ACKNOWTEDGMENT This study was -not my own effort, many hands were contribute for presenting it in this way. First, I am greatly indebted to my supervisor prof. Hamid Mohammed Mustafa, so my gratitudes and thanks go to him for hi kind help, splendid guidance and close supervision during the course of this study; his help left appreciable remarks through this study, My thanks should be extended to the working group (administration, supervision, engineering,.. .etc.) in the experimental urea plant in southern Khartoum for their concern and help in data collecting. The design group Eltigani. and Ahmed were a tremendous help presenting the first four chapters. My thanks should be extentend to them or their participation. Last, my thanks to all who helped me in anyway. 1
Transcript
ACKNOWTEDGMENT
This study was -not my own effort, many hands
were contribute for presenting it in this way.
First, I am greatly indebted to my supervisor
prof. Hamid Mohammed Mustafa, so my gratitudes
and thanks go to him for hi kind help, splendid
guidance and close supervision during the course
of this study; his help left appreciable remarks
through this study,
My thanks should be extended to the working
group (administration, supervision,
engineering,.. .etc.) in the experimental urea
plant in southern Khartoum for their concern and
help in data collecting.
The design group Eltigani. and Ahmed were a
tremendous help presenting the first four
chapters. My thanks should be extentend to them
or their participation.
Last, my thanks to all who helped me in anyway.
Najm Eldin
I ___________________________
1
CONTENTS
CHAPTER ONE
LITERATURE SURVEY ON UREA PRODUCTION ……………… …………
1—1 Introduction………………… ……………………………….
1—2 Properties of urea………………… ……………………..
1—3 Reaction kinetics………………… ………………………
l—4 Raw materials for urea production
1—5 Uses of urea………………… ……………………………….
CHAPTER TWO
MANUFACTURING PROCESS AND PROCESS DESCRIPTION ……………
2—1 introduction ………………… ……………………………
2—2 Solution recycle process………………… ……………
2—3 Once. through process ………………… ………………
2—4 Total recycle process………………… ………………..
2—5 Process description………………… …………………..
CHAPTER THREE
THE MATERIAL BALANCE………………… ……………………………
2
CHAPTER FOUR
THE ENERGY BALANCE………………………………………………………………
CHAPTER FIVE
1-2DESIGN OF THE REACTOR ………………… ………………
5—1 Introduction ………………… …………………………….
5—2 Chemical kinetics & rate equation ……………...
5—3 Design equation ………………… ……………………….
5—4 Calculations of the reactor size…………………
5—5 Mechanical design of the reactor…………………
CHAPTER SIX
SPECIAL TASK PROCESS CONTROL………………… ………………
6—1 Introduction ………………… …………………………
6—2 Control objectives………………… …………………
6—3 Typical control systems ………………… ……………
6—4 Reactor control………………… ………………………..
I1FFERENCES ………………… ……………………………………
3
CHAPTER ONE:-
I TERATURF SURVEY ON UREA PRODUCT IOI : -
1—1 Introduction:_
Urea is a crystalline material, soluble in water
and in alcohol. it contains 46% nitrogen. The
common method of manufacture of urea is to
combine ammonia and carbon dioxide under pressure
to form ammonium carbamate which is then
decomposed into urea and water, the un reacted
ammonia are
recovered and recycled to the synthesis
operation, The reaction is:
2NH3 +C02 z NH4C02 NH 2 NH2C0NH2 + H20
There are other methods for the production of urea
such as the ones through operation and partial
recycle operation. Capacity of the plant, production
facilities and economic considerations must be
taken into account.
1—2 Properties of urea:—
4
1—2-1 General Properties:
Ammonium carbamate is obtained by direct
reaction of ammonia (NH3 ) arid carbon dioxide
(Co2). The two reactions are carried out
simultaneously in high pressure reactor.
Recently urea has been used commercially as
cattle feed supp1yjnen and other appijcj113 for
urea: the manufacture of resins, solvents and some
medicines. -
1-2—2 Chemical properties:-. -
Urea has the formula
Urea is a weak base; at atmospheric pressure and at its
melting point urea-decomposes to ammonia, biuret
cyanuricacid C3 N3 (oH3,ammelide NH2 C(OH)2 and
triuret NH2 (CONH)2 COHN2 Biuret is the main and
the least desirable by-product in the commercial Synthesized urea.
Urea acts as monobasic substance and forms
salts with acids, with
5
NH
NHC OHNHC O
nitric acid it forms urea nitrate CO(NH2)11N03.
Solid urea is rather stable at room temperature.
and atmospheric pressure, heating under vacuum
at its melting point urea sublimes.
At180—1909C urea will sublime under vacuum and
be converted to ammonium cyanate NH4OCN, at
pressure of 100—200 atm. biuret will, revert to
urea when heated in pressure of ammonia.
Urea reacts with formaldehyde and forms compounds
such as monomethylol urea NH2 CONHCH3OH
diaethyloL , urea and others depending on mole
ratio and PH of the solution.
Urea reacts with hydrogen peroxide to give urea
peroxide CO(N112 )211202 while crystalline
powder, which is known under the trade name of
hyperol , and is used as an oxidizing agent.
6
Urea derivatives are used as medicines,
sedatives and hypnotics, the main derivative is
barbituric acid obtained from urea and malonic
acid (from its ester).
1—2—3 Physical properties:—
The following tables show the physical.
properties of urea.
table 1—3—1 shows the properties of saturated
aqueous solution
of urea:
Temperature °C Solubility in
water gm
urea/100 gm
solution
Density
g/cm
Viscocity
cp
H2O
vapour
pressure
mm Hg
7
0 41.0 1,120 2.63 4
20 . 51.6 1.147 1.96 13
40. 62.2 1.167 1.77 40
60 72.2 1.184 1.72 90
80 80.6 1,198 1.93 160
100 88.3 1.210 2.35 220
120 95.5 1.221 2.93 135
130 99.2 1.226 3.25 7
Table 1-2-i
The following table (1—2—2) shows the general
properties of urea
Property Assigned value
— Melting point 132. 7°C
—index of refraction nD20 1.484, 1.602
- specific gravity nD20 1,335
8
- crystalline formed
and hapit
Tetrazonal. needle or
prisms
— free energy of
formation at 25°C
—47.12 cal/gmole
— heat of fusion 60 cal/g,endothermic
— heat of solution in
water
58 cal/g
- heat of
crystallization 70%
aguous urea—solution
110 cal/g, exothermic
- bulk density
0.74 gm/cm
Table 1—2-2
Table 1—2—3 shows specific heat of ammonium carbonate.
Temperature co Specific heat [cal/g°c]
20 0.4
9
60 0.46
100 0.52
140 0.58
180 0.62
10
Table 1—2-4 shows specific heat of urea at different
temperatures.
Temperature [0C] Specific heat [cal/g0 c]
0 0. 344
50 0.397
100 0.451
150 0. 504
Table 1-2-4
Table 1—2—5 shows the properties of saturated solution
of urea in ammonia.
Temperature
[0C]
Urea in solution wt% Vapour
pressure of
solution
[ mmHg]
0 36 4
11
20
49
7
40 68 9.4
60 79 10.8
80 81 13.3
100 90 12.5
120 96 5
Table 1-2-5
Ammonium carbamate is of a inciting point of about
150°C and heat of fusion in the range of 4000 cal/gmol.
and it is soluble in water.
1-3 Reaction Kinetics:—
Ammonia reacts with carbon dioxide to form Ammonium
carbamate
by the reaction:
2NH3 + CO2 NH2C02NH4 (1)
This 2 is a third order reaction of equilibrium
constant Kp = PNH 32 PCO2
At high temperature and pressure
the reaction is instantaneous.
Ammonium carbamate is dehydrated to urea as follows:
NH2 CO2 NH4 NH2 CONII2 + H 20 (2)
carbamate urea
12
(1 — x)(a — 2x)
Frejacques made investigations of the equilibrium
conversion of Ammonium carbamate to urea with excess
ammonia and with excess water, in the temperature range
from 130 to 210°c. He derived the following formula for
the equilibrium constant
k = x(b + x)(1 +a + b — x)
where
K = equilibrium constant
X= mole fraction of total initial CO2 converted to urea
a = mole ratio of NH3 to CO2 in total mixture
b mole ratio of H2O to CO2 in the total initial mixture
13
before the urea reaction takes place
The overall given below will clarify the terms in the
above derivation
aNH3 + NH2O + CO2 xNH2CONH2 + (1 — x)NH4CONH2 +
(a — 2)NH3 3 + (b + x)H2O
The equilibrium conversion K can be increased either by
increasing the reactor temperature or by dehydrating
ammonium carbamate in the presence of excess ammonia.
2NH3 + CO2 NH2CONH2 + H2O
Excess ammonia shifts the reaction to the right side of
the overall equation and in the presence of excess
water the reaction is shifted to the left.
Mavrovic published a monographic presentation of the
reaction equilibrium constant (k). He found the actual
equilibrium constant at various temperatures given in
the table 1—4—1.
Temperature [oC] Reaction equilibrium constant
140 0.695
150 0.850
160 0.075
170 1.375
180 1.8
190 2.38
200 3.18
14
Table 1—3—1
1-4 Raw Materials For Urea Production :T
he raw materials for the production of urea are ammonia
and
carbon dioxide. Natural gas is the main source of
producing these raw materials. The carbon dioxide is
often passed over a catalyst to remove any remaining
traces of free oxygen, the main source of corrosion in
the urea reaction.
Ammonia synthesis gas is prepared by high-pressure
catalytic reforming of hydrocarbon feed gas, in the
primary reformer with super—healed stream and in the
secondary reformer with air to furnish nitrogen. The
amount of air is adjusted eventually to supply a H2/N2
molar ratio of 3:1. In the experimental plant in the
southern part of Khartoum, Naphtha is the main source
of producing the raw materials of urea production, and
this require very high cost.
15
1-5 Uses Of Urea;—
There are many uses of urea in our daily life:
1— The major uses of urea is its importance as a
fertilizer which mainly due to its high nitrogen
content (about 46%). Also urea solution is converted
into ammonium bicarbonate by the action of soil
bacteria.
Ammonium bicarbonate is quickly changed by soil acid
into a form stable against wash away loss. The ease of
application and unexistence of any residual matter in
16
the soil are some causes to make urea used as a good
fertilizer.
2- Urea is used in the manufacture of thermosetti.ng
resins, the most important of which is urea
formaldehyde resin.
3— Urea is also used in feeding of animals (cattle), it
can replace for up to one third of the total protein
intake by cattle.
4— Urea is also used as a softener for various
cellulose products, e.g. glassing, cellophane paper and
wood. Urea is also used as an additive to reduce the
Viscocity of starch and others. It is also used in the
manufacture of caffeine , ethyurea hydrazine , and many
other chemical industries.
17
CHAPTER – TWOMANUFACTURING PROCESS
AND
PROCESS DESCRIPTION
18
CHAPTER TWO:
1AMUFACTURING CESS AND PROCESS DESCRIPTION :
2-1 Urea can be produced by hydrolysis of cyammide
according to t he equation:
CN.NH2 + H2O C O(NH2)
At 33 to 53 atmosphere urea can also be formed by
heating ammonium carbamate. Equilibrium is obtained at
130 to 1500C with a 30 to 45% yield.
The common method of manufacture is to combine ammonia
and carbon dioxide under pressure to form ammonium
Carbamate which is then decomposed into urea and water.
The un reacted carbon dioxide and ammonia are recovered
and recycled to the synthesis operation.
Table 2—1—1 shows the number of processes available for
urea manufacturing:
Type of Dupant Pechiner
y
Montica
-
Inventa Chemica
l
operatio
n
carbamt
e
carbamte tini pa aqueous MEA
used
recycle
d
recycled rtial urea n— to abso
—
Reactor wit NH3 with oil liquid itrate rb C03H3
conditio & H2O recycle for NH3 Recycle
19
n d
Recover
y
Temperature 200 180 180 200 185
Pressure
[atm]
400 200 200 200 170
Lining silver lead stain— undis Silver
less closed
NH3;CO2; H2O; 5:1:0.73 2:1:0 3:4:1 2:1:0 6:1:0
CO conversion
in Autoclave 70 50 52 50 76
20
Total 70 50 88 50 76
NH3 conversion
in Autoclave
24 50 32 50 25
Total 24 50 72.5 50 73
2—2 Solution Recycled Process:—
The process employs aqueous urea nitrate to recover
ammonia
without CO current absorption of the carbon dioxide.
The ammonia and carbon dioxide are recompress and fed
with the make—up carbon dioxide and ammonia to
converter.
2—3 Once Through Process:
In this process, un reacted ammonium can be used by
recycling and un reacted carbon dioxide wasted or
utilized elsewhere. Recycling un reacted ammonia and
carbon dioxide may cause formation of solid ammonium
carbamate. The recycling of NH3 and CO2 results in
higher NH3/CO2 ratio. Ammonia conversion is 32% and this
is the least expensive process.
2—4 Total Recycle Process:—
The hot gases from decomposers are re circulated to the
21
reactor, there are different types adopted for total
recycle processes which can be briefed into:
(i) Hot gas mixture recycle:-
This method includes compression of the NH3/C02 gas
mixture in the five stages with air cooling between
stages. The gas mixture compressed to 2700C is very
corrosive and the tenPerat1jre control is very
difficult. This process is of high capital costs,
difficult operation and very high maintenance costs.
(ii) Separated gas recycle:-
This process involves the absorption of either ammonia
r carbon dioxide in a selective solvent, e.g. (mono
ethanol amine) and urea nitrate, resulting in their
sepaation and then re circulating back to the urea
synthesis reactor independently.
(iii) Carbonate solution recycle :T
his process involves the absorption of ammonia
and carbon
dioxide gas mixture iii water and recirculation
as aquous ammonia carbamate solution. The excess
ammonia can be stripped and re circulated
separately.
(iv) Carbamate slurry recycle: T
his process involves the reaction of the ammonia
and
22
carbon dioxide to form solid ammonium carbamate
in a non— aquous medium, such as mineral, oil or
liquid ammonia. The carbamate is re circulated
back t” the urea synthesis reactor.
A special purification is required to separate
the medium from the urea solution.
2—5 Process Description:—.
This description was carried up with reference
to the process flow sheet (fig. 2—1).
Liquid ammonia from vessels V—101 and V-105 were
combined, passed through several pumps,
preheated to 165°F by E-105 and fed to the
bottom of reactor R—l01. Carbon dioxide was
cooled to 101 °F by E—l01 and then compressed to
greater than 3615 psia
295°F by compressors. A liquid recycle stream
containing water, ammonia ,urea , ammonium
carbamate and some biuret also fed to the
reactor.
The reactor (R—l01) was operated adiabatically
at 3516 psia and 365°F. The total N113/C02 mole
ratic was 4.6. Half an hour is satisfying to the
residence time in the reaction. The percent of
the urea CO2 per total CO2 was 68%.
23
The mixture from the reactor was passed through
a pressure reduction valve to bring the pressure
to .bout 305 psia and the temperature to 250°F,
the stream was fed near the top of the high
pressure C-101. Heat was supplied to a re boiler
on the high pressure decomposer C—101 and about
85% of the ammonium carbamate was decomposed to
ammonia and carbon dioxide which exist with
water to the top of the decomposer.
24
The exit stream from the bottom of 101 was
cooled in E—107 and fed near the top of the low
pressure decomposer which operated at 43 psia.
85% of the entering carbamate was decomposed and
NH ,CO and water was the exit stream. The liquid
stream from the bottom was fed to gas separator
0—103.
The remaining carbamate decomposed in C—103
which was operated at 19 psia. Heat was supplied
to the liquid in the bottom of the gas separator
to increase the water content of the gas stream
leaving the top of 0-103. 76.4% by weight was
the amount of urea (with 8% of urea was biuret)
in the liquid stream from the bottom of C—103 •
The remaining (which was water) was cooled to
176°F, and fed to V—102 A, and through filter S—
1O1 A, preparation vessels V—103 A & B and S—101
B, the impurities were removed.
Urea solution was sent to crystallizers E-201 A
& B which was operating ,,at, 158°F. Heat of
crystallization was used to vaporize water in
25
the crystallizers. Urea slurry was fed to
centrifuges S—201 A & B, dried to 0.3% water by
weight in E—202.
Urea prills exited front the prill tower M-202
at 175°F were moved by convertor M—203 to
elevator M—301. Materials were removed by screen
14—302 and were melted in 14—303 and were
recycled to the process.
The mother liquor was fed to surge tanks T—201 A
& B and most of it was recycled to the
crystallizers and the other part was recycled to
the condenser E—116 where it was joined by the
gas stream from gas separator.
The liquid exit stream from the bottom of
condenser E—116 (containing ammonium carbamate,
ammonia, water, urea and biuret) was fed at the
top of low pressure absorber E—115. The gas
stream from C—102 was fed near the bottom of E—
115. Inerts were removed from the system and the
gas stream from 0—101 was fed to the bottom of
C-104.
In high pressure absorber 0—104, ammonia was
removed at the top and was condensed. Some of
the ammonia was returned to the column as reflux
26
and the other part was passed through surge
vessel V—105 and was recycle to reactor R—10l.
The bottom stream from 0—104 which contains
water, ammonia, and ammonium carbamate
was recycled to reactor 11—101.
A circulating stream in the high pressure
absorber was withdrawn from the bottom of C—104
and was passed through exchanger E—113, to
supply heat to the crystallizer. The stream was
then passed through cooler E—112 where the
remaining heat of absorbtion was removed and the
stream re—enters the column just above the lower
packed section.
27
Chapter
three
(The material
balance)
28
CHAPTER THREES —
THE MATERIAL BALANCE,—
To carry out the calculations of material
balance, Some
information was used in this calculation,
First it is necessary to know the
composij1 of recycle carbamate solution, which
have 34 mole Z water, mole X urea, 35 mole %
ammonium carbamate and mole X of ammonia.
Second, the degree of conversion of urea
C02/total CO2 i taken as 0.68, and NH3/Co2 ratio
is taken as 4.6.
Third, the conditions of operation are
such that the free CO2 and ‘*l3 in the feed are
completely converted t urea.
• Fourth, the liquid stream from bottom of
C103 contains about
• 76.4% urea and 23.6% water and the product
urea contains biuret which is 0.8% of the
product urea.
29
Fifth, mother liquor contains 0.35 kg water
per kg of urn 5% of the mother liquor is
recycled to E—116.
Sixth , a basis of 3. hour was used for
calculation and U product is then 31250 kg urea
and 0.8% of product urea is calculated as
biuret.
Seventh, streams (5), (7) and (9) contain
co2, NH3 and water vapour. The amount of CO2 and
NH3 were known fro carbamate decomposition and
amount of water was
• determined by trial and error procedure1 using
pseudocritical method. • -
Calculations of material balance are shown in
this chapter.
The material balance
30
* basis aria –hour
* Reactions forming urea are
CO2 + 2NH3 NH2CO2NH4
carbon dioxide + ammonia : ammonium
carbamate
NH2CO2NH4 NH2CO NIl2 NH2 +H2O ammonium
carbamate ammonium carbamate urea + water
2H2O + CONH2 NH2 +CONHCONH2 + NH3
urea biuret + ammonia
molecular weights :
Components M.W (Kg / kmo1)
Urea NH2CONH2 60
CARBAMATE NH2CO2NH4 78
Ammonia NH3 17
Carbon dioxide co2 44
Biuret nh2conhconh2 103
Water h2o 18
31
Materiab balance around the reactor r – 101 :-
Conditions : -
Conversition ( co2 in urea / total co2 ) = 0.68
Mole reaction including co2 & NH3 IN RECYCLE
STREAM NH3 / co2 = 4.6
Recycle carbonate composition :
Component Mole %
Water 34
Urea 10
Carbonate 35
Ammonia 21
From equation (1) and (2) production rate mole
of co2 = mole of urea = 31250
60 = 520.8 k mole /hr
Kgs of co2 = 520.8 x 44 = 22916.67 kg/hr
From equation (1)
32
Mole of NH3 = 2∗31250
60 = 1041.67 K MOLE/HR
Kgs of NH3 = 1041.67 X 17 = 17708.4 kg/hr
Assumption :-
Recycle carbamate = z K MOLE/HR
Recycle ammonia = x K MOLE/HR
0.68 co2 in urea /total co2
0. 68 = 520 .8+0.17520.8+0.17+0.35 z
Z = 813.86 kmole /hr
NH3 / CO2 ratio = 4.6
4.6 = 1041.67+X+0.217+0.35 Z X 2+0.1 Z X 252 .8+0.1 Z+0.35 Z
Ammonia in (4) = recycle ammonia + ammonia in carbamate solution + ammonia formed from formation of buiret = 36301.8 + 2895 + 36.48 = 39233.3 kg/hr
Buiret in (4) = (31250+4882 ) 0.70.993∗100
= 254.7 kg/hr
Stream (4)
Component Wt (kg/hr) Wt %
Urea
NH
Water
Carbamate
36132.0
39233.0
14433.0
254.7
32.2
35.0
12.8
0.2
36
22218.0 19.8
Total 112270.94 100%
Hint : figure (3 – 1) show the material balance around the reactor
37
Fig 3 -1 :Material balance Around the reactor
38
REACTOR R 101
4
1
3
19
18
2
Material balance around decomposers C—101, C—102 & 0—103:—
(see figure 3-2) Overall material balance:- stream(4) a stream(6) + stream(s). + stream(9) + stream (7) * Material balance around C—101 high pressure decomposer) overall balance
stream(4) = stream (6) + stream (5) * In C—101, 85Z of carbamate decomposed to kg/hr = 204.85 kmol/hr CO2 in (5) = 0.85 x 294.85 x 44 = 10653.39 kg/hr • NH3 in (5) a 0.95 x 294.85 x 2 x 17 = 0232.2 kg/hr
To calculate amount of water removed by C—101, pseudocritical method was used .
T PC = 0.32 * 304.2 + 0.65 * 405.6 + 0.03 * 647.3 = 380 0K
Pseudoreduced pressure P
PPR = P/PPR = 21/103.55 = 0.2
And pseudo reduced temperature T P R
T P R = T/T P R = 438/380.4 = 1.15
From compressability factor chart we find
Zmix = 0.96
The volume of gas = Zmix RT/P
= (0.96 * 8.314 *438) = 166.5 m3 /kmo
40
Using Dalton’s low
V mix = RTP ∑
i=1
n
zi y i
From charts
z(CO2) = 0.99
z(NH3) = 0.96
z(H2O) = 0.90
V mix = 8.314∗438 (0.32∗0.99+0.65∗0.96+0.03∗0.9)
21 = 167.8 m3 /kmol
Repeat this method using other values of amount of water removed and calculate the volume of gass by the two methods above . until the different be minimum and then find the correct amount of water . Then amount of water removed = 30 kmol = 540 kg/hr
NH3 in stream (5) = NH3 in (4) + NH3 formed from decomposition of carbamate =39233 + 8232.2 = 47465 kg/hr
Composition of stream (5) is
Component Wt (kg/hr) Wt %
NH
CO2
H2O
47465
10653
540
81
18.1
0.09
Total 58658 100
For decomposer C- 102 :
Stream (6)
41
Component Wt (kg/hr) Wt %
Urea
Biuret
Carbamate
Water
36132
254.7
3332.7
13893
67.3
0.48
6.21
26.01
Total 53612.4 100
Over all material balance around C - 102 STREAM (6) = STREAM (7) + STREAM (8) Also 85% of carbamate were decomposed .
42
Fig3 material balance around decomposer :
43
5 7 9
4
C103C102C101
6 8 10
As in C - 101 , using pseudocritical method we can find the amount of water removed in C - 102 = 50 kmol/hr = 900 kg/hr
NH3 in (7) = 0.85 * 3332.7∗2∗17
78 = 1234.8 kg/hr
CO3 in (7) = 0.85 * 3332.7∗44
78 = 1598 kg/hr
Stream (7)
Component Wt (kg/hr) Wt %
H2O
CO2
NH3
900
1589
1234.8
24.1
42.8
33.1
Total 3732.8 100
Stream (8)
Urea in stream (8) = 36132 kg/hr
Biuret in stream (8) = 0.15 * 3332.7∗78
78 = 500 kg
Water in (8) = water in (6) – water in (7)
= 13893 – 900
= 12993 kg/hr
44
Stream (8)
Component Wt (kg/hr) Wt %
Urea
Biuret
Carbamate
Water
36132.0
254.7
500.0
12993.0
72.94
0.51
1.0
26.0
Total 49879.7 100
Decomposer C- 103 material balance :-
Stream (8) = stream (9) + stream (10)
Stream (10) contains 76% urea , 23.3 % water and 0.7% Biuret .
H2O in (10) = 36132∗0.233
0.76 = 11077 kg/hr
Biuret in (10) = 36132∗0.007
0.76 = 254.7 kg/hr
45
Fig 3-3
Material balance Around the dryer E- 202
46
AIRE 202
25
26
27
Stream (10)
Component Wt (kg/hr) Wt %
Urea
Water
Biuret
36132
11077
254.7
76.0
23.3
0.7
Total 47463.7 100
Sream (9) = stream (b) – stream (10)
Stream (9)
Component Wt (kg/hr) Wt %
CO2
NH3
H2O
282
218
1916
11.7
9.0
79.3
Total 2416 100
Material Balance around dryer E- 202:-
See fig 3-3
47
Since stream (27) contains crystal composed of
0.3% water , then H2O in (27) = 31250+221∗0.3
100 = 318
kg/hr
Stream (25)
Component Wt (kg/hr) Wt %
Urea
Water
Biuret
31250
318
221
98.3
1.0
0.7
Total 31789 100
Material Balance around centrifuges & crystallizers ( E- 201 A& B, T201 A & B , S201 A & B ) : -
Water in (20) = water in (19) - water in (5) = 4981 – 540 = 4441 kg/hr
Stream (20) :-
Component Wt (kg/hr) Wt %
Urea
Water
CO2
NH3
4882
4441
1880
1427
38.6
38.2
14.9
11.3
Total 12630 100
Low pressure absorber E – 115 :-
Over all material balance
Stream (7) + stream (21) = inerts + stream (20)
NH3 in inerts = 218 + 1234.8 – 1427 = 25.8 kg/hr
CO2 in inerts = 282.1 + 1598 – 1880 = 0.1 kg/hr
Water in inerts = 3624.7 + 900 –4441 =83.7 kg/hr
54
Inerts stream :-
Component Wt (kg/hr) Wt %
Water
CO2
NH3
83.7
0.1
25.8
76.4
0.1
23.5
Total 109.5 100
55
Fig : 3 – 5 Material Balance Around Condenser & Absorber :
56
518
79
22
21
2019
C - 104 E - 115 E - 116
Chapter four
The energy Balance
57
CHAPTER FOUR : --
THE ENERGY Balance :
The general energy balance equation could be written as:—
accumulation of energy within the system] =[transfer of energy into system through the boundary]- [energy transfer through system boundary]+[energy generation within the system ]
- [energy conception Within the system The enthalpy of each stream was calculated by summing
enthalpies of its components with respect to a refer temperature (300c).
The enthalpy is given by:
∆ H=Mcp∆ t
where M = mass of stream in kg
Cp = mean heat capacity (kcal/kgoc)
∆ t = temperature difference
58
The Energy balance
Reactor R1Ol Energy balance:-
(see figure 4—1)
Overall enthalpy balance:
H2 + + H3 H19 + H4 (heat of solution of urea) + ∆ HS.
For the reaction of 23105.5 kg/hr of carbon dioxide and 54155 kg/hr of ammonia to produce 36132 kg/hr of urea, make 32% of the reactor products. Operating Conditions:— Temperature = 458°K Pressure = 3515 psia Conversion 0.72 Reactor No. : R1Ol Reactor Type : Continuous stirred tank reactor Reactor Size : 3 Volume : 38 m Length : 15 m Diameter : 2 m Shell thickness : 80 mm Ends Type : hemispherical shape. Material of construction stainless steel (316
78
SS)
CHAPTER FIVE: THE DESIGN OF THE REACTOR: 5—1 Introduction:— The aim in designing a reactor is to produce a specific product at a given rate from known reactants. In order to achieve the best design for the reactor, two most important, questions are to be settled: 1— the type of reactor to be used and whether its process is batch or continuous, and will the reactor operate isothermally or adiabatically ? 2- the physical, conditions of the reactants at the inlet of the reactor, such as pressure, temperature and compositions. Many requirements are satisfied by industrial chemical reactors: 1— chemical factors: involve kinetics of the reaction and sufficient residence time. 2— Mass transfer factors: with heterogeneous
79
reactions the reaction rate may be controlled by rate of diffusion of the reacting species. 3— heat transfer factors: removal or addition of heat of reaction. 4— Safety factors: confinement of hazardous reactants and products and control of the reaction and process conditions. The characteristics to classify reactor designs are: 1— mode of operation: whether is batch or continuous. 2— Phases present: which determine whether the reaction is homogenous or heterogeneous3— Reactor geometry: whether the reactor is tubular, stirred tank, plug flow or back—mixed reactor. 5-2 Chemical kinetics & rate equations:- The rate of the reaction decreases as the concentrations of the reactants decrease. To calculate the size of the reactor we need to know how the rate of the reactor, at any time or at any point in the reactor, depends on the concentrations of the reactants. The •reaction rate also increases rapidly with increasing temperature. Determination of the rate equation is very important to design a reactor. 5—3 Design equation:— This equation gives the relationship between the volume of the reactor, reaction rate and flow rate of the limited reactant. Its determination depends on the type of the reactor used. 5—4Calculations of the reactor size:
80
The calculations of the diameter and high of the reactor are shown in the following paragraphs:
5—4—1 Design and rate equation. The reactor Used in this project was continuous stirred tank reactor with design equation given by (5-1)
VF A O
= X
A−RA
where V = volume of reactants contained in the reactor [m] F AO
= flow rate of the limited reactant [kmol/sec]
X❑ = conversion of limited reactant (A) to
product A−R A
= rate of reaction [kmol/n1 .sec] Our reaction is a third order reaction which has a rate equation given by:
81
A-r = k * C3AP
(1−X )3
¿¿¿ (5 _ 2)
where K is the equilibrium reaction constant
∑A
X = No .of moles of PRODUCT−No . of molesof of reactantsno of moles of reactants
and CAO initial concentration of the limited reaction [kmol/m I 5-4-2 Given data: a. The residence time for forming urea = 30 min = 30 x 60 1800 sec b. The conversion of limited reactant (carbon dioxide) to product (urea) was given as 0.72 = 0.72 c. Flow rate of CO2 23105.5 kg/hr = 23105.5 0.146 kmol/sec 44 x 3600 d. Reactor temperature = 185°c e. Reactor pressure 3515 psia 5-4—3 Calculation of q & E. :— From our reactions:
2NH3 + CO2 NH2 NH4CO2NH2
NH4CO2NH2 NH2CONH2 + H2O 2NH3 + CO2 NH2CONH2 + H2O
2 moles 1 mole 1 mole 1 mole Number of moles of reactants = 3 Number of moles of products = 2
EA = 2 −¿3
¿ 3 = - 13
To calculate C we have:
82
PV nRT
P = N RTV = Crt
C = PRT
PCO2 = 3515 PSIA = 3515
14.7 * 101.3 = 24222.4 KN /M2
T = 146°c = 419°K R = 8.314 kmol/°K 3 CAO = 24222.415 = 6.95 kmol/m 419 x 8.314 5-4—4 Calculation of reaction constant (k):T o calculate the reaction constant (k) we need the residence time (t) and then the calculation was carried out according to the equation t = CAO X (5—3)
—rA (1 +EA XA)
from equation (5—2) 3
A-r = k cAO ¿¿
substitute equation (2) in equation (3) to give
t = CA 0
KC AO X A
3 (1+EA XA)3
(1−XA ) ¿¿
83
OR K = XTCAO
(1+EA X A)
2
(1−X A )3
= 0.72(1 – 0.24) 2 1800 * (6.95)2 (1 – 0.72)3
Then from equation (5—1) we can calculate the volume as follows: —
V = F AO X A
−R =
F AO
KC AO X A
3 (1+EA X A)
3
(1−X )3
= 0.146 * 0.72 (1 – 0.24) 3 = 28.5 m3
0.00022 * [6.95(1- 0.72)]3
Assume the reactor is 75% full V = 28.5 = 38 m3 0.75 5—4-5 Determination of diameter and height:- For our reactor, a closed cylindrical tank with formed heads on both sides were used. For high pressure vessels a hemispherical end was used. Hence the total volume of the tank is given by the combination of shell volume with the volume of the equivalent Cylinder for the ends ,i.e.
v = π4 Di3 l + π
6 Di3 (5 – 4)
where V = total volume of he vessel = 38 m3 D = internal diameter of the vessel L = hight of the vessel Assume the value of L/D ratio = 8
V = x π4 * 8Di3 + π
6 Di3
4 6 = 13 π Di3 (5—5) 6
84
Di = (6V/13 7c ) = (6 x 38/13 7c ) = 1.77 L = 1.77 x 8 = 14.2 m take Di = 2 m and L = 15 m
85
19
4
R - 101
FIG 5 – 1 MATERIAL & ENERGY BALANCE AROUND REACTOR R – 101
86
2 3NH3
CO2 = 231055
hi
ho
FIG 5 – 2 HEMISPHERICAL HEAD OF THE REACTOR R 101
5—5 Mechanical Design of The Reactor:— 5—5—1 Introduction:_ The basic data for mechanical design of a vessel are:— 1. vessel function 2. process materials and services 3. operating and design temperature and pressure
4. material of construction 5. vessel dimensions and orientation 6. type of vessel heads to be used 7. openings and connections required 8. specification of heating & cooling jackets or coils 9. type of agitator 10. specification of internal fittings. 5—5—2 Pressure, temperature & stress for design:— For vessels under internal pressure, the design pressure is taken as 5—10% above the working pressure, The working pressure P = 3515 Psia
87
Design pressure = 1.1 x 3515 = 3-367 Psia A suitable margin for fluctuation was added to the temperature to get the design temperature. Working temperature = 185°c take design temperature = 200 The design stress f was given, by the relation f 1/s x minimum tensile strength at working temperature where s is a safety factor. 5—5-3 Material of construction:_ - The important characteristics to be considered when selecting the material of construction are:— 1. mechanical properties 2. the effect of the physical properties on the mechanical properties 3. corrosion resistance 4. availability in standard sizes 5. ease of fabrication 6. cost. To prevent corrosion by ammonium carbamate, stainless steel is used as a material of construction (304—se) is used below 130°C (316—SS) is used above 130’c Since our reactor was operated at 185 C, stainless steel (316) was used for its design. 5—5—4 Shell wall thickness calculations:_ The following relations were used to calculate the minimum thickness needed for the shell wall:
z7 = ((π /4 )PD 2 - W π t(DI + t )
Z = p(Di + t) /2t
Z = ( z2 – z z7 +z7)
88
where f = design stress [ psia ] P = pressure [ psia ] t = shell thickness [in.] Di= internal diameter of. shell. [in.] w=total weight of vessel [ lb I z= longitudinal stress [ psia ] z = hoop stress [ psia ] -4]- = equivalent stress [ psia j For design the equivalent stress (n.) must not be greater than the design stress (f) f = 1/S x tensile strength S= safety factor taken as 1.4 The tensile strength for (316 stain—less steel) is 520 N/mm 75459 Psia. f = x 75459 = 53899 psia 1.4 The total equivalent weight of the reactor and its contents was given by W = Wv + Wf where Wv = weight of vessel
89
Wf weight of fluid inside the reactor Wf = V vessel ,•is the density of mixture based on the outlet stream (i.e. stream 4). According to the table below-we can calculate the density of the mixture in stream (4), (see chapter 2).
Component [kgmole/m] xi
Co2
NH3
Urea
H2O
Carbonate
17.66
37.59
22.25
55.40
21.05
0.00
0.35
0.32
0.13
0.20
Tale (5-1): Analysj8 of Components in stream (4)
Molar e ∑ fi xi = 31.69 kgmol/ average molecular weig1t of mixture = 43.09 kg/kmol fmix = 31.6s X 43.09 = 1305 kg/kmol = 0.049
Vvessel = (π4
D2L) + π6 Di
Di = 2 m = 80 in, L = 15 in = 600 in.
Vvessel = [π4(80)2 x 600] + π
6 (8)3 = 3284011,5 in3
4 6 WF = 3284011.5 X 0.049 = 160916.56 lb
90
WV = V * fsteel 489 lb/ft3 0.283 lb/in3 Vesteel = 7 L(D4 + t) = X 600(80 + t) W = V4 = 600(80 + t) x 0.283 = 533.44(80 + t) lb total weight W = 160916.56 + 33.44(80 + t)
= [6135978.7 — 169.8(80 + t)] / [t(80 + t)] z = 3867(80 + t)/2t = 1933.5(80 + t)/t ze = (a — n.. + n. ) ° The thickness (t) was calculated by trial and error method to obtain the suitable amount of the equivalent stress ( Trials: Trail No. t[in.] z [psia]1 2.00 68691.623 2 2..50 55299.356 3 2.75 50433.68 4 3.00 46377.09
91
minimum thickness required 2.75 in. 6.875 cm take reactor thickness 8 cm 5—5—5 Ends specjfjajo8: 9 Since the reactor wits operated at high pressure, hemispherical shape was used for designing the ends. It is the strongest shape, capable o resisting about twice the pressure of a torispherical head of the same thickness. The only disadvantage of the hemispherical heads was its forming cost, which is higher than the other shapes. For equal stress in the cylindrical section and hemispherical head of a vessel the thickness of the head need only be half that of the cylinder. The optimum thickness ratio is normally taken as 0.6. The thickness of the ends was, 8 x 0.6 = 4.8 cm take end thickness as 5 cm 5—5—6 Corrosion allowance:— Corrosion allowance should be based on experience with the material of construction. Stainless steel is very resistant to corrosion and needs no large allowance, we can take a corrosion allowance of 5mm for the purposes of this project. corrosion allowance = 0.5 cm 5—5—7 General observations:_ 1. Stainless steel is a very expensive material and may be economically Unacceptable for design, another material which
92
can be used instead. Generally, carbon steel is the most commonly used engineering material and can be suggested instead of (316—ss). Carbon steel need corrosion allowance greater than stainless steel. 55
2. The reaction operates adiabatically and there is no cooling or heating system. 3. For the economical purposes, elliptical dished type can take - place of the hemispherical heads. 4. A scaled representation for the reactor and ends were shown in figure (5-3)&(5-2). 56
93
94
5
585 * 10-2
Fig 5 – 3 AVERICAL SECTION FOUR REACTOR R - 101
Chapter – six95
Special task process control
CHAPTER SIX :-
SPECIAL TASK PROCESS CONTROL :-
6—1 Introduction: Control is accomplished by measuring the Cor1tIOi1ed variable and comparing this wits the desired value or et point, an adjusting the manipulated variable which has a direct effect on the controlled variable until the desired value is obtained Figure 6—1 & 6—2 Show Simple feed back control system.
96
D
Fig 6 – 1 simple feeds back control system .
Fig 6 – 2block diagram for the system in fig 6 - 1
97
* The measured value (B) is compared with the desired value (R) to produce an error ( ) such that R - B. The controller produces an output which a function of the magnitude • This is fed to control value in the steam line which make the necessary modif1catbon to satisfy the desired value. 6—2 Control- objective: We control in order to:— 1. keep the process variabe5 within the process limits 2. provide alarms and automatic shut—down systems 3. achieve the design product 0utpUt 4. maintain the product composition 0fl within the specified quality standards 5. operate at the lowest production cost, compared with the other objectives.
98
In a typical chemical processing plant, these objectives are achieved by a combiflat0fl of automat control, manual monitoring and laboratory analysis
6—3 Typical con0j 6—3-1 Level control:-In any equip where an interface exists between two phac05 some means of maintaining the interface at the required level must be provided. This may be incorporated by automatic control of the flow from the equjp011 A typical for level Control is shown in figure. (6—3). Fig. 6—3
Level control
6—3—2 Pressure control
99
1 : — It is necessary for most Systems handling vapour The method of control depends on the nature of the process Figure (6—4a&b) show typical Sc]1enes f pressure control
Fig. 6—4-b Pressure control of a Condenser by varying the heat transfer area.
100
Fig. 6-4-a Venting of non- condensable after a Condenser
6—3-3 Flow control:-It is usually associated with inventory control in a storage tank or other equipmentTo provide flow Control at a fixed speed and supplying a near in fig. (6—5 a&b).
(6—5 a&b).
Flow control for a reciprocating pump.
101
fc
Fig.6_5 b. Alternative scheme for a centrifugal compressor or Pump.
6—3—4 Temperature control:- it is necessary for most chemical system ha significant affect on phy0 and chemical Properties o product, reactor rate pressure, etc. Many devices are used for measuring temperature and each ha its own characteristics and liinitatio115._
102
a) expansion thermometer b) change of state thermometer c) Instruments exploiting electrical phenomena d) radiation and optical pyrometers * Thermocouple is the most common type of the temperature control see fig. 6—6.
6—4 Reactor control:_ - The schemes used for reactor control depends on the proc33 and type of the reactor. If the reactor dynamics are suitable the product composjti0 can be monitored continuously and the reactor COfldjj015 and feed flows controlled automatically to maintain the desired product composjtI0 and Yield. Reactor temperature will normally be controlled by regulating the flow of the heating or cooling medium. Material balance will be necessary to maintain the correct flow of reactants to the reactor. reactor and the flow of products un reacted materials from the exit Stream. • There are three streams entering into the reactor, and one Stream (2) containing carbon dioxide from it8 source was coztro1led by measuring the pressure in the bottom of the reactor -S and accordingly adjusting the flow of inlet CO to the reactor.
103
Stream (3) Containing ammonia from its source and from the recycled stream was controlled by regu1atjig globe valve for measuring the flow of ammonia. The exit stream (stream 4) was contro1ed by measuring the temperature in the top of the reactor. Stream (19) which is a recycled stream was controlled in the high pressure absorber and was entered to the reactor as a controllable stream.
Fig (6—7) shows a suggested control system for the reactor.
104
105
carbamate
From high pressure absorber
R 101
Fc
2
pc
REFERENCES 1. R.K.Sinnott Chemical Vol.6 (1983) 2. Coulson & Richardson Chem Eng Vol.3 (1979) 3. RH.Perry and C.Be1 Chem Eng. Ismail You Design of Urea Synthesis P1a Engineering stu for the degree of .Sc.(l984 5. Ib ElWaleed A1 Design of Urea Plant, year Project for the degree of B.sc.(1984 6. Encyclopedia of chemical Technology vol. 21
