Evaporator working principle and selection guide: Application of falling film evaporator and multi-effect evaporation

The evaporator is a chemical unit operating equipment that uses heat to partially vaporize the solvent in the solution, thereby concentrating the solution or crystallizing the solute. Evaporators are widely used in salt production, sugar production, dairy product processing, juice concentration, pharmaceutical intermediate production, seawater desalination and zero discharge treatment of wastewater. Through evaporation and concentration, the product concentration can be increased to reduce energy consumption in subsequent drying processes, and the solvent can also be recovered to achieve resource recycling.

Introduction to main types

Circulation evaporator is the oldest and simplest evaporator type. The material circulates naturally or forcedly in the system and is suitable for most conventional concentration needs. The material liquid in the falling film evaporator is evenly distributed from the top film distributor to the inner wall of each heating tube, and rapidly descends in a film shape along the tube wall. The solution residence time is short and is especially suitable for low-temperature concentration of heat-sensitive materials such as traditional Chinese medicine extracts, juices and dairy products. The scraper-type thin film evaporator is equipped with a high-speed rotating scraper in the heating cylinder to scrape the material on the heating surface into a constantly updated film. The evaporation intensity is extremely high, and the material stays in the heating zone for only a few seconds. It is an ideal equipment for processing high-viscosity materials and heat-sensitive products.

Multi-effect evaporation andMVRtechnology

Multi-effect evaporation is a thermally coupled energy-saving technology that operates multiple evaporators in series. The secondary steam generated by the first-effect evaporator is used as the heating source of the second-effect evaporator, and the latent heat of vaporization of the steam is utilized step by step. Compared with single-effect evaporation, the steam consumption of five-effect evaporation can be reduced by about 80%, and the energy saving benefits are very significant. Mechanical compression vapor recompression technology (MVR) is a more efficient energy-saving evaporation solution. The low-temperature secondary vapor is compressed and heated by an electric-driven compressor and then used as a heating heat source. The thermal economy is far better than that of multi-effect evaporation.

Key points for selection and design

The selection and design of the evaporator must be comprehensively determined based on material characteristics and process requirements. For heat-sensitive materials, falling film or thin film evaporators with short residence time and low evaporation temperature should be preferred. Materials with high viscosity or solid particles require forced circulation or scraper evaporators. The material selection of the evaporator needs to consider the corrosiveness of the material and the operating temperature. Common materials include304/316Stainless steel, titanium, Hastelloy and non-metallic materials such as graphite andPTFElining.