In our previous posts, we discussed various heat transfer equipment. Today, I want to dive deep into one of the most efficient pieces of equipment used in the Pharma and Chemical industries for concentration and solvent recovery: the Falling Film Evaporator (FFE).
Whether you are dealing with heat-sensitive APIs or looking for high-efficiency solvent recovery, understanding the FFE is crucial for any process engineer.
The Falling Film Evaporator is a sophisticated vertical shell-and-tube heat exchanger designed for high-efficiency evaporation, particularly under vacuum conditions. Unlike flooded evaporators where tubes are filled with liquid, the FFE operates by distributing the process fluid as a continuous, gravity-driven thin film—typically between 0.5 mm to 2.0 mm thick—along the inner walls of the tubes. The defining technical advantage here is the elimination of the hydrostatic head. In traditional evaporators, the weight of the liquid column increases the pressure at the bottom, which artificially raises the boiling point (boiling point elevation). In an FFE, since the liquid is only a thin film, the boiling happens at the exact saturation temperature corresponding to the vessel pressure. This allows the system to operate at very low temperature differences (dT), making it energy efficient and perfect for integration with Thermal or Mechanical Vapor Recompression (TVR/MVR) systems.
The key for a successful FFE operation lies in the Liquid Distribution System located at the top of the calandria. For the equipment to function, the liquid must be perfectly distributed to every single tube to ensure the entire inner surface is wetted. If the distribution is uneven, some tubes may receive too little liquid, leading to the formation of dry spots. These dry spots are a process engineer's nightmare; they cause localized overheating, which leads to product degradation, scaling, and eventual tube fouling. The flow regime within the film whether it is laminar, wavy-laminar, or turbulent—is determined by the Film Reynolds Number. It's important to note that maintaining a flow rate above the Minimum Wetting Rate is the primary safeguard against mechanical and process failure in these units.
From a thermal kinetic perspective, the FFE offers an exceptionally high heat transfer coefficient (U) because the thin film presents minimal resistance to heat flow. As the liquid descends, the heat from the shell side causes the solvent to flash into vapor. Interestingly, in most modern designs, the vapor and liquid travel co-currently (in the same direction) downwards. This high-velocity vapor core in the center of the tube actually helps push the liquid film against the walls, enhancing the heat transfer via shear stress. Because the liquid travels the length of the tube in a matter of seconds, the residence time is extremely low. This is the critical reason why FFE is the industry standard for concentrating heat-sensitive APIs, enzymes, and biological extracts that would otherwise decompose if held at high temperatures in a conventional batch reactor.
Before getting into the actual topic, lets have some Q & A's,
Why is it called a "Falling Film" evaporator?
Because the liquid flows down the inner walls of the tubes as a thin film under the influence of gravity, rather than filling the tubes completely.
What is the main advantage of FFE over Rising Film Evaporators?
FFE can handle highly heat-sensitive materials because it operates with a lower dT and does not require a "climb" against gravity, which reduces residence time.
What are "dry spots" and why are they dangerous?
Dry spots occur when the liquid film breaks. This leads to local overheating, which can cause product degradation, scaling, or "coking" inside the tubes.
What is the "Minimum Wetting Rate"?
It is the minimum liquid flow rate per unit perimeter of the tube (typically expressed in kg/m .s) required to maintain a continuous, unbroken film.
How does FFE handle vacuum operations?
FFEs are ideal for vacuum because they have a very low pressure drop. This allows for boiling at significantly lower temperatures, protecting the product.
Why is the distribution plate considered the most critical component?
If the plate is not level or the holes are clogged, the film will be uneven, leading to immediate loss of efficiency and potential equipment damage.
In which pharma processes is FFE commonly used?
Concentration of product rich layers, recovery of solvents from API mother liquors, and handling biological products like insulin or proteins.
What is the typical film thickness in an FFE?
The film is usually very thin, ranging from 0.5 mm to 2 mm, depending on the viscosity and flow rate.
Can FFE handle viscous liquids?
It is effective up to moderate viscosities (approx. 200 cP). For extremely viscous or "thick" fluids, a Wiped Film Evaporator (WFE) is a better choice.
What is the role of the vapor separator at the bottom?
Since the vapor and liquid exit the tubes together, the separator uses centrifugal force to pull the concentrated liquid to the bottom while the solvent vapor exits through the top.
What is the typical heat transfer coefficient (U) for FFE?
It is generally high, ranging from 1500 to 3000 W/m2 .K for water-based systems, depending on the Reynolds number of the film.
Designing of Falling Film Evaporator
When designing an FFE, we start with the Mass and Energy Balance.
1. Mass Balance
If F is the Feed rate, P is the Product rate, and E is the Evaporation rate:
2. Energy Balance (Heat Load)
The heat required (Q) is the sum of the heat to raise the feed to boiling point plus the latent heat of vaporization (Ζ):
3. Calculating Heat Transfer Area (A)
Using the heat transfer equation:
Where:
U = Overall Heat Transfer Coefficient.
LMTD = Log Mean Temperature Difference between the heating media and the process fluid.
4. Determining Tube Count (N)
To ensure a stable film, we calculate the number of tubes based on the Wetting Rate (πͺ):
Where D is the inner diameter of the tube. Usually, πͺ should be maintained between 0.25 to 1.0 kg/m.s for water-like liquids.
Scale-up From Lab/Pilot to Commercial
Scaling up an FFE is not just about increasing the area; it is about maintaining the film dynamics.
The Rule of Thumb: When scaling up, you must keep the Wetting Rate (πͺ) and the Tube Length constant or similar to maintain the same residence time and heat transfer characteristics.
Scale-up Example:
Suppose you have a pilot FFE with 10 tubes (1-inch diameter) processing 100 kg/hr of feed. You want to scale up to 500 kg/hr.
Step - 1:
Calculate Pilot Wetting Rate (πͺ):
πͺ = 100 / (10 x Ο x 0.0254) = 125 Kg/m. hr
Step - 2:
Calculate Commercial Tube Count (Nc)
To keep πͺ the same for 500 kg/hr
Nc = 500 / (πͺ x Ο x 0.0254) = 50 tubes
By keeping the tube diameter and length the same and simply increasing the number of tubes, you ensure that the heat transfer efficiency remains predictable.
That's it......!!
Hope you understood the concept of Falling Film Evaporator.
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