Problem 141 - A Complete Resource Book in Chemistry for JEE Main

Question

In context with the industrial preparation of hydrogen from water gas \(\left(\mathrm{CO}+\mathrm{H}_{2}\right)\), which of the following is the correct statement? (a) \(\mathrm{CO}\) and \(\mathrm{H}_{2}\) are fractionally separated using differences in their densities (b) \(\mathrm{CO}\) is removed by absorption in aqueous \(\mathrm{Cu}_{2} \mathrm{Cl}_{2}\) solution (c) \(\mathrm{H}_{2}\) is removed through occlusion with \(\mathrm{Pd}\) (d) \(\mathrm{CO}\) is oxidized to \(\mathrm{CO}_{2}\) with steam in the presence of a catalyst followed by absorption of \(\mathrm{CO}_{2}\) in alkali.

Step-by-Step Solution

Answer

Option (d) is correct: CO is oxidized to CO₂ with steam and absorbed by alkali.

Step 1: Understand Water Gas Components

Water gas is a mixture composed primarily of carbon monoxide (CO) and hydrogen (H₂). We need to separate the two gases to isolate pure hydrogen.

Step 2: Assess Separation Methods

The problem provides four options, each describing a method related to separating CO from H₂ in water gas. We need to evaluate which method aligns with common industrial practices.

Step 3: Evaluate Options

- **Option (a):** Fractional separation by density difference is ineffective as CO and H₂ have similar molecular weights and densities. - **Option (b):** CO does react with catalysts to form metal carbonyls, but aqueous Cu₂Cl₂ does not absorb CO in industrial practices. - **Option (c):** Occlusion with palladium (Pd) is a method used to purify hydrogen but is not used in the industrial scale for separating water gas components. - **Option (d):** CO and H₂ are commonly handled by converting CO to CO₂ using steam and a catalyst, followed by CO₂ absorption with an alkali.

Step 4: Identify Correct Industrial Practice

The industrial conversion of CO to CO₂ with steam, known as the water-gas shift reaction, followed by CO₂ absorption with an alkali or amine solution, is accurate. This method optimizes hydrogen separation in large-scale applications.

Key Concepts

Water Gas

Water gas is a crucial material in the industrial production of hydrogen. This gas is primarily a mixture of carbon monoxide (CO) and hydrogen (H₂). The process of creating water gas involves a reaction between carbon in the form of coal or coke and water vapor, also known as steam. This leads to the formation of a gaseous mix primarily composed of CO and H₂, with trace amounts of other gases.
Water gas is important because it's a useful intermediate for producing synthetic gases and as a source of pure hydrogen. Due to its composition, industrial processes leverage it to derive cleaner energy and more sophisticated fuels.
Understanding what water gas consists of is the first step in utilizing it efficiently for hydrogen production. As we delve into its components, it's clear that separating CO from H₂ becomes a pivotal step in industrial applications.

Water-Gas Shift Reaction

The water-gas shift reaction plays a crucial role in the hydrogen purification process. This chemical reaction transforms carbon monoxide (CO) and water vapor (H₂O) into carbon dioxide (CO₂) and additional hydrogen (H₂).
The reaction can be represented as follows: CO + H₂O → CO₂ + H₂.
This reaction is facilitated using catalysts, typically involving iron or chromium oxide. The shift reaction optimizes hydrogen production by converting CO, a less useful component, into additional hydrogen. It also simplifies the process of separating the resultant gases afterward.
Temperature settings are crucial in this reaction, as low-temperature shifts will yield a higher conversion efficiency. The shift reaction underpins modern hydrogen production as it increases the overall yield of hydrogen while simultaneously reducing CO emissions within the process.

Carbon Monoxide Separation

Separating carbon monoxide (CO) from hydrogen during industrial processes involves transforming CO to CO₂, making the separation easier. The conversion is vital to ensure the purity of hydrogen gas.
Beyond the water-gas shift reaction, various methods can handle this separation. Among these is the absorption of CO₂ in solutions that contain alkalis (such as sodium or potassium hydroxide). These solutions selectively remove CO₂ due to its acidic properties, while H₂ remains intact in gaseous form.
This technique is well suited for large-scale industrial applications, where purity and efficiency are paramount. It's necessary to ensure that the presence of CO does not interfere with hydrogen's end-use, especially in applications where high-purity hydrogen is needed.

Hydrogen Purification

Once isolated, hydrogen undergoes further purification to ensure its suitability for industrial and commercial uses. This purification is essential because even small amounts of impurities can affect the performance of hydrogen fuel cells and other technologies relying on hydrogen.
Methods for hydrogen purification include

• Cryogenic Distillation: Useful for achieving high purity by cooling gases to cryogenic temperatures.
• Pressure Swing Adsorption (PSA): Involves passing gas through materials that selectively adsorb specific components under pressure swings.
• Membrane Separation: Utilizes selective permeable membranes to filter hydrogen from other gases.

Each method has its own application scope depending on the required purity level and industrial demands. The choice often hinges on the scalability, cost, and purity needed for the intended hydrogen use. Thus, hydrogen purification is a field of dynamic technological advancement, continuously evolving to meet growing energy needs.