Gas Absorption and
Humidification in Cyclone Spray Towers H. F. JOHKSTONE AND H. E. SILCOX' Cniversity of Illinois, c'rbana, I l l .
Measurements were made in a model cyclone tower 011 the rates of absorption of sulfur dioxide by alkaline solutions and on humidification of the gas. The effect of entrance design, gas and liquor ielocities, and nozzle spacing are correlated in such a manner that the performance of large towers can be predicted. The towers provide high rates of absorption as well as good efficiency for the renioial of finely divided dust particles. They may be operated oiel a wide range of conditions and loading ratios of liquid to gas. Their simplicity and low pressure losses should make them useful in the chemical industry for treating large quantities of gases.
r i i a b r trarihfer corresponding t o IJUI~ slightly more thau UIW theuretical plate may be accomplished in a single unit. Consequently, the advantages of counterflow in approaching saturation of the liquid streams cannot be obtained. The number of transfer units available even in a short height is quite large, however, and 3lmost complete absorption of a solute gas can be obtained if the proper solvent is selected.
EQUIPMENT .A>D PROCEDURE
CYCLOKE spray toiver consists of a vertical cyliridrical chamber with a tangential gas inlet near the bottom and a central gas exit a t the top. A spray of small droplets of the scrubbing fluid is introduced at the axis of the chamber through nozzles on a manifold. The rotation of the gas in the chamber caused by the tangential entrance results in centrifugal forces \\-hich cause the spray droplets t o travel outward a t high velocities through the gas to the walls of the chamber. Such tower-. have found numerous applications in the removal of mist and d u s ~ . particles from large quantities of gases (2, 3, 4,9, 10, 11). For this purpose high removal efficiencies may be obtained for dust particles down t o 1 micron diameter, and some installations have heen reported t o deal with particles in the submicron range (.5). The possibility of using this type of scrubber for the absorptioil of gases ivas noted several years ago, when tests were run on ii commercial installation to determine the rate of Rhs(Jrpti0n ( i i sulfur dioxide from flue gas, when the circulating water ~va.. maintained on the alkaline side of neutrality ( 7 ) . The result. indicated high coefficients for both gas absorption and huniidifi. cation. During the war several installations were made bpecifically for gas absorption. These included toners for the IYcovery of chlorine, hydrogen fluoride, and sulfur dioxide, and for the removal of obnoxious odors (5). The design of most of tlie>e installations was htrictly empirical, and little exact informal i o r ~ has been available on their performance. The present study was undertaken t o provide :t theoretical viii experimental lxisis of design. Measurements were made 111 :I model tower on the rates of gas absorption and humidificat ioii, pre;sure drops, and flow patterns for several combinations of entrarices, nozzle spacings, and operating conditions. It is noit apparent that the cyclone spray toxver is a n important iieiv tool for the chemical industry in treating large quantitier of g;rbey. especially i n noiiregenerative processes for the removiil o f i m d l co:icerrtrations of solute gases where i t is not necesiai.)- t o saturate the solvent. I n its present design the tower provid+ mainly cross flox of the gas and liquid streams, and thercbt'o:,r 1
Psesent address, l l e s c k a n d Company, Inc., Rahway, S . J.
The model tower (Figures 1 a n d 2 ) consisted of a gas chamber and spray manifold. These lvere supplemented by holding tanks, a pump, filter, and flowmeter for t h e liquid stream, and a blower, the sulfur dioxide supply, and a flowmeter for the gas stream (1.4). T h e tower consisted of n tapered chamber 14 feet high, 28l,'2 inches in diameter at the bott,om, and 201/, inches in diameter at' t h e top. It was a one-twelfth scale model of a stack on a large power station. This design was chosen because of the possible application t o sulfur dioxide absorption from stack gases. The tower was constructed of cellulose acetate sheeting, 0.02 inch thirk, which was supported by an outbide metal frame. Twenty-four 1-inch ports were provided for withdrawing gas w n p l e s and measuring pressure drops and velocity traverses. These were located symmetrirally around the tower a t eight height>. The gas was blown ta,ngentially into the tower by mearid of it f a n t'hrough a 14-inch i.d. steel pipe and one of the inlet sections 4 o w n in Figure 3. Inlet 1 was a rectangular throat, 4>/: inches X 13l/2 inches, convergent from t h e 14-inch pipe in the horizontal dimension, with a deflector vane entirely n-ithin the cyclone. lnlet 2 was only slightly convergent, but the deflector vane, which extended outside of t h e cyclone, could be used t o control the mt ranee width nearly t o t h e full radius of the cyclone. The gas rate was measured by a standard orifice and recorded b y a Republic flowmeter. T h e exit gases xere removed through a 1-&inch c2entral outlet pipe at the top of the tower extending through t h e roof of the building. Sodium carbonate solution of vail-iiig conceiitratiori was intriiducetl into t h e tower through the spray manifold. This consisted 0 ; twenty t o fifty Spraco hollow-cone nozzles, rnotiel T55046, with 0.046-inch orific.es. T h e nozzles xere placed spir:xlly around t h e l'/?-inch standard pipe manifold, six to ii spiral , total of ninety-eight nozzle 110u i d 3 inches between spirals. 2 sitions nere provided SO t h a t t h e rffect of t h e spacing of the nozzles could he studied. The nozzles were pointed in the dirrctiori of the gas flow and a t an angle 30' above horizontal. The. liresaure .Lt the nozzles was 67 t o 70 pouuds per square inch g a w , :it which the capacity of each nozzle was 0.13 gallon per minute. T h e spray was relatively fine. Table I gives the approximate ilistrihut ion of drop sizes based on measurements a t several pressuiez: and interpolation of t h e data. The mass medisn ttiarneter of the drops was approximately 175 microns. The solution was supplied from a 220-gallon enamel-lined tali k t)y it centrifugal pump and was filtered prior to entering the spr:iy ti?anifold through a Channon-Fisher FreHo line filter, 31,!r-kc9h ~ i i p erize, with 0.0156-inch screen perforations. T h e liquid tiow \\.ah measured bj- a n orifice niet,er placed hetween t h e pump and the strainer. T h e spray impinged on the side wall and drained t o the enamel-lined t a n k through an outlet in the bottom of thr: tower. This outlet was covered with :i cloth bag t o prevcnt dirt from being circulated. T h e solution in t h e tank w ~ l and e iv:is cmntinuously niixed with a stirwr an11recycled to the t o n ~ r .
E!08
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1547
The pre-hCre drop ulong the iiianilold 2 to 3 pounds ye1 square inch, which \var sati.t:ictorily small. Salfur dioxide x a s introduced into t h e ga> atream direct1)- after the fan. I t was supplied from a cylinder through an orifice meter. T h e cylinder r a s placed in a thermostated irater bath on platform scales so that the total amount of t h e gas introduced during a run rould be measured directly and a comparison made with the analytical data. To :~llowa study of t h e amount of absorytiori 011 the tower \i-all, the nozzles were replawd \vith a 1-inch coil on the wall 92 i n c h above the bottom of t,he tower, and the solution n-ab sprayed onto the tower wall through 'llsinch holes spaced 11;* inches apart around the coil. Absorption measurements were made a t qeveral rates of gas flow and of liquid flondown the tower. Static pressure taps were placed iri the entrance duct, in the outlet pipe, and on the n-all of the tower near t h e top and bottom. Wetand dry-bulb thermometers and gas sampling tube8 were placed in t h e W i n c h gas inlet and outlet pipes (positions A and B , Figure 2). Velocity traverses across t h e toFer were made a t several points by means of a n .4lnor velometer. A small cellulose acetate pennant was attached t o t h e end of the velometer t o indicate the direction of gas flow. The velometer n-ay set a t t h e angle indicated by t h e pennant, and the direction of gas flow read on the protractor. In this way both the horizontal and vertical components of velocity were determined a t each point in t h e traverse. The concentration of sulfur dioxide in the inlet and outlet gas streams was determined by simultaneously drav-ing samples through absorption bot,tles containing standard sodium hydroxide solutions with excess neutral hydrogen peroxide, and measuring the volume of gas with met test meters. The solution circulated x'as maintained a t a pH above 8. T h e alkalinity as sodium carbonate was determined before and after each test run, and the depletion, of the water, was corrected for evapora used with the gas anal arid gas flow rate t o make a sulfur dioxide balanre 1Y;th the weight of gas from the tared cylinder. T h e halance for all runs was within 15% and in most cases mas within 5%. ABSORPTION THEORY
The absorption in the cyclone spray ton-er may be divided into t v o p3rallel mechanismst,hat is, absorption on the spray droplets and absorption on the v e t t e d wall. As a consequence the extent of absorption by each of these mechanisms may he considered zdditive, and the theory for each m:+y he considrred independently. For dilute gases the material bz1:iiice and pate equations for the total system gi1.r
The integral represents the total iiumber of transfer units ST, and the terms on the right represent the number of transfer units resulting and the n-all abfrom the spray absorption, sorption, S,. respectively. This equntion is applicable only when t h r driving force, p * - p , is the same for both mechanisms. For the cross flow process under consideration where p * is zern (for sulfur dioxide absorption) or is constant (for :\dialintic humidifiA\*f,
Figure 1 .
T-iew of Experimental Tower
809
INDUSTRIAL AND ENGINEERING CHEMISTRY
810
CE LLUCOSE ACETATE SHEET
Figure 2.
>lode1 (:yc*lotleTower and F l o
cation), the log nieiiti v:iluc. of p * - p will l w used f o i . k ~ ) t l ii l l ( * spi'ay and the wall al)>orption. ;\H~ORI'TIOX B Y SFRAY DROPLETS.The quantity of gas ahsorhed by R single drop as it passes through the gas depends in part upon the area of the drop, the velocity with vhich it, moves relative t o t,he gas, and the time required for it to reach the wall o!
Vol. 39, No. 7
the toiver. T h e spray from a given 1107zle does not contain drops of uniform size hut rather consists of :I wide rang(' of sizes, depending on the type of nozzlf. used itrid the prepsure of the liquid R t the nozzle. Furthermore, as the liquid lrxves tlie nozzle, it exists as highly UIIstable films and filaments which pro\?dt* surfaces for absorption. A41so, during SPRAY COIL their traverse of the gas the drops ma:. chaige in size because of division, co:iIescence, and evaporation. The velocitJn.ith which the drops move relative to the gas is not constant a t different radial positions ill the ton-er hut depends up011 NOZZLE MANIFOLD tlie centrifugal force t o which they ar(' wbjected and upon the frictional resistNGENTIAL GAS ;ince n-ith the gas. Both of these force. LUTlON OUTLET vary according to the velocity distrihuLUTION INLET tion in the toner and the distance froni the center of the cyclone chamher. Thr, velocity distribution, in turn, is affected hy the ga.; flow rate and the geometry 01. the tower. and by the dimensions of thi, gas inlet :ind tlie presence of vaiien :tiid htlffles. Thus there is a greater interdr~Lines 1)endenceof variables than is experienced in simple packed column ahsorptioii. Thr v a l i i t , i n o a t difficult t o establish is the mass transfer area i t i the t ~ ~ y at r r :in?. time. This value rannot he incorporated wit11 the mass transfer coefficient a s in simple column absorption, hecause it is markedly affected 11y the operating conditions. In the two systems studied, the data show t h a t the g w fi1111 contrihutes the major resistance t o mass transfer. M7e s11:111
'\
p
\
-
4
"
\
\
I
2
INLET
NO.1
--3
I
2
6
d,
+i +---
25"
+-
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1947
811
assume that the absorption coefficient, KO,,is a power function of the velocity of the droplets through the gases which is determined by the spinning velocity of the gases, and this in turn is a function of the velocity through the entrance. \Te shall also assume that the surface area of the droplets in the tower a t a n y time, A,, depends on the spinning velocity of the gases and is proportional to the diameter of the tower. It is also proportional to the liquor rate and is a function of the characteristic drop diameter. In these studies the nozzles used and the pressure were held constant so t h a t A , depended only on the velocity through the entrance and on the number of nozzles operating. For the evaluation of the data, therefore, the effect of the various opernting conditions may be reprePented by
KgaAs
=
( 2)
C1Gn(Scj-2i%f(d)
where II represents the number of nozzles operating. Exponent ni cont:iina the effect of the entrance velocity on the absorption coefficient and the absorption area. The usual function of t'he Schmidt grou11 is used to permit applicability of the equation to any tiiffusing substance ( 6 ) . A4BSORPrIOS BY KALL FILM. Similar considerations of ttir absorption by the liquid film on the wall of the to\\-er lead t ~ i h-gv,
=
CgGq(Scj-'/'
Figure 4. Reciprocal Plot of Wall 4bsorytion Coefficient to Show Absence of Liquid Film Resistance
i3)
Here it is nssumed that the wall ahwrption ccrefficient K,,, is a power function of the velocity of the gases a t tlir vci11. \vliich is a h :i fiinctinn of the vplority through the e n t i x i i w C O K R E L 4 ' I I O I O F RESL-LrS
The results have been correlated on the hasib of Equations 2 and 3 to show the effect of the operating conditions on m a s transfer in the particular tower studied and t o show that these equations may be used for predicting the performance of other cyclone towers. ABYORPTIOS ON WET TOJVER WALL. Table I1 s h o w the effect of gas velocity on the wall absorption coefficient. The wall velocities are those memured 113 inch from the wall a t the midpoint of the wetted height. The elope of the line on a logarithmic plot of absorption coefficient against imll velocity is 0.72. This exponent n-ab used for the reciprocal plot zhon-n in Figure 4.
The extrapolated line passes through the origin indicating the absence of a liquid film, or reaction resistance when sulfur dioxide is absorbed by alkaline solutions on a wetted wall. A similar conclusion was reached by Johnstone and Singh (8) for the absorption of sulfur dioxide by alkaline solutions in packed towers. The data on wall absorption are also plotted in Figure 5 on a logarithmic scale as number of transfer units against velocity of the gas a t the wall (lower line) and against molar gas rate a t the entrance (upper line). The slope of the lower line is -0.43. The wetted area of the wall in all of these measurementi, was 53 square feet. Since the rate of absorption should be proportional t'o the wetted area, the equation of the lower line may be written as
s, = 0.155A,Vw-0.42
(4)
The equation of the upper line in terms of mass rate of flow through the entrance is
N , = 0.0134 AWG-',a7 Diameter Range, SIirroni 5&15 15-25 25-35 35-45 45-55 S5 -65
Y o Drops
C r Liquid
K5--75 75-85 85-95 95-105 105-115 115- 125
Range, Micron. 115-135 135-145 145- 155 155-165 165-175 175-185 185-200 200-250 350-300 300-350 m-400
(51
Since the number of transfer units, by definition, is proportional to the ratio of the absorption coefficient to the molar mass rate of the gas, from the relation of the exponente the w d velocity must be proportional t o the 0.88 power of the gas velocity a t the entrance. That this is true is shown by the logarithmic plot of Figure 6 where the lower line, with a slope of 0.88, coincides closely with the observed values for inlet 1 with 41/*inch vane setting.
Y o Drops
Cc Liquids 25,000 20,000 17,000 14,000 11,500 10,000 11,200 20,000 8,500 5,000 2,250
0s4:-x
a5
TABLE 11. EFFE(.TOF T-ELOCITY OF GI* ABSORPTIOS (Entrnnce dilllensions 41, L X 13112 inches: ~ a iirea of wetted wall, 53 square feet. 7.8 through spray 'coil) Lh. Wall Moles, \-elordhsorpI XIiii.) i t y i - w , tion R a t e , Hun C'u. F t . , [Sq. F t . Ft./ L h . SO?) So. Min. Inlet) llin. Min. 109 110 111 11% 113
69% 864 1110 1330 1740
3.90 4.88 6.27 7.50 9.82
1150 1400 1750 2050 2600
0.0640 0 060% 0.0540 0.0489 0.0477
AT
WALLo s
W.41~.
n set e at 41'2 inrhea wide, gallons per minute
N W
03
KOWAU',
Lb.
l ~ ~ i , , Moles/ ~
htni. X 103
(Min.) (4tm.j
1 423 1 114 0 872 0.736 0 588
0.702 0.845 0.966 1.04 1.27
.\
ID
0 425 0.410 0.364 0 329 0 306
0.2
Figure 5. Effect of Wall Velocity and Entrance G a s Rate on Wall Absorption in Cyclone Tower
812
INDUSTRIAL AND ENGINEERING CHEMISTRY
1248 1925 7 07 0 0969 0 0.365 I ) 98 1102 1725 6 33 0 1116 0 0398 1 O B 701 1160 4 00 0 1716 n 0342 1 61 692 1150 3 97 0 1719 fl OB6i 1 54 708 1170 4 07 o 1700 n 035U 1 68 1145 1800 6 60 n io01 0 0348 1 OR 1573 2380 9 22 n 0722 il 0311, ( 1 , 8 2 2085 3070 12 27 0 0549 0.0290 (1 64 650 1080 3 73 0 1801 o on59 8 42 5 17 0 1338 0 0 0 6 i 2 99 892 1440 1990 2950 11 47 0 0574 0 0126 1 5 2 7 37 0 0898 0 0088 .I ;+'2 1288 2000 1970 2910 11 30 0 0589 0.0150 1 3 i 1275 2000 7 30 0 0923 0 0184 1 68 832 1350 4 73 0 1390 0 0110 2 54 2250 3270 13 07 0 0503 0 0158 1 15 1610 2450 9 30 0 0730 0.0179 1 . 4 0 3 78 0 1727 0 0079 3 08 660 1100 1195 1860 6 85 0 1094 0.0165 1 84 21'? 823 1615 4.73 0 . 1 4 7 i 0 0156 2 25 Y1/s 1051 2000 6 . 1 0 0.1163 0 0191 1 80 21,'~ 1306 2425 7 60 0 0832 0 0188 1 51 %I/? 1823 2910 10 65 0 0634 0 0180 1 27 d 35 818 1625 4 . 6 7 0 1390 0.0166 2.1'2 d 35 1099 2100 6 30 0 1073 0 0167 1 . 8 6 d 35 1505 2750 8 . 7 2 0 0732 0 0162 1 51 35 2235 1450 5 29 0 07iO 0 0217 1 '27 35 2220 1970 5 90 0 0775 0 0237 1 18 6 50 0 0809 0 0270 1 09 35 2125 2125 a I n l e t 1 a i t h entrance area of 0.422 q u a r e foot u>rd f o r run- l i square foot, respectively. b Nozzle pressure 66.5-68.5 pound. per rqiiilre inrli Distributed over entire ntanifnld. d No vane. 177 178 190 193 210 211 212 213 215 216 218 219 220 222 223 224 225 226 227 234 235 236 237 240 241 242
20 20 20 20 20 20 20 20 50 50 50 50 35 c 35 c 35c 35 35 35 35 35 35 35 35
0 19 0 74 0 '20 0.83 0 24 1 37 n 24 1 3 0 0 24 1 34 ( I 20 0 . 8 6 0.18 0.64 0.1; 0.47 0 30 x 12 0 26 2 . 7 3 n ir) 1 33 n 22 2 . 1 0 0 21 1,16 0 24 1 44 0 29 2 2 5 0 15 1 . 0 0 0 17 1 . 2 3 0 25 2 . 8 3 0 . 1 9 1 70
U 039h
0 0413 n 0685 0 0650 o 0670 0 0430 0 0320
n o m 0 0 0 0 0 0 0 0
0625 0546 0266 0420 0331 0412 0644
:1x y 39 , 11.3 41 5 41 1 38 8 :34 4 33.0
2i 1 27 3 23.9 22 7 21 2 2n 4 22 7 21.i
37'2
18 6 30.6 30.8 82 0 81.1 :10.6 29 3 29.6 30 0 19 2 25 9 27 3 26.6 26.8 2 5 U 30 9 24.7 30.6 2 6 . 0 29 9 25 2 28 6 26.2 30.0 27.2 31.2 251 r i ? i t .
:.> 25 :3 1 36 47 66 76
_-
7
-112 183
1625 1650 1700 1650 1650 1700 1726 1800 1825 1875 1825
15 15 10 10 6 2 -4 -7 -5 0 13
421 427 295 286 172 59 - 120 -219 - 159 0 411
1570 1594 1674 1625 1641 1699 1721 1787 1818 1875 1778
IO9
2250 2300 2300 2300 2400 2400 2460 2550 2600 2800 2400
35 35 25 20 -5 -7 - 13 - 13 - 10 7 10
1392 1320 973 7 87
- a08 - 293 - 551
-660 -453 341 125
1843 1884 2084 2162 2390 .2383 2386 2483 2561 2780 2398
28 31 41 49 66 74 84 106 136 199 198
2600 2750 2700 2700 2880 2750 2900 3000 3150 3300 3200
27 30 23
-3 8
1180 1375 1056 834 725 96 - 252 -417 - 548 - 172 444
2317 2382 2487 2568 2706 2747 2888 2970 3103 3297 3168
1625 1675 1675 1625 1650 1650 1625 1625 1650 1725 1600
25 24 24 18 15 11 1 - 11 - 11 -4 10
687 682 682 502 427 315 28 -310 -315 -121 278
1472 1530 1530 1545 1594 1620 1625 1596 1620 1722 1576
2000 2100 2050 2025 2000 2OM) 2050 2025 2000 2025 1950
5 2 3 17 16 10 7 2 -5 0 14
14 !
348 250 70 - 174 0 472
1992 2098 2048 1936 1922 1970 2048 2023 1992 2025 1892
2600 2700 2800 2850 2700 27W 2700 2750 2850 2900 2760
10 "0
889 923
3300 3400 3500 3400 3500 3500 3500 3550 3700 3800 3600
-4
18 15 2
-5
--810
5 I
4 6
-7 -6
-8 -6 0 9 16
'20 7 11 10 5 - 10 -5
-5 0 11
(4
107 591
5 j%
244
200
2 a4
- 329
- 284 -382 - 299 0 429
910 1163 427 649 609 305 - 609 - 309 - 322 0 688
2444 2538 2789 2844 2687 2681 2687 2723 2836 2900
2717
3171 8196 :3476 3339 3448 3486 :3448
:i53ii :3685
20 22 26
27 31 37 44 55 68 91
44
49 58 68 84 97 123 152 191)
270 34R
18 20 "2 25
29 34 39 44 54
76 85
R8
38 :3 9 :3 9 42 50 62 71 82 106 124 50 55
73 84 83 93 106 128 165 21; 253 83 8i 113 109 137 156 I76 216 282
l l f tlir m - r a 111 t l i t ) c->.c.liiile ileterminp. the vc,locity :it n.iiicli til? l i r l i i i t l il1,111ilet.: j1e:irtr:ite the 13rcnuse oi thi. clifficiilty g g:is vrlocitir; Ivliile t l i t . 1v:i. ciiicr:iting, :dl of the nw:isitr~iiioiit- of vriority :{tit1 m g l e of flow were nintlr o i i t l i v (dry p i - , It i y :issunid that essentinliy tlie c m i e :i:itterti \roiild i)Iit;nin irliile the sprays n'rrc O ~ I O T :itin#. Frir the tmwr drxscribed tlie pnttclrti of t i i i w prr-riited i n b'igure 8 is typical for l)otli iii1t.t (ita I I H P ~and f o r :ill settings of tlie eiitr:irict, v:incs. Table IV gives the velocity tr:i :-ever:il ratra of gas flow tlirouyli inlrt 1 . 'Flip horizontal components for the tn.o v:me ?etting> are shown gr:iphically in Figiire 9 for tr:ivcr++ : i t four heights, 10, 30, 50, and 70 inches :il)iivt. t i i t , bottom. The slopei of the lines for the d a t a taken 50 inches above the bottom of the to\vei,, :it which the flow should he smoothed out From tlic diiturtxince of the deflecting vane, varirs f i , o n i -0.1 to -0.25, For small cyclone dust rollrctors with the exit duct a t the same end :is thc. inlet, Shepherd and Lapple (18) foiind that tlir horizontal velocity component is proportional to R-0.5. The entire core of gas spine at :I velocity approximately equal t o that of the outside gas stream. The maximum.horizonta1 velocity is at R distance of about 4 inches from the center. Since the nozzles on the manifold extend to ahout 11i2 inches from the center line, they prol)ably act as a resistance to the spin :it thr. center and are responsible for the existence 01 tile niasimum horizontal velocity. I n any case, the h p i n ning of the core of gas is apparently caused by the transfer of momeritum from the main outhitie gas stream, for there is a sharp decrease in tht, angle of the spiral a t a distance from the ~ v n l equal l to the width of the vane opening. This is > h o w by the vertical velocity components; :iltliough some\\-hat more irregular than the horizontiit components, in general they indicate that t h r w is a downward flow of gas in a zone estrntling froni about 3 to 8 inches from the center. This general pattern was observed i n t case regardless of the inlet design or vatir $rtting. For wide vane settings, however, althou of f l m :tbove the horizontal plane \viis I hnlf t h r radius than it w w near the v-:ill of tlit, tower, it WNS never nrgiitivc in sign : c.on.t~clu(~iitly there was no net doiviiward flo\v. 1iitrgr:itions rif the vertical velocity tlistributioii curves :tgroed within lOy0with tlie gross flow of gas through t h e cyclone for the higher rates of Hoiv ])ut \vert' :w inuch a$ 50yo low at low gas veloc.itir*. Tlir CTktencc of the high -pin velocities i n the rore of gas within the main gas spiral is of intrre-t in the rgclone spray scrubher, because it r e d t s i n large centrifugal forces which hurl tlic .pi':iy droplets from the nozzle?.
.pr:iy
HUMIDIFIC.ATION \IE:ASIJRERlEYI'S
LVet- and dry-bulb temperature nieiisurenients were taken himultaneously with the gas analyses in the 14-inch inlet and outlet ducts (Table 111). I t is evident from the rhangr in the wet-bulb temperatures that the toiver vas not operated sdiahatirally hut thiit heat n-xq ahsnrhrd from the sinimunding
INDUSTRIAL A N D ENGINEERING CHEMISTRY
July 1947
815
warin air. In calculatiiig the number of transfer units it was assumed that the humidification took place in the loiver portion of t,he tower under adiabatic conditions and t h a t the heat transfer zone was essentially above the spray zone. The number of transfer units was then calculated using a logarithmic mean for p * - p in the integral of Equation '1. The use of the logarithniic mean driving force for the cross flo\\- process can be justified only when the temperature of the drop surface-that is, the net-bulb temperature of the gas-remains constant in y zone. The the ~ p r : ~ humidification huniitlifiration on the wall was calculntrd using Equation 5 with the Figure 8. Flow- Pattern of Gases in Cyclone proljer cwrrections for the Schmidt Heavy arrows represent actual velocities 50 inches above bottom at 1 l W cubic feet p.1 minute3 groul, for lvater vapor, aIld the nulniler ea dk light arrows represent horizontal and vertical components; angles show direetion h of transfer units in the spray \va+ low horizontal plane. found by difference. Berause of the difficulty of making lone s1)ray svruhbers if the results are to be generally a p accurate huruidity measurements under these conditioiih aiid t,tit> of plicable for design purpoees. Fortunately, data are available on close :rppro:tch to equilibrium in several of t h e runs, there ivas considerahle variation of the ratio .YT(H?O)'.Y ?(SO?),as shown the performanbe of two large towers for humidification and absorption of sulfur dioxide from flue gases when an alkaline soluin the la-t column of Table 111. The average value is 2.26 tion was circulated through the spray nozzles ( 7 ) . These proand the deviation of the mean is -0.062. The average is apvide a n opportunity to test some reasonable assumptions as to proxiinately equal to the ratio of the Schmidt groups for sulfur dioxide and \vatel, i i i air at 30" C.--that is, 1.34:0.607. the effect of the factors which have not been studied. Thus it The ubual value of - *I for ,the exponent on the Schmidt group may be assumed that the number of transfer units in the spray is directly proportional to the diameter of the lower and inversely rather than - 1, lion-ever, n-ill he rrt:~inrd tw(a:iiiw of the low t o the niass median diameter of the drops in the spray. If these accuravy of the humidity i1ie;tsureineiit.i. quantities for the small t,ower are substituted into Equations 6 (:0\11'4RISOR WITH L4R(;E (:Y(:LO%E S(:KCL(BP:Hh and 7 and the total liquid rate is introduced in place of the number of nozzles used a t constant pressure in the tests, the general AIthough Equations 6 and 7 are satisfactory for the cwrrelatwri equatior' becomes of the exnerimental data on the model toxver. more inforniatiori
is needrd on aeveral other fartor-
E RUI~
~
yelocity, Ft./Hin
\-
0 4001 4000
-I
1
W
4500
J
'f? 3000
-i
0
0 I 1500
2 3500
G
z
1
I
I
,
2500 2500.
l
l
l
1
1
2000
3
4
5
6
7 0
DISTANCE FROM A X I S OF
IO
15
20
TOWER,INCHES
I I I I 2
DISTANCE
4
3
5
-
e
I
j
:a-: 2000
2500
Figure 9.
1
3000
J
= 20002
/
>
i I
FROM AXIS OF
I
I l l
6 7 0
IO
15
20
TOWER, INCHES
Horizontal Velority Components in ( : y c - l o n e 'l'olter u ithout Spray % o d e s Operating, w i t h Vane Opening 2l/?I n c h e s Wide (left) and 1' Inches Wide ( r i g h t ) 0 10 inches aboFe bottom, tower radius 14 inches 0 30 inches ahovr bottom, tower radius 13' i inrhra -
-
a
50 inches ahme bottom, tower radius 13 inches i0 inrhrs above hottom, tower radius 12' 1 inrhrm
inlet and outlet duets; (bj expanaiou of the ga> xlieii it eiiter:. the cyclone chamber; (c) kinetir energy of rotation in the cyclone chamber; ( d ) wall friction of the gases in the cyclone chamber; and ( e ) contraction, friction, and expansion losses in the exit duct system. It is customary t,o expre.;s the over-all pressure loss in terms of the ratio t o the inlet vclority head. Kormally, for dust collecting cyclones, the loss as kinetic energg of rotation of the high velocity gas stream in the cyclone is responsible for mort of the pressure loss, and thih niay amount t,o several times the inlet velocity head. The over-all pressure loss across the cyclone spray scrubber \vas nieasured with water manometers for each of the absorptioii runs. T h e measurements were made both m-ith the sprays operating and for the same gas velocity with dry tower (Table T'I). The observed losses are all small, the largest value being about 3 inches of water for the dry tower at the highest velocity through inlet 1 11 ithout the deflecting vane. In every case the loss \vas smaller when the sprays were operat,ing than it, was 1%-iththe dry tover. Calculation of the momentum of the v a t e r leaviiig the nozzles shows t h a t i t x a s more than sufficient t o provide the angular momentum of t h e gas, provided that it was all directed in the direction of gas flow. T h e average pressure loss, expressed as number of inlet velocity heads, was approximately 1.2 for the dry t,ower and 0.8 for the spray tower with inlet, 1 with the 41/p-inch vane. These values were doubled when the deflecting vane was removed, as ivas expected from the results of previous workers (1, 15). X decrease in t h e width of t,he vane setting to 21/%inches also increased the pressure loss. A part of this increase was due t o the increased
rotational v l o c i t y rid described. T h e pressure 10-a~- \\-ere also greater with inlet 2 , \vliich gave higher rotational velocities than inlet 1 with the straight deflecting vane. In the measurements v i t h this entrance, the upstream pressure t a p was lomted in the entrance section near the hinge of the control vane. From the dimensions at this point and at the entrance for each hetting of the vane, a correction to the manometer reading \vas calculated so that the actual over-all pressure loss from the cyclone entrance t o the mit duct was found. In practice the recent installatioiis of cyclone spray twvers for dust collection 1vei-e designed with much higher entrance velocities and pressure losses than were used Jx-ith the model tower. Velocities up to 15,000 feet per minute are used in order to spin out the small droplets from the fine atomization which is necesw r y for the removal of extremely small dust particles. I n general it ha5 been found that the assumption of the over-all pressure loss equal to on? inlet velocity head is satisfactory for design estimates. The validity of this assumption, provided the correct inlet design is used, is established by the present data. Several installations were made v-ith scroll exit designs to recover the kinetic energy of rotation. For high velocities this may amount to a reduction of 10 to 207, of the pressure loss. ACKNOWLEDG>IENT
The work reliorted here is part of a n investigation in t h e Ehgineering Experiment Station and was sponsored in part by the Utilities Research Commission of Chicago. The authors acknowledge t h e helpful suggestions of A. W. Anthony, Jr., and the assistance in the experimental work of Oliver R. Kirby.
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1947 \ 011E\
C LATURE
.I= ,, au~fac~c, area oi >iiray droplets in tower a t any tinie, .q. ft. A , = surfac>earea of wett,ed wall, sq. ft.
b = tliffusivity of solute gas, consistent units Ci, C2 = constant coefficients in ronsistent units r of spray dropleb, mic.wiigas flow rate through duct a t toxyer entrance, 1)). mole.* !niin.) (sq.ft.) e loss across cyclone, inches oi watei, re expressed as number of entrance velocity head. K , , ~= niiis- transfer coefficient for spray droplets, 11). nioles ' j n i i n . ) i s q . ft.) (atm.) K , ,, = nia+ transfer coefficient for the wetted wall, Ib. moles: [niin.) 'sq. Et.) fatm.) L = liquid rate, gal.;niin. w , (1 = expnnential constaiits n = numher of nozzles = n u m h ~ . of r transfer units on spray surface &VT = total iiuniher of transfer unit:: in tover S,, = numher of transfer units on \retted vial1 5urface p = partial p r e ~ w r eof solute gas, atni. Subscript 1 refew to inlct gas, subscript 2 to outlet gas, anti aqtrriik to equilibrium preysure R = distanre from center of tower, ft. S = c.ro~s-sectiona1area of gas inlet, sq. ft.
G
=
rt at ton-er entrance, f t . miii.
817
I', = gas velocity near the na11, ft. i n i i i . = total pressure of qns, atni. = density of air, l h ~ m ft. ~. p u = viscosity of air, ronsist,ent u n i t %
T
LI1'ER.ATURE CITED
(1) .Ilden. .J. L., Ht,fii/iy oliri T ' t t f t i / f i t ; r ( y 35, 4& ( 1 9 3 5 , . ( 2 ) - h t I i o i i y , A . I\-., J:.. (.o Pease-hnthon? fcliiipincxit Co.), U. S. Patent 1,986,,013(Jan. 8, 1 0 3 5 ~ . ( 3 ) I / ) i d .2,155,853 f,.kpr, 2 5 , 19391. ( 4 ) 16id..2 , 2 8 1 , 2 6 1 (?.pi,. 2 $ , 1942). ( 5 ) Anthony, 1. K , JI ., private ro~iiiiiuiiication,AIarch 1946. (40) Chilton. T. H.. a n d Colhiirri, A. P., IND. E m . CHEM.,26. 1183 (1934).
( 7 ) .John3tone. H . F., :mri Kl:~iiis-titi~idt, l t . I,., T r a m . Am. Znst. Chrm. Engrs.. 31, 1'11 (19381. (8) Johnstoue. H. F . , aiid Yingh. h. I ) . . I Z D . l h c , (''HEAI,, 29, 286 (1937). (I)) Kleirisehinidt, I t . \',, ('hem. t t .\Id. Eng.. 4fj, 4h7 (1939'1. (10) Kleinschriiidt. 11. V., and .hthotiv, A TI-.,.TI... T r a m . .am.S o r . M ~ c h Engr,?., . 63, 349 (19.11). (11) Pea-e. F.F.. C . Y. P a t m t 1,99:!,7~i2 ( l . e l ~2li, . 1'1:45]. (12) Shepherd. C. B., a n d Lapple, C ' . I-:., I N D .l:scl. H HI:^,, 31, 972 (1939).
11X I'tid., 32. 12440 ( 1 9 4 0 t . (141 Silcox. H. E., Ph.D. t h e i s , I-iiiv. of Ill., 1942. PRESESTED before the Di 111 of Industrial and Engineering Pheiiiiitry a t rhe 109th l f e e t i n g of the . l \ r a ~ r r a v~ H F ; \ I I C & LS O C I E T Y , .Atlantic City, N. .1.
Vapor-Phase Nitration of Saturated Hydrocarbons J
H. B. HASS AND H. SHECHTER' Purdue Cniversity and Piirdiie Research E'oicndation, Lafayette, I n d .
S
I S C E 1930 considerable interest has been directed toward developing a new unit process, the vapor-phase nitration of aliphatic hydrocarbons. As a result the Commercial Solvents Corporation established a n efficient industrial process for the production of nitromethane, nitroethane, 1-nitropropane, and 2nitropropane by the reaction of nitric acid and propane a t temperatures above 400' C.; recently Imperial Chemical Industries, Ltd., announced the availability of these four nitroalkanes. On the bn4s of the investigations tvliich have been conducted at Purdue University and in many other laboratories during the past sixteen years, it is now possible to generalize about ninny of the complex actions of the vapor-phase nitration reaction. .Is a result a number of empirical rules based on quantitative experimental evidence have been formulated which characterize the reactions betn-een the various nitrating agents and saturated hydrocarbons. RULE 1
If pyrolytic temperatures are avoided during the nitration of alkanes or cycloalkanes, no carbon skeleton rearrangements occur. This rule is in harmony with the corresponding one for the monochlorination of alkanes. Sitration temperatures usually range from 150" to 475" C., and thus pyrolysis of the parent hydrocarbon does not constitute one of the principal high temperature reactions. However, coneiderable decomposition of the various nitrated and oxidized compounds occurs and results in the formation of olefins and degraded products. For example, pyrolysis of nitroethane and 1-nitropropane yields olefins, alde1
Present address, The Ohio S t a t e Cniversity, Columbus, Ohio.
hydes, carbon monoxide, carbon dioxide, and nitrogen but no loner nitroalkanes (18). The data presented in Table I indicate t h a t nitration of 2,2dimethylpropane, 2,2-dimethylbutane, 2,2,:3-trimethylhut:u1f~. and cyclopropane yields no product's resulting from structurd isomerization. Since nitroneopentane is obtained from neopcittime and neohexane, the reactions producing hydrogen or nll
