İki Ardışık Reaksiyon İçeren Tek Ve Çift Reaktif Bölgeli Reaktif Distilasyon Kolonlarının Kontrolü

Abstract

Günden güne artan çevresel ve ekonomik kaygılar, proses yoğunlaştırma ihtiyacını arttırmaktadır. Reaksiyon ve ayırma işlemlerinin aynı ünitede gerçekleşmesine ve dolayısıyla yatırım ve enerji maliyetlerinin düşmesine olanak sağlayan reaktif distilasyon kolonları da proses yoğunlaştırmanın kimya mühendisliği alanındaki en iyi örneklerinden birisidir. Literatüre bakıldığında, reaktif distilasyon kolonuyla ilgili tasarım ve kontrol çalışmalarının büyük çoğunluğunda tek kademeli reaksiyon sistemlerinin incelendiği görülmektedir. Ardışık iki reaksiyon içeren sistemlerin yatışkın hal tasarımının incelendiği çalışmaların sayısı ise çok az olup, bu çalışmaların tamamında tek reaktif bölgeli reaktif distilasyon kolonları kullanılmıştır. Yu ve arkadaşları, yakın tarihli bir çalışmalarında biri varsayımsal diğeri gerçek olmak üzere iki örnek için tek reaktif bölgeli ve çift reaktif bölgeli reaktif distilasyon kolonlarının yatışkın hal tasarımlarını incelemiş ve sonuç olarak çift reaktif bölgeli reaktif distilasyon kolonlarının ekonomik açıdan daha avantajlı olduğunu belirtmişlerdir [1]. Ancak çift reaktif bölgeli reaktif distilasyon kolonlarının tek reaktif bölgeli kolonlara göre daha üstün olup olmadığını söyleyebilmek için yatışkın hal tasarımlarıyla birlikte dinamik davranışları ve kontrol edilebilirlikleri de incelenmelidir. Bu çalışmanın amacı, yatışkın hal tasarımı Yu ve arkadaşları tarafından yapılmış olan, iki kademeli ardışık transesterifikasyon reaksiyonları yoluyla dietil karbonat üretiminin gerçekleştiği tek ve çift reaktif bölgeli reaktif distilasyon kolonlarının kontrol edilebilirliğinin karşılaştırılmasıdır. Bu amaçla, Aspen Dynamics simülasyon programı kullanılarak, iki sıcaklığa dayalı, üç farklı kontrol yapısı tasarlanmıştır. Tüm kontrol yapıları önce tek reaktif bölgeli reaktif distilasyon kolonları için kurulmuş, bu kontrol yapıları besleme akımları arasına bir oran kontrolü ekleyerek çift reaktif bölgeli reaktif distilasyon kolonları için modifiye edilmiştir. Sıcaklık kontrolü için kullanılan rafların seçimi, yatışkın hal duyarlılık analizi ile yapılmıştır. Kontrol yapılarının gürbüzlüğünün incelenmesi için üretim hızına ±%20, reaktan saflıklarına da %10 oranında bozan etkenler verilmiştir. Sonuçlar, tasarım maliyeti bakımından avantajlı olan çift reaktif bölgeli reaktif distilasyon kolonlarının, uygun kontrol ediciler kullanıldığında tek reaktif bölgeli reaktif distilasyon kolonlarına alternatif olacak nitelikte olduğunu göstermiştir.

The increasing environmental and economic concerns enhance the need of process intensification in chemical industry. Implementations of process intensification allow to improve process flexibility, safety, product quality and reduce capital and operating costs. Reactive distillation which combines reaction and separation in a single unit is one of the well-known examples of process intensification. This combination of two different operations in one unit reduces energy and operating costs. Reactive distillation columns offer several advantages to the conventional multi-unit reactor-column systems. The basic advantage of reactive distillation columns is reducing energy consumption. Owing to taking place reaction and separation in same zone, production and removal of products occur simultaneously, which means that conversion increases for reversible reactions. For exothermic reactions, the heat of reaction can be used for vaporization, which reduces the reboiler duty. Besides these, reducing catalyst requirements, avoidance of azeotropes, reducing by-product formation are the other significant advantages of reactive distillation systems. Nevertheless, having fewer valves and non-linear feature of the system make control of reactive distillation columns more complex. The literature review shows that the most of reaction systems for reactive distillation columns belong to two reactant-two product (A+B↔C+D) or two reactant-one product (A+B↔C) classes. Besides these, there are a limited number of studies on two-stage consecutive reaction systems as A+B↔C+D and C+B↔E+D. In all of these studies, reactive distillation columns with single reactive section have been investigated to separate two-stage consecutive reaction systems. Recently, Yu et al. (2014) compared the steady-state designs of reactive distillation columns with single and double reactive sections to separate two-stage consecutive reaction systems. In the case of reactive distillation column with double reactive section, common reactant, which is used in both reactions, is fed into both reactive sections. The other reactant is fed into only upper reactive section. So, it is predicted that while the first stage reaction takes place dominantly in the upper reactive section, the second stage reaction occurs principally in the lower reactive section. Adding one more reactive section provide more degrees of freedom such as number of stages for both reactive sections, number of stages between two reactive sections, splitting ratio of common reactant and the location of feed stages. More degrees of freedom result in not only consolidation of internal mass and energy integration between reaction and separation operations but also a better steady-state performance of reactive distillation column with double reactive section. They studied two different, one generic and one real, two-stage consecutive reaction systems. The real reaction system is the transesterification of dimethyl carbonate with ethanol to form diethyl carbonate. In both reaction systems, the distillation column with double reactive sections is designed to have the same number of stages and the same composition of products with the distillation column with single reactive section, which is designed to have the lowest total annual cost for the desired purity. Yu et al. reported that the reactive distillation columns with double reactive sections are more economic for both of the examples and can be a highly competitive alternative to conventional reactive columns for the separation of the two-stage consecutive reactions. However, in order to indicate that the reactive distillation column with double reactive sections is an alternative to reactive distillation column with single reactive section, dynamic performance of reactive distillation column with double reactive section must be investigated. The purpose of this thesis is to compare the control performance of reactive distillation columns with single and double reactive sections for transesterification of dimethyl carbonate with ethanol to form diethyl carbonate. In accordance with this purpose, three different two-point temperature control structures are designed by using commercial software Aspen Dynamics. Firstly, the control structures were designed for reactive distillation column with single reactive section and then they were modified for reactive distillation column with double reactive sections by adding a ratio control between two feed streams. The alternatives of every single control structure are formed by manipulating the ratios between feed streams instead of controlling them by manipulating the feed stream flowrates. It is anticipated that selection of feed ratio as manipulated variable for one of the temperature control loops provides required ratio of reactants in the case of feed composition disturbances. The dimethyl carbonate feed flow DMC, ethanol feed flow fed into top reactive section ETOH1 and vapor boilup VS, are flow controlled and used as production rate handle in control structures CS1, CS2 and CS3, respectively. In the all control structures, bottom and distillate flowrates are manipulated to control column base and reflux drum levels, respectively. Also reflux ratios are fixed and pressure is controlled by manipulating condenser heat duty. There are two and three valves left in the cases of reactive distillation column with single reactive section and reactive distillation column with double reactive sections, respectively. These manipulated variables are used to keep the product pruties on desired values. The selection of the tray locations for temperature control loops is made by using sensitivity analysis. The sensitivity analysis is based upon changes of controlled variables against the changes of potential manipulated variables. PI controllers are used in temperature control loops with a 60 sec time lag. Then, controller parameters are tuned using Tyreus-Luyben tuning method. The results of this thesis shows that CS1 is not a proper control structure for both reactive distillation columns with single and double reactive sections cases. It takes approximately fifteen hours to set new steady-state values for changes in both production rate and feed composition. The new steady-state values of product compositions in the case of CS2 and CS3 do not show significiant difference for changes in both production rate and feed composition. However, it is illustrated that CS2 has better performance based on ITAE values analyzed. Additionally, the results show that the manipulating the ratios between feed streams instead of manipulating the feed stream flowrates to control temperatures does not lead to an improvement in the control performances. The results also demonstrate that dynamic performance of reactive distillation column with double reactive sections is as good as reactive distillation column with single reactive section. Thus, the reactive distillation column with double reactive sections can be an alternative to conventional reactive distillation column with single reactive section for separation of the two-stage consecutive reversible reactions with regard to steady-state and dynamic performances.