납사분해공장 저온분리공정의 조업조건 최적화

송제훈, 최인석*, 박선원

한국과학기술원 화학공학과

삼성엔지니어링(주) 기술연구소*

Optimization of Operating Conditions

for Cold Recovery Processes of Naphtha Cracking Center

Jehoon Song, In Seok Choi*, and Sunwon Park

Dept. of Chem. Eng., KAIST

Samsung Engineering Co., LTD. R&D Center*

1. Introduction

Naphtha Cracking Center (NCC) is a plant in which ethylene and propylene are produced by thermal cracking of ethane and naphtha. It is well-known as a large scale plant consuming much energy and it is not easy to operate the plant because of its severe operating conditions. For this existing plants, significant economic benefit can be expected by operating at their optimal conditions at which the plant produces ethylene and propylene maximally but consumes utilities minimally which keeping the product within desired specifications. Even though Korea has the fourth largest ethylene production capacity in the world, on-line optimization is not implemented in most ethylene plants in Korea. This paper describes an attempt to optimize the cold recovery section of NCC.

2. Process Description

The ethylene plant to which optimization is implemented has the capacity of 300,000 MT/year and was built by Linde AG license. Main equipment of the cold recovery section of the naphtha cracking center can be listed as follows:

□Deethanizer(T3801, T3802) □Demethanizer(T4101) □C2 Fractionator(T4301) □Depropanizer(T5101)	□C3 Stripper(T5301) □C3 Fractionator(T5501) □Debutanizer(T5701)

The PFD of the cold recovery process of NCC is shown in Figure 1.

Figure . Cold recovery processes of NCC.

3. Modeling and Simulation

3.1 Components

The model includes 26 components shown in Table 1.

3.2 Thermodynamic Property Package

Peng-Robinson(PR) equation is used to represent the thermodynamic behavior of those components.

3.3 Specification of main columns

To define a column, we need to determine the number of theoretical trays, feed tray location, column specifications and their values. These are shown in Table 2.

3.4 Steady-state simulation

The model is verified by comparing steady-state simulation results with practical design data.

4. Optimization

4.1 Objective function

As an objective function for the optimization, we selected the total profit, that is, the revenue from the production of ethylene and propylene minus utility cost.

[eqn: Max ~~~SUM (C_e P_e + C_p P_p )][eqn: - SUM C_MP Q_MP][eqn: - SUM C_LLP Q_LLP][eqn: - SUM C_CW Q_CW]

The types of heating and cooling streams used in main columns are shown in Table 3.

Table 1 Components

No.	Component ID	Formula	Name
1	H2	H2	Hydrogen
2	CO	CO	Carbon Monoxide
3	CO2	CO2	Carbon Dioxide
4	H2S	H2S	Hydrogen Sulfide
5	CH4	CH4	Methane
6	C2H2	C2H2	Acetylene
7	C2H4	C2H4	Ethylene
8	C2H6	C2H6	Ethane
9	C3H4	C3H4	Propadiene
10	C3H6	C3H6	Propylene
11	C3H8	C3H8	Propane
12	C4H4	C4H4	Vinylacetylene
13	C4H6	C4H6	1,3-Butadiene
14	C4H8	C4H8	1-Butene
15	C4H10	C4H10	i-Butane
16	BENZENE	BENZENE	Benzene
17	TOLUENE	TOLUENE	Toluene
18	XYLENES	XYLENES	p-Xylene
19	EB	EB	Ethylbenzene
20	C5	C5	n-Pentane
21	C6	C6	1-Hexene
22	C7	C7	1-Heptene
23	C8	C8	1-Octene
24	C9	C9	n-Propylbenzene
25	OIL	OIL	1-Undecene
26	H2O	H2O	Water

Wash water means hot water which is heated in quench tower. It is excluded form the calculation of the total profit.

The prices of ethylene and propylene are set as 370$/MT and 300$/MT respectively. Money exchange rate was 1200｀/$ at that time. So [eqn: C_e] and [eqn: C_p] are 444｀/kg and 360｀/kg respectively.

Table 2 Number of trays and specifications of main columns

Column	Number of theoretical tray	Feed tray	Column specifications	Specifications value
T3801	28	1,10,12,14	Bottom Flow Rate (kg/h)	64900
T3802	36	13	Bottom Flow Rate (kg/h) Reflux Ratio	43980 1.632
T4001	7	1,7	-	-
T4101	46	11,16,22	Bottom Flow Rate (kg/h) Reflux Flow Rate (kg/h) Distillate Flow Rate (kg/h)	44050 11330 3014
T4301	120	1,92	C2H4 mole Fraction in Top	0.9995
T5101	33	14	Bottom Flow Rate (kg/h) Reflux Ratio	22100 0.933
T5301	5	1	Bottom Flow Rate (kg/h) C3H6 Recovery in Bottom	21790 0.9935
T5501	172	104	Bottom Flow Rate (kg/h) C3H6 mole Fraction in Top	1137 0.996
T5701	23	12	Bottom Flow Rate (kg/h) Reflux Ratio	6300 0.7390

MP steam (45bar, 390℃) is 11.956｀/kg, LLP steam (4.5bar, 180℃) is 10.525｀/kg, and cooling water is 0.02｀/kg. But these are not [eqn: C_MP], [eqn: C_LLP] and [eqn: C_CW], because the unit of them is '｀/kJ'. So we have to calculate the enthalpy change of each utility. The enthalpy change between the state of inlet and outlet can be obtained by calculation like this formula.

[eqn: DELTA H ~=~ int from T_0 to T_inlet C_P (T) ~dt - int from T_0 to T_outlet C_P (T) ~dt + DELTA H_vap]

We can also obtain the same value from the steam table.

Then steam cost is calculated like this.

[eqn: steam ~"cost" ~(｀/h) ~=~ {heat~ duty ~(kJ/h) } over {DELTA H ~(kJ/kg)} × steam ~unit ~"cost" ~(｀/kg)]

Cooling water cost calculation is similar to the steam calculation. But [eqn: DELTA H_vap] is not considered because there is no phase change in the case of cooling water.

4.2 Decision variables

Decision variables selected for this optimization are specification inputs to the main columns.

Table 4 Decision variables

Column	Column Specifications	Low Limit	Specification Values	High Limit
T3801	Bottom Flow Rate (kg/h)	32448.5	64900	71386.7
T3802	Bottom Flow Rate (kg/h) Reflux Ratio	21983.8 0.928	43980 1.632	48364.2 3.712
T4001	-	-	-	-
T4101	Bottom Flow Rate (kg/h) Reflux Flow Rate (kg/h) Distillate Flow Rate (kg/h)	22065.3 7216.32 1507	44050 11330 3014	52074.1 13711.0 6028
T4301	C2H4 mole Fraction in Top	0.99945	0.9995	0.99954
T5101	Bottom Flow Rate (kg/h) Reflux Ratio	1151.2 0.4375	22100 0.933	24312.6 1.750
T5301	Bottom Flow Rate (kg/h) C3H6 Recovery in Bottom	10896.7 0.9930	21790 0.9935	23972.7 0.9980
T5501	Bottom Flow Rate (kg/h) C3H6 mole Fraction in Top	588.60 0.9955	1137 0.996	2177.82 0.9964
T5701	Bottom Flow Rate (kg/h) Reflux Ratio	3830.35 0.3695	6300 0.7390	8426.77 1.478

Table 3 Utilities used in main columns

Column	Condenser	Reboiler
T3801	-	Wash Water
T3802	Propylene Recycle	LLP
T4001	-	-
T4101	Ethylene Recycle	Propylene Recycle
T4301	-	Propylene Recycle
T5101	Propylene Recycle	Wash Water
T5301	Propylene Recycle	Wash Water
T5501	Cooling Water	Wash Water
T5701	Cooling Water	LLP
Compressor	Steam Turbine
C4401 I	MP
C4401 II	MP
C4401 III	MP
C4401 IV	MP

4.3 Process constraints

Process constraints are shown in Table 5.

4.4 Optimization results

Optimization results are shown in Table 6. Steady-state values are compared with the optimized solutions. In Table 7, it is shown that economic benefit increased by 2.6% as a result of optimization of operating conditions. That is, if the plant is operated at  present operating conditions, the total profit is 208,000,000,000｀/year. We can increase the profit to 5,400,000,000｀/year by optimization of operating conditions without additional equipment investment.

4.5 Case studies

4.5.1 Case(1) : When propylene is more expensive than ethylene.

Actually case(1) was happened in June, 1998. Ethylene price was 290$/MT, and propylene was 295$/MT. In this case, economic benefit increases by 2.41% in comparison with the base case. That is, if the plant is operated at present operating conditions, the total profit is 197,000,000,000｀/year. But we can get more profit of 4,700,000,000｀/year by optimization of operating conditions without additional equipment investment.

4.5.2 Case(2) : When the plant is operated at 90% of its capacity.

When product prices are very low, the manager can determine to reduce production. Feed flow rates are reduced by 10% to reduce the production of ethylene and propylene. In this case, the base case is operating at the reduced feed flow rate by 10%. Economic benefit increases by 5% in comparison with the base case. That is, if the plant is operated at present operating conditions, the total profit is 188,000,000,000｀/year. But we can get more profit of 9,400,000,000｀/year by optimization of operating conditions without additional equipment investment.

5. Conclusion

Modeling, steady-state simulation, and optimization were implemented for the cold recovery processes of naphtha cracking center using HYSYS.

Table 7 Comparison between objective function values

		Base Case	Optimized Case
Objective Function Value (won/h)		2.37924e+07	2.44097e+07
T5501	C3H6 production(kg/h)	20672.7	22410.1
	C3H6 price (won/kg)	360	360
	C3H6 revenue (won/h)	7.44217e+06	8.06764e+06
	Qc (kJ/h)	8.81290e+07	1.12637e+08
	CW enthalpy (kJ/kg)	29.2740	29.2740
	CW price (won/kg)	0.02	0.02
	Qc cost (won/h)	60209.8	76953.7
T5701	Qr (kJ/h)	9.44806e+06	8.50135e+06
	LLP enthalpy (kJ/kg)	2184.30	2184.30
	LLP price (won/kg)	10.5250	10.5250
	Qr cost (won/h)	45525.3	40963.6
	Qc (kJ/h)	9.94645e+06	8.96524e+06
	CW enthalpy (kJ/kg)	25.0920	25.0920
	CW price (won/kg)	0.02	0.02
	Qc cost (won/h)	7927.99	7145.89
T4301	C2H4 production (kg/h)	37519.2	37533.6
	C2H4 price (won/kg)	444	444
	C2H4 revenue (won/h)	1.66585e+07	1.66649e+07
C4401 I	duty (kJ/h)	89799.0	89799.0
C4401 II	duty (kJ/h)	3.43072e+06	3.43072e+06
C4401 III	duty (kJ/h)	1.99093e+07	1.99183e+07
C4401 IV	duty (kJ/h)	1.26620e+06	1.26680e+06
	MP enthalpy (kJ/kg)	2596.59	2596.59
	MP cost (won/kg)	11.9560	11.9560
	total MP cost (won/h)	113713	113757
T3802	Qr (kJ/h)	1.79724e+07	1.86839e+07
	LLP enthalpy (kJ/kg)	2339.29	2339.29
	LLP price (won/kg)	10.5250	10.5250
	Qr cost (won/h)	80862.1	54063.1

The model was built on the basis of given practical process design data, and was verified by comparing steady-state simulation results with practical design data. As an objective function for the optimization, we selected maximization of total profit, that is, revenue from the production of ethylene and propylene minus utility costs. Column specifications was selected as decision variables. And overhead flow rate, reflux flow rate, and bottom flow rate had their constraints. The economic profit was increased by 2.59%(5,400,000,000｀/year) as a result of optimization of operating conditions. In the case that propylene was more expensive than ethylene, the economic profit was increased by 2.41%(4,700,000,000｀/year) as a result of the optimization. When the production was reduced  by 10%, the economic profit increased by 5%(9,400,000,000｀/year). If this optimization of operating conditions is carried out on a larger scale plant, more economic benefit is expected.

Acknowledgement

This work was supported by the Korea Science and Engineering Foundation (KOSEF) through the Automation Research Center at POSTECH and by the Korea Ministry of Trade, Industry and Energy through RaCER.

Table 6 Comparisons between base case and optimized cases

Column	Flow Rate or Mol Fraction	Low Limit		Base Case	Optimized Case		High Limit
T3801	Overhead(kg/h)	31694.3	≤	63413.3	62729.8	≤	69727.6
	Bottom(kg/h)	32448.5	≤	64899.9	64800.1	≤	71386.7
T3802	Overhead(kg/h)	44080.0	≤	55061.2	57611.4	≤	63366.0
	Reflux(kg/h)	27337.1	≤	34141.3	37440.3	≤	37588.5
	Bottom(kg/h)	21983.8	≤	43979.9	44629.0	≤	48364.2
T4101	Overhead(kg/h)	14094.9	≤	23293.9	22623.6	≤	26780.3
	Reflux(kg/h)	7216.32	≤	11329.7	12036.0	≤	13711.0
	Bottom(kg/h)	22065.3	≤	44050.1	44709.0	≤	52074.1
T4301	C2H4 frac. in Top	0.99945	≤	0.999501	0.9995	≤	0.99954
	Overhead(kg/h)	70982.8	≤	120230	120297	≤	134867
	Recycle(kg/h)	48474.6	≤	80766.1	80766.1	≤	93717.6
	Bottom(kg/h)	2928.85	≤	5719.10	5151.11	≤	9958.09
T5101	Overhead(kg/h)	21131.8	≤	42289.3	42256.2	≤	46490.0
	Reflux(kg/h)	10199.9	≤	20411.8	18544.2	≤	22439.7
	Bottom(kg/h)	11051.2	≤	22102.4	20917.1	≤	24312.6
T5301	Overhead(kg/h)	72.7500	≤	134.393	111.102	≤	458.325
	Bottom(kg/h)	10896.7	≤	21809.8	23597.1	≤	23972.7
T5501	C3H6 frac. in Top	0.9955	≤	0.99600	0.996	≤	0.9964
	Overhead(kg/h)	380867	≤	306075	380867	≤	390226
	Reflux(kg/h)	181733	≤	285403	358456	≤	363139
	Bottom(kg/h)	588.600	≤	1137.11	1186.98	≤	2177.82
T5701	Overhead(kg/h)	12557.5	≤	27480.1	24721.1	≤	30138.1
	Reflux(kg/h)	5336.70	≤	11677.9	9994.97	≤	15476.4
	Bottom(kg/h)	3830.35	≤	6300.14	6190.88	≤	8426.77

Table 5 Process constraints

Column	Low Limit		Flow Rate or Mole Fraction		High Limit
T3801	31694.3	≤	Overhead(kg/h)	≤	69727.6
	32448.5	≤	Bottom(kg/h)	≤	71386.7
T3802	44080.0	≤	Overhead(kg/h)	≤	63366.0
	27337.1	≤	Reflux(kg/h)	≤	37588.5
	21983.8	≤	Bottom(kg/h)	≤	48364.2
T4101	14094.9	≤	Overhead(kg/h)	≤	26780.3
	7216.32	≤	Reflux(kg/h)	≤	13711.0
	22065.3	≤	Bottom(kg/h)	≤	52074.1
T4301	0.99945	≤	C2H4 Fraction in Top	≤	0.99954
	70982.8	≤	Overhead(kg/h)	≤	134867
	48474.6	≤	Recycle(kg/h)	≤	93717.6
	2928.85	≤	Bottom(kg/h)	≤	9958.09
T5101	21131.8	≤	Overhead(kg/h)	≤	46490.0
	10199.9	≤	Reflux(kg/h)	≤	22439.7
	11051.2	≤	Bottom(kg/h)	≤	24312.6
T5301	72.7500	≤	Overhead(kg/h)	≤	458.325
	10896.7	≤	Bottom(kg/h)	≤	23972.7
T5501	0.99945	≤	C3H6 Fraction in Top	≤	0.99954
	380867	≤	Overhead(kg/h)	≤	390226
	181733	≤	Reflux(kg/h)	≤	363139
	588.600	≤	Bottom(kg/h)	≤	2177.82
T5701	12557.5	≤	Overhead(kg/h)	≤	30138.1
	5336.70	≤	Reflux(kg/h)	≤	15476.4
	3830.35	≤	Bottom(kg/h)	≤	8426.77