IBE fermantasyonundan butanol ve izopropanol saflaştırma prosesinin tasarımı ve kontrolü
Design and control of butanol and isopropanol purification process from IBE fermantation
Yazar: SERAP DEMİRDELEN
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:
Günümüzde artan enerji ihtiyacına, azalan enerji kaynaklarına ve çevresel kaygılara bağlı olarak alternatif temiz ve yenilenebilir yakıt arayışları başlamıştır. Biyokütle temelli biyobütanol, sahip olduğu üstün yakıt özellikleri sebebiyle iyi bir alternatif yakıt olarak karşımıza çıkmaktadır. Biyobütanol, aseton-bütanol-etanol (ABE) ve izopropanol-bütanol-etanol (IBE) fermantasyonları ile elde edilebilmesine karşın, izopropanolün asetona göre daha avantajlı bir yan ürün olması sebebiyle IBE fermantasyonu ile biyobütanol eldesine olan ilgi artmıştır. Fermantasyon yoluyla elde edilen ABE/IBE ürünlerinin saflaştırılması, ürün akımının yüksek oranda su içermesi ve ürünlerin su ile çeşitli azeotrop karışımlar oluşturması sebebiyle oldukça zorlayıcı bir ayırma prosesidir. Buna ek olarak IBE ürünlerin ABE ürünlerinden sayıca daha fazla azeotrop karışım içermesi, IBE ürünlerinin saflaştırılmasını daha karmaşık hale getirmektedir. Azeotropik karışımlar gibi karmaşık sistemlerin etkili bir şekilde ayrılmasında, distilasyon yaygın olarak kullanılan ve endüstriyel uygulamalarla da kendini kanıtlamış bir ayırma yöntemidir. Bu çalışmanın amacı, özelleşmiş ve hibrid distilasyon uygulamaları ile IBE fermantasyonu ürünlerinden bütanolün ve izopropanolün saflaştırılmasını sağlamaktır. Bu amaçla simülatör olarak Aspen Plus kullanılarak bir saflaştırma prosesi tasarımı yapılmıştır. Sonuç olarak tasarlanan IBE saflaştırma prosesi ile %99,5 saflıkta bütanol ve izopropanol elde edilmiştir. Ardından, yatışkın hal tasarımının dinamik kontrolünü sağlamak amacıyla farklı kontrol yapıları oluşturulmuştur. Birinci kontrol yapısında, ana ürün saflıkları dolaylı olarak sıcaklık kontrol edicilerle kontrol edilmektedir. İkinci kontrol yapısında ise ana ürün saflıkları doğrudan bileşen kontrol edicilerle kontrol edilmektedir. Oluşturulan kontrol yapılarını test etmek amacıyla sisteme üretim debisinde ±%20 ve besleme bileşiminde ±%3 oranında bozan etkenler verilmiştir. Sonuçlar incelendiğinde, her iki kontrol yapısının da prosesin genelini etkili bir şekilde kontrol edebildiği görülmüştür. Ancak birinci kontrol yapısında ürün saflıklarında çok küçük miktardaki sapmalar görülürken, ikinci kontrol yapısında ürün saflıklarında herhangi bir sapma yaşanmamıştır. Ancak ürün saflıklarının tekrar yatışkın hal değerine ulaşması, birinci kontrol yapısına kıyasla ikinci kontrol yapısında daha fazla zaman almaktadır.

Abstract:
Nowadays, the search for novel and clean energy sources has started due to the gradually decreasing fossil fuel resources, increasing energy demand and environmental concerns. Recently, studies on biomass based fuels such as biodiesel, bioethanol and biobutanol shows that biofuels have a potential to be an alternative to fossil fuel sources. Biobutanol is a clean and renewable energy source due to its advantages as a biofuel and its superior fuel potential (Dürre, 2007). It has been known that biobutanol can be obtained as a product of acetone-butanol-ethanol (ABE) and isopropanol-butanol-ethanol (IBE) fermentations (Jones and Woods, 1986). Since isopropanol is a by-product with superior properties compared to acetone, the interest in biobutanol production with IBE fermentation has increased.
Separation processes of ABE/IBE products have some challenges due to the great amount of water existing in the fermentation broth since alcohol products form azeotropic mixtures with water. The azeotropes existing in ABE fermentation broth are; butanol/water heterogeneous binary azeotrope and ethanol/water homogeneous binary azeotrope. In addition to these azeotropic mixtures, there is also isopropanol/water homogeneous binary azeotrope in IBE fermentation products. It is known that the azeotropes cannot be separated by conventional distillation. For this reason, enhanced separation methods such as extractive distillation, pressure-swing distillation, and hybrid separation systems consisting of decanter and distillation columns are used for the separation of azeotropic mixtures. In literature, there are many studies on the separation processes of the products from ABE fermentation. However, the studies on the separation processes of the products from IBE fermentation are limited.
The aim of this study is to design and control of butanol-isopropanol purification process from IBE fermentation by using complex and hybrid distillation separation methods. In order to conduct steady-state design and control simulations, Aspen Plus and Aspen Dynamics are used. The first part of the study includes steady-state design of the purification process. This purification process consists of six distillation columns, one decanter and two heat exchangers. The whole process can be divided into three main parts. First part includes removal of the excess water in the feed stream and separation of the azeotropes into two group. These two group of azeotropes are butanol/water heterogeneous binary azeotrope and isopropanol/water homogeneous binary azeotrope. In the second part, separation and purification of butanol from butanol/water heterogeneous binary azeotrope are carried out. In order to purify butanol, a hybrid system that consists of decanter and distillation columns is used. First, decanter is used in order to break this heterogeneous liquid-liquid azeotrope. Second, distillation columns are used in order to purify butanol from organic phase and water from aqueous phase. In the third part, separation and purification of isopropanol from isopropanol/water homogeneous binary azeotrope are carried out. In order to purify isopropanol, extractive distillation method is applied by using dimethyl sulfoxide as a solvent. First, isopropanol is obtained from the top of the extractive distillation column while water/solvent mixture is obtained from bottom of the column. Second, water/solvent mixture is separated in the solvent recovery column and entrainer is recycled to the extractive distillation column by mixing a solvent make-up stream. As a conclusion, butanol and isopropanol are obtained with 99.5% purity.
Total annual cost (TAC) is calculated for each process equipment. Results of economic analysis show that, the second column in the process which provides the separation of the azeotropes into two group as butanol/water and isopropanol/water has the highest capital cost. Also this separation is the most energy intensive step in the whole purification process. The first column in the process has the second place after the second column by means of both capital and energy costs. That shows removal of the excess water from the feed stream is an energy intensive process.
The second part of the study includes dynamic control of the purification process. With this aim, two control structures are formed as CS1 and CS2. In control structure CS1, butanol and isopropanol purities are inferentially controlled by temperature controllers (TC). In control structure CS2, butanol and isopropanol purities are directly controlled by composition controllers (CC). The way of the controlling purities of the main products is the only difference between CS1 and CS2. Except this, rest of the control structures are conducted by the same control strategy. Main feed stream flowrate is flow controlled and used as production rate handle in both control structures. Reflux drum and base levels of distillation columns are controlled by manipulating distillate and bottom flowrates, respectively; except that base level of solvent recovery column is controlled by manipulating make-up flowrate. First and second liquid levels of decanter are controlled by manipulating organic phase and aqueous phase flowrates, respectively. Condenser heat duties are manipulated to control pressure of the distillation columns. Appointed tray temperature of distillation columns are controlled by manipulating reboiler heat duty. Slop analysis is conducted in order to select the tray locations for temperature control loops. The trays with the greatest slop in the steady-state temperature profiles of the columns are selected.
PI controllers with 1 min and 3 min time lag are used for temperature and composition control, respectively. In order the obtain optimum controller parameters of PI controllers that are used in temperature and composition control, closed loop ATV test is applied. Then the controllers are tuned by using Tyreus-Luyben tuning parameters.
In order to test the dynamic behaviors of the control structures CS1 and CS2, disturbances in the production rate and feed composition are given to the system. Dynamic responces of the process to the disturbances are examined in three group. First, the changes in the purities of the butanol, isopropanol and dimethyl sulfoxide are observed against to disturbances.Second, the changes in the temperatures of the first, second and fourth columns are observed against to disturbances. Third, changes in the temperatures of the decanter feed stream and solvent feed stream are observed against to disturbances.
In control structure CS1, purities of the butanol and isopropanol have an offset as a result of change in the production rate and feed composition. The main reason of that situation is inferential control of purities by using temperature controllers. Unlike control structure CS1, there is no offset in the steady-state values of the product purities in control structure CS2 due to the direct control of the purities by using composition controllers. However, the settling times of the product purities to the steady-state values take longer in the control structure CS2.
As a result of this thesis, it is shown that the designed purification process can achieve butanol and isopropanol with hight purities, so the products can be used in biofuel applications. Also it is known that, the dynamic stability of the process is as important as the steady-state design of the process. Results also show that both control structure can achieve dynamic stability against the disturbances in the product rate and feed composition.