Mechanism prediction of flow-induced corrosion and optimization of protection measures in overhead system of atmospheric tower

doi:10.11949/j.issn.0438-1157.20180785

Abstract

Abstract:

Corrosion failures often occur at the overhead cooling system in the atmospheric tower in a domestic oil refinery. Based on the material balance principle, this study uses the reverse derivation method and process simulation to analyze the flow corrosion failure mechanism of the atmospheric pressure overhead cooling system, including dew point corrosion, ammonium salt crystal deposition scale corrosion, and multiphase flow erosion. Generally, water injection is a convenient and efficient measure to eliminate dew-point corrosion and under-deposit corrosion caused by ammonium salt crystallization. Nevertheless, due to the restricted water injection flow rate and various water injection modes for the heat exchangers, corrosion failure still occurred in the overhead cooling system of the atmospheric tower. By simulating the overhead system, the suitable one of three water injection modes (in the main process pipe, in the heat exchangers and by a programming controller) can be determined to realize the long-term stable operation of the overhead cooling system in the atmospheric tower according to a given injected water flow rate.

Key words: corrosion, crystallization, dew point, equilibrium, water injection, double-drum system

摘要：

国内某炼油厂常减压蒸馏装置的常压塔顶冷却系统换热器频繁出现腐蚀失效问题。基于物料衡算原理,采用逆向推导的方法及工艺过程模拟计算分析了该常压塔顶冷却系统的流动腐蚀失效机理，包括露点腐蚀、铵盐结晶沉积垢下腐蚀以及多相流冲刷腐蚀。注水是一种方便且非常有效的破除露点腐蚀和铵盐结晶沉积垢下腐蚀的工艺防护措施。但由于该炼油厂常压塔顶注水量有限，每台换热器采用不同的注水方式，依然出现了流动腐蚀失效问题。通过模拟计算，提出根据不同的注水量应选择不同的注水方式（总管注水、换热器定点注水和程控注水），从而实现该常顶冷却系统长周期稳定运行。

关键词: 腐蚀, 结晶, 露点, 平衡, 注水, 双罐系统

CLC Number:

TE 986

Changchun HE, Lei XU, Wei CHEN, Xiaofeng XU, Pengwei OUYANG. Mechanism prediction of flow-induced corrosion and optimization of protection measures in overhead system of atmospheric tower[J]. CIESC Journal, 2019, 70(3): 1027-1034.

何昌春, 徐磊, 陈伟, 徐晓峰, 欧阳鹏威. 常顶系统流动腐蚀机理预测及防控措施优化[J]. 化工学报, 2019, 70(3): 1027-1034.

Figures/Tables 14

Fig.1 Under-deposit corrosion caused by ammonium chloride crystallization

Fig.2 Process schematic of double-drum atmospheric tower overhead system

Fig.3 Corrosion morphology of U-tube heat exchanger

Table 1 Gas composition and flowrate in atmospheric tower overhead system

组分	体积分数/%	组分	体积分数/%
氢气	0	1-丁烯	0.12
空气	1.98	反-2-丁烯	0
甲烷	2.23	顺-2-丁烯	0.54
乙烷	9.34	碳五以上	26.23
乙烯	0	一氧化碳	0
丙烷	20.43	二氧化碳	2.41
丙烯	0	硫化氢	0.26
异丁烷	9.47	合计	99.98
正丁烷	26.97	流量/(kmol·h^-1)	205.84

Table 2 Second-stage oil process parameters and composition in atmospheric tower overhead system

流量/(t·h^-1)	温度/℃	压力/MPa	密度/(kg·m^-3)	HK^①/℃	10%/℃	50%/℃	90%/℃	KK^②/℃
74.1	40.3	0.076	672.8	25	36	72	126	146

Table 3 First-stage oil (output + reflux) process parameters and composition in atmospheric tower overhead system

流量/(t·h^-1)	温度/℃	压力/MPa	密度/(kg·m^-3)	HK/℃	5%/℃	10%/℃	50%/℃	90%/℃	KK/℃
133.8	90.1	0.127	726.4	48	74	83	118	161	167

Table 4 Sour water process parameters and composition in atmospheric tower overhead system

流量/

(t·h^-1)

温度/

℃

压力/

MPa

氨氮/(mg·L^-1)

硫/

(mg·L^-1)

Fig.4 NH4Cl crystallization temperatures versus Cl- concentrations in sour water

Fig.5 NH4Cl crystallization temperatures versus NH3 concentrations in sour water

Fig.6 Products of partial pressures of NH3 and H2S versus temperatures

Fig.7 Dew points of water phase versus H2O concentrations in overhead gas of atmospheric tower

Table 5 Relationship between percentage of liquid water and injected water flowrate

注水量/(t·h^-1)	液态水含量/%(mass)
21.2	5.26
22.0	8.76
22.3	10.01
23.6	15.05
25.1	20.22
26.7	25.09
28.5	29.92

Fig.8 Temperature profile of outer wall of heat exchanger tube bundle without water injection

Fig.9 Temperature profile of outer wall of heat exchanger tube bundle with injected water flowrate of 1000 kg·h-1

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