Annular groove graphite evaporators and reboilers

With their specific design our annular groove graphite evaporators and reboilers are especially adapted to the evaporation of highly corrosive chemicals. Annular groove evaporators & reboilers turn the liquid form of a chemical substance such as water or a solvent into its gaseous-form. It is often used to concentrate solutions by evaporation of the solvent. We offer annular groove falling film evaporators, as well as natural and forced circulation reboilers. Annular groove evaporators and reboilers transfer heat from condensing steam or thermal oil on service side to a boiling liquid on process side. Graphite discs are machined to create annular grooves on service side and large banana shaped channels on the process side. Heat is transferred by conduction through the impervious graphite left between the process and the service channels. Annular groove graphite evaporators consist in a stack of discs that are fused together.

The process of evaporation is used widely in the chemical process industry for a variety of purposes. These include the concentration of solutions, revaporization of liquefied gases, refrigeration applications, and generation of pure and mixed vapors for process applications. The term evaporators is usually reserved for the first of these applications, namely the vaporization of the solvent from a solution in order to concentrate the solution. Evaporators may be classified into falling film evaporators (in which evaporation takes place from the film interface with no nucleate boiling at the wall), natural or forced circulation evaporators or reboilers (in which wall nucleate boiling occurs over part or all of the heat transfer surface), flash evaporators, and direct contact evaporators.

Falling film evaporators

In falling film evaporators, the liquid product enters at the top of the evaporator. In the head, the product is evenly distributed into the heating holes. The liquid forms a thin film on the walls, flows downwards at boiling temperature, and partially evaporates. In most cases, steam is used for heating the evaporator. Thermal oil can also be used. The separation of the concentrated product from its vapor takes place in the lower part of the heat exchanger in the vapor/liquid separator.

Falling film evaporators versus flooded evaporators

Falling film evaporators have a number of advantages over their flooded evaporator counterparts. They require a lower amount of fluid as the entire process side doesn't need to be filled with liquid as a thin film is used to cover the surface. Falling film evaporators also show improved heat transfer characteristics over their flooded counterparts, particularly in cases with low heat flux. The residence time in falling film evaporators is extremely short thus creating a gentle evaporation especially under vacuum and subsequently preserving the quality of the product. Process control and automation is also easier in falling Film Evaporators. Thanks to their lower amount of fluid falling film evaporators react faster to changes in energy supply, vacuum, feed quantities, or concentrations.

However the fluid distribution for falling film evaporators may be challenging. The performance might be affected if the fluid is not evenly distributed over the surface. In addition due to the intimate contact of the liquid with the heating surface, falling film evaporators are sensitive to fouling from precipitating solids. The fluid velocity, typically low at the inlet is usually not sufficient to perform an effective self-cleaning. Falling film evaporators are therefore preferably used for the evaporation of clean, non-precipitating liquids.

Natural circulation evaporators

Natural circulation evaporation is based upon natural convection currents. Circulation through convection is achieved through bubble formation. Bubbles have a lower density and rise therefore through the liquid to promote upward lift in the evaporation vessel. Thermosyphon reboilers do not require pumping of the column bottoms liquid into the reboiler. Natural circulation is obtained by using the density difference between the reboiler inlet liquid and the reboiler outlet liquid-vapor mixture to provide sufficient liquid head to deliver the fluid into the reboiler. 

Physically natural circulation evaporators are usually rather short. Removing of the circulation pump reduces the operating costs, however due to characteristics of the system natural circulation evaporators have a long residence time and low flow rates, making their uses more limited than forced circulation evaporators. The most common application of natural circulation evaporators is as reboiler for distillation columns.

Forced circulation evaporators

In natural circulation evaporators, the liquid enters with low velocity and generally, the heat transfer coefficients are quite low. This is particularily true with viscous liquids. So, whenever we are dealing with a concentration of highly viscous and scale forming solutions there is no alternative but to use forced circulation evaporators. By increasing the velocity of the liquid flow through the evaporator heat transfer coefficients increases significantly. the high liquid velocity which is resulting from the pumps prevents the scale formation on heating surfaces.

Reboilers

Reboilers are heat exchangers typically used to provide heat to the bottom of distillation columns. They boil the liquid from the bottom of a distillation column to generate vapors which are returned to the column to drive the distillation separation. The heat supplied to the column by the reboiler at the bottom of the column is removed by the condenser at the top of the column.

Proper reboiler operation is vital to effective distillation. In a typical classical distillation column, all the vapor driving the separation comes from the reboiler. The reboiler receives a liquid stream from the column bottom and may partially or completely vaporize that stream. Steam usually provides the heat required for the vaporization.

Forced circulation reboilers

A forced circulation reboiler uses a pump to circulate the liquid through the reboiler. This is especially useful if the reboiler must be located far from the flash tank or if the liquid is viscous or temperature sensitive (subject to crystallization, polymerization, or degradation) by contact with high temperature heat transfer walls. High liquid recirculation rates are used to increase the heat transfer film coefficient on process side, reduce wall temperatures, thereby reducing polymerization on the wall and associated fouling.

Flash evaporators

In flash evaporation, the liquid is preheated at pressure and then flashed through a restriction into a vessel at lower pressure where vapor is formed in the process of restoring the liquid to its saturation temperature. 

Direct contact evaporators

In this type of evaporator, a hot gas is injected into a pool of liquid and causes it to evaporate, the vapor being carried away with the efflux gas. Such evaporators are relatively cheap and useful for the concentration of corrosive fluids, viscous liquids and slurries which may be difficult to handle in more conventional heat exchanger. Stripping columns are direct contact evaporators.

• Crop protection
• Fumed silica
• Silicones
• Titanium dioxide
• Fine chemicals
• Active pharmaceutical ingredients
• Copper electrolysis
• Epichlorohydrin
• Vinyl chloride monomer
• Fire retardants
• Flavors and fragrances
• Vitamins
• Isocyanates
• Viscose
• Polycarbonate
• Treatment, purification and concentration of spent acids
• and many more…