Brochure Forced Circulation Evaporator

Swenson Technology, Inc.

FORCED-CIRCULATION EVAPORATOR

     The liquor in a Forced-Circulation Evaporator is pumped through the tubes to minimize tube scaling or salting when precipitates are formed during evaporation. A Swenson forced-circulation evaporator (with a submerged inlet) complete with a single-pass vertical heat exchanger, elutriating leg, cyclone, and top mounted barometric condenser is shown in Fig. 8.
     Slurry is pumped from the bottom cone of the vapor body through the tubes of the vertical heat exchanger, where heat is added, and back into the vapor body where evaporation occurs. Sufficient slurry height (submergence) is maintained above the tangential inlet on the vapor body and above the top tubesheet of the heat exchanger to suppress mass boiling in the inlet and prevent surface (local) boiling on the tube surface. This is necessary to preclude salt precipitation on the tangential inlet and tubes.
     A high circulation rate is provided for adequate tube velocity to achieve good heat transfer. Therefore, lower slurry temperature rises are assured which minimize supersaturation of the solution. A sufficient quantity of salt crystals is suspended in the circulating system to provide seed crystals in the boiling zone for salt growth. Adherence to these basic principles of crystallization results in coarse crystals and minimal wall and tube salting, so less equipment washing is required. This conserves energy because less steam is required to boil wash water and this increases on-stream time for the evaporator.
     The circulating pump is usually of the axial-flow, single-elbow design, well-suited for the high flow rates and low pressure drops in Swenson designed circulating systems. These heavy duty pumps operate at low speeds, which reduce maintenance and minimize mechanical attrition of the salt crystals.
     Circulating piping interconnects the vapor body, the heat exchanger and the pump. Conical liquor chambers provide gradual, low pressure drop transitions from the circulating piping to the tube bundle which is particularly important for the establishment of uniform feed to the tubes. To provide for thermal expansion without expansion joints, the circulating pump base is spring-mounted. As an alternative, the entire pump may be hung from the circulating piping.
     The tangential inlet provides excellent mixing of slurry in the vapor body because of the circular motion it creates. Secondary vertical currents are also generated, mixing body slurry with the hotter slurry entering the vapor body to reduce the degrees of flash. This agitation minimizes salt buildup on the bottom cone of the vapor body. A swirl breaker is provided in the circulating slurry outlet.
     The vapor body is conservatively designed both in diameter and height. It is important to have an adequate free space above the liquor level to allow the liquor droplets entrained in the vapor leaving the boiling surface to reach equilibrium and return by gravity to the circulating slurry. The large diameters result in low vapor velocities which minimize entrainment and provide adequate retention time for salt growth.
     A mesh-type entrainment separator may be installed in the upper portion of the vapor body to reduce solids carryover to normally less than 50 parts per million parts of vapor. Swenson's sturdy mesh container design has virtually eliminated mesh falls into the vapor body. Other types of entrainment separators are also available.

     The elutriating leg, attached to the bottom cone of the vapor body, is a convenient device for thickening the slurry it receives from the vapor body and for salt crystal washing and classification. Slurry enters the top of the leg through a unique slurry-inlet device, which improves washing efficiency by reducing agitation in the leg.* Salt crystals are fluidized and washed in the leg with a portion of the feed liquor which enters the bottom cone and is distributed with a perforated, dished head. Smaller crystals are washed into the vapor body for additional growth and the larger crystals are discharged from a connection near the bottom of the leg.
     A Swenson designed, low-pressure-drop liquid cyclone is sometimes used to clarify liquor discharged from the evaporator. The driving force is the pressure drop across the circulating pump. Thickened slurry is returned through a wide-open cyclone underflow connection to the circulating piping before the pump suction.
     Another Swenson innovation is the direct-contact condenser mounted on the vapor body. A short piece of vertical pipe connects the vapor body with the condenser to minimize piping and pressure drop. This design also eliminates structural steel for support of a separate condenser. For cooling tower applications, the hotwell is elevated to permit gravity flow of water from the hotwell to the top of the cooling tower, thus eliminating the need for a pump.
     A Swenson forced-circulation evaporator with horizontal heat exchanger and a top-mounted stripping column is shown in Fig. 9. Reflux liquid is introduced on the top tray of the column to strip one or more compounds from the water vapor. Stripping columns are used for special applications and are provided either integral with the evaporator or as a separate column. The columns are for the recovery of valuable components from the water vapor and for the reduction of volatile pollutants. Swenson has supplied stripping columns with ballast and valve trays for Caprolactam and Boric Acid service. Spray columns have been used for the recovery of Fluorine generated along with the vapor during the concentration of Phosphoric Acid.
     Heat exchangers can be located on the discharge or suction side of the circulating pump.** In some cases, both discharge and suction-side heat exchangers have been provided in the same circulating system to maximize heat-transfer surface. Multiple circulating systems are provided for the same vapor body on large evaporators. Swenson designs include special provisions to reduce tube plugging caused by salt lumps.
     A variety of forced-circulation evaporator designs are available, with and without elutriating legs or cyclones. Different inlets and outlets are used on the vapor body, tailored for the particular application.
     For some applications, such as super-phosphoric acid and red liquor, it is necessary to use un-submerged inlet evaporators to reduce the volume in the system to a minimum or to minimize foaming. Because of the higher head created by the un-submerged inlet, it is sometimes necessary to use a mixed-flow circulating pump instead of an axial-flow pump.

*U.S. Patent No. 4,113,552
**U.S. Patent No. 3,303,870