Evaporator selection: which type of liquor is best handled in a long-tube vertical (rising-film) evaporator from an operability standpoint?

Introduction / Context:
Long-tube vertical (LTV) rising-film evaporators rely on vapor generation within vertical tubes to accelerate liquid upward, creating thin films and high heat-transfer coefficients. Equipment choice depends on fouling, foaming, viscosity, and crystallization tendencies of the liquor.

Given Data / Assumptions:

• LTV (rising-film) operation: nucleate boiling at tube bottoms, rapid film acceleration, short residence times.
• Normal tube lengths and diameters; reasonable heating steam pressures.
• Steady production without deliberate crystallization inside tubes.

Concept / Approach:
Foaming liquids actually benefit from the shear and rapid vapor disengagement in rising-film tubes; the motion suppresses persistent foam accumulation compared with quiescent pools. Conversely, scaling, salting, or highly viscous liquors hamper LTV: scale blocks tubes, salts crystallize in films, and high viscosity damps acceleration and lowers heat transfer. Such services are better suited to forced-circulation or falling-film designs with better control over velocities and shear profiles.

Step-by-Step Solution:
List behaviors: foaming vs. scaling vs. salting vs. high viscosity.Match to LTV strengths: strong vapor-driven film motion, short holdup, good heat-transfer coefficients → favorable for foamy, non-fouling feeds.Eliminate problematic feeds: heavy scaling/salting or very viscous streams that need forced circulation or scraped surfaces.

Verification / Alternative check:
Industry practice (e.g., paper mill black liquors) and standard design texts list foaming liquids among the common LTV applications (with antifoam control as needed).

Why Other Options Are Wrong:
(b) and (d) invite tube plugging; (c) lowers film Reynolds number excessively; (e) risks deposition and erosion; none exploit LTV advantages like foamy, relatively clean liquids.

Common Pitfalls:
Underestimating foaming tendencies at higher solids; overlooking need for proper vapor-head disengagement; ignoring recirculation alternatives for foulers.

Final Answer:
Foamy liquor