Gliserolden 1,2-propilen glikol üretim prosesinin tasarım ve kontrolü
Production of 1,2-propylene glycol from glycerol design and control
Yazar: ATİLLA ÖZCAN KARAAĞAÇ
Danışman: DOÇ. DR. DEVRİM BARIŞ KAYMAK
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı / Kimya Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Kimya Mühendisliği = Chemical Engineering

Özet:
Tükenen petrol rezervleri ve sera gazlarının birikimindeki artış dünya çapında çözüm aranan önemli sorunlardır. Bu sorunların çözümüne yönelik olarak yeşil kimyasalların kullanımı önem kazanmaktadır. Enerji ve hammadde kaynağı olarak biyokütle esaslı kaynakların kullanımı da diğer çözüm yollarından biridir. Bu çözüm özellikle gliserol arz fazlasının oluşmasını sağlamıştır. İlaç, gıda ve toksik olmayan kimyasalların üretimi gibi birçok alanda kullanılan propilen glikolün (C3H8O2), propilen oksit yerine gliserolden üretilebilmesi yeşil kimyasal kullanımında önemli bir adım sayılmaktadır. Literatüre göre uzun süredir bilinen bu proses, hem yatırım maliyeti hem de işletme maliyeti açısından uygulanabilir olmadığı için bugüne kadar tercih edilmemiştir. Literatürdeki proseslerin çoğu eski tip ya da patent aşamasında kalmış ve pahalı katalizörler tercih edilmiş gliserolden propilen glikol üretim tasarımlarıdır. Ancak yeni yapılan katalizör çalışmaları, bu poroses için daha az maliyetli katalizörler üzerinde yoğunlaşarak proses tasarımını iyileştirecek ve maddi dezavantajları tersine çevirecek yönde ilerlemektedir. Bu çalışmada gliserol esaslı bir propilen glikol üretim prosesi yeni çalışılan katalizör ve bu katalizöre bağlı reaksiyon fonksiyonları iletasarlanmış ve eşdeğeri olan propilen oksit esaslı klasik proses ile karşılaştırılmıştır. Operasyon koşullarında basınç, sıcaklık gibi farklılıklar ve farklı yan ürünler oluşmasına rağmen iki sistemde de benzer ekipmanlar kullanılarak proses tasarımları yapılmıştır. Hazırlanan proseslerin ekipman boyutlandırmaları ve enerji giderleri hesaplanarak ekonomik değerlendirme yapılmıştır. Ortaya çıkan sonuçlara göre toplam sabit sermaye maliyeti gliserol esaslı sistemde hala daha yüksek olmasına karşın toplam yıllık maliyet açısından propilen oksit esaslı sistem çok daha fazla yatırım gerektirmektedir. Çalışmanın ikinci aşaması daha az maliyetli çıkan yeşil kimyasal esaslı propilen glikol üretim prosesinin fabrika çaplı kontrolüdür.  Literatürdeki çalışmalardan yola çıkarak, sisteme ait kontrol edilen ve ayarlanan değişkenler seçilmiş ve eşleştirmeleri yapılmıştır. Seviye ve basınç gibi klasik ve çalışan kontrol ediciler literatür ayarlarında bırakılmış, diğer kontrolcü parametreleri otomatik ayar testi (Auto Tuning Test, ATV) ve Tyreus-Luyben yöntemi ile ayarlanmıştır. Çalışma sonucunda dinamik proses kontrolünün düzgün işleyebilmesi için tasarımsal değişikliğe gidilerek proses içerisinde propilen oksit esaslı sistem gibi çift yerine tek flaş tankı kullanılmıştır. Diğer yandan reaktör sıcaklık kontrolü için PI kontrolcü yeterli olmadığı için onun yerine PID kontrolcü tercih edilerek prosesin ilk yarısının kontrolü başarılı bir şekilde gerçekleşmiştir. Son olarak, prosesin ikinci yarısında üründeki en büyük safsızlık olan suyun bileşiminin kontrolünün bu proses için çok önemli olduğu anlaşılmıştır. Bunun için farklı kontrolcü kombinasyonları denenmiştir. Denemeler sonucunda suyun uzaklaştırıldığı ilk destilasyon kolonunun dip ürün hattından ölçülen safsızlık bileşiminin buharlaştırıcı kızgın buhar debisi ile riflaks debisinin eş zamanlı kontrolünün en iyi sonuç veren seçenek olduğu görülmüştür.

Abstract:
The depleted oil reserves and the accumulation of greenhouse gases are major problems that are sought for solution worldwide. The use of green chemistry has gained importance in solving these problems. In this context, the use of biodiesel as an automotive fuel is particularly important in the US and Europe. Although biodiesel is more expensive than petroleum derivatives, it promotes the production of governments around the world because of its renewable structure and lower emission values The use of biomass-based resources as a source of energy and raw materials is one of the solution. This solution in particular made it possible to produce excess glycerol supply. 
The production of propylene glycol (C3H8O2), which is used in many fields such as medicine, food and non-toxic chemicals, from glycerol instead of propylene oxide is an important step in the use of green chemicals and excess glycerol. 
Synthesis of propylene glycol at industrial scale from propylene oxide and water started in the 1930s. Mono, di and tri propylene glycol are formed in the ratio of 100: 10: 1, which is the most common result of the hydration of propylene oxide. Both acid and base, both homogeneous and heterogeneous catalysts can be used to increase reaction rates or propylene glycol selectivity. Process includes at least one reactor, flash units with propylene oxide recycle and distillation towers. 
Second way for the propylene glycol production is from glycerol by the hydrogenolysis reaction involving disruption of the C=O chemical bond and simultaneous hydrogen addition. Depending on the catalysts and the operating conditions applied, it is suggested that the reaction of glycerol hydrogenolysis proceeds via two mechanisms. The reaction route comprises a dehydration step of acetone on the acid sites and then a hydrogenation step on the metal fields. The second mechanism involves dehydrogenation, followed by 2-hydroxy acrolein dehydration and finally hydrogenation of propylene glycol on basic sites or basic media to glyceraldehyde. This process, which is known to be long-lasting according to the literature, has not been preferred until now because it is not feasible both in terms of investment cost and operating cost. Most of the processes in the literature are propylene glycol production form glycerol designs that are either of the older type or patented and expensive catalysts are preferred. 
New studies are going to reverse the material disadvantage and improve process design. Last couple of decades, process equipment designs and materials improved. Also, non-noble metals such as copper (Cu), cobalt (Co) and nickel (Ni) (cheap and plentiful) instead of noble metals (expensive and rare) such as platinum (Pt), rhodium (Rh), ruthenium (Ru), palladium (Pd), iridium (Ir) and rhenium (Re) have been researched for hydrogenolysis reaction catalyst for the last fifteen years. New catalysts are offer high yield and high selectivity (low amount of side product) that close to expensive catalysts
In this study, a glycerol-based propylene glycol production system was designed and compared with a classical and equivalent system (a reactor with flash and distillation units) based on propylene oxide by using Aspen Plus software. Despite the different operating conditions, economic evaluation was carried out using similar equipment in both systems. Moreover both system includes recycle streams for unreacted raw materials. 
After processes design, equipment sizing and cost calculations are done by literature. Working cost and fixed capital investment are calculated and updated by 2017 Chemical Engineering Plant Cost Index (CEPCI) and Marshall and Swift Cost Index (M&S). 
According to the economical evaluation results, although the total fixed capital cost is still higher in the new type system, the old system requires much more investment in terms of the total annual cost. 
The other part of the work is the plantwide control of the green chemical-based propylene glycol production system. Controllers of the system's equipment are selected according to recommendations and examples in the literature, adjusted by automatic adjustment test (ATV) and Tyreus-Luyben method. Controller settings and their dynamic tests are done by using Aspen Dynamics software in pressure driven mode.
Automatic adjustment test have developed by Åströmand Hägglund in 1984. In this method simple experimental test is used to determine ultimate gain (Kcu) and ultimate period (Pu). Feedback controller is temporarily replaced by an on-off controller (or relay). 
Controller settings adjusted by Tyreus-Luyben method instead of Zeigler-Nichols method. Zeigler-Nichols controller settings tend to produce oscillatory responses and large overshoots for set-point changes. However, Tyreus-Luyben method gives more conservative controller setting results.
In this study, the easiest method used to select the shelf for temperature controllers is the slope criterion. This is by far the easiest and most often applied method. The temperature profile at design conditions is plotted, and the "slope" of profile is examined to find the tray where this slope is the largest. Large changes in temperature from tray to tray indicate a region where compositions of important components are changing.
Different combinations of controllers and adjustments have been used at the points related to temperature and composition control that the conventional controller and adjustments cannot stabilize the system. The reason of unstabilized system is pressure difference of two flash tanks. In dynamic study this cause back flow at the flash tank liquid exits and overflowing/drying tanks. Normally this pressure difference can handle with controllers. However, glycerol to propylene glycol conversion process contains high pressure as working condition and gas recycle stream. Both of them make stabilizing of flash tanks pressure harder. As a result of the study, a single flash tank was used instead of a double in the process by changing the design to make the process control function properly. Since the PI controller is not sufficient for reactor temperature control because of gas recycle stream mostly. That's why the PID controller is preferred instead of PI controller in reactor's temperature controller. Finally, it is understood that the control of the composition of water, which is the largest impurity in the main product, is very important for this process. Therefore, five different controller combination tested individually. In the first two combinations, the water impurity in the bottom stream of the first column was controlled by reflux and hot steam flow of the first column respectively. At third case, the second column distillate impurity component was controlled by both the second column riffles flow and the first column evaporator utility. However, in this alternative, the process was attempted to control the impurity of the bottom of the first column and the top impurity of the second column at the same time, but the time difference between the impulse responses caused the oscillation. In the fourth structure, the composition of the second column top product stream is measured and the rifle flux is controlled. The first column bottom product stream composition was adjusted by controlling both the evaporator utility and the set point of the first column reflux controller. It is understood that this construction is fast enough, but the first column reflux controller has limited the first column bottom impurity control unit to adjust the impurity. First four combination of controllers couldn't control the propylene glycol impurity and/or stabilize the second column. It has been found that the simultaneous control of the evaporator's utility flow and reflux flow rate with measuring impurity composition from the bottom product line of the first distillation column from which the water was removed give the best result.