Question

In a feed forward multiple effect evaporator unit

• A. Viscosity of liquid is highest in first effect
• B. Transfer from effect to effect is done by pumps
• C. No pump is required to withdraw the product from the last effect
• D. No pump is required to transfer liquid from effect to effect

Hint:

Evaporator Feeding Configurations: - Forward Feed: Fluid moves down a pressure gradient. No intermediate pumps needed. However, highly viscous cold fluid enters where heating capacity is lowest. - Backward Feed: Fluid moves up a pressure gradient. Intermediate pumps are required between every single effect.

Answer 1

The Correct Option is D

Concept: A multiple-effect evaporator system links several individual evaporators in series to maximize steam economy. In a feed-forward configuration, both the thin liquid feed slurry and the heating steam enter the first effect together and flow in the same direction through the sequence of vessels. Crucially, each successive effect operates at a lower boiling temperature than the one before it. To maintain boiling at these lower temperatures, each vessel must be held at a progressively lower operating pressure: \[ P_1 \gt P_2 \gt P_3 \gt \ldots \gt P_n \] This pressure profile creates a natural driving force across the system.

Step 1: Analyzing fluid transfer between effects.
Because the pressure in the first effect (\(P_1\)) is higher than the pressure in the second effect (\(P_2\)), a natural pressure differential (\(\Delta P = P_1 - P_2 \gt 0\)) exists between the vessels. When the control valve on the liquid transfer line is opened, this pressure drop automatically drives the liquid from the first effect into the second effect. Consequently, no intermediate pumps are required to transfer liquid from effect to effect along the sequence. This confirms Option (D).

Step 2: Evaluating why alternative options are incorrect.
Let's review why the other statements are incorrect for a feed-forward configuration:

• Option (A): As water evaporates, the liquid concentrates as it moves through the system. The highest concentration—and therefore the highest viscosity—occurs in the final effect, not the first.

• Option (C): The final effect operates under a deep vacuum (\(P_n \lt P_{\text{atm}}\)). To remove the concentrated product from this low-pressure vessel and discharge it to atmospheric pressure, a discharge pump is required.
Thus, Option (D) is the only accurate statement.