Introduction

Acetone, also known as propane is a colorless liquid with a unique taste and odor (Bull, S.,

2010). It falls under the categorizing of ketones, which are organic compounds containing a

carbonyl group bonded to two hydrocarbon groups. It is a magnificent solvent for a wide

range of industrial materials including gums, waxes, dyestuffs and cellulosics (Dworkin, M.,

& Falkow, S., 2006). Most of the world’s acetone is obtained as a co-product of phenol

production by the cumene process.

The cumene-phenol process is the main source of acetone. In the past, there has been a

inadequacy of acetone, and it would have been uneconomic to satisfy acetone demand by the

accumulation of unsealeable phenol. The alternative route is where petrochemical is used to

to synthesis acetone. It was the first which have been pioneered by Exxon (Wittcoff, H., &

Reuben, B. G., 1996).

Chemical reactors are vessels designed to contain chemical reactions. It is the site of

conversion of raw materials into products and is also called the heart of a chemical process.

The design of a chemical reactor where bulk drugs would be synthesized on a commercial

scale would depend on multiple aspects of chemical engineering. Reactors are designed based

on features like mode of operation or types of phases present or the geometry of reactors.

Basically there are two main types of reactor; 1) batch and 2) continuous.

Batch reactors are normally used for most of the reactions carried out in a laboratory.

This procedure is also carried out in industry but usually in small scale products. An

alternative to a batch process is to feed the reactants continuously into the reactor at one

point, allowing the reaction to take place and withdraw the products at another point (CIEC,

2013). Some of common continuous reactors are Continuous Stirred Tank Reactor (CSTR),

Tubular Reactor, Fixed Bed Reactor (FBR) and Plug Flow Reactor (PFR).

In this case of acetone production, Plug Flow Reactor was chosen as acetone discharge as

product in vapor phase. PFR is applied when the reactions are in large scale, continuous

production and homogeneous and heterogeneous reactions. The reaction started with

isopropyl alcohol in liquid phase is fed in the inlet stream, where it undergoes catalytic

hydrogenation to acetone. The reactor exit gases consists of acetone, water, hydrogen and

unreacted isopropyl alcohol where they will undergo further separation process in scrubber

(Sinnot, R. K., 1993).

CHEMICAL REACTION

INTRODUCTION OF REACTOR

There are various types of reactors depending on the phases involved in the reaction. In

this process, an aqueous solution of isopropyl alcohol is fed into the reactor, where the stream

is vaporized and reacted over a solid catalyst. According to Turton et.al. (1998), a single pass

conversion of 85-92% with respect to isopropyl alcohol, with reactor conditions of 2 bar (200

kPa) and 350° C, is generally achieved. The reaction occurs in a packed bed reactor thus the

reactor design is modelled as in plug flow reactor (PFR).

Plug flow reactors which is also known as tubular reactors consist of a hollow pipe or

tube through which reactants flow. It consists of a cylindrical pipe with openings on each end

for reactants and products to flow through. Plug flow reactors are usually operated at steady-

state. Reactants are continually consumed as they flow down the length of the reactor. Figure

1 shows a schematic diagram of plug flow reactor.

Figure 4.0: Schematic diagram of plug flow reactor

In a plug flow reactor, reactants are fed to the reactor at the inlet and the product is

removed from the reactor at the outlet. The reaction takes place within the reactor as the

mixture moves through the pipe. The reaction is first order with respect to the concentration

of isopropyl alcohol and has an Arrhenius dependence on temperature with E= 72.38

MJ/kmol and A = 351,000 m3 gas/ m3.s (Mike et. al., 1998). The conversion is 90% with

respect to isopropyl alcohol.

There are many advantages of using PFR compared to other reactors such as Batch

reactor and Continuous Stirred Tank Reactor (CSTR). PFR advantages are as follows:

High conversion per unit volume

Low operating cost

Continuous operation

Good heat transfer

For this reaction, PFR was chosen because :

For gas phase reactions, PFR is more preferable.

For large operations and fast reactions PFR is used.

For conversion up to 90%, the performance of five or more CSTRs connected in series

approaches to that of PFR.

This is the reaction involved in the plug flow reactor

CH3-CHOH-CH3 CH3-CO-CH3 + H2

Isopropyl alcohol Acetone + Hydrogen gas

A B + C

To ensure the validity of the reaction and equations used in designing the reactor, few

assumptions according to Fogler, (2006) has been made:

Assumptions

Steady state

Isothermal

Adiabatic

Constant Pressure

Irreversible reaction

The mixture properties are uniformly distributed across the cross-section of the reactor

The values of operating conditions are as follows:

Reactor type : Isothermal

Conversion of isopropyl alcohol : 90%

Operating Temperature : 350 ⁰COperating Pressure : 200 kPa

Molar flowrate, FAO : 29.80 kmol/hr

PROCESS FLOWCHART

Figure 4.1: Process flow diagram in the PFR

SUMMARY OF MATERIAL BALANCE

n1, IPA = 29.89 kmol/hn2, H2O = 14.85 kmol/h(67% IPA,33% H2O)

n3, Acetone = 26.82 kmol/hn4, H2 = 26.82 kmol/hn5, H2O = 14.85 kmol/hn6, IPA = 2.98 kmol/h

REACTOR

Component Number of moles entering the reactor (kmol/hr)

Mass of component entering the reactor (kg/hr)

Number of moles leaving the reactor (kmol/hr)

Mass of component leaving the reactor (kg/hr)

IPA 29.80 1790.98 2.98 179.098Water 14.85 267.62 14.85 267.2Acetone - - 26.82 1557.7Hydrogen - - 26.82 53.64

STREAM CONDITION

Condition Inlet Outlet

A B C

Pressure (kPa) 200 200 200

Temperature (oC) 350 350 350

Phase Vapor Gas Gas

REACTION KINETICS

The reaction to form acetone from isopropyl alcohol is endothermic with a standard heat of reaction 62.9 kJ/mol. The reaction is kinetically controlled and occurs in the vapor phase over a catalyst.

k = A exp – Ea

RT

Where k = Reaction constant

A = Frequency factor

Ea = Activation energy

R = Gas constant (8.3144 kJ/kmol.K)

T = Absolute Temperature (623.15K)

There is only one single reaction in this process. The values of A, Ea and k for this reaction

are as follows:

Reaction A Ea k

1 3.51 × 105 m3 gas/m3

reactor.s

72380 kJ/kmol 0.3008 s-1

STOICHIOMETRIC TABLE

Species Symbol Initial Change Remaining Concentration

Isopropyl Alcohol (IPA)

A FA0 - FAOX FA=FAO(1-X) CA = CAO(1−X )

1+ξXAcetone B - +FAOX FB=FAOX

CB = CAO X1+ξX

Hydrogen C - +FAOX FC=FAOXCC =

CAO X1+ξX

Water (Inert) I FIO - FI = FIO CI = CIO

Assumption made: Assume elementary reaction (first order reaction) Assume limiting reactant is A Assume irreversible reaction

PARAMETER EVALUATION

YAO = F AO

FT

= 29.8044.65

= 0.667 (≡0.67)

δ = ¿ + ba ) -

aa

= ¿ + 11) - 1

= 1

Ɛ = YAO δ= 0.67 (1)= 0.67

CONCENTRATION OF EACH SPECIES

Assume Ideal gas,

PV = nRT

nV

= PRT

CT = PRT

= 200kPa

(623.15 K )8.314 m ³.Kpakmol .K

= 0.0386 kmol/m3

CAO = YAO CT

= 0.67 (0.0386 kmol/m3)= 0.02586 kmol/m3

(X = 0.9, YAO = 0.67, δ= 1, ξ= 0.67)

CA = CAO (1−X )

1+ξX

=0.02586(1−0.9)

1+0.67(0.9)= 0.001613 kmol/m3

CB = CAO X1+ξX

= 0.02586(0.9)1+0.67 (0.9)

= 0.01452 kmol/m3

CC=CAO X1+ξX

= 0.02586(0.9)1+0.67 (0.9)

= 0.01452 kmol/m3

CI = CIO

= CT - CAo

= 0.01274 kmol/m3

REACTOR VOLUME

Since the rate equation of reaction is -rIPA = K CIPA

In the form of conversion the rate equation becomes-rIPA = K CAO ¿)

Where ;

K = K0 exp [−EaRT

¿

Determination of k-value

k = 3.51x105 exp [−72380

(8.3144)(623.15)¿

= 0.3008 m3 gas

m3bulk catalyst . s

The volume of reactor can be calculated by taking a few steps as below:

Rate Law

-rA = kCA , CA = F AV

V= FT

F ¿ V o =

V OF¿(1+X )V O (1+X ) = V O F¿ ¿¿

V = VO (1+X)

-rA =k F AO

V O

(1−X )(1+X )

Design equation

FAO - FA + ∫r AdV = d N A

dt∫−r AdV = FAO Xd¿] = dFAOX−r A dV = FAO dX

dV = FAOdX−r A

∫0

V

dV = FAO ∫0

X dX−rA

V = FAO ∫0

X dX−rA

Combine both rate law and design equation

V = FAO ∫0

X dX−rA

= FAO ∫0X dX

kF AO

V O

(1−X )(1+ƐX)

V = V O

k ∫

0

X 1+εX1−X

dX

∫0

X (1+εX )dX1−X

= (1+ε) ln 1

1−X – εX

Where; VO = F AO

CAO

= 29.80kmol /hr

0.02586 kmol /m 3

= 1155.84 m3

hr x

1hr3600 s

= 0.3211 m3

s

Reactor Volume

Insert value ε = 0.67, X=0.9, V0 = 0.3157 m3/s

V = V O

k (1+ε) ln

11−X – εX

V = 0.32110.3008 (1+0.67) ln

11−0.9 – (0.67)(0.9)

= 3.4608 m3

Space time

τ = Vv O

= 3.4608m3

0.3211m3/s= 10.7780 s

CONCLUSIONIn this process, an aqueous solution of Isopropyl Alcohol is fed into the reactor and produce

Acetone and Hydrogen gas. The reactor operates at conditions of 2 bar (200 kPa) and 350°C.

The reaction occurs in a packed bed reactor thus the reactor design is modelled as in Plug

Flow Reactor. The reaction is a first order reaction. The k-value is calculated by using

Arrhenius’ law to get 0.3008 s-1. The volume of the reactor is calculated to be 3.4608 m3 and

the space time, τ is 10.7780 s. Based on the production of acetone which is 15,000 tonne of

Acetone/year via the dehydrogenation of Isopropyl Alcohol, the data that have been

calculated as summarized below are acceptable.

Type of reactor Plug Flow Reactor

Operating temperature 350 °C

Operating pressure 2 bar

K value 0.3008 s-1

Volume of reactor 3.4608 m3

Space time 10.780 s