chemical engineering Science, Vol. 43, No. 8, pp. 2073-2078, 1988 Printed in Great Britain

wLl!L2sOY/8x $3.w+O.w Pergamon Press plc

E x t e n d e d r e a c t or c o n c e p t for d y n a m i c D e N O x d e s ig n

D.W. Agar and W. Ruppel

BASF AG, Ludwigshafen, West Germany

Abstract An analysis of chemical reactors according to their use of internal and external heat and mass transfer reveals a new reactor configuration - a chromatographic reactor with periodically reversing flow_ The application of this configuration in the removal of nitrogen oxides from stack gases by selective catalytic reduction with ammonia is described_ The new reactor offers high NOx-removal rates without ammonia slip emissions and ameliorates problems associated with the fluctuations and distribution of the gas. The detailed kinetic modelling and preliminary experiments used to establish the feasibility of the process are presented. The necessary crite- ria for the use of this new reactor type and its relationship to the catalytic heat regenerator- reactor are discussed.

In many of the chemical reactors used in heterogeneous catalysis, the chemical reaction itself is only one of several processes taking place. In addition, one often has internal and external mass and heat transfer processes, which can be of critical importance for reactor performance and in subsequent processing steps. Distinctive reactor types can be distinguished according to the dominant transport process: whether it involves heat or mass, whether it is primarily inter- nal or external, whether it is diffusive or convective and its direction relative to the fluid stream (I). Such an analysis yields an interesting analogy: given a reactor with a certain heat transfer configuration, it is almost always possible to find a corresponding system for mass transfer. A generalised reactor consists of three basic components: the fluid being processed, the catalytic phase responsible for carrying out the reaction and the vessel in which the reac- tion occurs (Fig. 1). The catalytic phase has thermal (heat capacity, conductivity) and perhaps adsorptive properties in addition to catalysing the reaction(s). Similarly the vessel wall can be permeable to heat or mass and have ports through which convective transport takes place. In a tubular reactor, the individual properties of the catalytic phase and the wall can be distri- buted appropriately along the reactor length. The recently deveIoped catalytic heat regenerator (2) represents a reactor in which the thermal properties of the catalytic phase are exploited_ By using the catalyst as a thermal buffer it is able to assume the function of a heat exchanger, with the simplification and savings that this entai Is. The reactor utilises the well-known dynamic behaviour of thermal waves in exothermic systems to achieve reaction temperatures greatly in excess of the steady state adiabatic values. By periodically reversing the flow direction, the heat of reaction is retained within the bed and used to heat up the incoming feed gas. The reactor is especially suitable for non-autother- ma1 systems, i.e. dilute gases, and has already been employed commercially in sulphuric acid production. An analogous system involving equivalent mass transfer processes can also be envisaged, in which the adsorptive properties of the catalytic phase are utilised instead of its heat capacity (3). Reactants, rather than heat, are retained within the bed by means of their selective adsorption - a concept we11 known from chromatographic reactors (4) In order to prevent a breakthrough of the reactant being adsorbed, the reactant is supplied discontinuously to the feed and the flow is periodically reversed (Fig. 2). For the simple bimolecular, irreversible and isothermal reaction illustrated, a typical cycle comprises four phases. In the first, the reactant being adsorbed (B) is introduced in excess and forms a front within the bed, which gradually moves toward the exit. In this phase the other reactant (A) is completely converted_ In the second phase, no B is supplied - A, however, continues to react with the excess B previously adsorbed. The original breakthrough front continues its chromatographic progress down the reactor and a second front is formed due to the removal of adsorbed B via the reaction and due to its desorp- tion. The net effect is a diminishing zone of B moving through the reactor, reflecting a complex interaction between adsorption, desorption and reaction. The third phase is a repeat of the first, but with a reversal of the flow direction_ The existing B zone is overtaken by and sub- merged beneath the new front formed by the excess B now being supplied, thus producing a defined state symmetric to that at the end of the first phase. The fourth phase is identical to the second except that the flow is reversed. Following the fourth phase, the cycle is repeated. In

2073

2074 D. W. AGAR and W. RUPPEL E5

Fig. 1: Generalised reactor scheme for heterogeneous catalysis

= & A

Fig_ 2: Simplified flowsheet of a chromatographic reactor with periodically reversing flow

S.V. = 1300 h-1 T = 300 C

%L3 = 900 ppm

1X0-

-z 5: 600- -cl 3 400-

200-

Nttg- Beladung: 0.24 Gew. "b

Fig_ 3: Measurement and caIculation of NH curve shows the calculated results

adsorption behaviour on denitration catalyst. The ?of breakthrough experiment at 300 C and 1 bar on 77

cm3 post-desulphurisation 4 mm pitch monolithic catalyst with S.V. = 1 300 Nm3/m cat-h. At t = 0, 900 ppmV NH were added to the carrier gas (N +3% 02) being passed through the monlithic element. Alter total breakthrough (t = 7 260 s) NH was cut off and the adsorbed ammonia allowed to desorb. The breakthrough times fbr 5 8 ppmV NH measured in the reactor outlet using a rough indicator method were 5 400 s for adsorpt?on and 9 900 s for desorotion.

E5 Extended reactor concept for dynamic DcNOx design 2075

contrast to the usual chromatographic reactor, there is no regeneration phase, the reaction occurs over the whole cycle and removes the adsorbed component from the system. To control the duration of the phases one can either follow the B front in the first and third phases or the level of A in or before the reactor exit during the second and fourth phases. Regardless of which option is selected, the remaining phases are of fixed duration, the interval being determined by reactor design. The following criteria must be fulfilled if the reactor is to function properIy: 1. the adsorption must be selective and high enough to yieId reasonable cycle times

- for typical industrial catalyst loadings, this means uptake in excess of 0,l % by wt. 2. the rate of adsorption must be comparable to the.rate of reaction

- to ensure that suitable breakthrough fronts are formed 3. the rate of desorption must be slow

- this, with 2, defines the form of the adsorption isotherm, ideally it is rectangular 4. the reaction must be irreversible and free of side reactions

- the absence of a reactant in the end zone would lead to reverse reaction. The high resi- dence time of the adsorbed component could cause difficuIties with otherwise negligible sidereactions.

The chromatographic reactor with periodically reversing flow offers two interesting features: the high level of one reactant in the middle of the reactor (corresponding to the temperature in the catalytic heat regenerator) and the exclusion of this reactant from the product stream. Both characteristics are of interest in gas purification processes in which an impurity is removed by reaction with an externally introduced component_ In addition, the insensitivity of the reactor to the fluctuating concentration values common to such problems makes it attractive_ A periodic catalytic process with flow reversal has already been described for the removal of traces of oxygen from nitrogen and argon by the addition of hydrogen (5). Another suitable reaction is the seIective catalytic reduction of nitrogen oxides in stack gases using ammonia, the so-called SCR reaction - an important measure in air pollution control (6). The SCR reaction is described by the equation:

4 NO + 4 NH3 + O2 - 4 N2 + 6 Ii20 The reaction is carried out on a variety of catalysts, usually based on titanium, vanadium and tungsten oxides, at temperatures between 280 and 420 C. Various configurations are possible depending on the sequence in which dust remova1, desulphurisation and denitrification are carried out. The arrangements differ in the form and activity of the catalysts used, the reac- tion temperature and the energy demand3 A typical coal-fired power station in West Germany can achieve NO values of around 1 000 mg/m (calculated as NO firing, measures alone. In the Federal Republic of Ger Any basedo&-- 51~g~$$!~~h redulre; ;& coal-fired power stations of 300 MW or more achieve NO emissions of 200 mg/m3 or less. SCR processes represent the only proven technology available for this purpose. As ammonia emissions may not exceed 5 ppmV, the supply of ammonia has to be very carefully matched to the flow and NO concentration in the gas. Analytical problems: together with dramatic fluctuations in gas amount and composition and the difficulty of obtaining even distributions in the massive reactors involved, makes the operation of SCR reactors a formidable task. It has been known for some time that the SCR catalysts can adsorb considerable amounts of ammo- nia, a fact which must be taken into account when developing control procedures (7,8). The chromatographic reactor with periodic flow reversal enables this otherwise complicating factor to be used to the processs advantage. To establish the feasibility of using such a reactor, the adsorption of amnonia on commercially available denitrification catalysts was studied experimen- tally. Breakthrough and desorption curves measured at technically realistic temperatures and space velocities confirmed model results derived from the ammonia adsorption behaviour assumed in kinetic models of the SCR reaction (Fig. 3). For fine monoliths (with channels of 4 mm or less) the rate of adsorption could be shown to be comparable to the space time velocity and much faster than the rate of desorption. The amount of ammonia adsorbed (0,29 % by weight for a gas phase ammonia concentration of 2000 ppmV and a typical post-desulphurisation catalyst) and the absence of ammonia decomposition or oxidation with oxygen also fulfilled the necessary criteria. Having established rough values for the adsorption parameters, a dynamic model of the chromato- graphic reactor with periodic flow reversal was developed for the SCR system on the basis of kinetic data derived from an extensive study of SCR process data using a well established model (9) capable of describing dynamic and steady state behaviour over a large concentration range. The system, which is assumed to be isothermal, can be represented by the following equations: Ammonia adsorption NH3 + X _ NH3X (1) SCR reaction: NH3X + NO + l/4 02-N 2 f 3/Z H20 + X (2)

fluid phase mass balance: 4 G . -$$i = z _ Mi _ Nil5

yi z=o =y;

2076 D. W. AFAR and W. RUPPEL E5

catalyst phase mass balance: 2

dN. = QK x1

1 vij-wj j=l

Ni = - 13; . dci

Ni /x=0= O ax

Ni (x=6= Pi - (Ci - C *E - yi) 5

ammonia adsorption: dq =

r7Z r-D-. +Z + Qk . jf, Vi5 _ WJ

$S 0,6 = 0 (simplification warranted by catalyst physical properties)

reaction and adsorption kinetics: Ammonia adsorption: w1 = k,(T) . ( P,,,H - (1 - q) - 3 SCR-reaction: w2 = k*(T) . q - pNo

ad. 9) (7)

(8)

This non-linear boundary value problem is solved numerically usi ng an implicit finite difference method (10). The mesh points and the time step interval are auto smatically adjusted to match the movement of the adsorption and reaction fronts. This procedure 1 eads to a numerically stable and flexible algorithm, which is especially suited for the simulat ion of wavefront phenomena_ The solution of the equations gives the concentration profiles for NO and ammonia in the gas phase and for the adsorbed ammonia along the length of the reactor and across the catalyst wall as a function of time (Fig. 4). The sequence shown illustrates a complete half-cycle, i.e. up to the point at which the flow would be reversed, taken to be when the NO level in the reactor outlet reaches 100 ppmV.

(41

(5)

(6)

During the phase with ammonia, the reaction is confined to a small zone at the front of the reactor and in the outermost layer of the catalyst. This behaviour reflects the high diffusional resistances within the fine pores of the catalyst. The ammonia is only adsorbed downstream of the reaction zone and gives a breakthrough front much less influenced by diffusion than the reaction_ When the ammonia supply is cut off after 67 minutes, a time designed to prevent ammo- nia emerging in the reactor outlet, the ammonia level in the gas phase drops off almost immedia- tely to a low level_ A NO front is formed which steadily proceeds down the reactor, exhausting the adsorbed ammonia reserves as it goes. The downstream end of the adsorbed ammonia profile scarcely moves at all due to the slow desorption characteristics. The example demonstrates the basic feasibility of the process. The mean NO conversions and NH slip values obtained ( >99 % and

E5 Extended reactor concept for dynamic DeNOx design 2077

i= N O

t = 3 9 5 0 *

t - 7 8 0 0 s

Fig. 4: Development of NO and NH concentration profiles in the gas cc.1 and on the zatalyst cycle of &actor operation.

(q) during one half T = 300 C, P = 1 bar,

S.V. = 1 300 Nm/m cat-h, gas velocity = 0,34 m/s post-desulphurisation monolithic catalyst with 4 mm pitch.

2078 D. W. AGAR and W. RUPPEL

Notation:

E5

a C .

k 1

G ki k ppd ml N

: t T i X Y. zi

channel width , m concentrat ion of the species i in the3gas phase, kmol /m3 total ads ohase concentrat ion . kmol /m diffusion coefficient , mL/s specif ic mass flux in the gas phase, kg/m2.s rate constant for react ion j , kmol /bar .kg cat-s equi l ibrium constant for ammonia adsorpt ion , bar

_,

molecular weight of species i , kg/km01 mean molecular weight of the gas phase, kg/kmol specif ic molar flux of species i in the catalyst , kmol /m2.s part ial pressure, bar adsorbed ammonia as a fract ion of the saturat ion value t ime, s temperature, OK kinet ic rate expression for react ion j defined by equat ions 7 and 8, kmol /kg cat-s catalyst wal l width coordinate, m mass fract ion of species i in the gas phase catalyst bed length coordinate, m

Greek let ters

P

Subscripts and Superscripts

s

gas fi lm mass transfer coefficient fo5 species i , m/s e effect ive saturat ion ammonia adsorpt ion , kmol /m cat . i species half-breadth of the catalyst wal l , m j react ion

vi j stoichiometric coefficient3for species i in reaction j 0 inlet Pk catalyst bed densi ty, kg/m

(1)

(2)

(3)

(4)

(5)

(6)

(71

(81

(9)

(ID)

War, D.W. and Ruppel , W. , 1988, Erwei tertes Reaktorkonzept zur Entwicklung neuer Metho- den der Reakt ionsfuhrung, Chemie-Ingenieur-Technik , to be publ ished Matros, Yu . , 1985,Unsteady processes in catalytic reactors, Studies in Surface Science and Catalysis, 22 Agar , D-W. , Rumel , W. , Holderich , W. and Drews, R. , (BASF AG) , 1988, Verfahren zur Entst ickung von Rauchgasen , German patent appl ied for , Dec. 1987 Coca , J. and Lander S.H. , 1986, Doing chemistry in the gas chromatograph , Chemtech 16 ( l l) , 682 - 689

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Agar , D-W. , Jaeckh , C. and Gal lei , E. . (BASF AG) , 1985, periodisches Verfahren zur Fein- reinigung von Inertgasen , German patent no . DE 3 401 197 Kot ter , M. , Lintz, H. -G. , and Weyland . F. , 1986, selekt ive Redukt ion von St ickoxiden in Rauchgasen - Stand der Technik und neue Wege, Chem. -Trig. -Tech . , 58 (8) , 617 - 623 Zenz, J. and Helber , F. , (STEAG AG) , 1987, Verfahren zum AbschKden von St ickoxiden aus Rauchgasen , German patent no . DE 3 604 045 Ase, H. and Koyanagi , M. , 1987, periodic control of catalyt ic deni trif icat ion systems in refuse incinerator plant , Internat ional federat ion of automatic control , 10 th . Congress, Munich 1987, VDI/VDE-Gesel lschaft , DDsseldorf , Vol . 2, 326 - 331 Miyamoto, A., Yamazaki, Y., Hattori, T. , Inomata M. and Murakami , Y. , 1982, Study on the pulse react ion technique - VI kinetics of the reaction of NO wi th NH3 on vanadium pent- oxide catalyst, Journal of Catalysis, 74, 122 - 155 Butt, J.B., Weng, H.S. and Eigenberger , G. , 1975, Catalyst poisoning and f ixed bed reac- tor dynamics, Chemical Engineering Science, 30, 1341 - 1351 -