Pressure Drop in Heat Exchanger Networks

Transcription

1 Pressure Drop in Heat Exchanger Networks Topics: Heat Exchangers have (considerable) P Heat Transfer and Pressure Drop are related New Design: A new -way Trade-off Retrofit: Bottleneck Problems Basis: Graham T. Polley s work at UMIST, influenced by previous HTFS activities T. Gundersen DIV 01 Pressure Drop in Heat Exchanger Networks The Essence: Increased flow velocity affects both P (will increase) and heat transfer (reduced laminar layer) in exchangers kw 50 W A Q T. Gundersen DIV 0

2 Pressure Drop in Heat Exchanger Networks From basic Industrial Heat Engineering: Nu = f ( Re, Pr ) P = f (q) og h = f (q) For Shell & Tube Exchangers with turbulent Flow on the tube-side: Tube-side: P = K 1 A h.5 Shell-side: P = K A h 5.1 T. Gundersen DIV 0 Pressure Drop in Heat Exchanger Networks 1. New Design: Maximum allowed P i a new Stream Parameter instead of h i? A new -ways Trade-off between Area, Thermal and Mechanical Energy. Retrofit: Limitations in Pumps/ompressors will affect the Solutions we have chosen Bottleneck Problems in General T. Gundersen DIV 04

3 Operational Aspects Typical: Processes are designed & optimized based on given (fixed) data (flowrates, temperatures, pressures, etc.) But: Processes (and Heat Exchanger Networks) are: often operated off design (above/below) subject to disturbances to be started up and shut down The Result: The Process Engineer will over-design before the ontrol Engineer adds new Units for Manipulation T. Gundersen DIV 05 Various Operational Aspects ontrollability Ability of the Process, not the ontrol System Ability to handle operational Variations Flexibility Ability to cope with different Operating onditions Start-Up and Shut-Down Starting up from old onditions is challenging Switchability hange Operation from one ondition to another Environmental Aspects Safety Maintenance T. Gundersen DIV 06

4 Two important Aspects of Operability ontrollability of Processes Ability to handle Short Term Variations Withstand (unwanted) Disturbances Stability Issues Follow (wanted) Set-Point hanges On-line Optimization Flexibility of Processes Ability to handle Long Term Variations Undesirable Variations Fouling or Scaling) in Heat Exchangers Deactivation of atalysts Desirable Variations/hanges New Raw Materials and/or new Products hanges in Production Volume T. Gundersen DIV 07 Similarities/Analogies between Synthesis of Processes and ontrol Systems Levels Structure Parameters Process Production Site Process Equipment hoice of Units Matching Sequences Pressures Temperatures Flowrates ontrol Optimizing Advisory Basic ontrol Manipulators Pairing ontroller Types Gain Integral Time Derivate Time T. Gundersen DIV 08

5 Flexibility in Heat Exchanger Networks mp H H kw kw kw 0 kw T. Gundersen DIV 09 Flexibility in Heat Exchanger Networks mp H H kw kw kw 109 kw T. Gundersen DIV 10

6 Flexibility in Heat Exchanger Networks mp H H kw kw kw 9 kw T. Gundersen DIV 11 Flexibility in Heat Exchanger Networks In Summary: The Network Structure was Flexible (Resilient) for the ases when mp was 1.0 and 1.85 for Stream H1, but did not work when mp was 1.5 even with infinite Heat Transfer Area. The Reason: The Problem is Non-onvex, which happens when: the Pinch point changes there is a change in Mass Flowrates T. Gundersen DIV 1

7 WS-5: Design for Flexibility Q: How to handle Fouling? H 4 4 U 1 (W/m K) Exchanger 1 has fouling above Time (months) Ref.: Kotjabasakis and Linnhoff, Oil & Gas Jl., Sept T. Gundersen DIV WS-5: Fouling in Heat Exchangers 1: The Traditional Approach H 4 4 New Area: 148 m Energy Usage: onstant (the same) T. Gundersen DIV 14

8 WS-5: Fouling in Heat Exchangers : An alternative Solution H H 4 4 New Unit: Heater on Stream Energy Usage: From 1850 to 140 kw T. Gundersen DIV WS-5: Fouling in Heat Exchangers : Use Network Interactions H 4 4 New Area: 10 m Energy Usage: 15% Reduction!! T. Gundersen DIV 16

9 WS-5: Fouling in Heat Exchangers Summary Method/Approach Traditional Approach Alternative Solution Network Interactions Area 148 m New Heater 10 m Energy 0 + 1% - 15% Best Result obtained by using a Systems Approach T. Gundersen DIV 17 Summary of Various Topics Pressure Drop in Heat Exchanger Networks and a new -Way Trade-off (W A Q) ontrollability (Short Term Variations) Flexibility (Long Term Variations) A new Design Strategy for Fouling The importance of Topology is proven Process Integration has a Focus precisely on the Structural Aspects of Process Plants T. Gundersen DIV 18

10 Objectives from Energy ost to Equipment ost to Total Annualized ost and also Operability, including Flexibility ontrollability Switchability Start-up & Shut-down New Operating onditions and finally Environment, including Emissions Reduction Waste Minimization Expansions of PI Expansions of Process Integration T. Gundersen EXP 01 Expansions of PI Scope from Heat Exchanger Networks to Separation Systems, especially Distillation and Evaporation (heat driven) to Reactor Systems to Heat & Power, including Steam & Gas Turbines and Heat Pumps to Utility Systems, including Steam Systems, Furnaces, Refrigeration ycles to Entire Processes to Total Sites to Regions Expansions of Process Integration T. Gundersen EXP 0

11 Plants from ontinuous to Batch and Semi-Batch Projects from New Design to Retrofit to Debottlenecking Thermodynamics from Simple 1st Law onsiderations to Various nd Law Applications Exergy in Distillation and Refrigeration Expansions of PI Expansions of Process Integration T. Gundersen EXP 0 Expansions of PI Methods Pinch based Methodologies from Analogies from Heat Pinch for Heat Recovery and HP in Thermal Energy Systems to Mass Pinch for Mass Transfer / Mass Exchange Systems to Water Pinch for Wastewater Minimization and Distributed Effluent Treatment Systems to Hydrogen Pinch for Hydrogen Management in Oil Refineries Other Schools of Methods was discussed on a previous slide Expansions of Process Integration T. Gundersen EXP 04

12 Stages and Analogies in Methods Heat HeatPinch Mass Pinch Water Pinch Hydrogen Pinch T Heat Pinch Q Graphical Diagrams Representations and oncepts Performance Targets ahead of Design Pinch Decomposition Modeling Data Data Extraction Analysis Design Optimization Expansions of Process Integration T. Gundersen EXP 05 Expansions in Process Integration Strategic Planning Heat Integration Process Integration is much more than Pinch Analysis for Heat Exchanger Networks Pinch Analysis Optimization Methods ombined Methods onceptual Design Detailed Engineering Expansions of Process Integration T. Gundersen EXP 06

13 Wastewater Minimization Topic: Methods: Efficient Use of Wastewater Reuse, Regeneration and Recycling - both Targets and Design Water Pinch (discussed here) Mathematical Programming Ref.: Wang and Smith Wastewater Minimization, hem. Engng. Sci., vol. 49, pp , 1994 T. Gundersen WATER 01 T Wastewater Minimization Graphical Representation mass/heat analogy Q Q = mp T pr,out in,max 1 m = m HO pr,in out,max m T. Gundersen WATER 0

14 Example: Minimizing Wastewater Process ontamination in out Water Flowno. m (kg/hr) (ppm) (ppm) rate (t/h) (The concentrations are maximum values for water in and out) Ex.: ontamination in Process 1: m = m HO = 0,000 kg/h (100-0) 10-6 kg/kg = kg/hr T. Gundersen WATER 0 Minimum Fresh Water without Reuse (Zero inlet and maximum outlet concentrations for water) 0 t/h 50 t/h 7.5 t/h 5 t/h Process 1 Process Process Process t/h Process : m HO = m/ = (0 kg/hr)/(800-0) * 10 6 = 7.5 t/h T. Gundersen WATER 04

15 (ppm) Use of omposite urves m (kg/h) Target: 90 t/h Pinch m T. Gundersen WATER 05 Design that achieves Target 1 4 W T. Gundersen WATER 06

16 Maximize Alternative Designs One water source only Process 4 18 t/h Process c =. Process c = 100 c =. Process 7 t/h Process 64. t/h 5.7 t/h Process 1 c = c = c = 4 c = 800 Process c = 50 c = 100 Process t/h 0 t/h c = 800 Process 4 c = 100 Process 70 t/h 90 t/h 0 t/h 90 t/h T. Gundersen WATER 07 Use of Regeneration Target: 90 t/h Target: 46. t/h 100 Pinch m 100 m T. Gundersen WATER 08

17 Resulting Process Flowsheet 0 t/h =0 W 6. t/h omment: =100 Pr. Reg 1.1 t/h =5 Pr.1 1 Pr. = t/h Pr t/h 0.4 t/h Pr t/h Design is based on targeted flowrate, regeneration placed at pinch and the use of stream grid from HENS T. Gundersen WATER 09 Use of Regeneration and Recycling Target: 46. t/h m Flow: 7.7 t/h 5.7 t/h R m T. Gundersen WATER 10

18 Resulting Process Flowsheet =0 =100 W Pr t/h =417 omments: Reg =5 5.6 t/h 5.7 t/h = t/h Pr. Pr t/h = t/h Pr.4 4 Design is based on targeted flowrate, regeneration (Reg.) in one step only and from a higher concentration. 0 t/h T. Gundersen WATER 11 Summary of Wastewater Example Fresh Regen. ase Description (t/h) (t/h) 1 Fresh Water to all Processes Reuse of Water from Targets Regeneration and Reuse Regeneration and recycling Savings: 1. Reduced fresh water consumption (reduced pumping costs and debottlenecking if there is water shortage). Reduced wastewater to effluent treatment (reduced investment and operating cost for treatment plant) T. Gundersen WATER 1

19 Main Results from Pinch Analysis T Heat Pinch Q,min Q H,min Q The oncept of omposite urves Applicable whenever an Amount has a Quality Heat & Temperature, Mass & oncentration, etc. A Two Step Approach: Targets ahead of Design A fundamental Decomposition at the Pinch Water Pinch Water min m Final Summary T. Gundersen SUM 01 Objectives for using Process Integration Minimize Total Annual ost by optimal Tradeoff between Energy, Equipment and Raw Material Within this trade-off: minimize Energy, improve Raw Material usage and minimize apital ost Increase Production Volume by Debottlenecking Reduce Operating Problems by correct rather than maximum use of Process Integration Increase Plant ontrollability and Flexibility Minimize undesirable Emissions Add to the joint Efforts in the Process Industries and Society for a Sustainable Development Final Summary T. Gundersen SUM 0