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 SCHEMEREACTION MODEL OF SUNDARAM & FROMENT

Rxn No. (i) Reaction

1. C2H6 C2H4 + H2

2.2C2H6 C3H8 + CH4

5. C3H6 C2H2 + CH4

6.C2H2 + C2H4 C4H6

8.C2H4 + C2H6 C3H6 + CH4

Rxn. No. (j) Reaction

3 C3H8 C2H4 + CH4

4 C3H8 C3H8 + H2

7 2C3H6 3C2H4

Gasoline

DNG (x4)

Ethane (x1)

Propane (x2)

Butadiene

Propylene

Gas oil (x3)Ethylene

Fuel (x7)

Rec Propane (x6)Rec Ethane (x5)

EthyleneCracker

Methanee

Fuel oil

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 RUNSr. No. Variable Optimum value

1. X1 1.0909e52. X2 0.03. X3 0.04. X4 0.05. X5 72727.2736. X6 0.07. X7 58864.8198. F 9.9065e59. Et 1.6136e6

10. P 25072.72711. B 48436.36412. G 18054.54513. Pro 1.7052e614. feed 7.1455e5

SENSITIVITY ANALYSIS:RUN IVCAPACITY : 400000 lb/hr.

ETHYLENE : 100000 lb/hr.

PROPYLENE : 20000 lb/hr.

Sr. No. Variable Optimum value1. X1 1.2e52. X2 03. X3 04. X4 05. X5 800006. X6 07. X7 646588. F 1.0897e69. Et 1.775e6

10. P 2758011. B 5328012. G 1986013. Pro 1.875e614. Feed 7.86e5

SENSITIVITY ANALYSIS:RUN IICAPACITY: 400000 lb/hr.

ETHYLENE: 50000 lb/hr.

PROPYLENE : 20000 lb/hr.

Sr. No. Variable Optimum value1. X1 600002. X2 03. X3 04. X4 05. X5 400006. X6 07. X7 32793.8668. F 5.4486e59. Et 8.8750e5

10. P 1379011. B 2664012. G 993013. Pro 9.3786e514. feed 3.93e5

SENSITIVITY ANALYSIS:RUN IIICAPACITY : 400000 lb/hr.

ETHYLENE : 200000 lb/hr.

PROPYLENE : 10000 lb/hr.

Sr. No. Variable Optimum value1. X1 2.181e52. X2 03. X3 04. X4 05. X5 1.4545e56. X6 07. X7 1.168e58. F 1.981e69. Et 3.227e6

10. P 5014511. B 9687212. G 3610913. Pro 3.41e614. feed 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