Multiple Effect Evaporators
Falling Film Evaporators
Falling film technology has certain advantages over rising film technology. In multiple effect systems, a key advantage is that the technology allows for very low delta T on a body without losing stability. As a result, the technology is suited for seven or eight effect operation and also for applications with fixed steam pressure and system vacuum such as a pre-evaporator system. The advantage is provided at the cost of increased system complexity and energy consumption. Because of the increased instrumentation, piping and pumps, a falling film system is more expensive than a rising film system of similar capacity.
Rising Film Evaporators
Rising film evaporator technology is also referred to as Long Tube Vertical (LTV) technology. This technology has historically been used for kraft pulp mill multiple effect evaporation systems. The process utilizes the change in buoyancy of the liquor upon heating and evaporation to drive the liquor through the tubes. A typical body has a vertical tube bundle with a vapour head mounted on top of it with internal chevron type separators. There are several advantages of LTV technology: the simplicity of the system; there are no recirculation pumps and the instrumentation is limited. The simplicity of the system results in low installed capital cost.
Pre-Evaporation
Pre-evaporation systems utilize low-level waste heat to evaporate weak liquor prior to the main evaporator train. Aside from using waste heat, one of the main advantages of a pre-evaporation system is condensate segregation. The ideal scenario for a mill with a pre-evaporation system is to use separate liquor storage tanks for the virgin weak liquor and liquor that has been through the pre-evaporator. The pre-evaporator strips the bulk of the TRS and VOC's from the liquor resulting in an evaporator system condensate that is substantially cleaner.