|
|
VAPOR COMPRESSION FLASH EVAPORATION
VACOM
Description of Operation
Wastewater is preheated across a secondary heat exchanger and diverted into a primary recirculation loop. Within the primary recirculation loop, the temperature is elevated to create separation of steam generated from the liquid. The wastewater is continuously pumped out of a disengagement "flash" vessel through a primary heat exchanger and back into the vessel across a flash diffuser located above the liquid level in the vessel. Flows and velocities within the primary recirculation loop are designed to create turbulent self-cleaning action within the primary heat exchanger. The system is also designed to eliminate evaporation within the primary heat exchanger and create a steam separation or "flash" zone within the vessel only.
Steam generated from the boiling wastewater is recycled and the heat recovered by evacuating the steam off the disengagement vessel through a blower, which imparts a pressure and temperature rise to the steam. The steam is blown back into the primary heat exchanger where it transfers all of its latent heat of vaporization and completely condenses. Pressures and temperatures within the disengagement vessel and primary heat exchanger are automatically controlled.
Steam that is evacuated from the disengagement vessel passes through a mist eliminator. Below the mist eliminator is a spool section, which confines the boiling pool and also allows for enough headspace to disengage froth or foam, developed during active boiling, from the liberated steam.
As mentioned, the steam gives up its latent heat to the incoming wastewater while being blown into the primary heat exchanger and condenses. Pressurized by the blower, the condensate exits the primary heat exchanger, and then passes through a steam trap followed by a secondary or pre-heat exchanger. The secondary exchanger is used to recover heat from the condensate by preheating the incoming wastewater. Condensate, now effluent, exits the system after the secondary heat exchanger at approximately 95-100 degrees Fahrenheit.
Heat losses of 2 – 5% are expected. Make-up heat for heat losses and heat required for initial start-up needs to be supplied in the form of low pressure ( 15-25 PSIG ) steam. Make-up steam is introduced through a control valve, which is automated via a temperature controller, and introduced indirectly into the primary exchanger. As set point temperature is reached the steam valve closes. As (0.1) degree of temperature is lost, the valve re-opens to maintain temperature. The temperature controller also initiates high and low temperature alarm functions.
The wastewater feed stream is continuously recirculated from the VACOM skid and back again to the head of the feed pump. The feed rate to the system is controlled to match the evaporation rate via level controls housed in an external standpipe attached to the disengagement vessel. The level controls are used to measure the liquid level within the disengagement vessel, indicate high and low level alarm functions, and automatically shut the system down should and alarm function occur and to control the feed/recirculation modulating valve. As the wastewater evaporates, the liquid level drops. The modulating valve controls the quantity of feed entering the recirculation loop. The modulating valve controls the level of liquid within the disengagement vessel to within ½".
Contaminants are continuously recirculated and concentrated within the vapor compression distillation system. These concentrates are periodically purged from the system by actuating a discharge valve located on the primary recirculation loop. The valve is controlled from a timer, within the PLC, set to achieve the ratio of concentration relative to the volume required for the application. As the valve opens, the recirculation pump pumps concentrates form the system. The concentrate valve can also be controlled automatically via on-line concentrate analytical devices depending on the application.
The system is designed for unattended operation and controlled with a programmable logic controller (PLC). The standard control system (Allen Bradley SLC 500) and program optionally provides for integration of customer supplied feed and concentrate tanks and their respective level devices.
Air actuated valves, motor starters and associated alarms and system notifications are controlled through the PLC via pressure transducers, temperature controllers with RTD’s and flow meters. Provided for safety, as independent devices, are temperature and level switches, and vacuum and pressure relief valves. |
