Alternatif dimetil karnonat üretim prosesinin kontrolü
Process control of a novel dimethyl carbonate production process
Yazar: EDA KILIÇ
Danışman: DOÇ. DR. DEVRİM BARIŞ KAYMAK
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı / Kimya Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2021
Konu: Kimya Mühendisliği = Chemical Engineering

Özet:
Geleneksel hidrokarbon yakıt kaynaklarının yakın gelecekte sınırlı hale geleceği öngörülmektedir. Petrol tüketiminin artış hızı sebebiyle atmosferde bulunan kirleticiler de artacaktır. Devletler çevre kirliliğini önlemek için sıkı emisyon kuralları uygulamaktadır. Emisyon sorunlarını azaltabilmek için geleneksel hidrokarbon yakıtların yanı sıra yeni alternatif yakıtlar veya katkı maddelerinin üretimi oldukça önemlidir. Toksisitesi düşük ve biyolojik olarak kolay bozunan bir kimyasal olan dimetil karbonat (DMC) son yıllarda artan bir önem kazanmıştır. DMC kimyasal sentez için "çevreye zararsız bir yapı taşı" olarak, karbonilasyon ajanı olarak fosgenin ve alkilasyon ajanı olarak dimetil sülfatın yerini almak için yaygın olarak kullanılabilir. Ayrıca, DMC'nin çok iyi bir benzin katkı maddesi ve boyama çözücüsü olduğu bulunmuştur. Tüm bunlar DMC üretim proseslerinin tasarımı üzerine araştırmayı büyük ölçüde motive etmiştir. DMC üretmek için fosgenasyon, üre esterifikasyonu, etilen karbonat esterifikasyonu, metanolün oksidatif karbonilasyonu ve CO2 ile metanolden DMC'nin doğrudan sentezi gibi çeşitli yöntemler mevcuttur. Son yıllarda çevresel ve ekonomik sebepler, entegre ve hibrit ayırma proseslerinden oluşan proses yoğunlaştırma teknolojisine duyulan ilgiyi artırmıştır. Reaktif distilasyon kolonları, reaksiyon ve distilasyon işlemlerini tek bir ekipmanda birleştirerek enerji maliyetlerini düşüren proses yoğunlaştırmanın kimya endüstrisindeki en iyi örnekleri arasında yer almaktadır. 
Tek kademeli reaksiyon sistemleri üzerine yapılmış olan tasarım ve kontrol çalışmalarının sayısı daha fazla olmakla birlikte iki kademede gerçekleşen reaksiyon sistemlerinin yatışın hal tasarımının ve kontrolünün incelendiği çalışmaların sayısı daha azdır. Bu çalışmanın amacı, literatürde yatışkın hal tasarımı önerilmiş olan, iki kademeli ardışık dolaylı alkoliz reaksiyonları sonucu dimetil karbonat üretiminin gerçekleştiği reaktif distilasyon kolonlarının Aspen Dynamics simülasyon programı kullanılarak proses kontrolünün sağlanmasıdır. Tasarlanan iki farklı kontrol mekanizmasının gürbüzlüğünü incelemek amacıyla, besleme debisinde ± %20 ve besleme bileşiminde %10 büyüklüğünde bozan etkenler uygulanarak, prosesin bu değişimlere gösterdiği dinamik cevaplar incelenmiştir. Kontrol mekanizmasının bozan etkenler karşısında yeniden yatışkın hale oturma süresi yaklaşık 5 saat olarak bulunmuştur. Bu süre sonunda proseste takibi yapılan birçok parametrenin beklenen değişimleri göstererek uygun bir değere oturduğu gözlemlenmiştir.

Abstract:
Conventional hydrocarbon fuel resources are predicted to become limited in the near future. Environmental pollutants are also estimated to raise due to the increasement rate in oil consumption. Strict emission rules are applied by governments in order to prevent environmental pollution. In order to reduce emission problems, it is very important to produce new alternative fuels or additives in addition to traditional hydrocarbon fuels. Dimethyl carbonate (DMC), a low-level toxic and easily biodegradable chemical, has gained increasing importance in recent years. DMC can be widely used as an "environmentally harmless building block" for chemical synthesis, to replace phosgene as the carbonylation agent and dimethyl sulfate as the alkylation agent. Also, DMC has been found to be a useful gasoline additive and paint solvent, which has greatly motivated research on DMC. Various processes are available to produce DMC, such as phosgenation, urea esterification, ethylene carbonate esterification, oxidative carbonylation of methanol, and direct synthesis of DMC from CO2 and methanol. 
In recent years, interest in process intensification technology, which consists of integrated and hybrid separation processes, has increased for environmental and economic reasons. Reactive distillation columns are among the best examples in the chemical industry of process intensification, which reduces energy costs by combining reactor and distillation in one equipment. There are many advantages of reactive distillation operation. Improved selectivity, increased conversion rate for equilibrium reactions due to simultaneous separation of the products from the column, better temperature control, reduction of reboiler duty, especially thanks to the use of heat obtained in exothermic reactions in distillation, avoidance of azeotropes and reduced costs. Reactive distillation process has been developed for chemical equilibrium limited reactions such as esterification, transesterification, polymerization and has been successfully applied on a large scale. However, there are some limiting factors that should be considered in the operation of this procedure. The relative volatility of the reactants and products should be such that the products are easily removed from the reactive region of the column while the reactants remain in the column. The other major limitation is the need for a match between the temperatures suitable for the reaction and separation, since the reaction and separation take place in a single unit and at the same pressure. If the separation takes place at low temperatures, it will result in low reaction rates, very large holdup or the need for large quantities of catalyst. If the temperature required to carry out the separation process is high, this results in very low chemical constants in exothermic reversible reactions and it may be difficult to achieve the desired conversion. Also, high temperatures can cause undesirable side reactions. Therefore, reactive distillation may not be economical in both low and high temperature situations. Additionally, less valves are placed in order to operate and control the column and system shows non linear feature. This situation makes control structure more complicated.
According to the literature researches, reactive distillation columns are used mostly reactions with two reactant-two product (A+B↔C+D) or two reactant-one product (A+B↔C). Nevertheless, the lack of research examples for reactive distillation for consecutive reaction systems as A+B↔C+D and C+B↔E+D. When the literature studies are examined, it is seen that the reaction systems placed in one stage attract more attention and the design and control studies on these systems are much more. The number of studies examining the steady state design and control of two-stage reaction systems is less.  The aim of this study is to design and control the process of reactive distillation columns, in which dimethyl carbonate is produced as a result of two-stage sequential indirect alcoholysis reactions, the steady state design of which was made by Shi et al. They propose five different process equipment configurations. First of all, continuous stirred-tank reactor and conventional distillation (CSTR+CD) column combination is simulated in ChemCad software. Then, instead of CSTR+CD, reactive distillation column configuration is designed under near-neat operation. In order to improve heat integration, they offer three different heat-integrated RD+CD configurations. Excess amount of MeOH reactant is used in these three configurations.
In the first heat-integrated model, CD column overhead is used for reducing the RD cloumn reboiler duty. Because of the fact that increasement in capital cost is higher than the operating cost reduction, first heat-integrated model is not feasible. In order to reduce the energy consumption, CD column operation pressure can be reduced, but another challenging issue under low pressure is PC-PG azeotrope. For separation of the PC-PG binary mixture, CD column needs to be designed to has several trays. One of the best economical advantage can be provided by running the CD column top under high pressure to prevent azeotrope while operating the bottom under low pressure. The second heat-integrated model is designed according to this method. The top section of the conventional distillation column is processed at higher pressure and temperature conditions than bottom of the column. Two different ways can be applied for heat integration. One way is to use the latent heat of CD overhead flow  in CD column reboiler. If the condenser duty is greater than the reboiler duty, the additional latent heat can be released to the reboiler of the RD column. The other way is to use the CD overhead latent heat in the RD cloumn reboiler as the last heat-integrated model. As per ChemCad results, heat-integrated model 2 and 3 shows high benefit and advantageous in azeotrope characteristic of the PC-PG. In terms of heat integration, the last process shows the best economic results on simulation program.
The purpose of this thesis is to offer control mechanism over reactive distillation columns for transesterification of propylene carbonate with methanol to form dimethyl carbonate. In accordance with this purpose, two different control mechanisms are created by using commercial software Aspen Dynamics. Firstly, the process control mechanisms were designed for reactive distillation column with single-end temperature control mechanism, then modified for reactive distillation column with dual temperature control mechanism. For temperature control circuits, the selection of the tray locations temperature control loops is made by using slope of the temperature profile. The time delay in PI controllers of the temperature control loops is defines as 60 sec and controller parameters are tuned using Tyreus-Luyben tuning method. In order to examine the robust operation of the control structure for the two different control mechanisms applied, the response of 20% increase and decrease in production rate and 10% disturbance in reactant purity were examined. The response time of the process to get a new balance according to manipulations is about 5 hours for different parameters.