Biyodizeldeki Suyun Reçine ile Uzaklaştırılması: Adsorpsiyon İzotermi, Kinetiği ve Termodinamik İncelemesi

Öz

Fosil kökenli yakıtların kullanımına ilişkin ciddi çevresel kaygılar, her geçen gün alternatif temiz yakıtlara olan ilgiyi artırmaktadır. Biyodizel, uygun bir katalizör eşliğinde, bir alkolle trigliseritlerin transesterifikasyonu yoluyla üretilen, temiz, biyolojik olarak bozulabilen, toksik olmayan ve yenilenebilir bir yakıttır. Biyodizel üretimi sırasında, yan ürün olarak oluşan gliserin ayrıldıktan sonra, ham biyodizelin belli uluslararası yakıt standartlarını (EN 14214 ya da ASTM D6751) karşılaması için saflaştırılması gerekir. Biyodizelde safsızlıkların varlığı, sadece motor performansını önemli ölçüde etkilemekle kalmaz, aynı zamanda onun kullanımı ve depolanmasını da zorlaştırır. Bu nedenle ham biyodizelin saflaştırılması bir zorunluluk olarak karşımıza çıkar. Ham biyodizeli saflaştırmak için, hem ıslak hem de kuru yıkama yapılabilmesine rağmen, genellikle kullanılan yöntem ıslak yıkama (su ile yıkama) yöntemidir. Ancak bu yöntemde kullanılan suyun, biyodizelden çok iyi uzaklaştırılması gerekir. Biyodizel üretimi için yağların geleneksel transesterifikasyonunda, su her zaman olumsuz etkiler yaratır. Biyodizeldeki suyun varlığı, sabun oluşumuna neden olur, katalizörü tüketir, katalizörün etkinliğini azaltır ve bunların tümü düşük dönüşüme neden olur. Ayrıca üretilen yakıtın kalitesini de olumsuz etkiler.

Bu çalışmada, biyodizeldeki suyun, katyonik Dowex HCR-S iyon değiştirici reçine ile adsorpsiyon mekanizması çalışılmış ve prosesin adsorpsiyon dengesi, adsorpsiyon kinetiği ve adsorpsiyon termodinamiği incelenmiştir. Bu kapsamda Langmuir, Freundlich, Temkin ve Dubinin-Radushkevich izotermleri denenmiş ve deneysel verilere en uygun izotermin, R² değeri 0.997 olan Langmuir izotermi olduğu bulunmuştur. Ayrıca, maksimum adsorpsiyon kapasitesi, 53.48 mg g^-1 olarak tespit edilmiştir. Kinetik verilerin analizleri, yalancı birinci mertebe ve yalancı ikinci mertebe kinetik modelleri kullanılarak yapılmıştır. Elde edilen sonuçlar, adsorplama prosesinin, yalancı ikinci mertebe kinetik modele en iyi uyduğunu göstermiştir. Çalışmada, ayrıca adsorpsiyon termodinamiği de incelenmiştir. Buna göre, adsorpsiyon prosesinin kendiliğinden gerçekleştiği (∆G^o < 0) ve ekzotermik olduğu (∆H^o < 0) belirlenmiş ve Dowex HCR-S katyonik reçinenin biyodizelden su adsorpsiyonu için, uygun bir adsorbent olduğuna karar verilmiştir. 

Anahtar Kelimeler

• Biyodizel
• Dowex HCR-S reçine
• su adsorpsiyonu
• izoterm
• kinetik
• termodinamik

Removal of Water from Biodiesel with Resin: Isothermal, Kinetic and Thermodynamic Investigation of Adsorption

Abstract

The serious environmental concerns about the use of fossil-sourced fuels has increased the interest in alternative clean fuels with each passing day. Biodiesel is a clean, biodegradable, non-toxic and renewable fuel produced by transesterification of triglycerides with an alcohol in the presence of a suitable catalyst. During production of biodiesel, after by-product glycerin is removed, raw biodiesel must be purified to reach certain international fuel standards (EN 14214 or ASTM D6751). The presence of impurities in biodiesel does not only significantly affect motor performance, but simultaneously makes use and storage difficult. As a result, purification of raw biodiesel is a necessity. To purify raw biodiesel, in spite of the use of both wet and dry washing, generally the wet washing method (with water) is used. However, the water used in this method must be removed very thoroughly from the biodiesel. Water always causes negative effects on traditional transesterification of oils in production of biodiesel. The presence of water in biodiesel causes the formation of soap, consumes the catalyst, reduces the efficiency of the catalyst and this all leads to low transformation. Additionally, it negatively affects the quality of the fuel.
In this study, the adsorption mechanism of the water in the biodiesel with the cationic Dowex HCR-S ion exchange resin was studied and the adsorption equilibrium, adsorption kinetics and adsorption thermodynamics were investigated for the process. Within this scope, the Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherms were used and the most appropriate isotherm for experimental data was found to be the Langmuir isotherm with R2 value 0.997. Additionally, maximum adsorption capacity was identified as 53.48 mg g-1. Kinetic data were analyzed using pseudo-first order and pseudo second-order kinetic models. The results obtained show the adsorption process fitted the pseudo-second order kinetic model best. The study also investigated the adsorption thermodynamics. Accordingly, the adsorption process was determined to occur spontaneously (∆Go < 0) and be exothermal (∆Ho < 0). It was concluded that Dowex HCR-S cationic resin was an appropriate adsorbent for adsorption of water from biodiesel.  

Keywords

• Biodiesel
• Dowex HCR-S resin
• water adsorption
• isotherm
• kinetics
• thermodynamics

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