4.2.7 Manufacture of Ethanol: Fermentation vs Hydration of Ethene | 乙醇的制造：发酵法与乙烯水合法

📚 4.2.7 Manufacture of Ethanol: Fermentation vs Hydration of Ethene | 乙醇的制造：发酵法与乙烯水合法

Ethanol (C₂H₅OH) is an important organic compound widely used as a biofuel, a solvent, in alcoholic beverages, and as a feedstock for many chemical syntheses. In IGCSE Edexcel Science (Chemistry), students must be able to describe two main industrial methods for manufacturing ethanol: fermentation of sugars and the hydration of ethene. Understanding both routes, including their raw materials, conditions, equations, advantages, and limitations, is essential for grasping the interplay between renewable and non-renewable resources in the chemical industry.

乙醇（C₂H₅OH）是一种重要的有机化合物，广泛用作生物燃料、溶剂、酒精饮料以及许多化学合成的原料。在IGCSE Edexcel 科学（化学）中，学生必须能够描述两种主要的乙醇生产方法：糖类发酵和乙烯水合。理解这两种路线的原料、条件、化学方程式以及各自的优缺点，对于掌握化工行业中可再生与不可再生资源之间的相互关系至关重要。

---

1. Introduction to Ethanol | 乙醇简介

Ethanol belongs to the alcohol homologous series and has the molecular formula C₂H₅OH. It is a colourless, volatile liquid that mixes completely with water. Its ability to act as a fuel, a solvent, and an intermediate in the production of esters and other chemicals makes its manufacture a key industrial process. The two main pathways considered in the Edexcel specification are fermentation (a biological process) and direct hydration of ethene (a petrochemical process).

乙醇属于醇类同系物，分子式为C₂H₅OH。它是一种无色、易挥发且与水完全互溶的液体。乙醇可用作燃料、溶剂，并作为生产酯类及其他化学品的中间体，因此其制造是关键的工业过程。Edexcel 大纲考察的两条主要路线是发酵（一种生物过程）和乙烯直接水合（一种石化过程）。

Despite both producing the same chemical, the two methods differ fundamentally in their energy sources, carbon footprints, reaction speeds and product purity. These differences provide a rich context for discussing sustainability, equilibrium and rates of reaction in IGCSE Chemistry.

尽管两种方法生产相同的化学物质，但它们在能源来源、碳足迹、反应速率和产品纯度方面存在根本区别。这些差异为在IGCSE 化学中讨论可持续性、平衡和反应速率提供了丰富的背景。

---

2. What is Fermentation? | 什么是发酵？

Fermentation is an anaerobic biological process in which microorganisms such as yeast break down carbohydrates into ethanol and carbon dioxide. In industrial ethanol production, the yeast Saccharomyces cerevisiae provides the enzymes that catalyse the conversion of glucose into ethanol. Because the yeast is living, the process must occur under carefully controlled conditions that keep the organism active without killing it.

发酵是一种厌氧生物过程，微生物如酵母将碳水化合物分解为乙醇和二氧化碳。在工业乙醇生产中，酿酒酵母（Saccharomyces cerevisiae）提供将葡萄糖转化为乙醇的酶。由于酵母是活的，该过程必须在严格控制的条件下进行，以保持微生物活性同时避免其死亡。

Historically, fermentation has been used for thousands of years to produce alcoholic drinks. The same principle is applied today on a large scale to manufacture bioethanol for fuel blending. The maximum concentration of ethanol achievable by fermentation alone is around 10–15% by volume, because higher ethanol concentrations become toxic to the yeast.

历史上，发酵已被用于生产酒精饮料数千年。如今，同样的原理被大规模应用于生产用于燃料混合的生物乙醇。仅通过发酵能够达到的最高乙醇浓度约为10–15%（体积分数），因为更高的乙醇浓度会对酵母产生毒性。

---

3. Raw Materials and Conditions for Fermentation | 发酵的原料与条件

The main raw material for fermentation is a carbohydrate source rich in glucose, sucrose or starch. Common feedstocks include sugar cane juice (sucrose), corn starch or cellulose from plant biomass. Any starch must first be broken down into simpler sugars through hydrolysis before yeast can act upon it.

发酵的主要原料是富含葡萄糖、蔗糖或淀粉的碳水化合物来源。常见原料包括甘蔗汁（蔗糖）、玉米淀粉或来自植物生物质的纤维素。任何淀粉都必须先通过水解分解为更简单的糖，然后酵母才能对其进行作用。

The necessary conditions for fermentation are: an aqueous solution of sugar, temperatures around 25–35°C, a slightly acidic pH (around 4–5) and the absence of oxygen. A warm temperature is needed to provide activation energy for the yeast enzymes, but temperatures above 40°C denature these enzymes and stop the reaction. The absence of oxygen ensures that yeast resorts to anaerobic respiration, producing ethanol instead of carbon dioxide and water.

发酵所需的必要条件是：糖的水溶液，温度在25–35°C左右，微酸性pH（约4–5），以及缺氧环境。需要温暖的环境为酵母酶提供活化能，但温度超过40°C会使酶变性并终止反应。缺氧确保酵母进行无氧呼吸，生成乙醇而非二氧化碳和水。

Once the fermentation is complete, the resulting mixture contains ethanol, water, leftover sugars and yeast cells. The ethanol is separated by fractional distillation, exploiting the difference in boiling points between ethanol (78°C) and water (100°C).

发酵完成后，所得混合物含有乙醇、水、残余糖分和酵母细胞。乙醇通过分馏分离，利用乙醇（78°C）和水（100°C）之间的沸点差异。

---

4. Chemical Equation of Fermentation | 发酵的化学方程式

The overall equation for the fermentation of glucose is:

葡萄糖发酵的总方程式为：

C₆H₁₂O₆(aq) → 2 C₂H₅OH(l) + 2 CO₂(g)

This reaction is exothermic and is catalysed by enzymes present in yeast. Notice that one mole of glucose produces two moles of ethanol and two moles of carbon dioxide. The evolution of CO₂ bubbles is often a visible sign that fermentation is active.

该反应为放热反应，并由酵母中的酶催化。注意，一摩尔葡萄糖生成两摩尔乙醇和两摩尔二氧化碳。二氧化碳气泡的产生通常是发酵正在进行的可见标志。

Since the process occurs in aqueous solution, the ethanol formed is dilute. Subsequent distillation is required to obtain pure ethanol. It is important to recall that distillation itself does not alter the carbon footprint of the overall process but does add an energy cost.

由于该过程在水溶液中进行，生成的乙醇是稀溶液。要获得纯乙醇，必须进行后续蒸馏。重要的是要记住，蒸馏本身并不会改变整个过程的碳足迹，但确实会增加能源成本。

---

5. What is Hydration of Ethene? | 什么是乙烯水合？

Hydration of ethene is an addition reaction in which water (in the form of steam) adds across the carbon–carbon double bond of ethene to produce ethanol. This is a petrochemical route because ethene is obtained from the cracking of long-chain hydrocarbons derived from crude oil or natural gas. Unlike fermentation, no living organisms are involved.

乙烯水合是一种加成反应，水（以蒸汽形式）加成到乙烯的碳碳双键上生成乙醇。这是一条石化路线，因为乙烯来源于从原油或天然气中获得的长链烃的裂解。与发酵不同，该过程不涉及活体生物。

The reaction is reversible and is carried out in the gas phase. Because ethene is an unsaturated hydrocarbon, the addition of water is a typical electrophilic addition. The process is continuous, operates at a high temperature, and can produce ethanol of very high purity in a single step.

该反应是可逆的，并在气相中进行。由于乙烯是不饱和烃，水的加成是典型的亲电加成。该过程是连续的，在高温下操作，并且可以一步生产出高纯度的乙醇。

---

6. Raw Materials and Conditions for Hydration | 水合的原料与条件

The essential raw materials for the hydration of ethene are ethene (C₂H₄) and steam (H₂O). Ethene is produced from fossil feedstocks via cracking. A phosphoric acid (H₃PO₄) catalyst adsorbed onto a solid silica support is used to speed up the reaction. A typical temperature range is 300–350°C, and the pressure is maintained at about 60–70 atmospheres.

乙烯水合所需的基本原料是乙烯（C₂H₄）和水蒸气（H₂O）。乙烯通过裂解从化石原料中制得。使用负载在固体二氧化硅载体上的磷酸（H₃PO₄）催化剂加速反应。典型的温度范围为300–350°C，压力维持在约60–70个大气压。

These conditions represent a compromise: a higher temperature favours a faster rate but also shifts the equilibrium towards the reactants because the forward reaction is exothermic. A high pressure favours the forward reaction (fewer moles of gas on the product side) but increases plant costs. The chosen conditions therefore optimise yield and economic viability.

这些条件代表了一种折中：较高温度有利于提高反应速率，但由于正向反应放热，也会使平衡向反应物方向移动。高压有利于正向反应（产物侧气体摩尔数更少），但增加了设备成本。因此，所选择的条件在产率和经济可行性之间进行了优化。

Only about 5% of the ethene is converted in a single pass through the reactor. Unreacted ethene and steam are recycled back into the system, improving the overall atom economy and making the process efficient enough for large-scale production.

单次通过反应器时，只有约5%的乙烯发生转化。未反应的乙烯和蒸汽被循环回系统，提高了整体原子经济性，使该过程足够高效以进行大规模生产。

---

7. Chemical Equation of Hydration | 水合的化学方程式

The chemical equation for the hydration of ethene is:

乙烯水合的化学方程式为：

C₂H₄(g) + H₂O(g) ⇌ C₂H₅OH(g)

This reversible reaction is exothermic in the forward direction (ΔH is negative). The double arrow (⇌) indicates that an equilibrium mixture is established. The phosphoric acid catalyst does not appear in the equation because it is not consumed, but it is crucial for achieving a practical reaction rate.

该可逆反应正向放热（ΔH为负）。双箭头（⇌）表示建立了平衡混合物。磷酸催化剂不出现在方程式中，因为它没有被消耗，但对于达到实用的反应速率至关重要。

Notice the contrast with fermentation: here, all reactants and the product are small molecules, and the reaction is a direct addition with 100% atom economy theoretically. In practice, the need for recycling reduces the overall effective atom economy slightly.

注意与发酵的对比：此处，所有反应物和产物都是小分子，反应是直接的加成，理论原子经济性为100%。实际上，由于需要循环，使得总体有效原子经济性略有降低。

---

8. Batch versus Continuous Process | 间歇式与连续式工艺

Fermentation is typically a batch process. A vessel is loaded with sugar, yeast and water, left for several days under controlled conditions, then emptied and cleaned before the next batch begins. This allows easy monitoring, but results in downtime and variable product quality if not managed carefully.

发酵通常是一个间歇式过程。将糖、酵母和水装入容器，在受控条件下放置数天，然后清空、清洗，再进行下一批次。这便于监控，但会导致停工时间，如果管理不当，还会造成产品质量波动。

Hydration of ethene, in contrast, is a continuous process. Reactants are fed constantly into the reactor, and product is continuously removed. This is more suitable for large-scale, cost-effective production. It also delivers a more consistent ethanol purity and is easier to automate, reducing labour costs.

乙烯水合则是一个连续过程。反应物被不断地送入反应器，产物被连续移除。这更适合大规模、高经济效益的生产。它还能提供更一致的乙醇纯度，且更易于自动化，从而降低劳动力成本。

The choice between batch and continuous methods directly influences production capacity, energy efficiency and overall cost. In the context of IGCSE, recognising the operational differences helps explain why hydration is favoured for industrial solvent-grade ethanol while fermentation remains important for beverage alcohol and biofuels.

在间歇式和连续式方法之间的选择直接影响生产能力、能源效率和总成本。在IGCSE的背景下，认识到操作上的差异有助于解释为什么水合法更受工业级溶剂乙醇的青睐，而发酵法在饮料酒精和生物燃料中仍然重要。

---

9. Feedstock: Renewable vs Non-renewable | 原料：可再生与不可再生

Fermentation utilises biomass that can be regrown on a relatively short timescale, making it a renewable route. Plants such as sugar cane and corn absorb CO₂ from the atmosphere during photosynthesis, and this offsets some of the CO₂ released when the bioethanol is burned, contributing to a potentially lower net carbon footprint.

发酵利用的是可在相对较短时间内再生的生物质，因此是一条可再生路线。甘蔗、玉米等植物在光合作用过程中从大气中吸收CO₂，这抵消了燃烧生物乙醇时释放的部分CO₂，从而可能实现较低的净碳足迹。

Hydration of ethene relies on crude oil or natural gas, which are non-renewable fossil resources. Using these feedstocks contributes to depletion of finite reserves and generally results in a higher carbon footprint unless carbon capture technologies are employed. The ethene itself requires energy-intensive cracking processes.

乙烯水合依赖原油或天然气，这些都是不可再生的化石资源。使用这些原料会加剧有限储量的枯竭，并且通常会导致较高的碳足迹，除非采用碳捕获技术。乙烯本身就需要高能耗的裂解过程。

This renewable versus non-renewable contrast is a central theme in examinations. Students are often asked to evaluate which method is ‘greener’ and to discuss the trade-offs between sustainability, cost and purity.

可再生与不可再生的对比是考试的核心主题。学生经常被要求评价哪种方法更“绿色”，并讨论可持续性、成本和纯度之间的权衡。

---

10. Product Purity and Yield | 产品纯度与产率

Fermentation directly yields an ethanol concentration of only about 10–15% in the fermentation broth. Fractional distillation can concentrate this to about 96% ethanol (the azeotropic mixture with water). Further dehydration is required to obtain absolute ethanol. This multi-step downstream processing adds cost and energy consumption.

发酵直接在发酵液中产生的乙醇浓度仅为约10–15%。通过分馏可将其浓缩至约96%的乙醇（与水形成的恒沸混合物）。要获得无水乙醇还需进一步脱水。这种多步下游处理增加了成本和能耗。

Hydration of ethene, by contrast, produces a more concentrated ethanol vapour stream. After condensation, very high purity ethanol can be obtained, often exceeding 99% with minimal purification. The continuous nature of the process also means that the yield per unit time is high, despite the low single-pass conversion.

相比之下，乙烯水合产生的乙醇蒸气流浓度更高。冷凝后可直接得到高纯度乙醇，通常经过最低限度纯化即可超过99%。该过程的连续性也意味着尽管单次转化率低，单位时间的产量仍然很高。

In terms of atom economy, fermentation has a lower value because it produces CO₂ as a by-product. The hydration reaction has a theoretical atom economy of 100% because all reactant atoms end up in the desired product, making it an inherently ‘greener’ reaction in terms of atom usage.

就原子经济性而言，发酵较低，因为它产生CO₂作为副产物。水合反应的理论原子经济性为100%，因为所有反应物原子最终都进入目标产物，使得就原子利用而言，它本质上是一个更“绿色”的反应。

---

11. Environmental Impact and Energy Considerations | 环境影响和能源考量

Fermentation can be considered carbon-neutral over a short cycle if the biomass is sustainably sourced, because the CO₂ released during fermentation and fuel combustion is recently captured from the atmosphere. However, energy used in fertiliser production, harvesting, distillation, and transport often reduces this carbon neutrality significantly.

如果生物质来源可持续，发酵在短周期内可视为碳中和，因为发酵和燃料燃烧释放的CO₂是近期从大气中捕获的。然而，化肥生产、收割、蒸馏和运输所消耗的能源通常会显著降低这种碳中和程度。

The hydration route consumes large amounts of energy to maintain high temperatures and pressures, and depends on a fossil-derived feedstock. Unless the energy comes from low-carbon sources, its overall greenhouse gas emissions are higher. However, it avoids the land use and water consumption associated with growing crops for fermentation.

水合路线消耗大量能源以维持高温高压，并依赖化石衍生原料。除非能源来自低碳来源，否则其总体温室气体排放量更高。但它避免了发酵法种植作物所需的大量土地和水资源消耗。

Both methods present environmental challenges: fermentation competes with food production for arable land, while hydration contributes to fossil resource depletion and CO₂ emissions. In examinations, a balanced evaluation that considers local factors (such as availability of biomass or cheap petrochemicals) is encouraged.

两种方法都带来了环境挑战：发酵与粮食生产争夺耕地，而水合则导致化石资源枯竭和CO₂排放。考试中，鼓励学生进行平衡的评价，并考虑当地因素（如生物质或廉价石化产品的可获得性）。

---

12. Summary Table: Fermentation vs Hydration | 对比总结表：发酵与水合

Aspect	Fermentation	Hydration of Ethene
Raw materials	Carbohydrates (sugar/starch)	Ethene + steam
Feedstock type	Renewable (biomass)	Non-renewable (petroleum)
Conditions	25–35°C, anaerobic, yeast	300–350°C, 60–70 atm, H₃PO₄ catalyst
Reaction type	Biological, batch	Chemical, continuous
Product purity	~10–15%, needs distillation	Very high, >99%
Atom economy	Lower (CO₂ by-product)	100% (theoretical)
Carbon footprint	Potentially lower if sustainable farming	Higher (fossil-dependent)
Land use	Large agricultural area required	Minimal direct land use

This table captures the key criteria that appear frequently in IGCSE Edexcel Science questions. Students should be able to pick out these contrasts and link them to broader topics such as sustainability, catalysis, reversible reactions and energy resources.

此表概括了IGCSE Edexcel 科学试题中经常出现的关键标准。学生应能够找出这些对比，并将其与更广泛的主题联系起来，如可持续性、催化作用、可逆反应和能源资源。

Published by TutorHao | Science Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）