Ardışık reaktif distilasyon kolonlarında gerçekleşen biyodizel üretim prosesinin tasarımı ve kontrolü
Design and control of the biodiesel process produced in consecutive reactive distillation columns
Yazar: EKİN YENER
Danışman: DOÇ. DR. DEVRİM BARIŞ KAYMAK
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı / Kimya Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Kimya Mühendisliği = Chemical Engineering

Özet:
Biyodizel; etanol ya da metanol gibi alkollerin bitkisel yağlar ile reaksiyonundan oluşan, yenilenebilir ve fosil yakıtlara alternatif bir yakıttır. Reaktif distilasyon kolonları ise ayırma ve reaksiyon işlemlerini aynı ünitede gerçekleştirmeleri sayesinde yatırım ve enerji maliyetlerinin düşmesine katkıda bulunan ekipmanlardır. Bu çalışmanın amacı, ardışık reaktif distilasyon kolonlarında yüksek saflıkta (>99.6%) biyodizel üreten prosesin tasarım ve kontrolünü simule etmektir. Bunun için, Aspen Plus HYSYS 9.2 simülasyon aracı olarak kullanılmıştır. Yatışkın hal tasarımı Aspen Plus kullanılarak yapılırken, dinamik hal kontrolü Aspen Dinamik kullanılarak yapılmıştır. Proses temel olarak iki reaktif distilasyon kolonu bir ısı değiştirici ve bir dekanter içerir. Reaktif distilasyon kolonlarında sırasıyla, serbest yağ asitlerinin esterifikasyonu ile gliseridlerin metanol ile reaksiyonu olan transesterifikasyon reaksiyonları gerçekleşmektedir. Esterifikasyon reaksiyonunda atık yağların içeriğini temsil olarak oleik asidin metanol ile reaksiyonu gerçekleşirken, transesterifikasyonda trioleinin metanol ile reaksiyonu gerçekleşir ve biyodizel ile yan ürün olan gliserol elde edilir. Niyobiyum oksit esterifikasyon reaksiyonunda katalist olarak kullanılırken, Mg(OCH3)2 transesterifikasyon reaksiyonu için katalist olarak kullanılır. Tasarım parametreleri belirlenirken Cisneros ve diğ.(2016) makalesi baz alınmış, prosesteki üst ve alt akımdan alınması beklenen maddelerin relatif uçuculuk ve kaynama noktaları göz önüne alınarak, makaledeki uygunsuz sonuçlar düzeltilmiştir. Birinci kolon için üç farklı durum (toplam kondenserli, kısmi kondenserli ve kondensersiz) ve ikinci kolon için iki farklı durumun (tek metanol beslemesi, çift metanol beslemesi) maliyet hesapları gerçekleştirilmiştir. Sistemin maliyet hesabı üzerinden en uygun durum, nihai durum olarak belirlenmiştir. Çalışılan proses sonucunda biyodizel ASTM standardı gereğince, 0.9974 saflığında elde edilmiştir. Tesisin güvenliğini sağlamak ve son ürünün istenen saflıkta kaldığından emin olmak için; proses kontrol edilmelidir. Bu proseste iki farklı kontrol yapısı oluşturulmuştur. İlk kontrol yapısı beş seviye kontrolörü, iki basınç kontrolörü, iki sıcaklık kontrolörü, üç debi kontrolörü, iki oran kontrol edici içerirken ikinci kontrol yapısında sıcaklık kontrol edicileri yerine kompozisyon kontrol ediciler tasarlanmıştır. Sıcaklık ve kompozisyon kontrol edicilerin nihai kazanç ve periyot değerleri kapalı çevrim ATV testi sayesinde elde edildikten sonra, Tyreus-Luyben eşitlikleri yardımıyla uygun kazanç ve integral zaman sabiti değerleri belirlenmiştir. Sistemin gürbüzlüğünü ölçmek için karışım giriş debisine ±%20 ve karışım içerisindeki oleik asit bileşenine ±%15 bozan etken verilmiş ve bozan etkenlere karşı sistemin sonuçları incelenmiştir.

Abstract:
By the population growth, energy resources has been decreasing day by day. Interest for renewable, clean energy resources has been rised considering the increase in consumption of fossil fuel which is not enough for the growing population, and increase in environmental damage. Thus, usage of renewable energy sources which is alternative for fossil fuel should be increased such as bioethanol, biobutanol and bi odiesel. Biodiesel is a most common example for renewable, alternative fuel to fossil fuel which can be produced through reaction of mild alcohol such as methanol or ethanol with vegetable oils. 
On the other hand, integration of process in chemical industries has been enhanced in order to reduce cost and environmental impact. Thanks to intensification of chemical processes, both operating and capital costs can be reduced and also product quality and process safety can be improved. Reactive distillation columns having separation and reaction process in a single unit is one of the common example for the process integration. When compared with conventional multi-unit reactor system, reactive distillation columns provide several advantages such as reducing energy consumption, avoidance of azeotropes, decreasing of catalyst usage. Owing to continous removal of the product coming from reaction, conversion limitation of reaction equilibrium for reversible reaction is prevented. Furthermore, if exothermic reaction occurs in the column, then the reaction heat can be used for vaporization which provides to decrease reboiler heat duty. 
Aim of the study is to simulate production of biodiesel having high purity (>99.6%) in reactive distillation column (RDC) and ensure control of the process. To achive this aim, Aspen Plus v9.2 was used as the process simulation tool. Steady state design of the process is achived by using Aspen Plus while dynamic contrallability is executed by aid of Aspen Dynamics. Fot the all reactive distillation column, RADFRAC option were selected in the tool. 
The process consist of two main reactive distillation columns, one heat exchanger and one decanter unit. The reactive distillation process consist of two reaction including esterification of the free fatty acids (FFA) and transesterification of glycerides with methanol. In this study, biodiesel was produced by using waste cooking oil and methanol. Because, when the economic analysis of commercial biodiesel production is examined, the biggest contribution comes from plant size and vegetable oil costs.  Waste cooking oil, on the other hand, are a powerful alternative as raw material for this reaction due to both being cheaper than pure, fresh vegateble oils and reducing environmental damages while being destroyed. 
For the esterification reaction in RD-1, oleic acid which represents waste cooking oil content mostly, was reacted with methanol whereas in transesterification reaction in RD-2, triolein was reacted with methanol to produce biodiesel and glycerol as a byproduct. Niobium oxide solid catalyst were used in esterification one reactive zone while alkali catalyst which is Mg(OCH3)2 were used in transesterification one reactive zone. 
Kinetic parameters for the both reaction including esterification and transesterification were built separately. Esterification reaction is second degree with respect to oleic acid and first degree with respect to methanol. While the reaction rate constant created, the reaction was accepted as irreversible. Since, reversible hydrolysis reaction is assumed as negligible. On the other hand, transesterification reaction is reversible and the kinetic parameters for the reaction were formed by aid of Arrhenius equation. 
Thanks to decanter unit in the process, biodiesel and glycerol which is produced by transesterification in RD-2 were separated.  
In this study, Cisneros et al.(2016) was the main guidance paper to determine design parameters. Cisneros et al.(2016) claims that, the bottom stream has 54.5 kmol/hr methanol which is equal to 97.4% of unreacted methanol coming from esterification reaction while top stream consist of 99.9% water. However, when the boiling point and relative volatility of the triolein, methanol and water examined, it is seen that methanol, which is the lightest key, should be obtained from the upper stream, while the heaviest key, triolein, should be obtained from lower stream. 
Therefore, in this study, overall methanol  which is not reacted with oleic acid during esterification reaction, was obtained from upper stream of the first column, while unreacted triolein was obtained from the lower stream of first column.
Total annual cost for the three situation (total condenser, partial condenser, no condenser) in first column and two situation (one methanol feed, two methanol feed) in second column were calculated. According to total annual cost calculation, first column with no condenser and second column with one methanol feed were detected as best options. At the end of the final process, 0.9974 biodiesel purity was obtained based on ASTM standard. In order to provide maintenance of product purity at desired level and ensure safety for the plant, control system should be created. Two distinct type control scheme were designed naming CS-1 and CS-2. First control scheme have five level controllers, two pressure controller, two temperature controller, three flow controller and two feed to feed ratio controller. On the other hand, in second control system, temperature controllers were replaced with composition controllers. All level controllers were selected as P-only controller whereas the remaining type controller including pressure, temperature, flow and composition were selected as PI type. Closed loop ATV test was used to determine ultimate parameters (KU and PU) and Tyreus-Luyben equation was used for determination of controller gain and integral time constant for both temperature and composition controllers which is belong to CS-1 and CS-2 respectively. In order to measure robustness of the system, ±%20 distrubances on feed flowrate and ±%15 composition change in oleic acid fraction in feed were performed and the results were examined.
The process has continued its stable behaviour when facing with distrubance factors. When ultimate absoulate errors of CS-1 and CS-2 control structures compared with each other, it is seen that, ultimate product purity which is controlled by second control structure has lower off-set. While first system which is controlled by CS-1 sets to new steady-state value approximately nine hours, second system which is controlled by CS-2 sets to new steady-state value approximately four hours. Therefore, it can be said that CS-2 can be a better option to control the process.
Consequently, the study indicates that the biodiesel having desired purity can be obtained from waste cooking oil by using two reactive distillation column consisting of esterification and transesterification reaction  and also the process can be controlled with both control structures naming CS-1 and CS-2. Dynamic performance of the CS-2 is better than CS-1 considering settling time and off-set.