photochemical

1. GAS ABSORPTION WITH PHOTOCHEMICAL REACTION
V. V. MAHAJANI and M. M. SHARMA*
Department of Chemical Technology, University of Bombay,Malunga Road, BombaH 01%India
(Received 22 January 1980;accepted 31 Juiy 1980)
Abstract-Gas-liquid reactions can be activated by high energy radiations (photons). When gas absorption is
accompaniedby pseudo-firstorder reaction furtherenhancement in the specific rate of absorptioncan be realiscd
for situations where either the reactive species in the liquid phase is activated or the dissolved solute gas is
activated.
INTRODUCTION
Gas absorption with reaction activated by high energy
radiations (photons) is of considerable practical value.
Recently, Fischer [1] has reviewed industrial application
of photochemical syntheses. Chlorination of n-alkanes,
side chain chlorination of alkylbenzenes, sulfochlorina-
tion of n-alkanes, sulfoxidation of n-alkanes, nitrosation
of cycle-alkanes etc. are some typical examples of pho-
tochemical syntheses involving gas absorption. Photo-
chemical reactions also find application in waste water
treatment[2-4]. It has been reported that the enhance-
ment in the rate of reaction can be realised by the UV
activation in the case of ozone treatment of waste water
containing undesirable pollutants (inorganic and organic).
Under certain conditions, the reaction becomes mass
transfer controlled [4].
In some cases, the reaction between solute gas and
reactive species in the liquid phase may .be sufficiently
fast enough to occur in gas-liquid film. The high energy
radiations (Photons) may further enhance the specific
rate of absorption of gas. It was thought desirable to
consider the above situation wherein gas absorption is
accompanied by fast pseudo-first order reaction. In
general, this problem can be classified into two main
situations namely (1) the solute gas reacts simultaneously
with the liquid reactant and its activated form and (2) the
dissolved solute gas gets activated and both the species
(namely, the dissolved and activated dissolved gas) react
with the liquid reactant.
For the sake of simplicity, we shall consider the fol-
lowing irreversible reaction taking place in a gIass wetted
wall column:
A + B ----% products.
Figure 1 shows the schematic representation of the
reactor and the concentration profiles. The following
assumptions are made: (1) The film theory of mass
transfer is applicable. (2) The reaction occurs entirely in
the gas-liquid film; no dissolved solute exists beyond the
film thickness 8 and the concentration of liquid reactant
B is the same throughout (that is-there is no depletion
*Author to whom correspondenceshould be addressed.
of liquid reactant at the gas-liquid interface)..~. (3) The
rectilinear co-ordinate system for mass and photon
transfer is valid. (4) Monochromatic beam of radiation is
present. (5) The Beer-Lambert law holds for photon
transfer. (6) The activation step is first order with respect
to light absorbed.
CA!33 I: PHOTOCAEMICAL ACllVATION OF SPECIES B
We shall first consider the situation wherein the liquid
reacting species B gets activated due to photons.
The reaction scheme is as follows:
*t
A + B - products
B+hv*2B
*3
(1)
(2)
B + A- products. (3)
The concentration profiles of A are B are shown in Fig. 1.
The relevant differential equation is as follows:
D__,$= k,AB+ k>Afi
Boundary conditions
atx=O A=A*, B=BO, dBc=O (5)
atx=b A=O, B=B,. (6)
By applying stationary state principle to 8, we have
k,Z, = k2AB (7)
. . BEE
5
where, I, is light absorbed per unit volume.
Now, 1, = Ifi*
L ={Zoexp(-a,(6-x)B)}a,B.
(9)
(10)
595

2. 5% V. V. hdAHAlANI and M. M. SHARMA
where
@AS PHASE
Fii. 1. Gas absorption with fast pseudo-first order reaction
activated by highenergy radiations: Wetted wall column reactor:
Concentration profiles.
Substituting eqns (8) and (9) in eqn (2), we get:
D,, $$ = klAB + kJ,ce~B exp [-a&(8 - x)B]. (11)
Since there is no depletion of B, kJ? is constant.
Further, k2B10a, is assumed to be constant over the film
thickness 8.
Let & = kIB
& = kJi,cxAB
I
WI
and Z = aAB
Substituting quantities frOM eqn (12) in eqn (1l), we get:
DA $$ = &A + &exp [-Z(6 - ~11
1
(13)
The solution of eqn (13) with the heIp of boundary
conditions (5) and (6) gives the concentration profile of A
as:
A =
I
[A* - fl exp (-Zs)] sinh t/a(S -x) - @ sinh d/rrx
sinh -a6
+ fi exp [2(x - S)]. (14)
The specific rate of absorption of A, RA, is given by:
RA=[A*-B tanh%‘aa +&
exp (-ZS)]V(&tDA) daD*
(15)
(17)
Under these circumstances
RA = t/(DAkl)[A* - j3 exp (-ZS)]
+ 2/W&) exp (d(a)&)
- D&Z exp (-Z6). (18)
In the absence of photochemical activation (that is-
when /!I= 0). we get the following equation which is the
same as that for gas absorption accompanied by fast
pseudo-first order reaction:
RA = A*d(D&. (19)
Thus, it can be seen from eqn (18) that the specific rate
of absorption is not directly proportional to the inter-
facial concentration of the solute gas for a first order
reaction when activation of B is taken into con-
sideration.
CASEII: PHOTOCHEMICALACTNATIONOFDlssoLWDA
In principle, we can have photochemical activation of
dissolved A. Typical examples of solutes subject to this
type of activation are CL NOCl, OS, etc.
kI
A + B -product (1)
A+hdL-A (20)
k3
A + B-product. (21)
The relevant differential equation with the boundary
conditions is as follows:
d2A
DA u = kl AB + k&.
Boundary conditions
cm
-D&Z exp (-Z&j (16)
atx=O. A=A*, B=Bo, $f=O (23)

3. Gas absorptionwith photochemicalreaction 597
x=8, A=O, B = B,, (24) CONCLUSION
High energy radiations can substantially enhance the
I, = IP* (25) specific rate of absomtion in some situations where eas
= &[exp (-aAA(S - x)]aAA.
1~
(25)
absorption is accombanied by fast pseudo-first order
reaction. There is need for experimental work in this
Substituting (25) in (22), we get
area.
DA 2 = k,AB + k,arAIo exp (-a,A(S - x)]. (26)
Here we shall make a simplifying assumption of constant
light intensity throughout the film thereby neglecting the
terms containing aA(6 -x) in the expansion of
exponential term in eqn (26). Therefore, eqn (26) takes
the form:
D !fb = k AB + &&,a~A
a dx2 ’
= [k,B + k&cr,]A. (28)
The bracketted quantities are constant and hence we
get the specific rate of absorption of A, R,+ after solving
eqn (28) with boundary conditions (23) and (24) as:
R,, = A*q(D/,(k,B + kJa,aA)). (29)
In this case, where we have activation of the solute gas
A, the specific rate of absorption is proportional to the
interfacial concentration A* in the situation wherein the
light intensity is constant throughout the film.
There is a real dearth of useful data in the literature to
show the actual extent of enhancement in the specific
rate of absorption due to photochemical activation for
both the cases. Additional cases of general order reac-
tions or where the activation process is zero order in A
can also be considered.
A!
B
D*
I0
I,
k
RA
X
Z
_
NOTATION
concentration of gas in the liquid phase, gmole/cm3
saturation concentration, gmolelcm3
concentration of the liquid reactant, gmole/cm’
diffusion coefficient, cm’lsec
intensity of light absorbed, Ein/cm’ set
light absorbed per unit volume
reaction rate constant
specific rate of absorption, gmolelcm*sec
distance in the liquid film from gas-liquid interface,
deiyed by eqn (12)
on the top indicates activated species
Greek symbols
u defined as k,lD,
a* molal absorptivity
B ti(F-a)
6 gas-liquid film thickness, cm
&? &DA
PA attenuation coefficient
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