换热壁面碳酸钙吸附与脱水行为的分子动力学

doi:10.16085/j.issn.1000-6613.2021-1943

摘要/Abstract

摘要：

碳酸钙污垢具有较高的热阻，换热壁面碳酸钙的沉积会导致换热器效率显著下降，因此，换热壁面碳酸钙的形成机理与抑制是换热器设计研究的重点。本文采用分子动力学方法模拟分析过饱和溶液中碳酸钙在高温铜金属壁面上的吸附与脱水行为。模拟结果表明，在碳酸钙向壁面吸附过程中Ca—C配位数先增加后趋近于定值，离子的水合数先减少后趋近于定值。吸附在金属壁面的碳酸钙内部结构未发生明显变化，碳酸钙为具有一定水合数的非晶体结构。壁面温度越高，吸附的碳酸钙脱水越完全，Ca—C和Ca—O的配位数越高，高温壁面吸附的碳酸钙从水合结构向无水晶体转化。当壁面温度提高到800K，离子水合数接近于0，Ca—O配位数约为6，与宏观尺度下无水碳酸钙晶体的Ca—O配位数相接近。

关键词: 碳酸钙, 吸附, 脱水, 模拟, 表面

Abstract:

Calcium carbonate fouling has a higher thermal resistance, and deposition of calcium carbonate on the heat exchange surface will lead to the reduction of heat exchanger efficiency, thus the formation and suppression of calcium carbonate are the focus of heat exchanger design. In this work, molecular dynamics method was adopted to analyze the adsorption and dehydration behavior of calcium carbonate on the copper metal surface in supersaturated calcium carbonate solution. The results showed that Ca—C coordination number firstly increased and then approached a constant value during the adsorption process of calcium carbonate to the surface, while the ion hydration number firstly decreased and then approached a constant value. The structure of calcium carbonate adsorbed on the metal surface did not changed significantly, and calcium carbonate was an amorphous structure with a certain hydration number. Increasing the surface temperature, the dehydration of the calcium carbonate was more complete, coordination numbers of Ca—C and Ca—O become higher, and the calcium carbonate adsorbed on the high-temperature surface transforms from a hydrated structure to anhydrous crystals. When the surface temperature was 800K, the ion hydration number was close to 0, and the Ca—O coordination number was approximate 6, which was close to the Ca—O coordination number of anhydrous calcium carbonate crystals at the macro scale.

Key words: calcium carbonate, adsorption, dehydration, simulation, surface

中图分类号:

TK124

肖毅, 王兵兵, 于旭亮, 王鑫, 蔡汉友. 换热壁面碳酸钙吸附与脱水行为的分子动力学[J]. 化工进展, 2022, 41(8): 4077-4085.

XIAO Yi, WANG Bingbing, YU Xuliang, WANG Xin, CAI Hanyou. Molecular dynamics simulation on adsorption and dehydration behavior of calcium carbonate on heat exchange surface[J]. Chemical Industry and Engineering Progress, 2022, 41(8): 4077-4085.

图/表 17

图1 金属壁面碳酸钙吸附过程物理模型

表1 水分子内原子相互作用势能参数

项目	类型	k/eV·?^-2	r₀/?	k_θ/eV·rad^-2	θ₀/(°)
H_w—O_w	Harmonic	45.93	1.012
H_w—O_w—H_w	Harmonic			3.291	113.24

表2 碳酸根分子内相互作用势能参数

项目	类型	k/eV·?^-2	r₀/?	k_a/eV·rad^-2	k_bb/eV·?^-2	k_ba/eV·?^-1·rad^-1	k₂/eV·?^-2	k₄/eV·?^-4
C—O	Harmonic	40.8493	1.3042
O—C—O	class2			13.234	12.818	1.53319
C—O/O/O	Distance						13.647	360.0

表3 各原子电荷量与原子量

原子	电荷量/e	原子量
H_w	0.41	1.008
O_w	-0.82	16.000
Ca	2	40.080
C	1.123285	12.010
O	-1.041095	16.000
Cu	0	63.550

表4 分子间势能参数

Lennard-Jones	ε/eV		σ/?
O_w—O_w（lj/cut）	0.00674		3.16549
O_w—Ca（lj/mdf）	0.00095		3.35
O_w—Cu（lj/cut）	0.05252		2.72029
Ca—Cu（lj/cut）	0.09211		2.35409
C—Cu（lj/cut）	0.03956		2.98960
O—Cu（lj/cut）	0.04969		2.68805
Cu—Cu（lj/cut）	0.4093		2.3377
Buckingham	A/eV	ρ/?	C/eV·?⁶
Ca—O（buck/mdf）	3161.63	0.27151	0
Ca—C（buck/mdf）	120000000	0.12	0
O—O（buck/mdf）	63840.20	0.19891	27.899
O—O_w（buck/mdf）	12534.46	0.2020	12.090
O—H_w（buck/mdf）	396.30	0.2170	0

图2 水分子沿垂直于壁面方向的数密度分布

图3 0.6mol/L溶液中碳酸钙向金属壁面吸附过程演化

图4 0.6mol/L碳酸钙溶液5ns和70ns时刻壁面的吸附情况

图5 溶液中钙离子与铜原子相互作用势能

图6 0.6mol/L碳酸钙溶液吸附层内不同种类离子数随时间演化

图7 溶液中不同团簇离子数及溶液中团簇总数随时间的演化（包括离子和离子对）

图8 不同浓度下碳酸钙吸附过程演化图像

图9 Ca2+扩散系数和吸附层内水分子数随时间的演化

图10 Ca—C配位数与水合数随时间的演化(a)和初始时刻（1ns）与最终时刻（70ns）Ca—Ca径向分布函数(b)

图11 800K壁温时最终时刻壁面碳酸钙吸附形态（绿色代表钙，碳酸根中的碳和氧分别由蓝色和红色表示，水分子中的氢和氧分别由白色和红色表示）

图12 不同壁面温度工况下Ca—C配位数、水合数和Ca—O配位数随时间的演化

图13 不同壁面温度时碳酸钙脱水最终时刻Ca—Ca径向分布函数

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