Operation and Design of Diabatic Distillation Processes

Abstract

Diabatisk drift af en destillationskolonne indebærer, at varme udveksles på en eller flere bunde i kolonnen. En udbredt metode til at realisere diabatisk operation er ved brug af indre varmeveksling, hvilket resulterer i den varmeintegrerede destillationskolonne (engelsk forkortelse: HIDiC). Ved at operere forstærkersektionen ved et højere tryk, kan en temperaturdrivkraft opnås, således at en varmeovergang fra forstærkersektionen til afdriversektionen kan realiseres. Dette resulterer i, at den krævede mængde energi, som fjernes fra kondensatoren og tilføres i kedlen, reduceres betydeligt.For destillationskolonner med to produkter, anses HIDiC som et favorabelt alternativ til den konventionelle destillationskolonne (engelsk forkortelse: CDiC). Energibesparelser på op til 83 % er rapporteret for HIDiC’en sammenlignet meden CDiC, mens de rapporterede økonomiske besparelser er op til 40 %. Imidlertid, eksisterer en enklere varmeintegreret destillationskolonnekonfiguration, som udnytter kompression til at opnå en direkte varmeintegration mellem den øverste damp og kedlen. Denne konfiguration kaldes den mekaniske dampgenkompressionskolonne (engelsk forkortelse: MVRC). Energimæssige og økonomiske besparelser af lignende størrelsesorden som for HIDiC’en er rapporteret for MVRC’eren. Derfor er det vigtigt at udvikle værktøjer og metoder til at udvælge den mest favorable destillationskolonnekonfiguration. Bidragene fra dette arbejde kan opdeles i tre dele. Den første del involvereridentificering af den foretrukne destillationskolonnekonfiguration (CDiC, MVRC,or HIDiC) for en given blanding, der skal separeres. Sammenhænge mellem fysiske parametre, designvariable, og simple evalueringsindikatorer undersøges via simuleringersstudier. Simuleringersstudierne omfatter casestudier, hvor forskellige blandinger bliver udført i forskellige destillationskolonnekonfigurationer. De betragtede blandinger er industrielt relevante og deres termodynamiske egenskaber afviger betragteligt fra hinanden. HIDiC’en har vist sig at være den foretrukne konfiguration, hvad angår driftsomkostninger, for blandinger med normal kogepunktsforskelle under 10 kelvin.Den anden del omfatter en undersøgelse af den teknologiske gennemførlighed af en HIDiC. Indflydelsen af valget af måden, hvorpå HIDiC kolonnen arrangeres, på kolonnenkapaciteten undersøges. Dette dækker over undersøgelser af kravet tilbundareal, samt risiko for væskelækage og -oversvømmelse på bundene. Endvidere er evnen til at opnå stabil drift undersøgt ved systematisk at designe et stabiliserende kontrollag efterfulgt af et tilsynsførende kontrollag. Stabil drift, hvad angår kolonne kapacitet og setpunktsporing, demonstreres ved simulering. Den sidste del dækker over de udviklede simuleringsværktøjer og -metoder. En ny destillationskolonnemodel præsenteres i en generisk form, således at alle de betragtede destillationskolonnekonfigurationer kan beskrives inden for de samme modelrammer. De betragtede destillationskolonnekonfigurationer er:• Den konventionelle destillationskolonne (CDiC)• Den mekaniske dampgenkompressionskolonne (MVRC)• Den varmeintegrerede destillationskolonne (HIDiC)• Sekundær refluks og fordampningskolonne (SRVC)På grund af den generiske karakter af modelleringsrammen, kan sammenligningsstudier af destillationskolonnekonfigurationer fortages på systematisk og konsistent vis. For yderligere at forenkle sammenlingingsstudier af destillationkolonnekonfigurationerer en konceptuel designalgoritme formuleret, som leder til en systematisk bestemmelse af designvariablene. Det konceptuelle design af varmeintegrerede destillationkolonnekonfigurationer er udfordrende som følge af det øgede antal af designvariablesammenligning med CDiC’en. Endeligt, er en Matlab-implementering af modellen, samt en database over de betragtede konfigurationer og separationer, etableret.Diabatic operation of a distillation column implies that heat is exchanged in one or more stages in the column. The most common way of realising diabatic operation is by internal heat integration resulting in a heat-integrated distillation column (HIDiC). When operating the rectifying section at a higher pressure, a driving forcefor transferring heat from the rectifying section to the stripping section is achieved. As a result, the condenser and reboiler duties can be significantly reduced. For two-product distillation, the HIDiC is a favourable alternative to the conventional distillation column. Energy savings up to 83% are reported for the HIDiC compared to the CDiC, while the reported economical savings are as high as 40%. However, a simpler heat-integrated distillation column configuration exists, which employs compression in order to obtain a direct heat integration between the top vapour and the reboiler. This configuration is called the mechanical vapour recompression column (MVRC). Energy and economic savings of similar magnitude as the HIDiC are reported for the MVRC. Hence, it is important to develop methods and tools for assisting the selection of the best distillation column configuration.The contributions of this work can be divided in three parts. The first part involves the identification of the preferred distillation column configuration (CDiC,MVRC, or HIDiC) for a given mixture to be separated. Correlations between physicalparameters, distillation column design variables, and preliminary feasibility indicators are investigated through simulations studies. The simulation studies include case studies, where different mixtures are separated in different distillation column configurations. The considered mixtures are industrially relevant and their thermodynamic behaviours vary considerable from one another. The HIDiC was found to be the preferred configuration in terms of operating expenditures for mixtures of normal boiling point differences below 10K.The second part involves the investigation of the technological feasibility of the HIDiC. The impact on the column capacity (required tray area, entrainment flooding,weeping) of different column arrangements of the internal heat transfer is investigated. Furthermore, the ability to achieve stable operation of a concentric HIDiC is investigated by systematically designing a regulatory control layer and a supervisory control layer. Stable operation, in terms of column capacity and set point tracking, is demonstrated by simulation.The final part covers the developed simulation tools and methods. A new distillationcolumn model is presented in a generic form such that all the considered distillation column configurations can be described within the same model framework.The following distillation column configurations are considered:• The conventional distillation column (CDiC)• The mechanical vapour recompression column (MVRC)• The heat-integrated distillation column (HIDiC)• The secondary reflux and vaporisation column (SRVC)The generic nature of the modelling framework is favourable for benchmarking distillation column configurations. To further facilitate benchmarking of distillation column configurations, a conceptual design algorithm was formulated, which systematicallyaddresses the selection of the design variables. The conceptual design of the heat-integrated distillation column configurations is challenging as a result of the increased number of decision variables compared to the CDiC. Finally, themodel is implemented in Matlab and a database of the considered configurations, case studies, pure component properties, and binary interaction parameters is established