In pulp and paper manufacturing, the spent cooking liquor (“black liquor”) must be concentrated before recovery boiler firing.\r Which evaporator arrangement is conventionally used to concentrate black liquor efficiently?

Introduction:
Black liquor from kraft pulping contains dissolved lignin, hemicelluloses, and inorganic chemicals. Before chemical recovery, it must be concentrated from low solids to much higher solids. Energy efficiency is crucial because evaporation is steam-intensive.

Given Data / Assumptions:

• Feed is dilute black liquor requiring concentration.
• Target is an energy-efficient, industry-standard method.
• No crystallization step is routinely required for black liquor concentration.

Concept / Approach:
Multiple-effect evaporation reuses vapor from one effect as the heating medium for the next, dramatically cutting steam consumption relative to a single effect. Black liquor plants typically use several effects (often 5–7) in forward, backward, or mixed feed configurations, sometimes with falling film or forced-circulation effects to handle viscosity rise at high solids.

Step-by-Step Solution:
Identify energy goal: minimize steam per kg of evaporation.Compare options: single effect wastes latent heat; multiple effects cascade it.Select the standard: multiple effect evaporator trains are conventional.

Verification / Alternative check:
Mill process flow diagrams universally show multi-effect evaporator sets upstream of concentrators and recovery boilers.

Why Other Options Are Wrong:

• Single effect (with or without crystallizer): Uneconomical and crystallization is not a normal step for black liquor.
• Crystalliser options: Black liquor solids are concentrated, not crystallized, prior to firing.

Common Pitfalls:
Ignoring viscosity and fouling; liquor properties dictate effect type and temperature profile but not the multi-effect principle.

Final Answer:
multiple effect evaporator.