Fluid Bed Absorbers

THE TURBOSCRUBBER^®'S FLUID BED TURBULENT CONTACT ABSORPTION (FBA or TCA) SYSTEM BRINGS MASS, HEAT & PARTICLE TRANSFER INTO THE 21^ST CENTURY

Fluid Technologies (Environmental) Ltd. (FTL) and joint venture partner Osprey Environmental Technologies Ltd. (OET) have together been developing and evolving FTL's fluidised bed gas cleaning technology - TURBOSCRUBBER^® - since the early 1990s. In that time they have installed well over one hundred scrubber and stripper plants using the fluidised bed principle in many countries including notably the U.K., Germany and the U.S.A. where the market is generally most receptive to innovative process technologies.

The Turboscrubber^® system uses different hollow plastic elements of varying shape, marketed as Turbofill^® , Turboid^® & TurboPak ^®depending on shape, to generate a three phase fluidised bed in which increased Reynolds numbers (turbulence) in the gaseous phase and the liquid phase provoke an intense enhancement of turbulent action which in turn leads to substantial increases in overall mass and heat transfer coefficients and interphase surface renewal rates. Over many years FTL and OET have researched and tested the complex mechanisms and processes involved in mass and heat transfer, with and without chemical reaction, in a three phase fluidised bed system. The role of element (plastic particle) shape, density and size have been intricately studied by F.T.L., leading to U.S. process patent 5,588,986 (Mass or Energy Transfer Process using Fluidized Bed) and further modelled by FTL & OET along with the detailed chemical engineering, energy related and physico-chemical phenomena inherent in the multifaceted systems for absorption, desorption, heat recovery and fine particulate dedusting encountered in process engineering.

The ultimate aim of this work was to be able to accurately model and predict all necessary variables, whether pressure drop, mass & heat transfer coefficients or reaction kinetics, in order to accurately design Turboscrubber^® plants for all manner of industrial applications. Today the FTL/OET process design model is complete. Available as windows based software, known as ScrubMaster^® , it combines the know how built up along with standard engineering methods to give optimum designs for tower, absorber or desorber construction. The software can also be used as a diagnostic tool for non fluidised bed systems employing fixed packing or sieve plates to promote the interface crucial for efficient mass and heat transfer. The mathematical models developed by FTL/OET map not only the three phase motion of complicated fluid beds and plates but also the more laminar & transition regime flow patterns encountered with fixed packed towers and columns.

Fluid Bed History

Over the years many people have postulated theories about fluidised beds for absorption and desorption. When reviewing the literature one is struck by the enormous diversity of equations and correlations available. This is severely confusing to the practising chemical engineer and has undoubtedly contributed to the view that fluidised beds were little understood novel devices, often designed using rules of thumb or 'black arts', and at best applicable only occasionally.

Virtually all the previous work undertaken has been confined to the use of spherical elements, usually in the range of 20 to 50mm diameter, and most notable amongst the 'early' researchers (1950-1988) are Douglas⁽¹⁾, Vunjak-Novakovic , Vukovic & Littman⁽²⁾ , Kito, Tabei & Murata⁽³⁾, O'Neill, Nicklin, Morgan & Leung⁽⁴⁾ , Muroyama & Fan ⁽⁵⁾ and Calvert⁽⁶⁾.

When the fluid bed concept was initially used it was principally applied in the U.S.A. and Canada and was often limited to combined mass transfer and particulate duties on the large scale ( e.g. power plants & aluminium smelters ) where conventional systems such as those employing random packings would have fouled up rapidly by a combination of particulate deposition and precipitation. When the fluid bed tower was adequately 'designed' it would function not only as an absorber but also as a particle deduster upstream of 'dedicated' particulate removal equipment (e.g. electrostatic precipitators or bag filters). Frequently however, designs did not work out satisfactorily. A prime example, cited by Ling-Shih Fan is his book "Gas-Liquid-Solid Fluidization Engineering"⁽⁷⁾ is the F.G.D plant at Conesville, Ohio where fluid bed towers employing spherical balls had finally to be converted to open spray towers. The problems in these large towers were caused by (a) bed instability with spherical balls, leading to 'gulf-streaming' and wholesale channelling causing a serious reduction in both mass transfer and particulate removal efficiency,

(b) the consequent mass breakage of spheres due to the accelerated number of collisions caused by the 'gulf-streaming' and by the inherent problem of uneven (thinned) wall thickness unavoidable when using blow moulded 'stretched' spherical balls, and (c) the high bed and tower heights and pressure drops required with spherical balls to achieve even modest efficiencies.

Years later the writer, Howard Davis of FTL, was able to examine these problems in detail especially when compared to a new non spherical shape ( an oblate spheroid or 'two dimensional' ellipsoid), known later as the Turbofill^® invented by Herr Wolfram Ruff (8) a practising German chemical engineer working on fluid bed towers in the aluminium industry. Working together on the concept of fluidised beds and the reasons for the considerable improvements ( in mass transfer, fine particulate removal and bed stability) discovered by Ruff, when using the even walled 'ellipsoids', Davis & Ruff jointly produced a new international patent (9) in 1990 for the control and use of the fluid bed process using ellipsoidal shapes.

Fluid Bed Practise

Where the Turboscrubber^® technology using Turboid^® or TurboPak^® packing really comes into its own is where simultaneous transfer processes are required. For example the replacement of a two unit operation such as a venturi deduster or bag filter followed by a packed column by a single Turboscrubber^® is now commonplace. The sub micron particulate removal efficiencies have been shown to compare very favourably when compared directly with venturi scrubbers in full scale tests

Performance Data

Venturi & Packed Tower Combination versus TURBOSCRUBBER

(A Real Case Comparison of Equivalent Efficiency Systems)

 

A Typical Packed Tower Absorber

Versus TURBOSCRUBBER

Generalised HTU Comparisons Between Different Fluid Bed and Fixed Bed Packings. (click to here for graph)