Adiabatic Fixed-Bed Reactors

Practical Guides in Chemical Engineering

Jonathan Worstell

• Publisher: Butterworth-Heinemann
• Year: 2014
• Print ISBN: 9780128013069
• eBook ISBN: 9780128014660
• Topic: Technology & Engineering
• Subtopic: Industrial Management
• Index: Technology & Engineering

Practical Guides in Chemical Engineering are a cluster of short texts that each provides a focused introductory view on a single subject. The full library spans the main topics in the chemical process industries that engineering professionals require a basic understanding of. They are 'pocket publications' that professional engineers can easily carry with them or access electronically while working.Each text is highly practical and applied, and presents first principles for engineers who need to get up to speed in a new area fast. The focused facts provided in each guide will help you converse with experts in the field, attempt your own initial troubleshooting, check calculations, and solve rudimentary problems.Adiabatic Fixed-bed Reactors covers the fundamentals of fixed-bed reactors, including various types and their physical properties. Applications of each device type are discussed, as well as trouble-shooting Solid-supported Catalysts. This text is ideal for any engineer who is new to working with fixed-bed reactors and needs to know the basics quickly and easily.- Practical, short, concise information on the basics will help you get an answer or teach yourself a new topic quickly- Supported by industry examples to help you solve a real world problem- Single subject volumes provide key facts for professionals

Chapter 1

Introduction

This chapter discusses the concept of catalysis and the reasons for designing, building, and operating fixed-bed reactors. It also presents the types of fixed-bed reactors available for use and relates reactor type to the heat transfer requirements of the chemical process. This discussion occurs via examples from the chemical processing industry. It also introduces the classification of fixed-bed reactors as adiabatic or nonadiabatic. This chapter also introduces the concepts of catalyst productivity and selectivity.

Keywords

Catalyst; fixed-bed reactor; adiabatic; nonadiabatic; catalyst productivity; selectivity

1.1 Slow Reactions and Catalysts

Many reactions are too slow to be commercially viable, no matter how much we desire the product. Over the centuries, we have learned that adding a particular chemical, such reactions proceed at commercially viable rates. We call that particular chemical a “catalyst.” A catalyst is a substance that accelerates a specific reaction toward its equilibrium but remains unchanged after the reaction achieves equilibrium.¹ Many times the catalyst and product can be in the same phase at the end of the reaction. In other words, the catalyst contaminates the product and must be removed before the product is sold. Achieving this separation can be expensive, which increases the asking price for the product. Separation costs can actually make the product commercially unviable.

Two options exist for alleviating the problem of catalyst separation: (1) increase catalyst productivity to the point that only parts per million of it remain in the product and (2) make the catalyst stationary relative to product formation.

The first option may provide the simplest process, provided we can find a catalyst with sufficient productivity. Catalyst productivity is the rate at which reactant disappears or product appears

• per number of catalytic sites;

• per catalyst surface area;

• per catalyst volume or per catalyst weight.

The chemical processing industry (CPI) generally defines catalyst productivity as rate per unit catalyst weight.²

The chemical process could be as simple as charge the reactor with reactant, inject catalyst, stir, empty the reactor, then sell the product. While such a process certainly has its advantages, it also has some major disadvantages. First, since the catalyst remains in the product, its components must appear in the product specification, even if present as parts per million. If the product is a chemical intermediate, which means it undergoes further reaction, then any change in its specification must be communicated to its users and must gain their approval. In many cases, the processes using such chemical intermediates are sensitive to those parts per million of catalyst. In some cases, those parts per million are beneficial to the process; in most cases, they are a detriment to the process. Thus, the parts per million catalyst in the product must be controlled, which represents a manufacturing cost. A greater disadvantage is we cannot change the catalyst without informing our customers since we will have to alter the product specification. If we adopt a more cost-effective catalyst, our customers are going to want some or all of that cost savings, i.e., they will want a reduction in product price. Or—if those catalyst parts per million are beneficial to the customer, then the customer may simply refuse our request to change catalyst, thereby forcing us to supply current product for the life of the contract. But, the greater disadvantage of this option is the product tells the world what catalyst we used to manufacture product. Such insight allows them to use a similar catalyst or to develop a more productive catalyst.

Second, if the product enters the food chain, either as an ingestible or as a food container or wrapper, then it must undergo significant testing and costly certification by a variety of government agencies. Once a product gains regulatory approval, any change to its process or specification can represent recertification, which is a costly proposition. Thus, improving catalyst performance or changing the catalyst becomes problematic due to all the regulatory issues raised by such a change.

Third, what to do with product not meeting specification? This situation occurs when the catalyst is not as productive as it should be. In this case, the product contains too much catalyst. Diluting the “off-spec” material with “on-spec” product is the most likely action. Diluting in this fashion can require considerable time if the volume of off-spec material is large.

The second option is to immobilize the catalyst relative to process flow. In this case, the catalyst is stationary and the process fluid flows over it or through it. To immobilize a catalyst, we must adsorb, coprecipitate, or attach by some other means the active chemical component of the catalyst onto a solid. The solid must provide enough surface area to meet the productivity requirements of the process. In other words, the solid-supported catalyst must possess a productivity sufficient for economic viability. The solid must also possess enough strength to support its own weight when contained in a vessel. If these requirements are met, then we can install a fixed-bed reactor in our process.

There are several advantages to using a fixed-bed reactor in a catalyzed process. First, we know where the catalyst is—most of the time. Second, we do not have to recover the catalyst from any process stream or from our product. Third, product registration with a government agency may not be required, or, if required, will be simplified considerably since we do not leave trace amounts of catalyst in the product. This advantage can reduce the time to develop a given process and reduce development costs considerably since registration with a government agency may not be required. Fourth, we do not have to disclose the catalyst’s components in our product specification since the product does not contain them. This advantage allows us to decide whether to patent our catalyst or use it as a trade secret. Also, we do not have to inform customers of any catalyst change, unless so stipulated by the sales contract. Fifth, this option expands the choice of catalyst since the catalyst does not have to possess extraordinarily high catalytic productivity. Sixth, solid catalysts can be regenerated. Regeneration is a procedure that returns catalytic activity to or close to its original value. Regeneration generally involves burning organic chemical deposit off the catalyst’s solid surface, then chemically reducing the metal component of the catalyst before exposing it to process fluid.

Fixed-bed reactors also possess disadvantages. First, solid-supported catalytic productivity declines with time for a variety of reasons. In many cases, it is difficult to distinguish which mechanism is causing the catalytic deactivation. Thus, solid-supported catalysts require periodic regeneration or replacement. Second, solid-supported catalysts can move en masse when the screens retaining the catalyst bed fail. When the retaining screens fail, catalyst flows into downstream piping and equipment. Such a screen failure initiates a massive maintenance effort to rehabilitate the process. Third, the solid-supported catalyst can be crushed by its own weight, thereby plugging the fixed-bed reactor. Fourth, the catalyst can attrite, i.e., form dust or fines, which escape the reactor and enter the downstream process. Such fines can blind filters, plug control valves, or accumulate in process dead-legs. In general, such fines cause havoc in the process.

When evaluating the advantages and disadvantages of fixed-bed reactors, the former generally overpowers the latter, making fixed-bed reactors the most popular method for catalyzing reactions in gas or liquid process streams.^2(pp5–7)

1.2 Fixed-Bed Reactor Classification

We generally classify fixed-bed reactors as adiabatic or nonadiabatic. Adiabatic and nonadiabatic are thermodynamic terms describing energy exchange. Energy is an extensive variable, thus it depends upon the quantity of material being investigated. We call a given quantity of material a “system” and we idealize it as separated by real or imaginary walls from the rest of the universe, which we call the “surroundings.” By separating the system in this manner, we can control or observe energy exchange between it and its surroundings. A system may be isolated, in which case energy, either as heat or work, is not exchanged between the system and its surroundings. An adiabatic system exchanges work, but not heat, with its surroundings.³ Thus, the walls of an adiabatic system are heat opaque; they are insulated. But, systems with no or poor insulation can be adiabatic if any change within them occurs more rapidly than does achieving a new energy equilibrium with their surroundings, which is the case for fixed-bed reactors: localized events occur, then disappear, within fixed-bed reactors before a new energy equilibrium can be established with their surroundings.⁴ Adiabatic fixed-bed reactors are the reactor of choice for solid-supported catalysts due to their simple design, straightforward construction, and “hassle-free” operation.

Nonadiabatic fixed-bed reactors exist. They are used primarily in processes that are highly exothermic, such as oxidation of hydrocarbons. These reactors require heat transfer along the catalyst bed, which necessitates a high heat transfer surface area to reaction volume (HTSA/RV) ratio. Filling a small diameter pipe, i.e., a tube, with solid catalyst, then submerging it in a flowing fluid is the easiest method for achieving a high HTSA/RV ratio. However, one tube is commercially unviable, but multiple tubes in one container or shell can be commercially viable. Such fixed-bed reactors resemble upended heat exchangers. Multitube fixed-bed reactors are also used for endothermic reactions, steam reforming being an example.

1.3 Processes Operating Fixed-Bed Reactors

1.3.1 Ammonia Synthesis

Civilization started with the advent of agriculture. As agriculture became more productive, the human population increased, thereby increasing the demand for agricultural products. The use of fertilizers was and is an integral component for increasing agricultural productivity. Nitrogen- and phosphorus-containing chemicals are the most important ingredients of a fertilizer. Plants use the nitrogen to synthesize purines and pyrimidines, which form the base pairs in plant DNA, and the phosphorus, as phosphate, forms the DNA backbone from which the purines and pyrimidines extend.

Until the third quarter of the nineteenth century, animal waste and decaying vegetation supplied agriculture with nitrogen-containing fertilizer. Guano from Chile and Peru provided most of the nitrogen-containing animal waste. Decaying vegetation came from plants rotting in fields. By mid-nineteenth century, demand for nitrogen-containing chemicals was beginning to overtake their supply. The burgeoning steel industry came to agriculture’s rescue. Steel requires coke which comes from the destructive distillation of coal. Coal comes from plants; thus, nitrogen-containing chemicals are in it. The steel industry recovered those nitrogen-containing chemicals and sold them to agriculture as fertilizer. However, by 1870 or so, guano, decaying vegetation, and coke-derived nitrogen could not meet fertilizer demand. About 1870, chemical companies started mining saltpeter, then modified it for agricultural use. The fertilizer market stabilized until 1900. Around the turn of the century, reports began to appear claiming the global supply of saltpeter reserve was about 30 years. Shortly thereafter, chemists in Germany realized that air represented an unlimited supply of nitrogen for ferti...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• Chapter 1. Introduction
• Chapter 2. Fundamentals of Fixed-Bed Reactors
• Chapter 3. Catalyst Deactivation
• Chapter 4. Improving Fixed-Bed Reactor Performance
• Chapter 5. Scaling Fixed-Bed Reactors
• Chapter 6. Regeneration of Solid-Supported Catalysts
• Appendix. Foundation of Dimensional Analysis