Chemical Engineering - Heat Transfer - Discussion

@Seriyala.

In backward feed, the concentrated viscous liquid can be handled at a higher temperature at the first effect. It usually offers a larger evaporation capacity than forward feed, but it may be a disadvantage from the view of product quality. It is used when feeding it cold since it saves large quantities of steam and increases the economy.

Normally, a backward feed multi-effect evaporator saves a large quantity of steam and increases the economy.

Which is the correct answer?

Please, Explain me.

The backward feed arrangement is a very common arrangement. In this arrangement, the dilute liquid is fed to the last effect and then pumped through the successive effects to the first effect. The method requires additional pumps (generally one pump in between two effects). Backward feed is advantageous and gives higher capacity than the forward feed when the concentrated liquid is viscous because the viscous fluid is at the higher temperature being in the first effect. However, this arrangement provides lower economy as compared to forward feed arrangement.

Answer is D: Lower economy, according to backward and forward multiple effect evaporator backward feed often gives a higher capacity than forward fee when the thick liquor is viscous, but it may give a lower economy than forward feed when the feed liquor is cold.