Page 178 - CW E-Magazine (4-2-2025)

Special Report Special Report

Making profi ts fl ow: Demystifying fl ow reactors depends on more factors than just the Scaling-up a reaction in Batch Reactor
kinetic rate constant (k) and concen-
When scaling up a batch reaction,
tration. Some reactions are limited by the reaction time often increases due
Reactions performed in fl ow reac- DARSHAN CHHEDA mass-transfer and heat-transfer consi- to limitations in heat and mass trans-
tors are always faster compared Tonight’s Show: Process Engineer – R&D Flow Chemistry derations: mass-transfer controlled, fer, rather than changes in intrinsic
“to batch reactors!” This statement Magic of Flow Reactors Amar Equipment Pvt. Ltd. heat-transfer controlled, reaction kinetics. The intrinsic kinetics should
astounds every rational chemist and Email: darshan.chheda@amarequip.com controlled, or more commonly, by remain constant if conditions are main-
chemical engineer. a combination of these factors. tained (same temperature and pres-
DR. VISHWANATH H. DALVI sure). While the reaction rate constant
It is particularly astonishing when Advisor – Flow Technology When we describe a reaction as remains constant (ideally), changes
people make such claims. The design Amar Equipment Pvt. Ltd. kinetically controlled, it means the occur due to reactor geometry and
Faculty, Institute of Chemical Technology
equation for a batch reactor is identical Mumbai reaction rate depends on the intrinsic volume increases, particularly in the
to that of a plug fl ow reactor. In theory, Email: vishwanath.dalvi@amarequip.com reaction kinetics-the molecular reaction decrease of heat transfer area per unit
if a reaction achieves a certain conver- pathway, activation energy, tempera- volume and mixing effi ciency.
sion after ‘X’ minutes of reaction time, DR. CHANDRAKANTH GADIPELLY ture, and pressure. The reaction rate then to the solid catalyst phase limits the
it should achieve the same conversion Principal Research Scientist – R&D is sensitive to the reaction temperature reaction rate). Heat transfer limitations
Flow Chemistry
after ‘X’ minutes of residence time, Amar Equipment Pvt. Ltd. (and pressure to some extent in highly Larger reactors suffer from lower
provided the temperature and pressure Email: chandrakanth.gadipelly@amarequip.com compressible fl uids like gases). In heat-transfer limited reactions, surface area-to-volume ratios. This cre-
of the reaction remain constant, i.e., k, This raises several questions: Why the rate is constrained by how quickly ates heat transfer limitations because
the reaction rate constant, remains the make such a claim? Do reactions pared to batch reactors? If the tempera- A mass-transfer controlled reaction heat can be removed from or added to less surface area is available to remove
same. (Fig.1) actually run faster in fl ow reactors com- ture and pressure remain unchanged, means the reaction rate is limited by how the reaction system. (Note: This is a or add heat relative to the reaction
what causes this acceleration? What quickly the reactants/reagents/products user-imposed limitation; the reaction volume. The resulting temperature
dC drives the transition to fl ow reactors?
Design Equation for a batch reactor: A = –kC A Design Equation for a Plug-fl ow reactor: dC A n can be transported to or from the reaction is deliberately throttled, for example, gradients can cause certain parts to be at
n
dt = –kC A Do reactions inherently perform better
dT site. This typically occurs at phase bounda- by restricting the fl ow of one or more non-optimal temperatures for the reac-
Assuming fi rst-order kinetics: n = 1 Assuming fi rst-order kinetics: n = 1 in fl ow reactors than in batch reactors? ries, such as gas-liquid, liquid-liquid, or reactants). This typically occurs in highly tion. This can slow reactions in cooler
solid-liquid interfaces. While the reaction exothermic or endothermic reactions. regions and lead to inconsistent product
Concentration profi le for A (wrt time): Concentration profi le for A (wrt residence time/length): The answers to these questions are proceeds rapidly when reactants reach the Insuffi cient heat transfer rates can lead quality, such as charring at hot spots.
C (t) = C e –k(T)t – more complex than one might expect. reaction site, the rate-determining factor is to thermal runaways in exothermic
A A,O C (T) = C e k(T)T The determining factor is the regime
A
A,O
Rearranging for time: Rearranging for residence time: under which a reaction operates (Fig. 2). the speed at which they arrive there. This reactions or inadequate conversion in
behaviour is common in heterogeneous endothermic reactions (e.g., nitrations or
t = –1 In C (t) –1 C (T) Without delving into excessive detail, reactions (e.g., hydrogenation, where oxidations where localized overheating
A
A
k(T) C A,O T = k(T) In C
A, O it is important to understand that not the transfer of hydrogen gas into the could accelerate the reaction uncontrol-
all reactions are limited by the rate liquid phase containing the substrate and lably).
Fig. 1: Design equations for Batch and Plug Flow reactors of intrinsic reaction. The conversion
Hand holes for
Kinetically Controlled Regime Mass Transfer Controlled Regime Heat Transfer Controlled Regime charging reactor
UA∆T LMTD
r = m r = k a (C , - C ) r = ∆UH Rxn
k C A
A, Surface
A Bulk
L
Where, Where, Connection for
k is the kinetic rate constant; k is the mass transfer coeffi cient; Where, heating or
L
m is order of the reaction a is the interfacial area per unit volume; U is the overall heat transfer coeffi cient; cooling jacket “See! Scale-up is so easy”
A is the heat transfer area;
C A, Bulk is the concentration of reactant A in the ∆T is the log mean temperature difference; Mass transfer limitations
bulk phase; ∆H LMTD is the enthalpy of the reaction Larger reactors typically cannot
C A, Surface is the concentration of reactant A on Rxn Agitator achieve the same level of mixing as a
the reaction site The reaction is deliberately throttled (slowed (Lab-scale : Round Bottom Flask)
down) to allow enough time for heat transfer round-bottom fl ask, where mixing is
surfaces to remove heat faster and more thorough. Poor mixing
(Pilot-scale: Stirred-Tank Reactor) in large-scale reactors leads to concen-
tration gradients, resulting in slower
Fig. 2: Rate equations for different regimes reaction rates in regions where reactants/

178 Chemical Weekly February 4, 2025 Chemical Weekly February 4, 2025 179

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