Fluidized Bed Processing

Summary

Fluidized bed processing is a technique that mixes solid particles with either a liquid or a gaseous stream to form a mix that behaves like a fluid. This mix can be made to flow, keep a uniform temperature, and facilitate interaction among solid and fluid particles, supporting heat and mass transfers. The early 1940s saw its first major application in fluid catalytic cracking units that increased a refinery's yield of high-quality gasoline and aviation fuel. The technique has become widely used in the process industry, including the synthesis of polyethylene and polypropylene, key feedstocks of the chemical industry. It is also used in the food and pharmaceutical industries, at power plants, and for waste incineration.

Definition and Basic Principles

Fluidized bed processing is a technique developed from the fact that if small, solid particles encounter a quickly moving fluid of liquid or gaseous nature, the resulting mixture, including the solids, behaves like a fluid itself. This provides numerous advantages in process engineering.

For fluidized bed processing to work properly, the solids have to exist in the form of tiny particles, often called granules, ranging in size from 1–2 millimeters to as little as 0.02 millimeters. The smaller the particles are, the easier it is to get them into a fluidized state. The only exceptions are food particles, which can be much larger because of their low density.

To achieve fluidization and to make use of this physical conversion process, a fluid medium is injected into a bed of solid particles. As the fluid rises at its own superficial velocity—roughly meaning its own speed, which is measured by dividing the volumetric flow rate of the fluid by the cross-sectional area of the bed of solids—it encounters the solid particles. Once the rising fluid exerts a drag force greater than the net weight of the solid particle, which is called minimum fluidization, the solid particles begin to behave like the rest of the fluid. Typically, they become entrained in the fluid and rise with it to the top of the unit. Because of the laws of thermodynamics, the solids quickly attain the same temperature as the fluid medium. Because of their large shared surface area, solids and fluid medium particles can interact very efficiently.

Background and History

On December 16, 1921, German chemist Fritz Winkler was the first to use the principle of fluidized bed processing in a coal-gasification experiment based on small-grained lignite. By 1926, the German company BASF, which employed Winkler, began commercial coal gasification based on Winkler's new technique. Fluidized bed processing has remained an important part of coal gasification and has been applied to the gasification of biomass.

The second most important discovery was the use of fluidized bed processing for the catalytic cracking of long hydrocarbons at petroleum refineries. This was done to obtain more valuable gasoline and aviation fuel from crude oil distillate. American scientists Donald L. Campbell, Homer Martin, Eger Murphree, and Charles Tyson filed their patent for the process on December 27, 1940. It was granted on October 19, 1948. Based on their research, a pilot plant started in Baton Rouge, Louisiana, in May 1940, and the commercial plant began operations on May 25, 1942. Since then, fluid catalytic cracking has been at the heart of every modern refinery.

In each subsequent decade, fluidized bed processing has found new applications in the chemical, petrochemical, pharmaceutical, and food-processing industries. The technique is important in the quest for cleaner power generation and even the production of carbon nanotubes.

How It Works

Bed Preparation. To begin the process, solid particles are placed or injected onto a bed in a holding unit. This is called a packed or a fixed bed, until fluid is applied. The bed is typically in a reactor, boiler furnace, or another part of a processing unit, such as the catalytic riser of a refinery's fluid catalytic cracker. There are different kinds of beds, the most common being stationary or bubbling beds or more complex circulating beds.

Typically, the solid particles of the bed are distinguished by their size and density. They are commonly classified into four categories, known as Geldart groups because they were first proposed by English chemical engineer and professor Derek Geldart in 1973. The larger and denser the particles are, the more energy is needed to fluidize them.

In the case of fluidized bed combustion, there is a difference between solid bed materials (which will not be burned), solid sorbents, and the solid particles that serve as fuel. Solid particles can be inserted into the bed in a continuous process, even if there is already a fluid mix. This is very commonly done, rather than individual batch processing, to save time and energy.

Fluidization. Once the solids are in the bed, fluid is injected into the bed at a high velocity through a distributor, which is commonly either a grid or a porous plate at the bottom of the bed. The fluid can be in either liquid or gas form, as defined in physics. Very often, superheated steam or, in the case of fluid catalytic cracking, evaporated hydrocarbon molecule chains are used.

Because of the velocity with which the fluid flows through the bed to the top of the containing unit, it will exert a drag force on the solids in the bed. Minimum fluidization begins once the solids rise with the fluid medium and become entrained in it as they leave the bed for the top. If a gaseous liquid is used as a fluid medium, bubbles will form in the bed, giving it the characteristic look of boiling water in a process called aggregative fluidization. If a liquid fluid is used, the bed of solids will expand continuously and uniformly as solids rise within the liquid fluid to the top. This is called particulate fluidization.

To measure and control fluidization, process engineers measure the so-called superficial velocity with which the fluid charges through the bed. It is obtained by dividing the volumetric flow rate of the fluid by the cross-sectional area of the bed and corresponds roughly to the speed with which the fluid medium is pressed through the bed. It is typically given in meters per second. Each type of solid has its own minimal fluidization velocity that must be reached for the solids to begin to fluidize.

Processing. In the fluid state, the processing of solids and fluid occurs as designed for each application. This is aided by the uniform heat of the fluid mix, its transportability through pipes, and the high rate of surface interaction between solid and fluid medium particles. The remaining solids and fluid medium are typically separated at the end of the desired processing.

Applications and Products

Fluid Catalytic Cracking (FCC). Traditionally, this is the key industrial application of fluidized bed processing. Both atmospheric and vacuum distillation of crude oil at a refinery leave behind a large percentage of low-value residue instead of desired gasoline, diesel, or aviation fuel. This distillation residue leaves behind long hydrocarbon molecule chains.

Cracking creates much more of the shorter hydrocarbon chains, which yield desirable fuels such as gasoline. The American invention of the FCC in 1942 quickly spread to refineries worldwide after World War II ended in 1945.

The great technical advantage of fluid catalytic cracking is that the long hydrocarbon chains, typically light fuel (or gas) oil, come into contact with the solid catalyst particles in a fluidized, rather than a stationary, bed environment. That means that the total surface area between the catalyst and oil particles is much larger than possible if the catalyst were just fixed to the floor or sides of the reactor. In the cracker's catalytic riser, tiny catalyst particles are surrounded by the oil particles and swim in a fluid stream together for a few crucial fractions of a second.

In this case, it is the fluid medium—the fuel oil preheated to its evaporation point—that undergoes the value-adding conversion. Its long hydrocarbon chains are quickly cracked when encountering the catalyst particles in the fluid mix. The fluid is sent to a reactor. Cyclones at the top of the reactor separate the gaseous hydrocarbon fluid from the spent catalyst and transport it into a distillation unit. The solid catalyst particles are collected at the bottom of the reactor and sent to a regenerator, from where they are fed back into the catalytic riser in a continuous process.

Fluidized Bed Combustion (FBC). This is the oldest application that has gained new attention in the quest for cleaner power generation and waste incineration. Its basic goal is to convert the energy of solid fuels, such as coal, biomass, or waste materials, through gasification, steam generation, or incineration using fluidized bed combustion.

An advanced power plant can use FBC to increase fuel efficiency and lower harmful emissions. Inside the combustor, there is a bed of inert material that will not be burned itself, typically a form of sand. After solid fuel particles are added to the bed, a mixed solid sorbent is added to absorb potentially harmful substances released during combustion. When coal is used as fuel, limestone or dolomite is used as sorbent to absorb sulfur that would otherwise react and be emitted as sulfur oxide, a strong atmospheric pollutant. The solid particles are fluidized by the injection of a fluid, typically heated air. There are two kinds of arrangements for the bed of solids that are hit by the fluid—a bubbling fluid bed or a circulating fluid bed.

The two great advantages of the FBC are that it can be fired by a wide variety and mix of solid fuel particles, including coal, solid wastes, biomass, or natural gas. Also, it generates much less pollution because of the use of sorbent in the fluid to catch sulfur. It can burn fuel at lower temperatures where oxygen and nitrogen of the air-fluid do not yet react to form the pollutant nitrogen oxide.

A simple FBC operates at atmospheric pressure as the solid fuel particles are burned in the fluidized state, and the sorbent particles bind potentially harmful by-products. In a more advanced design, the fluid of generally hot air is pressurized. Now, combustion creates a hot pressurized gas flow that fires a gas turbine, while steam generated during combustion fires a steam turbine for maximum energy yield in what is called a combined-cycle power plant. This accounts for one of the most efficient and least polluting modes of power generation.

Chemical Applications. Fluidized bed processing has become widely used in the chemical industry. It is important, in particular, for synthesizing polyethylene and polypropylene, key basic plastics used for packaging, textiles, and plastic components. Fluidized bed reactors are also used for the industrial production of monomers, such as vinyl chloride or acrylonitrile, which are used to make plastics. These reactors also produce polymers such as synthetic rubber and polystyrene. The advantages of uniform heat transfer, great surface interaction, and transportation as fluid, whether in liquid or gaseous form, have made fluidized bed processing very valuable for contemporary chemical industry processes.

Pharmaceutical and Food Processing. Because of its excellent properties facilitating material and heat transfer, fluidized bed processing has become an important application for the coating or drying of pharmaceuticals. It is also used for batch granulation of pharmaceuticals. In the food-processing industry, the technique is used especially for creating individually quick-frozen (IQF) products. Because of their low density, individual food particles as large as bite-size diced vegetables can be fluidized and frozen quickly to maintain their taste. Complete food packages, as well as individual food particles, are also frozen, blanched, cooked, roasted, or heat sterilized in fluidized bed processing.

Mineral Processing. This older application dating back to the 1950s is used to decompose or purify ores through calcination or the roasting and pre-reduction of ores. Fluidized bed processing is also used in cement manufacture.

Careers and Course Work

Students interested in a career in designing, developing, or supervising the operation of fluidized bed processes should take high school courses in natural science, computer science, and mathematics. Refineries, power plants, and chemical and pharmaceutical companies employ skilled technicians to operate fluidized bed processes, and an associate's degree in science or engineering is a good foundation. Universities and research institutes also hire technicians to assist their researchers in the field.

A bachelor of science or arts degree in physics or chemistry, and perhaps a double major in computer science, is very useful for a career in the field. Students interested in food processing could also major in biology. An engineering degree is an excellent foundation for an advanced career, especially in chemical, electrical, mechanical, or computer engineering. Students should take classes in process engineering, chemical process modeling, and safety engineering and understand thermodynamics well.

Any master of engineering or doctoral degree in chemistry, physics, or chemical engineering is good preparation for a high-level career. A PhD in one of the sciences can lead to an advanced research position with a university, company, or government agency.

For a career in private industry, some sense of economics is also helpful. A willingness to work outside the United States is a plus for those interested in applications in the refining or the power generation industry, as many contemporary greenfield plants using the latest technology are built abroad. For this reason, a semester or two of studying abroad is advised.

Social Context and Future Prospects

As global demand for high-end petroleum products such as gasoline, diesel, and jet fuel continues, optimization of fluidized bed processes in FCCs will remain important. The drive to develop more efficient catalysts and processes, increase desirable product yield, lower energy consumption, and reduce emissions will provide for an exciting field of research in this technique.

The quest for a zero-emission power generation plant would bring enormous prestige and business benefits for its developer. In the future, the growth in global energy consumption may be satisfied with renewable energy sources. However, experts project that fossil fuels will be the dominant means of energy for much of the twenty-first century. Chemical-looping technology has been developed to bridge the use of fossil fuels and the transition to renewable energy.

As the chemical and petrochemical industries have grown, so have applications for well-designed, specialized fluidized bed processes in their production processes. The same holds true for the pharmaceutical and food processing industries, where qualitative process advantages have been sought as well as new modes for innovative processes.

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