Unit 6 Chemical and Process Thermodynamics-3_化学工程与工艺专业英语音频资料_免费在线阅读收听下载

《化学工程与工艺专业英语》
由十岁董悠然小朋友用纯正英式英语为您朗读、陪您一起学习，让纯净的童声唤起您内心对英语的热爱。我们一起来大声朗读专业英语吧！

3. The Thermodynamic Laws
The First Law. The first law of thermodynamics is simply a statement of the conservation of energy. As shown in Fig 3-1, the sum of all the energy leaving a process must equal the sum of all the energy entering, in the steady state. The laws of conservation of mass and energy are followed implicitly by engineers designing Unfortunately, taken by itself, the first law has led to much confusion when attempting to evaluate process efficiency. People talk of energy conservation being an important effort, but in fact, no effort is required to conserve energy –it is naturally conserved.

The conclusions which can be drawn from the first law are limited because it does not distinguish among the various energy forms. Shaft work introduced by a reflux pump will leave a column as heat to the condenser just as readily as will heat introduced at the reboiler. Some engineers have fallen into the trap of lumping all forms of energy together in attempting to determine process efficiency. This is obviously not justified—the various energy forms have different costs.
The Second Law. There are many different statements of the second law as applied to cycles in which heat is converted into work. At this point, a more general statement is desirable: The conversion of energy from one form to another always results in an overall loss in quality. Another is: All systems tend to approach equilibrium (disorder). These statements point out the difficulty in expressing the second law. It cannot really be done satisfactorily without defining another term describing quality or disorder.

That term is entropy. This property of state quantifies the level of disorder in a fluid, body, or system. Absolute zero entropy is defined as the state of a pure, crystalline solid at absolute zero temperature. Each molecule is surrounded by identical molecules in a perfectly ordered structure at rest. Motion, randomness, contamination, uncertainty, all adds disorder and therefore contribute to entropy. Conversely, order is valuable, whether in the clarity of a gem stone, the purity of a chemical product, the cleanliness of a living space, or the freshness of air and water. Order commands a high price and can be creates only by applying work. Most of our work is expended in creating or restoring order in the home, the workplace, and environment. High entropy in the environment is one of externalized costs of manufacturing.

The purpose of every productive process is to reduce entropy by separating mixtures into pure products, reducing uncertainty in our knowledge, or creating works of art from raw materials. In general, there is a progression of decreasing entropy from feedstock to products. However, this is an uphill struggle inasmuch as the natural tendency is for entropy to increase as systems approach equilibrium.

The driving force for the decreasing in entropy required of production is a concomitant increase in entropy by a greater amount in the rest of the universe. Generally speaking, this increase is sustained within the same plant and is therefore responsible for the decrease in product entropy. Whereas the entropy decrease resides in the conversion of feedstock unto products, the greater increase is indicated by the conversion of fuels, electricity, air, and water, into combustion products, wastewater, and waste heat.

The bottom lines of fig.3-1 describe the second law, just as the middle line describes the first law. The total entropy of all streams leaving a process must always exceed that of all streams entering. If the entropy were to balance, as do the mass and energy, the process would be reversible, i.e. it could function as well to backward. Reversible processes are only theoretically possible, requiring dynamic equilibrium to exist continually-they are not productive. Furthermore, if the inequality were reversed, i.e. if there were a not entropy decease, all the arrows would also be reversed and the process would be forced to run backward. In essence, it is the entropy rise that drives the progress; It is the same driving force that makes water flow downhill, heat flow from hot to cold, vessels leak, glass break, metals corrode. In short, all things approach equilibrium with their surroundings.

The first law requires the conservation of energy and gives equal weight to all types of energy changes. While no process is immune to its authority, this law does not recognize the quality of energy nor does it explain why spontaneously occurring processes are never observed to spontaneously reverse themselves. The repeatedly confirmed observation that work may be completely converted into heat but that the reverse transformation never occurs quantitatively leads to the recognition that heat is a lower quality of energy. The second law with its origins deeply rooted in the study of the efficiency of heat engines recognizes the quality of energy. Through this law the existence of a heretofore unrecognized property, the entropy, is revealed, and it is shown that this property determines the direction of spontaneous change. The second law is in no way diminishes the authority of the first law; rather it extends and reinforces the jurisdiction of thermodynamics.

The Third law. The third law of thermodynamics provides an absolute scale of values for entropy by stating that for changes involving only perfect crystalline solids at absolute zero, the changes of the total entropy is zero. This law enables absolute values to be stated for entropies.
THE END

Unit 6 Chemical and Process Thermodynamics-3

Unit 6 Chemical and Process Thermodynamics-3

用户评论