REAKSI HETEROGEN KATALITIK

REAKSI HETEROGEN KATALITIKKATALISZat yang dapat mempengaruhi kecepatan reaksi tetapi tidak ikut terlibat dalam reaksi1. meningkatkan atau menurunkan kecepatan reaksi. Yang menurunkan disebut inhibitor.2. Mempengaruhi arah atau selektivitas 3. Jumlah katalis tidak berubah selama reaksi

Jenis katalis : homogen dan heterogen

Katalis homogenKompleks Cobalt dalam pembuatan n dan isobutil aldehidKatalis heterogenReaksi reaktankatalisSintesis amoniaN2 , H2Fe dengan AL2O3 dan KOksidasi amoniaNH3 , O2Pt - RhOksidasi SO2SO2 , O2V2O5 atau PtHidrogenasi minyakH2 , minyak tak jenuhNicrackingFraksi minyak bumiSi, Al dg molecular sievepolimerisasietilenAl-alkil dan TiCl4dehidrogenasiEtil benzeneOksida FeTempat terjadi reaksiReaksi terjadi pada batas muka fluida-padatan. Katalis harus mempunyai luas muka yang besarStruktur berporiContoh : katalis Si-Al mempunyai volum pori 0,6 cm3/g dengan radius 4 nm.Luas muka : 300 m2/gJenis katalis Katalis berporiContoh :Ni dalam hidrogenasi minyakPt-Al dalam reforming naftaFe dalam sintesis amonia2. Molecular sieveHanya dapat dimasuki oleh molekul berukuran kecilContoh : senyawa Si-AlZeolit3. MonolithicKatalis dalam bentuk plate dan disusun secara paralelContoh : Pt dalam oksidasi amonia

Syarat katalis untuk industriSifat mekanik baikDapat diproduksi ulangStruktur stabilTahan terhadap racun dan foulingAktivitas dan selektifitas Distribusi ukuran baikFluida dapat mengalir dengan baikBiaya rendahSifat termal baik

Komponen katalis industriSenyawa bersifat katalis : katalis dan promotorBiasanya senyawa logam dengan titik leleh rendah dan distabilisasi dengan bahan refraktori seperti Al, Cr, dan MgPenyangga katalis : menyediakan badan dan kekuatan dan sering berfungsi untuk menurunkan konsentrasi katalis yang mahalBiasa berupa keramik .Stabilisator : berupa kristal , berguna untuk mencegah sintering dan kehilangan muka aktif

Teknik Penyiapan KatalisPresipitasi atau pembentukan gel dari bahan aktif melalui interaksi larutan air dari dua atau lebih senyawa kimiaImpregnasi dari suatu carrier dengan menggunakan larutan yang mengandung senyawa katalis yang diinginkan

Sifat support/carrierTidak mahalTersedia dalam jumlah banyakCukup berpori sehingga memudahkan dispersi katalis sampai permukaan dalamBebas dari komponen yang mungkin meracuni katalisStabil pada kondisi operasi dan regenerasiCukup kuat terhadap panas dan peristiwa mekanisTidak mudah ausInert terhadap komponen pada umpan, produk maupun arus regenerasiNon katalitik terhadap reaksi samping yang tidak diinginkan.Tipe bahan supportBahan dengan luas muka rendah (sampai 1 m2/g)keramik silika aluminaBahan dengan luas muka tinggi (10 -100 m2/g)bahan alam dan sintetis bentuk serbukdiatome, batu apung, turunan lempung, bauksit, karbon aktif, xerogel dari Si-Al

Bentuk katalisPowder : ukuran 100 mikronMicrosphere : 40 -150 mikronCocok untuk slurry dan fluidized bed

Bentuk katalisSphere : 1 5 mmReaktor slurry dan fluidized bed Granula : 40 mikron 1 cmReaktor fixed bed

Bentuk katalisExtrudat (pelet) : 0,3 8 mmReaktor Fixed bedMinilith : panjang - inch

Langkah reaksi katalitikTrasfer massa (difusi) reaktan dari badan fluida ke permukaan luar katalisDifusi reaktan ke dalam pori-pori katalisAdsorpsi (Chemisorption) reaktan pada permukaan katalisReaksiDesorpsi hasil dari permukaan katalisDifusi hasil keluar poriTransfer massa hasil dari permukaan luar ke badan fluida

Deaktivasi katalisFoulingterjadi dengan cepat dan disebabkan oleh dekomposisi atau degradasi reaktan atau hasil pada permukaan katalis.deposit terbentuk pada permukaan sehingga menutupi permukaan dan poriContoh : fouling dalam reaksi cracking sehingga perlu regenerator

2.Poisoningterjadi karena permukaan katalis berubah akibat terjadinya chemisorption bahan yang tidak mudah dihilangkan.biasa disebabkan oleh adsoprsi sejumlah kecil pengotor umpan.jika proses adsorpsi reversibel dapat dihilangkan dengan reaktivasi misal mengubah suhu atau komposisi umpan.jika proses irreversibel maka pengaktifan dengan proses kimia atau penggantian katalisDeaktivasi katalis3. Agingkehilangan aktivitas akibat perubahan luas permukaan katalis karena pertumbuhan kristal atau proses sinteringREAKTORReaktor fixed bedExamplesDirect oxidation of methanol to formaldehyde over ferric molybdate catalyst at 330~380 C, 1.2 ~ 1.5 bar, and 7000~12000 h-1 GHSV. Production of vinyl acetate over palladium/silica catalyst at 150-175 C and about 5-10 bar.

Fluidized bedExamplesThe manufacture of acrylonitrile by the ammoxidation of propylene over complex metal-oxide catalysts at 400-500 C and 1.5 bar.

Trickle columnExamplesHydrogenation of pyrolysis gasoline over supported nickel catalyst at 60~140 C, H2 partial pressure of 15~30 bar, and 1~6 h-1 LHSV. Production of isopropanol by the addition of water to the propylene over a solid sulfonic acid ion exchanger at 130~160 C, with a pressure of 80~100 bar.

Slurry bubble column reactorExamplesHydrogenation of benzene to cyclohexane with Raney-Nickel catalyst at 200 C and 20~30 bar

AluminaAlumina supports have a wide range of surface areas and pore volumes. The supports can be treated for excellent stability at high temperatures to avoid agglomeration/sintering of surface metals. Appropriate for intermediate pH.

AluminaTitaniaAlumina/Titania have the advantages of alumina with the additional functionality of Titania

AluminaZirconiaAlumina/Zirconia supports have the pore structure of Alumina with the additional functionality of Zirconia

Carbon HybridsCarbon Hybrid Supports are a new class of catalyst supports that combine the features of carbon and solid oxide supports to provide catalytic surfaces with unique properties

Ceria/ZirconiaCeria/Zirconias supports are designed for applications that require a large oxygen storage capacity coupled with a high rate of oxygen release. Appropriate for high pH conditions

ClayClay supports are inexpensive and can provide some catalytic functionality at higher temperatures.

SilicaSilica supports are designed to have the broadest range of surface areas and pore volumes. Silica can be surface-treated to minimize additional catalytic effects. These materials can be used over a wide range of pH conditions

SilicaAluminaSilica/Aluminas supports are designed to have a broad range of surface areas coupled with an acid function. The acid site concentration is determined by the Si/Al ratio of the support

SilicaTitaniaSilica/Titania supports have the pore structure of silica combined with the functionality of titania.

SilicaZirconiaSilica/Zirconia supports have the pore structure of silica with an additional acid function

ZeoliteZeolite supports are designed for reactions that can take advantage of the shape-selectivity or size exclusion of the zeolite pore. Typical pore sizes are between 4 and 8 Angstroms

General features: The catalytic particles are packed in their positions in a fixed bed reactor. It is most practical for large-scale, relatively slow reactions not involving large heat generation or consumption. The fluid moves downward through the reactor in nearly plug flow.

Reaction phases : Gas or liquid

Operation : Continuous

Particle size and shape : Pressure drop determines particle size and shape. Larger particles will produce less pressure drop, more diffusion limitations.

Heat of reaction : Moderate. Intermediate cooling or heating may be necessary for highly exothermic or endothermic reactions

General features: The catalytic particles are supported by an upflow of gas. This reactor provides easy loading and removing of catalysts, so it is good when the catalysts must be removed and replaced frequently. It is less expensive to construct but hydrodynamics are very complex. A high conversion with a large throughput(high space-time yield) can be obtained. Particles should be attrition-resistant and not agglomerate. Cyclones are usually needed to return the fines to the bed.

Reaction phases : Gas

Operation : Continuous

Particle size and shape : 20-100 micron average particle size with a narrow size distribution is generally recommended. Spherical shape may be used and they need to be attrition resistant.

Heat of reaction : Excessive heat of reaction can be easily controlled in fluidized bed reactor with good temperature uniformity.

General features: The catalytic particles are fixed in their positions and liquid/gas reaction mediums flow downward concurrently through the packed bed of particles. Distributor design may be needed for even distribution of gas and liquid over the column(bed).

Reaction phases : Gas and liquid

Operation : Continuous

Particle size and shape : No specific requirement for particle size and shape. Larger particles will produce less pressure drop, more diffusion limitations.

Heat of reaction : Moderate. Excessive heat of reaction can be controlled by partial vaporization of liquid medium.

General features: The particles are suspended in a mechanically stirred liquid and also by the uprising gas bubbles. The reaction involves complex mass transfer from the gas bubbles to liquid medium, and finally to the catalytic particles.

Reaction phases : Liquid and gas

Operation : Batch

Particle size and shape : Size of < 200 micron is used for good suspension. Particles must be attrition resistant.

Heat of reaction : Excessive heat of reaction is absorbed through liquid medium and exchanged by external cooling jacket. The catalyst suspension may be circulated for better heat removal.