OPTIMISATION
OF
ETHYLENE CRACKER
HEMENDRA KHAKHAR
PROJECT SUMMARY
• REQUIREMENT OF SIMULATOR:
PREDICTION OF CONVERSION AND YIELD OF PRODUCTS
WITH CHANGE IN OPERATING CONDITIONS
• SUNDARAM AND FROMENT MODEL:
MOLECULAR REACTION SCHEME ( 8 REACTIONS )
• CONCERNS OF OPTIMISATION:
DOWNSTREAM PLANT CAPACITIES
PLANT FEED CAPACITY
PRICE OF CRUDE OIL
CYCLIC FLUCTUATION IN PETROCHEMICAL PRICES
AVAILABLE REACTOR TIME
INTRODUCTION
• FEED STOCKS FOR THERMAL CRACKING:
ETHANE, PROPANE, LPG, NAPHTHA,GAS OIL,
CRUDE OIL, ETC.
• MAIN PRODUCTS: ETHYLENE, PROPYLENE
• BY PRODUCTS: BUTENE, HYDROGEN, GAS OIL,
FUEL OIL
• OPERATING VARIABLEAS:
TEMPERATURE
PARTIAL PRESSURE OF H/C
RESIDENCE TIME
ETHANE CRACKING FURNACE
PROBLEM DESCRIPTION
• FEED: ETHANE/PROPANE/LPG
• CAPACITY : 200000 lb/hr.
• MAIN PRODUCTS :
ETHYLENE: 100000 lb/hr.
PROPYLENE:20000 lb/hr.
• DOWNSTREAM PLANTS:
ETHYLENE GLYCOL
POLYVINYL CHLORIDE
HIGH DENSITY POLYETHYLENE
POLYPROPYLENE
ISSUES FOR OPTIMISATION
• LARGE NO. OF FEED & PRODUCT STREAMS
NECESSITATING CAREFUL CHOICE OF FEEDS
AND CONDITIONS
• INCREASING FLEXIBILITIES OF THE REACTORS
CLAIMED BOTH FOR FEEDSTOCK UTILISATION
AND EFFLUENT DISTRIBUTION
• RECYCLING STREAMS
• RUNLENGTH OF REACTORS
• RELIABLE PREDICTION OF EFFLUENT
DISTRIBUTION
• REVAMPING OF UNITS
OPTIMISATION MODEL
• THREE WAYS FOR SCALING UP :
DIRECT EXPERIMENTAL SIMULATION
EQUIVALENT TIME AS A DESIGN
PARAMETER
MATHEMATICAL MODELLING
• AVAILABLE MODELS:
EMPIRICAL MODEL
MOLECULAR ( STOICHIOMETRIC ) MODEL
MECHANISTIC MODEL
ADOPTED MODEL
•
•
•
•
•
•
DIFFICULTIES ENCOUNTERED;
1. STIFFNESS OF DIFFERENTIAL EQUATIONS
2. UNKNOWN INITIAL VALUES OF DEQ
STOICHIOMETRIC MODEL:
SUNDARAM & FROMENT MOLECULAR MODEL
MATERIAL BALANCE:
dFi/dz = -  (Siri) * dt2 /4
• ENERGY BALANCE:
dT/dz = 1/ Ficpi { Q(Z)* dt + dt2/4*ri *(- Hi)
REACTION SCHEME
REACTION MODEL OF SUNDARAM & FROMENT
Rxn No. (i)
1.
2.
5.
6.
8.
Reaction
C2H6
2C2H6

C2H4

+
C3H8
H2
+
CH4
C3H6

C2H2
+
C2H4

C4H6
C2H4
+
C2H6

C3H6
C2H2
+
Rxn. No. (j)
CH4
Reaction
3
C3H8

C2H4
+
CH4
4
C3H8

C3H8
+
H2
7
2C3H6

3C2H4
+
CH4
Rec Ethane (x5)
Rec Propane (x6)
Gasoline
Ethane (x1)
Ethylene
Cracker
Propane (x2)
Butadiene
Propylene
Gas oil (x3)
Ethylene
DNG (x4)
Methane
e
Fuel oil
Fuel (x7)
FORMULATION OF OBJECTIVE
FUNCTION
• OBJECTIVE FUNCTION, PROFIT,
f = PRODUCT COST - FEED COST - ENERGY COST
• INEQUALITY CONSTRAINTS:
CRACKER CAPACITY : 200000 lb/hr.
ETHYLENE PROCESSING CAPACITY : 100000 lb/hr.
PROPYLENE PROCESSING CAPACITY : 20000 lb/hr.
• EQUALITY CONSTRAINTS:
ETHANE RECYCLE BALANCE
PROPANE RECYLCE BALANCE
ENERGY BALANCE
RESULTS OF ACTUAL RUN
Sr. No.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Variable
X1
X2
X3
X4
X5
X6
X7
F
Et
P
B
G
Pro
feed
Optimum value
1.0909e5
0.0
0.0
0.0
72727.273
0.0
58864.819
9.9065e5
1.6136e6
25072.727
48436.364
18054.545
1.7052e6
7.1455e5
SENSITIVITY ANALYSIS:RUN IV
CAPACITY : 400000 lb/hr.
ETHYLENE : 100000 lb/hr.
PROPYLENE : 20000 lb/hr.
Sr. No.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Variable
X1
X2
X3
X4
X5
X6
X7
F
Et
P
B
G
Pro
Feed
Optimum value
1.2e5
0
0
0
80000
0
64658
1.0897e6
1.775e6
27580
53280
19860
1.875e6
7.86e5
SENSITIVITY ANALYSIS:RUN II
CAPACITY: 400000 lb/hr.
ETHYLENE: 50000 lb/hr.
PROPYLENE : 20000 lb/hr.
Sr. No.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Variable
X1
X2
X3
X4
X5
X6
X7
F
Et
P
B
G
Pro
feed
Optimum value
60000
0
0
0
40000
0
32793.866
5.4486e5
8.8750e5
13790
26640
9930
9.3786e5
3.93e5
SENSITIVITY ANALYSIS:RUN III
CAPACITY : 400000 lb/hr.
ETHYLENE : 200000 lb/hr.
PROPYLENE : 10000 lb/hr.
Sr. No.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
Variable
X1
X2
X3
X4
X5
X6
X7
F
Et
P
B
G
Pro
feed
Optimum value
2.181e5
0
0
0
1.4545e5
0
1.168e5
1.981e6
3.227e6
50145
96872
36109
3.41e6
1.4291e6
SUGGESTION FOR FUTURE
WORK
• RADICAL REACTION MODEL:
• ETHANE CRACKING:
49 REACTIONS,11 MOLECULAR SPECIES, 9 RADICAL
SPECIES
• PROPANE CRACKING:
80 REACTIONS,11 MOLECULAR SPECIES, 11 RADICAL
SPECIES
•
DEVELOPMENT OF GEAR ALGORITHM TO
SOLVE STIFF DEQ
• REGOROUS ENERGY BALANCE(BOTH
COMBUSTION AND PROCESS SIDE ) AND
MOMENTUM BALANCE REQUIRES
COMPUTATIONAL FLUID DYNAMICS