Process Operability Class Materials Copyright © Thomas Marlin 2013 Operating Window Basic flowsheet Design with Operability FC 1 LC Copyright © Thomas Marlin 2013 The copyright holder provides a royalty-free license for use of this material at non-profit educational institutions

PROCESS OPERABILITY : 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 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 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis 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 feasible

OPERATING WINDOW The variables in the plot can be 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 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.

OPERATING WINDOW Minimum heating valve opening is “hard” Maximum feed 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 Minimum heating valve opening is “hard” Maximum feed valve opening is “hard” Minimum feed valve opening is “soft” (The valve can be fully closed) feasible

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. Note: This shows a range of set points that can be achieved (without disturbances). T A Reactant Solvent Coolant infeasible feasible infeasible A  B -rA = k0 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)

Design Basis Memorandum. Define Oper. Window OPERATING WINDOW Class Workshop: Discuss the operating window for this non-isothermal CSTR. Design Procedure Set goals and design specifications Select process technology Define process structure (sequence) Simulate the flowsheet Design equipment 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. The flowsheet typically involves basic M&E balances, equilibrium and rate processes. It does not consider practical issues for achieving the operation. Equipment design achieves the base case flowsheet (plus other concerns). This sets the “capacity” of the plant.

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 the frame OPERATING WINDOW The frame defines the “size” of the operating window. These are typically physical bounds, equipment operation and stream specifications. Determine the constraints (limitations) that define the frame (boundary) of the window Process variable 2 feasible Process variable 1

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 the frame OPERATING WINDOW Class Workshop: Determine typical constraints that affect the operating window for a distillation tower. FR FV xB xD

OPERATING WINDOW Class Workshop: Distillation Constraints xD FR FV xB 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 the frame OPERATING WINDOW Class Workshop: Distillation Constraints Maximum cooling capacity FR FV xB xD Maximum and minimum liquid and vapor flow rates Product composition Pumping, pipe, valve capacity Maximum and minimum liquid and vapor flow rates Maximum heating Flow pipe, valve capacity Minimum natural circulation to reboiler Product composition

The exchanger exists to cool this stream 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 Size of Oper. Window OPERATING WINDOW The design specification will define a boundary of the operating window. Heat exchanger Q = U AY (T)lm Hot process fluid into shell Cooling water into tubes The exchanger exists to cool this stream What are the “worst case” operating conditions we would use to design the heat exchanger?

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 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. Highest flow rate, Highest temperature Lowest flow rate, Highest temperature Hot process fluid into shell Cooling water into tubes Lowest temperature Greatest fouling, Lowest U How do we determine values?

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 Size of Oper. Window OPERATING WINDOW The design specification will define a boundary of the operating window. 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 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis 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 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 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis 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? Class Workshop: Complete the following table. Small equipment* Large equipment Just satisfies base case Advantages Disadvantages * = small equipment just satisfies base case design point

OPERATING WINDOW Class Workshop: Complete the following table. 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 Size of Oper. Window OPERATING WINDOW Class Workshop: Complete the following table. Small equipment Low capital cost Most efficient at base case Achieve “precise” operation (smaller equipment to adjust) Advantages Cannot achieve higher capacity Cannot compensate for large range of disturbances Cannot achieve fast transition (no overshoot in manipulated variable) Disadvantages

OPERATING WINDOW Class Workshop: Complete the following table. 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 Size of Oper. Window OPERATING WINDOW Class Workshop: Complete the following table. Large equipment Can achieve higher capacity Can compensate for likely range of disturbances Can achieve faster transition (allows overshoot in manipulated variable) Advantages High capital cost Likely lower efficiency at base case and lower production rates Might not achieve “precise” operation at base case Disadvantages

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 Size of Oper. Window OPERATING WINDOW In general, we want a large operating window. Why not design and construct equipment with very large capacities? 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.

OPERATING WINDOW Some designs increase 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 Size of Oper. Window OPERATING WINDOW Some designs increase the operating window Centrifugal pumps - Configurations to increase the operating window Flow rate Head Typical pump head curve Series Parallel Pumps provide “pressure (head)” and “flow”. How do we select the correct option, if needed?

OPERATING WINDOW Some designs that increase 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 Size of Oper. Window OPERATING WINDOW Some designs that increase the operating window Centrifugal pumps - Configurations to increase the operating window Series: This configuration provides higher pressure at (approximately) the same flow rate. Series Parallel: This configuration provides higher flow rate at (approximately) the same pump exit pressure. Parallel

OPERATING WINDOW Some designs that increase 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 Size of Oper. Window OPERATING WINDOW Some designs that increase the operating window 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 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transitions 7. Dynamic Performance 8. Monitoring & diagnosis 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. 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 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 After the frame has been established, we check the internal points. Are there any “donut holes”? Determine whether the process and equipment function correctly everywhere within the window. Process variable 2 feasible Process variable 1

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 Orifice meter heating FC Cold (20C) liquid Velocity increases; Bernoulli says that pressure decreases Any concerns about this design?

Sensors: Principles of the orifice meter Porifice Measure pressure drop                                                                                        Porifice=P1 – P3 pressure Distance 

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 = density A = cross sectional area Relate the pressure drop to the flow rate Bernoulli’s eqn. General meter eqn. Installed orifice meter (requires density measurement) 0 = aver. density C0 = constant for specific meter Installed orifice meter (assuming constant density) Most common flow calculation, does not require density measurement

Sensors: Principles of the orifice meter When an orifice meter is used, the calculations in yellow are performed. Typically, they are not shown on a process drawing. “Measured value” to flow controller K FC Multiply signal by meter constant K Take square root of measurement  Measure pressure difference P liquid cooling

Sensors: Are there limitations to orifices? v = velocity F = volumetric flow rate f = frictional losses = density A = cross sectional area Relate the pressure drop to the flow rate General meter eqn. We assume that the meter coefficient is constant. The flow accuracy is acceptable only for higher values of flow, typically 25-100% of the maximum for an orifice Cmeter Reynolds number

Sensors: Is there a downside to orifices? What is a key disadvantage of the orifice meter? Ploss = P1 – P2 Non-recoverable pressure drop Pressure loss! When cost of pressure increase (P1) by pumping or compression is high, we want to avoid the “non-recoverable” pressure loss. pressure Porifice=P1 – P3 Distance 

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 Orifice meter heating FC Cold (20C) liquid 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 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 FC heating Cold (20C) 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 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 Any concerns about this design? Hint: Describe the condition of the liquid in the bottom of the tower Bubble point What happens when the pressure is reduced? Bottom tray Bottoms product reboiler Centrifugal pump

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 Centrifugal pump Bottoms product Pressure drop due to the velocity increase in the eye of the pump Pressure drop due to flow frictional losses reboiler What happens in the pump?

Basic concept of a centrifugal pump http://www.britannica.com/EBchecked/topic-art/632655/7035/Volute-centrifugal-pump http://www.sprayingequipmentsupply.com/pumps/centrifugal-pumps.html

Basic concept of a centrifugal pump Constant speed Impeller diameter 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 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 Let’s prevent bubbles from forming. Equipment must function correctly within the operating window Centrifugal pump 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 reboiler This liquid head increases the pressure at the inlet to the pump and prevents cavitation. Centrifugal pump Bottoms product 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 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. 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. 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

Fortunately, engineers have lots of relevant experience! 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!

“For well tested process, safety factors can approach 0%” * Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transients 7. Dynamic Performance 8. Monitoring & diagnosis OPERATING WINDOW INDUSTRIAL PRACTICE 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, McGraw-Hill, New York, 1983 (pg 209)

OPERATING WINDOW INDUSTRIAL PRACTICE Key Operability issues 1. Operating window 2. Flexibility/ controllability 3. Reliability 4. Safety & equipment protection 5. Efficiency & profitability 6. Operation during transients 7. Dynamic Performance 8. Monitoring & diagnosis OPERATING WINDOW INDUSTRIAL PRACTICE 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.

OPERABILITY : 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 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