Showing papers in "Chemical Engineering in 1997"

Advanced distillation saves energy and capital

Abstract: Although new separation methods are continuously being explored, distillation remains the most frequently used separation process. It is well known that distillation is both energy intensive and inefficient. Compared to conventional column sequences, one particular configuration--the fully thermally coupled distillation column--can save significant amounts of energy by reducing thermodynamic losses. In such a setup, a number of columns (typically two) are linked together through vapor and liquid streams without reboilers or condensers between the columns. One practical application of the fully thermal coupled column has been known for a long time. However, lack of design experience and fear of operational and control problems, have prevented its widespread use to date. In addition to energy savings, divided-wall columns can save capital costs, by reducing the number of column shells, reboilers and condensers. The divided-wall column can be applied with real benefits in a variety of refinery, gas separation and chemical processes. Generally, the technology should be considered in any ternary (three-component) separation, as a possible low-cost alternative to conventional schemes. Generic guidelines for the use of divided-wall columns are discussed. The application of the divided-wall column for natural gas liquids separation is also presented.

Bioaugmentation: Put microbes to work

Abstract: The murky ponds of wastewater treatment plants are a natural habitat for microbiological growth The indigenous biomass is an asset to the treatment process because it breaks down organic wastes But as the amount of organic loading increases and discharge limits decrease, reinforcements are needed to gird the biomass and boost its degradation efficiency Bioaugmentation provides that kind of support by altering the composition of the biomass Like biostimulation, which supplements biomass with organic and inorganic nutrients and inducers, bioaugmentation fortifies biomass, but with microorganisms, or bugs, that have been isolated and selectively adapted to degrade specific compounds Added to a wastewater treatment system, the microbes enhance the biomass` ability to respond to certain situations or to tackle contaminants not broken down by their indigenous counterparts This results in improved treatment The paper discusses the selection of microbes, applications, performance expectations, maintenance dosing, and promising research

Aerobic versus anaerobic : Wastewater treatment

Abstract: Biological wastewater treatment facilities are designed to emulate the purification process that occurs naturally in rivers, lakes and streams. In the simulated environment, conditions are carefully manipulated to spur the degradation of organic contaminants and stabilize the residual sludge. Whether the treatment process is aerobic or anaerobic is determined by a number of factors, including the composition of the wastewater, the degree of stabilization required for environmental compliance and economic viability. Because anaerobic digestion is accomplished without oxygen in a closed system, it is economical for pretreatment of high-strength organic sludge. Before the effluent can be discharged, however, followup treatment using an aerobic process is required. Though it has the drawback of being energy intensive, aerobic processing, the aeration of organic sludges in an open tank, is the primary method for treatment of industrial and municipal wastewater. Aerobic processes are more stable than anaerobic approaches and can be done rather simply, particularly with trickling filters. Gradually, the commercialization of modular systems that are capable of aerobic and anaerobic digestion will blur the distinctions between the two processes. Systems that boast those capabilities are available now.

Critical length helps calculate compressible flow

Abstract: Calculations relating to compressible fluid flow incorporate non-linear equations. Trial-and-error solution is often required and arduous to perform. Convergence to the right root can be slow. Even computer programs can fail to perform. This situation is most frustrating in the vicinity of sonic or critical conditions. Application of the critical length (a length where the gas exits at the speed of sound) concept allows the situation to be analyzed quickly. This article provides a computation method based on dimensionless quantities that will always converge monotonically to the solution from the safe side. In thermodynamic terms, that means that the entropy will never surpass its possible maximum.

Hate statistics ? Try neural networks

Abstract: To illustrate the capabilities of today's neural network (NN) tools, a pair of chemical reactions (preparation of dicyandiamide by reacting cyanamide with ammonia at ph 9.2 and 180°C; sulfonation of naphthalene with 100% sulfuric acid) will be evaluated by both regression analysis and an NN. Then, some general guidelines for setting up NN analysis will be proposed.

Catalytic distillation extends its reach

Abstract: Since the early 1980s, catalytic distillation processes have been selected by more than a hundred operators for various applications. Since such a unit performs both reaction and distillation simultaneously, a combined column can replace a separate, fixed-bed reactor and distillation column, thereby eliminating equipment and reducing capital costs. And, compared to the conventional approach, catalytic distillation may also improve other factors, such as reactant conversion, selectivity, mass transfer, operating pressure, oligomer formation and catalyst fouling. The constant washing of the catalyst by liquid flowing down the column and the distillation of high-boiling foulants results in extended catalyst life. Four selective hydrogenation applications of catalytic distillation are discussed: Butadiene selective hydrogenation combined within an MTBE unit; Pentadiene selective hydrogenation; C{sub 4} acetylene conversion; and Benzene saturation.

Advanced distillation saves energy & capital. Part 1

Abstract: This article reveals how a new twist - a novel column design - in an old process can save energy and capital.

Difficult liquid-liquid separations

Abstract: This article discusses the basic configurations and operation of a high-performance coalescer. To understand the technology, it is also necessary to understand the physical mechanisms of coalescence, including interfacial tension, concentration of surface-active compounds, steric repulsion and electrostatic-charge effects on particles. These concepts are discussed in this article.

Solid-liquid separation via filtration

Abstract: Solid-liquid separation is a truly ubiquitous unit operation. It can be found in all parts of a manufacturing process, from raw material purification, through product separation, all the way to waste management. Yet, common as it is, solid-liquid separation is generally not well understood, partly because of the complex nature of fluid-particle systems. Because of this lack of knowledge, such operations are often the problem area or bottleneck in a plant. Yet, with the growing emphasis on process efficiency and waste management, there is an enormous need for more understanding in this technology. It is the objective of this article to present an overview of common solid-liquid separation operations, so that users are aware of all the available options. This article will cover filtration techniques, in which a septum is used to separate particles from a liquid.

Breaking down the data wall: vertical integration in the CPI. Part 1

Abstract: This article discusses vertical integration and what it will mean to the chemical engineer.

Enzyme catalysis: Cleaner, safer, energy efficient

Abstract: Protein catalysts, more commonly referred to as enzymes, are the driving force behind the myriad of chemical reactions occurring in living organisms. By using their ability to distinguish between similar biochemical compounds and optical isomers (enantiomers), with virtually complete discrimination, enzymes are efficient catalysts, making them an attractive alternative for synthetic ones. Tapping into the natural abilities of enzymes, the chemical process industries (CPI) are beginning to realize that enzymes are not only effective for catalyzing reactions of natural compounds within living systems, but that they can also be used to catalyze reactions of unnatural compounds. Enzymes are novel among catalysts in that they are capable of directing asymmetric transformations with complete activity under ambient conditions. As a result, bioconversions, such as the hydroxylation of unactivated hydrocarbon centers, to give alcohols in high optical purity, have few counterparts in traditional chemical catalysis. And unlike most chemical manufacturing catalysts, enzymes work in water, at ambient temperature and near neutral pH. Also, they are easy to dispose of, since they are composed of biodegradable protein. Thus, biocatalysts are the ideal green catalyst, producing less waste and consuming less energy.

The many faces of ion-exchange resins

Abstract: Ion-exchange resins have been used commercially for over 60 years. Softening and demineralization of water for boiler feed and process use were then, and continue to be, the most familiar and widespread applications of ion-exchange resins throughout the chemical process industries (CPI). Several types of membrane-based technologies, such as electrodialysis, reverse osmosis and, more recently, electrodeionization are recognized as alternative methods for water treatment. Yet, modern versions of ion-exchange resins remain a major player in water treatment. In addition, these versatile materials can be found performing a wide range of tasks in both aqueous and nonaqueous environments. Some of these diverse applications include: acid or base catalysis; manufacture of high-purity solvents and reagent chemicals; separation of by-products of fermentation processes; deacidification of organic solvents; high-purity water production for semiconductor manufacture; recovery of valuable waste from dilute process effluents; controlled release of pharmaceutical products; and chromatography, both on the analytical and the industrial scale. The key to understanding the potential of ion-exchange resins is to look beyond their exchange and adsorptive characteristics, and to see their fundamental nature. In other words, it`s necessary to first consider them as spherical, particulate reactive polymers that perform chemical reactions.

Catalytic distillation extends its reach. Part 2

Abstract: This article shows how combining reaction and distillation in a single column can slash capital costs, and improve productivity. Four selective hydrogenation applications of catalytic distillation are discussed: (1) butadiene selective hydrogenation combined within an MTBE (methyl tert-butyl ether) unit; (2) Pentadiene selective hydrogenation; (3) C 4 acetylene conversion; (4) Benzene saturation.

Enhancing safety through risk management

Abstract: A systematic, risk-based approach to safety design is discussed. Such an approach can help eliminate those hazards that pose intolerable risks and mitigate the potential consequences of such hazards. Guidance for identifying tolerable risk levels is also provided.

Sizing up heat transfer fluids and heaters

Abstract: While steam is commonly used in the chemical process industries (CPI) because of its low cost and easy accessibility, it is not always a suitable source of heat. For example, steam systems operating at high temperatures require extremely high pressures, making the use of steam impractical. Heat transfer fluids in combination with fired heaters, on the other hand, can handle the high temperatures without having to deal with the high pressures. Petroleum-derived heat transfer fluids are limited to bulk operating temperatures of about 600 F. For temperatures up to 750 F, synthetic organic fluids are commonly used. Silicone-based fluids, which are less susceptible to fouling than synthetic organic fluids, are also an option for systems that need to be heated to 750 F. They may have use in applications where both high-operating temperature and pumpability at low temperatures are required. However, silicone-based fluids are typically more expensive than other heat transfer fluids. This article covers the selection of synthetic organic fluids operating at temperatures above 600 F. Design and selection of liquid tube-type fired heaters are also discussed.

Variables search : A simple technique for spotting the key parameters

Abstract: The manufacture of most chemical-process products involves a long list of variables, such as temperatures, pressure, catalyst quality, and concentrations of raw material and in-process streams. When something goes wrong during production, it is often hard to single out the variables responsible. This article shows an attractive experimental-design scheme for doing so, known as variables search.

Keep solids in suspension

Abstract: Mixing is an important operation in the CPI. It is not synonymous with agitation. Mixing is a random distribution into and through one another of two or more initially separate phases. Within that broad definition is the important specialty area of liquid-solid dispersion. This paper addresses the dispersion of solids in lower concentrations that don`t affect the rheological properties of the fluid. The just suspended condition represents the lowest grade of complete suspension, but this level of agitation is the most efficient for solids-liquid agitation. Higher mixing speeds waste energy. Undersized mixers need replacing. The top-entering mixer has a long history in the CPI and the environmental area. Many suspension studies were run with this type. These papers result in empirical correlations for just suspension conditions to scale up from laboratory measurement. Variables considered are the agitation speed, liquid and solids physical properties, solids concentration, system geometry and impeller type. Lately, submersible mixers are becoming more popular, but there are no published sizing methods. This article will explain how to define the critical hydraulic conditions in the tank to reach just solids suspension for a submersible agitator of the type described here as FJFA (Free Jet Flow Agitator).

Efficiently generate steam from cogeneration plants

Abstract: As cogeneration gets more popular, some plants have two choices of equipment for generating steam. Plant engineers need to have a decision chart to split the duty efficiently between (oil-fired or gas-fired) steam generators (SGs) and heat recovery steam generators (HRSGs) using the exhaust from gas turbines. Underlying the dilemma is that the load-versus-efficiency characteristics of both types of equipment are different. When the limitations of each type of equipment and its capability are considered, analysis can come up with several selection possibilities. It is almost always more efficient to generate steam in an HRSG (designed for firing) as compared with conventional steam generators. However, other aspects, such as maintenance, availability of personnel, equipment limitations and operating costs, should also be considered before making a final decision. Loading each type of equipment differently also affects the overall efficiency or the fuel consumption. This article describes the performance aspects of representative steam generators and gas turbine HRSGs and suggests how plant engineers can generate steam efficiently. It also illustrates how to construct a decision chart for a typical installation. The equipment was picked arbitrarily to show the method. The natural gas fired steam generator has a maximum capacity of 100,000 lb/h,more » 400-psig saturated steam, and the gas-turbine-exhaust HRSG has the same capacity. It is designed for supplementary firing with natural gas.« less

Soil contamination : Dealing with petroleum spills

Abstract: Soil is often contaminated when petroleum leaks out of storage tanks or is lost during loading and transfer operations. This article presents a brief review of the constituents found in the most common petroleum products, and discusses their toxicity and behavior in soil. It also reviews some analytical methods and state-mandated cleanup protocols for petroleum-contaminated soil. Such information is needed by engineers and other professionals charged with the task of investigating or remediating sites with petroleum-laden soil. At existing and formerly used industrial facilities, gasoline, diesel fuel, and fuel oils No. 1, No. 2 and No. 6 are routinely handled onsite. The potential hazards posed by the main constituents of these products can be compared. To simplify the comparison among them certain constituents can be considered representative compounds within each type of fuel. Their behavior can then be examined on the basis of properties, including solubility, vapor pressure and carbon partition coefficient.

Pollution prevention by design

Abstract: Waste is a common problem throughout the companies comprising the chemical process industries (CPI). It is especially a concern to those who manage solid waste, wastewater systems, hazardous waste or air-pollution-control devices--typically systems for handling end-of-the-pipe wastes. Regardless of its type, waste is almost always composed of purchased raw materials that did not become part of a final product. For example, some chemical manufacturers, such as liquid latex producers, have found that their wastewater stream is a dilute form of their primary product. Much of this waste is the result of process inefficiencies that cause entrapment and loss of raw materials. These are conventionally disposed through equipment cleaning. However, rethinking and improving process design, piping and vessel design and material-handling techniques can improve process efficiency. The result is reduced waste generation, reduced operating costs, cleaner equipment and increased profits.

Reactor design considerations. Part 1. Part 2

Abstract: Part 1 explains how the geometry of scale-up short changes the larger unit of heat transfer area. Part 2 shows the mathematics of scale-up to predict the times for temperature adjustment steps at the larger scales.