Chlorination Process
(A) Common Features of Chlorination:
(1) Basically chlorine is used as chlorinating agent during chlorination. Hence chlorine
supply is important basic feature for chlorination. Chlorine is generally available in
75 kg and 900 kg cylinder. 900 kg Cl2 cylinder is called as “toner”. As chlorine is
stored in liquid form under pressure, it is need to evaporate before passing it to
reactor.
(2) Chlorination reactions are highly exothermic where ΔHr ranges between 25 to 30
kcal/gmol.
(3) During chlorination, HCl gas is obtained as the by-product.
(4) As HCl gas is the by-product of reaction and it is highly corrosive in nature, hence in
environmental and safety point of view, HCl gas can’t be vented directly in
atmosphere. So proper absorption system for HCl should be designed.
(5) HCl leaving the reactor is saturated with organics. So after condensation, these
condensed organic contents should be recovered and recycled back to reactor.
(6) The organic phase from reactor (i.e. mostly mono-chloro-benzene, MCB, in present
case) contains traces of HCl. This HCl is stripped off from reaction mass i.e. HCl
stripping is done using stripper.
(7) Recovery of organics from HCl absorption system is necessary.
(B) Preliminary Process System (PPS) for manufacture of Mono-chloro-benzene
from Benzene and Chlorine:
To develop this PPS, use following guidelines / hints/ description.
(i)
(ii)
(iii)
(iv)
(v)
Chlorine is stored in chlorine toner. Chlorine from toner is passed to chlorine
evaporator to evaporate the chlorine and then this gaseous chlorine is passed
to reactor.
Benzene (in liquid form) from Benzene day tank (DT) is passed to reactor
(chlorinator).
Reaction between chlorine and benzene takes place in chlorinator and organic
phase mostly containing mono-chloro-benzene (MCB) along with few amount
of di- and tri- chloro-benzene (DCB and TCB) is produced; also HCl gas is
produced as by-product.
Gaseous phase (mainly containing HCl gas and few amount of organics
saturated with HCl) from reactor is passed to series of condensers (2 or 3
condensers), where, organics get condensed which are recycled back to
chlorinator while HCl gas stream is further passed to HCl absorption system.
Organic phase from chlorinator contains traces of HCl. Therefore this organic
phase before passing to further separation system, it is passed to stripper
where HCl traces get stripped off from organic phase. This HCl, stripped off
from stripper is also sent to HCl absorption system. Organic phase now free
from HCl is passed to further purification i.e. to series of distillation column,
where main product MCB gets separated from traces of DCB, TCB and
unconverted Benzene if any.
(C) Preliminary Process System (PPS) for manufacture of chloro-nitro-benzene
from Benzene and Chlorine:
In this case, first MCB is produced using chlorination process i.e. chlorination of benzene
using chlorine gas as chlorinating agent; further nitration of this MCB is carried out using
mixed acid as nitrating agent to produce chloro-nitro-benzene.
Therefore, to draw PPS for this process, first draw PPS for chlorination as described
above and connect it to PPS of nitration process as described in nitration process.
(D) Special type of reactor used for chlorination:
For chlorination process special type of chlorinator is used which is of type Bubble
Column Reactor with external Heat exchanger.
To show this bubble column reactor, draw reactor (cylindrical, vertical arrangement),
show the liquid and bubbles inside it. Liquid (Benzene) is fed from top or from side inlet
of reactor.
Here, as it is gas-liquid type reaction, CL2 gas (evaporated in Cl2 evaporator) is passed in
reactor from bottom using sparger. Cl2 gas is sparged in benzene liquid so that it bubbles
in liquid and good contact between gas and liquid takes place for reaction to occur.
As reaction is exothermic, we need to continuously remove this heat of reaction, for
which external heat exchanger is used. The whole reaction mixture (gas+liquid) is
continuously circulated from reactor to heat exchanger to reactor. For this circulation,
pump is used. Once the reaction is completed, the reaction mixture (product) is removed
from reactor through bypass line provided on circulation line somewhere at the top of
reactor.
Draw heat exchanger as vertical shell and tube type heat exchanger beside the bubble
column reactor.
(** Refer the diagram of bubble column reactor shown at the time of lecture).
(E) HCl Absorption system:
As HCl gas is by-product of chlorination process and as it is being corrosive in nature, it
is needed to absorb this HCl gas using suitable solvent like water or any other. The gas
free from HCl then further can be vented in atmosphere.
For drawing the diagram for this absorption system, use following guidelines.
(i)
HCl gas from condensers and stripper, is sent to 1st tail gas tower (TGT). The
water from water day tank is passed to this tail gas tower, which will absorb
HCl.
(ii)
(iii)
(iv)
HCl absorbed in solvent is removed from bottom of 1st TGT and passed to
falling film absorber (FFA) (from top of FFA), which is arranged below 1st
TGT. For a particular time duration, this HCl solution is circulted from FFA
to 1st TGT to FFA. To remove the heat of absorption, cold water is circulated
around FFA.
After certain time, HCL solution from FFA is removed from its bottom and
passed to settler cum separator, located below FFA. In this settler cum
separator, HCL acid gets separated from traces of organic phase if any. HCL
acid is sent to storage tank (ST).
Tail gases free from HCl gas, are taken out from top of 1st TGT and passed to
2nd TGT arranged in series of 1st TGT. From this 2nd TGT, traces of HCl if any
got escaped along with tail gases from 1st TGT, are absorbed in alkali solution
contained in alkali solution tank located below 2nd TGT. For some time this
alkaline solution is circulated using pump, to 2nd TGT to alkaline solution
tank. Once all the traces of HCl removed from tail gases, remaining tail gases
almost free from HCl gas, are removed from top of 2nd TGT and vented to
atmosphere.
(** for TGT, FFA and Settler cum separator, refer the diagrams shown at the time of
tutorial)
(F) Separation of Organic Phase from stripper:
We need to separate main product MCB from traces of other by-products like DCB or
TCB and unconverted benzene, from organic phase coming out from stripper. To draw it,
use following guidelines:
(i)
(ii)
(iii)
(iv)
Organic phase (containing mostly MCB and free from HCl gas) from HCl
stripper is sent to 1st distillation column (DC1), which separates unconverted
Benzene from other product components.
Benzene separated in DC1 is removed from its top and sent to ST. remaining
organic phase is taken out from bottom of DC1 and sent to 2nd distillation
column (DC2).
DC2 separates MCB from top, which is further sent to ST, however bottoms
of DC2 are sent to 3dr distillation column(DC3).
Traces of DCB gets separated from top of DC3 and sent to ST, however
bottoms of DC3 is residue stream.
** While drawing the PPS, show all equipments / tanks by rectangular or square box as
necessary.
** Below PPS, make a legend showing list of equipments in PPS, for e.g. DT1 – day tank
for H2SO4, C – Reactor, etc.
***While writing answer to such questions like to draw PPS or above various diagrams,
there is no need to write this all description as described above. Use this description just
as a reference to draw the various diagrams.
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