Quencher TOWER Design

DESIGN OF QUENCHER

CHAPTER No.7 DESIGN OF QUENCHER 7.1 QUENCHER:The word quencher means “sudden cooling”. It may be used for various purposes, e.g. for  hardening of solid surfaces, for stopping further reactions etc. In our case quenching of the reactor product is needed for sudden cooling, for removing impurities and to avoid side reactions. Cooling by liquid quenching is essentially accomplished accomplished by introducing the hot gases into a liquid contacting device. When the liquid evaporates the energy necessary necessary to vaporie the liquid is obtained at the e!pense of hot combustion gases, resulting in a reduction of gas temperature. The temperature of the combustion gases discharge from the quencher is at the adiabatic saturation temperature of the combustion gases if the operation is adiabatic and the gas leaves the quencher saturated with water vapors.

7.2 WHY QUENCHING? Quenching is choosing in contrast to heat echanger !ecause:

". If an e!changer e!changer is used large large area area is required. required. #. $lso, $lso, direct conta contact ct is more more efficien efficientt than indirec indirectt contact. contact. %. The fine partic particles les from the the reactor reactor will also also be removed removed which may may otherwise otherwise erode e!changer tubes. Quenching is "i##erent #ro$ coo%ing in the sense that: ". Cooling Cooling is done in a comparati comparatively vely longer longer time. time. #. It also also does not not stop stop the side side reaction reactionss complet completely ely.. %. While While in quencher quencher formation formation of by&prod by&products ucts are are complete completely ly stopped. stopped.

7.& 'Y(E) *+ QUENCHER: There are three types of quenchers that may be employed. ". 'pra pray tow tower erss #. (entur nturee scrub scrubbe bers rs

90

Production of Acrylonitrile

DESIGN OF QUENCHER %. )ac*ed towers

7., )EEC'I*N CRI'ERI: 'pray tower is selected from the above mentioned types of quencher as+ ". 'pray towers can be used for gas absorption, which removes impurities. #. The main advantage of spray towers over other scrubber is their completely open design. It is simple to construct. This feature eliminates many of the scale build up and plugging problems associated with other scrubbers. %. This is an ine!pensive controlled device primarily used for gas conditioning cooling or humidification-. . It requires very little space and only that amount of water is used that is needed to maintain the desired temperature of the gases at the discharge. /. Their installation and operation cost are generally considered to b less than that for  other cooling method. 0. 'pray towers are very effective in removing pollutants particles from reactor- if the pollutants are highly soluble.

7./ 0EERI') *+ EN'URI )CRU33ER): ". #. %. .

In venture scrubber contact area available for water and gases is less. $ precooler is to be used when venture scrubber is used for removing particulates. There construction is not so simple. 1arge amount of water is required for cooling.

7.4 0EERI') *+ (C5E0 '*WER: ". #. %. .

In pac*ed tower pressure drop is higher. )ac*ing material increases the cost of the tower. It is less efficient than cooler. )roblems li*e plugging, fouling and channeling are associated with it.

7.7 )(RY '*WER):'pray towers or spray chambers consist of empty cylindrical vessels made of steel or  plastic and noles that spray liquid into the vessels. 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

91

Production of Acrylonitrile

DESIGN OF QUENCHER countercurrent flow. Countercurrent flow e!poses the outlet gas with the lowest pollutant concentration to the freshest scrubbing liquid.  2any noles 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 noles is to ma!imie 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. 3owever, 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 noles to produce droplets that are usually /44 to ",444 5m in diameter. $lthough small in sie, these droplets are large compared to those created in the venturi scrubbers that are "4 to /4 5m in sie. In a spray tower, absorption can be increased by decreasing the sie of the liquid droplets and6or increasing the liquid&to&gas ratio 167-. 3owever, to accomplish either of these, an increase in both power consumed and operating cost is required. In addition, the physical sie of the spray tower will limit the amount of liquid and the sie of droplets that can be used. Cooling hot gases with the acidic quench is relatively simple and required very little space. 8nly that amount of water is used that is needed to maintain the desire temperature of the gases at the discharge. Their installation and operating costs are generally considered to be less than that for other cooling methods.

7.6 )(RY N*E:$ spray nole is a device that facilitates the formation of spray. When a liquid is dispersed as a stream of droplets atomiation-, it is called a spray. 'pray noles are used to achieve two primary functions+ increase liquid surface area to enhance evaporation, or distribute a liquid over an area. It is a device which ma*es use of the pressure energy of a liquid to increase its velocity through an orifice and brea*s it into drops. 9ach nole type has a specific type of characteristics and capabilities and is designed for use under certain application conditions.

9

Production of Acrylonitrile

DESIGN OF QUENCHER

7.8 'Y(E) *+ )(RY N*E): 'pray noles can be categoried into several types based on the energy input used+

1. H9"rau%ic sra9: The hydraulic spray nole utilies the liquid *inetic energy as the energy source to brea*  the liquid into droplets. This type of spray is less energy consuming than a gas atomied or twin&fluid spray nole. $s the fluid pressure increases the flow increases and the drop sie decreases. :ut this leads to problems in selecting a droplet sie and to achieve a certain flow rate at a given pressure. To overcome this situation a special hydraulic nole 1echler  'pillbac* ;ole- has been developed. This nole can vary the liquid flow rate at a particular droplet sie and pressure. This nole creates a better and optimum control on the liquid spray and in certain applications can eliminate the need of e!pensive compressed air.

2. Gas ;air< ato$i=e" sra9: $ir, steam or other gases can brea* up a liquid to form a spray, with the gas providing a source of energy. I.

Interna% $iing

Internal mi!ing noles mi! fluids inside the nole. The gas atomied spray utilies a gaseous source to brea* the liquid to the droplets. The internal mi!ed twin fluid spray can utilie two different ways for spraying liquid+ In the #irst t9e, the liquid impinges upon a surface for impact to brea* the liquid stream and then the air is mi!ed to atomie it. The advantage of this process is to reduce the amount of air required to generate the droplets but the downside is that the over time the impact surface becomes eroded and effects the spray droplet sie pattern. The nole life can be very short if the liquid has impurities in it. In the secon" t9e, the liquid is bro*en into droplets by using only gas. The advantage of  this type is that the nole lasts longer but the downside is that this type of spray needs more gas to generate the same sie of droplets. II.

Eterna% $iing

9!ternal mi!ing noles mi! fluids outside the nole.

9!

Production of Acrylonitrile

DESIGN OF QUENCHER This type of spray nole may require more air, but the mi!ing and atomiation of liquid ta*es places outside the nole. If a liquid is atomied using any gas which may react with the liquid, it is possible that the reaction may damage the inside of the nole. This type of nole is most beneficial for a liquid which may evaporate inside the mi!ing chamber of an internal mi! nole or using steam to atomie the liquid. $lso this nole is suitable of spraying viscous fluid. &. Rotar9 ;sinning "isc (s > 4./ m6s 7as density > G @ 0.?#@ *g6 m% 2ass flow rate of gas > W 7 > ""A?@ *g6hr  rea o# the Quencher:

$rea > W7 6 %044 (s G $rea > ""A?@6 %044 ! 4./ ! 0.?#@ $ > ?."@0 m# 0ia$eter o# the Quencher :

$ > B # 6  ?."@0 > %.""0 ! # 6   > %."? m og ean 'e$erature 0i##erence +

D T 12T >

D T" & D T# ln D T"6 D T#-

D T 12T > @#." oC )uer#icia% ass e%ocities:

Eor 7as 'tream,

Eor 1iquid 'tream,

7 > mass flow   area 7 > 0/#?.#0 *g6m# .hr 1 > mass flow $rea 1>

"?"# ?."@0

1> #4@" *g 6m# .hr  Rate *# Heat 'rans#er:

q > mo C p DT

9#

Production of Acrylonitrile

DESIGN OF QUENCHER Where mo > molar flow rate of liquid stream > ?@/.? *mol 6hr  q > heat rate required to vaporie the water to discharge temperature C p > A./% F 6*mol .G  DT > @#." oC q > %44%?#?."%" F6hr Heat 'rans#er Coe##icient:

= > 4.% 74.@ 14.  4./

H.. equation 

Where

 > height of Juencher  = > 3eat Transfer Coefficient o%u$e o# Quencher +

( > q 6 = ! D T 12T H..equation / Ca%cu%ation o# Height o# Quencher :

(>$! H..equation 0 Combining equations , / and 0 we get the height of the Juencher  ( > q !  4./ 6 4.% 7 4.@ ! 1 4. ! D T 12T 4./

 6 4.% 7 4.@ ! 1 4. ! D T 12T

$!>q!

 4./ > q 6 4.% 7 4.@ ! 1 4. ! $ )utting all the values,  4./ > %.//%?  > "#.0% m o%u$e o# Quencher:

( >$ ! 

K

( > ""0.4"? m%

9$

Production of Acrylonitrile

DESIGN OF QUENCHER

7.11 )(ECI+IC'I*N )HEE': I"enti#ication

Item

Juencher

Item L

J&"4"

Type

'pray Type

 ;o. of item

" +unction

Juenching the reactor outlet stream 0esign seci#ication

7as inlet Temperature

##4 oC

7as outlet Temperature

@/  oC

1iquid inlet Temperature

/  oC

1iquid outlet Temperature

0/  oC

8perating )ressure

"A# *)a

esign Temperature

%/4 oC

esign )ressure

#44*)a

iameter

%."? m

3eight

"#.0% m

2aterial

Carbon 'teel @" : /

(olume

""0.4" m % Quenching $echanis$

Juenching media

3 #'8 %4 M-

Juenching 'ystems

;oles

9%

Production of Acrylonitrile