishes to stop off-site treatment of wastewater to avoid cost of off-site treatment ($110,000/year) and alleviate legal liability concerns in case of transportation accidents or inadequate treatment of the wastewater. The objective of this problem is to eliminate or reduce to the extent feasible off-site wastewater treatment. For capital budget authorization, the plant has the following economic criterion: Payback period 5
Fixed capital investment Annual savings # 3 years
ð6:15Þ
where Annual Savings 5 Annual avoided cost of off-Site treatment 2 Annual operating cost of on-site system In addition to the information provided by Problem 3.1, the following data are available: Economic Data • Fixed cost of extraction system associated with S2, $ 5 120,000 (Flowrate of wastewater, kg/s)0.58 • Fixed cost of adsorption system associated with S3, $ 5 790,000 (Flowrate of wastewater, kg/s)0.70 • Fixed cost of stripping system associated with S4, $ 5 270,000 (Flowrate of wastewater, kg/s)0.65 • A biotreatment facility that can handle 0.35 kg/s wastewater has a fixed cost of $240,000 and an annual operating cost of $60,000/year. Technical Data Water may be recycled to two sinks; the seal pot and the water-jet compression station. The following constraints on flowrate and composition of the pollutant (heavy organic) should be satisfied: Seal Pot • 0:10 # Flowrate of feed water ðkg=sÞ # 0:20 • 0 #Pollutant content of feed water ðppmwÞ #500
Makeup to Water-Jet Compression Station • 0:18 # Flowrate of makeup water ðkg=sÞ # 0:20 0#Pollutant content of makeup water • ðppmwÞ#50 6.2. Consider the magnetic tape manufacturing process previously dexscribed by problem 4.7 and shown by Fig. 6.23. In this process (Dunn et al., 1995), several binders used in coating are dissolved in 0.09 kg/s of organic solvent. The mixture is additionally mixed and suspended with magnetic pigments, dispersants, and lubricants. A base file is coated with the suspension. Nitrogen gas is used for blanketing and to aid in evaporation of the solvent. As a result of thermal decomposition in the coating chamber, 0.011 kg/s of solvent are decomposed into other organic species. The decomposed organics are separated from the exhaust gas in a membrane unit. The reject stream leaving the membrane unit has a flowrate of 3.0 kg/s and is primarily composed of nitrogen, which contains 1.9 wt./wt.% of the organic solvent. The coated film is passed to a dryer where nitrogen gas is employed to evaporate the remaining solvent. The exhaust gas leaving the dryer has a flowrate of 5.5 kg/s and contains 0.4 wt./wt.% solvent. The two exhaust gases are mixed and disposed of. In order to recover the solvent from the exhaust gases, three are three candidate MSAs. The equilibrium data for the transfer of the organic solvent to the jth lean stream is given by y 5 mjxj where the values of mj are given in Table 6.4. A minimum allowable composition difference of 0.001(kg organic solvent)/(kg MSA) is to be used. The annualized fixed cost of a mass exchanger, $/year, may be approximated by 18,000 (gas flowrate, kg/s)0.65. The value of the recovered solvent is $0.80/kg of organic solvent. In addition to the environmental problem, there is also an economic incentive to recover the solvent from the exhaust gases. The value
Gaseous Emission GTotal = 8.5 kg/s 0.93% Solvent
Decomposed Organics 0.011 kg/s Magnetic Pigments
Solvent 0.09 kg/s Resin Premixing
Dispersants and Lubricants
Slurry Premixing
G1 = 3.0 kg/s 1.90% Solvent
G2 = 5.5 kg/s 0.40% Solvent
N2 Dispersion
Coating
Binders Resins
Additives
Base Film
FIGURE 6.23 Schematic representation of a magnetic tape manufacturing process (Dunn et al., 1995).
N2 Drying Dry Tape Product
SUSTAINABLE DESIGN THROUGH PROCESS INTEGRATION
212
6. COMBINING MASS-INTEGRATION STRATEGIES
of 0.09 kg/s of solvent is approximately $2.3 million/year. The objective of this problem is to develop a minimum-cost solution that minimizes the usage of fresh solvent in the process. The solution strategies may include segregation, mixing, recycle and interception. To assess the economic potential of the solution, evaluate the payback period for your solution. 6.3. A process manufactures 42,000 kg/h of finished wood panels (El-Halwagi, 2017). The process as shown by Fig. 6.24 involves the following steps: 1. Log Processing (Green-End Operations): 60,000 kg/h on an oven-dry (OD) basis of hauled logs are first sprayed with 5000 kg/h of warm water to soften
the logs (and to deice in cold areas). The softened logs are sorted, debarked, and flaked. The “green” strands leaving the flakers are sorted via screens, then forwarded to the drying system. The wood processing residuals (e.g., bark, offspec strands/chips, and sawdust) are burned in specially designed furnaces to utilize their heating value. 2. Strand Drying: Rotary dryers using 100,000 kg/h of hot air are used to reduce the moisture content of the green strands to about 4%. The high temperature causes the vaporization and/or chemical conversion of the wood constituents into volatile organic compounds (VOCs) that include hazardous air pollutants
TABLE 6.4 Data for the MSAs Stream
Upper bound on flowrate LCj (kg/s)
Supply composition (mass fraction) xjs
Target composition (mass fraction) xtj
mj
εj mass fraction
Cj $/kg MSA
S1
N
0.014
0.040
0.4
0.001
0.002
S2
N
0.020
0.080
1.5
0.001
0.001
S3
N
0.001
0.010
0.1
0.001
0.002
Water 15,000 kg/h Particulate-Free Dryer Off-Gas
Quench WESP
WESP Wastewater 14,000 kg/h Dryer Off-Gas Warm Water 5000 kg/h Logs 60,000 kg/h
Debarked Wood Log Sorting and Debarking
Log Softening
Wastewater
Residual Wood
Green Strands
Settling WESP Sludge 500 kg/h (40% water)
Phenol
Resin MDI Binders Formation and Wax Hot Phenol Air Formaldehyde Resin
Flaking
Off-Spec Strands and Sawdust
Formaldehyde 220 kg/h
Dried Strands
Mixing Blending
Forming
Mat Trimming Mat Trimmings
Hot Pressing
Engineered Wood 42,000 kg/h
FIGURE 6.24 A block flow diagram for the engineered wood manufacturing process (El-Halwagi, 2017).
SUSTAINABLE DESIGN THROUGH PROCESS INTEGRATION
Press Fumes
213
REFERENCES
(HAPs) such as methanol, formaldehyde, and acetaldehyde. The flowrate of the released HAPs depends on the drying temperature as follows: Formaldehyde released from the dryers ðkg=hÞ 513:67 1022 T 76:85 753 #T ðKÞ#873 ð3:121aÞ Acetaldehyde released from the dryers ðkg=hÞ 5 11:17 1022 T 69:48 753# TðKÞ# 873 ð3:121bÞ Methanol released from the dryers ðkg=hÞ 510:51 1022 T 67:32 753#TðKÞ#873
ð3:121cÞ
where T is the inlet drying temperature that is allowed is vary between 753 and 873 K. The hot gases leaving the dryers are fed to a wet electrostatic precipitator (WESP) to remove the particulate matters. Water is fed to the WESP at the rate of 15,000 kg/h to quench the gases, facilitate the capture of fines, and wash the system from condensing tars that can form a fire hazard. Settling tanks are used to separate the aqueous stream leaving the WESP into sludge and wastewater. 3. Resin formation: Phenol formaldehyde resin is manufactured by reacting phenol with formaldehyde. This reaction consumes 220 kg/h of formaldehyde. The resin is mixed with methane diisocyanate (MDI) binders and wax to form an additive solution to be sprayed on the dried strands. 4. Blending: The dried strands are mixed with resin binders and wax to facilitate further
processing and to decrease moisture absorption in the final product. 5. Forming: The resin-treated strands are formed into layers that are oriented at right angles. When the desired number of the layers is deposited, mats of specific dimensions are trimmed. 6. Pressing: Heated presses are used to compact the mat of the cross-directional strands into boards. The fumes from pressing include HAPs such as formaldehyde (5 kg/h), acetaldehyde (1 kg/h), and methanol (4 kg/h). Using mass integration strategies, determine the targets and show the implementation strategies for: 1. Minimizing fresh water usage 2. Minimizing HAP emissions
References Dunn, R.F., El-Halwagi, M.M., Lakin, J., Serageldin, M., 1995. In: Griffith, E.D., Kahn, H., Cousins, M.C. (Eds.), Selection of Organic Solvent Blends for Environmental Compliance in the Coating Industries. Proceedings of the First International Plant Operations and Design Conference, Vol. III. AIChE, New York, pp. 83107. El-Halwagi, M.M., 2017. A return on investment metric for incorporating sustainability in process integration and improvement projects. Clean Technol. Environ. Policy. 19, 611617. El-Halwagi, M.M., 1997. Pollution Prevention Through Process Integration. Academic Press, San Diego. El-Halwagi, M.M., 1999. Sustainable pollution prevention through mass integration. In: Sikdar, S., Diwekar, U. (Eds.), Tools and Methods for Pollution Prevention. Kluwer pub, pp. 233275. El-Halwagi, M.M., Hamad, A.A., Garrison, G.W., 1996. Synthesis of waste interception and allocation networks. AIChE J. 42 (11), 30873101. El-Halwagi, M.M., Spriggs, H.D., 1998. Solve design puzzles with mass integration. Chem. Eng. Prog. 94 (2544), 1998.
SUSTAINABLE DESIGN THROUGH PROCESS INTEGRATION
6 Combining Mass-Integration Strategies 6.1 INTRODUCTION The previous three chapters set the stage for developing mass-integration strategies. Chapter 3, Benchmarking Process Performance Through Overall Mass Targeting, presented several approaches to the determination of overall mass targets depending on the type and extent of available data. These targets are identified ahead of detailed design. As mentioned in Chapter 3, Benchmarking Process Performance Through Overall Mass Targeting, these targets can be attained by a combination of strategies, where impact is a measure of the effectiveness of the proposed solution in partially reaching the target. The cumulative impact of all the strategies yields the desired target. Acceptability is a measure of the likelihood of a proposed strategy to be accepted and implemented by the plant. Chapters 4, Direct-Recycle Networks: Graphical and Algebraic Targeting Approaches, and 5, Synthesis of Mass-Exchange Networks, gave important classes of systematic mass-integration tools associated with identifying such strategies. Direct recycle was covered by Chapter 4, Synthesis of Mass-Exchange Networks, as a key element in no/low-cost strategies involving segregation, mixing, and rerouting of streams without the addition of new equipment. The synthesis of massexchange networks was described in Chapter 5, Synthesis of Mass-Exchange Networks, as an illustration of the addition of new units (interception) and the selection or substitution of solvents to achieve a separation task. This chapter combines the tools presented in the previous three chapters. First, the process representation from a species viewpoint is presented. Then, the combination of mass-integration tools is demonstrated through the applicability to a case study on the production of acrylonitrile.
Sustainable Design Through Process Integration DOI: http://dx.doi.org/10.1016/B978-0-12-809823-3.00006-0
6.2 PROCESS REPRESENTATION FROM A MASS-INTEGRATION SPECIES PERSPECTIVE Once an overall mass target is determined, it is necessary to develop cost-effective strategies to reach the target. For a given target, there are numerous design decisions that must be judiciously made. These include addressing the following challenging questions (Fig. 6.1): • What are the optimum stream-rerouting strategies? • Should any streams be segregated, mixed, or rerouted? Which ones? • Which streams should be recycled/reused? To which units? • Should design variables and/or operating conditions of existing units be altered? Which ones? To what extent? • Is there a need to add/replace units? Which ones? Where to add/replace? • Should interception (e.g., separation) devices be added? Which streams should be intercepted? To remove what? To what extent? • Which separating agents should be selected for interception? • What is the optimal flowrate of each separating agent? • How should these separating agents be matched with the rich streams (i.e., stream pairings)? Because of the prohibitively large number of alternatives involved in answering these questions, a systematic procedure is needed to extract the optimum solution(s) without enumerating them. This is the role provided by mass integration. Mass integration is a holistic and systematic methodology that provides a
199
© 2017 Elsevier Inc. All rights reserved.
200
6. COMBINING MASS-INTEGRATION STRATEGIES
Cost/Impact
Target
Technology Changes (New Chemistry, New Processing Technology, etc.) Equipment Addition/Replacement (Interception, Separation, etc.)
Material Substitution (e.g., Solvent, Catalyst) Acceptability
Modest Manipulation of Units (e.g., Moderate Changes in Operating Conditions)
Minor Structural Modifications (Segregation, Mixing, Recycle, etc.)
FIGURE 6.1 Hierarchy of mass-integration strategies (El-Halwagi, 1999).
Sources
Segregated Sources
Separating Agents
Sinks/ Generators (Units)
Sources (Back to Process)
#1
#2 . . .
SPecies Interception Network (SPIN)
. . .
Nsinks
Separating Agents (to Regeneration and Recycle)
FIGURE 6.2 Process from a species perspective (El-Halwagi et al., 1996).
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. Mass integration is based on fundamental principles of chemical engineering combined with system analysis using graphical and optimization-based tools. In order to develop detailed mass-integration strategies, let us represent the process flowsheet from a species viewpoint (e.g., El-Halwagi and Spriggs, 1998; El-Halwagi et al., 1996) as shown in Fig. 6.2. For each targeted species, there are sources (streams that carry the species) and process sinks (units that can accept the species). Process sinks include reactors, separators, heaters/coolers, biotreatment facilities, and discharge media. Streams leaving the sinks become, in
turn, sources. Therefore, sinks are also generators of the targeted species. Each sink/generator may be manipulated via design and/or operating changes to affect the flowrate and composition of what each sink/ generator accepts and discharges. Stream characteristics (e.g., flowrate, composition, pressure, temperature, etc.) can be modified by adding new units that intercept the streams prior to being fed to the process sinks and condition their properties to the desired values. This is performed a speciesinterception network (SPIN) that may use mass- and energy-separating agents. Interception denotes the utilization of new unit operations to adjust the composition, flowrate, and other properties of certain process streams to make them acceptable for existing process sinks. A particularly important class of interception
6.2 PROCESS REPRESENTATION FROM A MASS-INTEGRATION SPECIES PERSPECTIVE
Sources
Segregated Sources
Mass Separating Agents in
Sinks/ Generators
201
Sources (Back to Process or Terminal Discharge)
1
2
Species Interception Network (Special Case: Mass-Exchange Network)
Nsinks
Mass Separating Agents out (to Regeneration and Recycle)
FIGURE 6.3 A process from a species viewpoint when MSAs are used for interception.
devices is separation systems. These separations may be induced by the use of mass-separating agents (MSAs) and/or energy-separating agents (ESAs). A systematic technique is needed to screen the multitude of separating agents and separation technologies to find the optimal separation system. The synthesis of MSA-induced physical-separation systems has been
covered in Chapter 5, Synthesis of Mass-Exchange Networks, through the synthesis of mass-exchange networks (MENs). Other interception systems are covered throughout the book. The MEN can be used to prepare sources for recycle. When MSAs are used for interception, Fig. 6.2 is revised as shown in Fig. 6.3 by placing a MEN instead of the SPIN.
E X A M P L E 6 . 1 A P P L I C AT I O N O F M A S S I N T E G R AT I O N T O D E B O T T L E N E C K A N A C RY L O N I T R I L E P R O C E S S A N D R E D U C E WAT E R U S A G E A N D D I S C H A R G E Acrylonitrile ðAN; C3 H3 NÞ is manufactured via the vapor-phase ammoxidation of propylene: catalyst
C3 H6 1 NH3 1 1:5O2 ! C3 H3 N 1 3H2 O: The reaction takes place in a fluidized-bed reactor in which propylene, ammonia, and oxygen are catalytically reacted at 450 C and 2 atm. The reaction is a single pass with almost complete conversion of propylene. The reaction products are cooled using an indirect-contact heat exchanger that condenses a fraction of the reactor off-gas. The remaining off-gas is scrubbed with water, then decanted into an aqueous layer and an organic layer. The organic layer is fractionated in a distillation column under slight vacuum induced by a steam-jet ejector. Fig. 6.4 shows the process flowsheet along with pertinent material balance data.
The wastewater stream of the plant is composed of the off-gas condensate, the aqueous layer of the decanter, the bottom product of the distillation column, and the condensate from the steam-jet ejector. This wastewater stream is fed to the biotreatment facility. Since the biotreatment facility is currently operating at full hydraulic capacity, it constitutes a bottleneck for the plant. Plans for expanding production of AN are contingent upon debottlenecking of the biotreatment facility by reducing its influent or installing an additional treatment unit. The new biotreatment facility will cost about $12 MM in capital investment and $0.7 MM/year in annual operating cost, leading to a TAC of $1.9 MM/ year using a 10-year linear depreciation. The objective of this case study is to use mass-integration techniques to devise cost-effective strategies to debottleneck the biotreatment facility.
SUSTAINABLE DESIGN THROUGH PROCESS INTEGRATION
202
6. COMBINING MASS-INTEGRATION STRATEGIES
EXAMPLE 6.1
5.0 kg AN/s 5.1 kg H2O/s + Gases
O2 NH3
Water 6.0 kg H2O/s
(cont’d) Steam-Jet Ejector
B FW 1.2 kg H2O/s
Condensate
Steam Boiler
34 ppm NH3 0.2 kg AN/s 1.2 kg H2O/s
Tail Gases to Disposal
Reactor
Scrubber
C3H6
AN to Sales
Off-Gas Condensate 14 ppm NH3 0.4 kg AN/s 4.6 kg H2O/s
18 ppm NH3 4.6 kg AN/s 6.5 kg H2O/s
10 ppm NH3 4.2 kg AN/s 1.0 kg H2O/s
Distillation Column
1 ppm NH3 3.9 kg AN/s 0.3 kg H2O/s
Decanter
Aqueous Layer 25 ppm NH3 0.4 kg AN/s 5.5 kg H2O/s
Bottoms 0 ppm NH3 0.1 kg AN/s 0.7 kg H2O/s
20 ppm NH3 1.1 kg AN/s 12.0 kg H2O/s Wastewater to Biotreatment
FIGURE 6.4 Flowsheet of AN production. The following technical constraints should be observed in any proposed solution: Scrubber 5:8 # flowrate of wash feed ðkg=sÞ # 6:2
ð6:1Þ
0:0 # ammonia content of wash feed ðppm NH3 Þ # 10:0 ð6:2Þ
Boiler Feed Water (BFW) Ammonia content of BFW ðppm NH3 Þ 5 0:0
ð6:3Þ
AN content of BFW ðppm ANÞ 5 0:0
ð6:4Þ
10:6 # flowrate of feed ðkg=sÞ # 11:1
ð6:5Þ
Decanter
Distillation Column 5:2 # flowrate of feed ðkg=sÞ # 5:7
ð6:6Þ
0:0 # Ammonia content of feed ðppm NH3 Þ # 30:0 ð6:7Þ 80:0 # AN content of feed ðwt:% ANÞ # 100:0
ð6:8Þ
Furthermore, for quality and operability objectives the plant does not wish to recycle the AN product stream (top of distillation column), the feed to the distillation column, or the feed to the decanter. Three external MSAs are considered for removing ammonia from water: air (S1), activated carbon (S2), and an adsorbing resin (S3). The data for the candidate MSAs are given in Table 6.1. The equilibrium data for the transfer of the pollutant from the waste stream to the jth MSA is given by, y1 5 mj xj ; ð6:9Þ where y1 and xj are weight-based parts per million of ammonia in the wastewater and the jth MSA, respectively.
Solution The first step in the analysis is to identify the target for debottlenecking the biotreatment facility. An overall water balance for the plant (Fig. 6.5A) can be written as follows: Water in 1 Water generated by chemical reaction 5 Wastewater out 1 Water losses Since the wastewater discharge is larger than the freshwater flowrate, it is possible, in principle, to bring
SUSTAINABLE DESIGN THROUGH PROCESS INTEGRATION
203
6.2 PROCESS REPRESENTATION FROM A MASS-INTEGRATION SPECIES PERSPECTIVE
EXAMPLE 6.1 TABLE 6.1
(cont’d)
Data for MSAs of the AN Problem
Stream
Upper bound on flowrate LCj
Supply composition (ppmw) xsj
Target composition (ppmw) xtj
S1
N
0
S2
N
S3
N
mj
εj ppmw
Cj $/kg MSA
Crj $/kg NH3 removed
6
1.4
2
0.004
667
10
400
0.02
5
0.070
180
3
1100
0.01
5
0.100
91
(A) ACRYLONITRILE PLANT Scrubber Water 6.0 kg/s
Wastewater 12.0 kg H2O/s Water Generation 5.1 kg/s Water Loss (with AN Product) 0.3 kg H2O/s
BFW 1.2 kg/s
(B)
ACRYLONITRILE PLANT Scrubber Water 6.0 kg/s
No Fresh Water B FW 1.2 kg/s
Wastewater 4.8 kg H2O/s
Water Generation 5.1 kg/s
Water Loss (with AN Product) 0.3 kg H2O/s
FIGURE 6.5 Establishing targets for biotreatment influent: overall water balance (A) Before and (B) After Mass Integration.
wastewater to a quality that can substitute for fresh water using segregation, mixing, recycle, and interception. Furthermore, sink/generator manipulation can be employed to reduce flowrate of fresh water. Hence, freshwater usage in this example can in principle be completely eliminated, and for the same reaction conditions and water losses, the target for wastewater discharge can be calculated from the overall water balance as follows (Fig. 6.5B): Target of minimum discharge to biotreatment 5 5:1 2 0:3 5 4:8 kg=water=s:
ð6:10Þ
As can be seen from Fig. 6.5B, the corresponding target for minimum usage of fresh water is potentially zero. Fig. 6.6 illustrates the gap between current process performance and benchmarked mass targets. Having identified this target, let us now determine how to best attain the target. It is also necessary to sequence and integrate the solution strategies.
First, we develop no/low-cost strategies starting with minor process modifications. An operating parameter that can be altered to reduce freshwater usage (and consequently wastewater discharge) is the flowrate of the feed to the scrubber. As given by constraint (6.1), the flowrate of fresh water fed to the first scrubber may be reduced to 5.8 kg/s. This is a minor process modification that involves setting the flow control valve. Compared to the current usage of 6.0 kg/s, the net result is a reduction of 0.2 kg/s in water usage (and discharge). These results are shown in Fig. 6.7. To track water, the wastewater discharge is described in terms of kg H2O/s (not as total flowrate of wastewater including other species). Next, we consider other no/low-cost strategies including segregation, mixing, and direct-recycle opportunities. First, we identify the relevant sources and sinks. Once the streams composing the terminal wastewater are segregated, we get four sources that can be potentially recycled. Fresh water used in the scrubber
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6. COMBINING MASS-INTEGRATION STRATEGIES
EXAMPLE 6.1
(cont’d)
Current Discharge 12.0 kg/s Current Fresh 7.2 kg/s How? How? 4.8 kg/s Target Discharge 0.0 kg/s Target Fresh
FIGURE 6.6 Benchmarking water usage and discharge.
Current Discharge 12.0 kg/s 11.8 kg/s
Minor process modification
Current Fresh 7.2 kg/s
How?
Minor process modification
7.0 kg/s
How?
4.8 kg/s Target Discharge
0.0 kg/s Target Fresh
FIGURE 6.7 Reduction in water usage and discharge (expressed as kg H2O/s) after minor process modification. Air
Carbon Resin Aqueous Layer
Off-Gas Condensate Scrubber Aqueous Layer Feed to Biotreatment
Fresh Water to Scrubber
Distillation Bottoms
Ejector Condensate
SPecies Interception Network (SPIN)
Boiler/ Ejector
Ejector Condensate
Fresh Water to Boiler Air to AN Condensation
Carbon Resin to Regeneration and Recycle
FIGURE 6.8 Segregation, mixing, interception and recycle representation for the AN case study. and the boiler provides two more sources. In order to reduce wastewater discharge to biotreatment, fresh water must be reduced. Hence, we should focus our attention on recycling opportunities to sinks that employ fresh water, namely, the scrubber and the boiler. Fig. 6.8 illustrates the sources and sinks involved in the analysis.
Direct-recycle strategies are based on segregating, rerouting, and mixing of sources without the use of new equipment. Hence, there is no SPIN involved in direct recycle. Therefore, for direct recycle Fig. 6.8 is revised by eliminating the SPIN as shown in Fig. 6.9. Because of the stringent limitation on the BFW (no ammonia or AN),
205
6.2 PROCESS REPRESENTATION FROM A MASS-INTEGRATION SPECIES PERSPECTIVE
EXAMPLE 6.1
(cont’d) Aqueous Layer
Off-Gas Condensate Scrubber Aqueous Layer Feed to Biotreatment
Distillation Bottoms Ejector Condensate
Fresh Water to Scrubber
Boiler/ Ejector
Fresh Water to Boiler
Ejector Condensate
FIGURE 6.9 Schematic representation of segregation, direct recycle, and mixing for the AN example (El-Halwagi, 1997). Sink Data for the AN Example
300.0
Maximum inlet load, 1026 kg NH3/s
BFW
1.2
0.0
0.0
10.0
58.0
TABLE 6.3
150.0 100.0 Boiler 58.0 50.0 Scrubber
Source Data for the AN Example
Source
Inlet Inlet load, Flowrate composition 1026 kg (kg/s) of NH3 (ppm) NH3/s
Distillation bottoms
0.8
0.0
0.0
Off-gas condensate
5.0
14
70.0
5.7
25
142.5
Jet-ejector condensate 1.4
34
47.6
Aqueous layer
200.0
Load (10
Scrubber 5.8
250.0 kg NH3/s)
Maximum inlet composition of NH3 ppm
–6
Sink
Flowrate (kg/s)
a
a
Since the flowrate of the feed to the scrubber has been reduced by 0.2 kg/s as a result of minor process modification, the flowrate of the aqueous layer is assumed to decrease from 5.9 to 5.7 kg/s.
no recycled stream can be used in lieu of fresh water (segregation, mixing, recycle, and interception can reduce, but not eliminate, ammonia/AN content). Consequently, in Fig. 6.9 none of the process sources are allocated to the boiler. Hence, the boiler should not be considered as a sink for recycle (with or without interception). Instead, it should be handled at the stage of sink/generator manipulation. This leaves us with the five segregated sources and two sinks (boiler and scrubber). The data for the sources and sinks are summarized in Tables 6.2 and 6.3, respectively. Using these data, the material-recycle pinch diagram is constructed by first developing the sink composite curve (Fig. 6.10), then the source composite curve (Fig. 6.11), and the materialrecycle pinch diagram (Fig. 6.12). As can be seen from
0.0 0.0
2.0 1.2
4.0
6.0
8.0 7.0
10.0
12.0
14.0
16.0
18.0
Flowrate (kg/s)
FIGURE 6.10 Sink composite diagram for the AN example. 300.0 260.1 250.0 Load (10–6 kg NH3/s)
TABLE 6.2
Jet-Ejector Condensate
212.5 200.0 Aqueous Layer
150.0 100.0 70.0 50.0
Distillation Bottoms
0.0 0.0 0.8
2.0
Off-Gas Condensate 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 5.8
11.5 12.9 Flowrate (kg/s)
FIGURE 6.11 Source composite diagram for the AN example.
Fig. 6.12, when direct recycle is used the following targets can be obtained: Minimum fresh water 5 2:1 kg=s
ð6:11aÞ
Maximum direct recycle 5 4:9 kg=s Minimum waste discharge 5 8:0 kg=s
ð6:11bÞ ð6:11cÞ
As mentioned earlier, to track water it is useful to express the flowrate of waste discharge as kg H2O/s
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6. COMBINING MASS-INTEGRATION STRATEGIES
EXAMPLE 6.1 (not as total flow including other species). This can be readily calculated by noting that the reduction in freshwater usage is equal to the reduction in wastewater discharge (expressed as kg H2O/s). Since, Reduction in fresh water usage 5 7:2 2:1 5 5:1 kg=s ð6:11dÞ We have Minimum waste discharge ðexpressed as kg H2 OÞ=sÞ 5 12:0 2 5:1 5 6:9 kg H2 O=s
ð6:11eÞ
300.0 Wastewater = 8.0
Load (10–6 kg NH3/s)
250.0 Direct Recycle = 4.9
200.0 150.0
Source Composite Diagram
100.0 Sink Composite Diagram
50.0 0.0 0.0
Pinch Point
the boiler sink on the source-sink mapping diagram. Following the lever arm rules including sink-feed conditions and source-prioritization rules described in Chapter 3, Benchmarking Process Performance Through Overall Mass Targeting, the following statements can be made: • The composition of the feed to the boiler should be set to its maximum value (10 ppm). • The use of the sources should be prioritized as follows: distillation bottoms, off-gas condensate, aqueous layer, then jet-ejector condensate. Hence, we start by considering the use of the first two sources (shortest fresh arms): distillation bottoms and off-gas condensate. The flowrate resulting from combining these two sources (5.8 kg/s) is sufficient to run the scrubber. However, its ammonia composition2 as determined by the lever-arm principle is 12.1 ppm, which lies outside the zone of permissible recycle to the scrubber. As shown by Fig. 6.15, the maximum flowrate 7.0 Flowrate of a Source/Feed to a Sink (kg/s)
These results are shown by Fig. 6.13. It is beneficial to detail the direct-recycle strategies to attain the identified targets. Towards that end, the source-sink mapping diagram can be used to identify which sources should be recycled to which sinks. Fig. 6.14 is a source-sink mapping representation of the problem. As mentioned earlier, because of the stringent requirements on the feed to the boiler sink (0.0 composition of any pollutant), it is not possible to replace any of the feed to the boiler through recycle (even with interception). Consequently, we will not represent
(cont’d)
7.0
6.2 Scrubber
5.8 5.0
4.0
6.0
8.0
5.0
Off-Gas Condensate
4.0
4.0
3.0
3.0
2.0 1.0 0.8
10.0 12.0 14.0 16.0 18.0
1.0
Distillation Bottoms
0.0 0
Flowrate (kg/s)
2.0 1.4
Ejector Condensate
0.0 2.0
6.0 Aqueous Layer
5
10
15
20
25
30
35
Composition (ppm NH3)
Fresh = 2.1
FIGURE 6.12 Material recycle pinch diagram for the
FIGURE 6.14 Sourcesink mapping diagram for the AN
AN example.
example.
Current Discharge Minor process modification
Current Fresh
Direct Recycle
7.2 kg/s 7.0 kg/s
How?
2.1 kg/s
6.9 kg/s
Minor process modification Direct Recycle How?
Target Discharge
0.0 kg/s Target Fresh
FIGURE 6.13 Reduction in water usage and discharge (expressed as kg H2O/s) after minor process modification and direct recycle.
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6.2 PROCESS REPRESENTATION FROM A MASS-INTEGRATION SPECIES PERSPECTIVE
EXAMPLE 6.1 Flowrate of a Source/Feed to a Sink (kg/s)
New Feed to Scrubber
6.2 6.0
Scrubber 5.8
Aqueous Layer
5.0
Off-Gas Condensate
4.0
Fraction of Off-Gas Condensate to be Recycled
7.0
300.0
6.0
250.0
5.0 4.1 4.0
3.0
3.0
2.0 1.7
2.0
Fresh Water Distillation Bottoms
0.9 0.8 0.0 0
5
10
Ejector Condensate Fraction of Off-Gas Condensate to be Discharged
14 15
Load (10–6 kg NH3/s)
7.0
(cont’d)
25
30
Off-Gas Condensate
Intercepted Load = 12*10–6kg NH3/s Intercepted Off-Gas Condensate
2.0
4.0
6.0
8.0
10.0 12.0 14.0 16.0 18.0
Fresh = 1.2
35
FIGURE 6.15 Direct-recycle strategies for the AN example. of the off-gas condensate to be recycled to the scrubber3 is determined as follows: Arm of Gas Condensate Total Arm Flowrate of Recycled Gas Condensate Flowrate of Scrubber Feed
100.0 Boiler 70.0 58.0 50.0
Flowrate (kg/s)
FIGURE 6.16
y (ppm NH3)
5
150.0
0.0 0.0
1.0 0.0
20
200.0
ð6:12Þ
content has to be reduced. As can be seen from Fig. 6.16, in order to fully recycle the off-gas condensate the ammonia load to be removed is 12 1026 kg/s. The composition of the intercepted off-gas condensate is the slope of the intercepted stream, which is 11.6 ppm. Therefore, the interception task is to reduce the composition of ammonia in the off-gas condensate from 14.0 ppm to 11.6 ppm. The same result can be obtained algebraically as follows:
i:e:; 10 2 0 Flowrate of Recycled Gas Condensate 5 14 2 0 5:8 Hence, flowrate of recycled gas condensate 5 4.1 kg/s and the flowrate of fresh water is 0.9 kg/s (5.80.84.1). Therefore, direct recycle can reduce the freshwater consumption (and consequently the influent to biotreatment) by 5.1 kg/s. This is exactly the same target identified from the material-recycle pinch analysis as described by Eq. (6.11d). With the details of the direct-recycle strategies determined, it is possible to determine the implementation cost. The primary cost of direct recycling is pumping and piping. Assuming that the TAC for pumping and piping $80 is mUyear and assuming that the total length of piping is 600 m, the TAC for pumping and piping is $48,000/year. Since not all the off-gas condensate has been recycled, there is no need to consider recycle from any other source (since they have longer fresh arms). Therefore, there are no more direct-recycle opportunities and we have exhausted the no/low-cost strategies. Next, we move to adding new units and we consider interception. Before screening interception devices, it is necessary to determine the interception task. For all of the off-gas condensate to be recycled to the boiler, its ammonia
Intercepting the off-gas condensate.
Load removed from off-gas condensate 5 flowrate of off-gas condensate ðsupply composition 2 target compositionÞ
ð6:13aÞ
i.e., 12 1026 5 5:0 ð14:0 target compositionÞ Therefore, Target ðinterceptedÞ composition of ammonia in off-gas condensate 5 11:6 ppm
ð6:13bÞ
The same result may also be obtained through the sourcesink mapping diagram as shown in Fig. 6.17. Alternatively, it may be calculated as follows: ð5:0 kg=sÞyt ppm NH3 1 0 5 10:0 ppm NH3 5:8 kg=s i.e., yt 5 11:6 ppm:
ð6:13Þ
In order to synthesize an optimal MEN for intercepting the off-gas condensate, we construct the pinch diagram as shown in Fig. 6.18. Since the three MSAs lie completely to the left of the rich stream, they are all thermodynamically feasible. Hence, we choose the one with the least
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6. COMBINING MASS-INTEGRATION STRATEGIES
EXAMPLE 6.1 7.0
7.0
Flowrate of a Source/Feed to a Sink (kg/s)
Off-Gas Condensate
6.2
Scrubber
6.0
cost ($/kg NH3 removed), namely, the resin. The annual operating cost for removing ammonia using the resin is:
6.0
5.8
5 New Feed to Scrubber
5.0
5.0 4.1
4.0
kg Liquid kg NH3 ð14:0 1026 2 11:6 1026 Þ kg Liquid s
91
4.0 Intercepted Off-Gas Condensate
3.0
3.0
2.0
2.0
1.0
1.0
0.8
Distillation Bottoms 0.0
0.0 0
5
11.6
10
14
15
y (ppm NH3)
FIGURE 6.17
Determination of interception task for the off-
gas condensate. 12.0 Mass Exchanged (10–6 kg NH3/s)
(cont’d)
10.0 8.0 6.0
Off-Gas Condensate
4.0 2.0 0.0 0
3
6S1
9
12
0.1S2
2.3
4.4
6.6
145S3
295
445
595
295
595
895
1415 y 8.7 x1 745
x2
1195 1495
x3
FIGURE 6.18 The mass-exchange pinch diagram for the AN case study.
$ s 5 $34;437=year: T3600 8760 kg NH3 year
ð6:14Þ
The annualized fixed cost of the adsorption column along with its ancillary equipment (e.g., regeneration, materials handling, etc.) is estimated to be about $85,000/year. Therefore, the TAC for the interception system is $119,437/year. As a result of minor process modification, segregation, interception, and recycle, we have eliminated the use of fresh water in the scrubber, leading to a reduction in freshwater consumption (and influent to biotreatment) by 6.0 kg/s. Therefore, the target for segregation, interception, and recycle has been realized (Fig. 6.19). Next, we focus our attention on sink/generator manipulation to remove freshwater consumption in the steam-jet ejector. The challenge here is to alter the design and/or operation of the boiler, the ejector, or the distillation column to reduce or eliminate the use of steam. Several solutions may be proposed including: • Replacing of the steam-jet ejector with a vacuum pump. The distillation operation will not be affected. The operating cost of the ejector and the vacuum pump are comparable. However, a capital investment of $75,000 is needed to purchase the pump. For a five-year linear depreciation with negligible salvage value, the annualized fixed cost of the pump is $15,000/year. • Operating the column under atmospheric pressure, thereby eliminating the need for the vacuum pump. Here a simulation study is needed to examine the effect of pressure change.
Current Discharge 12.0 kg/s
Minor process modification
11.8 kg/s
Current Fresh Direct Recycle
7.2 kg/s 7.0 kg/s
6.9 kg/s 6.0 kg/s
Minor process modification Direct Recycle
Interception How?
4.8 kg/s
2.1 kg/s Interception 1.2 kg/s
Target Discharge
How?
0.0 kg/s Target Fresh
FIGURE 6.19
Reduction in water usage and discharge (expressed as kg H2O/s) after minor process modification, direct recycle,
and interception.
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6.2 PROCESS REPRESENTATION FROM A MASS-INTEGRATION SPECIES PERSPECTIVE
EXAMPLE 6.1
Tail Gases to Disposal
Reactor
Scrubber
C3H6
AN to Sales 23 ppm NH3 5.1 kg AN/s 5.8 kg H2O/s
Off-Gas Condensate 14 ppm NH3 0.4 kg AN/s 4.6 kg H2O/s
To Regeneration and Recycle 10 ppm NH3 0.5 kg AN/s 5.3 kg H2O/s
Vacuum Pump
21 ppm NH3 4.7 kg AN/s 1.0 kg H2O/s
1 ppm NH3 4.6 kg AN/s 0.3 kg H2O/s
Distillation Column
Decanter
Adsorption Column
Resin
Aqueous Layer 25 ppm NH3 0.4 kg AN/s 4.8 kg H2O/s
11.6 ppm NH3 0.4 kg AN/s 4.6 kg H2O/s
Wastewater to Biotreatment
Bottoms 0 ppm NH3 0.1 kg AN/s 0.7 kg H2O/s
25 ppm NH3 0.4 kg AN/s 4.8 kg H2O/s
• Relaxing the requirement on BFW quality to a few parts per million of ammonia and AN. In this case, recycle and interception techniques can be used to significantly reduce the freshwater feed to the boiler and, consequently, the net wastewater generated. Fig. 6.20 illustrates the revised flowsheet with segregation, interception, recycle, and sink/generator manipulation. As can be seen from the figure, the flowrate of the terminal wastewater stream has been reduced to 4.8 kg H2O/s. This is exactly the same target predicted in Fig. 6.5B. In order to refine the material balance throughout the plant, a simulation study is needed. This is an effective use of simulation in coordination with process synthesis and integration. Fig. 6.20 is a tradeoff diagram (commonly referred to as the Pareto chart) for the cumulative reduction in wastewater and the associated TAC. The decision makers may decide to adopt all the strategies leading to the ultimate target for reducing wastewater flowrate or may decide to implement some of the strategies depending on the economic, technical, environmental, and safety factors (Fig. 6.21). We are now in a position to discuss the merits of the identified solutions. As can be inferred from Fig. 6.20, the following benefits can be achieved: • Acrylonitrile production has increased from 3.9 kg/s to 4.6 kg/s, which corresponds to an 18% yield enhancement for the plant. This production increase
240 Total Annualized Cost, $1000 (year)
NH3
FIGURE 6.20 Optimal solution to the AN case study.
Tail Gases to Disposal
5.0 kg AN/s 5.1 kg H2O/s + Gases
O2
(cont’d)
Unit Replacement (Vacuum Pump)
200
182 167
160 Interception (Addition of Adsorption Column)
120 80
Segregation and Direct Recycle
40
48
0 0
5.1 2 4 6 Reduction in Wastewater Flowrate (kg/s)
7.2
8
FIGURE 6.21 The Pareto diagram for total annualized cost of mass integration strategies versus reduction in wastewater flowrate entering the biotreatment facility. is a result of better allocation of process streams; the essence of mass integration. For a selling value of $0.6/kg of AN, the additional production of 0.7 kg AN/s can provide an annual revenue of $13.3 million/year! • Freshwater usage and influent to biotreatment facility are decreased by 7.2 kg/s. The value of fresh water and the avoidance of treatment cost are additional benefits. • Influent to biotreatment is reduced to 40% of current level. Therefore, the plant production can be expanded 2.5 times the current capacity before the biotreatment facility is debottlenecked again.
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6. COMBINING MASS-INTEGRATION STRATEGIES
EXAMPLE 6.1 Clearly, this is a superior solution to the installation of an additional biotreatment facility. It is instructive to draw some conclusions from this case study and emphasize the design philosophy of mass integration. First, the target for debottlenecking the biotreatment facility was determined ahead of design. Then, systematic tools were used to generate optimal solutions that realized the target. Next, an analysis study was performed to validate or refine the results. This is an efficient approach to understanding the global insights of the process, setting performance targets, realizing these targets, and saving time and effort by focusing on the big picture first and dealing with the details later. This is a fundamentally different approach than using the designer’s subjective decisions to alter the process and check the consequences using detailed analysis. It is also different from using simple end-of-pipe treatment solutions. Instead, the various species are
(cont’d)
optimally allocated throughout the process. Therefore, objectives such as yield enhancement, pollution prevention, and cost savings can be simultaneously addressed. Indeed, pollution prevention (when undertaken with the proper techniques) can be a source of profit for the company, not an economic burden.
1
Algebraically, this composition can be calculated as follows:
ð5:0 kg=sÞð14 ppm NH3 Þ 1 0 5 12:1 ppm NH3 5:8 kg=s 2 Again, algebraically this flowrate can be calculated as follows: Flowrate of recycled off-Gas condensate314 ppm NH3 1010 5:8 kg=s 510 ppm NH3 :
where the numerator represents the ammonia in recycled off-gas condensate, distillation bottoms (none) and fresh water (none). Hence, flowrate of recycled off-gas condensate 5 4.14 kg/s.
6.3 HOMEWORK PROBLEMS
compensate for water losses with the wet cake (0.08 kg water/s) and the shredded tires (0.12 kg water/s). The mixture of filtrate and water makeup is fed to a high-pressure compression station for recycle to the shredding unit. Due to the pyrolysis reactions, 0.08 kg water/s is generated. The plant has two primary sources for wastewater: the decanter (0.20 kg water/s) and the seal pot (0.15 kg/s). The plant has been
6.1. Let us revisit the tire-to-fuel process described in Problem 5.1. Fig. 6.22 is a more detailed flowsheet. Tire shredding is achieved by using high-pressure water jets. The shredded tires are fed to the process while the spent water is filtered. The wet cake collected from the filtration system is forwarded to solid waste handling. The filtrate is mixed with 0.20 kg/s of freshwater makeup to
To Atmosphere
Gaseous Fuel Condenser
0.20 kg/s
Reactor Off-Gases Light Oil
Tires Shredded Tires Water-Jet Shredding
Filtration
Pyrolysis Reactor
Flare
Decanter
Wastewater to Treatment
Separation
Seal Fresh Pot Water 0.15 kg/s Flare Gas
Finishing
Wet Cake (to Solid Waste Handling)
Compression
Fresh Water 0.20 kg/s
FIGURE 6.22 Schematic flowsheet of tire-to-fuel process. SUSTAINABLE DESIGN THROUGH PROCESS INTEGRATION
Wastewater to Treatment
Liquid Fuels
211
6.3 HOMEWORK PROBLEMS
shipping the wastewater for off-site treatment. The cost of wastewater transportation and treatment is $0.01/kg leading to a wastewater treatment cost of approximately $110,000/year. The plant wishes to stop off-site treatment of wastewater to avoid cost of off-site treatment ($110,000/year) and alleviate legal liability concerns in case of transportation accidents or inadequate treatment of the wastewater. The objective of this problem is to eliminate or reduce to the extent feasible off-site wastewater treatment. For capital budget authorization, the plant has the following economic criterion: Payback period 5
Fixed capital investment Annual savings # 3 years
ð6:15Þ
where Annual Savings 5 Annual avoided cost of off-Site treatment 2 Annual operating cost of on-site system In addition to the information provided by Problem 3.1, the following data are available: Economic Data • Fixed cost of extraction system associated with S2, $ 5 120,000 (Flowrate of wastewater, kg/s)0.58 • Fixed cost of adsorption system associated with S3, $ 5 790,000 (Flowrate of wastewater, kg/s)0.70 • Fixed cost of stripping system associated with S4, $ 5 270,000 (Flowrate of wastewater, kg/s)0.65 • A biotreatment facility that can handle 0.35 kg/s wastewater has a fixed cost of $240,000 and an annual operating cost of $60,000/year. Technical Data Water may be recycled to two sinks; the seal pot and the water-jet compression station. The following constraints on flowrate and composition of the pollutant (heavy organic) should be satisfied: Seal Pot • 0:10 # Flowrate of feed water ðkg=sÞ # 0:20 • 0 #Pollutant content of feed water ðppmwÞ #500
Makeup to Water-Jet Compression Station • 0:18 # Flowrate of makeup water ðkg=sÞ # 0:20 0#Pollutant content of makeup water • ðppmwÞ#50 6.2. Consider the magnetic tape manufacturing process previously dexscribed by problem 4.7 and shown by Fig. 6.23. In this process (Dunn et al., 1995), several binders used in coating are dissolved in 0.09 kg/s of organic solvent. The mixture is additionally mixed and suspended with magnetic pigments, dispersants, and lubricants. A base file is coated with the suspension. Nitrogen gas is used for blanketing and to aid in evaporation of the solvent. As a result of thermal decomposition in the coating chamber, 0.011 kg/s of solvent are decomposed into other organic species. The decomposed organics are separated from the exhaust gas in a membrane unit. The reject stream leaving the membrane unit has a flowrate of 3.0 kg/s and is primarily composed of nitrogen, which contains 1.9 wt./wt.% of the organic solvent. The coated film is passed to a dryer where nitrogen gas is employed to evaporate the remaining solvent. The exhaust gas leaving the dryer has a flowrate of 5.5 kg/s and contains 0.4 wt./wt.% solvent. The two exhaust gases are mixed and disposed of. In order to recover the solvent from the exhaust gases, three are three candidate MSAs. The equilibrium data for the transfer of the organic solvent to the jth lean stream is given by y 5 mjxj where the values of mj are given in Table 6.4. A minimum allowable composition difference of 0.001(kg organic solvent)/(kg MSA) is to be used. The annualized fixed cost of a mass exchanger, $/year, may be approximated by 18,000 (gas flowrate, kg/s)0.65. The value of the recovered solvent is $0.80/kg of organic solvent. In addition to the environmental problem, there is also an economic incentive to recover the solvent from the exhaust gases. The value
Gaseous Emission GTotal = 8.5 kg/s 0.93% Solvent
Decomposed Organics 0.011 kg/s Magnetic Pigments
Solvent 0.09 kg/s Resin Premixing
Dispersants and Lubricants
Slurry Premixing
G1 = 3.0 kg/s 1.90% Solvent
G2 = 5.5 kg/s 0.40% Solvent
N2 Dispersion
Coating
Binders Resins
Additives
Base Film
FIGURE 6.23 Schematic representation of a magnetic tape manufacturing process (Dunn et al., 1995).
N2 Drying Dry Tape Product
SUSTAINABLE DESIGN THROUGH PROCESS INTEGRATION
212
6. COMBINING MASS-INTEGRATION STRATEGIES
of 0.09 kg/s of solvent is approximately $2.3 million/year. The objective of this problem is to develop a minimum-cost solution that minimizes the usage of fresh solvent in the process. The solution strategies may include segregation, mixing, recycle and interception. To assess the economic potential of the solution, evaluate the payback period for your solution. 6.3. A process manufactures 42,000 kg/h of finished wood panels (El-Halwagi, 2017). The process as shown by Fig. 6.24 involves the following steps: 1. Log Processing (Green-End Operations): 60,000 kg/h on an oven-dry (OD) basis of hauled logs are first sprayed with 5000 kg/h of warm water to soften
the logs (and to deice in cold areas). The softened logs are sorted, debarked, and flaked. The “green” strands leaving the flakers are sorted via screens, then forwarded to the drying system. The wood processing residuals (e.g., bark, offspec strands/chips, and sawdust) are burned in specially designed furnaces to utilize their heating value. 2. Strand Drying: Rotary dryers using 100,000 kg/h of hot air are used to reduce the moisture content of the green strands to about 4%. The high temperature causes the vaporization and/or chemical conversion of the wood constituents into volatile organic compounds (VOCs) that include hazardous air pollutants
TABLE 6.4 Data for the MSAs Stream
Upper bound on flowrate LCj (kg/s)
Supply composition (mass fraction) xjs
Target composition (mass fraction) xtj
mj
εj mass fraction
Cj $/kg MSA
S1
N
0.014
0.040
0.4
0.001
0.002
S2
N
0.020
0.080
1.5
0.001
0.001
S3
N
0.001
0.010
0.1
0.001
0.002
Water 15,000 kg/h Particulate-Free Dryer Off-Gas
Quench WESP
WESP Wastewater 14,000 kg/h Dryer Off-Gas Warm Water 5000 kg/h Logs 60,000 kg/h
Debarked Wood Log Sorting and Debarking
Log Softening
Wastewater
Residual Wood
Green Strands
Settling WESP Sludge 500 kg/h (40% water)
Phenol
Resin MDI Binders Formation and Wax Hot Phenol Air Formaldehyde Resin
Flaking
Off-Spec Strands and Sawdust
Formaldehyde 220 kg/h
Dried Strands
Mixing Blending
Forming
Mat Trimming Mat Trimmings
Hot Pressing
Engineered Wood 42,000 kg/h
FIGURE 6.24 A block flow diagram for the engineered wood manufacturing process (El-Halwagi, 2017).
SUSTAINABLE DESIGN THROUGH PROCESS INTEGRATION
Press Fumes
213
REFERENCES
(HAPs) such as methanol, formaldehyde, and acetaldehyde. The flowrate of the released HAPs depends on the drying temperature as follows: Formaldehyde released from the dryers ðkg=hÞ 513:67 1022 T 76:85 753 #T ðKÞ#873 ð3:121aÞ Acetaldehyde released from the dryers ðkg=hÞ 5 11:17 1022 T 69:48 753# TðKÞ# 873 ð3:121bÞ Methanol released from the dryers ðkg=hÞ 510:51 1022 T 67:32 753#TðKÞ#873
ð3:121cÞ
where T is the inlet drying temperature that is allowed is vary between 753 and 873 K. The hot gases leaving the dryers are fed to a wet electrostatic precipitator (WESP) to remove the particulate matters. Water is fed to the WESP at the rate of 15,000 kg/h to quench the gases, facilitate the capture of fines, and wash the system from condensing tars that can form a fire hazard. Settling tanks are used to separate the aqueous stream leaving the WESP into sludge and wastewater. 3. Resin formation: Phenol formaldehyde resin is manufactured by reacting phenol with formaldehyde. This reaction consumes 220 kg/h of formaldehyde. The resin is mixed with methane diisocyanate (MDI) binders and wax to form an additive solution to be sprayed on the dried strands. 4. Blending: The dried strands are mixed with resin binders and wax to facilitate further
processing and to decrease moisture absorption in the final product. 5. Forming: The resin-treated strands are formed into layers that are oriented at right angles. When the desired number of the layers is deposited, mats of specific dimensions are trimmed. 6. Pressing: Heated presses are used to compact the mat of the cross-directional strands into boards. The fumes from pressing include HAPs such as formaldehyde (5 kg/h), acetaldehyde (1 kg/h), and methanol (4 kg/h). Using mass integration strategies, determine the targets and show the implementation strategies for: 1. Minimizing fresh water usage 2. Minimizing HAP emissions
References Dunn, R.F., El-Halwagi, M.M., Lakin, J., Serageldin, M., 1995. In: Griffith, E.D., Kahn, H., Cousins, M.C. (Eds.), Selection of Organic Solvent Blends for Environmental Compliance in the Coating Industries. Proceedings of the First International Plant Operations and Design Conference, Vol. III. AIChE, New York, pp. 83107. El-Halwagi, M.M., 2017. A return on investment metric for incorporating sustainability in process integration and improvement projects. Clean Technol. Environ. Policy. 19, 611617. El-Halwagi, M.M., 1997. Pollution Prevention Through Process Integration. Academic Press, San Diego. El-Halwagi, M.M., 1999. Sustainable pollution prevention through mass integration. In: Sikdar, S., Diwekar, U. (Eds.), Tools and Methods for Pollution Prevention. Kluwer pub, pp. 233275. El-Halwagi, M.M., Hamad, A.A., Garrison, G.W., 1996. Synthesis of waste interception and allocation networks. AIChE J. 42 (11), 30873101. El-Halwagi, M.M., Spriggs, H.D., 1998. Solve design puzzles with mass integration. Chem. Eng. Prog. 94 (2544), 1998.
SUSTAINABLE DESIGN THROUGH PROCESS INTEGRATION