Influence of real phase equilibria on the sizing of pressure relief devices

Transcription

1 Influence of real phase equilibria on the sizing of pressure relief devices ACHEMA CONGRESS th June 2015, Frankfurt am Main Stephan Dreisch, Frank Westphal, Monika Christ

2 Motivation / Facts to start with Influence of real phase equilibria on pressure relief devices - Bachelor Thesis by Stephan Dreisch, 2014, in continuation of a study by Christ, 2009 Influence of improper input data on the size of a safety valve for a tempered system Wrong temperatures at sizing conditions can lead to the most decisive deviations for the sizing result Sizing often based on vapour pressure curves of idealized systems Influence of real phase equilibria was not investigated systematically until now 15 th June 2015 consilab Gesellschaft für Anlagensicherheit mbh Slide 2

3 Design case example Scenario: failure of external heating safety valve pressure M max. allowable pressure during relief steam max. allowable working pressure failure of steam control closing pressure working pressure time 15 th June 2015 consilab Gesellschaft für Anlagensicherheit mbh Slide 3

4 Objective Investigation of the influence of phase equilibria data of real mixtures on size of relief areas Development of a practical procedure to consider real phase equilibria for sizing of pressure relief devices Possible quality improvement by considering real phase equilibria for sizing calculations of pressure relief devices 15 th June 2015 consilab Gesellschaft für Anlagensicherheit mbh Slide 4

5 Approach Selection of three binary solvent mixtures with different characteristics and measurement of phase equilibrium data by using a Vapour-Liquid-Equilibrium apparatus (VLE) Transfer of the phase equilibrium data into a simulation software (ChemCAD ) and determination of the phase equilibrium parameters Vent sizing according to ISO : Safety devices for protection against excessive pressure Sizing of safety valves for gas/liquid two phase flow Comparison of the vent sizing results to those based on idealised systems and evaluation of occurring errors 15 th June 2015 consilab Gesellschaft für Anlagensicherheit mbh Slide 5

6 Selected binary systems Acetone/Chlorobenzene Boiling point difference of 76 K at 1013 hpa Miscible in any concentration Large deviation from ideal phase equilibrium behaviour Water/Toluene (azeotrope, nearly immiscible) 2-Propanol/Toluene (azeotrope, miscible) Measurements via VLE-Apparatus at two set pressures (1.5 and 3.0 bar abs ) Sample analysis via HPLC and refractometer 15 th June 2015 consilab Gesellschaft für Anlagensicherheit mbh Slide 6

7 VLE-Apparatus storage tank temperature sensor (evaporator) Cottrell-pump return line: vapour phase return line: liquid phase sampling: liquid phase boiling vessel sampling: vapour phase 15 th June 2015 consilab Gesellschaft für Anlagensicherheit mbh Slide 7

8 mass fraction of acetone in gas phase Results: acetone/chlorobenzene at 1.5 bar 1,0 YX-diagram for an acetone/chlorobenzene system at 1.5 bar abs 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 mass fraction of acetone in liquid phase Measured values ChemCAD-Simulation Idealised 15 th June 2015 consilab Gesellschaft für Anlagensicherheit mbh Slide 8

9 temperature [ C] Results: acetone/chlorobenzene at 1.5 bar 150 Txy-diagram for an acetone/chlorobenzene system at 1.5 bar abs ,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 mass fraction of acetone Measured values - liquid phase Measured values - gas phase ChemCAD - liquid phase ChemCAD - gas phase 15 th June 2015 consilab Gesellschaft für Anlagensicherheit mbh Slide 9

10 Summary of the experimental results Good agreement of measured boiling points of pure substances and simulated values Good correlation of the trends in the YX- und Txy-diagrams comparing the measured and simulated values of the material systems acetone/chlorobenzene and 2-propanol/toluene Most of the measured equilibrium temperatures are higher than the simulated temperatures 15 th June 2015 consilab Gesellschaft für Anlagensicherheit mbh Slide 10

11 Determination of simulation parameters (Binary Interaction Parameters - BIP) Fit of the NRTL parameters (BIP) to measured VLE data using ChemCAD Simulations in ChemCAD using different NRTL parameters at pressure levels 1.5 and 3.0 bar abs : BIP from ChemCAD database BIP-Regression 1 via ChemCAD (minimisation of the absolute errors) BIP-Regression 2 via ChemCAD (minimisation of the relative errors) 15 th June 2015 consilab Gesellschaft für Anlagensicherheit mbh Slide 11

12 temperature [ C] Results: acetone/chlorobenzene at 1.5 bar 150 Txy-diagram for an acetone/chlorobenzene system at 1,5 bar abs comparison of different NRTL-parameters (BIP) ,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 mass fraction of acetone Measured values BIPs from ChemCAD database BIP-Regression 1 (abs.err.) BIP-Regression 2 (rel.err.) 15 th June 2015 consilab Gesellschaft für Anlagensicherheit mbh Slide 12

13 Summary of the simulation results Successful determination of the NRTL parameters using measured VLE data of the investigated systems Good fit of the simulated trends with the trends of the measured values Large differences in equilibrium temperatures (deviations up to 10 K) 15 th June 2015 consilab Gesellschaft für Anlagensicherheit mbh Slide 13

14 Vent sizing calculation (according to ISO ) Design Case: external heating / vapour system Basic data: Vessel Volume: 10 m³ Heating Area: 15 m² Liquid Surface Area: 3.8 m² Steam Temperature: 200 C Heat Transfer Coefficient: 800 W/(m²K) Calculations for set pressures of 1.5 bar abs and 3.0 bar abs Determination of the required relief area for single phase vapour flow two-phase flow Physical properties of pure components from ChemCAD - database 15 th June 2015 consilab Gesellschaft für Anlagensicherheit mbh Slide 14

15 required relief diameter for two phase flow [mm] Results: acetone/chlorobenzene (set pressure: 3.0 bar abs ) 200 Calculated relief diameter for an acetone/chlorobenzene system at 3.0 bar abs two phase flow ,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 mass fraction of acetone in liquid phase Measured values BIP from ChemCAD database BIP-Regression 1 (abs.err.) BIP-Regression 2 (rel.err.) Idealised 15 th June 2015 consilab Gesellschaft für Anlagensicherheit mbh Slide 15

16 Deviation of the sizing results Deviation of the sizing results derived from NRTL models in relation to the results derived from calculations with measured data from VLE apparatus: system flow regime NRTLsimulations idealised min max min acetone/chlorobenzene (set pressure 3.0 bar abs ) two phase -3.2% 11.6% -25.3% vapour -2.7% 12.5% -19.2% Negative values: sizing based on simulated data < sizing based on measured data 15 th June 2015 consilab Gesellschaft für Anlagensicherheit mbh Slide 16

17 Summary Equilibrium temperature and vapour phase composition do have impact on the required relief diameter temperature effect is higher Optimization of NRTL-parameters can lead to deviations of up to 15% Vent sizing with idealised material systems can lead to smaller relief areas not conservative! The lowest temperature at the set pressure should be taken into account to receive conservative relief areas Recommendation: be careful with idealisation of phase equilibria data try to use realistic values 15 th June 2015 consilab Gesellschaft für Anlagensicherheit mbh Slide 17

18 Thank you for your attention. Stephan Dreisch, B. Sc. T +49(0) F +49(0) stephan.dreisch@consilab.de consilab Gesellschaft für Anlagensicherheit mbh Industriepark Höchst, G Frankfurt am Main