Ind. Eng. Chem. Res. 1993,32, 1904-1913
1904
Synthesis of HCN in a Solid-Electrolyte-Cell Reactor Elizabeth A. McKenna, Alexandros Othoneos, Nikolas Kiratzis, and Michael Stoukides' Department of Chemical Engineering, Tufts University, Medford, Massachusetts 02155
A kinetic model was prepared for the simulation of the production of HCN in a yttria-stabilizedzirconia cell that uses CH4-NH3 mixtures as fuel. The model considers 14 surface reactions occuring simultaneously at the anode. The effects of temperature, 0%flux, inlet gas composition, and inlet gas velocity on the HCN yield and on the overall exothermicity of the system were examined. It was found that the cell can operate without external heat supply if the CHJNH3 feed ratio is higher than 1.25. Two different cell designs were examined, one in which the anode is inside the zirconia tube, and one in which air flows inside the tube and the fuel flows outside perpendicular to the direction of air flow. The latter design exhibits certain additional advantages over the traditional tubular cell design.
Introduction Hydrogen cyanide, a key building block for a variety of chemicals, is produced today at an annual rate exceeding 600 million pounds in the US alone (Kirk-Othmer, 1978). The dominant route for its manufacture is the Andrussow and air react over process according to which CH4, "3, a platinum-based catalyst at 1050-1100 OC: CH,
+ NH, + 1.50,
-
HCN + 3H20
(1)
The effluent stream containsabout 8% HCN and a number of byproducts such as H2, CO, and C02. The catalyst is a pad of 90% Pt-10% Rh woven screens (Waletzko and Schmidt, 1988). The gauze thickness is 3-5 mm, and with the high gas velocities employed, the contact times achieved are of the order of a few milliseconds only (Waletzko and Schmidt, 1988; Satterfield, 1980). A large number of reactions occur simultaneously,and the kinetic analysis of the system is a challenging problem. Several kinetic studies have been reported (Pan and Roth, 1968; Pan, 1971),but only recently the works of L. D. Schmidt and his co-workers (Schmidt et al., 1985; Hasenberg and Schmidt, 1985, 1986, 1987) as well as several studies on catalyst activation, characterization, and performance (Cowans et al., 1990; Schmidt and Luss, 1971; Suarez and Loffler,1986)have given a good insight into the mechanism of this complex catalytic system. The second commercial route for HCN synthesis is the strongly endothermic Degussa (or BMA) process in which methane and ammonia react over platinum catalysts in the absence of air at 1150-1250 OC: CH4 + NH,
-
HCN
+ 3H2
(2)
The off-gas stream contains more than 20% HCN. The heat for the endothermic reaction is provided by external heating of banks of ceramic tubes suspended in a furnace. The catalyst is deposited on the inside wall of the tubes (Satterfield, 1980). The absence of oxygen reduces the number of reactions taking place, and the only byproduct produced to an appreciable extent is molecular N2 from Typical contact times are 1 the decomposition of "3. order of magnitude higher than those of the Andrussow reactors (Koberstein, 1973). Although there are several advantages of the Degussa over the Andrussow process (higher HCN yield, higher NH3 conversion per pass, fairly high purity of the byproduct H2), external heating is required with the
* To whom correspondence should be addressed.
heating cost contributing significantly to the overall economics of the system (Kirk-Othmer, 1978). The energy requirement of HCN synthesis led to the idea of producing it in a high-temperature solid-electrolyte cell (Kiratzisand Stoukides, 1987). Oxygen ion conducting solid electrolytes offer the advantage of operation at temperatures where several industrially important reactions take place. Two porous thin (5-BO pm) film electrodes are deposited on the two sides of an 0% conducting solid electrolyte, e.g., yttria-stabilized zirconia (YSZ), the most common 0%solid-state conductor (Stoukides, 1988).One of the electrodes (cathode) is exposed to an oxygencontaining gas, and the other is exposed to the reactant gas. A spontaneous flow of 02-is created due to the difference in chemical potential of oxygen across the solid electrolyte. An external resistive load connected to the two electrodes converts chemical energy directly into electrical energy. If the appropriate catalyst-anode is selected, the reaction can produce useful chemicals. Concerning the HCN formation these reactions could be stoichiometrically written as cathode:
(3/2)02+ 6e
anode: 30%+ CH,
-
+ NH,
30% HCN + 3H2O + 6e
(3) (4)
Such a device could offer an alternative to the two major processesfor HCN synthesis. Furthermore, in such a hightemperature solid-electrolyte cell, (a) nitrogen of the air is avoided and so are possible reactions of nitrogen at these elevated temperatures and (b) undesired gas-phase reactions of 0 2 are also avoided. What remains to be seen is if indeed such an autothermal operation with high HCN yield can be achieved in a solid-electrolyte cell. The feasibility of cogeneration of electrical energy and HCN in such a cell has been demonstrated experimentally (Kiratzisand Stoukides, 19871,and preliminary modeling results for its isothermal operation have already been presented (McKennaand Stoukides, 1992). In the present paper a mathematical model is presented which simulates the performance of a reactor cell that produces HCN by using CH4-NH3 mixtures as fuel. The model examines the conditions under which the cell can operate at maximum HCN yield without external heat supply. A comparison between model predictions and experimental results (Kiratzis, 1991) is also presented.
0888-5885/93/2632-1904$04.00/00 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No.9,1993 1905
HCN +
“ I
*
P&iW
CHI
Figure 1. Schematic diagram of tubular reactor design (TRD).
The Model The Tubular Reactor Design (TRD). Two cell designs have been examined. Figure 1is a schematic of a tubular reactor cell. Yttria-stabilized zirconia (YSZ), a solid-state 0” conductor, is used as the electrolyte. Methane and ammonia are fed inside the YSZ tube which has inside diameter d,, wall thickness e , and tube length L. A platinum film is deposited on the inside cylindrical surface to serve as anodic electrode. The choice for cathodic electrode does not affect the present simulation results, and therefore it can be any metal or conducting perovskite typically used in solid oxide fuel cells (Grosz et al., 1991). The cathode is exposed to air. The anode (catalyst) surface area is A, and the reactor volume (anode side) is V . Thus the cell can be considered a Degussa reactor in which the ceramicperipheral walls are permeable to oxygen. At the cathode the overall reaction can be considered to be the charge-transfer conversion of oxygen into oxygen ion 0%:
-
(1/2)0, + 2e 0” (5) At the anode many reactions can take place. Waletzko and Schmidt published their results on the simulation of industrial HCN synthesisreactors (Waletzkoand Schmidt, 1988). Thirteen simultaneous unimolecular and bimolecular surface reactions were considered, and the individual rates for these reactions were experimentally obtained in previous studies. Despite the complexity of the system and the many sources of experimental data used, the predictions of their model for either type of operation (with and without oxygen) are in very good agreement with industrial reactors. The model of Waletzko and Schmidt (WS) served as a basis for our simulation. In order to adapt the above model to the fuel celloperation: (a) A term proportional to the electric current I was added in the oxygen balance. (b) A term proportional to IZR, (where R, is the ohmic resistance of the cell) was added in the overall energy balance. Furthermore, it was assumed that R, is independent of I and therefore, for a given zirconia wall thickness, it is a function of temperature only. Also, in the adiabatic operation, it was assumed that all the heat evolved (both from chemical reactions and from cell resistance) is used to heat up the gas flowing over the anode. (c) A reaction of HCN hydrolysis was added. Hence, 14 chemical reactions were considered to occur at the anode. The kinetic rate expressions for these reactions are shown on Table I and are the ones used in the WS model; the HCN hydrolysis kinetics (reaction 13 in Table I) were provided in aprivate communication (Schmidt, 1990).Ten
chemical species were considered,namely H2,02, N2, CHI, “3, HCN, H20, CO, C02, and NO. (d) It was assumed that 0%first undergoes the chargetransfer reaction at the anode, and then adsorbed atomic oxygen reacts with various species on the platinum surface. (e) Since under certain conditions-mainly at low 0% fluxes-the molar flow rate could change significantlywith conversion, the gas velocity over the anode was allowed to vary with the extent of reaction. Hence, at each increment a new velocity was calculated assuming ideal gas law. With the above assumptions the material balance in the tubular reactor design (TRD) for any component j except oxygen can be written in differential form as dPjldz = (ARTIVNou)xniri For oxygen, the mass balance equation is:
+
dPoJdz = ( A R T / V N o u ) x n i r i JRT/uFd,
(6) (7)
where u = local gas velocity (cm/s), z = distance from gas inlet (cm) (Hence, z varies between 0 and L),NO = Avogadro’s number (=6.023 X moleculeslmol), i = number of reactions (1-14), ri = reaction rate (molecules/ (cm2 s)), ni = stoichiometric coefficient, Pj = partial pressure of species j (Torr), J = current density (A/cm2), and F = Faraday’s constant. Note that the oxygen flux through the electrolyte (mol of 02/(cm2-s))is given by the equation
Go, = J/4F
(8)
The energy balance for isothermal operation can be written in differential form as (9) dQIdz = @pe/d, + ( A / V N o ) z ( A H i r i ) where Q = amount of heat accumulated (W/cm2)and p = cell resistivity (ohm.cm). The temperature dependence of p is given by the equation p = PO exp(E,dRT), where PO and Eelare constants. AHi = heat of reaction i (J/mol). For adiabatic operation the mass balances are the same while the energy balance is written as
dT/dz = (A/VN,,upC,)z(AHiri)
+ @pe/dcupC,,
(10) wherep and C, are average values of the gas density (mol/ cm3) and specific heat (J/(mol K)),respectively. The above model equations 6,7,9, and 10can be written in a dimensionless form as dYj/dz* = Wrj/ Wbt
(6’)
d YoJdz* = Wd,/ Wb,+ Well Wbt
(7’)
dQ*/&* = HJHbt
+ HellHbt
(9’)
dT*/dz* = HrlHbt+ He1/H,, (10’) where Yj stands for the mole fraction of species j , Q*and T* are dimensionlessheat flux and temperature (=T/To), respectively,and z* stands for dimensionlessreactor length (=z/L).The quantities Wrj, W, Wd, and Welare molar flow rates (molls), and similarly, H,,Hbt and H1 . have units of energy per unit time (W). The Cross-Flow Design (CFD). As it will be shown later, the results obtained with the TRD indicated that the cell operation is sensitive to certain design parameters and particularly to the tube diameter d,. An alternative type of reactor, the cross-flow design (CFD), was then tested. A schematic diagram of that design is shown in Figure 2. It consists of several parallel YSZ tubes of length 1 and outside diameter do. The CH4-NH3 mixture flows outside the tubes in a direction perpendicular to the
1906 Ind. Eng. Chem. Res., Vol. 32, No. 9, 1993 Table I. Reactions and Reaction Rates. no. reaction
rate expression (molecules/(cm28))
-.
(3/2)H2 + (1/2)N2
1
NHs
2
NH3 + CH4
3
NHs + (5/4)02
4
NHs + (3/2)NO
-
5
(1/2)02 + H2
H2O
6.
CHI + (3/2)02 --c CO + 2H20
+
HCN + 3H2
+
-
NO + (3/2)H20
NO + H2
8
NO
9
NO + CO --c (1/2)N2+ C02
+
2.1 x 10l6e ~ p ( l O 8 5 0 / T ) P ~ , P ~ ~ ~ ~ ~
(5/4)N2 + (3/2)H20
7
+
(1+ 0.044 exp(2390/T)PCH,/PNHy)'
(1/2)N2 + H2O
(1/2)N2 + (1/2)02
+
1 5 x 10-10 exp(15000/T)PcH4
3.5 x IO" exp(7300/T)P,P,,
[I + 2.7 X lo-' exp(9750/T)PNo+ 15 exp(1100/T)Pho.'12 5.53 X 1OI6 exp(-2625/T)PN, 1 + 6.95 X lo-' exp(4125/T)PNo+ 1.56 exp(4775/T)Po,
3.5 x 1017exp(2~0/T)P,.,,-, 1+4x
a
-
10
co + (1/2)02
11
C&
12
CO + H2O .-+COS+ H2
13
HCN + H2O .-+ NH3 + CO
14
CHI + 3N0
+ NO
-t
c02
exp(15000/T)P~~
2.5 X 1015e~p(16000/Z")P~#~,
HCN + (1/2)H2 + H2O
-t
(3/2)N2 + CO + H2O
Rate expressions and kinetic constants (except 13) are taken from Waletzko and Schmidt (1988).
cylinder axis; air flows inside. There is a space h between tubes for the reacting gas flow. Regardless of the model predictions there are certain practical advantages of this design over the TRD: (a) The anodic electrode (which serves as a catalyst and is therefore subject to deactivation) is on the outside, and it is easier to either examine or regenerate it. (b) Higher fuel velocities can be more easily achieved. Recent studies showed that as the diameter of the ceramic tubes decreases, the flow becomes laminar (Kiratzis and Stoukides, 1991). (c) It is relatively easy to impose different operating conditions in each tube (Figure 2) if desired, while it is difficult to do so along the length of a single YSZ tube (Figure 1). It is mainly the last two reasons that modeling of the CFD was preferred over the monolith-type fuel cell which offers a number of attractive features (Michaels et al., 1986; Debenedetti et al., 1984; Vayenas et al., 1985). Assuming that h
