化学肥料，取代了只能靠农作物获得的化学元素（如氮、磷、钾），促使现代农业增产。第二，生产了许多保护植物的药品。如灭虫剂，它们减少了农作物的病虫危害。第三，生产许多兽医药品，保护家畜免收病害或者治愈它们所得的传染病。
（2）、健康。化学医药部门生产的药品及它们的医疗技术已经帮助我们保持我们健康，因此我们是最大的受益者。比如抗生素可以治疗细菌感染，甚至可以延续生命；再如维生素β可以降低血压。
（3）、服饰 。现在的合成纤维的性能比以前的服饰材料（如麻，棉）有了很大的改善。衬衫，裙子以及外套是用的确良或者尼龙制成的，它们防皱，易洗，易干，不需要烫平并且比天然原料便宜。
随着科技的发展，纤维制品可以根据时尚设计要求，染成多种颜色。现在，几乎可以染成光谱中的任何一种颜色，如果某一个合适的颜色不能的得到，将通过改变原料试然，直到颜色满意为止。
在工业范畴，有另一个显著的优点，即印染的服饰不易褪色，也就是颜色不易受洗涤影响。
（4）、住房。娱乐及交通运输 在住房中大量使用了现代的合成加聚物，这些加聚物非常坚固的塑料正在取代传统的木质材料，因为它们轻并且不需维护（就是说，它们抵御气候和不用油漆的能力，其它的聚合物，例如甲醛或聚氨脂，能减少散热，从而具有节能作用。
同样，化学工业对运输业的贡献是运输业得到了主要的提高。因此，附加的添加剂像抗氧化剂和发动机的油粘性的改变，使得速度大幅度的提高，从300到6000再到12000海里/小时。调查研究表明，原因在于原油和动物油的润滑，从而使它们的流动性增大。尤其是汽车运输业中，聚脂和塑料的广泛应用于制造跑道、车轮、座椅垫子还有它们的覆盖等等。－现在超过了40％。
所以，虽然在化学工业对世界公众和我们生活的主要贡献只有简单的回顾，如果没有这些工业产品，我们的生活会有很大的不同。制作业的水平和化学工业的发展是衡定一个国家发展水平的重要指标。
3、化学工业的研究和发展
在世界的发展中，化学工业快速发展的一个主要原因是世界对它的巨大需求和在研究发展中的投资（R&D）。典型的数字占到5％的销售收入，是精深的医药领域投资的几乎两倍。注意我们这里引用的百分比不是利润的而是销售收入。收入包括原料费用、主管及员工的薪水。在过去，这大笔的资金应用的好，将会生产出许多有价值的产品而投入到消费市场，如合成聚脂产品，包括尼龙、聚脂、医学用药及农用药品。通常一种产品进入市场在最初几年遭到排斥，通过一个过渡期，研究部门将会逐渐减少，剩下一些高水平的研究机构。 化学工业是一个非常高技术的工业，它集中了在电子和工程中的最新科技的优势。计算机都得到了各种类型的广泛的应用，从化工设备的自动化控制到化合物的分子结构样式，到实验室分析仪器的控制。
个别生产设备的产量范并不高，每年仅有几吨的产量。而工业的纺织及石油化工厂的产量极大，，每年达到50 0000吨。这些公司需要巨额投资，就算是那时的个体生产部门在当今也值250 000 000美元。另外，这些生产部门广泛使用自动化设备，使得生产效率比人工有很打大幅度的提高。
主要的化工公司大都是跨国的，它在全球的大多数国家运转它的销售市场，而且它们在许多国家都设立了加工部门。伴随着经营全球化、国际化光明前景，化学工业在不断增长，主要是通过在别的国家设立的加工部门或者在那设立的跨国分公司经营的。
（选自：艾伦希顿，化学工业.第二版，布莱基父子公司，1997）
Reading Material 12
Principles of Momentum Transfer
1. Introduction
The flow and behavior of fluids is important in many of the unit operations in process engineering. A fluid may be defined as a substance that does not permanently resist distortion and, hence, will change its shape. In this text gases, liquids, and vapors are considered to have the characteristics of fluids and to obey many of the same laws*
In the process industries, many of the materials are in fluid form and must be stored, handled, pumped, and processed, so it is necessary that we become familiar with the principles that govern the flow of fluids and also with the equipment used. Typical fluids encountered include water, air, C02 , oil, slurries, and thick syrups.
If a fluid is inappreciably affected by changes in pressure, it is said to be incompressible. Most liquids are incompressible. Gases are considered to be compressible fluids. However, if gases are subjected to small percentage changes in pressure and temperature, their density changes will be small and they can be considered to be incompressible.
Like all physical matter, a fluid is composed of an extremely large number of molecules per unit volume. A theory such as the kinetic theory of gases or statistical mechanics treats the motions of molecules in terms of statistical groups and not in terms of individual molecules. In engineering we are mainly concerned with the bulk or macroscopic behavior of a fluid rather than with the individual molecular or microscopic behavior.
In momentum transfer we treat the fluid as a continuous distribution of matter or as a \ large enough number of molecules so that a statistical average is meaningful and the macroscopic properties of the fluid such as density, pressure, and so on, vary smoothly or continuously from point to point.
The study of momentum transfer, or fluid mechanics as it is often called, can be divided into two branches : fluid statics, or fluids at rest, and fluid dynamics, or fluids in motion. In other sections we treat fluid statics; in the remaining sections, fluid dynamics. Since in fluid dynamics momentum is being transferred, the term \2. Fluid Flow
The principles of the statics of fluids are almost an exact science. On the other hand, the principles of the motions of fluids are quite complex. The basic relations describing the motions of a fluid are the equations for the overall balances of mass, energy, and momentum, which will be covered in the following sections.
These overall or macroscopic balances will be applied to a finite enclosure or control volume fixed in space. We use the term \enclosure. The changes inside the enclosure are determined in terms of the properties of the streams entering and leaving and the exchanges of energy between the enclosure and its surroundings.
When making overall balances on mass, energy, and momentum we are not interested in the details of what occurs inside the enclosure. For example, in an overall balance average inlet and outlet velocities are considered. However, in a differential balance the velocity distribution inside an enclosure can be obtained with the use of Newton's law of viscosity. 3. Laminar and Turbulent Flow
The type of flow occurring in a fluid in a channel is important in fluid dynamics problems. When fluids move through a closed channel of any cross section, either of two distinct types of flow can be observed according to the conditions present. These two types of flow can be commonly seen in a flowing open stream or river. When the velocity of flow is slow , the flow patterns are smooth. However, when the velocity is quite high, an unstable pattern is observed in which eddies or small packets of fluid particles are present moving in all directions and at all angles to the normal line of flow.
The first type of flow at low velocities where the layers of fluid seem to slide by one another without eddies or swirls being present is called laminar flow and Newton's law of viscosity holds. The second type of flow at higher velocities where eddies are present giving the fluid a fluctuating nature is called turbulent flow.
The existence of laminar and turbulent flow is most easily visualized by the experiments of Reynolds. Water was allowed to flow at steady state through a transparent pipe with the flow rate controlled by a valve at the end of the pipe. A fine steady stream of dye-colored water was introduced from a fine jet as shown and its flow pattern observed. At low rates of water flow, the dye pattern was regular and formed a single line or stream similar to a thread. There was no lateral mixing of the fluid, and it flowed in streamlines down the tube. By putting in additional jets at other points in the pipe cross section, it was shown that there was no mixing in any parts of the tube and the fluid flowed in straight parallel lines. This type of flow is called laminar or viscous flow.
As the velocity was increased, it was found that at became dispersed and the pattern was very erratic. This type of flow is known as turbulent flow. The velocity at which the flow changes is known as the critical velocity. 4. Reynolds Number
Studies have shown that the transition from laminar to turbulent flow in tubes is not only a function of velocity but also of density and viscosity of the fluid and the tube diameter. These variables are combined into the Reynolds number, which is dimensionless.
Re=D v p/u
where Re is the Reynolds number, D the diameter in m, p the fluid density in kg/m3, u the fluid viscosity in Pa ? s, and v the average velocity of the fluid in m/s (where average velocity is defined as the volumetric rate of flow divided by the cross-sectional area of the pipe).
The instability of the flow that leads to disturbed or turbulent flow is determined by the ratio of the kinetic or inertial forces to the viscous forces in the fluid stream. The inertial forces are proportional to pu2 and the viscous forces to u v/D. and the ratio pv2/(/uv/D)is the Reynolds number Dvp/pL.
For a straight circular pipe when the value of the Reynolds number is less than 2100, the flow is always laminar. When the value is over 4000, the flow will be turbulent, except in very special cases. In between, which is called the transition region, the flow can be viscous or turbulent, depending upon the apparatus details, which cannot be predicted.
5. Simple Mass Balances
In fluid dynamics fluids are in motion. Generally, they are moved from place to place by means of mechanical devices such as pumps or blowers, by gravity head, or by pressure, and flow through systems of piping and/or process equipment. The first step in the solution of flow problems is generally to apply the principles of the conservation of mass to the whole system or to any part
of the system. We will consider an elementary balance on a simple geometry. Simple mass or material balances were followed.
input = output + accumulation
since, in fluid flow, we are usually working with rates of flow and usually at steady state, the rate of accumulation is zero and we obtain
rate of input = rate of output (steady state)
(Selected from: Christie J. Geankoplis, Transport Processes and Unit Operations* 2nd Edition, 1983. )
材料12
动量传递原则
1.介绍
流体的流量和行为在工程学的单元操作过程中起着非常重要的作用。流体被定义为一种不能永久保持抗变形的物质，也就是说，它的形状可以改变。在这一章 里气体，液体和蒸汽也认为有这个特点，也遵守一些同样的原则。
在工业过程，我们有许多的液态物质需要我们的储存，使用，传输和加工，所以我们有必要熟悉流体的管理原则和设备容器的使用原则。我们常遇到的典型的流体包括水，气体，CO2,油，泥浆和粘稠糖浆。
如果一种流体的压力改变时只受到微笑的变化，我们就说这是不可压缩流体。许多的流体是不可压缩的。气体被认为是可压缩的流体。当然，如果气体对于压力和温度的改变受影响度非常小，我们就称为不可压缩气体。
像所有的物质，流体也是由每体积极其多的分子组成的。像气体分子运动论或者统计力学对待运动的分子都是就统计团体而不是单个分子来说的。在工程上，我们更关心的是大量的或者说宏观的流体行为而不是单个的或者微观粒子的行为。
在动力传递中，我们把流体看成是连续分部的物质或者是“连续统一体”。当一个非常小的体积里包含足够的分子量使得统计平均是有意义的，流体的密度，压力等宏观的性能是流畅和连续的，这样我们把这种流体看做是连续的物质的方法是有效的了。
一种动量传递的研究，或者说是流体力学，可分为两个分支：流体静力学和流体动力学。在其它的一些领域，我们视为流体静力学，在另外一些，我们视为流体动力学。在流体动力学动量在传递，我们常称动力传递或者传输。
2.流体流动
静态流体的原则几乎是一门精密科学。在另一方面，动态流体的原则是十分复杂。动态流体的基本关系的描述就是整个质量平衡方程，能量和动量将在后面章节中说到。宏观平衡将被应用到限制外壳和控制空间的体积。我们之所以用“全部”这个词是因为我们想从外面去整个的描述它的平衡。流体里面的改变取决于流体的流进和流出还有环境和内部之间的能量交换。
当我们在质量，能量和动量上取得平衡时，我们将不感兴趣其内部发生些什么。例如，对于全部来说进出流体的流速也是要被考虑的。但是，在微分方程里流体内部的速度分布也是要通过牛顿黏性定律加以考虑的。 3 层流和湍流
流体在管道中的流动类型对于流体动力学问题是很重要的。当流体通过封闭的管道截面时，两种不同类型的流体可以通过观察得出。两种类型的流体在开放得小溪或是河流中很常见。当流体的流速缓慢，流体的模式顺畅。然而，当流体的速度非常的高的时候，我们可以观察到漩涡或者是各个角度和方向的小包流体粒子的不稳定的模式。
在第一种低流速的流体类型，层流在缓慢流动没有涡流的出现，我们称为层流，它也遵守牛顿黏性定律。第二种高流速产生涡流的流体类型我们称为湍流。
层流和湍流的存在是可以很容易通过雷诺茨试验直观看到的。水允许在末端有阀门的管道里保持平稳的流速流过管道。我们把着色的液体引入好的液体中观察流动情况。当水德流速很慢时，我们可以看到着色的液体很有规律的保持线状。没有横向混合流体，它们以流线形通过管道。在其它的管道截面增加一个喷射机，结果显示流体保持平行流线，在管道的任意截面没有产生混合。这种我们称为层流或者说是黏性流。
当流速增加时，我们发现流体开始混乱模式开始波动。这种类型的流体我们称为湍流。流体开始改变时的速度我们称为临界流速。 4雷诺系数
研究发现，层流转变为湍流不仅仅是与流速有关，还与密度和黏性，管道的直径有关。这些变量综合在一起产生了一个数值很小的雷诺系数。
Re=D v p/u
Re是雷诺系数，D是管道的直径，p是液体的密度，单位是kg/m3,u是液体的黏性，单位是Pa ? s，v是液体的平均流速m/s
影响和干扰流体的不稳定因素取决于流体惯性力与黏性力的比值。惯性力与pv^2成比例，粘性力与uv/D成比例，比值pv^2/(uv/D)就是雷诺系数Dvp/u
对于一个直圆管，雷诺系数小于2100的话，流体就是层流。当值超过4000，除了一些特殊的例子外就都是湍流。在这两者之间，称为过渡流，可以使层流也可以是湍流，取决于装置的细节，不能够预测。 5简单的物质平衡
在流体力学中液态是流动的。通常，它们通过泵或者是鼓风机装置或者重力，压力，管道系统或者过程设备使它们从一个地方到另外一个地方。解决流体问题的第一步是将质量守恒原理应用于整个系统或者是部分系统。我们将考虑几何上的初步平衡。简单的质量平衡如下：
输入 = 输出 + 累积 在流体流动中，我们经常面对一种稳定状态，这时我们将 累积 视为0，就有：
输入率 = 输出率 （稳态）
（选自Christie J. Geankoplis，传输过程和单元操作第2版，1983）