PERENGKAHAN KATALITIK BIO-OIL MENJADI FRAKSI BAHAN BAKAR DAN ANALISIS TERMODINAMIKANYA

SUNARNO, Prof. Ir. Arief Budiman, MS, D.Eng;Prof. Ir. Rochmadi, SU, Ph.D; Prof. Ir. Panut Mulyono, M.Eng, D.Eng

2018 | Disertasi | S3 ILMU TEKNIK KIMIA

Biomassa merupakan sumber energi alternatif yang sangat menjanjikan. Tandan kosong sawit adalah sumber biomassa yang jumlahnya berlimpah di Indonesia. Biomassa ini mengandung selulosa, hemiselulosa dan lignin yang dapat dikonversikan menjadi bio-oil melalui proses pirolisis. Namun bio-oil ini belum dapat menggantikan bahan bakar transportasi karena mengandung senyawa oksigenat sehingga viskositasnya tinggi, kandungan oksigen tinggi, nilai kalor rendah dan bersifat korosif. Untuk itu dibutuhkan proses untuk meningkatkan kualitas bio-oil, diantaranya adalah perengkahan katalitik bio-oil. Tujuan penelitian ini adalah mempelajari pengaruh suhu dan panjang bed katalis terhadap yield dan komposisi serta kinetika dan eksergi pada perengkahan katalitik bio-oil. Perengkahan katalitik bio-oil ini dijalankan pada tekanan atmosferik dengan suhu 450 sampai 600 derajat celsius di dalam reaktor fixed-bed yang berisi katalis silika-alumina dengan ketebalan 1 sampai 4 cm. Rangkaian alat terdiri dari pompa umpan, vaporiser, reaktor, tungku pembakaran, kompor gas LPG, kondensor dan pompa air pendingin. Sebelum pemanasan, 10 gram katalis silika-alumina diisikan ke dalam bed katalis. Reaktor dipanaskan dengan bahan bakar LPG sampai suhu operasi yang diinginkan. Kemudian gas nitrogen dialirkan dengan laju alir 400 ml/menit, dilanjutkan bio-oil dialirkan dengan laju alir 8 ml/menit dan dihentikan sampai volume 50 ml. Uap yang dihasilkan dikondensasi dan cairan yang terkondensasi ditampung dalam erlenmeyer. Komposisi produk cairan dianalisis dengan gas chromatography-mass spectroscopy (GC-MS), sedangkan komposisi produk gas dianalisis dengan gas chromatography (GC). Hasil penelitian menunjukkan bahwa suhu dan ketebalan katalis berpengaruh tehadap yield produk dan kinetika perengkahan katalitik bio-oil. Dengan kenaikan suhu dan ketebalan katalis, yield oil turun, sementara yield gas naik. Data percobaan ini didekati dengan 3 model yaitu Model 1, Model 2 dan Model 3. Dengan Model 1 dapat digunakan untuk menentukan konstanta kecepatan reaksi perengkahan katalitik bio-oil pada tiap suhu percobaan, dalam hal ini sesuai dengan persamaan Arrhenius. Sedangkan Model 2 dan 3 dapat dipakai untuk menjelaskan langkah reaksi yang dominan pada perengkahan katalitik bio-oil. Disamping pendekatan kinetika, pada penelitian ini juga dianalisis termodinamikanya. Dengan konsep exergy loss menunjukkan bahwa exergy loss terbesar berasal dari vaporiser dan kondensor, yang disebabkan oleh perubahan suhu dan fasa. Berdasarkan konfigurasi simulasi menyatakan bahwa pemanfaataan panas reaksi sebagai media pemanas preheater 1 dapat menurunkan exergy loss total dari 2.367,76 menjadi 1.890,69 kJ/kg.menit.

Among many alternatives, biomass is a renewable energy source. As an agricultural country, one of the most abundant biomass in Indonesia is palm empty fruit bunches (EFB). This type of biomass has a very competitive price because it is produced as a solid waste from the palm oil industry. Moreover, EFB contains cellulose, hemicellulose and lignin, all of which can be converted into bio-oil through pyrolysis. On the other hand, bio-oil produced from biomass pyrolysis has not been able to substitute transportation fuel since it contains a high proportion of oxygenated compounds. These oxygenated compounds have various undesired properties on bio-oil such as thermal instability, high viscosity, low heating value and corrosiveness. Consequently, prior to being used as an energy source, the effort to upgrade bio-oil quality is required. Reducing the oxygenated compounds in bio-oil can be achieved through catalytic cracking. The aims of this research was to study the effect of operation temperature and length of catalyts bed on yield and composition of product, determine the kinetics and exergy in catalytic cracking of bio-oil. The catalytic cracking of bio-oil was performed under atmospheric pressure from 450 to 600 degree celcius in a tubular reactor packed with a silica-alumina catalyst bed that had a depth of 1 to 4 cm. This system consist of two tube reactors (inner diameter = 70 mm, length = 300 mm), a liquid feed system, a liquid feed pump, furnace, stove, a gas of LPG system, a condenser and a cooler water pump. Before the heating, 10 g of the silica-alumina catalyst with a particle size 6 mm was uniformly filled in the catalyst bed. Reactor was heated with LPG fuel until setted temperature of operation. Futhermore, nitrogen was flowed with flow rate of 400 ml/min, then bio-oil was flowed with volumetric rate of 8 ml/min and stopped to volume of 50 ml. The vapor produced was condensed in water cooled condenser and collected in the erlenmeyer. The compositions of the liquid and gas products were analyzed using gas chromatography-mass spectroscopy (GC-MS) and gas chromatography (GC), respectively. The results show that temperature and catalyst bed thickness affected the product yield and kinetics of bio-oil catalytic cracking. With an increase in the catalyst bed thickness and temperature, the oil yield decreased, while the gas yield increased. The reaction rate constants of bio-oil cracking were calculated by using kinetic Model 1, which satisfied the Arrhenius equation. Kinetic Models 2 and 3 can be employed to determine the dominant reaction step in bio-oil catalytic cracking. The thermodynamic analysis using the concept of exergy loss, shows that the highest exergy loss derives from vaporizer and condenser which occured the temperature and phase changes. From the simulation configurations that utilizing reaction heat as media preheater 1 can decrease the exergy loss total of 2,367.76 into 1,890.69 kJ/kg.min.

Kata Kunci : Bio-oil, catalytic cracking, kinetic, silica-alumina, exergy