Effect of phase-equilibrium uncertainties on the process design of selected C2 and C3 alcohol separation systems: a Monte Carlo approach

Date
2017-03
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH SUMMARY: Process design uncertainties can significantly influence the safety and reliability of separation processes. It is broadly accepted that the process engineer must not only specify the best available thermodynamic model to obtain reasonable results, but also quantify the effects of uncertainties in thermodynamic models and data on the final process design. Phase equilibrium correlations are reported as the most significant source of property uncertainties. Thus, how can one best account for the thermodynamic model parametric input uncertainty and the propagation of said uncertainty through the process simulation model? In this work, the effect of phase equilibrium uncertainties on the process design of the dehydration of C2 and C3 alcohols using extractive and azeotropic distillation was investigated. The extractive distillation of diisopropyl ether (DIPE) and isopropanol (IPA) with 2methoxyethanol as the solvent, as well as the heterogeneous azeotropic distillation of ethanol and water using DIPE as entrainer, were considered. Firstly, a systematic evaluation of thermodynamic models was performed. The objective was to identify the model that offered the closest prediction of experimental data of the underlying system (What is the best model?). The performance of the NRTL activity coefficient model in predicting the phase equilibria of the DIPE / IPA / 2-methoxyethanol extractive distillation system was of a high degree of accuracy. The prediction of the azeotrope temperature and composition were improved, although marginally, with the Hayden O’Connell and Nothnagel equations of state. However, this benefit was not extended to the binary vapour-liquid equilibrium (VLE) correlation ability of the model, thus the NRTL model was used. For the DIPE / ethanol / water azeotropic distillation system, the evaluation process revealed that the NRTL activity coefficient model offered largely excellent results, with a high degree of accuracy apparent in the azeotrope and phase envelope predictions. The inclusion of liquidliquid equilibrium (LLE) data provided a meaningful improvement of the model's ability to predict experimentally measured equilibrium data, confirming the usefulness of the NRTL model for this system. Secondly, a combined computer-based approach of stochastic models and process simulation was used to assess the effect of phase equilibrium uncertainties on the sizing of key process equipment. The Monte Carlo simulation technique generated a set of random input variables that represent the range of parametric uncertainty. For each system a process model was developed and the simulation solved for each unique set of input parameters with Aspen Plus® v8.8. The results were subsequently combined to develop cumulative distribution functions (CDF) for each design output of interest e.g. reboiler duty, heat exchanger surface area or column diameter and thus used to estimate the confidence level of the design. For the DIPE / IPA / 2-methoxyethanol system, it was observed that the extraction column uncertainty was predominantly in the bottom section of the column and was mainly related to the reboil rate. The design confidence could be improved to an acceptable level through a marginal increase of the reboil rate. The investigation further determined that the recovery column uncertainty was also limited to the bottom section of the column and only reboil ratio was of concern. The recovery column condenser and reboiler design confidence were 45% and 82%, but a small increase in duty restored the design confidence to the required levels. It was therefore concluded that the design of the extractive distillation process for the separation of the diisopropyl ether + isopropanol azeotrope with 2-methoxyethanol is acceptable and the identified risk areas can easily be resolved. For the DIPE / ethanol / water system, it was observed that the azeotropic distillation column geometry was not significantly impacted by the phase equilibria uncertainty, but that reboil ratio, bottoms flow rate and condenser surface area were. It was further noted that the dilute component uncertainties were high in the decanter, but did not appear to effect the overall performance of the decanter as the ratio of organic to aqueous liquid phase was high. Lastly, in the top section of the recovery column the key design output variables sensitive to phase equilibrium uncertainty were those related to condenser thermal requirements and that the effect of the phase equilibria uncertainty on the column geometry was negligible. In this work, it was thus shown that a systematic uncertainty quantification process based on a Monte Carlo approach reveals the effect of phase equilibrium uncertainty on the process design of C2 and C3 low molecular weight alcohol separation systems. The approach presented can be used to facilitate decision making in fields related to safety factor selection.
AFRIKAANS OPSOMMING: Proses ontwerp onsekerhede kan die veiligheid en betroubaarheid van skeidingsprosesse aansienlik beïnvloed. Dit word algemeen aanvaar dat die proses ingenieur die beste beskikbare termodinamiese model moet spesifiseer om redelike resultate te verkry. Verder word daar ook van hom 'n kwantitatiewe maatstaf vereis om die onsekerhede in termodinamiese modelle en data op die finale proses ontwerp te verreken. Die korrelasies van fase-ewewig word beskou as die belangrikste bron van toestand onsekerhede. Die vraag is dus hoe om die termodinamiese model parametriese insette onsekerheid en die voortsetting van hierdie onsekerheid deur die proses simulasiemodel die beste te verreken. In hierdie tesis word die effek van fase-ewewig onsekerhede in die proses ontwerp van skeiding stelsels met lae molekulêre gewig alkohol beskou. Die twee sleutel azeotropiese skeidings tegnieke wat uit die literatuur geïdentifiseer is, is ekstraktiewe en azeotropiese distillasie. Die effek van fase-ewewigte onsekerhede oor hierdie stelsels is nog nie deeglik bestudeer nie. Eerstens is daar 'n sistematiese evaluering van termodinamiese modelle uitgevoer. Die doel was om 'n model te identifiseer wat die akkuraatste voorspelling gee van die eksperimentele data wat aangebied word by die oorweging van die fase-ewewigte van die lae molekulêre gewig alkohol stelsels van belang vir hierdie tesis (Wat is die beste model?). Die NRTL aktiwiteitskoëffisiënt model was baie akkuraat in die voorspelling van die fase-ewewigte van die di-isopropiel eter / iso-propanol / 2-metoksie-etanol ekstraktiewe distillasie stelsel. Die voorspelling van die azeotropiese temperatuur en samestelling is effens verbeter met die Hayden O'Connell en Nothnagel toestandsvergelykings. Maar hierdie voordeel is nie uitgebrei word na die binêre VLE korrelasie vermoë van die model nie. Wat die DIPE / etanol / water azeotropiese distillasie stelsels betref, het die evalueringsproses aangetoon dat die voorgestelde NRTL aktiwiteitskoëffisiënt model grootliks uitstekende resultate lewer, met 'n hoë graad van akkuraatheid, soos wat duidelik blyk uit die azeotropiese en fase koevert voorspellings. Die insluiting van VVE data lewer 'n betekenisvolle verbetering van die vermoë van die model om eksperimentele ewewig data te voorspel. Die resultate bevestig die nut van die NRTL model vir lae molekulêre gewig alkohol azeotropiese stelsel modellering. 'n Gekombineerde rekenaargebaseerde benadering deur stogastiese modelle en proses simulasie is gebruik om die effek van fase-ewewig onsekerhede op die grootte bepaling van die belangrikste proses toerusting te evalueer. Die Monte Carlo simulasie tegniek het 'n stel ewekansige inset veranderlikes gegenereer wat die variasie in die parametriese onsekerheid verteenwoordig. Dit is gevolg deur 'n proses simulasie vir elke unieke stel insette parameters met Aspen Plus® v8.8 te voltooi. Die resultate is daarna gekombineer om kumulatiewe verdelingsfunksies (KVF) te ontwikkel vir elke ontwerp uitset van belang bv. opkoker las, hitteruiler oppervlakte of kolom diameter. Die KVF is gebruik is om die betroubaarheidsvlak van die ontwerp te voorspel. By die di-isopropiel eter / isopropanol / 2-metoksie-etanol stelsel is dit opgemerk dat die ekstraksiekolom onsekerheid oorwegend in die onderste gedeelte van die kolom was en hoofsaaklik verband hou met die opkooktempo. Die ontwerp vertroue kan verbeter word tot 'n aanvaarbare vlak deur 'n marginale hoër opkooktempo. Die ondersoek het verder bepaal dat die herwinningskolom onsekerheid ook beperk was tot die onderste deel van die kolom en net die opkook verhouding was ’n probleem. Die herwinningskolom kondenseerder en opkoker ontwerp vertroue het gewissel van 45% tot 82%, maar 'n klein toename in termiese werksverrigting herstel die ontwerp vertroue tot die vereiste vlakke. Daar is dus tot die gevolgtrekking gekom dat die ontwerp van die ekstraksie distillasie proses aanvaarbaar en die geïdentifiseerde risiko areas maklik opgelos kan word vir die skeiding van die di-isopropiel eter + isopropanol azeotrope met 2-methoksie-etanol. By die DIPE / etanol / waterstelsel is dit opgemerk dat die azeotropiese distillasiekolom uitleg nie beduidend beïnvloed is deur die fase-ewewigte onsekerheid nie, maar wel deur die opkook verhouding, bodem vloeitempo en kondenseerder oppervlakte. Dit is verder opgemerk dat die verdunde komponent onsekerhede hoog was in die skeidingsdrom, maar blykbaar nie die algehele prestasie van die skeidingsdrom affekteer het nie, aangesien die verhouding van organiese tot waterfase onsekerheid laag was. Laastens, in die boonste deel van die herwinningskolom was die sleutel ontwerp uitsetveranderlikes sensitief vir fase ewewig onsekerheid wat verband hou met kondenseerder termiese vereistes. Die effek van fase-ewewig onsekerhede op die kolom uitleg was gering. In hierdie tesis is dit dus aangetoon dat 'n sistematiese onsekerheids kwantifiseringsproses wat gebaseer is op 'n Monte Carlo benadering, die effek van fase-ewewig onsekerheid op die proses ontwerp van C2 and C3 lae molekulêre gewig alcohol skeidings stelsels toon. Die benadering wat aangebied is, kan gebruik word om besluitneming te vergemaklik om ontwerpstoelating in die ontwerp van toerusting te kies.
Description
Thesis (MEng)--Stellenbosch University, 2017.
Keywords
Uncertainty, Process design, Chemical equilibrium, Phase rule and equilibrium, Separation of alcohols, Monte Carlo method
Citation