Flow Reactor Systems

Summary

Flow reactor systems provide the heart of chemical reactors that enable the continuous conversion of various reactants, very often in the presence of catalysts, into desired new products. Contemporary chemical reaction engineering has designed a wide variety of flow reactor systems that have been customized for the specific chemical conversion processes they facilitate. Flow reactors have become essential in process industries as they provide a very effective way of converting raw materials into desired products. The world's demand for gasoline, for example, could hardly be met without the flow reactor enabling fluid catalytic cracking of low-value hydrocarbons into high-value hydrocarbons derived from crude oil.

Definition and Basic Principles

Flow reactor systems are designed to allow the continuous operation of chemical reactors by having reactions take place while the reactants flow through the reactor. This is possible when the reaction times of the desired processes are so short that they can occur while the reactants move through the reactor. A chemical reactor is a unit that houses the process where raw materials, consisting of feed molecules called reactants, react with each other, often supported by catalysts, to form desired products.

In a flow reactor, reactants are loaded continuously at a steady pace and react while flowing through the reactor, and the ensuing products of the reaction are taken off continuously. This enables a steady-state operation of the reactor and eliminates the downtime associated with a stop-and-go, or batch, process. Unlike a flow reactor, a batch reactor is loaded and unloaded for each separate reaction run. Generally, the shorter the reaction time of the process, the more advantageous a flow reactor is over a batch reactor.

Because their true advantages appear only if flow reactors are customized for the exact reaction they facilitate, there is a great variety of flow reactor systems. Designing contact modes for reactants with catalysts, which is required for most reactors, has led to further design variations. Typically, catalysts either remain stationary as reactants flow along or through them in a packed- or fixed-bed reactor or move with reactants in a fluidized bed reactor.

Background and History

The history of flow reactor systems is closely related to the development of chemical reaction engineering, a branch of the young discipline of chemical engineering. With the rise of the chemical industry, beginning in the mid-nineteenth century with the Industrial Revolution, processes were designed that could be run with reactors in either the batch or the flow mode. An early example is Belgian chemist Ernest Solvay's plant that manufactured soda ash from brine and limestone in 1863. Russian German chemist Wilhelm Ostwald's groundbreaking work on catalysts, for which he won the Nobel Prize in Chemistry in 1909, laid the foundation for catalyst use in chemical reactors for which flow reactors are well suited.

The invention of the Haber-Bosch process for industrial ammonia production in 1909, the invention of coal liquefaction to obtain synthetic fuels by German chemist Friedrich Bergius in 1913, and the invention of the Fischer-Tropsch process in 1925 all utilized chemical reactions well suited for flow reactors. In the 1930s, academic work on chemical engineering, including flow reactors, flourished in particular in the United States at the Massachusetts Institute of Technology (MIT) and in Germany. Gerhard Damköhler, who taught physical chemistry at the University of Göttingen from 1936 to 1944, discovered the Damköhler numbers, which describe the timescale of chemical reactions in a flow reactor. The start of the world's first commercial fluid catalytic cracking unit in Baton Rouge, Louisiana, in 1942 marked a milestone in the industrial application of flow reactors. Octave Levenspiel's influential textbook on chemical reactors, Chemical Reaction Engineering, first published in 1962 and since updated, has been credited with teaching generations of chemical engineers.

How It Works

For all the existing variety in flow reactor systems, there are some basic models. These can be distinguished by how they design the flow of the reactants or how they bring together reactants and catalysts.

Continuous-Flow Stirred Tank Reactor (CSTR). In this flow reactor, a tank reactor is continuously fed with reactants that exist in a single fluid phase, which can be either a gas, liquid, or slurry (solids thickly suspended in liquids). The tank reactor may include a catalyst, and the reactants are mixed with a stirring propeller. Ideally, this complete mix creates the desired product, which is continuously removed from the tank. In practice, perfect mixing can be hampered by stagnant regions along tank edges or the involuntary creation of a fast bypass flow of little-mixed reactants. There are different techniques to maximize mixing in a CSTR. Baffles are commonly added to the walls of CSTRs to direct flow to the propeller. Mixing can also be done at multiple stages where reactants flow from one mixing zone to the next inside the tank reactor. A pump can create additional circulation of the reactant and product flow.

CSTRs generally provide uniform product quality, and their operating temperature can be well controlled. Product yield is often lower than with other reactors. This is often improved by using two or three CSTRs after each other to ensure most reactants form a product.

Plug Flow Reactor (PFR). Typically, in a PFR, reactants are mixed in a single fluid phase before they are sent on a journey down a tubular flow reactor. As each reactant of a single plug, or single drop, completely fills the tube's cylinder and flows with the same speed, ideally, there is no upstream or downstream mixing of reactants of different plugs. In an ideal PFR, all reactions occur in each single plug as it flows along the reactor tube. PFR tubes are bent for tubular flow conditions. In practice, there are possible deviations from ideal flow within a PFR. A common form is recirculation of reactants causing dead spots, or the development of laminar (layered) flow, where reactants closer to the reactor wall flow slower than those in the middle of its tube. This requires corrective design resolutions. Another challenge is hot spots in a reactor part that demands special cooling.

PFRs can deal with very high volumes of reactants and quickly deliver very high product yields. Able to operate at high temperatures, PFR heat control has remained challenging. While PFRs have long maintenance intervals, their eventual servicing is more costly than for CSTRs or batch reactors.

Catalytic Multiphase Reactors. Flow reactors can be designed to deal with reactants existing in one or two phases, gas and liquid, and a solid-phase catalyst. There are two basic designs.

In a packed- or fixed-bed reactor, the reactants flow through catalysts that remain stationary. Typically, an area of the reactor tube of a PFR is filled with catalysts in the form of small pellets that are held in place in a stationary bed and can range in size from millimeters to centimeters. The reactants flow through this bed. If both gas and liquid reactants flow downward through the reactor, or gas and liquid move in a countercurrent flow, this is called a trickle bed reactor as the liquid can wet the catalyst for the gas reactants.

The most challenging flow reactors to design are fluidized bed reactors. Here, the catalyst particles begin to move together with the flow of the reactants. The key advantage is that the catalysts can be removed, regenerated, and added without having to stop the process as with a packed bed. Therefore, fluidized bed reactors are a good solution for processes with high catalyst degradation requiring their speedy exchange.

Depending on the speed with which catalysts are entrained and move with the fluid reactants, there are different kinds of fluidized bed reactors. They range from incipiently fluidized bed to bubbling bed and turbulent bed to fast fluidized and finally pneumatic or transport bed.

Applications and Products

Flow reactor systems are widely used in process industries, particularly oil and gas, petrochemicals, and chemicals. Their relatively higher cost, compared with batch reactors, is generally warranted whenever enabling a continuous process is of material benefit. This is especially true for plants with reactors that truly operate continuously, day and night, sometimes for months or years at a time without a maintenance shutdown. Flow reactors tend to be customized for each process they facilitate. Their complexity tends to increase, from a basic CSTR or PFR to a fluidized bed reactor, for example, whenever the optimization of product quality and yield gained by highly sophisticated flow reactor design and operation significantly exceeds the greater costs of specialization.

Oil and Gas Industry. One of the most important applications of a flow reactor system is to enable fluid catalytic cracking at an oil refinery. It is only because long hydrocarbon molecule chains that remain after distillation of crude oil can be broken, or cracked, very efficiently in a fluidized bed reactor that the world's demand for lighter hydrocarbon products, such as gasoline or jet fuel, can be satisfied. In addition, oil refineries employ a variety of other flow reactor systems to facilitate many of their other processes. Catalytic reforming to increase gasoline quality is done both in packed and fluidized bed reactors.

Isomerization and polymerization for higher-octane gasoline and aromatization for petrochemicals are typically done using packed-bed reactors. Hydrocracking, to increase gasoline yield from crude-oil distillates, and desulfurization, essential to clean gasoline and diesel fuels of polluting sulfur oxides, is done in packed-bed or trickle-bed reactors, or in a fluidized bed reactor for desulfurization.

Manufacture of synthetic fuels, such as gasoline or jet fuel from coal gasification in the Fischer-Tropsch process, one of the oldest applications of flow reactors, or the methanol to gasoline process, is done using a bubble bed or a slurry reactor, respectively.

Petrochemical Industry. The petrochemical industry could not operate without the manifold flow reactor systems employed in its processes. The manufacture of such key industrial petrochemicals as ethene (commonly known as ethylene) oxide and dichloride, essential feedstocks for the plastics industry, is done utilizing packed bed reactors. The same is true for the production of maleic and phthalic anhydride, building blocks for resins and plasticizers, respectively, which is done either in packed or fluid bed reactors. Polyethylene, a material for plastic bags and other packaging applications, is also made in either tank or tube flow reactors. The same is true for the petrochemical polypropylene, used for plastics, textiles, and even banknotes, and polystyrene and polyvinyl chloride (PVC), essential for the plastics industry. Flow reactors are the blood vessels of the petrochemical industry.

Inorganic Bulk Chemicals. Flow reactors, from less expensive but reliably packed bed reactors to more complex fluid bed reactors, have enabled the economical production of inorganic bulk chemicals used in the chemical industry. The manufacture of ammonia, one of the most common bulk chemicals used especially for fertilizers and a key part of agriculture, saw with the Haber-Bosch process one of the first commercial uses of a flow reactor. A sulfuric acid plant uses a flow reactor for the manufacturing step of oxidation of sulfur dioxide to arrive at the widely used final product employed for lead-acid batteries, in refining, ore processing, or even wastewater treatment. Similarly, nitric acid, used for fertilizers, explosives, or chemical synthesis; hydrogen peroxide, a powerful bleaching agent; and sodium borohydride, which is used to replace chlorine-based bleaching of paper products, are all manufactured using packed-bed or fluidized bed flow reactors.

Other Uses. Flow reactors are used in virtually every process industry in a very wide variety of applications. Wood processing relies on customized flow reactors for its key processes, particularly those involving bleaching. Increasingly, organic fine chemicals such as pharmaceuticals, cosmetics, herbicides, and pesticides are produced in flow rather than batch reactors. Biochemical processes such as fermentation or biological wastewater treatment have come to employ slurry reactors or packed-bed reactors. Metal-extraction processes such as roasting of sulfide ores, production of crystalline silicon and titanium dioxide, and uranium processing all use flow reactors. The same holds true for gas cleaning and combustion processes. Even solid waste can be incinerated in a flow reactor.

Research. Apart from industrial applications, flow reactors are used in research, especially on the laboratory or pilot scale. They are employed to analyze the mechanism and kinetics, or study of motion, of chemical reactions. They provide data in process simulation and aid the investigation of process performance before large-scale industrial flow reactors are designed and built for new or improved applications. They are also subject to testing new ideas for efficiency and process optimization.

Careers and Course Work

Flow reactor systems are designed by chemical reactor engineers. They work in a subfield of chemical engineering and should combine a good understanding of chemistry and engineering. A key career decision is whether to work in the chemical industry or academia. Operation and maintenance of flow reactors also require very skilled technicians.

Students interested in eventually designing, supervising construction and operation, or operating and servicing a flow reactor should take science courses in high school, particularly chemistry and physics, as well as mathematics and computer courses. The same is true for those who want to pursue research in the field.

There are many opportunities for technicians who benefit from grounding in the sciences. An associate's degree in science, applied science, or engineering is a very good basis for a flow reactor technician's career path.

A bachelor of science in chemical engineering is the most straightforward basis for an advanced career in flow reactor systems. However, a mechanical, electrical, or computer engineering degree can also be helpful. At the same time, a bachelor of science or arts in chemistry with at least some courses in engineering or an engineering degree with an additional minor in chemistry is similarly beneficial. Given the rising importance of computational fluid dynamics (CFD) and numerical mathematics for the field, a degree in computer science or mathematics can also be advantageous.

Postgraduate study of chemical reaction engineering is suitable preparation for an advanced career. When choosing a university for postgraduate work, a student should look for professors with expertise and renown in chemical reaction engineering who lead an active research community. A PhD in chemical engineering with an emphasis in computational science and engineering is good preparation for top research in flow reactor systems.

Social Context and Future Prospects

Without flow reactors, there would not be sufficient fuel for the world's road and air transport needs, sufficient fertilizer to enable crops to grow to feed the world's people, nor the many plastics-based applications contemporary humanity utilizes. Flow reactors provide efficient and economical means to derive desired products from raw materials, including crude oil, natural gas, sulfur, oxygen, and water. The contemporary world depends on the work of flow reactors to such an extent that their absence would decisively change the very nature of society in all industrialized and industrializing nations.

Because flow reactors are essential for core processes defining contemporary industry, society, and culture, significant global research has been conducted into improving their operations and customizing them for new chemical reaction processes. Theoretically, the rise and development of CFD has greatly aided flow-reactor data analysis and simulation for scale-ups from laboratory to industrial size. Contemporary research focuses on the design of transitional, or dynamic, and oscillating operation of catalytic flow reactors. Advanced systems engineering of processes has yielded successes such as reactive distillation—integrating the chemical reaction with the subsequent product distillation in the same reactor. There has been a quest to discover innovative forms for reactors, such as monoliths or fiber reactors, to work with microwaves or ultrasound as energy sources and utilize supercritical reaction media. Research into microreactors has been especially exciting.

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