Fluid Solid Reactions Questions and Answers

Answer: C

Correct answer: True

It is the model’s defining assumption: the reaction is confined to a sharp boundary between fully converted ash and entirely unreacted core, because nothing penetrates the core. Where the solid is porous the reaction occurs throughout the particle instead, and the progressive conversion model applies rather than this one - which is the first question to ask before fitting either.

Answer: D

The diffusion path lengthens as the core shrinks, so the rate falls even at constant conditions. This is why ash diffusion control gives such a strong dependence on particle size. The path lengthens as the core shrinks.

Answer: A

Both are true and the reason gives the mechanism, and the last part of the conversion therefore takes disproportionately long, which matters greatly when a high conversion is required. The last stages take the longest.

Answer: B

The first two are correct. The third is wrong, since the relation between time and size is non-linear and using the mean size gives a systematically wrong answer. The distribution must be integrated. Small particles finish long before large ones.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the cause. Because the conversion time depends non-linearly on radius, the average conversion of a mixture is not the conversion of the average particle - the small particles finish long before the large ones and then contribute nothing further, while the large ones dominate the time to reach high overall conversion. A size distribution must therefore be integrated over rather than replaced by a mean.

Answer: D

The fluid must cross each of them in turn, and any one can dominate the overall rate. Identifying which one does is the whole of understanding the kinetics, and the order cannot be rearranged since each resistance lies physically inside the one before it.

Answer: A

Ore roasting and coal combustion both exploit this, and the vigorous mixing prevents local overheating. The solids residence time distribution is broad, which means some particles leave before they are fully converted.

Answer: B

Both are true and the reason gives the consequence: complete conversion of every particle is impossible in a single well mixed stage, which is why several stages in series are sometimes used. Some particles leave almost immediately.

Answer: C

The first two are correct. The third is inverted, since well mixed solids means some particles leave almost at once, while a moving bed gives every particle the same time. Mixing and conversion pull against each other.

Answer: D

The iron in ammonia synthesis is a catalyst and is not consumed, so the reaction is catalytic rather than a fluid solid reaction proper. The other three all transform the solid itself. A catalyst is regenerated each cycle.