Ind. Eng. Chem. Res. 1990, 29, 1492-1502
1492
Savage, P. E.; Javanmardian, M. Effect of Reactant Size Reductions on Catalytic Reaction Rates. Ind. Eng. Chem. Res. 1989,28,381. Smith, J. M. Chemical Engineering Kinetics, 3rd ed.; McGraw-Hill Co.: New York, 1981. Ternan, M.; Packwood, R. H. Catalyst Technology for Reactors Used to Hydrocrack Petroleum Residua. In Chemical Reactor Design and Technology; de Lasa, H. I., Ed.; Martinus Nijhoff Publishers: Dordrecht, Holland, 1986. Velo, E. Ph.D. Thesis, Department of Chemical Engineering, Universitat Polit6cnica de Catalunya at Barcelona, 1990 (in preparation). Velo, E.; Puigjaner, L.; Recasens, F. Inhibition by Product in the Liquid-Phase Hydration of isobutene to tert-Butyl Alcohol: Ki-
netics and Equilibrium Studies. Ind. Eng. Chem. Res. 1988, 27, 2224.
Wakao, N.; Smith, J. M. Diffusion in Catalyst Pellets. Chem. Eng. Sci. 1962, 17,825. Wheeler, A. In Advances in Catalysis; Academic Press: New York, 1951; Vol. 3. Wilke, C. R. Estimation of Liquid Diffusion Coefficients. Chem. Eng. Prog. 1949, 45, 218. Received for review June 29, 1989 Revised manuscript received February 5, 1990 Accepted February 21, 1990
Absorption of Nitrogen Oxides in Alkaline Solutions: Selective Manufacture of Sodium Nitrite Naresh J. Suchak, Kanhaiya R. Jethani, and Jyeshthraj B. Joshi* Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India
Absorption of nitrogen oxides was studied in a pilot-scale 150-mm-i.d., 3000-mm-long packed column. An aqueous solution of sodium hydroxide (5% w/w) was used as a solvent. Superficial liquid velocity was varied in the range 2.0-7.0 mm/s, and the inlet solvent temperature was varied in the range 50-90 "C. The conversion of NaOH, the selectivity of sodium nitrite, and the temperature were monitored along the length of the column. A mathematical model has been developed for an adiabatic operation. The gas-phase reactions and equilibria, gas-phase mass transfer, interface equilibria, and liquid-phase reactions were included in the mathematical model. The parametric sensitivity was investigated. The gas-phase mass-transfer coefficient was found to play a significant role on the selectivity in addition to the temperature, the total partial pressure of NO,, and the ratio of divalent to tetravalent nitrogen oxides. A comparison has been presented between the model predictions and the experimental observations. 1. Introduction
Inorganic nitrites, namely, sodium, calcium, and ammonium nitrite, are manufactured by absorption of NO, gases in corresponding carbonates or hydroxides. NO, gases are available mainly from nitric acid plants based on the oxidation of ammonia. Other soupces Lve flue gases, off-gas from steel pickling plants, and organic process plants like nitration, oxidation, etc. In commercially marketed nitrite, it is essential to have the nitrate content as low as possible for applications like diazo dyes and printing and bleaching textile fabrics. It is desirable to improve the selectivity with respect to nitrite in order to reduce the loss of fixed nitrogen as nitrates and reduce the costs involved in further purification. Formation of nitrite and nitrate in the absorption column is due to the combined effect of complex equilibria, several competing reactions, and physical mass transfer. For maximizing nitrite yields, it is important to systematically investigate all the factors affecting the selectivity. In order to design the NO, absorption system and to arrive at its performance characteristics, modeling of NO, absorber incorporating most of the effects is inevitable. Carta (1984) has reported a mathematical model for isothermal conditions. He has clearly brought out the factors affecting selectivity. The major aim of this work is to present experimental data from a pilot-scale packed column for the manufacture of sodium nitrite. This column was operated under adiabatic conditions. A mathematical model has been developed for the adiabatic conditions. The role of the gas-side mass-transfer resistance has been brought out. A comparison between model
* To whom
Table I. Gas-Phase Reactions equilib and rate const, atm-l eq 1 log k, = 652.115" - 0.7356 2 log K2 = 2993/T - 9.226 3 log K3 = 2072/T - 7.234 4 log K4 = 2051.171T - 6.7328 log K5 = 2003.8/T - 8.757 5
predictions and experimental observations has been presented. 2. Mathematical Model 2.1. Model for Overall Rate of Absorption. 2.1.1. Gas-Phase Reactions and Equilibria. The NO,% consist of a mixture of various gases, namely, NO, NO2,N203, Nz04,and N20b The gas phase also contains water vapor, and hence, in addition to the above gases there exist oxyacids HN02 and HN03. Nitric oxide undergoes irreversible oxidation with oxygen, forming nitrogen dioxide. Oxidation reaction is second order with respect to NO and first order with respect to oxygen. The oxidation reaction is expressed as
Complex equilibria prevail in the gas phase, which can be characterized by the following equations:
(3)
NO(,) + NOz,,
+ H20,,, & 2HN02c1,
all correspondence should be addressed. 0888-5885/90/2629-1492$02.50/0
0 1990 American Chemical Society
(4)
Ind. Eng. Chem. Res., Vol. 29, No. 7 , 1990 1493 The reaction rate constant I t , and the gas-phase equilibria constants Kz-K5 are reported by Joshi et al. (1985) and are summarized in Table I. In the absorber, the gas-phase composition continuously changes due to the absorption, the oxidation of NO, the exothermicity of reactions, and the vaporization of water. Gas-phase equilibria between various gaseous species can be expressed by eight equations. The eight variables are the moles of NO, NOz, N2O3, N204,HN03, H20, and HNOz and the total moles of gas per mole of inerts. From eqs 2-5, we write the following four relationships:
K3 =
K4 =
YN,o,YT
(7)
YNOY N O , ~ T
(8)
YNOY N O , ~ H , ~ ' T YHNO,~ YNOYT YNO; YH$T
The total number of moles in the gas per mole of inerts is obtained by adding the contributions due to all gaseous species. YT = YNO+ Y N O + ~
'N204
+
Y*NO
=
YNO~ + ~ Y N ~+ o ,~ Y N , O + ~'HNO3 yNO
(23)
2N02- + H2O
(25)
N204(1, + 20H-
NOz- + NO3- + H 2 0
(26)
HN03,,,+ OHHNOqI, + OH-
-
NO3- + H 2 0
(27)
+ H2O
(28)
NO2-
(11)
2.1.5. Mass Transfer with Chemical Reaction in the Liquid Film. It has been illustrated by Joshi et al. (1985) that the absorption of NO2, N203,and Nz04is accompanied by chemical reaction within the liquid film. The volumetric rates of absorption in the liquid film are [for details, see Joshi et al. (1985)l
(12)
RaNO2 ~~NO~'2(2/33(k~~N02)''2(PiNOz)3'2 (29)
+ YHNO~
Y*H*O = yHzO + 0.5YHN03 + O-~YHNO,
-
N2O3,,, + 20H-
+
+ yN201 + O * ~ Y H N-O0.5YHN03 ~
f ( T ,concentration of NaOH)
2.1.4. Liquid-Phase Reactions. Nitrogen oxides when absorbed in an alkaline medium form respective nitrite and nitrate ions. The following reactions occur in the liquid phase: 2N02(1,+ 20H- NO2- + NO3- + H 2 0 (24)
+ YN~+ o ~YHNO~ + YHNO~ + YHzO YO, 1.0 (10)
Let us define three new parameters, namely, Y *N, Y *NO, and Y *H20. Y *N means the total moles of reactive nitrogen per mole of inerts and Y *NO is the total moles of divalent reactive nitrogen per mole of inerts. Y*N = YNO+
In the manufacture of nitrites, the liquid-phase reactive component is an aqueous alkaline solution. The reaction between an alkali and HNOz or HN03 has been assumed to be instantaneous. Therefore, there is no free HN03 or HNOz in the liquid phase, and the equilibrium vapor pressures of HN03 and HN02 are also zero. Further, in the industrial absorber, water vapor is present at its saturation value. The vapor pressure data of water over alkaline solutions have been reported as a function of temperature and concentration of alkali in the literature (Liley et al., 1984): P ~ H ~= O
YHNO~~YT
K, =
(22)
(13)
R ~ N =~~HN,o,((~D)N,o,) o ~ 1'z~i~2~4
(30)
The solution of nonlinear algebraic equations (eqs 6-13) yields the concentration of all the components in the gas phase. 2.1.2. Rates of Gas-Phase Mass Transfer. Simultaneous mass transfer and chemical reactions occur in the gas phase as well as in the liquid phase. The volumetric rates of gas-phase mass transfer are given by the following equations:
RaN20, = ~HN,o,((~D)N,o,) 1'2~i~z~s
(31)
RgNO =
(ItGa)NO[P0NO
- PiNOl
RgNOz = (kGa)NOz[PoN02- PiN02] RgNZO, = RgNzOs =
(ItGa)N20,[P0N20,
- PiN20,]
( ~ G a ) N ~ O s [ P o N , O s - PiN2031
(14)
(15) (16)
=
(kGa)HNO~[PoHNO~l
(18)
RgHNOz
=
(kGa)HNO~[PoHNO~l
(19)
= (kCa)H20b0H20- PiH201
R~HNO =,
(17)
RgHNOa
bH2O
It may be noted that the nitrous and nitric acid vapors undergo an instantaneous irreversible reaction. Volumetric rates of absorption of HN03 and HN02 constitute two parts: (1) Physical mass transfer in the gas phase and (2) an enhancement in physical rate of mass transfer due to the formation of HN02 and HN03 in the gas film. Carta (1984) has assumed no gas-side resistance and therefore flat partial pressure profiles of NO, NO2,and HzO across the gas film. On the basis of this assumption, the solution of diffusion equation for mass transfer of HN02 in the gas film yields the following equation:
(20)
2.1.3. Interface Equilibria. Since NO, NO2,N203,and Nz04are always in equilibrium, the following equations hold at the interface:
k, is the forward rate constant of eq 4. When gas-side resistance is assumed to be negligible, the partial pressures of NO, NO2, and H20 are the same as that in the bulk. However, in the presence of gas-side mass-transfer resistance, the concentration profiles of NO, NOz, and H20 exist across the gas film. Hence, it is more rational to incorporate the average of the bulk and interface partial pressures of NO, NOz, and H 2 0 for calculating the en-
1494 Ind. Eng. Chem. Res., Vol. 29, No. 7 , 1990
hancement term. On the basis of average partial pressures, the rate of HNO, is given by the following equation: R~HN= O ?( ~ G ~ ) H N O . P ' H N O ~+ 1 aDHN02k4 P ~ N O P ~ N O ~ ~+H P'NOP'NOJJ'H~O ~O 2 2 RT(k c) HNO?
(32b)
Carta (1984) has neglected nitrate formation due to the absorption of HN03. At high temperature, the partial pressure of H 2 0 a t the interface is substantial. The contribution of HN03 in the gas phase then becomes significant toward the formation of nitrate. Some HN03 is formed within the bulk of the gas phase, and an additional amount is formed in the film. The latter gives enhancement in the rate of mass transfer. The absorption rate of HN03can be estimated by using a similar approach as that used for HN02. The rate of HN03 absorption is given by the following equation:
2.1.6. Overall Volumetric Rates of Absorption. For obtaining the overall rate of absorption, we need to solve eqs 14-23 and 29-33 simultaneously. It may be noted that there are 15 equations, whereas there are 17 unknowns, namely, (i) the volumetric rates of gas-phase mass transfer, RgNOt k N 0 p RgNz04,RgN 0 RgHN03,RgHNO and RgH20; (ii) the partial pressures ol d0, NO2,N204,d203,and H2O , p'Np,, and p'H8; and (iii) at the interface, piNo, p ' ~ qp'Np,, the rates of mass transfer with chemical reaction in the liquid film, Ra, h ~ ~ oh ,~ ,, , and Ram,. Two more equations are needed for making the model consistent. These are provided by the material balance of divalent and tetravalent nitrogen oxides across the interface. The NO2 balance at the interface gives (RgNo, - R h o 2 ) = ~ . O ( R ~ N-~RgNzo,) O, + ( R ~ N ,-O ~ RgN203)+ 1/2(RaHNOz- RgHN02) + 3/2(RaHN03 - RgHN03) (34) 7
9
The NO balance at the interface gives RgNo = (RaN203- RgN20,) + 1/2(RaHN02- RgHNO,) 1 /2(RaHN03 - RgHN03) (35) Equation 23 can be solved independently while eqs 14-22 and 29-35 are nonlinear algebraic equations. In order to avoid the possible difficulty in convergence under varied parametric conditions, the set of 16 equations were reduced to 2 where the 2 variables are interface partial pressures of NO and NO2. The following are the reduced two equations: A ~ ( P ' N o , )+~ AZ(P'NO,)~P'NO A~(P'NO,)~'~P'NO 4AG'NO,(P'NO)~ + ASP'NOP'NO~+ AGP~NO = 0 (36)
B1(Pi~O)'4- B ~ @ ' N ~ ) ~ P 'B$&o N~,
2.2. Model for the Column Performance. 2.2.1. Model Equations. The packed column has been modeled for simulation studies. The following assumptions were made: (i) The gas and liquid phases flow in a countercurrent plug flow manner. (ii) The liquid holdup is uniform throughout the column. (iii) The gases follow ideal gas behavior. (iv) The values of the effective interfacial area (a) and the gas-side mass-transfer coefficient (k&) are uniform throughout the equipment. (v) There are no radial gradients in the concentration and temperature. (vi) The column is operating at steady state. 2.2.2. Mass Balance. The mass balance across a differential height dh at height h from the bottom results in the following differential equations. Component Balance in the Gas Phase. (a) divalent nitrogen balance
(b) balance for total reactive nitrogen
B4(Pi~o2l3 =0 (37) (c) water vapor balance
where 3 a2DHN03k5 . A, = P1H20 8 RT(kGa)HN03 A2
= 2(a~N204[(kD)NzOl11'2 + (kGu)Nz04)K2 A3
= ~HNo,~'~[~/~(~D)No,]~'~
(d) oxygen balance
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1495
Component Balance in the Liquid Phase. (e) balance for hydroxide ions
Table 11. Values of Constants in Equation 47 Dackings a /3 25-mm ceramic Raschig rings 0.64 0.48 25-mm PVC" Raschig rings 0.735 0.41 25-mm ceramic Intalox saddles 0.7 0.48 25-mm PPb Intalox saddles 0.75 0.45 25-mm SS' pall rings 0.58 0.38 0.47 25-mm PPb pall rings 0.68
(f) nitrate balance
Y
2.22 1.58 2.98 2.31 1.90 2.35
"PVC = poly(viny1 chloride). b P P = polypropylene. 'SS = stainless steel. Table 111. Heats of Reaction
(g) nitrite balance
(44) Equations 38-44 are coupled linear differential equations. The total mass balance is established across the differential height on solving these equations simultaneously. 2.2.3. Estimation of Design Parameters. Mahajani and Joshi (1989) have developed general design procedures and derived rational correlations for hydrodynamics and mass-transfer characteristics in packed columns. The following correlation gives the value of effective interfacial area. For metallic pall rings a = 2.9 x 104d -0.777V 0.538 (45) P L For ceramic intalox saddles a = 7.43
x 104dp-09~v L0.620
(46)
For the gas-side mass-transfer coefficient, the following correlation has been proposed: kGUpO, = 4.21~ L VG' Y VLYD:~
(47)
where a,P, and y depend upon size and type of packings employed. Table I1 gives these values for 25-mm packings of various types. The diffusivities of gases vary with the temperature and pressure. For this purpose, the following correction was used:
D, = D,,25*c(T/298.2)1.75(101.33/P)
(48)
The selection of H(kD)'I2 values for NO2, N2O3, and N204will be discussed later. 2.2.4. Heat Balance. The oxidation of NO, the formation of Nz03and N204in the gas phase, the absorption of NOz, N203,and N204,and the liquid-phase reactions are exothermic. The decomposition of HN02,however, is endothermic. Because of these reactions, significant heat changes occur. The value of physicochemical parameters such as reaction rate constants, equilibrium constants, solubilities, vapor pressures, and diffusivities depend upon the temperature. Heats liberated because of various reactions in the gas and liquid phases have been included in Table 111. It will be assumed that the heat liberated is taken up by the liquid phase and the interface is always saturated with water vapor. If QT is the total heat change per unit time in the differential element, then the temperature change under adiabatic conditions is given by
AT = QT/m°Cpo
(49)
where AT is the temperature change across the differential element.
std heat of reaction (25 "C), kcal/kmol X 10-3 (A) Reactions in the Gas Phase AHl = -27.22 2NOk) + 0 2 2N0zb, A H 3 = -13.61 2NOZb) &4c, AH, = -9.55 NO,) + NOz NzO3 A H 6 = -8.55 3N02,) + HZlfj(,) 2HN03 + NO(,) AH, = -9.80 NO,,) + NOzb,+ H20(,, !hNOzk,
- ,-
- -
+
(B) Reactions in the Liquid Phase eq 63 eq 64 eq 65 eq 66 eq 67
AH8
-49.82
A H 9 = -36.13 AH10
= -30.16
A H 1 2 = -19.46 AH11
= -31.00
(a) Contribution to the Heat Changes from the Gas Phase. These are due to the following steps: (i) The rate of formation of N203from NO and NO2 in the differential element is given by (N2OJf = (NzOJe - (N203)i + (N203)a (NzOJf
G[(yN,O,)e - (yNzOs)il
+ RaN2O3Sdh (50)
The rate of heat generation due to N203formation is 81 (N203)fm4 (51) (ii) The rate of formation of N204 from NO2 in the differential element is given by (N204)f = (N204)e - (Nz04)i + (N204)a (N2O4)f= G[(YN,O,)e - (yN20,)il
+ RaN,O,S
dh (52)
The rate of heat generation due to Nz04formation is Q2 = (N204)fm3 (53) (iii) The rate of formation of HN03 in the differential element is given by (HN03)f = (HNOJe - (HNO3)i + (HNOJa = G[(YHN03)e- (YHN03)il + RaHNOsS dh (54) The rate of heat liberated due to HNO, formation is Q3 = (HNO3)&& (55) (iv) The rate of formation of HN02 in the differential element is given by (HN02)f = (HN02)e - (HNOJi + (HNOz), = ~ [ ( Y H N o , )-, (YHN02)il + R ~ H N odh, ~ (56) The rate of heat liberated due to H N 0 2 formation is Q4 = ( H N O A m , (57) (v) The rate of heat liberated due to the oxidation of NO is estimated from the knowledge of the rate of NO oxidation. It may be noted that NO is used in the N203and HN02 formation and it is liberated during HN03 forma-
1496 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 Table IV. List of H(kD)'12at 25 "C and Activation Energy Correlationa set 3 (Newman set 1 (Joshi et al., 1985) set 2 and Carta, 1988)
H(kD)'I2
EIR lP/2(2/3kD)'/2
EIR
set 4
set 5
set 6
N2°3
1.57x 10-5 1285.3
2.5 X lo4 -4564.16
2.5 x 10-4 -4564.16
2.5 X lo4 -4564.16
2.5 X 10'' 0
2.5 X '0 1 -4564.16
2X 1285.3
2 x 10-7 -4564.16
NO2 2 x 10-7 -4564.16
2 x 10-7 -4564.16
2 x 10-7 0
2 x 10-7 -4564.16
8.1 X lo4 1285.3 +ve (eq 33)
8..1X lo4 1285.3 +ve (eq 33)
3.3 x 104 1285.3 +ve (eq 33)
8.1 X lo4 -4564.16 +ve (eq 33)
8.1 X 10" 1285.3 +ve (eq 33)
8.1 X lo4 1285.3 0
N2°4
H(kD)'i2
EIR RaHN03
3.1. Estimation of Gas-Phase Composition. Equations 6-13 form a set of eight simultaneous algebraic equations. The eight unknowns are (moles per mole of inerts of) NO, NO2, Nz04,N203,HN03, HNOZ,and H 2 0 and the total moles in the gas phase. Equations were ( G [ ( Y N O ) i - (YN0)e - (YN203)e + ( y N z 0 3 ) i - '/2(yHNO2)e + solved numerically by the Newton-Raphson matrix me1 thod for nonlinear equations. /2(YHNOz)i + 1/(YHN03)e - 1/Z(YHN03)iI 3.2. Estimation of Interface Composition. For the [RaNz03 + ?!ZRaHNO,- '/RaHN031S dh! (58) estimation of interface composition, we need to solve 16 The rate of heat generation due to NO oxidation is given equations simultaneously (eqs 14-22 and 29-35). The intricate algebraic reductions of these 16 equations into by 2 polynomial equations (eqs 36 and 37) has obviated the 85 = [(NO)OIm1 (59) need for the scaling algorithm. The variables selected, (b) Heat Changes Due to Water Evaporation. The namely, the interfacial partial pressures of NO and NO2, rate of water evaporation in the differential element was are always present in greater magnitude than the other obtained by the material balance: components of NO,. These two equations were numerically solved by the Newton-Raphson and Guass-Jordan (H2O)v = [(HZO), - (H2O)il + 1/(HNOJf + method. The main objective behind using this composite '/2(HN02)f= G[(YH,o),- ( y H z O ) i + '/(YHNOJ~technique that is algebraic and analytical in nature was 1/(YHN03)i + 1/2(YHN0z)e - %(YHNOz)il + [1/2RaHN03+ to achieve a single positive finite solution with minimum ~/~R~HN dho ~(60) ]S numerical iterations. Substituting these values in eqs 21 and 22 results in interfacial partial pressure values of N204 Q6 = ( H 2 0 ) v ~ w (61) and N203. The total heat change in the gas phase is given by 3.3. Estimation of H(kD),'12Values. The absorption of NO2, N203, and N204 in water has been extensively QC = 81+ Q 2 + Q3 + Q4 + Qrj + Qs (62) studied in the literature. Joshi et al. (1985) have critically (c) Heat Changes Due to Absorption with Chemical reviewed the previous work and have suggested the values Reaction. In the absence of information on the heats of given in Table IV, set 1. The value of H ( k D ) 1 / 2for N204 dissolution of gases, the information on heats of formation at 25 "C (8.1 X lo4) was selected from the work of of reactants and products was used to compute the heats Kramers et al. (1961). The activation energy for N204was of reactions. calculated from two values of H(kD)'I2 a t 20 and 30 O C . Reactions under consideration for the heat balance are Kameoka and Pigford (1977) have shown that the rates (1) 2N02,,, + 2NaOH(,, of absorption of N204in water and aqueous NaOH solution (up to 0.8% w/w) were practically the same. Newman and NaN03,,, + NaN02,,,+ H20(1, (63) Carta (1988) have found a lower value of H(kD)1/2(3.3 X (2) N204(,,+ 2NaOHo) lo+) for Nz04 absorption a t higher concentrations of NaOH solution (24% w/w). Both of these values of HNaN03,,, + NaNoz(,,+ HzO(1) (64) (kD)'I2for N204(8.1 X lo+ and 3.3 X lo*) were used for (3) N&,, + 2NaOHo) 2NaNOZ,,,+ HzO(l, (65) estimating the selectivity of nitrite. From Figure 1, it is clear that the contribution of N204toward nitrite forma(4) HN02&,+ NaOHo) NaN02,,,+ HzO(l) (66) tion is always less than 5%. If the activation energy for (5) HN03,1,+ NaOH(1, NaNO,,,, + H20(1) (67) NOPand N203is assumed to be the same as that for N204, set 1 represents H(kD)'l2 values for the absorption of NO, The heats of reactions are summarized in Table 111. in aqueous NaOH solutions at 25 "C. Recently, Newman The heat liberated due to the absorption with chemical and Carta (1988) have measured H(IzD)'/~values for N2O3 reaction in the liquid phase is expressed as and N204in aqueous NaOH (24% w/w) solution at 25 "C. QL = (RaNo,m, + R a ~ ~ o , m +sR ~ N ~ O + ~ ~ ~ I O Set 3 of Table IV gives these values. The activation energy for N203was calculated from two H ( k D ) 1 / 2values a t 25 RaHNO3m11 RaHNozAH1,)S dh (68) and 40 "C. The activation energy for NO2was considered The total heat change in the differential volume is the to be the same as that for N203,and that for N204was the summation of all the heat changes. same as in set 1. (69) QT = QC + QL In order to investigate the parametric sensitivity, an additional four sets were constructed. The estimated 3. Method of Solution values of the activation energies for H(kD)'12work out to The method of solution involved the following steps: be (i) E = 2553.9 kcal/kmol ( E / R 1285.3K) for Nz04from
tion. The mass balance for NO across the differential element is given by (NO), = [(NO)i - (NO),] - (N203)f - '/(HN02)f + '/(HN03)f =
-
--
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1497
“
Y
5100
T O T A L PARTIAL PRESSURE OF
0 IA.
NO^= 8-00 K N / M ’
0
P x
g
80
IL
’“1 y’/
/
6
0
V
0 w
TOTAL P R E S S U R E
NO,
W
L
101.33 K N / M 2
TOTAL P A R T I A L P R E S S U R E O F
P$o
= 8-00 K N / M ’ Plo,
=
1.5
GAS S I D E M A S S T R A N S F E R C O E F F I C I E N T = 3xld’k molc/(m’.s(KH/m?
IL a
E F F E C T I V E IN T E R F A C I A L A R E A = 150 m2/m SOLVENT
= 5%
NaOH (wt.1
\ -e
10
I-
4W K
O
20
NO’ ,
30
40
50 60 T E M P E R A T U R E ,OC
70
80
90
Figure 1. Effect of temperature on the relative contribution of different species toward the formation of nitrite.
the work of Kramers et al. (1961) and (ii) E = -9068.9 kcal/kmol ( E / R = -4564.1 K) for N2O3 from the work of Newman and Carta (1988). The values of the parameters in sets 2 and 4-6 were estimated by using these two activation energies. For instance, in set 2, the value of H(kD)1/2for N203was taken from the work of Newman and Carta (1988) and for N2O4 from the work of Kramers et al. (1961). The activation energy for NO2 was assumed to be the same as that for N203 In set 4, it has been assumed that the activation energy for N203can also be used for NO2and N204 In set 5, the values of the activation energy for NOz and N203have been assumed to be zero. In the first five sets, the rate of absorption of HN03 was calculated by using eq 33. In the last set, this rate was set at zero according to the observations of Newman and Carta (1988),the other parameters remaining the same as in set 2. 3.4. Solutions of Ordinary Differential Equations. Seven simultaneous differential equations (eqs 38-44) were solved by the fourth-order Runge-Kutta method. 3.5. Solution of Heat Balance Equations. From steps 3.1 and 3.2, the concentration of all the species are known at the inlet and exit of a differential height. The rates of chemical reaction and mass transfer are also known. Assuming that all the sensible heat is taken up by the liquid phase, the change in temperature across the differential height was estimated. For this purpose, eqs 49-69 were used. 4. Discussion on the Mathematical Model 4.1. Local Selectivity. The formation of nitrite is advantageous as compared to the nitrate formation. The liquid-phase reactions that result in the formation of nitrite and nitrate are given by eqs 24-28. Nitrite is formed in reactions 25 and 28, and nitrate is formed in reaction 27.
T E M PER ATUR E , “C
Figure 2. Effect of temperature on the relative contribution of different species toward the formation of nitrate.
Both are simultaneously formed in reactions 24 and 26. For the selectivity toward nitrite, the formation of N203 and HN02 needs to be favored whereas the formation of N204and HN03 needs to be suppressed. Therefore, the relative formation of N203,N204,HN02, and HN03 depends upon the total concentration of NO,, the ratio of divalent and tetravalent nitrogen oxides, the temperature, and the total pressure. The effect of these parameters on the selectivity of nitrite was estimated by using the mathematical model developed in the previous section. Figure 1 shows the relative contributions of NO2,N203, Nz04,and HN02toward the formation of nitrite. The bold graphs in all the figures were obtained by using the parameters of set 2. It can be seen that HN02 has the major role. Figure 2 shows a similar graph for nitrate formation. It can be seen that, at low temperatures, N204is responsible for the nitrate formation. At relatively high temperatures, HN03 becomes responsible for the nitrate formation. The values of equilibrium constants have been summarized in Table I. The rates of absorption with chemical reaction are given by eqs 29-31. The overall result of these reactions is as follows: (i) The formation of N204and HN03 decreases with an increase in temperature. (ii) The formation of N203decreases, and HN02 increases with an increase in temperature. (iii) The formation of N204and HN03 increases with an increase in the partial pressure of total NO, and also with an increase in the mole ratio of NO2 to NO. (iv) The formation of N203and HN02 increases with a decrease in the partial pressure of total NO, and with an increase in the mole ratio of NO to NO2 (v) The rates of absorption of Nz03and Nz04depend upon the selection of physicochemical parameters (the respective values of H(kD)’lZ). As pointed out earlier, set 3 corresponds to the H(kD)ll2values for 24% (w/w) sodium hydroxide solution (Newman and Carta,1988). The principal
1498
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990
I
/
I
'
/E 1 I
TOTAL P A R T I A L P R E S S U R E OF NO, = 8.00 K N / M z
1.0
2.0
3.0
4.0
=
25'C
TOTAL P A R T I A L PRESSURE OF NO,, = 0.08 *TOTAL PRESSURE
I/III/ II11, / /1
6 o0
TEMPERATURE
5.0
SET- 2 1
-------- S E T -
6.0
R E L A T I V E PARTIAL PRESSURE OF DIVALENT TO TETRAVALENT NITROGEN O X I D E S , NO*/NO:
1.0 2.0 3.0 4.0 5.0 6.0 R E L A T I V E P A R T I A L P R E S S U R E OF D I V A L E N T TO TETRAVALENT NITROGEN O X I D E S , NO*/NO:
Figure 3. Effect of the ratio of partial pressures of divalent to tetravalent nitrogen oxides and temperature on the selectivity of nitrite.
Figure 4. Effect of the ratio of partial pressures of divalent to tetravalent nitrogen oxides and total pressure on the selectivity of nitrite (for details of set 1 and set 2, refer to Table IV).
difference between sets 1 and 2 is the nature of variation of H(kD)'/2with temperature. In set 1, H(kD)'12 increases with an increase in temperature, whereas in set 2, H(kD)'12 decreases with an increase in temperature. It can be seen from Figures 1-6 that the selectivity based on set 1 parameters is generally lower than those based on set 2 parameters. Further, the effects of the total pressure of NO, and the ratio of NO to NOz are analogous in both of the sets. However, in the case of set 2, the selectivity is higher at 25 O C compared with that at 50 "C, and again it increases a t 7 5 O C . In the case of set 1, the selectivity continuously increases with an increase in temperature. From the foregoing discussion, it is clear that the relative concentrations of HNOZ,NZ0? NOz, HNO,, and Nz04at the interface govern the selectivity with respect to nitrite formation. The relative values of concentration varied with gas-side mass-transfer coefficient under otherwise identical conditions. Therefore, it was thought to be desirable to investigate systematically the effect of kGa. The results are shown in Figures 7-9. The following points may be noted from these figures: (i) From Figure 7 it can be seen that, a t high NO, concentrations in the gas phase (>25%), the selectivity decreases with an increase in ~ G U .At a certain critical value of kGa, a minimum in selectivity is observed. A further increase in kGu results into an increase in selectivity. The same behavior was observed a t three levels of NO, concentration. (ii) From Figure 8 it can be seen that the effect of kGa on selectivity is large when the ratio of NO/N02 in the gas
phase is small. The effect of temperature is shown in Figure 9. It can be seen that the effect of kGa on selectivity decreases with an increase in temperature. 5. Experimental Work Experiments for the absorption of NO, gases were conducted in a continuous countercurrent manner and under adiabatic conditions in a stainless steel 304 packed column of 150-mm i.d. and 3000-mm packed height. Stainless steel 304 (16-mm) pall rings were used as packings. In order to measure the concentration and temperature profile, five sample points and thermowells were provided in the packed column. The schematic diagram of the experimental setup is shown in Figure 10. NO, gases were obtained as a product gas in the commercial manufacture of isonicotinic acid by the oxidation of y-picoline. YOOH
The total volumetric flow rate of gases was measured by using a calibrated orifice meter. The flow rate of inert was estimated to be 5.37 X 10" kmol/s, and the flow rate of NO, gas was found to be 2.51 X 10" kmol/s. The ratio of divalent to tetravalent nitrogen oxides was found to be 2.5. Aqueous sodium hydroxide (5% w/w) was prepared in a 1000-L stainless steel 304 vessel. The solution was passed through a precalibrated rotameter and a heat ex-
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1499
I 90
6.
3.
-
-
::
-
IW
K I-
r 80U
----_-------____ TEMPERATURE
=
25.C
T O T A L PRESSURE = i o i . 3 3
KN/M'
-- -- - - - SET-1
-S E T - 2
SET-2
60
0
16 20 24 T O T A L P A R T I A L P R E S S U R E OF N O , , K N / M '
4
8
12
28 I
1
1
I
I
I
I
b
8
12
16
20
24
28
2
T O T A L P A R T I A L P R E S S U R E OF N O , , K N / M ~
'""!
P ~ o / P ~ o o3.0;
Figure 6. Effect of total partial pressure of NO, and temperature on the selectivity of nitrite (for details of set 1 and set 2, refer to Table IV).
90-
P c
K
-z I-
8 4 I
0 U
->-> I-
I-
V
y
80-
W
lA
7010 60
0
SYMBOL
P'. K N / M 2
0
10.0
h
25.0
V
35.0
0
55.0
1.0 I
2.0 I
TOTAL PRESS.
3.I0
4.I0
5.0 1
= 101.33
KN /M2
6.0 I
7.0 I
8 0
8 12 16 20 24 28, T O T A L P A R T I A L PRESSURE OF N & , K N / M
32
Figure 5. Effect of total partial pressure of NO, on the selectivity of nitrite.
changer. The temperature of the solution was maintained within fl "C. The outgoing liquid was collected in a 500-L storage vessel. Sufficient time (10 times the residence time) was allowed for attaining steady state in the packed column. Samples of the liquid were collected, and temperatures were noted after achieving steady state. Gas analysis was done by absorption in bubblers containing sodium hydroxide and hydrogen peroxide solutions. Liquid samples were analyzed by using an ion chromatograph (HPIC DIONEX 2010-i). The chromatograph was precalibrated with synthetic mixtures of NaN02 and NaN03.
GAS S I D E M A S S T R A N S F E R COEFFlClENT, k g p x 10'
Figure 7. Effect of gas-side mass-transfer coefficient and total partial pressure of NO, on the selectivity of nitrite.
6. Results and Discussion The temperature profiles were measured at three superficial liquid velocities and three inlet temperatures of the liquid. The values of the gas-side mass-transfer coefficient at superficial liquid velocities of 40-110 mm/s and at the temperature range 50-90 OC were in the range 1 X 10-3-1.56 X kmol/(m3 s (kN/m2)). Some experimental results for temperature profiles are shown in Figure 11. The temperature profiles predicted by the model are also shown in Figure 11. It can be seen from Figure 11 that the model predictions and experimental observations are generally in good agreement. There are two types of temperature profiles. At lower temperature,
1500 Ind. Eng. Chem. Res., Vol. 29, No. 7 , 1990 GAS OUTLET
T SP CP CY TC HE TOTAL PRESSURE
0
=
101.33 K N / M '
TI LL
- THERMOWELL - SAMPLE POINT - CENTRIFUGAL PUMP - CONTROL VALVE - TEMPERATURE CONTROLLER
- TRIPLE wiin
-
-
PASS HEAT EXCHANGER
s m z AREA
TEMPERATURE TRANSMITTER L i a u i o L E V E L INDICATOR
2.0 3.0 L.0 5.0 6.0 GAS SIDE MASS TRANSFER COEFFICENT, keg x10'
1.0
Figure 8. Effect of gas-side mass-transfer coefficient and relative partial pressures of divalent and tetravalent nitrogen oxides on the selectivity of nitrite.
1000 LITERS
ROTAMETER 0 - 2 0 0 cc/s
CP
Figure 10. Schematic diagram of the experimental setup.
I
TOTAL PRESSURE
I
101.33 K N I M '
62-
1
0 76 p 7b0".
0
0
0
0
0
0
70-
," 3
66-
2 a
62-
0" 5 8 1
I
I
I
1
I
1.0 2.0 3.0 4.0 5.0 6.0 GAS S I D E M A S S T R A N S F E R C O E F F I C I E N T , k 6 c x 1 0 3
w
50 -
c 54
Figure 9. Effect of gas-side mass-transfer coefficient and temperature on the selectivity of nitrite.
46
the temperature continuously decreases with an increase in distance from the bottom. However, a t high temperatures, a maximum is observed in the profile. The cooling effect a t the bottom (increase of temperature with height) is due to water evaporation. It may be noted that the gas phase was introduced a t the ambient temperature of 30 "C and was saturated with water vapor. In the column, the temperature of gas increases, and its capacity to hold water also increases. A t relatively high temperatures, the extent of water evaporation is more, and this results in the reduction in temperature as the liquid phase descends in the bottom region. However, the gas phase becomes practically saturated a short distance from the bottom, and all the exothermic heat is used for increasing the temperature of the liquid phase. This explains the occurrence
38
-
42-
E X P E R I MENTAL
-
S I MULATION
34-
30,
I
I
I
I
I
1
I
of the maximum in the temperature profile. It may be noted that the model predictions also bring out this essential feature of the temperature profile. The gas-phase composition and the liquid-phase concentrations were measured at five places along the column height. It was observed that most of the gas-phase conversion occurs in the bottom 150-mm height. Above this
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1501 Table V. Nitrite Formed (Grams/Liter) in the Bottom 150-mmSection of the Packed Column temp, O C flow, cm3/s expt set 1 set 2 set 3 set 4 50 60 12.32 11.77 12.16 11.85 11.79
set 5 13.33
set 6 12.156
100
SET-b SET-5 SET-I
-
r\
0
-:: -
100-
SET-6
0
0
0
SET-6 SET-b
SET-2
-s
95-
W I-
-
1
SET-1
K
c
SET-l
W I-
;95c
AT I N L E T
= 50°C E X P E R I M E N TAL
J S I M U L A T l ON
8 5LO
50
60
70
80
90
100
110
L I Q U I D FLOW RATE , c c / s
Figure 12. Comparison of predicted and experimental values of nitrite selectivity: temperature = 50 O C (for the details of sets, refer to Table IV).
height, the change in gas- and liquid-phase compositions was nominal. Therefore, the comparison between model predictions and experimental observations was made only at the height of 150 mm. Though the agreement at higher heights was excellent, it was because of the low levels of conversion. Table V gives the experimental values of nitrite formed in the bottom 150 mm. The predicted values on the basis of parameters in the six sets are also given in Table V. It can be seen that the predictive capability of all the sets is generally very good as far as the formation of total nitrite is concerned. This is because the formation of nitrite is controlled by the absorption rate of HN02 However, the difference arises when we compare the predictive ability of different sets for the selectivity of nitrite. When the selectivity is more than 95%, the model predictions become very sensitive to the nitrate formation. The experimentally observed values of the selectivity a t three temperature levels and three liquid flow rates have been shown in Figures 12-14. The predictions of set 2 are reasonably good, whereas the predictions of set 6 are excellent at all the temperatures and liquid flow rates. The parameters in set 6 (as in Table IV) have been recently suggested by Newman and Carta (1988). These authors assume that the formation of nitric acid in the gas phase does not occur. This assumption has been included in set 6, whereas in set 2 the rate of nitrate formation was estimated according to eq 33. 7. Conclusions (1)A mathematical model has been developed for the
absorption of NO, gases in aqueous alkaline solutions. All the aspects such as gas-phase equilibria, gas-phase mass transfer, interface equilibria, and liquid-phase mass transfer with chemical reaction have been included in the model. Heats of reactions, absorption, and evaporation
1
I
85.
I
I
1
T E M P E R A T U R E OF AT I N L E T
0
= 9O'C
I
1
Liauio
EXPERIMENTAL
-S I M U L A T I O N
I ZY
50
I
60
Liauio
I
I
I
1
90
100
110
I
70 80 FLOW R A T E
,
CC/S
Figure 14. Comparison of predicted and experimental values of nitrite selectivity: temperature = 90 "C (for the details of sets, refer to Table IV).
have been considered in the heat balance. (2) The formation of nitrite occurs mainly due to the absorption and reaction of HN02 and Nz03,whereas the
1502 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990
nitrate is formed d u e t o t h e absorption a n d reaction of NOz, N204,and HNO,. T h e relative contributions of these species t o t h e formation of nitrite a n d nitrate have been quantified with respect t o temperature. T h e dependence on temperature, NO*/NO,* ratio, and partial pressure of NO, was evaluated. (3) T h e gas-side mass-transfer coefficient was found t o play an important role in deciding the selectivity of nitrite. T h e selectivity decreases with an increase in kGa;a further increase in kGa results in a minimum followed by a n increase in selectivity for all levels of NO, at different temperatures. This could be generalized for NO, with a N O * / N 0 2 * ratio higher t h a n 1. However, at lower NO*/NOZ* ratio, a n increase in t h e value of kGa resulted in an increase in selectivity. (4)T h e absorption of NO, gas in t h e aqueous solution of sodium hydroxide (5% w/w) was carried out in 150mm-i.d. packed column. T h e experimentally measured temperature profiles were found t o agree with model predictions. T h e formation of total nitrite was also favorably predicted by t h e mathematical model at three levels of temperature a n d three levels of liquid flow rate. The experimental values of selectivity of nitrite were found t o agree with t h e model when t h e parameters suggested by Newman a n d Carta (1988) were employed.
Nomenclature a = interfacial area, m2/m3
C’,
= average specific heat of the liquid phase, kcal/(kmol K)
dh = differential height of the column d, = diameter of the packings, mm D, = diffusivity of component x, m z / s EG = fractional gas holdup G = flow rate of the inerts, kmol/s H ( k D ) 1 / 2= absorption factor for the fast pseudo-first-order reaction, kmol/(m2 kN/m2 s) H,(kD),1/2= absorption factor for the fast pseudo-first-order reaction for component x, kmol/(m2 kN/m2 s) AH = standard heat of reaction, kcal/kmol AHl = standard heat of reaction 1, kcal/kmol = standard heat of reaction 2, kcal/kmol LW~ = standard heat of reaction 3, kcal/kmol ~6 = standard heat of reaction 5, kcal/kmol .AH7 = standard heat of reaction 4, kcal/kmol AHs = standard heat of reaction 63, kcal/kmol AH9 = standard heat of reaction 64, kcal/kmol AHlo = standard heat of reaction 65, kcal/kmol AH,,= standard heat of reaction 66, kcal/kmol AHl2 = standard heat of reaction 67, kcal/kmol kG = gas-side mass-transfer coefficient, kmol/(m2 s (kN/m2)) k , = forward reaction rate constant for reaction n k - , = backward reaction rate constant for reaction n K , = equilibrium constant for reaction n h = height from the bottom of the packed bed. m L = flow rate of liquid, kmol/s L , = latent heat of vaporization of water, kcal/kmol NO* = total moles of divalent nitrogen oxides NO2* = total moles of tetravalent nitrogen oxides p l P = partial pressure of component x a t the gas-liquid interphase per mole of inert
a3
pox= partial pressure of component x in the bulk of the gas phase PT = total pressure of gas, kN/m2 Q1 = rate of heat generation due to N203formation, kcal/s Qz = rate of heat generation due to N 2 0 4formation, kcal/s Q3 = rate of heat generation due to H N 0 3 formation, kcal/s Q4 = rate of heat generation due t o HNOz formation, kcal/s Q5= rate of heat generation due to oxidation of NO, kcal/s Q 6 = rate of heat changes due to evaporation of water, kcal/s QT = total heat changes, kcal/s QG = total heat changes in the gas phase QL = total heat changes in the liquid phase R = universal gas constant, (m3 kN/m*)/(kmol K) Ra, = rate of absorption of component x, kmol/(m3 s) Rg, = rate of gas-phase mass transfer of component x, kmol/(m3 s) S = cross-sectional area of the column, m2 T = temperature, K VL = superficial liquid velocity, m/s V, = superficial gas velocity, m/s X, = kmoles of x ions per kmole of water (X), = component X a t the exit of the differential volume (X)i = component X a t the inlet of the differential volume (XIf = component X formed in the differential volume (X), = component X oxidized in the differential volume (X), = component X vaporizied in the differential volume X(g)= component X in the gas phase X,,) = component X in the aqueous phase YT = total moles of gas per mole of inert Y , = moles of gaseous component x per mole of inert (Y,), = Y , a t the exit of the differential height ( Yx)l= Y , a t the inlet of the differential height Y *N = kmoles of reactive nitrogen per kmole of inerts Y *NO = kmoles of divalent nitrogen per kmole of inerts AT = temperature difference, K
Greek Symbols LY = coefficient of eq 7 ,9 = exponent of superficial gas velocity, eq 7 y = exponent of superficial liquid velocity, eq 7 Registry No. NO,, 11104-93-1;sodium hydroxide, 1310-73-2;
sodium nitrite, 7632-00-0.
Literature Cited Carta, G. Role of HNOz in the Absorption of Nitrogen Oxides in
Alkaline Solutions. Ind. Eng. Chem. Fundam. 1984,23,260-264. Joshi, J. B.; Mahajani, V. V.; Juvekar, V. A. Absorption of NO, Gases. Chem. Eng. Commun. 1985,33, 1-93. Kameoka, V.; Pigford, R. L. Absorption of Nitrogen Dioxide into Water, Sulfuric Acid, Sodium Hydroxide and Alkaline Sodium Sulfite Aqueous Solutions. Ind. Eng. Chem. Fundam. 1977, 16, 163-169.
Kramers, H.; Blind, M. P. P.; Snoeck, E. Absorption of Nitrogen Tetroxide by Water Jets. Chem. Eng. Sci. 1961, 14, 115-125. Liley, P. E.;Reid, R. C.; Buck, E. Physical and Chemical Data. In Perry’s Chemical Engineer’s Handbook; Green, D. W., Ed.; McGraw-Hill Book Co.: New York, 1984. Mahajani, V. V.; Joshi, J. B. Generalised Process Design of Packed Towers-A Quick Approach. Unpublished literature, 1989. Newman, B. L.;Carta, G. Mass Transfer in Absorption of Nitrogen Oxides in Alkaline Solutions. AIChE J. 1988, 34, 1190-1199. Received for review May 8, 1989 Revised manuscript received January 30, 1990 Accepted February 23, 1990
