Separation of mixed alcohols from Fischer-Tropsch aqueous by-product: design, optimization and control of fraction cutting

doi:10.16085/j.issn.1000-6613.2021-2512

Abstract

Abstract:

Aqueous by-product from Fischer-Tropsch process contains C₁—C₈ alcohols (methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol and 1-octanol) and water (>95%) in which C₂—C₈ alcohols form the minimum-boiling azeotropes with water. The complete separation of alcohol-water mixture is of great importance. However, the process still remains a challenge due to difficulty in separation and huge energy consumption. Therefore, it has been a topic of interest for academia and industry. Noticeably, C₂—C₃ alcohols/water form homogeneous azeotropes whilst C₄—C₈ alcohols/water form highly heterogeneous azeotropes. Based on these characteristics, the two columns-sidestream decanter process was designed to achieve the precise separation of C₁—C₃ alcohols/water mixture, dehydrated C₄—C₈ alcohols mixture and water. C₄—C₈ alcohols/water mixture from the sidestream entered the decanter to break the distillation boundary. Water rich phase returned to the sidestream distillation column, while alcohols rich phase entered stripper column to obtain highly dehydrated C₄—C₈ alcohols mixture. Total annual cost (TAC) targeted steady-state optimization revealed that the two columns-sidestream decanter sequence has a 14.79% reduction in TAC and a 15.96% energy saving compared with the conventional three-column sequence. Furthermore, the control structure of the two columns-sidestream decanter sequence was established. The result of dynamic simulation showed that robust control was achieved with the combination of a concentration controller and a feed-forward ratio control structure. This work provides the design schemes and useful guides for the efficient separation of mixed-alcohol from Fischer-Tropsch aqueous by-product.

Key words: Fischer-Tropsch synthesis, mixed alcohols, separation, simulation, energy saving, optimization

摘要：

费托合成水相副产物主要为C₁~C₈醇（甲醇、乙醇、丙醇、丁醇、戊醇、己醇、庚醇和辛醇）与水的混合物，其中水的质量分数高达95%，且C₂~C₈醇与水均形成最低共沸物。此类醇水混合物的完全分离虽具重要价值，但难度大、能耗高，一直是学界和工业界的关注热点。本研究充分利用C₂~C₃醇水混合物的均相共沸物特性和C₄~C₈醇水混合物的高度非均相共沸物特性，提出两塔-侧线分相器工艺：通过侧线精馏塔实现C₁~C₃醇水、富C₄~C₈醇水和水的精准馏分切割；富C₄~C₈醇水混合物通入分相器以打破精馏边界，其中富水相返回侧线精馏塔，富醇相进入汽提塔，得到无水C₄~C₈醇混合物。基于年度总成本（total annual cost，TAC）的稳态优化表明，与常规三塔粗分流程相比，两塔-侧线分相器工艺能够降低TAC 14.79%，节约能耗15.96%。进一步，建立了两塔-侧线分相器工艺的控制结构，动态模拟表明，结合浓度控制器与前馈比例的控制结构表现出良好的控制性能。

关键词: 费托合成, 混合醇, 分离, 模拟, 节能, 优化

CLC Number:

TQ028.4

HUANG Yang, ZHANG Jiajun, LI Jiateng, XIA Ming, XU Chunjian. Separation of mixed alcohols from Fischer-Tropsch aqueous by-product: design, optimization and control of fraction cutting[J]. Chemical Industry and Engineering Progress, 2022, 41(10): 5200-5213.

黄洋, 张稼骏, 李家腾, 夏铭, 许春建. 费托合成水相副产物混合醇分离：馏分切割工艺设计及控制[J]. 化工进展, 2022, 41(10): 5200-5213.

Figures/Tables 23

References 38

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组分	分子式	组成（质量分数）/%	沸点/℃	共沸物中的醇质量分数/%	共沸物沸点/℃	溶解度（水）
甲醇	CH₄O	0.96	65.40	—	—	混溶
乙醇	C₂H₆O	2.00	78.01	95.6	78.16	混溶
正丙醇	C₃H₈O	1.00	97.20	69.0	87.63	混溶
正丁醇	C₄H₁₀O	0.83	118.66	56.5	91.78	微溶
正戊醇	C₅H₁₂O	0.53	137.80	45.5	94.30	微溶
正己醇	C₆H₁₄O	0.25	157.40	25.0	97.80	微溶
正庚醇	C₇H₁₆O	0.04	176.30	17.0	98.70	微溶
正辛醇	C₈H₁₈O	0.01	195.20	10.0	99.40	不溶
水	H₂O	94.38	100.00	—	—	—

组分	分子式	组成（质量分数）/%	沸点/℃	共沸物中的醇质量分数/%	共沸物沸点/℃	溶解度（水）
甲醇	CH₄O	0.96	65.40	—	—	混溶
乙醇	C₂H₆O	2.00	78.01	95.6	78.16	混溶
正丙醇	C₃H₈O	1.00	97.20	69.0	87.63	混溶
正丁醇	C₄H₁₀O	0.83	118.66	56.5	91.78	微溶
正戊醇	C₅H₁₂O	0.53	137.80	45.5	94.30	微溶
正己醇	C₆H₁₄O	0.25	157.40	25.0	97.80	微溶
正庚醇	C₇H₁₆O	0.04	176.30	17.0	98.70	微溶
正辛醇	C₈H₁₈O	0.01	195.20	10.0	99.40	不溶
水	H₂O	94.38	100.00	—	—	—

共沸物	模拟值/（kg/kg，℃）	实验值/（kg/kg，℃）
乙醇/水	0.9581/0.0419，78.17	0.956/0.044，78.16
丙醇/水	0.6914/0.3086，87.73	0.690/0.310，87.63
丁醇/水	0.5742/0.4258，92.64	0.565/0.435，91.78
戊醇/水	0.4492/0.5508，95.87	0.455/0.545，94.30
己醇/水	0.2793/0.7207，98.21	0.250/0.750，97.80
庚醇/水	0.1614/0.8386，99.21	0.170/0.830，98.70
辛醇/水	0.1159/0.8841，99.52	0.100/0.900，99.40

共沸物	模拟值/（kg/kg，℃）	实验值/（kg/kg，℃）
乙醇/水	0.9581/0.0419，78.17	0.956/0.044，78.16
丙醇/水	0.6914/0.3086，87.73	0.690/0.310，87.63
丁醇/水	0.5742/0.4258，92.64	0.565/0.435，91.78
戊醇/水	0.4492/0.5508，95.87	0.455/0.545，94.30
己醇/水	0.2793/0.7207，98.21	0.250/0.750，97.80
庚醇/水	0.1614/0.8386，99.21	0.170/0.830，98.70
辛醇/水	0.1159/0.8841，99.52	0.100/0.900，99.40

参数	公式
塔壳费用/USD	(M&S/280)×596.115d^1.066H^0.802
塔板费用/USD	(M&S/280)×12.69d^1.55H
换热器费用^①/USD	(M&S/280)×101.3A^0.65(2.29+3.75F_d)
塔高(H)/m	1.2h (N_T-2)
换热面积(A)^②/m²	Q/(UΔT)
低压蒸汽(500kPa,160℃) /USD·GJ^-1	13.28
冷却水(30~35℃)/USD·GJ^-1	0.354
M&S	1638.2(2018)^[34]