CIESC Journal ›› 2020, Vol. 71 ›› Issue (10): 4720-4732.doi: 10.11949/0438-1157.20200698

• Process system engineering • Previous Articles     Next Articles

Multi-objective operation optimization of olefin separation process for MTO plant

Lu YANG(),Shuoshi LIU,Xiaoyan LUO,Siyu YANG,Yu QIAN()   

  1. School of Chemical Engineering, South China University of Technology, Guangzhou 510640, Guangdong, China
  • Received:2020-06-03 Revised:2020-07-09 Online:2020-10-05 Published:2020-07-30
  • Contact: Yu QIAN E-mail:ceceyanglu@mail.scut.edu.cn;ceyuqian@scut.edu.cn

Abstract:

In modern coal processing industries, methanol-to-olefins (MTO) is an important equipment. Its olefin separation process is facing with problems such as the change of raw materials, the loss of olefin products and the high consumption of utilities. Operation optimization is required to achieve maximum benefits under the circumstance of quality assurance and requirements. This article takes the pre-depropanized olefin separation process of Lummus as the research object. And the optimization objectives are the total yield of ethylene and propylene as well as the total energy consumption. Modeling, simulation and multi-objective optimization of the process are conducted. Non-dominated sorting genetic algorithm (NSGA-II) is used to solve multi-objective optimization problem. The simultaneous optimization of 15 operational variables is achieved. Under the current yield, the optimal operation point is found by reducing the reflux ratio of low pressure depropanizer, deethanizer and 1# propylene tower and so on. The results show that the optimal operating point can reduce energy consumption by 20 MW compared with the existing operating point. The optimization interval of each operation variable corresponding to different trade-off points is determined by the comprehensive analysis of decision variables. It is also found that distillation equipment can operate in different optimal operation intervals.

Key words: methanol-to-olefins, olefin separation, multi-objective optimization, olefin yield, energy conservation

CLC Number: 

  • TQ 221.2

Fig.1

Schematic diagram of olefin separation process"

Table 1

Operating parameters of olefin separation process"

塔名称压力/MPa(G)塔顶/塔底温度/℃
DP11.8515.8/80
DP20.7714.4/79.2
DM2.65-11.1/13.8
DE2.40-20.4/62.7
ET1.64-34.3/-11.6
PT21.9451.9/59.1
PT11.8045.8/51.8
DB0.3746.8/92.6

Table 2

Material balance of olefin separation process"

Stream

Temperature/

Pressure/

MPa(G)

Mass flow/

(t/h)

Mole flows/(kmol/h)Mole fraction/%
H2,N2,COCH4C2H6C2H4C3H8C3H6C4H10C4H8C4H6C5CH3OCH3
feed 112.22.13947.8911900.350.410.7133.273.0245.120.5412.570.313.640.05
feed 211.51.8749.1216385.856.261.0460.81.4722.770.081.550.040.130.02
1-34.11.64138.521373trace0.010.0499.950trace00000
2461.803389030trace0.0200.3599.6400000
340.10.36910.518800000.10.154.0892.852.250.50.07
489.20.4073.3748000001×10-120.010.62099.370
1-34.21.64138.52137300.010.0499.950000000
245.81.80338903000.0200.3599.6300000
339.60.36910.518800000.10.154.0992.872.240.50.07
492.60.4073.37480000000.010.62099.370

Table 3

Optimization variables and their constraints of olefin separation process"

符号变量下限上限
x1高压脱丙烷塔(DP1)塔顶采出量, kg/h101000105000
x2高压脱丙烷塔(DP1)回流比1.181.23
x3低压脱丙烷塔(DP2)回流比1.01.4
x4低压脱丙烷塔(DP2)塔顶采出量, kg/h76008240
x5脱丁烷塔(DB)回流比0.951.2
x6脱丁烷塔(DB)塔顶采出量, kg/h1020010700
x7脱甲烷塔(DM)塔顶采出量, kg/h41504350
x8脱甲烷塔(DM)丙烷流量, kg/h1620017400
x9脱乙烷塔(DE)回流比1.31.6
x10脱乙烷塔(DE)塔顶采出量, kg/h4000043000
x11乙烯塔(ET)塔顶采出量, kg/h143000147000
x12乙烯塔(ET)中间测线采出量, kg/h3700040000
x132#丙烯塔(PT2)塔顶采出量, kg/h524000536000
x141#丙烯塔(PT1)塔顶采出量, kg/h3735039000
x151#丙烯塔(PT1)回流比14.018.0

Fig.2

Conceptual block diagram of the olefin separation process with NSGA-II algorithm"

Fig.3

Pareto-optimal front of olefin yield and total energy consumption"

Table 4

Operating parameters of the optimal point and operating point on the Pareto frontier map"

符号变量

A

(150.49 MW, 33.59%)

B

(155.22 MW, 34.44%)

QP

(153.42 MW, 34.23%)

Q*

(174 MW, 34.15%)

QPQ*

偏差%

x1DP1塔顶采出量, kg/h101220101860101740104450-2.6
x2DP1回流比1.2191.2191.2191.2190
x3DP2回流比1.3321.0001.0241.203-14.8
x4DP2塔顶采出量, kg/h7680816080707960+1.5
x5DB回流比1.0380.9620.9540.988-3.3
x6DB塔顶采出量, kg/h10370102801028010500-2.1
x7DM塔顶采出量, kg/h4240422542254255-0.7
x8DM丙烷流量, kg/h17000163601633016940-3.6
x9DE回流比1.301.371.411.50-6.7
x10DE塔顶采出量, kg/h41800408904079040580+0.5
x11ET塔顶采出量, kg/h145080146670146650147050-0.3
x12ET中间测线采出量, kg/h396004000040000400000
x13PT2塔顶采出量, kg/h528415524650524315531350-1.3
x14PT1塔顶采出量, kg/h37460390003852038340+0.5
x15PT1回流比15.0015.2015.0818.00-16.2

Table 5

Comparison of the optimal point and operating point on the Pareto frontier map at 80 generations"

位置项目主要设备节能占比主要设备节能相对变化率
再沸器冷凝器总和再沸器冷凝器总和
ADP11.33%2.75%4.08%5.03%-27.31%11.18%
DP20.42%-0.13%0.29%5.04%1.75%1.81%
DB-0.75%-0.89%-1.64%-9.58%10.12%-9.86%
DE0.99%1.12%2.11%3.03%-5.27%3.91%
PT25.70%91.06%3.31%18.38%
PT139.76%45.60%53.57%-18.31%
other4.10%1.88%
BDP11.22%2.30%3.52%3.67%-18.27%7.70%
DP21.29%0.64%1.93%12.39%-6.78%9.72%
DB-0.34%-0.48%-0.82%-3.43%4.42%-3.95%
DE2.28%1.93%4.21%5.55%-7.25%6.22%
PT29.04%86.88%4.20%14.01%
PT134.34%43.50%36.96%-13.95%
other4.28%1.73%
QPDP11.22%2.37%3.48%3.93%-19.97%8.35%
DP21.21%0.60%1.76%12.43%-6.75%9.72%
DB-0.28%-0.43%-0.69%-3.08%4.14%-3.64%
DE1.96%1.54%3.39%5.07%-6.15%5.49%
PT28.04%90.74%4.09%15.62%
PT137.15%45.55%42.58%-15.56%
other3.64%1.55%

Fig.4

Multi-objective operation optimization of propylene tower"

Fig.5

Optimal values of reflux ratio corresponding to the Pareto-optimal front"

Fig.6

Optimal values of separating column’s distillate rates corresponding to the Pareto-optimal front"

Fig.7

Optimal values of product column’s distillate rates corresponding to the Pareto-optimal front"

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