develop men t
Recovery of Nitrogen Oxides by Silica
E. GORDON FOSTER'
AND
FARRINGTON DANIELS
UNIVERSITY OF WISCONSIN, MADISON, WIS.
4. T h e nitrogen dioxide is recoverrd by adsorption on the silica gel.
w o r k was undertaken to develop an economical process for the recovery of the 1 to 1.5% nitric oxide produced by a thermal process for the fixation of atmospheric nitrogen which was developed at the University of Wisconsin. Basic data are presented for the dehydration and adsorption steps of a recovery process in\olving cooling of the product gas from the fixation furnace in a spray tower, dehydration of the gas in silica gel drScrs, catalytic oxidation of the nitric oxide content of the gas to nitrogen dioxide, and adsorption of the nitrogen dioxide h 5 silica gel and desorption to gibe a concentrated nitrogen dioxide. Of chief interest is a method developed for corrclation of the rate of adsorption of nitric dioxide on silica gel. The effect of silica gel depth, temperature, particle size, and gas velocity on rates of adsorption are reported and correlated in terms of F , the average rraction saturation of the entire gel bed, and E,, the average adsorption efficienq during the adsorption c)cle. The data indicate that diffusion of the adsorbed nitrogen dioxide into the solid aclsorhent may be the rate-controlling step.
5. Xitrogen dioxide is released later by heating.
In t h e devorption process t h e nitrogen dioxide is removed eit,lier b y heat>ingthe gel in a heat exchanger or hy recirculating a hot mixture of nitrogen oxide and air through t,he gel bed. .After desorption the bed is cooled b y the recirculation of cold air and the adsorption cycle is then repeated. T h e silica gel is such a 1mor heat conductor t h a t recirculation of gases through the bed and through heat exchangers is advantageous. Provision is iiiade for removing the heat evolved in the adsorption process i ) c ~ n n s ethis heat is large and a small rise in temperature deryeases the amount of nitrogen dioxide adsorbed. T h e desorbed nitrogen dioxide, in concentrated form, may then be absorbed in water in small absorption ton-ers, or it may tic condensed t o give liquid nitrogen tctroside. , Dnta are presented giving the capacity of a number of different .ilica gels for t,he adsorption of water and of nitrogcn dioside. Data are also given xvliich shov the effects of temperature, gel ilepth, nnd particle size on the adsorption efficiency for nitrogen tlioside. Tivo quantities, the fraction-saturation of the gel and the avcrage adsorption efficiency are correlated as a function of a S A thermal process for t,lie fixation of atmosphrric* iiitrogrn ratio involving the bed depth, particle size, concentration of inlet developed a t the Uni ity of \Viseonsin ( . i ) over 1% of tlie gas, and adsorption capacity of the gel. nitrogen in air n-as converted into iiitric oxide by licatiiig air ir-ith T h e rate of adsorption of nitrogen dioside on silica gel is a e ~ fraction of the rate calculated from t h e resistance of t,he gaseous fuel above 2100' C. A c l o u b l ~lied of Inagnesiii ~ ) ~ l ~ l ~ lsinall served to preheat the air and to chili the products of CoiiibuPtiou gas film alone. This fact, together with a strong dependency fast enough t o prevent. dissociation into nitrogen and o\;\-gcln. I t of tlie adsorption rate upon temperature, indicates t h a t diffusion was recognized from t,hc beginning of the n-ork in 1939 t!l;it it of the adsorbed nitrogen dioxide into the solid adsorbent may be probably would be too expensive to recover 1.5 t o 2.070 nitrogen the rate-controlling step just as liquid film diffusion is frequently dioxide in standard n-ater absorption t o w r a . Aiccordiiialy :t the rate-controlling step in the analogous absorption of a gas b y a search for a n improved method of recovering the nitrogen oxides liquid. was carried on simultaneously n.ith the development of :i s&iit:iI)le refractory and furnace capable of operating a t 2100' C . I'mDRYING AND OXIDATION O F NITRIC OXIDE liniinary work on t h e recovery of nitrogen osidrs from an electric I n the early experiments the furnace gases contained 2.07'0 arc hy silica gel ( 5 ) offered dTicient promise to carry nut t,he c7arbon dioxide, 6.0% water, 15.7% osygen, 1.0% nitric oxide, research on the adsorption of nitrogen oxides suninxiriae~li n tlie mid s h o u t i 5 % nitrogen. T h e temperature of the gases ranged present report. T h e results have been useful in citrrying o:lt from 100" t o 350" C. depending on the time after reversing the experiments on the recovrry of nit,ric oside on a larger mil(,. tlirection of air flow through the double-pebble bed furnace. DurI n t h e recovery process : I S c:trried out, in a pilot plant :it t h e ing t,he sprag cooling and dehumidification there is fiome loss of University of JTisconsin t h e fluc gas leaving the nitrogtn iix:ltion fixed nitrogen l)y the reactions furnace and containing 1 t o 1.5% nitric oxide !vas subjected to fivr operations: NO 1 / 2 0 2 eNO, 1. T h e flue gas iE first cooled and partially dehumidified by a cold water spray NO2 XO HZO ~e2HIVOs 2. T h e dehumidification is completed with silica gel. 3. T h e nitric oside contained in the dried flue gas is oxidized T h e oxidation of nitric oxide is slow and b y means of a rapid t o nitrogen dioxide in the presence of silica gel. rate of water flon- in the spray tower it is possible t o obtain a high rate of heat transfer and dehumidification with a minimum 1 Present address, E. I. dri P o n t de S e i n o r i r s 9: Co , I n c . , Wilmington. contact time of the gases in the tower. Preliminary experiments Del.
I
+
+
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INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1951
shotved a loss of less than 10% of the 1% nitric oxide in the furnace gases. Calculations based on short increments in a spray tower 5 feet high and 9 inches in dianieter using furnaw gas containing lyOnitric oxide, obtained by burning 1 part of methane in GO parts of air, gave a t,otal loss of nitric oxide of about 127,.
TIME, MINUTES
Figure 1.
Loss of Fixed Nitrogen
Silicn gel used to dry w.et gases low in NO content
67 standard e n . leet/niin.-square feet 6 , 7 inches
Linear velocity
Deqth of bed Initial concn. S o .
0.84" Temp. (isothermal operation) 25' CP yo relative humidity of entering gases 50% Additional Data 07
Linear Yelocity, Ft./.\Iin.
Depth
67
Inches 1.3
Initial 4S O 0.80
164
3.3
0 . 84
of Bed.
Humydity Temp., Entering C. Air Comments Saliiple exit gas CUI2.5 90 lected after 2 niin. SI, apparent difference i n enterinn arid exit concn. of f i x ~ dnirrogen 25 90 Saniple exit gas collected after 2 1 , ' ~ niin. X o apparent difference i n entering and exit concn of fixed nitrogen
The dehumidification of the furnace gaPes is completed in silica gel dryers. BJ- virtue of t'he following facts it is possible to dry the furnace gas b y adsorption of the water vapor without a t the same time sustaining excessive loss of nitrogen oxides: 1. Silica gel has practically no adsorption capacity for nitric oyide as such. 2. Although dry silica gel containing adsorbed nitrogen dioside catalyzes the oxidation of nitric oxide to nitrogen dioxide, silica gel saturated Kith water vapor does not catalyze the oxidation. 3. Silica gel has a much higher adsorption capacity for water vapor than for nitrogen dioxide. This selective adsorption can be improved by the proper selection of silica gel. 4. T h e rate of adsorption of water vapor by silica gel is much greater than the rate of adsorption of nitrogen dioxide.
T h e silica gel bed acts essentially as a selective adsorber of water vapor from nitric oxide and air, similar t o the chroniatographic adsorpt,ion of differently colored materials in solutions. When a gas stream containing nitric oxide and oxygen and water vapor is passed through a deep bed of silica gel, the water is removed in t,he first part of the bed and nitrogen dioxide is found farther along in t,he gel bed. The complete dehumidification of the furnace gas by silica gel without escessive loss of nitrogen oxides lyas shown b y the following experiment. A gas mixture containing 0.8-i70 nitric oxide in air, 90% saturated with water vapor a t 25' C., was passed through a layer of silica gel 6.7 inches deep contained in a */*-inch diameter tube immersed in a wat,er bath at 25' C. Samples of exit gas were analyzed for nitrogen oxides with the results shown in Figure 1. T h e loss of fised nitrogen is represented by the shaded portion of t h e graph. At the end of 55 minutes the
987
dew point of the gas was still less than 0' C. It is eatirnatrtl from curves of Dodge and Hougeii ( 7 ) for the adsorption o f water vapor by silica gel t h a t the gel, under isothermal conditions, would continue t o dry the gas for i 3 minutes t o a final drying efficiency of 95%. With this drying cycle, the average loss of oxides of nitrogen would be less than 5% of the nitrogen oxides fed over the entire drying cycle. T h e mechanism of the slow oxidation of nitric oxide in t,lie gas phase is not completely understood. It is being studied furt,her. When a mixture of nitric oxide and air is passed through a bed of dry silica gel for a vhile the reaction is accelerated n hundredfold or more. A possible catalytic mechanism is t h a t the adsorbed nitrogen dioside formed by the oxidat,ion of the nitric oxide acts as a solvent for the nitric oxide b y forming nitrogen trioxide. T h e dissolved nitrogen trioxide could then react with oxygen diffusing t o the surface of the adsorbed solution. In support of this mechanism it vias observed t h a t when nitric oxide was passed into a silica gel bed containing adsorbed nitrogen dioxide, the color of the gel changed from brown to green, indicating the formation of nitrogeu trioside. T h i s catalytic effect has been studied by others ( 3 , 8 ) . It was observed in this work that a silica gel bed 1 to 3 feet deep a t 25" C. was sufficient t o catalyze the oxidation of pract,ically all of the nitric oxide when the gases were passed through the bed at the rate of 50 to 100 cubic feet per minute per square foot of bed cross section. It was observed also t'hat silica gel saturated with water vapor does not appreciably catalyze the oxidation, whereas silica gel saturated with nitrogen dioxide i p an effective catalyst. ADSORPTION ISOTHERMS OF NITROGEN DIOXIDE ON SILlCA G E L
The remainder of this report is concerned n-it11 the quantitative adsorption and desorption of nitrogen dioxide on different types of d i r n gel. r . I l i p rrcovcry of nitrogm osides by silica gel is riot II(.\V 1,5:i. .In investigation of the adsorptive capacity of silica gel l'or nitrogen dioside was reported by Ray ( 1 0 ) arid i)y Alniquist, (;u(lily, and Braham ( 2 ) . T h e per cent weight of adsorkd iiitrogeri dioside by the silica gel is low, however, anti sirice the economics of the adsorption process depend to a considerable extent upon the capacity of the adsorbent, attempts were made to find :I silica gc.1 r i t h a high adsorbent capacity for nitrogcn dioxide. In studying the adsorption with different silica gels a stream of t i r i d air containing about 1% nitrogen dioxide was passed at measured velocities through the silica gel bed enclosed in a vc'rtical 1-inch glass tube surrounded by a large thermostat regulated t o 0.01" C. The nitrogen dioside was generated by passing air through an electric arc between iron electrodes operated by a high voltage transformer. The all-glass system included a time ctiamt)er between the arc and t,he adsorption bed, large enough t o permit, nearly complete oxidation of the) nitric oside t o nitrogen dioxide. T h e amount of nitrogen dioxide adsorbed was determined by weighing the gel before and after the adsorption. Thc roni'cntrntion of nitrogen dioxide in t,hr gas stream before and after adsorption n-as determined with a photoelectric colorimcter (S), calibrated with different amounts oi nitrogen dioside det(~rniinet1 analytically. The results of the selection tests for the best adsorbents are summarized in Table I. As can he seen from this table there \vas a wide variation in the adsorption capacity of silica gels for nitrogen dioxide. The best comniercial silica gel for nitrogen dioxide adsorption had over tnice as much adsorption cnparity for nitrogen dioxide as did the poorest. A special silica gel, prepared by approaching the gelation point (neutral pII) from the basic side rather than from the acid side as is custoinary, had approximately a n 80 Or, greater adsorpt'ion capacit,j- for nitrogen dioxide than the best comniercial gel. This gel, howelrr, h x d t h e
988
INDUSTRIAL AND ENGINEERING CHEMISTRY T ~ B LI E
Vol. 43, No. 4
.kUSORPrIOY O F S I T R O G E S DIOXIDE B Y
DIFFERENT
SILICA GELS 4~ 25" C. S o . oi .idsorption Cycles
1
2
1 2 3 1
4
1
x
4 1 1
3 4 1
PN9, Vrn. Hg I;
Figure 2.
Nitrogen Dioxide Adsorption Capacity of Silica Gel 5
disadvantage of a lowcr density and a greater friability than any of the commercial gels. Adsorption isotherms for t.he hwt adsorbents, silica gels 5 and 7 , are given in Figures 2 ;inti 3, respectively. Several of the adsorbents which had been tested for nitrogen dioxide adsorption were also tested for the adsorption of water vapor. Isotherms were measured by the static method. The data are not plotted. Those adsorbents which had a high capacity for nitrogen dioxide from a 1% gas also had n high capacity for water vapor at a per cent relative humidity of less than 5. There was, however, no correlation between thc : i t i m p tion capacities for 170 nitrogen dioxide and water vapoi' itt the high relative humidities normally encountered in the dehumitlification of air-for example, silica gel 1 had only 35% as much adsorption capacity for 1% nitrogen dioxide as silica gel 7: but had l Z O 7 0 of the adsorption capacity of 7 for n-ater vapor at a 60% relative humidity. Silica gel 1 mould, therefore, be useful in the dehumidification step of the recovery process whew it is desired t o keep the adsorption of nitrogen oxides a t a minimuin.
G. S O r
per 100 G , Gel
1.67 1.27 2 40 2 3.5 2 34 1.94 1.88 1.88 1.88 2.59 2.41 2.60 2.48 2.23
Other Silica Gpls
G r w n ,and rxtracred 3 hours n i t h aqueous HCI csy. g r . , 1.18, a t 50' C . Lamisilite (verniiculte extracted with acid, iiifg. by Peoples Gas, Light, and Coke Co., Chicago. Ill.) Chabazite, activated by heating f o r 1 hour a t 300' C. Chabazite, extracted 1 hour with HSOa (sp. gr.. 1.42). washed, dried, and heated 0.5 hour a t 2000 Bentonite extracted 2 . 5 Iioiirs with 25% HzSOd a t t h e boiling point .4sbestos extracted w i t h boiling HSOa ( a p . gr., 1.2) for 20 niin., washed, dried, and heated a t ?000 .\ad-extracted soda glass 1. Soda glass (alkali: silicate ratio; 1:'2), 6-14 mesh. extracted with 8 N HCI a t (0' C . iintil t h e alkali was cornpletely neutralized. and then washed, dried, a n d heated a t 180' C. .icid-extracted soda glass 2
c.
c.
1
4 81
2 5
4 65
1 1
2 3
1.7; 4.11 3.79 3.14
1
1.21
2
4.36
1
1.26
1
2.70
5
3 ,3 2.6
5
4 40
c d l y corw1:irc.d in tc'rnis of the fractioii-satiirLLtion of the gel t)ed, F , the average adsorption efficiency over the entire adwrption cycle, E,,., and the ratio Drc- ot the lied, poun
of riitrogizii dioxidr per pound 01' nitrogen dioxide-free gcl. Lis defined by Isquatioris 3 and 4, is the amourit of nitrogen dioxide adsorbed in the ciitirc bed divided by the pounds of nitrogeii dioxide in the feed gas which is fed to the bed during the adsorption cycle. I.' and Eav.have lieen calrulntrd by graphical integration of t l i r d a t a given in F i p u w t o I). iiirlusivc! nrcording t o ' I.:qrintions 3 :in(\ 6. ('1
\vli(src tlic.
ii(~n. s>.iiil)olslinvr
t l i c foIlor\.iiig siKiiificuiicv:
p = :ii =
liulli delisit?- of grl, pouiitls p c cul)ic ~ foot8 nittogen dioxide content of air, p o u n d s of nit,rogen dioxide.
f;
=
7
=
per pound of nit,rogen dioxide-frrc :iir a t the entrance t o the gel bed mass velocity of drj- and nitrogeii dioxide-free air, pounds per hour per square foot time, hour::
If F and Ea>. :we i i o \ \ - graphed as fuiictious of .r and thc tireal;. point efficiency E,,, i t is readily eecn that, fixing w a ,p , vi, and G ( [ c a s p : !/% are normill>- f i x d ;inyway hy the recovery problem
INDUSTRIAL AND ENGINEERING CHEMISTRY
990
Vol. 43, No. 4 i 'J
IOCC
2000
3300
4ooo
S.c.f./ f
5ooo
6000
7000
t.2
1>3 1\I
ing &am or heat consumption), the n r c e s s a q bed depth, the length of the ndsorpt,ioii cyck. and the break-point efficiency a t which t h c a
cycle should be changed are rendilj- r:ilculatetl. Conversely, if a given per cent rcwvery pw cycle is desired for a given tied t l q i t h , the pc.r cent saturation of the gel, thr lengtli of tliv adsorption cycle, and the breakpoint efficiency can also be calculated by g i ~ ~ pre:tding h and :I simple arithmetical calculat,ion. T h e effect, of the gel bed depth, .r, upon li' ani1 E,,., as given hy Figures 9 and 10, ea11 bc prodieted from t,he theory of adsorption developed by Hougen and .\larshall (9). The effect of adsorption capwit-, as expressed by the ratio c8/c(, however. cannot, as will be shown lat,er. According t o the t,heory of Hougen and Marshall, adsorption from a fluid stream flowing through a stationary granular bed is g o i w n e d by the following equations:
' Temperature: ' ~ ~ ~
15.00 p ~3*F0.05.C. t. . '
Inlet Conc. N%: S.c.f./ f t.- min.: Av. Pressure:
x
2
;6€
0
1.19 %
44.7
7 5 0 Mm.Hg
6 - 8 Tyler Mesh
c
z
S.c.f. I f t.'
l>- H , , i.01, water adsorption = 0.610/0.0348 = 17.5.
1
llir
~ v l i t ~ the r c ~ nelv symbols h a w the following significaiiw : /;
=
diffusion coefficient for diffusion of a d s o r h d iiitrogrii diosido \Tithin the gel particle, pounds of gt.1 p~ hour p r foot
EngFnyring process development
Flow Rates through Soybean Flakes I
GRAVITY PERCOLATION OF'HEXANE MISCELLA
DAVID CORNELL', BLAW-KNOX C O . , PITTSBURGH, PA. DONALD L. KATZ, UNIVERSITY OF MICHIGAN, ANN ARBOR, Hexane is used to extract oil from soybean flakes 11) pcrcolation through beds of flakes. This paper prewnts C Y perimental data on rates of percolation f o r hexane rniscella under specified conditions. The data are correlated by the procedure debised 1)) Brownell and Katz for porous beds; this inbolves a lcnowledge of bed porosity, particle diameter and shape, and properties of the miscella. The correlation permits the presentation of charts for predicting hexane miscella percolation rates under conditions for commercial equipment.
MICH.
M
A S Y so!.lieau cstructiou plants u,w cquipiuent i n whicli hrsane is :rllon.rd to percolate by gravity t,hrough a h t l of so?-hean f l d i p i . The flow rat.c at which the liquid w i l l p n s ~ through thew Halw beds is of interest to equipment designer? and plant operators alikc. Kenyan, liruse, and Clark ( 7 ) give operating d a t a for a basket estractor handling 405 tons per d a y of soybean flakes. Their d a t a show that the mass velocit'y of iniseella through the extraction baskets is about 2200 pounds per Jquare foot per hour on t,he count,ercurrent side, and about 1600 pounds per square foot per hour on the concurrent sidc of thi. :
Pi?-ent address. L-niversity of AIichigan, Ann Arbor, Mich.
