Alternatif Biyoetanol Saflaştırma Proseslerinin Kontrolü

Abstract

Biyoetanol, biyokütleden biyokimyasal bir reaksiyonla genel olarak elde edilen alternatif bir yakıttır. Biyoetanol; temizleme, ekstraksiyon, işleme, sakarifikasyon, fermantasyon, damıtma ve dehidrasyon adımları ile üretilir. Etanol hammadde, katkı maddeleri ve çözücü olarak da kullanılabilir. Bu nedenle, biyokütleden elde edilen etanol geleceğin yakıtı olarak kabul edilmektedir. Avantajlarından en önemlisi çevre açısından yararlı olan, yenilenebilir enerji kaynaklarından üretilmesidir, bunun nedeni; benzinden daha düşük sera gazı emisyonlarını açığa çıkarmasıdır. Etanol aynı zamanda yüksek oktan sayısına, geniş yanıcılık sınırlarına ve benzinden daha yüksek buharlaşma ısıları vardır. Buna ek olarak, benzin katkı maddesi olarak kullanılabilir ve hatta doğrudan kullanılabilir. Tez iki aşamadan oluşmaktadır. İlk aşamada, seçilen üç biyoetanol ayırma prosesi Aspen Plus'ta simüle edilmiştir. Proseslerin ilki ön yoğunlaştıncı kolon, ekstraktif kolon, solvent geri kazanım kolonu ve yoğunlaştırıcı kolonu içeren dört kolonlu bir prosestir. Birinci kolonda, fermentasyon suyundan % 85 etanol ve % 15 su içeren karışım elde edilirken, saf etanol üretmek için etilen glikol ikinci kolona gönderilir. İkinci kolonun distilatından susuz etanol elde edilirken, kolonun dip akımı çözücü geri kazanımı için bir sonraki kolona gönderilir. Solventin küçük bir miktarının, bu geri dönüşüm sırasında kaybını önlemek için telafi olarak makeup eklenir. Solvent geri kazanım kolonundan su ve azetropik karışım elde edilir. Buradaki azeotropik karışım ilk kolona geri gönderilir. Ikinci proses (CLR), üç kolondan oluşmaktadır: ön yoğunlaştıncı kolon, ekstraktif kolon, solvent geri kazanım kolonu. Dört kolonlu sistemden farkı bir kolon indirgenmesi bunu takiben üçüncü kolonun distilatının birinci kolona gönderilmesidir. Son proses SSVR denilen iki kolonlu prosestir. Burada ön derişiklendirme kolonu aynı çalışırken ekstraktif kolon buhar yan akımına sahiptir ve bu akımla birinci kolna dönüş yapar. Ektraktif kolonun distilatı saf etanol içerirken; dip akım solvent içerir ve sisteme geri beslenir. Aspen Dynamics'e gönderilmeden önce gerekli kolon boyutlandırılmaları yapılarak yapılar Aspen Dynamics'e gönderilir. Yeterli literatür araştırması sonucunda proseslere kontrol yapıları kurulmuştur. Yapılara ± %20 besleme akış ve %0.4 ve %0.6 mol besleme kompoziyonu distürbansı uygulanmaktadır ve veriler 10 saat boyunca toplanmaktadır. Elde edilen veriler sonucu MATLAB'te grafikler oluşturularak incelenmiştir. Sistemlerin distürbanslara karşı verdiği cevaplar çok düşük değişimlere sahiptir ve kısa zamanda yatışkın hale ulaşmıştır. Sonuç olarak her üç yapının da dinamik davranışlarının iyi olduğu gözlemlenmiştir.

Bioethanol is an alternative fuel obtained generally by biochemical reaction of biomass. Bioethanol is produced efficiently and economically with cleaning, extraction, treatment, saccharification, fermentation, distillation and dehydration steps of sugarcane, corn, wheat and cellulose, simultaneously. Ethanol can be used as raw material, additives and solvent, such as cosmetics, sprays, perfumery, paints, medicines, food, varnishes and explosives industries. Therefore, ethanol produced from biomass is regarded as the fuel of the future. Due to the fact that ethanol has important advantages like it is produced from renewable energy sources that are environmentally beneficial; it has the lower greenhouse gas emissions than gasoline. Ethanol has also a higher octane number, wider flammability limits, and higher heats of vaporization than gasoline. Furthermore, it can be used as additive with gasoline and also used directly. On the contrary, the major disadvantages of ethanol are including lower energy density, lower vapor pressure and miscibility with water. Several alternative processes are applied to produce bioethanol: ordinary distillation, pervaporation, adsorption, pressure-swing distillation, extractive distillation, azeotropic distillation, liquid–liquid extraction, adsorption as well as hybrid methods combining these options. In this thesis, the simulation and control of bioethanol production processes using extractive distillation method are studied. The thesis consists of two stages. In the first stage, the processes selected are simulated in Aspen Plus using the data in the relevant article. Three bioethanol separation processes formed by Errico et al have been selected. The first one is a four-column configuration which includes the preconcentrator column, the extractive distillation column, the solvent recovery column, and the concentrator column. In first column, fermentation broth is converted into the azeotropic mixture, and also the mixture is sent to the second column to produce pure ethanol using ethylene glycol as a solvent. While this is obtained from the distillate of the second column, the bottom of the column is sent to the next column for solvent recovery. A small amount of fresh solvent is added as make up to prevent any losses of solvent during this recycle. The distillate of the solvent recovery column is separated as water and an azetropic mixture and also the mixture is turned back to the first column in the last column. The second configuration is called conventional separation sequences with liquid recycle (CLR) and also consists of three columns: preconcentrator, extractive and solvent recovery column. While the same sequences occurs in both preconcentrator and extractive column, changes are made in the solvent recovery column. The solvent is obtained from the bottom of the solvent recovery column and is turned to the second column (extractive column) not to the first column. The last configuration is called SSVR, includes two column: preconcentrator column and extractive column. The preconcentrator column is performed same in the other processes. In the extractive column, , pure ethanol is obtained from the distillate, the solvent is recovered at the bottom. The vapor side stream includes a mixture of water and ethanol and also is turned to the preconcentrator column. Before being sent to Aspen Dynamics, column sizing is applied to the columns of these three structures to determine the diameter and length of the vessel. Then, the procedure for "exporting" is performed. Three process control structure has been established by examining the control structure in the literature. In the control structures of four column and three column configurations: reflux drum levels for all columns are controlled by manipulating the distillate flow rates in the first configuration. In the CLR and SSVR, the control of the partial condenser is applied. The base levels for all columns except the solvent recovery column are controlled by manipulating the bottoms flow rates. The base level for recovery column is controlled by manipulating the makeup flow rate. The top pressures of both columns are controlled by manipulating the corresponding condenser duties. The entrainer flow rate is ratioted to the azeotropic feed and the ratio is controlled by manipulating the bottoms flow rate of the recovery column. Reflux ratios are held constant in each column at their nominal values during disturbances. The fresh feed to the preconcentrator column is flow control in order to guarantee the constant flowrate. The entrainer feed temperature is controlled by manipulating cooler duty. The reboiler duties of both columns are used to control the temperature in a particular stage of each column. In the two column process, reflux drum level for extractive column is controlled by manipulating the distillate flow rate. The reflux drum level for preconcentrator column is controlled by manipulating reflux. The base level for preconcentrator column is controlled by manipulating the bottoms flow rates. The base level for second column is controlled by manipulating the makeup flow rate. The top pressures of both columns are controlled by manipulating the corresponding condenser duties. The entrainer flow rate is ratioted to the azeotropic feed and the ratio is controlled by manipulating the bottoms flow rate of the recovery column. Reflux ratio is held constant in extractive column at their nominal values during disturbances. Distillate flow rate of the preconcentrator column is ratioed to the reflux flow rate. The fresh feed to the preconcentrator column is flow control in order to guarantee the constant flowrate. The entrainer feed temperature is controlled by manipulating cooler duty. The reboiler duties of both columns are used to control the temperature in a particular stage of each column. The temperature of the vapor sidestream is controlled by manipulating the bottom of the second column. After the design of the structures, two type distorbances are given to the processes: ethanol composition disturbances and Fresh feed flow disturbances. Ethanol composition disturbances, from 5 to 6 mol% ethanol and from 5 to 4 mol% ethanol, for 10 hours. Therefore, fresh feed flow disturbances of ±20% are applied for 10 hours. The results are recorded and shown by using MATLAB. Dynamic responses of the all systems are given in the Figures. The designed three control structures are affected from disturbance with small changes and soon stabilize and so the systems give good dynamic behaviours.