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首页> 外文会议>2002 ASME International Mechanical Engineering Congress and Exposition , Nov 17-22, 2002, New Orleans, Louisiana >REDUCING THE DEPOSITION OF SCALE IN THE EVAPORATOR OF A MECHANICAL VAPOUR RECOMPRESSION SYSTEM FOR CONCENTRATION OF PULP MILL EFFLUENTS
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REDUCING THE DEPOSITION OF SCALE IN THE EVAPORATOR OF A MECHANICAL VAPOUR RECOMPRESSION SYSTEM FOR CONCENTRATION OF PULP MILL EFFLUENTS

机译：减少用于制浆厂废水浓缩的机械蒸汽压缩系统蒸发器中的水垢沉积

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A numerical model was developed, validated with the aid of pilot evaporator tests, and used to assess practical methods of minimizing scaling observed in a mechanical vapour recompression (MVR) plant at Millar's Western's Meadow Lake, Saskatchewan pulp mill. On average, 8,000 m~3/day of effluent (approximately 7 m~3/Bone Dry Tonne product) resulting from bleached-chemi-thermo-mechanical wood processing, are purified and the recovered water is returned to the plant while the effluent is concentrated to 75% TD&SS in a recovery boiler. The evaporators are used in the first stage of the feed concentration process. The system uses a heat pump principle: steam produced during the boiling of the falling liquid film is mechanically compressed and condensed on the outer surface of a vertical tube evaporator. Most of the condensation and compression heat is recovered. Reducing the rate of scale deposition and increasing the interval between two successive cleaning operations of vertical evaporators used in the MVR scheme was identified as an essential component of operation costs and given special attention. To assist the mill in assessing practical methods for achieving this goal an experimental pilot evaporator and a numerical model were developed and used first at the Alberta Research Council in Edmonton, Canada, and then at the mill location. The mill uses a different model for control and supervision of system parameters. The magnitude of the (critical) temperature difference (CTD) across the laminar sub-layer of boiling liquid film is calculated and is recommended in this paper to be used to quantify the fouling tendency. Further to recommendations resulting from previous experimental investigations as well as in this study, the mill introduced additional process control parameters to reduce and maintain the temperature drop across the effluent boiling film to a maximum range of 2-4 ℃. In addition to CTD, the wall (top-bottom) axial temperature difference (ATD) has been identified as another criteria for assessing potential scale deposition during evaporation-concentration. Calculations and experimental measurements performed with the pilot evaporator suggest that increasing the circulation rate of effluent pumped from the sump to feed the liquid film at the top of evaporator tubes has a positive effect on reducing the CTD and the ATD. During four months of laboratory investigations with a pilot evaporator, non-uniform liquid distribution among vertical evaporator tubes of the evaporator was observed and is discussed separately. This paper will present the model and mill observations and summarize the main results and suggested practical strategies for reducing the rate of scale deposition and improving the system economics.
机译：开发了一个数值模型，并通过试点蒸发器测试进行了验证，并用于评估使在萨斯喀彻温省Millar's Western的Meadow Lake的机械蒸汽再压缩（MVR）厂中观察到的水垢最小化的实际方法。漂白，化学-热机械加工木材所产生的废水平均为8,000 m〜3 / day /天（约7 m〜3 / Bone Dry吨产品），净化后所回收的水返回工厂。在回收锅炉中浓缩至TD＆SS的75％。蒸发器用于进料浓缩过程的第一阶段。该系统采用热泵原理：在下降的液膜沸腾过程中产生的蒸汽在垂直管式蒸发器的外表面被机械压缩并冷凝。大部分冷凝和压缩热被回收。在MVR方案中使用的降低垂直蒸发器的水垢沉积速率并增加两次连续清洁操作之间的间隔被确定为操作成本的重要组成部分，并给予了特别关注。为了帮助工厂评估实现该目标的实用方法，开发了实验性先导蒸发器和数值模型，并首先在加拿大埃德蒙顿的艾伯塔研究委员会使用，然后在工厂使用。工厂使用不同的模型来控制和监督系统参数。计算了沸腾液膜层流子层的（临界）温差（CTD）的大小，建议在本文中用于量化结垢趋势。除了先前的实验研究以及本研究提出的建议外，该工厂还引入了附加的过程控制参数，以降低并维持流出的沸腾膜上的温度降到最大2-4℃。除CTD外，壁（上下）轴向温度差（ATD）已被确定为评估蒸发浓缩过程中潜在水垢沉积的另一个标准。用先导蒸发器进行的计算和实验测量表明，增加从集水槽泵出的污水的循环速率以在蒸发器管的顶部馈入液膜对减少CTD和ATD有积极作用。在使用先导蒸发器进行的四个月的实验室研究过程中，观察到蒸发器的垂直蒸发器管之间液体分布不均匀，并分别进行了讨论。本文将介绍模型和轧机的观测结果，并总结主要结果，并提出降低结垢速率和提高系统经济性的实用策略。

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