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Difficulty: MediumCarbon: Allotropes, Coal Distillation, and Industrial Fuel Gases

During the industrial production of water gas, steam is blown over red-hot coke at elevated temperatures (1000∘C1000^\circ\text{C}). Why must the steam supply be periodically interrupted to blow air over the coke bed instead?

  1. The reaction between steam and coke is endothermic, which continuously cools the coke bed below the required reaction temperature.Answer
  2. B
    Steam acts as a homogeneous catalyst that shifts the position of dynamic equilibrium back toward reactants.
  3. C
    Air is required to convert amorphous carbon into graphite isomers before steam can react.
  4. D
    The gaseous water gas product expands beyond the molar volume of 22.4 dm³ per mole, stopping the flow of steam.

Answer

The steam supply must be interrupted because the reaction between steam and red-hot coke (C(s)+H2O(g)→CO(g)+H2(g)\text{C}_{(s)} + \text{H}_2\text{O}_{(g)} \rightarrow \text{CO}_{(g)} + \text{H}_{2(g)}) is strongly endothermic. This lowers the temperature of the coke bed below 1000∘C1000^\circ\text{C}, requiring air to be blown in so that carbon burns exothermically in oxygen (C+O2→CO2\text{C} + \text{O}_2 \rightarrow \text{CO}_2) to reheat the bed.
The reaction C(s)+H2O(g)→CO(g)+H2(g)\text{C}_{(s)} + \text{H}_2\text{O}_{(g)} \rightarrow \text{CO}_{(g)} + \text{H}_{2(g)} is endothermic, absorbing heat from the red-hot coke. To maintain the high temperatures necessary for continuous reaction, the steam blow is periodically stopped and an air blow is introduced to exothermically burn some coke and reheat the bed.

Step-by-Step Solution

1
Analyze the thermochemical nature of the water gas reaction.
The formation of water gas (CO+H2\text{CO} + \text{H}_2) from coke and steam absorbs heat (ΔH>0\Delta H > 0).
Endothermic reactions remove heat from the reaction vessel, causing the solid coke bed temperature to drop.
2
Determine the industrial necessity of alternating steam and air runs.
Blowing air causes exothermic combustion of carbon, restoring the red-hot temperature necessary for water gas synthesis.
High temperatures (1000∘C1000^\circ\text{C}) are required to maintain high reaction rates and favor product yield.

Key Concept

Industrial production and energetics of water gas synthesis from coal/coke
Estimated Time:1m 0s
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Topics

Carbon: Allotropes, Coal Distillation, and Industrial Fuel Gases

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