STUDI KINERJA ZONA PEMBAKARAN SECONDARY REFORMER PABRIK AMONIAK PUSRI II DENGAN COMPUTATIONAL FLUID DYNAMICS (CFD)

Alfa Widyawan, Muslikhin Hidayat, ST, MT, Ph.D

2012 | Tesis | S2 Teknik Kimia

Secondary reformer merupakan salah satu peralatan proses dalam pabrik Amoniak Pusri II yang berfungsi sebagai tempat terjadinya reaksi antara steam dan metana untuk menghasilkan hidrogen. Reaksi ini merupakan reaksi endotermis dan kebutuhan panas reaksi dipenuhi oleh reaksi pembakaran metana dan oksigen yang terjadi di zona pembakaran secondary reformer. Tahun 2011 kinerja secondary reformer menurun dengan ditandai adanya kenaikan konsentrasi metana keluar dari secondary reformer di atas 0,3 % mol. Kenaikan konsentrasi metana diperkirakan karena adanya masalah pada mixing antara metana dan oksigen di zona pembakaran. Dari pemeriksaan saat TA 2012 terlihat adanya kerusakan pada hexagonal bricks di sisi tengah outlet zona pembakaran. Oleh karena itu perlu dilakukan studi mengenai distribusi kecepatan, suhu dan konsentrasi aliran fluida yang berguna untuk mengevaluasi kinerja zona pembakaran secondary reformer. Penelitian ini bertujuan untuk mempelajari distribusi kecepatan, suhu dan konsentrasi fluida di outlet zona pembakaran secondary reformer dengan menggunakan software Computational Fluid Dynamics (CFD) yaitu ANSYS 13.0 (Fluent). Asumsi kondisi steady state dan bagian dalam burner dalam kondisi baik. Model turbulensi yang digunakan adalah realizable k-epsilon. Model species yang digunakan adalah non premix combustion. Perhitungan penyelesaian numeris dilakukan dengan 2 nd order upwind scheme. Tahapan simulasi meliputi pembuatan geometri zona pembakaran, pembuatan mesh, penyelesaian numeris serta analisis hasil. Mesh independency didapatkan pada mesh sebanyak 21535 sel. Pemilihan variabel dilakukan berdasarkan kondisi relevan yang terjadi pada proses di secondary reformer. Variabel yang diteliti berupa variasi laju alir umpan (62300-75000 kg/jam), suhu udara umpan (440 – 500 o C), jarak burner terhadap outlet zona pembakaran (1033 – 1093 mm) dan sudut burner (35 – 50 o ). Hasil simulasi CFD menunjukkan bahwa mixing antara metana dan oksigen terjadi dengan cukup baik sehingga proses yang terjadi (dengan asumsi burner dalam kondisi baik) tidak menyebabkan terjadinya kerusakan hexagonal bricks di outlet zona pembakaran secondary reformer. Hasil simulasi juga menunjukkan bahwa kenaikan laju alir umpan dan suhu udara umpan akan menurunkan konsentrasi metana dan oksigen di outlet zona pembakaran. Penambahan jarak burner terhadap sisi outlet zona pembakaran menyebabkan distribusi laju alir fluida dan konsentrasi metana lebih merata di setiap sisi outlet. Penurunan sudut burner menyebabkan laju alir fluida lebih merata, namun distribusi konsentrasi metana tidak terlalu baik.

a Secondary reformer is one of the mmonia plant process equipments in Pusri II where a reaction occurs between steam and methane to produce hydrogen. The reaction is endothermic and heat for this reaction is fulfilled from combustion reaction between methane and oxygen in combustion zone of secondary reformer. The secondary reformer performance was decreased in 2011, showed by the increase of methane concentration out from secondary reformer above 0,3 % mole. This case was estimated because of mixing problem between methane and oxygen in combustion zone. There was a damage of hexagonal bricks at the center of combustion zone outlet when checked at TA 2012. Therefore, it is necessary to study velocity distribution, temperature and concentration of fluid in order to evaluate the performance of the secondary reformer combustion zone. This research aimed at study the distribution of velocity, temperature and concentration of the fluid at the outlet of the secondary reformer combustion zone with Computational Fluid Dynamics (CFD) of ANSYS, 13.0 (Fluent) software. There were some assumptions used in the simulation. The internal part of burner was in a good condition and the process was steady state. The simulation used in the simulation was realizable k-epsilon turbulence model and non premixed combustion species model. The differential equation were numerically solved with the used of 2 nd order upwind scheme. The CFD simulation consisted of combustion zone geometry development, meshing, numerical solution and results analysis. Mesh independency was obtained at 21535 mesh cells. The variables of this simulation were feed flowrate (62300-75000 kgs/h), air feed temperature (440-500 °C), the distance between burner and the outlet of combustion zone (1033-1093 mm) and burner’s angles (35-50 o ) . The effect of these variables to the concentration distribution of methane and oxygen at the outlet of combustion zone were evaluated. The CFD simulations showed that there was a good mixing between methane and oxygen, so that it would not lead to a damage of hexagonal bricks at the secondary reformer combustion zone outlet. The results also showed that the increase of feed flowrate and air feed temperature would decrease the concentration of methane and oxygen at the outlet of combustion zone. The increase of distance between burner and the outlet of combustion zone would cause the distribution of the fluid flowrate and methane concentration more even at each spot of the outlet. The decrease of burner’s angle would lead to more even fluid flowrate distribution, but not methane concentration distribution.

Kata Kunci : secondary reformer, zona pembakaran, burner, CFD, realizable kepsilon, mixing, distribusi konsentrasi metana