Spray Tower

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Spray towerFrom Wikipedia, the free encyclopedia

Typical countercurrent-flow spray tower.

A spray tower (or spray column or spray chamber ) is gas-liquid contactor used to achieve

the mass and heat transfer between a continuous gas phase (that can contain dispersed solid

particles) and a dispersed liquid phase. It consists of empty cylindrical vessel made of steel or plasticand nozzles that spray liquid into the vessel. The inlet gas stream usually enters the bottom of the

tower and moves upward, while liquid is sprayed downward from one or more levels. This flow of

inlet gas and liquid in the opposite direction is called countercurrent flow.

This type of technology can be used for example as a wet scrubber for air pollution control.

Countercurrent flow exposes the outlet gas with the lowest pollutant concentration to the freshest

scrubbing liquid. Many nozzles are placed across the tower at different heights to spray all of the gas

as it moves up through the tower. The reasons for using many nozzles is to maximize the number of

fine droplets impacting the pollutant particles and to provide a large surface area for absorbing gas.

Theoretically, the smaller the droplets formed, the higher the collection efficiency achieved for both

gaseous and particulate pollutants. However, the liquid droplets must be large enough to not be

carried out of the scrubber by the scrubbed outlet gas stream. Therefore, spray towers use nozzles

to produce droplets that are usually 500 1000 m in diameter. Although small in size, these dropletsare large compared to those created in the venturi scrubbers that are 10 50 m in size. The gas
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velocity is kept low, from 0.3 to 1.2 m/s (1 4 ft/s) to prevent excess droplets from being carried out ofthe tower.

In order to maintain low gas velocities, spray towers must be larger than other scrubbers that handle

similar gas stream flow rates. Another problem occurring in spray towers is that after the droplets fall

short distances, they tend to agglomerate or hit the walls of the tower. Consequently, the total liquidsurface area for contact is reduced, reducing the collection efficiency of the scrubber.

Crosscurrent-flow spray tower

In addition to a countercurrent-flow configuration, the flow in spray towers can be either

a cocurrent or crosscurrent in configuration.

In cocurrent- flow spray towers, the inlet gas and liquid flow in the same direction. Because the gas

stream does not "push" against the liquid sprays, the gas velocities through the vessels are higher

than in countercurrent-flow spray towers. Consequently, cocurrent- flow spray towers are smaller

than countercurrent-flow spray towers treating the same amount of exhaust flow. In crosscurrent-

flow spray towers, also called horizontal-spray scrubbers, the gas and liquid flow in directions

perpendicular to each other.

In this vessel, the gas flows horizontally through a number of spray sections. The amount and quality

of liquid sprayed in each section can be varied, usually with the cleanest liquid (if recycled liquid is

used) sprayed in the last set of sprays.

Particle collection [edit] Spray towers are low energy scrubbers. Contacting power is much lower than in venturi scrubbers,

and the pressure drops across such systems are generally less than 2.5 cm (1 in) of water. The

collection efficiency for small particles is correspondingly lower than in more energy-intensivedevices. They are adequate for the collection of coarse particles larger than 10 25 m in diameter,although with increased liquid inlet nozzle pressures, particles with diameters of 2.0 m can be

collected.

Smaller droplets can be formed by higher liquid pressures at the nozzle. The highest collection

efficiencies are achieved when small droplets are produced and the difference between the velocity
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of the droplet and the velocity of the upward-moving particles is high. Small droplets, however, have

small settling velocities, so there is an optimum range of droplet sizes for scrubbers that work by this

mechanism.

This range of droplet sizes is between 500 and 1,000 m for gravity-spray (counter current)

towers .[1] The injection of water at very high pressures 2070 3100 kPa (300 450 psi) creates afog of very fine droplets. Higher particle-collection efficiencies can be achieved in such cases since

collection mechanisms other than inertial impaction occur .[2] However, these spray nozzles may use

more power to form droplets than would a venturi operating at the same collection efficiency.

Gas collection [edit] Spray towers can be used for gas absorption, but they are not as effective

as packed or plate towers. Spray towers can be very effective in removing pollutants if the pollutants

are highly soluble or if a chemical reagent is added to the liquid.

For example, spray towers are used to remove HCl gas from the tail-gas exhaust in

manufacturing hydrochloric acid. In the production of superphosphate used in

manufacturing fertilizer, SiF 4 and HF gases are vented from various points in the processes. Spray

towers have been used to remove these highly soluble compounds. Spray towers are also used for

odor removal in bone meal and tallow manufacturing industries by scrubbing the exhaust gases with

a solution of KMnO 4.

Because of their ability to handle large gas volumes in corrosive atmospheres, spray towers are also

used in a number of flue-gas desulfurization systems as the first or second stage in

the pollutant removal process.

In a spray tower, absorption can be increased by decreasing the size of the liquid droplets and/or

increasing the liquid-to-gas ratio (L/G). However, to accomplish either of these, an increase in both

power consumed and operating cost is required. In addition, the physical size of the spray tower will

limit the amount of liquid and the size of droplets that can be used.

Maintenance problems [edit] The main advantage of spray towers over other scrubbers is their completely open design; they have

no internal parts except for the spray nozzles. This feature eliminates many of the scale buildup and

plugging problems associated with other scrubbers. The primary maintenance problems are spray-

nozzle plugging or eroding, especially when using recycled scrubber liquid. To reduce these

problems, a settling or filtration system is used to remove abrasive particles from the recycled

scrubbing liquid before pumping it back into the nozzles.

Summary [edit]
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Spray towers are inexpensive control devices primarily used for gas conditioning (cooling or

humidifying) or for first-stage particle or gas removal. They are also being used in many flue-gas

desulfurization systems to reduce plugging and scale buildup by pollutants.

Many scrubbing systems use sprays either prior to or in the bottom of the primary scrubber to

remove large particles that could plug it.

Spray towers have been used effectively to remove large particles and highly soluble gases.

The pressure drops across the towers are very low usually less than 2.5 cm (1.0 in) of water; thus,the scrubber operating costs are relatively low. However, the liquid pumping costs can be very high.

Spray towers are constructed in various sizes small ones to handle small gas flows of 0.05 m/s(106 ft/min) or less, and large ones to handle large exhaust flows of 50 m/s (106,000 m/min) or

greater. Because of the low gas velocity required, units handling large gas flow rates tend to be large

in size. Operating characteristics of spray towers are presented in the following table .[3]

Operating characteristics of spray towers

Pollutant Pressure drop

(p) Liquid-to-gas ratio (L/G)

Liquid-inlet

pressure (p L

Gases 1.3 7.6 cm of

water0.07 2.70 l/m (0.5 20 gal/1,000 ft) 70 2800 kPa

Particles0.5 3.0 in ofwater

5 gal/1,000 ft is normal; >10 when usingpressure sprays

10 400 psig
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Gambar 1. Menunjukkan sebuah menara lawan arus-arus semprot khas..

Jenis teknologi adalah bagian dari kelompok dari polusi udara kontrol kolektif disebut sebagai scrubberbasah. Aliran arus balik mengekspos gas outlet dengan konsentrasi polutan terendah pada cairanscrubbing segar. Nozel Banyak ditempatkan di menara pada ketinggian yang berbeda untukmenyemprot semua gas ketika bergerak ke atas melalui menara. Alasan menggunakan nozel banyakadalah untuk memaksimalkan jumlah tetesan halus berdampak pada partikel polutan dan untuk

menyediakan luas permukaan yang besar untuk menyerap gas.

Secara teoritis, semakin kecil tetesan terbentuk, efisiensi pengumpulan yang lebih tinggi dicapai untukkedua polutan gas dan partikulat. Namun, tetesan cairan harus cukup besar untuk tidak dilakukan dariscrubber dengan aliran gas keluar digosok. Oleh karena itu, menara semprot menggunakan nozel untukmenghasilkan tetesan yang biasanya 500-1000 m dengan diameter. Meskipun ukurannya kecil, tetesanini adalah besar dibandingkan dengan yang dibuat di scrubber venturi yang 10-50 pM dalam ukuran.Kecepatan gas tetap rendah, 0,3-1,2 m / s (1-4 ft / s) untuk mencegah tetesan kelebihan dari yangdilakukan menara.

Untuk mempertahankan kecepatan gas rendah, menara semprot harus lebih besar dari scrubber lainyang menangani tingkat aliran gas aliran serupa. Masalah lain yang terjadi di semprot menara adalahbahwa setelah tetesan jatuh jarak pendek, mereka cenderung menggumpal atau memukul dindingmenara. Akibatnya, luas permukaan cairan total untuk kontak berkurang, mengurangi efisiensi koleksiscrubber. Selain konfigurasi lawan arus aliran, aliran dalam menara semprot dapat berupa cocurrentatau crosscurrent di konfigurasi.
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Gambar 2. Aliran semprot menara crosscurrent.

Dalam cocurrent menara dengan aliran semprot, gas inlet dan aliran cairan ke arah yang sama. Karenaaliran gas tidak "mendorong" terhadap semprotan cair, kecepatan gas melalui pembuluh lebih tinggi

dari arus balik di menara dengan aliran semprot. Akibatnya, cocurrent menara dengan aliran semprotlebih kecil dari arus balik aliran semprot menara mengobati jumlah yang sama aliran gas buang. Dalamcrosscurrent menara dengan aliran semprot, juga disebut horisontal-semprot scrubber, gas dan alirancairan dalam arah tegak lurus satu sama lain. Pada gambar 2, gas mengalir horizontal melalui sejumlahbagian semprot. Jumlah dan kualitas cairan disemprotkan di setiap bagian bisa bervariasi, biasanyadengan cairan bersih (jika cairan daur ulang yang digunakan) disemprotkan pada set terakhir darisemprotan.

Pada menara semprot (spray tower), gas kotor masuk dari bagian dasar akibat adanya tekanan. Gasmembumbung ke atas, sementara dari atas disemprotkan air melalui pipa air yang dilengkapi dengansprayer sehingga air yang keluar merupakan butiran-butiran halus yang memenuhi menara. Karenaadanya gaya berat, butiran-butiran air akan turun sementara gas naik bersama udara. Gas yangterkandung dalam udara bereaksi dengan air dan turun ke bawah kemudian ditampung dan dialirkan ketempat tertentu yang nantinya akan diolah kembali. Udara dan gas yang bersih keluar melalui cerobongatas.

Menara tower ini mampu digunakan hingga 3 sampai 4 tahun. Perawatannya pun tidak rumit. Cukupdengan pengecekan minimal 6 bulan sekali, kemudian dilakukan platting jika ada tanda-tanda akanterjadi korosi.

Jika mengaitkan antara keberhasilan metode ini dengan persentase keberhasilannya, maka perlu adanya

keterkaitannya dengan pihak lain, yaitu pemerintah. Dalam hal ini pemerintah telah mengadakan suatuprogram yang disebut Clean Air Act. Clean Air Act dibuat oleh pemerintah dan menambah pajak bagiindustri yang melakukan pencemaran udara.
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Gambar 3. Tata penempatan spray tower di dalam cerobong asap.

Menara semprot adalah perangkat kontrol murah terutama digunakan untuk pengkondisian gas(pendingin atau pelembab) atau untuk tahap pertama atau penghapusan partikel gas. Mereka jugadigunakan di banyak gas cerobong sistem desulfurisasi untuk mengurangi penumpukan plugging danskala oleh polutan. Banyak sistem scrubbing menggunakan semprotan sebelum atau di dasar scrubberutama untuk menghilangkan partikel besar yang bisa pasang.

Menara semprot telah digunakan secara efektif untuk menghilangkan partikel besar dan gas yang sangatlarut. Penurunan tekanan yang melintasi menara yang sangat rendah - biasanya kurang dari 2,5 cm (1,0dalam) air, dengan demikian, biaya operasi scrubber relatif rendah. Namun, biaya pemompaan cairanbisa sangat tinggi.

Menara Semprot dibangun dalam berbagai ukuran - yang kecil untuk menangani gas kecil mengalir dari0,05 m / s (106 ft / min) atau kurang, dan yang besar untuk menangani arus knalpot besar 50 m / s(106.000 m / menit) atau lebih besar . Karena kecepatan gas yang rendah diperlukan, unit menanganitingkat aliran gas besar cenderung besar ukurannya. Karakteristik operasi dari menara semprot disajikanpada Tabel 1.

Karakteristik Spray Tower

Polutan PenurunanTekanan(p)

Cair ke Gasrasio (L/G)

Cair ke Inlettekanan(PL)

Removalefisiensi

Aplikasi
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Gas 1.3-7.6 cmair

0.07 2.70 l/m(0.5-20gal/1,000 ft)

70 2800 kPa

50-90% +(efisiensitinggi hanyasaat gassangatmudahlarut)

Pertambangan,Industri

Kimia, Proses industri

Boiler dan Incinerator

Besi dan Industri baja

Partikel 0.5-3.0 didalam air

5 gal / 1.000 ft adalahnormal;> 10saatmenggunakansemprotantekanan

10 400 psig 2 8 mdiameter

Menara semprot dapat digunakan untuk penyerapan gas, tetapi mereka tidak seefektif dikemas ataumenara piring. Menara semprot dapat sangat efektif dalam menghilangkan polutan jika polutan yangsangat larut atau jika reagen kimia ditambahkan ke cairan.

Misalnya, menara semprot digunakan untuk menghilangkan gas HCl dari knalpot ekor gas dalampembuatan asam klorida. Dalam produksi superfosfat digunakan dalam pupuk manufaktur, SiF4 dan gasHF yang dilepaskan dari berbagai titik dalam proses. Menara semprot telah digunakan untukmenghilangkan senyawa ini sangat larut. Menara semprot juga digunakan untuk menghilangkan bau dimakan tulang dan lemak industri manufaktur dengan menggosok gas buang dengan larutan KMnO4.

Karena kemampuan mereka untuk menangani volume gas besar di atmosfer korosif, menara semprot juga digunakan dalam sejumlah gas cerobong sistem desulfurisasi sebagai tahap pertama atau keduadalam proses penghapusan polutan. Dalam sebuah menara semprot, penyerapan dapat ditingkatkandengan mengurangi ukuran tetesan cair dan / atau meningkatkan rasio cair ke gas (L / G). Namun, untukmencapai salah satu dari ini, kenaikan kedua daya yang dikonsumsi dan biaya operasi diperlukan. Selainitu, ukuran fisik dari menara semprot akan membatasi jumlah cairan dan ukuran tetesan yang dapatdigunakan.


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