ENCH600015
Perpindahan Massa
ABSORPSI GAS (Kuliah Pendahuluan - 2015)
oleh:
Setijo Bismo – DTK FTUI
Garis Besar Topik Pembahasan • Pendahuluan Absorpsi • Prinsip-prinsip Dasar Absorpsi • Contoh-contoh Aplikasi di Industri • Kesetimbangan Gas-Cair • Operasi Satuan untuk Absorpsi Gas: a). Kolom Isian (Packed Tower) b). Kolom Talam (Tray Column, Plate Column)
• Perpindahan Massa di antara Fasa
Pendahuluan • Peristiwa ABSORPSI ¨ di antara GAS dan CAIRAN. • Fluida yang meng-ABSORPSI: CAIRAN ¨ ABSORBEN • Fluida yang di-ABSORPSI: GAS ¨ ABSORBAT • ABSORBAT (solutes) mengalami ABSORPSI dari fasa GAS ke fasa CAIRAN. • ABSORPSI tidak merusak (secara kimiawi) fasa GAS. • Sederhananya: peristiwa perpindahan (massa) gas KONTAMINAN menuju cairan (absorben). • DESORPSI (Stripping) adalah peristiwa kebalikan dari ABSORPSI
Pendahuluan: Skematis Sistem Absorpsi-Desorpsi
Pendahuluan: Garis Operasi dan Kesetimbangan Absorpsi
Bawah Menara 0,01
(Tower Bottom)
Garis Operasi 0,02
(Operating Line)
Garis Kesetimbangan (Equilibrium Line)
0,03
Puncak Menara (Tower Top)
0,01
0,02
0,03
0,04
Pendahuluan: Garis Operasi dan Kesetimbangan Absorpsi-Desorpsi 0,5 0,45 0,4
Theoretical Stages
0,35 0,3
Equilibrium curve
0,25
P (X F , Y1 )
0,2 0,15
Operating Line
0,1 0,05 0,0
Q ( X n , Yn+1 ( = Ys ) ) 0,05
0,1
0,15
0,2
0,25
0,3
0,35
0,4
Pendahuluan: Garis Operasi dan Kesetimbangan Absorpsi-Desorpsi
Prinsip-prinsip Dasar Jenis Cairan Pengontak (ABSORBEN) yang dipilih bergantung pada: 1. KELARUTAN Absorbat (gas kontaminan) di dalam cairan pengontak yang dipilih. ¨ Air: NH3, Asam Asetat, Aseton ¨ Heksana: Bahan organik (herbal, FG) 2. Reaktivitas Kimiawi antara gas dan cairan. ¨ Larutan Basa/Soda: gas asam, HCl, SO2 ¨ Menghasilkan GARAM
Aplikasi Industri 1. Absorpsi SO2 dalam gas buang menggunakan larutan alkali. 2. Hidrogenasi minyak sayur di industri makanan. ¨ gas H2 digelembungkan ke dalam minyak ¨ absorpsi 3. Penyingkiran gas CO2 dari SynGas dengan cara mengabsorsinya dengan larutan K2CO3 panas, MEA, MDEA, DEA, TEA, atau campurannya (di industri pupuk/ammonia). 4. Absorpsi (CH3)2S (dimetil sulfida) di dalam industri proses/makanan.
Tentang Kesetimbangan Gas-Cair
(1/3)
• Sebagai contoh: sistem SO2-udara-air. • Sejumlah gas SO2, udara dan air ditempatkan dalam suatu kontainer (“kontaktor) tertutup dan dikocok terus berulang-kali pada suhu tertentu sampai tercapai keadaan kesetimbangan. • Kemudian, sampel gas dan cairan di atas dianalisis untuk menentukan “tekanan parsial” (pA) dari gas SO2 dalam campuran gas dan “fraksi mol” (xA) zat terlarut (kita sebut absorbat atau solute) di dalam cairan (kita sebut absorben).
Tentang Kesetimbangan Gas-Cair • Plot kurva kesetimbangannya adalah sbb:
Kesetimbangan Sistem SO2-Air pada suhu 293 K (20 ºC)
(2/3)
Tentang Kesetimbangan Gas-Cair
(3/3)
• Relasi kesetimbangan antara pA (fasa gas) dan xA (fasa cair) dapat dinyatakan dalam suatu Persamaan garis lurus yang dikenal sebagai Hukum Henry pada konsentrasi rendah:
pA = H xA dengan H = konstanta Hukum Henry (frasi-mol gas/fraksi-mol cairan)
Data Kesetimbangan Lainnya... Beberapa data kesetimbangan dengan air untuk gasgas lainnya yang banyak digunakan di industri proses dapat disajikan berikut ini.. Sumber: Appendix A.3 (Geankoplis, Transport Process and Separation Process Principles, 4th ed., Prentice Hall)
Sistem Absorpsi Perancangan dan atau Rancang-bangun dari Sistem Pemroses Absorpsi (Peralatan), umumnya melibatkan dua hal berikut ini: 1. Kolom atau Menara Isian (Packed Bed Column or Packed Tower) 2. Kolom Talam (Tray or Plate Column)
OTK #1: MENARA ISIAN Peralatan paling umum yang digunakan dalam operasi absorpsi gas adalah MENARA ISIAN
OTK #1: Morfologi Menara Isian Di dalam Peralatan Menara Isian terdapat: 1. Kolom slindris atau MENARA 2. Inlet gas inlet dan “spasi distributor” di bagian bawah menara 3. Inlet cairan (ABSORBEN) dan distributor di bagian atas menara 4. Keluaran-keluaran gas dan cairan berturut-turut pada bagian atas dan bawah menara 5. Isian (packing) menara – terdiri dari sekumpulan padatan inert dengan bentuk tertentu.
OTK #1: Morfologi Menara Isian
OTK #1: Kolom Isian di Laboratorium
Apa itu di dalam Kolom Isian? •
Inlet Cairan
- ABSORBEN atau PELARUT (solvent ) atau weak liquor yang dimasukkan di bagian atas menara dan distribusikan secara merata - pendistribusian dilakukan menggunakan distributor yang berada di bagian atas isian. - “membasahi” secara merata (uniform) permukaan isian (asumsi)
•
Distributor
•
Inlet Gas
- sekumpulan (set) pipa-pipa berlubang (Fig. 18.1) - dapat berupa “spray nozzles” dalam suatu menara besar - masuk di bagian bawah menara melalui bagian “spasi distribusl”, yang letaknya tepat di bawah isian - mengalir ke atas (upward) menembus isian (packing) berlawanan arah (countercurrent) dengan aliran cairan.
PACKINGS •
The packing - provides a large area of contact between the liquid and gas - encourage intimates contact between the phases
•
Common dumped packings is shown in Figure 18.2.
PACKINGS • •
Hollow or irregular packing units – high void spaces Intalox saddles – the shape prevents pieces from nesting closely together - Increases the bed porosity
•
Porosity or void fraction: 60 – 90%
•
3 principal types: i) dumped packings, (0.25 – 3 inch) ii) stacked packings, (2 – 8 inch) iii) structured/ordered packings.
•
Made from: plastic, metal or ceramic
Structured Packing
Ceramic Intalox Saddle Packing
Contact between liquid & gas • Good contact between liquid & gas is the hardest to meet esp. in large tower • Channeling – occur at low liquid rates - some of the packing surface dry - chief reason for the poor performance - severe in tower filled with stacked packings - less severe in dumped packings - can be minimized by having the ratio of tower diameter to packing diameter, 8:1
FLOODING • Occur in countercurrent flow towers Inlet gas flow rate is too high It interferes with the downward flow of the solvent liquid. Cause an upward flow of the liquid through the tower
• Most absorbers are designed to operate at no more than 70% of maximum gas velocity that can cause flooding. • Factors that may lead to flooding: 1. high inlet gas flow rates 2. low liquid circulation rates 3. small diameter towers
Pressure Drop & Limiting Flow rates • Figure 18.4 shows typical data for the pressure drop in a packed tower. • Pressure drop is due to fluid friction • Pressure drop - common way of determining if flooding is occuring / something else goes wrong inside the absorber. • The graph is plotted on logarithmic coordinates for ΔP (inches H20/ft packing) versus the gas flow rate, Gy (lb/ft2.h)
Loading & Flooding Point • Point K is the loading point • Point L is the flooding point for the given liquid flow. • Loading point is a point where liquid hold up starts to increase and caused a change in the slope of the pressure drop • Flooding point is a point where the gas velocity will result in the pressure drop start to become almost vertical. Liquid rapidly accumulates, the entire column filled with liquid.
Unit operation 2: PLATE COLUMN • Plate Column absorbers distribute a contacting liquid over plates situated one above the other. • The contacting liquid flows downward through the column from one plate to the other in a stepwise fashion. • The inlet gas rises through each plate through openings in the plate and comes into contact with the liquid. • Usually, a layer of foam and froth is formed above each plate resulting from the mixing of liquid and gas.
• The gas not absorbed rises through the foam layer to the next plate for another stage of absorption. • Plate column absorbers result in a high removal efficiency since there are multiple stages of contact between liquid and gas. • More expensive than packed bed towers. • The advantages of plate columns are usually not justified in small operations where a packed bed tower will suffice.
• Plate columns have certain advantages over packed bed towers: a) plate columns can handle high gas flow rates accompanied by a low liquid flowrate with little chance of flooding. b) little chance for channeling inside of a plate column compared to a packed bed tower. c) sediment build-up often can be easily removed in plate column absorbers (packed bed towers are harder to clean).
Prinsip Absorpsi dalam Pengelolaan Lingkungan ¨¨,¨¨ /120 §§+§§ Konvensional dan Non-Konvensional ¨ OTK
Setijo Bismo PerMas – 2015 eBook: Principles of Chemical Separations with Environmental Applications
General Purposes:
Unit Operation in Pollution Prevention (Environmental Separation)
Based upon sources of pollution and the nature of polluted sites (air, land, or water), environmental separations can be categorized as follows. #1. Clean up of existing pollution problems Examples: z z z z z
surface water contamination: (a). organics, (b). anorganics, (c). metals, etc. groundwater contamination: (a). organics, (b). metals, etc.) airborne pollutants: (a). SOx, (b). NOx, (c). CO, etc.) soil clean-up: (a). solvent contamination, (b). heavy metals, etc.) continuing discharges to the environment: (a). automobiles, (b). industrial processes (chemical, nuclear, electronics, engineering, etc.).
General Purposes:
Unit Operation in Pollution Prevention (Environmental Separation)
#2. Pollution prevention Examples: z
chemically benign processing: (a). hybrid processing, (b). use of water instead of hydrocarbon/fluorocarbon solvents, (c). alternative chemical synthesis routes
z
use of separation steps: (a). reduction in downstream processing steps, (b). (b). eliminate solvent use (membranes instead of extraction, for example), (c). eliminate purge streams (internally remove contaminants so purge stream is not needed), (d). recovery and recycle instead of discharge (organics, water).
Hierarchy for Pollution Prevention in Environmental Engineering Figure [#01] below portrays a hierarchy for pollution prevention in Environmental Engineering. It is apparent that the difficulty of implementation decreases from top to bottom. Note that, the first four approaches on the hierarchy involve chemical separations (mass transfer operations). The Chemical Manufacturers Association has published a strategy for addressing pollution minimization or elimination in chemical processing facilities very similar to Figure below. They suggest, in priority order: 1. Source reduction ¨ Process changes to eliminate the problem. These process changes can include: z
z z
Reducing by-product formation through changes in processing and/or catalyst usage. This step can include changes in raw materials used. Better process control to minimize processing variations which lead to additional discharges. New processing flowsheets to minimize unwanted product generation and/or release.
2. Recycle ¨ If source reduction is not feasible, then recycle: z z z
within the process within the plant off-site.
3. Treatment ¨ Post-process waste treatment prior to discharge to minimize the environmental impact. Some recent researches and observations describe more than 50 pollution prevention strategies that do not require large investment costs.
The Role of Chemical Separations in Industries ¾ The use of chemical separations is already very important in many industries. ¾ Many industries include: biotechnology, metals recovery and purification, fuels, chemical processing plants and feedstocks, municipal sewage treatment, and microelectronics. ¾ For these and other industries, the efficiency of the separation steps is often the critical factor in the final cost of the product.
The Role of SEPARATION In Pollution Prevention
Separations as Unit Operations
The Constraints • The separation cost is often related directly to the degree of dilution for the component of interest in the initial mixture. • This cost includes the fact that most separations use 50 times the minimum energy requirement based on the ideal thermodynamic requirements. • To put the energy consumption in perspective, the chemical and petroleum refining industries in the US consume approximately 2.9 million barrels per day of crude oil in feedstock conversion. • One method to visualize this cost factor is with the Sherwood plot shown in Figure [#2] below.
The Constraints One method to visualize this cost factor is with the Sherwood plot shown in Figure [#2] below.
Figure [#2]. Sherwood plot. Reproduced with permission of National Academy Press. This log–log plot shows that there is a reasonable correlation between the initial concentration of a solute in a mixture and its final price. For environmental applications, this correlation would translate to the cost of removal and/or recovery of a pollutant based on its initial concentration.
Remember: view of Separation Processes Separation processes (environmental separations) can apply to the clean-up of existing problems as well as pollution prevention. The cost of separations is directly related to the degree of dilution in the feed stream. The three primary functions of separation processes are purification, concentration, and fractionation. Separations use thermodynamic equilibriumand/or mass transfer (rate-) based analysis.
Sulphur Problems in Industries Our principal sources of energy – fossil fuels – are all contaminated to some extent with sulfur compounds. When these fuels are burned, the sulfur compounds are burned to sulfur oxides, which are emitted to the atmosphere in the flue gas. In the atmosphere, these oxides are converted into the sulfur acids that are a principal cause of acid rain. Separations technology plays a critical role in limiting sulfur-oxide pollution from sulfur-bearing fossil fuels. This technology is sufficiently advanced that there are no inherent technological limits to removing more than 95 percent of the sulfur present in natural gas, crude oil, and coal – many processes exist for accomplishing this before, during, or after combustion. The principal barriers to nearly complete sulfur removal are cost and practicality.
Sulphur in Mineral Fossil (Fuels) Industries (#1) 1. Natural Gas. The principal sulfur contaminant of natural gas is another gas – hydrogen sulfide. Because it is extremely toxic, civil authorities have long forbidden significant levels of this compound in natural-gas pipelines. Hydrogen sulfide is removed from natural gas by a variety of commercial processes including reaction with aqueous solutions of oxidants, absorption into aqueous solutions of bases, distillation, and selective permeation through membranes. The end product of these processes is elemental sulfur, which can be sold and, in some cases, is worth more than the co-produced natural gas. In 1984, about 24,000 tons (24 million kilograms) of sulfur was produced from natural-gas wells in the United States.
Sulphur in Mineral Fossil (Fuels) Industries (#2) 2. Petroleum. Sulfur can also be recovered from crude oil with technology that relies on the reaction of hydrogen with sulfur-containing compounds in crude oil (hydrodesulfurization) and permits modern refiners to turn 3 percent sulfur crudes into liquid product with no more than 0.5 percent sulfur. About 26,000 tons of saleable by-product sulfur was produced from crude oil in 1983.
Sulphur in Mineral Fossil (Fuels) Industries (#3) 3. Coal. Coal can be partially desulfurized before combustion. Washing and magnetic separation are effective in reducing the content of iron sulfide, the principal inorganic sulfur contaminant, by up to 50 percent or somewhat higher. However, there are also organic sulfur compounds in coal, and a feasible means of removing them has not yet been found. Accordingly, combustion of coal produces a flue gas that contains significant amounts of sulfur oxides, which must be removed from the gas if sulfur pollution is to be minimized.
Sulphur Removal Devices in Industries Flue-gas scrubbers are proven but expensive separation devices for removing sulfur from combustion gases. The new dry-scrubber technology removes about 90% of the sulfur in a flue gas by contact with a lime slurry in a specially designed combination spray dryer and reactor. The reaction product is a dry calcium sulfate– sulfite mix that is environmentally benign. Larger users favor the wet-scrubber technology, which is capable of removing up to 90 percent of the sulfur with a lime slurry in a contactor column. Separations technology has made a substantial contribution to reducing the sulfur pollution problem associated with the burning of fossil fuels. The principal barrier to further alleviation of this problem is economic and will respond to improved technology gained through further research and development [1].
Could you identify all the names of them??
Could you identify all the names of them??
