PENGARUH LUAS PERMUKAAN DAN LEBAR PORI KARBON AKTIF PADA SISTEM ADSORBED NATURAL GAS (ANG)

Andy Cahyadi, Ir. Imam Prasetyo, M.Eng., Ph.D.

2013 | Tesis | S2 Teknik Kimia

Gas alam merupakan salah satu bahan bakar alternatif pengganti BBM karena ramah lingkungan, memiliki nilai HHV tinggi, dan cadangan yang melimpah di seluruh dunia. Kendala utama pemanfaatan gas alam adalah sistem penyaluran dan penyimpanan gas alam. Hingga saat ini ada tiga teknologi untuk penyimpanan gas alam: liquefied natural gas (LNG), compressed natural gas (CNG), dan adsorbed natural gas (ANG). Dalam beberapa tahun terakhir, ANG telah menarik perhatian sebagai teknologi alternatif dibandingkan CNG dan LNG. Gas alam mayoritas terdiri dari gas metana (85-95%) dengan sejumlah kecil etana dan senyawa hidrokarbon berat lain sehingga karakteristiknya mirip dengan metana. Dalam penelitian ini, CNG digunakan sebagai pengganti gas alam karena komposisinya sama. Jumlah gas yang teradsorpsi di sistem ANG tergantung pada jenis dan sifat adsorben. Penelitian sebelumnya menunjukkan bahwa karbon aktif adalah adsorben terbaik untuk sistem ANG. Tujuan penelitian ini adalah mengevaluasi efek struktur pori adsorben (luas permukaan BET dan lebar pori) terhadap banyaknya metana dan CNG teradsorpsi dan tersalurkan. Penelitian ini juga membahas panas adsorpsi dan lebar pori optimum sebagai faktor penting dalam kinerja ANG. Untuk mengetahui pengaruh luas permukaan adsorben, dilakukan percobaan adsorpsi metana dan gas alam pada tiga karbon aktif dengan luas permukaan BET dari 1.000-3.000 m2/g (karbon Maxorb, RTBPF, and Ajax). Sedangkan untuk mempelajari pengaruh lebar pori dilakukan percobaan adsorpsi metana pada dua buah karbon aktif dengan luas permukaan BET sama tetapi lebar pori berbeda (karbon Ajax and RPF-EG2). Data adsorpsi dan desorpsi gas metana dan CNG pada semua karbon aktif diukur dengan sistem adsorpsi secara statik volumetris pada kisaran tekanan (0-4 MPa) dan suhu operasi tangki ANG (303-323 K). Data kesetimbangan adsorpsi didekati dengan baik oleh model isoterm Langmuir, Freundlich, Sips, Unilan, dan Toth. Model Toth merupakan model yang paling mendekati hasil penelitian. Urutan karbon aktif yang mengadsorpsi metana paling banyak adalah Maxorb > RTBPF > Ajax > RPF-EG2. Hasil penelitian menunjukkan jumlah metana teradsorpsi proporsional terhadap luas permukaan dan lebar pori juga menentukan jumlah metana teradsorpsi. Urutan berdasarkan panas adsorpsi berkebalikan dari urutan berdasarkan jumlah metana teradsorpsi. Metode Grand Canonical Monte Carlo (GCMC) digunakan untuk menghitung isoterm adsorpsi dan lebar pori optimum untuk metana dalam mikropori berbentuk slit pada berbagai kisaran suhu (303-323 K) dan lebar pori (4-60 A). Molekul metana dimodelkan sebagai molekul bulat Lennard-Jones dan persamaan potensial Steele 10-4-3 digunakan untuk menghitung energi interaksi antara molekul metana dan dinding adsorben. Hasil simulasi dan penelitian yang mirip menunjukkan bahwa model dapat menjelaskan mekanisme adsorpsi metana di karbon aktif dengan baik. Simulasi menunjukkan lebar pori optimum untuk adsorpsi metana adalah 8,35 Ã… (303 K), 8,51 Ã… (313 K), dan 8,79 Ã… (323 K).

Natural gas (NG) is one of the best alternative fuel to replace gasoline due to its clean properties, higher HHV, cheap price, and its vast proved reservoirs in the world. The main problem to NG utilization are its transportation and storage system. So far, there are three known technologies for on-board NG storage: liquefied natural gas (LNG), compressed natural gas (CNG) and adsorbed natural gas (ANG). In recent years, ANG has attracted considerable attention as a possible alternative to CNG and LNG. NG consists of mainly methane (85-95%) with a minor amount of ethane, and higher-order hydrocarbon compounds, therefore, the characteristics of NG are similar to methane. In this experiment, we used CNG as the substitute for NG because they have the same composition. The amount of adsorbed gas in ANG system depends on adsorbent types and properties. Previous experiment showed that activated carbon is the most potential material for ANG storage adsorbent. The objective of this study is to evaluate the effect of activated carbon pore structure (BET surface area and pore width) to the amount of adsorbed methane. We also discuss the heat of adsorption as well as the optimum pore width which are important factors in ANG performance. In order to evaluate the effect of adsorbent surface area, adsorption of methane and CNG in three activated carbons with 1,000-3,000 m2/g BET surface area (carbon Maxorb, RTBPF, Ajax) has been done. Another experiment of methane adsorption in two activated carbon with same BET surface area but different pore width (Ajax and RPF-EG2) is used to evaluate the effect of pore width. The methane and CNG adsorption and desorption equilibrium data were measured by a static volumetric adsorption system at operational pressures ranges (0-4 MPa) and temperature ranges (303-323 K). The experimental data is well fitted by Langmuir, Freundlich, Sips, Unilan, and Toth models. The Toth model provided the best fit to the experimental adsorption isotherms. The activated carbon which have the biggest adsorption intake is ordered as follows: Maxsorb > RTBPF > Ajax > RPF-EG2. The results indicates that the amount of adsorbed methane is proportional to the surface area while pore width also have effect on it. The heat of adsorption is ordered as the opposite of the order of the amount of adsorbed methane. Grand Canonical Monte Carlo (GCMC) method has been used to calculate the adsorption isotherms and optimum pore width for a simple model of methane confined in slit carbon micropores at various temperatures (303 K-323 K) and pore widths (4-60 A). Methane molecules are modeled as the Lennard-Jones spherical molecules, and Steele’s 10-4-3 potential is used to represent the interaction between the methane molecule and the adsorbent wall. Good agreement between simulated and experimental data indicates that our model represents well the mechanism of methane adsorption on an activated carbon. Our simulation found that the optimum pore width for methane adsorption are 8,35 Å (303 K), 8,51 Å (313 K), and 8,79 Å (323 K).

Kata Kunci : adsorpsi metana dan gas alam, GCMC, isoterm, karbon aktif, lebar pori optimum, luas permukaan BET