Reaktor dan Bioreaktor
Pendahuluan
Reaktor adalah jantung dari setiap proses fermentasi, konversi enzim konversi kimia.
Merancang bioreaktor merupakan pekerjaan yang kompleks, bergantung pada prinsip ilmiah dan teknik serta aturan praktis (rule of thumbs).
Menentukan aspek reaktor dan operasinya melibatkan beberapa keputusan penting.
i. Konfigurasi reaktor. Misalnya, haruskah reaktor berupa tangki berpengaduk atau bejana yang digerakkan udara tanpa agitasi mekanis?
ii. Ukuran reaktor. Berapa ukuran reaktor yang diperlukan untuk mencapai tingkat produksi yang diinginkan?
iii. Kondisi pemrosesan di dalam reaktor. Kondisi reaksi seperti suhu, pH dan tekanan oksigen terlarut yang harus dipertahankan dalam bejana, dan bagaimana parameter- parameter ini akan dikontrol? Bagaimana kontaminasi dapat dihindari?
iv. Mode operasi. Apakah reaktor dioperasikan secara batch atau sebagai proses aliran kontinu? Haruskah substrat diberi makan sebentar-sebentar? Haruskah reaktor dioperasikan sendiri atau secara seri dengan yang lain?
Pendahuluan
• Reaktor Adiabatik: tangki yang diisolasi untuk meminimalkan perpindahan panas, kenaikan atau penurunan suhu berasal dari saluran masuk dan terjadi karena panas reaksi.
• Reaktor Batch : tangki yang digunakan untuk reaksi kimia tidak memiliki aliran umpan atau limbah. Reaktor diaduk dengan baik dan biasanya dijalankan baik secara isotermal maupun secara adiabatik. Variabel perancangan utama reaktor ini adalah berapa lama reaktan dibiarkan tetap di reaktor untuk mencapai tingkat konversi yang diinginkan.
• Reaktor tangki berpengaduk kontinu (Continuous Stirred Tank Reactor, CSTR):
terkadang disebut reaktor tangki berpengaduk aliran kontinu, mixer ideal, atau reaktor aliran campuran, semua istilah tersebut menggambarkan reaktor dengan input dan output material yang kontinu. Konsentrasi keluaran diasumsikan sama dengan konsentrasi di setiap titik dalam reaktor.
• Reaktor aliran (Plug Flow Reactor, PFR): disebut juga reaktor aliran sempurna.
Reaktor aliran memiliki input dan output material yang kontinu. Asumsi aliran pada reaktor ini umumnya membutuhkan aliran turbulen.
Klasifikasi Reaktor
Berdasarkan konfigurasi operasional.
• Batch: reaktor batch dioperasikan dengan semua bahan ditempatkan di reaktor sebelum dimulainya reaksi, dan semua bahan dikeluarkan setelah reaksi selesai. Tidak ada penambahan atau penarikan bahan selama proses reaksi.
• Semi-batch: reaktor semi-batch menggabungkan istilah batch dan tangki berpengaduk kontinu. Reaktor ini pada dasarnya beroperasi secara batch tetapi memiliki input atau output yang kontinu selama operasi.
• Reaktor Aliran Kontinu: reaktor aliran kontinu mewakili kelompok reaktor terbesar berdasarkan klasifikasi operasional. Beberapa reaktor aliran kontinu digunakan secara industri.
Klasifikasi Reaktor
Reaktor Aliran Kontinu
• Reaktor tangki pengaduk kontinu (CSTR) melibatkan pengumpanan reaktan ke dalam tangki berpengaduk dengan output produk secara simultan.
• Reaktor aliran plug (PFR) terdiri dari pipa atau tabung panjang.
Campuran reaksi bergerak ke bawah tabung sehingga terjadi perubahan konsentrasi sepanjang reaktor.
• Dalam reaktor dengan aliran daur ulang, bagian dari aliran keluar dikembalikan ke saluran masuk reaktor. Reaktor dengan aliran daur ulang memungkinkan operasi terus menerus dalam CSTR dan PFR.
Klasifikasi Reaktor
Klasifikasi Reaktor
Reaktor juga dapat diklasifikasikan berdasarkan jumlah fase yang ada di reaktor.
• Homogen: Reaktor homogen hanya mengandung satu fase di seluruh reaktor.
• Heterogen: Reaktor heterogen mengandung lebih dari satu fase.
a. Gas-cair b. Padat gas c. Cair-padat
d. Gas-cair-padat
Konfigurasi reaktor multifase sangat dipengaruhi oleh operasi perpindahan massa. Setiap tipe reaktor yang disajikan di atas dapat dioperasikan sebagai reaktor multifase.
Klasifikasi Reaktor
Klasifikasi reaktor juga dapat dibuat berdasarkan jenis reaksi.
• Katalitik: reaksi yang membutuhkan katalis untuk memperoleh kondisi laju yang diperlukan untuk desain reaktor tertentu.
• Non-catalytic: reaksi yang tidak menggunakan katalis homogen atau heterogen.
• Autocatalytic: skema reaksi di mana salah satu produk dapat meningkatkan laju reaksi keseluruhan.
• Biologis: reaksi yang melibatkan sel hidup (enzim, bakteri, atau ragi), bagian sel, atau produk dari sel yang diperlukan untuk reaksi.
• Polimerisasi: reaksi yang melibatkan pembentukan rantai molekul, baik pada fasa padatan atau dalam larutan.
Primary Reactors
• There are five primary reactor designs based in theory: batch, semibatch, continuous-stirred tank, plug flow, and fluidized bed. The operating expressions for these reactors are derived from material and energy balances, and each represents a specific mode of operation.
BATCH
• Batch processes are the easiest to understand since they strongly relate to
"cookbook" technology. You put everything in at the beginning and stop the reaction at some time later. This cookbook technology allows for immediate production of a new product without extensive knowledge of the reaction kinetics.
• The reactor is characterized by no addition of reactant or removal of product during the reaction. Any reaction being carried out with this constraint, regardless of any other reactor characteristic, is considered batch.
BATCH
The assumptions for batch operation are 1) the contents of the tank are well mixed,
2) reaction does not occur to any appreciable degree until filling and startup procedures are complete, and
3) the reaction stops when quenched or emptied.
The reactor can be operated with either a homogeneous or heterogeneous reaction mixture for almost any type of reaction.
BATCH_ADVANTAGES-DISADVANTAGES.
• The primary advantages of the batch reactor are simplicity of design, which allows for tremendous flexibility, and integration of the performance equation over time. The simplicity of design, usually a stirred tank, makes operation and monitoring easy for the majority of reactions.
BATCH_ADVANTAGES-DISADVANTAGES.
• One of the traditional disadvantages of the batch reactor has been the labor required between runs for emptying and filling the tank.
• The major disadvantage of batch reaction now is the hold-up time between batches. Although the actual reaction time necessary to process a given amount of feed may be substantially less than for a time-averaged reactor such as a CSTR, when the hold-up time is added, the total process time may be greater. Other disadvantages of the batch reactor are dependent on the particular type of reaction being considered, such as whether the reaction is in parallel or series.
SEMI-BATCH
• The semi-batch reactor is a cross between an ordinary batch reactor and a continuous-stirred tank reactor. The reactor has continuous input of reactant through the course of the batch run with no output stream. Another possibility for semi-batch operation is continuous withdrawal of product with no addition of reactant.
SEMI-BATCH
• Physically, the semi-batch reactor looks similar to a batch reactor or a CSTR. Reaction occurs in a stirred tank, with the following assumptions: (1) the contents of the tank are well mixed, and (2) there are no inlet or outlet effects caused by the continuous stream.
SEMI-BATCH_ADVANTAGES-DISADVANTAGES
• The temperature-controlling features of this reaction scheme dominate selection and use of the reactor. However, the semi-batch reactor does have some of the advantages of batch reactors: temperature programming with time and variable reaction time control.
• The temperature conditions and the batch nature of this reactor are the primary operational difficulties and make the reactor impractical for most reactions, even for computer-controlled systems. The majority of reactions considered for semi- batch are highly exothermic and, as such, are dangerous and require special attention.
CONTINUOUS-STIRRED TANK
• The continuous-stirred tank reactor (CSTR) has continuous input and output of material. The CSTR is well mixed with no dead zones or bypasses in ideal operation. It may or may not include baffling. The assumptions made for the ideal CSTR are (1) composition and temperature are uniform everywhere in the tank, (2) the effluent composition is the same as that in the tank, and (3) the tank operates at steady state
CONTINUOUS-STIRRED TANK_ADVANTAGES-DISADVANTAGES
The advantages for CSTRs include
• steady-state operation,
• back mixing of heat generated by exothermic reactions, which increases the reaction rate and subsequent reactor performance,
• avoidance of reactor hot spots for highly exothermic reactions, making temperature easier to control,
CONTINUOUS-STIRRED TANK_ADVANTAGES-DISADVANTAGES
The advantages for CSTRs include
• favoring lower-order reactions in parallel reaction schemes,
• economical operation when large volumes require high contact time, and
• enhancement of heat transfer by mixing.
CONTINUOUS-STIRRED TANK_ADVANTAGES-DISADVANTAGES
The disadvantages for CSTRs
• For the kinetics of decreasing rate with increasing conversion (most reactions), isothermal CSTRs have lower product composition than plug flow reactors.
• Additional disadvantages of CSTR are that larger reactor volumes are usually required, compared with other reactor schemes, and that energy for agitation is required in the tank, increasing operating costs .
PLUG FLOW
• This reactor has continuous input and output of material through a tube. The PFR can be imagined as a tube, but not all tubular reactors respond as PFRs. The assumptions need to be verified with experimental data.
PLUG FLOW
Assumptions made for the plug flow reactor (PFR) are
• material passes through the reactor in incremental slices (each slice is perfectly mixed radially but has no forward or backward mixing between slices; each slice can be envisioned as a miniature CSTR),
• composition and conversion vary with residence time and can be correlated with reactor volume or reactor length, and
• the reactor operates at steady state.
PLUG FLOW_ADVANTAGES-DISADVANTAGES
The advantages of a PFR include
• steady state operation,
• minimum back mixing of product so that concentration remains higher than in a CSTR for normal reaction kinetics,
• minimum reactor volume in comparison with CSTR (since each incremental slice of the reactor looks like an individual CSTR, we can operate at an infinite number of points along the rate curve),
PLUG FLOW_ADVANTAGES-DISADVANTAGES
The advantages of a PFR include
• application of heat transfer in only those sections of the reactor where it is needed (allowing for temperature profiles to be generated down the reactor), and
• no requirement for agitation and baffling.
FLUIDIZED BED
• Fluidization occurs when a fluid is passed upward through a bed of fine solids. At low flow rates the gases or liquids channel around the packed bed of solids, and the bed pressure drop changes linearly with flow rate.
• At higher flow rates the force of the gas or liquid is sufficient to lift the bed, and a bubbling action is observed. During normal operation of a fluidized bed the solid particles take on the appearance of a boiling fluid.
FLUIDIZED BED
• The reactor configuration is usually a vertical column.
• The fluidized solid may be either a reactant, a catalyst, or an inert.
• The solid may be considered well mixed, while the fluid passing up through the bed may be either plug flow or well mixed depending on the flow conditions.
• Bubble size is critical to the efficiency of a fluidized bed.
FLUIDIZED BED_ADVANTAGES-DISADVANTAGES
• The fluidized bed allows for even heat distribution throughout the bed, thereby reducing the hot spots that can be observed in fixed-bed reactors.
The small particle sizes used in the bed allow high surface area per unit mass for improved heat and mass transfer characteristics. The fluidized configuration of the bed allows catalyst removal for regeneration without disturbing the operation of the bed. This is particularly advantageous for a catalyst that requires frequent regeneration.
FLUIDIZED BED_ADVANTAGES-DISADVANTAGES
• Several disadvantages are associated with the fluidized bed. The equipment tends to be large, gas velocities must be controlled to reduce particle blowout, deterioration of the equipment by abrasion occurs, and improper bed operation with large bubble sizes can drastically reduce conversion.
AUTOCLAVE
• The autoclave reactor is a small cylindrical reactor, built to withstand high pressures, used to evaluate the kinetics of high temperature, high-pressure reactions and the production of small quantities of specialty chemicals. The reactor is typically packed with a supported catalyst, and reactant is added by injection. Pressure in the system is elevated by increasing the temperature of the autoclave. Additional pressure, if needed, can be obtained with the injection of additional gaseous reactant or an inert
BLAST FURNACE
• The blast furnace, a vertical shaft kiln, is the oldest industrial furnace.
Reactant enters in the top of the shaft and falls down through a preheating section, a calcinating section, past oil, gas, or pulverized coal burners, through a cooling section, with the product ash falling through a discharge gate.
BUBBLE COLUMN
• The bubble column is a tower containing primarily liquid (>90%) that has a gas or a lighter liquid sparged into the bottom, allowing bubbles to rise through the column. The column may contain staging, which enhances the mass transfer characteristics of the reactor. In countercurrent operation the reactor is particularly attractive for slightly soluble gases and liquid- liquid systems. With co-current flow and a highly baffled column, the reactor has mass transfer characteristics similar to those of a static mixer.
The reactor may sometimes contain a solid suspended in the liquid phase.
CHEMOSTAT—TURBIDOSTAT
• The chemostat is a biological CSTR where the substrate concentration in the tank is maintained constant. The turbidostat is similar to the chemostat except that the cell mass in the reactor is kept constant. The primary distinction between the two reactors is the control mechanism used to maintain continuous operation. A unique feature of a biological CSTR is the washout point. When the flow rate is increased so that the microbes can no longer reproduce fast enough to maintain a population, the microbes wash out of the tank, and the reaction ceases. This washout point represents the limits of maximum flow rate for operation.
DIGESTOR
• The digestor is a biological reactor used mainly for the treatment of municipal and industrial wastes. Wastes are fed continuously to the digestor, where some solids settle to the bottom of the tank, and other solids are matted and lifted to the surface by the gases produced during the fermentation. In an aerobic digestor the mat is broken and mixed by gas circulation. The solid sludge in the bottom of the tank is raked down a conical bottom and pumped from the tank. A fraction of the sludge is recycled back to the digestor to maintain a steady microbial population.
EXTRUDER
• For reactions that require high temperature and pressure for short periods of time, the extruder is ideal. The reactant is fed to a screw type device that narrows toward the exit. Friction in the extruder produces high temperatures and pressures, and the product is forced out dies at the end of the extrusion tube. This type of extruder is referred to as a dry extruder. If steam is injected along the extrusion tube, the reactor is referred to as a wet extruder.
FALLING FILM
• Falling-film reactors have a liquid reactant flowing down the walls of a tube with a gaseous reactant flowing up or down (usually countercurrent). This reactor is particularly advantageous when the heat of reaction is high. The reaction surface area is minimal, and the total reaction heat generated can be controlled.
FERMENTOR
• The term fermentation is used to describe the biological transformation of chemicals. In its most generic application, a fermentor may be batch, continuous-stirred tank (chemostat), or continuous plug flow (immobilized cell). Most industrial fermentors are batch. Several configurations exist for these batch reactors to facilitate aeration. These include sparged tanks, horizontal fermentors, and biological towers.
GASIFIERS
• A gasifier is used to produce synthesis gas from carbonaceous material. The solid is packed in a column, and gas is passed up through the bed, producing a mixture of combustible products, primarily methane, hydrogen and carbon monoxide, with a low to medium BTU content.
RECYCLE
• A recycle reactor is a mode of operation for the plug flow reactor in reaction engineering terms. Recycle may also be used in other configurations involving a separation step. In plug flow some percentage of the effluent from the reactor is mixed back into the feed stream. The reason for this is to control certain desirable reaction kinetics. The more recycle in a plug flow reactor, the closer the operation is to a stirred-tank reactor. Therefore, with recycle it is possible to operate at any condition between the values predicted by either CSTR or PFR.
• Pelajari contoh soal 13.1 dan 13.2 di buku Bioprocess Engineering Principles, Pauline M. Doran.
