pb-sse11 dynsim and dynamic simulation for process relief ... · • dynamic simulation for...
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PB-SSE11 DYNSIM and Dynamic Simulation for Process Relief Calculations
Presented by Tom ScholtenSenior Process EngineerAscent Engineering
Slide 3
Dynamic Simulation Introduction
• Dynamic simulation for determining maximum required relief rate
• Refinery alkylation unit de-isobutanizer revamp example
• Relief rates by steady-state calculation affected flare and header
• Dynamic simulation can provide
– More accurate relief rate calculations
– Insight into the relief event
– Reduce capital costs
• Dynamic simulation will require extra engineering schedule
Slide 4
Dynamic Simulation Presentation
• Review traditional steady-state analysis
• Dynamic simulation advantages
• Alkylation de-isobutanizer column study
• Total power failure case
• Cooling water failure case
• Summary
Slide 5
Traditional Steady-State Analysis
• Unbalanced heat method is our typical steady-state analysis
• Basis is a steady-state simulation
• Modify material and heat flows based on the scenario
• Calculate a heat of vaporization at the top of the tower
• Convert excess energy to material flow that must be relieved
• Gross tower overhead also used to compare to previous calculations
Slide 6
Unbalance Heat Method - A Sample Calculation
NORMAL DESIGN DATA Flow Rate Enthalpy Heat Flow
LB/HR BTU/LB MMBTU/HRIN Feed 379,683 54 20.42
Feed 27,670 137 3.78Feed 2,126 260 0.55Reboiler 28.53Reboiler 26.65
Total: 409,479 79.94
OUT Ovhd distillate 249,400 31 7.85Bottoms 160,079 137 21.88Offgas 0 0.00Water 0 0.00Ovhd Condenser 50.20
Total: 409,479 79.94
Deisobutanizer Relief Load Calculation - Cooling Water Failure
Slide 7
Unbalance Heat Method - A Sample Calculation
RELIEF DATAFlow Rate Enthalpy Heat Flow
LB/HR BTU/LB MMBTU/HR
IN Feed 379,683 54 20.42Feed 27,670 137 3.78Feed 2,126 260 0.55Reboiler 17.35Reboiler 37.93
Total: 409,479 80.03
OUT Ovhd distillate 204,305 199 40.66Bottoms 205,174 133 27.28Offgas 0 0.00Water 0 0.00Ovhd Condenser 0.00
Total: 409,479 67.94
0 Delta 12.09Enthalpy on stage 4 - Liq = 87Enthalpy on stage 4 - Vap = 201
Duty Heat of VapMMBTU/HR BTU/LB LB/HR
Energy Accumulation 12.1 113.7 106,315Ovhd Distillate Material Accumulation 204,305Total Relief Rate 310,620
Deisobutanizer Relief Load Calculation - Cooling Water Failure
Slide 8
Advantages of Dynamic Simulation
• More accurate calculation of maximum required relieving rate
• Better understanding of the relief event
• Opportunity for designing mitigation devices through HIPS
• Possibly reduced capital costs in flare, flare header, and flare laterals
• Multiple towers can show a time sequence for relief that could reduce the maximum flare header flow rate
Slide 9
Requirements of Dynamic Simulation
• Dynamic Simulation requires additional:
– Details of process control scheme
– Project schedule
– Engineering time
– Engineering expense
Slide 10
Rules for Dynamic Simulation
• API 521 allows the use of dynamic simulation for determination of maximum relief loads
• Dynamic simulation can be used wherever steady-state methods are used
• Sensitivity analysis to the dynamic control response is required
• No credit for reduced relief load can be taken for process control response
• The dynamic simulation must match the steady-state simulation
Slide 11
Alkylation De-Isobutanizer Column System
NC4 Recycle
FC
LC
FC
FC
FC
FC
LC
LC
LC
LC
TC
FC
FC
PC
FI
IsobutaneTo Alkys
IsobutaneTo Storage
Alkylate
NC4/IC4From Butamer
Slide 12
Dynsim Simulation Model
Slide 13
Dynsim Simulation Model
Slide 14
Total Power Failure Case
Alkylate effluent feed to the DIB tower stopped Outside butane/isobutane feed to the DIB tower stopped Butane recycle feed to the DIB tower stopped Cooling water to first overhead condenser stopped Cooling water to second overhead condenser stopped Cooling water to iso-butane cooler stopped DIB tower recycle pump stopped Temperature control valve to low temperature reboiler normal operation Steam flow control valve to high temperature reboiler normal operation DIB tower recycle control valve open DIB tower pressure control valve closed DIB overhead accumulator level control valve closed DIB tower bottoms flow control valve closed Iso-butane flow control valve to AlkyA closed Iso-butane flow control valve to Alky B closed
Slide 15
Total Power Failure Case Results
Calculated Maximum Relieving Rates for the DIB tower for the Total Power Failure Case
Maximum Relief Rate Calculation Method DIB Total Power Failure Case Relief Rate lb/hr Dynamic Simulation Method 85,000 Unbalanced Heat & Material Balance Method
486,220
Gross Tower Overhead Method 336,423
The alkylation de-isobutanizer column light end component load is one of the largest in the refinery.
Slide 16
Total Power Failure Case
Slide 17
Total Power Failure Case
Slide 18
Total Power Failure Case
Slide 19
Dynamic Simulation Process Insight
• The dynamic behavior of the low and high pressure steam reboilers resulted in the reduced calculated maximum relieving rate
• Common sump with level control on the recirculating side
• During total power failure the recirculating sump overflows the weir to the low pressure sump
• The thermal driving force is eliminated on the low pressure reboilers
• Loss of the energy from the low pressure reboilers reduces the maximum relieving rate
Slide 20
Cooling Water Failure Case
Alkylate effluent feed to DIB tower continues Outside butane/isobutane feed to DIB tower continues Butane recycle feed to DIB tower continues Cooling water to first overhead condenser stopped Cooling water to second overhead condenser stopped Cooling water to Iso-butane cooler stopped DIB tower recycle pump running Temperature control valve to low temperature reboiler normal operation Steam flow control valve to high temperature reboiler normal operation DIB tower recycle control valve minimum position DIB tower pressure control vlave minimum position DIB tower level control valve minimum position DIB tower bottoms flow control valve normal operation Iso-butane flow control valve to Alky A minimum position Iso-butane flow control valve to Alky B minimum position
Slide 21
Cooling Water Failure Case Results
Calculated Maximum Relieving Rates for the DIB tower for the Cooling Water Failure Case
Maximum Relief Rate Calculation Method DIB Cooling Water Failure Case Relief Rate lb/hr Dynamic Simulation Method 211,700 Unbalanced Heat & Material Balance Method
310,620
Gross Tower Overhead Method 336,423
Slide 22
Dynamic Simulation Process Insight
• The difference in the column bottoms flow rate between the steady-state model and the dynamic model accounts for the reduced maximum required relief rate in the dynamic simulation calculation
• The bottoms flow increases in the dynamic simulation to a value larger than the steady-state flow rate to maintain reboiler heat input
Slide 23
Summary
• Dynamic simulation can be used to calculate maximum required relieving rates for overpressure protection
• Dynamic simulation provides: – Insight into the relief event– More accurate calculation of maximum required relieving rate– Opportunity for designing mitigation devices through HIPS– Possibly reduced capital costs in flare, flare header, and flare laterals
• The dynamic simulation requires additional project schedule, engineering time and engineering expense
• The increased engineering is often easily justified on vessels with large relieving rates
