摘要
在用水量极大的工业循环冷却水系统中,结垢是一个长期存在,影响系统安全运行的主要问题之一。水垢一般是由具有反常溶解度的难溶盐或微溶盐组成。在循环冷却水系统中,最常见的难溶盐或微溶盐有碳酸钙、硫酸钙、磷酸锌及硅酸镁等。当这些盐类呈过饱和状态时,在一定条件下,就可能在换热器表面以水垢的形式析出。其中,碳酸钙是循环水系统中最常见、危害最大的水垢。通常我们主要讨论碳酸钙垢的抑制问题。目前,添加阻垢剂的方法已经成为国内外循环冷却水系统中控制结垢的主要手段。因此,阻垢剂的开发与合理利用是循环冷却水处理的关键问题。能否准确、快速地评定阻垢剂的阻垢性能以及对碳酸钙结晶动力学的研究就显得十分重要。
论文首先概述了国内外常用的评定阻垢剂阻垢性能的方法,比较了几种方法的优缺点。并对碳酸钙结晶的机理进行了阐述。
本试验研究了一种新型的评定阻垢剂阻垢性能的方法,这种方法是一种基于对溶液电导率的监测来评定阻垢剂阻垢性能的方法。试验是在恒温、密闭的条件下,通过对碳酸钙过饱和溶液电导率的测定,找出电导率开始下降的点,即溶液中开始析出沉淀的点,计算有阻垢剂和无阻垢剂时碳酸钙的临界过饱和度值来比较阻垢剂的阻垢性能。临界过饱和度值越大,阻垢剂的阻垢性能越好。通过阻垢剂的浓度和相对过饱和度值的关系图可以找出理想的阻垢剂以及合适的剂量。试验表明,电导率监测法具有试验仪器简单、操作方便、快速、准确、精度高等优点,可以比较不同阻垢剂和相同阻垢剂在不同浓度下的阻垢效果。
在碳酸钙结垢的动力学研究中,通过测定五种不同过饱和度的碳酸钙溶液的电导率随时间的变化来获得动力学的试验数据。本试验把碳酸钙结晶析出的过程分为两个阶段进行研究:晶核生长和晶粒长大。试验表明,晶核生长和晶粒长大的速率可以用一级反应速率方程来表示,并可以求出反应的速率常数和活化能。在阻垢剂和温度对碳酸钙结晶影响的试验研究中表明,温度、阻垢剂对碳酸钙结垢的反应速率产生影响。反应温度升高,晶核生长、晶粒长大的速率常数增大。在一定的过饱和度范围内,加入一定浓度的阻垢剂可以延长反应的诱导期,并抑制碳酸钙晶体的生长。从整个结晶过程来看,温度和阻垢剂对晶核生长的反应速率产生较大影响,对晶粒长大反应速率影响较小。原因是晶粒长大的速率常数很小,过饱和溶液中一旦有晶核生成,就开始进入不可逆的晶粒长大过程,即溶液中晶粒长大的条件是有晶核的存在,而温度、阻垢剂对之影响甚微。试验过程中,晶核生长和晶粒长大的反应级数并不因为温度的改变和阻垢剂的加入而发生改变。
It is particularly a problem in the cooling-water system where the formation of calcium carbonate is a common and main concern of endangering security in the cooling-water system. In most cases, scale is mainly composed of infusible material with abnormal solubility. In the cooling water system, the most common scale is carbonate, sulphate, phosphate, silicate. In some certain condition, when the scale assume supersaturation state, they are likely to precipitate on the heater surface. Calcium carbonate is the most usual and unsafe scale in the cooling water system. We commonly research into the problem of controlling calcium carbonate scale. At present, adding scale inhibitors is the main method of controlling scale in cooling water system. A method of evaluation scale inhibitors rapidly and exactly is therefore crucial. And an understanding of the crystallization kinetics of calcium carbonate is essential.
Firstly, the common evaluation methods of scale inhibitors and their advantages and disadvantages are summarized. Then the crystallization mechanics of calcium carbonate is presented.
A new evaluation method of scale inhibitors has been developed and tested. This method is based on solution conductivity measurement, In the given water, at the constant temperature and airtight conditions, the conductivity values of the solution were measured, the point of the precipitation located. By calculating the critical supersaturation value, we can determine the stabilizing property of scale inhibitors and determine the optimum concentration. The experimental results showed that the conductivity method has the merits of simple experimental devices, convenient operation, rapid, accurate ect. We can evaluate the efficiency of different scale inhibitors at different concentration.
In the kinetic studies of calcium carbonate crystallization, the process is divided into two processes: nuclei formation and crystal growth. This method is based on the conductivity measurement of the supersaturation solution. It showed that the rate of nuclei formation and crystal growth can be denoted as first order reaction rate equation and the rate constants and activation energy values can be calculated. In the study, the results indicated that a certain amount of scale inhibitors and temperature have influence on the rate of crystallization. The rate constants increase when temperature gets higher. Scale inhibitors can postpone the induction period and retard the scale formation. Temperature and scale inhibitors influence mainly on nuclei formation in the whole reaction process. It is because of the small rate constant of crystal growth that the crystal growth is irreversible reaction when nuclei appear in the supersaturation solution. That is, the condition of crystal growth is the existence of nucle
i. Temperature and scale inhibitor have little influence on crystal growth. In the experiment, the reaction order can't change for temperature and scale inhibitors.
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