Presentation on theme: "NETWORK PINCH ANALYSIS PART II"— Presentation transcript:
1NETWORK PINCH ANALYSIS PART II PROCESS OPTIMIZATIONIntroducing Process Integration for Environmental Control in Engineering CurriculaNETWORK PINCH ANALYSIS PART IICreated by:Ana Carolina HortuaTexas A&M UniversityCollege Station, TX.
2PURPOSEThe purpose of this module is to describe basic concepts and techniques necessary in the application of direct recycle as a strategy for the optimization of a process performance.Direct recycle is considered a low-cost strategy since the companies are not required to invest a large amount of capital into equipment or technologies in order to reduce significantly their fresh resource consumption and/or waste production.
3MODULE STRUCTURE This module is made up of three tiers: Tier I: Basic ConceptsTier II: Application of Direct RecycleConcepts (Case Study)Tier III: Open-Ended Problem
5DEFINITIONS PROCESS INTEGRATION: It is a holistic approach to process design and operation that emphasizes the unity of the process and optimizes its design and operation.Refers to the identification of performance benchmarks ahead of detailed design. In other words, targeting determines how far we can push the process performance without specifying how it may be reached.TARGETING:
6DEFINITIONSRECYCLE:Refers to the utilization of a process stream (e.g. a waste or a low-value stream) in a process unit.SEGREGATION:Refers to avoiding mixing of streams. Segregation of streams with different compositions avoids unnecessary loss of driving force streams. Thus, improving the performance of process units and may allow the streams to be recycle directly into the units.
7DEFINITIONS (F) (T) FRESH LOAD (F): Correspond to the quantity of the targeted species in streams entering the process.WholePlant(F)TERMINAL LOAD (T):Correspond to the load of the targeted species in streams designated as waste streams or point sources of pollution.WholePlant(T)
8DEFINITIONS SOURCE: A stream which contains the targeted species. SINK:An existing process unit/equipment such as reactors, separators, etc., which can accept a source.
9RECYCLE STRATEGYThis is an important strategy in the optimization of a process because through this technique we can determine the target for minimum usage of fresh resource (Fmin), maximum material reuse and minimum discharge to waste (Tmin); as a result of the reutilization of process streams.Total Fresh Load (F)Process StreamsTotal Terminal Load (T)RecoveryNetworkWhole PlantOUTINMaximum Recycle (R)Fmin = F - RTmin = T - R
11DIRECT RECYCLEThis technique is based on the rerouting of process streams directly to process units without the addition of new equipment.Direct recycle must respect the constrains such as feed flowrate, composition, etc., for each process units involved in the recycle analysis in order to keep the same level of product quality as well as the safety process operation.Therefore, in order to fulfill with these constrains the process streams can be segregated, mixed and/or distributed in several ways depending on the specific case.
12DIRECT RECYCLE REPRESENTATION SourcesSegregated SourcesSinksConstrains on feed flowrate and compositionSINK?SINKSINK
13DIRECT RECYCLEThe next design questions should be answered in order to apply direct recycle strategy successfully :Should a stream (source) be segregated and split?To how many fractions? What should be the flowrate of each split?Should streams or splits of streams be mixed? Towhat extent?What should be the optimum feed entering eachsink? What should be its composition?What is the minimum amount of fresh resourceused?What is the minimum discharge of unused processsources?
14DIRECT RECYCLE HOW TO IDENTIFY BOUNDS ON SINKS The previous design questions can be solved by the application of a graphical procedure called Source-Sink Mapping Diagram and Level-Arm Rules.However, before describing the methods mentioned above, it is necessary to identify the bounds (limits) on flowrate and composition for the sink(s) that is/are going to accept the recycle stream(s).HOW TO IDENTIFY BOUNDS ON SINKSThe flow rate and composition bounds on sinks can be determined based on several considerations such as:From physical limitations (e.g. flooding flowrate, weeping flowrate, channeling flowrate, saturation composition, etc.).From manufacturer’s design data.From technical constrains (e.g., to avoid scaling, corrosion, explosion, buildup, etc.)From historical data.
15HOW TO IDENTIFY BOUNDS ON SINKS (continued) Example:By taking into account a process with a number of process sources that can be considered for possible recycle and replacement of the fresh material and/or reduction of waste discharge. Each source, i, has a given flowrate, Wi, and a given composition of a targeted species, yi. Available for services is a fresh (external) resource that can be purchased to supplement the use of process sources in sinks. Each sink, j, requires a feed whose flowrate, Ginj, and an inlet composition of a targeted species, zinj, that must satisfy certain bounds.FeedGinjzinjUnit jSource iWiyi
16HOW TO IDENTIFY BOUNDS ON SINKS (continued) As mentioned in the previous problem statement, there are bounds on flowrate and composition entering each sink. In this example, the bounds were set using historical data and they are described as follows :Flow Rate Bounds:GminjGmaxjTimeGminj < Ginj < GmaxjWhere:j=1,2,…,NsinksGminj and Gmaxj are the lower and upper bounds on admissible flowrate to unit jComposition Bounds:zminj < zinj < zmaxjWhere:j=1,2,…,Nsinkszminj and zmaxj are the lower and upper bounds on admissible composition to unit jzmaxjzminjTime
17zinj zinj +1 HOW TO IDENTIFY BOUNDS ON SINKS (continued) Constrain propagation:In some cases, the constrains on a sink j are based on critical constrains for another unit (j+1). Therefore, to determine the constrains for unit j, it is necessary to use a process model that relates the inlets for both sinks.Example:Unknown ConstraintsKnown Constraintszminj+1 < zinj+1 < zmaxj+1zminj < zinj < zmaxjzminj < zinj < zmaxjUnit jUnit j+1zinjzinj +1From process model:zinj = 3zinj+10.03 < zinj < 0.040.09 < zinj < 0.012
18Source-Sink Mapping Diagram In order to determine if process streams can be direct recycle to a specific process unit or sink a graphic technique called Source-Sink Mapping Diagram was developed.This diagram is constructed by plotting the flowrate versus composition for each targeted species. On the source-sink mapping diagram the sources are represented by shaded circles and sinks are represented by unshaded circles.The constrains on flowrate and composition are respectively represented by horizontal and vertical bands and the intersection of this two bands provides a zone of acceptable load and composition for recycle. As shown in Fig. 1.
19Source-Sink Mapping Diagram Figure 1: Source-Sink Mapping DiagramRange of acceptable composition on sink “S”sinksourceSbaCompositioncFlowrateSource “a” can be directly recycle to sink “S”Range of acceptable flowrates on sink “S”El-Halwagi, 1997
20Lever-Arm Rules Flowrate Wa+Wb Wb Wa ya ys yb Composition As we can see from Fig. 1, just source “a” can be directly recycle to the sink (s) but we still have two more sources “b” and “c”, which contain the targeted specie. Therefore, to create a mixed stream that fulfills with the flowrate and composition constrains for sink (s), the sources “a” and “b” will be mixed using the Lever-Arm Rules as follows in Fig. 2:Figure 2: Mixing of sources “a” and “b”FlowrateResultingMixtureSourcebWa+WbSourceaWbWayaysybEl-Halwagi, 1997Composition
21Lever-Arm RulesAs seen in Fig. 2, the result of mixing the source “a” and “b”, which have a flowrate Wa and Wb and a composition ya and yb respectively, is a mixture with a flowrate Wa + Wb and a targeted composition ys. This resulting mixture fulfills with the constrains for sink (s).Applying a material balance for the targeted species around the mixing operation we get the following equation:ys(Wa + Wb) = yaWa + ybWb (1)From equation (1), we obtained:Similarly:Wa yb - ys arm for aWb ys - ya arm for b=Wa arm for aWa + Wb Total arm=Arm for a = yb - ysArm for b = ys - yaTotal Arm = yb - ya
22Lever-Arm Rules Flowrate Wa Wa+Wb Wb Composition ya ys yb Arm for b The lever-arm for the sources and resulting mixture are represented in Fig. 3.Figure 3: Lever-Arm Rules for mixingFlowrateSourceabResultingMixtureWaWa+WbWbCompositionyaysybArm for bArm for aTotal ArmEl-Halwagi, 1997
23Lever-Arm Rules Applications Lever-Arm rules for fresh resource:One effective method to reduce fresh resource consumption is by mixing the fresh resource stream with a process stream. But to determine what should be the appropriate composition of the feed entering the sink (s), the lever-arm rules should be applied as shown in Fig. 4:Figure 4: Lever-Arm Rules for fresh resourceFlowrateSourceaFreshFeed toSink jFresh ArmTotal ArmyfyaZ feed to sinkComposition
24Lever-Arm Rules Applications Based on the equations showed in the previous slide, we can see that the flowrate of fresh resource is minimized when the feed composition (zfeed to sink) entering the sink (s) is maximized. This analysis leads us to the following rule:Sink Composition Rule:When a fresh source is mixed with process sources (s),the composition of the mixture entering the sink shouldbe set to a value that minimizes the fresh arm.Example:To reduce the fresh resource consumption in sink (s), the fresh resource stream is mixed with a process stream “a” to obtain a mixture, which satisfies the composition and flowrate requirements for sink (s). The composition constrains for the sink (s) are zmin < zin < zmax.What should be the composition of the feed entering the sink?
25Lever-Arm Rules Applications Figure 6: Sink Composition RuleSink SSourceaWhat should be the composition of the feed entering the sink? Zmin, Zavg or Zmax????FlowrateFreshZ minZ maxZ avgZ maxFlowrateSourceaFreshMinimumfresh armSink SApplying the sink composition rule, the composition of the inlet feed should be Zmax because with that value we have the shortest fresh arm.
26Lever-Arm Rules Applications When there are two or more process sources that can be recycle to reduce the fresh usage, it is necessary to determine the order in which they should be used.Example:Supposed we have three process sources (a, b and c) that can be mixed with the fresh source. Which source should be recycled first in order to minimize the use of fresh resource?SourceaSourcebSourcecSink S???FreshFlowrateyFZ smaxyaybyc
27Lever-Arm Rules Applications In order to solve the problem previously mentioned, a prioritization rule was developed using the concept of fresh arm as follows:Source Prioritization Rule: To minimize the usage of the fresh resource, recycle of the process sources should be prioritized in order of their fresh arms starting with the source having the shortest fresh arm.According with the source prioritization rule, the source a should be used first until it is completely recycled before using source b.Sourcea (1st)SourceB (2nd)SourceC(3rd)Sink SFreshFlowrateShortest Fresh ArmyFZ smaxyaybyc
28Process Before Recycle Recycle AlternativesUsing the concepts explained in direct recycle strategies, we can set the target performance of a process. However, depending on the process this target may be attain for more than one recycle alternative, as shown in the next example:Example: To minimize the terminal load discharged of the process shown below, two different recycle alternatives could be applied.Alternative 1Process Before RecycleFk,1Tk,112Tk,1Tk,2231Fk,1Fk,2Fk,2Tk,23Alternative 2Note: To generate an effective recycle strategy all the recycle streams should be rerouted to units process that employ fresh resources (In our case: S1 and S3).Tk,1231Fk,1Fk,2Tk,2
30Material Recycle Pinch Diagram Reference: El-Halwagi, M. M., F. Gabriel, and D. Harell, “Rigorous Graphical Targeting for Resource Conservation via Material Recycle/Reuse Networks”, Ind. Eng. Chem. Res., 42, (2003)Material Recycle Pinch Diagram is a method use to determine the target performance of a process as a result of direct recycle without detailing the recycle strategies.Before describing the targeting procedure the following considerations should be taken into account:Composition Constrain for Sink j:zminj < zinj < zmaxj Where: j=1,2,…,NsinksImpurities Entering the Sink j:Where Gj is the feed flowrate entering to the sink jMsinkj = Gj zinjConstrain on Load:0 < Msinkj < Mmaxj Where: j=1,2,…,Nsinks
31Material Recycle Pinch Diagram In order to apply the Material Recycle Pinch Diagram the nextprocedure should be follow:Rank the sinks in ascending order of maximum admissible composition of impurities.zmax1 < zmax2 < …. < zmaxj.... < zmaxNSINKSRank sources in ascending order of impurities composition y1 < y2 < …. < yi.... < yNSINKSPlot the sink composite curve. In order to generate this curve each sink is represented as an arrow by plotting the maximum load of impurities for each sink (Msinkj = Gj zinj ) versus its flowrate, as shown in Fig. 7.
32Material Recycle Pinch Diagram Figure 7: Sink RepresentationLoadMsink,max3S3Msink,max2S2Msink,max1S1G1G3G2FlowrateReference: El-Halwagi, M. M., F. Gabriel, and D. Harell, “Rigorous Graphical Targeting for Resource Conservationvia Material Recycle/Reuse Networks”, Ind. Eng. Chem. Res., 42, (2003)
33Material Recycle Pinch Diagram Sink Composite Curve (Fig. 8) - is the outcome of superposing the sink arrows in ascending order starting with the sink, which has the lowest composition of impurities (zmaxj).Figure 8: Sink Composite curveG1 +G1G3S2S1S3Msink,max1Msink,max2Msink,max3G2 +G1 +G2LoadFlowrateSourceCompositeCurve
34Material Recycle Pinch Diagram Generate the source composition curve by plotting the load of each source (Msourcei = Wj yi ) versus its flowrate. This curve starts with the source that has the least composition and the rest of the sources will be plotted in ascending order using superposition, as shown in Fig.9:Figure 9: Source Composite CurveLoadMsource3M source2Msource1W1W2W3Flowrate
35Material Recycle Pinch Diagram Locating the pinch point. The pinch point is found by placing both composite curves in the same diagram (Fig. 10a), then the source composite stream is moved horizontally until it touches the sink composite stream. The point where both curves unite is called the pinch point (Fig. 10b).Figure 10 a : Sink and Source Composite CurvesLoadSink Composite CurveSourceComposite CurveFlowrate
36Material Recycle Pinch Diagram Figure 10 b : Material Recycle Pinch DiagramLoadMaterial RecyclePinch PointSourceComposite CurveSink Composite CurveFlowrateMinimumFreshMaximumRecycleMinimumWasteReference: El-Halwagi, M. M., F. Gabriel, and D. Harell, “Rigorous Graphical Targeting for Resource Conservation viaMaterial Recycle/Reuse Networks”, Ind. Eng. Chem. Res., 42, (2003)
37Material Recycle Pinch Diagram Identify the targets for Minimum Fresh Usage, Maximum Direct Recycle and Minimum Waste Discharge. Those targets are determined using the Material Recycle Pinch Diagram as follows:The target for Minimum Fresh Usage is the flowrate of sink below, which there are not sources (fig. 10b).The target for Maximum Direct Recycle is the overlapped area region of process and sources (fig. 10b).The target for Minimum Waste Discharge is the flowrate above the sources, which there are not sinks. (fig. 10b)
38Design RulesIn order to attain the optimum target for minimum fresh usage and minimum waste discharge, the following three design rules are required:1. No flowrate should be passed through the pinch.Figure 11: Passing Flow through the Pinch.LoadSink Composite CurveSourceComposite CurveFreshRecycleWasteFlowrateMinimumFreshMinimumWasteReference: El-Halwagi, M. M., F. Gabriel, and D. Harell, “Rigorous Graphical Targeting for Resource Conservation viaMaterial Recycle/Reuse Networks”, Ind. Eng. Chem. Res., 42, (2003)
39Design RulesAs we can see in Fig. 11, the sink stream is not touching the source stream; this fact allows a flowrate (α) to pass through the pinch point, which will cause an increase in the consumption of fresh resource as well as in the waste discharge in the amount equal to the (α). Therefore, it will increase the cost operation twice since we are consuming more resource and producing more waste.No waste should be discharged from sources below thepinch.3. No fresh resource should be used in any sink above the pinch.
40Targeting for Impure Fresh In the case where the fresh resource is not pure, the same targeting procedure explained previously applies. However, the main difference now is that the source composite curve will not be locus on the horizontal axis as was shown before, instead it will be locus on a straight line emanating from the origin. The slope of this line is the composition of impurities in the fresh (yfresh ) Fig.12.Figure 12: Material pinch Diagram when fresh resources is ImpureFlowrateMaterial RecyclePinch PointSink Composite CurveSourceComposite CurveMinimumFreshMaximum RecycleWasteLoadLocusReference: El-Halwagi et al., 2003
41Material Recycle Pinch Diagram Based on Properties Reference: Kazantzi, V. and M. M. El-Halwagi, “Targeting Material Reuse via Property Integration”,Chem. Eng. Prog., 101(8) 28-37(2005)
42Material Recycle Pinch Diagram Based on Properties As we have seen so far, we have just considered the composition and flowrate constrains to evaluate the sink performance; however, there are many process units, specially those that work with solvents, which their performance are based in properties such as viscosity, solubility, density, volatility, etc. Therefore, in order to apply direct recycle as a process optimization strategy, a graphic property integration method was developed to track properties instead of compositions as was study previously.
43Material Recycle Pinch Diagram Based on Properties Before describing the targeting procedure, consider the following process with:A number of sinks Nsinks, which require a feed with a given flowrate Gj and an inlet property, pinj. The feed entering to each sink should satisfy the following constrain:A number of sources Nsources, which have a given flowrate i, and a given property, pi. They can be considered for recycle to reduce fresh usage.A fresh resource whose property value is pfresh, and it can supplement the use of process sources in sinks.Property constrain:pminj < pinj < pmaxj Where: j=1,2,…,Nsinks (1)
44Material Recycle Pinch Diagram Based on Properties In order to develop a procedure that allows us to determine the optimum target performance of the process stated before, the following rules should be taken into account.The resulting property of mixing two or more source streams will be evaluated according to the next equation:F * Ψ ( p ) = ΣFi * Ψ( pi ) (2)iWhere F is the total flowrate of the mixture, which is given by:F = ΣFi (3)And, Ψ( pi ) is the property-mixing operator, which can be evaluated from first principles or estimated through empirical or semi-empirical methods.1. Mixing Rule:
45Material Recycle Pinch Diagram Based on Properties Example: Consider the mixing of two liquid sources whose flowrates are F1 and F2, volumetric flowrates are V1 and V2, and densities are ρ1 and ρ2. Suppose that the volumetric flowrate of the mixture is given by V = V1 + V2 (4).Applying the definition of density and designating the total flowrate of the mixture by F, we obtain:F = F1 + F2 (5) comparing with equation (2)ρ ρ1 ρ2We can define the density-mixing operator as:Ψ( pi ) = 1 (6)ρ i
46Material Recycle Pinch Diagram Based on Properties After the mixing operator is defined, the sink property constrain (eq. 1) can be restated as follows:And considering the special case where the fresh source has a property operator larger than all other streams, the sink constrain is rewritten as:The equations shown above gives us the basis to derive optimality rules for maximum recycle of process to sinks, as explained in the following slide.Ψminj < Ψinj < Ψmaxj (7)Ψfresh < Ψinj < Ψmaxj (8) Where: Ψfresh = Ψ( pfresh) (9)
47Material Recycle Pinch Diagram Based on Properties Reference: Kazantzi, V. and M. M. El-Halwagi, “Targeting Material Reuse via Property Integration”,Chem. Eng. Prog., 101(8) 28-37(2005)2. Sink Optimality Condition:When a fresh source is mixed with a reused material, the inlet property operator to the sink should be assigned to its maximum feasible value, that is described in Fig. 13 using the lever-arm rules:Figure 12: Sink Optimality ConditionFlowrateTo minimize the consumption of fresh resource Ψin = Ψ max.Appling the lever-arm rule:Ffresh = Ψi - Ψ maxjGj Ψi - Ψ freshFfresh = Fresh arm for 1Gj Total arm from i to freshSourceiSink jGjFreshMinimumfresh armΨ freshΨ maxPropertyOperator
48Material Recycle Pinch Diagram Based on Properties Reference: Kazantzi, V. and M. M. El-Halwagi, “Targeting Material Reuse via Property Integration”,Chem. Eng. Prog., 101(8) 28-37(2005)3. Source Prioritization Rule:The use of the process sources should be prioritized starting with the source having the last value of property operator and sequenced in increasing order of the property operator of the sources (Fig.13).Figure 13: Source Prioritization RuleFlowrateSource i +1Sink jSource iGjFreshFresh arm for iFresh arm for i+1Ψ freshΨ maxΨiΨi +1PropertyOperator
49Material Recycle Pinch Diagram Based on Properties Based on the optimality conditions for sources and sinks, thenext graphical targeting procedure is developed:Targeting Procedure:Rank the sinks in ascending order of Ψmaxj,Ψ max1 < Ψ max2 < …. < Ψ maxj.... <Ψ maxNSINKS (10)and calculate the maximum admissible property load (Uj), for each sink using the following equation:Uj= Gj* Ψmaxj (11)Where Gj is the required flowrate for each sink.
50Material Recycle Pinch Diagram Based on Properties Creating a sink composite curve using the required flowrate for each sink (Gj) and the calculated values of the maximum admissible loads (Uj), as shown in Fig.14.Figure 14: Fresh LocusG1G2FlowrateG3U1LoadU1 + U2U1 + U2 + U3SinkCompositeCurveSink3S3S2Sink2Source CompositeCurveSink1Reference: Kazantzi, V. and M. M. El-Halwagi, “Targeting Material Reuse via Property Integration”,Chem. Eng. Prog., 101(8) 28-37(2005)
51Material Recycle Pinch Diagram Based on Properties Evaluating the property operator of fresh (Ψfresh) using equation 9, then a locus of the fresh line is drawn starting from the origin with a slope of Ψfresh Fig.15.Figure 15: Sink Composite Curve Based on Properties.G1G2FlowrateG3U1LoadU1 + U2U1 + U2 + U3SinkCompositeCurveFresh LineReference: Kazantzi, V. and M. M. El-Halwagi, “Targeting Material Reuse via Property Integration”,Chem. Eng. Prog., 101(8) 28-37(2005)
52Material Recycle Pinch Diagram Based on Properties Calculating the value of the property operator for each source (Ψi) and then ranking the sources in ascending order of (Ψi).Ψ 1 <Ψ2 < …. < Ψ3.... <Ψi …. <Ψi NSources (12)In addition, the property load of each source (Mi) is calculated using the next equation.Mi = Fi * Ψ( pi ) (13)Where Fi is the flowrate and Ψ( pi ) is the property operator for each source .
53Material Recycle Pinch Diagram Based on Properties Generating the source composite curve using the flowrate of each source and the calculated values of the property operator Ψi,, as seen in Fig. 16.Figure 16: Sink Composite Curve Based on PropertiesM1 + M2This curve starts with the source, which has the lowest property operator and the rest of the sources will be plotted in ascending order using superposition.Source2LoadSource CompositeCurveM1Source1F1F2FlowrateReference: Kazantzi, V. and M. M. El-Halwagi, “Targeting Material Reuse via Property Integration”,Chem. Eng. Prog., 101(8) 28-37(2005)
54Material Recycle Pinch Diagram Based on Properties Placing both composite curves in the same diagram Fig. 17:Figure 17: Sink and Source Composite CurvesG1G2FlowrateG3U1LoadU1 + U2U1 + U2 + U3SinkCompositeCurveFresh LineSource CompositeReference: Kazantzi, V. and M. M. El-Halwagi, “Targeting Material Reuse via Property Integration”,Chem. Eng. Prog., 101(8) 28-37(2005)
55Material Recycle Pinch Diagram Based on Properties Locating the pinch point. The pinch point is found by placing the source composite curve onto the fresh line and then sliding it to the left until the source composite stream touches the sink composite stream. The point where both curves unite, is called the pinch point (Fig. 18).Figure 18: Pinch DiagramFlowrateLoadProperty-BasedMaterial ReusePinch PointSinkCompositeCurveSource CompositeFresh LineReference: Kazantzi, V. and M. M. El-Halwagi, “Targeting Material Reuse via Property Integration”,Chem. Eng. Prog., 101(8) 28-37(2005)
56Material Recycle Pinch Diagram Based on Properties Identifying the targets for Minimum Fresh Usage, Maximum Direct Recycle and Minimum Waste Discharge. Those targets are determined using the Material Recycle Pinch Diagram, as shown in Fig.19:Figure 19: Pinch Diagram (Targeting)LoadProperty-BasedMaterial ReusePinch PointSource CompositeCurveSinkCompositeCurveFresh LineFlowrateMinimumFresh UsageMaximum RecycleMinimum WasteReference: Kazantzi, V. and M. M. El-Halwagi, “Targeting Material Reuse via Property Integration”,Chem. Eng. Prog., 101(8) 28-37(2005)
57Material Recycle Pinch Diagram Based on Properties Design Rules:The same three design rules explained in the Material RecyclePinch Diagram directly apply to the Material Recycle PinchDiagram Based on Properties, which are as follows:No flowrate should be passed through the pinch.2. No waste should be discharged from sources below the pinch.No fresh resource should be used in any sink above the pinch.Reference: Kazantzi, V. and M. M. El-Halwagi, “Targeting Material Reuse via Property Integration”,Chem. Eng. Prog., 101(8) 28-37(2005)
59Case Study Tapioca Starch Manufacturing Reference: Tia., Thongchai Srinophakun Water-Waste Management of Tapioca Starch Manufacturing Using Optimization Technique. Science Asia, pp 57-67, February (2000)Tapioca Starch ManufacturingTapioca Starch is obtained from the processing of tuberous roots of the manioc or cassava plant. This process can be divided in four stages, which are:Washing and peeling of the roots, rasping them and straining the pulp with addition of water.Rapid removal of the fruit water and its soluble and replacing them with pure water to prevent deterioration of the pulp. This stage includes sedimentation and washing of the starch in tanks or settling tables.The removal of water by draining, centrifuging and drying.Grinding, bolting and other finishing operations.Tapioca starch process is described in detailed in the following flowsheet Fig. 20.
61Case StudyAs seen in Fig. 20, the processes of washing, rasping and grinding do not required the use of fresh water to operate because the water is used to wash the adhering dirt to the roots as they go through the mentioned processes. After that, the slurry is sent to the screening process in order to separate the fiber and other insoluble particles such as protein, fat, etc. Then the soluble particles are removed with water in the separating process.On the other hand, the discharged water from screening and separating processes contain a high value of starch. Therefore, in order to recover this important component those streams are sent to the ‘final screen’. The stream resulting from the final screen process is rich with starch and is returned to the grinding process while the fiber is sent to the press process to recover water. Fresh pulp is sold as a cattle food.Taking into account the information given for tapioca starch manufacturing process, determine the recycle strategies that minimize the usage of fresh water.
62Case StudySolution:In order to solve the previously stated problem the next procedureis followed:Identifying the sinks/process units which consume fresh water, and as we can see in Fig. 20 they are: Screen 1, Screen 2, Separator 1, Screen 3 and Separator 2.Identifying the bounds in composition and flowrate for each one of the process units mentioned above. To determine these bounds a mathematical model was developed using the following assumptions:There are two main components in the manioc roots: starchand water.Only one single contaminant is considered: COD (ChemicalOxygen Demand)The composition of starch loss in drying is the same as theydry starch product.
63Composition Constrains (Based on COD mg/l) Case StudyAs a result of the application of the mathematical model the following process constrains were determined (Table 1):Table 1: Process ConstrainsProcess UnitsFlowrate Constrains(Tons/hr)Composition Constrains (Based on COD mg/l)Screen 115≤ Flowrate of Feed to Screen 1 ≤ 200≤ Composition of COD in Feed to Screen 1 ≤ 5Screen 225≤ Flowrate of Feed to Screen 2 ≤ 330≤ Composition of COD in Feed to Screen 2 ≤ 4Separator 180≤ Flowrate of Feed to Separator 1 ≤ 720≤ Composition of COD in Feed to Separator 1 ≤ 0.00Screen 38≤ Flowrate of Feed to Separator 1 ≤ 100≤ Composition of COD in Feed to Screen 3 ≤ 3.5Separator 242≤ Flowrate of Feed to Separator 2 ≤ 500≤ Composition of COD in Feed to Separator 2 ≤ 0.00
64Case StudySelecting the process streams (sources) that can be recycle to the chosen process units. The criteria used to select the streams is based on the COD value, amount of starch and amount of protein. This criteria is applied to the four wastewater streams that are leaving the process as follows:The stream leaving the washing and rasping units can not bereused in any unit due to its high content of COD.The discharged water from Separator 1 can not be reused inany unit except in washing process since this watercontains high protein.The streams leaving the dewater and Separator 2 units canbe consider for recycle.
65Case Study Generating the Source-Sink Mapping Diagram Fig. 21: Figure 21: Source-Sink Mapping Diagram for the Tapioca Starch Case Study80Separator 1 (S3)72706050Separator 2 (S4)42Flowrate, tons/hr40Separator 1 wastewater 1(R2)3330Screen 2 (S2)2520Screen 1 (S1)1510Screen 3 (S4)8.0Dewater wastewater (R1)5.03.5Concentration of Contaminant, mg/lt
66Case Study Applying Lever-Arm Rules. Flowrate, tons/hr Calculations for Screen 1For S1, R1 has the shortest water fresh arm80Separator 1 (S3)7270Fresh Water Used in S1 = tons/hr60Flowrate to be recycled from R1 to S1 should be:5015 – 3.46 = tons/hrSeparator 2 (S4)42Flowrate, tons/hr40 All of R1 is used in S1 (5 tons/hr)33Screen 2 (S2)30Separator 1 wastewater 1(R2)2520Screen 1 (S1)1510Screen 3 (S4)8.0Dewater wastewater (R1)5.03.5Concentration of Contaminant, mg/lt
67Case Study Applying Lever-Arm Rules. Flowrate, tons/hr Calculations for Screen 1 (continued)For S2, After using all R1, R2 has the next shortest arm80Separator 1 (S3)Applying a water balance:72705* *8 + Fresh Water in S1* = 15*5605.312 tons/hr50Separator 2 (S4)42Fresh water in S1 = 15 – 5 – 5,312 = 4,68 ton/hrFlowrate, tons/hr403330Screen 2 (S2)Separator 1 wastewater 1(R2)2520Screen 1 (S1)1510Screen 3 (S4)8.0Dewater wastewater (R1)5.03.5Concentration of Contaminant, mg/lt
68Case Study Applying Lever Arm Rules. Flowrate, tons/hr Calculations for Screen 280Separator 1 (S3)7270Fresh Water Used in S2 = tons/hr60Flowrate to be recycled from R2 to S2 should be:50Separator 2 (S4)25 – 12.5 = 12.5 tons/hr42Flowrate, tons/hr40 tons/hr of R2 are used in S233Screen 2 (S2)30Separator 1 wastewater 1(R2)2520Screen 1 (S1)1510Screen 3 (S4)8.0Dewater wastewater (R1)5.03.5Concentration of Contaminant, mg/lt
69Case Study Applying Lever Arm Rules. Flowrate, tons/hr Calculations for Screen 380Separator 1 (S3)7270Fresh Water Used in S3 = 4.5 tons/hrFlowrate to be recycled from R2 to S3 should be:608 – 4.5 = 3.5 tons/hr50 3.5 tons/hr of R2 are used in S3Separator 2 (S4) 8.7 tons/hr of R2 will go to waste sincethey can not be used in other sink,42Flowrate, tons/hr403330Screen 2 (S2)Separator 1 wastewater 1(R2)2520Screen 1 (S1)1510Screen 3 (S4)8.0Dewater wastewater (R1)5.03.5Concentration of Contaminant, mg/lt
70Case Study Case Study R1 R2 Fresh Water Waste Applying Lever Arm Rules.For the process units Separator 1 and Separator 2 nothing can be done to reduce their consumption of fresh water since those sinks are not allowed to accept a feed flowrate with any concentration of contaminant.Solution to the minimization of fresh water consumptionR15 tons/hrScreen 1R25.312 tons/hr30 tons/hr12.5 tons/hrScreen 23.5 tons/hrFresh Water12.5 tons/hrScreen 34.68 tons/hr21.7 tons/hr4.5 tons/hrWaste8.7 tons/hrTotal Fresh Water Consumption = = tons/hr
71Case Study Case Study R1 R2 Fresh Water Waste Applying Lever Arm Rules.Alternate Solution:R15 tons/hr3 tons/hrScreen 16.937 tons/hr5.063 tons/hrR230 tons/hr12.5 tons/hrScreen 22 tons/hr12.5 tons/hrFresh Water1.875 tons/hr21.7 tons/hr4.125 tons/hrScreen 3Waste8.7 tons/hrThe same target for minimum fresh usage is reached using a different recycle strategy.
72Inlet Composition (Based on COD mass fraction) Case StudyCase StudyApplying Lever Arm Rules.The same problem is now solve using the Material-Recycle Pinch Diagram. The important data for sources and sinks are summarized in Tables 2 and 3.Table 2 : Sources Data for the Tapioca Starch Case StudySourceFlowrate(Tons/hr)Inlet Composition (Based on COD mass fraction)InletLoad (Tons/hr)R1(Dewater 1)50.0650.325R2(Separator 1)300.082.4
73Maximum Inlet Composition (Based on COD mass fraction) Case StudyCase StudyApplying Lever Arm Rules.Table 3 : Sink Data for the Tapioca Starch Case StudySinksFlowrate(Tons/hr)Maximum Inlet Composition (Based on COD mass fraction)Maximum InletLoadSeparator 172Separator 242Screen 380.0350.28Screen 2250.041.0Screen1150.050.75
74Case Study Case Study Applying Lever Arm Rules. Using the data given in the Tables 2 and 3, the sink and source composite curve are constructed as shown in Figures 22 and 23:Figure 22: Source Composite Curve for the Tapioca Starch Case Study3.02.7252.52.0Separator 1Load, tons/hr1.5SourceCompositeCurve10.50.325DewaterDewater102030354050607080901001201301401501601705Flowrate, tons/hr
75Case Study Case Study Applying Lever Arm Rules. Load, tons/hr Figure 23: Sink Composite Curve for the Tapioca Starch Case Study3.02.52.032.0Load, tons/hrSinkCompositeCurveScreen 11.51.281Screen 20.5Screen 3Separator 2Separator 10.2810203040506070809010012013014015016017042114122147162Flowrate, tons/hr
76Case StudyCase StudyApplying Lever Arm Rules.Next, both curves are placed in the same diagram and the source composite curve is slid horizontally to the right until it touches the sink composite curve as shown in Fig. 24:Figure 24: Material Recycle Pinch Diagram for the Tapioca Starch Case StudyWaste = 8.73.02.5Material RecyclePinch Point2.0Load, tons/hr1.5SinkCompositeCurve1SourceCompositeCurve0.5Fresh Water = 135.7102030405060708090100120130140150160170180Flowrate, tons/hr135.7162170.7
77Case Study Case Study Applying Lever Arm Rules. As demonstrated by applying Direct Recycle Strategy, the maximum consumption of fresh water reduction that can be reached without the use of new equipment is from 162 tons/hr to tons/hr. The same targeted value was reached by using the Material Recycle Pinch Diagram and Direct Recycle Strategy.
78Case Study TIER III Open Ended Problem Applying Lever Arm Rules.TIER IIIOpen Ended Problem
79Case Study Open Ended Problem Applying Lever Arm Rules. Problem Statement:Consider the metal degreasing process shown in Fig.25:Figure 25: Microelectronics Manufacturing FlowsheetReference: Kazantzi, V. and M. M. El-Halwagi, “Targeting Material Reuse via Property Integration”,Chem. Eng. Prog., 101(8) 28-37(2005)
80Case Study Open Ended Problem Applying Lever Arm Rules. In this process, a fresh organic solvent is used in the degreaser and the absorber. A reactive thermal processing and solvent regeneration system is used to decompose the grease and the organic additives and regenerate the solvent from the degreaser.The liquid product of the solvent regeneration system is reused in the degreaser, while the gaseous product is passed through a condenser, an absorber and a flare. The process produces two condensate streams: Condensate I from the solvent regeneration unit and Condensate II from degreaser. The two streams are currently sent to hazardous waste disposal.Since these two streams have many desirable properties that enable their possible use in the process, it is recommended their recycle/reuse to be considered. The absorber and the degreaser are the two process sinks. The two process sources satisfy many properties required for the feed of two sinks. An additional property should be examined; namely Reid Vapor Pressure (RVP), which is important in characterizing the volatility, makeup and regeneration of the solvent.
81Lower Bound on RVP (atm) Upper Bound on RVP (atm) Case StudyOpen Ended ProblemApplying Lever Arm Rules.The mixing rule for vapor pressure (RVP) is giving by the following expression:The pertinent information regarding the sinks in study can be seen in Table 5.Table 5 : Flow Rates and Bounds on Properties of SinksSinkFlowrate(kg/s)Lower Bound on RVP (atm)Upper Bound on RVP (atm)Degreaser5.02.03.0Absorber4.0
82Open Ended Problem Applying Lever Arm Rules. The RVP for Condensate I is a function of the thermal regeneration temperature as follows:RVPCondensate I =Where RVP Condensate I is the RVP of Condensate I in atm and T is the temperature of thermal processing system in K. The acceptable range of this temperature is 430 to 520 K. At present, the thermal processing system operates at 515 K leading to an RVP of 6.0. The data for Condensate I and Condensate II are given in Table 6.
83Open Ended Problem Applying Lever Arm Rules. Table 6 : Properties of Process Sources and FreshSourcesFlowrate (kg/s)RVP (atm)Process Condensate I4.06.0Process Condensate II3.02.5Fresh SolventTo be determined2.0Using Direct Recycle Strategies, identify the target for minimum usage of fresh solvent and minimum waste discharge for this case study.
84END OF TIER III CONGRATULATIONS This is the end of Module 18. Applying Lever Arm Rules.CONGRATULATIONSThis is the end of Module 18.Please submit your report to your professor for grading.
85REFERENCES Applying Lever Arm Rules. El-Halwagi, M.M. Pollution Prevention through ProcessIntegration. Acadamic Press. 1997El-Halwagi, M. M., F. Gabriel, and D. Harell, “Rigorous Graphical Targeting for Resource Conservation via Material Recycle/Reuse Networks”, Ind. Eng. Chem. Res., 42, (2003)Kazantzi, V. and M. M. El-Halwagi, “Targeting Material Reuse via Property Integration”, Chem. Eng. Prog., 101(8) 28-37(2005)Suvit Tia., Thongchai Srinophakun Water-WasteManagement of Tapioca Starch Manufacturing UsingOptimization Technique. Science Asia, pp 57-67, February(2000)