Multiphase CFD Modeling Simulation of a Deaerator

Deaerators serve a critical role in modern generating boilers by removing dissolved oxygen in boiler feedwater that can otherwise cause accelerated corrosion in boiler tubes, condensate lines and process piping.  They also serve the role of preheating boiler feedwater and thus improving the efficiency of the power cycle.  Deaerators are typically ASME certified pressure vessels and operate at pressures between 5 and 15 psig.  Modern deaerators can achieve feedwater oxygen concentrations as low as 7 parts per billion. 

How They Work

The two most common types of deaerators are the tray-type and the spray-type with the spray-type gaining in popularity due to its higher efficiency and wider range of operation.  The typical spray-type deaerator is a horizontally oriented vessel and functions as a continuous flow reactor.  Boiler feedwater to be deaerated enters the tank through overhead spray nozzles and deaerated boiler feedwater leaves via piping ordinarily located on the opposite end of the tank.  Low pressure steam is injected through a sparger located near the bottom of the vessel.  Along the way, feedwater is heated to its boiling point via contact with the steam and thusly stripped of its condensable gases.  Excess steam is vented to the atmosphere.

Improving Efficiency

Improved deaerator designs can lead to competitive advantage by offering lower capital and operating costs and more reliable performance.  We have recently been working with a client interested in evaluating alternative deaerator designs with the aim of increasing the thermal transfer efficiency between steam and water phases and thusly lowering steam consumption by 20% .  Computational Fluid Dynamics (CFD) modeling has been used to virtually test various combinations of sparger, vessel, spray header, and baffles designs in order to meet this objective.

Multi-phase CFD Modeling

Deaerators present an obvious challenge to CFD technology due to the complex interactions between steam, water and surrounding air and have previously been considered too complex for optimization using CFD modeling analysis.  In the common Eulerian CFD approach, individual phases are treated as physics continua where the governing equations are expressed in Eulerian form.  The Volume of Fluid (VOF) method has recently extended this model by allowing neighboring continua where boundaries are immiscible, such as free-surface flows.   This approach captures the movement of the interface between the fluid phases and is often used for marine applications. 

In contrast, Lagrangian multi-phase methods solve the equations of motion for representative parcels of a dispersed phase as they pass through an Eulerian continua.  The Lagrangian method is intended for systems that consist mainly of a single continuous phase carrying a relatively small volume of discrete particles, droplets, or bubbles.  Lagrangian models for solids, droplets or bubbles can be two-way coupled with surrounding continue.  That is, Lagrangian particles interact with the Eulerian continuums dynamically. 

A New Approach

New multi-phase interaction models available in STAR-CCM+ now allow Lagrangian parcels to impinge upon VOF interfaces, thus adding mass and energy to the respective continua in realistic ways.  The video below demonstrates a fully-coupled, multi-phase CFD modeling simulation of a fictional deaerator geometry.  The spray droplets are colored according to the droplet diameter (with the color scale on left) and steam bubbles according to the bubble diameter (via the color scale at the bottom). 

The introduction of these new methods in combining multi-phase techniques paves the way for CFD application on a whole new class of technologies that were previously thought to be too complex for study, including separators, coating processes, fluidized bed reactors, liquid film formation where sprays are present, and many others.  Contact us to find out just how economical the CFD driven fluid dynamic optimization of your process technology can be. 

Stewart BibleComment