Process Operability Class Materials Operating Window Basic flowsheet
Process Operability Class Materials Operating Window Basic flowsheet Design with Operability LC 1 FC 1 Copyright © Thomas Marlin 2013 The copyright holder provides a royalty-free license for use of this material at non-profit educational institutions
Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis PROCESS OPERABILITY : THE OPERATING WINDOW In this Lesson, we will learn • What is an Operating Window? - Flash Drum, CSTR • What defines the “Frame”? - Distillation • How can we set equipment capacity (the operating window) to achieve desired operation? - Equipment capacity: Heat exchanger, pump - Alternative Equipment: Pump, flash • How do we determine if operation is possible within the window? - Pump, distillation
Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection Define Oper. Window OPERATING WINDOW The range of achievable steady-state operations. This is affected by manipulated and disturbance variables. The limitations can be due to equipment (e. g. , maximum flow), safety, product quality, etc. Flash Drum Example 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis feasible
Key Operability issues Define Oper. Window OPERATING WINDOW 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis The variables in the plot can be • Set points of controlled variables • Disturbance variables The frames (boundaries) of the window can be • “hard” constraints that cannot be violated • “soft” constraints than can be violated at a (usually large) economic penalty Class Workshop: Determine the category for each of the constraints for the flash drum.
Key Operability issues 1. Operating window Define Oper. Window OPERATING WINDOW Minimum heating valve opening is “hard” 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection feasible 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Minimum feed valve opening is “soft” (The valve can be fully closed) Maximum feed valve opening is “hard”
Key Operability issues 1. Operating window 2. Flexibility/ controllability Define Oper. Window OPERATING WINDOW Class Workshop: Discuss the operating window for this non-isothermal CSTR. Note: 3. Reliability 4. Safety & equipment protection Solvent infeasible 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis This shows a range of set points that can be achieved (without disturbances). T feasible A infeasible Reactant Coolant A B -r. A = k 0 e -E/RT CA What do you note about the shape of the operating window?
Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Define Oper. Window OPERATING WINDOW Class Workshop: Discuss the operating window for this non-isothermal CSTR. • We can determine the operating window using modelling (flowsheeting) • If the plant exists, we could determine the operating window empirically (but maybe make off-specification products) • The operating window is not always a polygon • The operating window is not always 2 dimensional (can be much higher dimension) • Operation can occur outside the window during transients (or when assumptions are violated)
Define Oper. Window OPERATING WINDOW Class Workshop: Discuss the operating window for this nonisothermal CSTR. Design Procedure • Set goals and design specifications • Select process technology • Define process structure (sequence) • Simulate the flowsheet The flowsheet typically involves basic M&E balances, equilibrium and rate processes. It does not consider practical issues for achieving the operation. • Design equipment Equipment design achieves the base case flowsheet (plus other concerns). This sets the “capacity” of the plant. The design must define the range of operations (set points and disturbances) to be achieved. We can accept less than full production rate or top efficiency for extreme situations. We must document specifications and range or operations and review with all stakeholders! These “specifications” are in the Design Basis Memorandum.
Key Operability issues 1. Operating window 2. Flexibility/ controllability Define the frame OPERATING WINDOW The frame defines the “size” of the operating window. These are typically physical bounds, equipment operation and stream specifications. 3. Reliability Determine the constraints (limitations) that define the frame (boundary) of the window 4. Safety & equipment protection 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process variable 2 5. Efficiency & profitability feasible Process variable 1
Key Operability issues Define the frame OPERATING WINDOW 1. Operating window 2. Flexibility/ controllability Class Workshop: Determine typical constraints that affect the operating window for a distillation tower. 3. Reliability 4. Safety & equipment protection x. D 5. Efficiency & profitability FR 6. Operation during transitions FV 7. Dynamic Performance 8. Monitoring & diagnosis x. B
Key Operability issues 1. Operating window Define the frame OPERATING WINDOW Class Workshop: Distillation Constraints 2. Flexibility/ controllability Maximum cooling capacity 3. Reliability 4. Safety & equipment protection Maximum and minimum liquid and vapor flow rates x. D Product composition FR 5. Efficiency & profitability Pumping, pipe, valve capacity 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Maximum and minimum liquid and vapor flow rates FV Maximum heating Flow pipe, valve capacity Minimum natural circulation to reboiler x. B Product composition
Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability Size of Oper. Window The design specification will define a boundary of the operating window. Heat exchanger 4. Safety & equipment protection 5. Efficiency & profitability OPERATING WINDOW Q = U AY ( T)lm Hot process fluid into shell Cooling water into tubes The exchanger exists to cool this stream 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis What are the “worst case” operating conditions we would use to design the heat exchanger?
Key Operability issues 1. Operating window 2. Flexibility/ controllability Size of Oper. Window OPERATING WINDOW The design specification will define a boundary of the operating window – The Worst Case gives the largest area for heat exchange. 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability Lowest flow rate, Highest temperature 8. Monitoring & diagnosis Hot process fluid into shell Cooling water into tubes 6. Operation during transitions 7. Dynamic Performance Highest flow rate, Highest temperature Lowest temperature Greatest fouling, Lowest U How do we determine values?
Key Operability issues 1. Operating window 2. Flexibility/ controllability Size of Oper. Window OPERATING WINDOW The design specification will define a boundary of the operating window. 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Consider the flow system. What variables must we determine? What is the “worst case” we would use to design the system, specifically the required pump outlet pressure?
Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection Size of Oper. Window OPERATING WINDOW The design will define a boundary of the operating window - Worst case gives the largest pump. Highest vessel pressure Highest flow, largest friction factor Lowest level (lowest head) P Highest pressure drop 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Highest pressure drop What variables must we determine? - Pipe diameter - by guideline (Liq: 1 m/s, Gas: 30 m/s) - Pump horsepower - from highest flow rate and PP and the lowest suction pressure
Key Operability issues 1. Operating window 2. Flexibility/ controllability Size of Oper. Window OPERATING WINDOW In general, we want a large operating window. Why not always design and construct equipment with very large capacities? 3. Reliability 4. Safety & equipment protection Class Workshop: Complete the following table. Just satisfies base case Small equipment* Large equipment 5. Efficiency & profitability Advantages 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Disadvantages * = small equipment just satisfies base case design point
Key Operability issues 1. Operating window Size of Oper. Window OPERATING WINDOW Class Workshop: Complete the following table. 2. Flexibility/ controllability Small equipment 3. Reliability 4. Safety & equipment protection Low capital cost Advantages Most efficient at base case 5. Efficiency & profitability Achieve “precise” operation (smaller equipment to adjust) 6. Operation during transitions Cannot achieve higher capacity 7. Dynamic Performance 8. Monitoring & diagnosis Disadvantages Cannot compensate for large range of disturbances Cannot achieve fast transition (no overshoot in manipulated variable)
Key Operability issues 1. Operating window Size of Oper. Window OPERATING WINDOW Class Workshop: Complete the following table. 2. Flexibility/ controllability Large equipment 3. Reliability 4. Safety & equipment protection Can achieve higher capacity Can compensate for likely range of disturbances Advantages 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance Can achieve faster transition (allows overshoot in manipulated variable) High capital cost Disadvantages Likely lower efficiency at base case and lower production rates Might not achieve “precise” operation at base case 8. Monitoring & diagnosis
Key Operability issues 1. Operating window 2. Flexibility/ controllability Size of Oper. Window OPERATING WINDOW In general, we want a large operating window. Why not design and construct equipment with very large capacities? 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis So, we design plants that have “just the right” capacity in “the right places”. We have to consider the Boundaries and the Internal Points of the operating window. The following class workshops demonstrate examples of equipment designs that achieve operability with acceptable cost through modest modifications to the process structure.
Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability Size of Oper. Window OPERATING WINDOW Some designs increase the operating window Centrifugal pumps - Configurations to increase the operating window Typical pump head curve Series Head 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions Parallel Flow rate 7. Dynamic Performance 8. Monitoring & diagnosis Pumps provide “pressure (head)” and “flow”. How do we select the correct option, if needed?
Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection Size of Oper. Window Some designs that increase the operating window Centrifugal pumps - Configurations to increase the operating window Series 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis OPERATING WINDOW Parallel Series: This configuration provides higher pressure at (approximately) the same flow rate. Parallel: This configuration provides higher flow rate at (approximately) the same pump exit pressure.
Key Operability issues 1. Operating window Size of Oper. Window OPERATING WINDOW Some designs that increase the operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis The vapor flow rate is usually small. However, in some cases (e. g. , start up) , it is 20 times more that its typical value. What do we do?
Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability Size of Oper. Window OPERATING WINDOW We provide a larger pipe and valve in parallel. The pressure control will adjust the small valve first, then the large valve. 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis The vapor flow rate is usually small. However, in some cases, it is 20 times more that its typical value. What do we do?
Key Operability issues 1. Operating window 2. Flexibility/ controllability “Holes” in Oper. Window OPERATING WINDOW After the frame has been established, we check the internal points. Are there any “donut holes”? 3. Reliability 4. Safety & equipment protection 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Process variable 2 5. Efficiency & profitability feasible Process variable 1 Determine whether the process and equipment function correctly everywhere within the window.
Key Operability issues 1. Operating window 2. Flexibility/ controllability “Holes” in Oper. Window Equipment must function correctly within the operating window heating 3. Reliability 4. Safety & equipment protection OPERATING WINDOW Orifice meter FC Cold (20 C) liquid 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Any concerns about this design? Velocity increases; Bernoulli says that pressure decreases
Sensors: Principles of the orifice meter pressure Porifice=P 1 – P 3 Distance Measure pressure drop
Sensors: Principles of the orifice meter Nice visual display of concept. In practice, pressure difference is measured by a reliable and electronic sensor = Porifice From: Superior Products, Inc. http: //www. orificeplates. com/
Relate the pressure drop to the flow rate v = velocity F = volumetric flow rate f = frictional losses r= density A = cross sectional area Bernoulli’s eqn. General meter eqn. Installed orifice meter (requires density measurement) Installed orifice meter (assuming constant density) 0 = aver. density C 0 = constant for specific meter Most common flow calculation, does not require density measurement
Sensors: Principles of the orifice meter “Measured value” to flow controller Multiply signal by meter constant K Take square root of measurement Measure pressure difference K FC P liquid cooling When an orifice meter is used, the calculations in yellow are performed. Typically, they are not shown on a process drawing.
Sensors: Are there limitations to orifices? v = velocity Relate the pressure drop to the flow rate F = volumetric flow rate f = frictional losses r= density A = cross sectional area General meter eqn. Cmeter We assume that the meter coefficient is constant. The flow accuracy is acceptable only for higher values of flow, typically 25100% of the maximum for an orifice Reynolds number
Sensors: Is there a downside to orifices? What is a key disadvantage of the orifice meter? Ploss = P 1 – P 2 pressure Pressure loss! Porifice=P 1 – P 3 Distance Nonrecoverable pressure drop When cost of pressure increase (P 1) by pumping or compression is high, we want to avoid the “non-recoverable” pressure loss.
Key Operability issues 1. Operating window 2. Flexibility/ controllability “Holes” in Oper. Window Equipment must function correctly within the operating window heating 3. Reliability 4. Safety & equipment protection OPERATING WINDOW Orifice meter FC Cold (20 C) liquid 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis The fluid can partially vaporize. The pressure difference will not reliability indicate the flow rate! Velocity increases; Bernoulli says that pressure decreases
Key Operability issues 1. Operating window 2. Flexibility/ controllability “Holes” in Oper. Window OPERATING WINDOW Equipment must function correctly within the operating window heating 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis FC Cold (20 C) liquid Simple solution, • Locate flow measurement where the pressure is highest and temperature lowest. • Ensure that flashing does not occur - design calc’s
Key Operability issues 1. Operating window 2. Flexibility/ controllability “Holes” in Oper. Window OPERATING WINDOW Equipment must function correctly within the operating window Any concerns about this design? 3. Reliability Hint: Describe the condition of the liquid in the bottom of the tower 4. Safety & equipment protection 5. Efficiency & profitability Bottom tray 6. Operation during transitions 7. Dynamic Performance What happens when the pressure is reduced? Bottoms product Centrifugal pump 8. Monitoring & diagnosis Bubble point reboiler
Key Operability issues 1. Operating window 2. Flexibility/ controllability “Holes” in Oper. Window OPERATING WINDOW Equipment must function correctly within the operating window 3. Reliability Centrifugal pump 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Bottoms product reboiler Pressure drop due to the velocity increase in the eye of the pump Pressure drop due to flow frictional losses What happens in the pump?
Basic concept of a centrifugal pump http: //www. britannica. com/EBchecked/topicart/632655/7035/Volute-centrifugal-pump http: //www. sprayingequipmentsupply. com/pumps/cent rifugal-pumps. html
Basic concept of a centrifugal pump Impeller diameter Constant speed Towler, G. and R. Sinnott (2008) Chemical Engineering Design, Elsevier-Butterworth-Heinemann, page 254
Basic concept of a centrifugal pump http: //hiramada. wordpress. com/2009/07/07/introduction-to-centrifugal-pump-technical-selection/
Key Operability issues 1. Operating window 2. Flexibility/ controllability “Holes” in Oper. Window OPERATING WINDOW Equipment must function correctly within the. Let’s prevent bubbles from operating window forming. 3. Reliability Centrifugal pump 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis Bottoms product reboiler Cavitation: The liquid partially vaporizes. As the pressure increases in the pump, the vapor is subsequently condensed. This collapsing of bubbles (cavitation ) causes noise, vibration and erosion - all of which are bad.
Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis “Holes” in Oper. Window OPERATING WINDOW Equipment must function correctly within the operating window NPSHR: The manufacturer must define the minimum net positive suction head required. The process engineer must design to provide it. NPSHA>NPSHR Centrifugal pump Bottoms product reboiler This liquid head increases the pressure at the inlet to the pump and prevents cavitation. NPSHA
Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance “Holes” in Oper. Window OPERATING WINDOW Equipment must function correctly within the operating window NPSHR: The manufacturer must define the minimum net positive suction head required. The process engineer must design to provide it. How? • Elevate the liquid above the pump (two ways) • Reduce friction losses • Subcool the liquid (careful of added pressure drop) This is issue when liquid is at (near) its bubble point. Give examples when this is the situation in chemical processes. 8. Monitoring & diagnosis From: Woods, D. R. , Process Design and Engineering Practice, Prentice -Hall, 1995
Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis “Holes” in Oper. Window OPERATING WINDOW Equipment must function correctly within the operating window This is issue when liquid is at (near) its bubble point. Give examples when this is the situation in chemical processes. We deal with liquids at their bubble points often, for example, • Distillation/stripper bottoms • Distillation/absorber condensers and OH drums • Flash drums • Concentration by boiling • Vapor compression refrigeration • Reactor cooling by solvent vaporization From: Woods, D. R. , Process Design and Engineering Practice, Prentice -Hall, 1995
Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis OPERATING WINDOW INDUSTRIAL PRACTICE Regrettably, no systematic method is used in practice First, define the range over which the plant must operate. Consider most demanding conditions. Second, solve flowsheet for the limiting cases Third, design equipment to function for each of the limiting cases; may have to change structure. Fourth, ensure that interior is operable. Fifth, add features to achieve other operability features (on list at left), as needed Fortunately, engineers have lots of relevant experience!
Key Operability issues OPERATING WINDOW 1. Operating window INDUSTRIAL PRACTICE 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transients 7. Dynamic Performance 8. Monitoring & diagnosis SAFETY FACTORS: Couldn’t we just design for the base case and multiply every capacity by a safety factor, (1+ X/100) ? (X = 25%, 35%, 50%, …). This is not engineering! Any single factor would be too small for some equipment and too large for others. After applying the proper procedure, a small safety factor can be employed for modelling uncertainty, based on experience. Typical values are 10 -15%. “For well tested process, safety factors can approach 0%” * * Valle-Riestra, J. F. (Dow Chemical Co. ), Project Evaluation in the Process Industries, Mc. Graw-Hill, New York, 1983 (pg 209)
Key Operability issues OPERATING WINDOW 1. Operating window INDUSTRIAL PRACTICE 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transients 7. Dynamic Performance 8. Monitoring & diagnosis SAFETY FACTORS: Some “safety factor” is built into the design procedure. After we have calculated the required pipe diameter, valve diameter, vessel size, motor power etc. , we purchase the closest available size. Since the manufactured sizes are discrete, we select the next largest size. This provides some safety margin.
Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis OPERABILITY : THE OPERATING WINDOW In this Lesson, we will learn • What is an Operating Window? - Flash Drum, CSTR • What defines the “Frame”? - Distillation • How can we set equipment capacity (the operating window) to achieve desired operation? - Equipment capacity: Heat exchanger, pump - Alternative Equipment: Pump, flash • How do we determine if operation is possible within the window? - Pump, distillation
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