讲义教程说明06-6063heat and power integration

1、Heat and Power IntegrationTasks Requiring Input or Removal of HeatTemperature change (heating, cooling)Sensible heatConstant heat capacityHeat capacity varies with temperaturePhase change (evaporation, condensing, melting, freezing, absorption, stripping)Latent heatSingle componentConstant PressureP

2、ressure drop ponent mixtureDew pointBubble pointHeat of reactionEndothermic, exothermicIsothermal, nonisothermalFCp, Hvap, or HrxnTSensible, const CpSensible, Cp dec with TIsothermal ReactionNonisothermal ReactionLatent, single componentLatent, binary, Hvap H Hvap LDew PointBubble PointArea = QTherm

3、al UtilitiesHeating UtilitiesFired furnaceHot oil (Dowtherm)Condensing steam (typically available at several temperatures (pressures) Tempered waterAirCooling UtilitiesBoiler feed water (generating steam)AirTempered waterCooling tower waterRiver waterChilled waterRefrigerationABCA+B=CB, CBBComponent

4、Boiling PointdegCMolecular WeightCpKcal/kg/CHvapKcal/kgA10018.01.0500B12028.81.0100C6046.81.075Hrxn = 40 Kcal/kg AABCA+B=CB, CBB50801502020130150260RR=2.027625C25C80C60C120C30C90CABCB, CBB50801502020130150260RR=2.027625C25C60C120C30C90C7134568A+B=C80C2StreamTypeTin, CTout, CFCpFHvapHrxnQ,Kcal/min1Ho

5、t Sensible12080208002Hot Reaction8080200020003Hot Latent606029250292504Hot Sensible603013039005Cold Latent12012027600276006Cold Sensible809015015007Cold Sensible25805027508Cold Sensible258080440012010080604020FCp, Hvap, or Hrxn10012345678275044008002000150027600292503900T12010080604020Q1000012345678

6、275044008002000150027600292503900T12010080604020Q100001234567+871508002000150027600292503900THot Composite CurveCold Composite Curve12010080604020Q100001234567+871508002000150027600292503900TIntegratable HeatMin App TempThe “Pinch”Heating Utility RequirementCooling Utility Requirement12010080604020Q

7、100001234567+871508002000150027600292503900TMaximumIntegratable HeatZero ApproachTempMinimum Heating UtilityMinimum Cooling Utility2890028600ABCA+B=CB, CBB5080150202013015026027625C25C80C60C120C30C90CTempera-ture IntervalHot StreamsCold StreamsQnetInitial PassQin = 0Final PassQpinch = 0Qin0289001205

8、-27600-27600130090-1201600-27000190080-9016-1300-283006008022000-26300260060-807, 8-2600-289000603292503502925030-6047, 803502925025-307, 8-650-30028600Before Integration Total Heating Utility 36250 Total Cooling Utility 35950After Integration Total Heating Utility 28900 Total Cooling Utility 28600

9、Maximum Integratable Heat 7350QTHeating UtilityCooling Utility11232111112121211AboveBelowPinchIntegratable HeatSignificance of the PinchNever transfer heat across the pinchFinite Tpinch increases utility demands above minimum (increases operating costs)Breaks the heat integration network design prob

10、lem into two separate partsQTHeating UtilityCooling Utility11232111112121211AboveBelowGenerating Heat Exchanger NetworksBase CaseJust use utilitiesMinimum number of exchangersPossibly minimum capital costMaximum operating costVertical matchingMinimum utility usageHowever, extreme and possibly more e

11、xpensive utilitiesMaximum temperature difference driving forceMinimum total areaVery many small exchangersVery high exchanger capital costOperating and Capital CostsUtility costsLinear with QHeating utility cost generally increases with increasing temperatureCooling utility cost generally increases

12、with decreasing temperatureCapital costsC = Fp Fm a A b (b 1.0)Q = U A TLM (countercurrent)Q = U A Ft TLM (other than perfectly countercurrent; U-tube, multipass, etc.)TLM = (TA - TB) / ln(TA/TB) 1/U = 1/Uh + xw/k + 1/UcTypical One-side Heat Transfer CoefficientsBTU/hr/ft2/FMaterialUBoiling/condensi

13、ng water600Boiling/condensing organic400Liquid water400Liquid organic200Liquid brine100Vapors50Solids (not in exchangers)20Slightly different shell side or tube sideFor each Network Design ProblemMinimum number of exchangers possible is Nh + Nc + NU NNWRules for reducing number of exchangersLoop bre

14、akingStream splittingSuperstructure optimization via mathematical (MINLP) programmingToo many simplifying assumptions such as minimum total area desirable goal, exchanger cost linear with area, constant U, same materials of construction, etc.Some commercial tools (e.g., ASPEN Energy Analyzer)No simp

15、le design algorithm with guaranteed economic and engineering optimalityTrial and Error Heat Exchanger Network DesignStart at the pinch and consider heat matches separately above and below pinch in two passes (differences in second pass are shown in parentheses in what follows)Above (below) the pinch

16、, total QHotStreams QColdStreams (total QColdStreams QHotStreams) and only heating (cooling) utility is absolutely requiredChoose a hot (cold) stream just above (below) the pinch, or if the hot (cold) stream crosses the pinch choose only the portion above (below) the pinch to processQT11232111112121

17、211FCp, Hvap, or HrxnTPinchCold StreamsHot StreamsPropose a match against some cold (hot) stream colder (hotter) in temperatureAttempt to meet the entire hot (cold) stream requirement with one single matchConsider not matching across the pinch (or a utility penalty will result)Between any two stream

18、s, generally two matches are possible, one with a greater average temperature difference (will result in a smaller exchanger) and another with a lesser average temperature difference (will result in a larger exchanger)FCp, Hvap, or HrxnTQHQC, ovlp, sm TQHQC, ovlp, lg TQHQC, no ovlp, sm TQHQC, no ovl

19、p, lg TQHQC, ovlp, lg TMin Apch TMin Apch TQHQC, no ovlp, splitIf either end of the match involves a close approach temperature, calculate an approach temperature that optimizes utility saved vs. exchanger cost given actual pressures, materials of construction, and overall heat transfer coefficient

20、for the specific stream pairA five-year payback implies that as the approach temperature gets smaller, a decrease in $1/yr in utility cost (hot and cold) is justified by an incremental increase of no more than $5 in exchanger capitalSelect a match and then continue to work with any residual segments

21、 of either the hot or cold streamAt the beginning of the procedure while working with hot (cold) streams just above (below) the pinch, the number of match opportunities that do not involve transferring heat across the pinch is limitedAs hot (cold) streams farther from the pinch are processed, the nu

22、mber of match possibilities is increasedLarger temperature difference choices result in smaller exchangers, but the remaining unintegrated cold (hot) segments will be satisfied with hotter (colder) and possibly more expensive utilitiesSmaller temperature difference choices result in larger exchanger

23、s, but the remaining cold (hot) segments will be cooler (hotter) and may be satisfied with cooler (hotter) and possibly less expensive utilitiesIf a stream requires several matches to satisfy its heat load consider designs that either involve consecutive exchangers, or split the stream into parallel

24、 streams resulting in parallel exchangersContinue process until all of the hot (cold) streams above (below) the pinch have been integrated.Assign all remaining cold (hot) streams above (below) the pinch to heating (cooling) utilitiesConsider reducing the number of matches (fewer small exchangers)Avo

25、id complex networks (difficult to control)Favor matches between streams physically close together (minimize piping costs)Favor matches between streams close in flowsheet (disturbances may partly compensate)Some matches may be prohibited e.g., over safety concerns if the heat transfer device should l

26、eakEvaluate the network and then repeat procedure making different match choicesQTHeating UtilityCoolingUtilities11111111111.4.611Exchanger Ends With Approach Temperature ConsiderationsSplit StreamHeating UtilityGreater than MinimumCooling UtilityGreater than MinimumCross PinchHeat TransferMultiple

27、SegmentsBetween Stream PairsRelatively Simple NetworkResulting Network may Use more than the Minimum Utility RequirementHeat was transferred across the pinch (for convenience or simplification)Integration was not selected because utilities were desired to enable start-upIntegration was not selected

28、because utilities were used to be able to reject disturbancesIntegration was not selected because network was too complex (or insufficiently robust)Integration was not selected because it divided streams into too many small segmentsHeat Exchanger Network EvaluationCompare actual amount of heat integrated with maximum integratable heatCompare actual utility cost to no-integrati