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MATERIAL BALANCE:
The kinetics aspects of the process were discussed and now we shall carry out the material balance on a 600 tpa plant of Benzaldehyde. Material balance is based on the law of conservation of mass i.e. total mass input is equal to total mass output. It gives us the amout of compounds needed and specific amount of products to be formed [8]. Section 1 gives the calculation of the material balance of the plant. The summary of the calculation is given in table 4.1 to 4.5 in section
2. 4.1 CALCULATIONS OF MATERIAL BALANCE:
The material balance for the plant is done as follows capacity of plant 600 tpa.
Assuming no of working days 300 days per year.
Reactions involved are:
Toluene + Cl2---bluelight-------> Benzyl chloride + HCl ----- i
Toluene + 2Cl2---blue light---------> Benzalchloride + 2HCl ----- ii
Toluene + 3Cl2 ---blue light----------->Benzotrichloride + 3HCl ----- iii
F = Fresh Toluene feed.
R = Recycle Stream.
M= Mixed Stream.
Let,
XR1 be mole fraction of toluene in Recycle Stream.
XR2 be mole fraction of Benzylchloride in Recycle Stream.
XR3 be mole fraction of Benzalchloride in Recycle Stream.
Assumption –
There is no Benzotrichloride in Recycle stream, that is all Benzotrichloride is separated by distillation. Toluene in fresh feed is 100% pure.
Now, F + R = M {Overall mole Balances}
Total Toluene entering the chlorinator. = F + RXR1
Total Benzyl chloride entering the chlorinator = RXR2
Total Benzalchloride entering the chlorinator = RXR3
Toluene + Cl2----------> Benzyl chloride + HCl ----- i
Toluene + 2Cl2------------> Benzalchloride + 2HCl ----- ii
Toluene + 3Cl2 -------------->Benzotrichloride + 3HCl ----- iii
Assuming that in 1st reaction 55% of total toluene entering is consumed.
In 2nd reaction 22% of total toluene entering is consumed.
In 3rd Rxn 1% of total toluene entering is consumed
and the conversion of toluene is 78%, and no nuclear product is being formed in chlorinator.
TOLUENE BALANCE:
Toluene in product (P) = Toluene unreacted.
Toluene in product (P) = 0.22 (F + RXR1) ---------------- 1
Assuming that 98% of toluene is separated by distillation and recycled.
Toluene in recycle = 0.98 [ 0.22 (F + RXR1)] = 0.2156 [F + RXR1] ---------------- 2
But toluene in recycle = RXR1 ---------------- 3
Equating equation 2 and 3, we get,
RXR1 = 0.2156 [F + RXR1] RXR1
= 0.2748 F (1- 0.2156) RXR1
= 0.2156 F = 0.7844 RXR1
therefore RXR1 = 0.2748 F ---------------- 4
Total toluene coming to chlorinator = F + RXR1 = F+ 0.2748 F = 1.2748 F ---------------- 5
BENZYLCHLORIDE BALANCE:
Benzyl chloride in recycle stream = RXR2 ---------------- 6
Total Benzyl chloride entering chlorinator = RXR2 ---------------- 7
Now Benzyl chloride formed by Rrn (ii)
Toluene + Cl2 ------------>Benzalchloride + HCl.
Since 55% of toluene entering is consumed in above reaction.
Benzyl chloride = 55% of toluene entering
= 0.55 [ 1.2748 F] by equation 5.
= 0.70114 F. ---------------- 8
Assuming 50% of Benzyl chloride in recycle stream, reacts with chlorine to form Benzalchloride by following stoichometry.
Benzylchloride + Cl2 ------------->Benzalchloride + HCl.
Benzalchloride formed by above reaction = 0.5 (RXR2). ---------------- 9
Unreacted Benzyl chloride of recycle stream = RXR2 – [0.5(RXR2)]
= 0.5 RXR2 ---------------- 10
Benzylchloride in product(P) = Benzalchloride formed by reaction by equation 8 +Unreacted of recycle stream by equation 10
= 0.70114 F + 0.5 RXR2 ------------- 11
Assuming 97% of Benzylchloride is separated by distillation and recycled.
Benzylchloride in recycle = 0.97 [0.70114 F + 0.5 RXR2]
= 0.680 F + 0.485 RXR2------------- 12
But Benzylchloride in recycle = RXR2
therefore Equating equation 6 and 12 we get.
RXR2 = 0.680 F + 0.485 RXR2
(1- 0.485) RXR2= 0.680 F.
0.515 RXR2=0.680 F.
RXR2 = 0.680 F / 0.515.
thereforeRXR2 = 1.32 F ------------- 13
This is total Benzyl chloride coming to chlorinator.
BENZALCHLORIDE BALANCE:
Benzalchloride in recycle stream = RXR3
Benzalchloride in feed is zero = 0
Total Benzalchloride entering chlorinator = RXR3
Benzalchloride formed by reactions.
(i) From toluene chlorination:
Toluene + 2Cl2------------>Benzalchloride + 2HCl
Since 22% of Toluene entering is getting consumed in above reaction.
therefore By stoichometry Benzalchloride formed by above reaction.
= 0.22 [Toluene Entering]
= 0.22 (1.2748 F) by equation 5
= 0.280456 F ------------- 14
(ii) Chlorination of Benzyl chloride ( in recycle):
Since 50% of Benzylchloride in recycle stream is chlorinated to form Benzalchloride by following reaction.
Benzylchloride + Cl2 -------------->Benzalchloride + HCl.
therefore Benzalchloride formed = 0.5 (Benzyl chloride in recycle)
= 0.5 (RXR2)
but, RXR2 = 1.32 F from equation 13
therefore Benzalchloride formed = 0.5 (1.32 F) = 0.66 F ------------- 15
Assuming that 15% of Benzal chloride in recycle shown is getting reacted with chlorine to form Benzotrichloride.
therefore Total Benzalchloride in product (P) = (Benzalchloride in recycle) + (Benzalchloride formed by reaction.) – 15% (Benzalchloride in recycle).
= RXR3 + (0.66 F + 0.280456 F) – 0.15 (RXR3)
Total Benzal chloride in product = 0.85 RXR3 + 0.940456 F. ------------- 16
Assuming that 90% of Benzalchloride is removed by distillation 10% of Benzal chloride comes in the recycle stream (by top).
therefore Benzalchloride in recycle stream = 0.1 [ Benzalchloride coming out of reactor].
= 0.1 [ 0.85 RXR3 + 0.940456 F]
= 0.085 RXR3+0.0940456 F --------- 17
But, Benzalchloride in recycle stream = RXR3 --------- 18
therefore Equting equation 17 and 18 we get,
RXR3 = 0.085 RXR3 + 0.0940456 F RXR3
= 0.085RXR3 + 0.0940456 F
(1- 0.085) RXR3= 0.0940456 F.
therefore RXR3 = 0.10278 F. --------- 19
This is Benzal chloride in recycle stream = RXR3 = 0.10278 F.
Now, Benzal chloride obtained ofter distillation: = 90% [Benzal chloride in the Product(P) ]
= 0.9 ( 0.85 RXR3 + 0.940456 F)
= 0.9 (0.85 (0.10278 F) + 0.940456 F)
= 0.9 (0.08736 F + 0.940456 F)
= 0.9 ( 1.0278 F)
therefore Benzal chloride obtained after distillation = 0.92502 F. --------- 20
BENZOTRICHLORIDE BALANCE:
Benzotrichloride in recycle stream = 0
Benzotrichloride in feed is = 0
therefore Benzotrichloride entering the reactor is zero.
Benzotrichloride formed by reactions:
i) By chlorination of Toluene :
Toluene + 3Cl2 ----------->Benzotrichloride + 3HCl.
therefore 1% of Toluene entering is getting consumed by above reaction.
therefore By stoichometry Benzotrichloride formed = 0.01 [Toluene entering]
= 0.01 [1.2748 F] by equation 5
= 0.012748 F. --------- 21
ii) By chlorination of Benzal chloride:
Benzal chloride + Cl2 ------------->Benzotrichloride + HCl.
Since, 15% of Benzal chloride in recycle stream is getting consumed by above reaction. therefore Benzotrichloride formed = 0.15 [ Benzal chloride in recycle]
= 0.15 [RXR3]
= 0.15 [0.10278 F]
= 0.015417 F. --------- 22
therefore Total Benzotrichloride formed = 0.012748 F + 0.01541 F
= 0.028165 F. --------- 23
therefore Total Benzotrichloride in product (P) = 0.028165 F.
Assuming all Benzotrichloride is separated by distillation and there is no Benzotrichloride in the recycle stream.
HCL BALANCE:
HCl in recycle stream = 0
HCl in F = 0
therefore HCl entering the reactor = 0
HCl formed by the reactions:
i) By chlorination of toluene to Benzyl chloride.
Toluene + Cl2---blue light-------------> Benzyl chloride + HCl.
Since, 55% of Toluene entering is consumed is above reaction.
Therefore by stoichometry.
therefore HCl formed = 55% [Toluene entering]
= 55 [ 1.2748 F]
= 0.70114 F. --------- 24
ii) By chlorination of Toluene to Benzal chloride:
Toluene + 2 Cl2---blue light------------> Benzal chloride + 2 HCl.
Since, 22% of Toluene entering is getting comsumed in above reaction.
therefore By stoichometry HCl formed = 2 [0.22[1.2748 F]]
= 2(0.280456 F)
= 0.560912 F. --------- 25
iii) By chlorination of Toluene to Benzotrichloride:
Toluene + 3Cl2 ---blue light-------------->Benzotrichloride + 3 HCl.
Since, 1% of Toluene entering is getting consumed in above reaction.
therefore By stoichometry HCl formed = 3 [ 0.01[1.2748 F]]
= 3 (0.012748 F) = 0.038244 F. --------- 26
iv) By chlorination of Benzyl chloride (recycle stream) to Benzal chloride:
Benzyl chloride + Cl2---blue light--------------> Benzal chloride + HCl.
therefore 50% of Benzyl chloride in recycle is getting reacted by above reaction.
therefore By stoichometry HCl formed. = 0.50 [ Benzyl chloride in recycle]
= 0.50 [ RXR3]
= 0.50 [ 1.32 F]
= 0.66 F. --------- 27
v) By chlorination of Benzyl chloride (recycle stream) to Benzotrichloride:
Benzal chloride + Cl2 ----blue light---------------> Benzotrichloride + HCl.
therefore 15% of Benzal chloride is getting consumed by above reaction.
therefore By stoichometry, HCl formed. = 0.15 [ Benzal chloride in recycle]
= 0.15 [ RXR3] = 0.15 [ 0.110278 F].
= 0.015417 F. --------- 28
therefore Total HCl formed = 0.70114 F + 0.560912 F + 0.038244 F + 0.66 F + 0.015417 F
= 1.975713 F. --------- 29
We can absorb this HCl in water to give 30% HCl solution (by weight).
Therefore, HCl on weight basis is = 1.975713 F(36.5)
= 72.1135 F (weight basis)
The water required for absorption can be calculated by the following.
Now, 72.1135 F = 0.3 ( Total solution)
Total solution = 72.1135 F / 0.3
= 240.378 F.
Water required, = 0.7 (240.378 F)
= 168.2648 F (weight basis)
therefore Water required = 168.2648 F (weight basis)
CHLORINE BALANCE:
By stoichometry of reactions we know that,
chlorine reacted = HCl formed.
Since, HCl formed = 1.975713 F
therefore Chlorine reacted = 1.975713 F
Note: Assuming all chlorine fed is getting reacted as the equation stated before.
MATERIAL BALANCE FOR DISTILLATION:
The chlorinated mass obtained, has to be purified by distillation. The bottom product contains mostly Benzal chloride and Benzotrichloride.
Feed to distillation column = product (P) from chlorinator.
(P) = (Toluene) + (Benzyl chloride) + (Benzalchloride) + (Benzotrichloride)
Toluene Balance :
Toluene in P = Toluene in R + toluene in W.
0.22 (F + RXR1) = RXR1 + Wtoluene
0.22 (F + 0.2748 F) = 0.2748 F + Wtoluene
0.280456 F – 0.2748 F =Wtoluene
Wtoluene = 0.005656 F = toluene in bottoms W
Benzylchloride Balance:
Benzylchloride in P = Benzylchloride in R+ Benzylchloride in W
0.70114 F + 0.5 RXR2 = RXR2 + Wbenzylchloride
0.70114 F + 1.32 F (0.5) = 1.32 F + Wbenzylchloride
Wbenzylchloride = 0.04114 F = benzylchloride in bottoms W
Benzalchloride Balance:
Benzalchloride in P = Benzal chloride in R + Benzal chloride in bottoms W
0.85 RXR3 + 0.940456 F = RXR3 + Wbenzalchloride
0.85 (0.10278 F) + 0.940456 F = 0.10278 F + Wbenzalchloride
1.027819 F – 0.10278 F = Wbenzalchloride
therefore Wbenzalchloride = 0.925039 F = Benzal chloride in bottoms W
Benzotrichloride Balance:
Benzotrichloride in P = Benzotrichloride in R + Benzotrichloride in bottoms W
0.028165 F = 0 + Wbenzotrichloride
therefore Wbenzotrichloride = 0.028165 F = Benzotrichloride in bottoms W
Since Benzotrichloride is not recycled.
Distillation column overall material balance:
OVER ALL MOLE BALANCES:
Feed P = Distillate R + Bottoms W
| COMPOUNDS | FEED (P) | DISTILLATE (R) | BOTTOMS (W) |
| TOLUENE | 0.280456 F | 0.27480 F | 0.005656 F |
| BENZYLCHLORIDE | 1.361140 F | 1.32000 F | 0.041140 F |
| BENZALCHLORIDE | 1.027819 F | 0.10278 F | 0.925039 F |
| BENZOTRICHLORIDE | 0.028165 F | 0.00000 | 0.028165 F |
| TOTAL | 2.697580 F | 1.69758 F | 1.000000 F |
Toluene
0.280456 F = 0.2748 F+ 0.005656 F
Benzylchloride
1.36114 F =1.32 F +0.04114 F
Benzalchloride
1.027819 F =0.10278 F+ 0.925939 F
Benzotrichloride
0.028165 F =0 0.028165 F
Total 2.69758 F 1.69758 F 1.0 F
P = R + W
MATERIAL BALANCE FOR HYDROLYSER:
The mixture W (bottoms) is hydrolysed by sodium carbonate to form benzaldehyde. Hydrolysis is carried out batch wise and the organic layer is sent to batch distillation for further purification of benzaldehyde,
Capacity of plant is = 600 tpa
therefore Benzaldehyde = 2000 kg/day (assuming no. Of working days = 300)
65 day are for shutdown, mantainances etc.
Assuming no of batches = 1 per day.
The bottoms of distillation column is sent to storage tank for 24 hrs. And then contents of tank are sent to hydrolyser.
therefore In 24 hrs. Contents of tank are :
BzCl2 = benzalchloride = 24 (0.925039 F) = 22.2009 F
BzCl3 = benzotrichloride = 24 (0.028165 F) = 0.67596 F
BzCl = benzylchloride = 24 (0.04114 F) = 0.98736 F
Toluene = 24 (0.005656 F) = 0.135744 F
These are distillation bottoms collected in storage tank during 24 hrs. And sent to hydrolyser.
Reactions involved:
C6H5CHCL2 + Na2CO3 + H2O-----------> C6H5CHO + 2NaCl + H2O + CO2
C6H5CHCL3 + 2Na2CO3 + H2O ------------> C6H5COONa + 3NaCl + H2O + 2CO2
By stoichiometry of reaction (i) one mole of Na2CO3 and water is required to hydorlyde 1 mole of Benzal chloride. But we should also consider solubillity of Na2CO3 in water.
Solubility of Na2CO3 in water = 0.455 gm Na2CO3 /gm H2O
= {0.455 (1/106) gm mole of Na2CO3 / (1/18) gm mole of H2O}
= 0.07726415 gm mole of Na2CO3 / gm mole of H2O
from stoichimetry
for (22.2009 F ) Benzalchloride , Na2CO3 required = 22.2009 F ----------( by reaction 1 )
For (0.67596 F ) Benzotrichloride, Na2CO3 required = 2 (0.67596 F) ------(by reaction 2 )
= 1.35192 F
therefore Total Na2CO3 required = 22.2009 F + 1.35192 F
= 23.55282 F.
Adding 10% excess Na2CO3
therefore Total Na2CO3 added = 0.1(23.55282 F) + 23.55282 F
= 25.908102 F.
By solubility of Na2CO3 in water.
One mole of water dissolves only 0.077264005 mole of Na2CO3.
therefore how much moles of water will required to dissolve (25.908102 F) of Na2CO3.
therefore water required = 335.3191696 F
adding 20% excess water = 0.2 (335.3191696 F) + 335.3191696 F
Total water added to hydrolyser = 402.3830036 F.
therefore By stoichiometry
Total benzaldehyde formed = 22.2009 F ----------- (reaction 1 )
Total sodium benzoate formed = 0.67596 F -------- (reaction 2 )
Total NaCl formed = 2(22.2009 F) + 3(0.67596 F) = 46.42968 F
Total CO2 formed = 22.2009 F + 2 (0.67596 F) = 23.55282 F
Total water present after hydrolysis = 402.3830036 F
Total Na2CO3 remaining after hydrolysis = 2.355282 F
Now solubility of benzaldehyde in water = 0.8%
= 0.8 gm/100 gm water
= 0.008 gm benzaldehyde / gm water
= 0.008 (1/106.12) / (1/18)
= 0.001356954 gm mole of benzaldehyde / gm mole of H2O
Total water = 402.3830036 F
therefore Benzaldehyde in inorganic (aqueous layer ) = 0.546015383 F
therefore Benzaldehyde in organic layer = 22.2009 F – 0.546015383 F
= 21.65488462 F.
Now solubility of water in benzaldehyde = 1.5 gm H2O / 100 gm benzaldehyde
= 0.015 (1/18) gm mole H2O / (1/106.12) gm mole benzaldehyde
= 0.088433333 gm mole H2O / gm mole of benzaldehyde
therefore water in organic layer = (21.65488462 F) 0.088433333
= 1.91501363
therefore water in inorganic layer = 402.3830036 F – 1.91501363 F
= 400.46799 F
This water is associated with Na2CO3 as well as NaCl.
therefore moles of Na2CO3 / moles of H2O = 2.3552282 F / 402.3830036 F
= 0.005853333 moles of Na2CO3 / mole of water
therefore Na2CO3 in organic layer i.e. associated with water in organic layer
= 1.91501363 F (0.005853333)
= 0.011209213 F
therefore Na2CO3 in organic (aquesous layer) = 2.355282 F –0.011209213 F
= 2.344072786 F
Now NaCl associated with water .
moles of NaCl / mole of water = 46.42968 F / 402.3830036 F
= 0.115386782 moles of NaCl / mole of water.
therefore NaCl in organic layer i.e. associated with water in organic layer
= (1.91501363 F) (0.115386782)
= 0.22096726 F.
therefore NaCl in inorganic layer = 46.42968 F – 0.22096726 F
= 46.20871274 F.
Assuming solubility of CO2 in organic and inorganic phase is negligible.
The organic layer from hydrolyser is sent to a batch distillation unit.
BATCH DISTILLATION:
Assuming 95% recovery of Benzaldehyde.
Benzaldehyde in top product =[distillate collected in 24 hrs]
= 0.95 (21.65488462 F)
= 20.57214039 F.
Now,
[20.57214039 F] = 2000 kg / Day = (2000/106.12) kg mole / Day
= 18.84658877 kg mole / Day
F = 18.84658877 / 20.57214039
F = 0.91612192 kg mole / hr
therefore benzaldehyde in top product = 20.57214039 F
= 20.57214039 (0.91612192)
= 18.84658875 kg moles in 24 hrs.
= 18.84658875 (106.12) = 2000 kg.