Showing papers in "Chemical Engineering in 1994"

How to disperse gases in liquids

Abstract: Mixing of a gas in a liquid is required in fermentation operations and a variety of oxygenation and hydrogenation processes. Agitation increases the mass transfer between the gas and the liquid phase. Gas-liquid reactors equipped with agitators are often operated at high power input and large gas holdup, making these units among the most difficult to design. Modern high-efficiency and concave-blade disc impellers provide the proper balance of flow, turbulence and shear for most applications. For an overview of the state of the art in gas-liquid agitation, a number of excellent reports are available. This article aims to present practical design guidelines while explaining the basics of gas-liquid mixing. It expands on design procedures presented in 1976 by Hicks and Gates and capitalizes on recent advances in flow-visualization techniques, development of new gas-dispersion impellers and improved understanding of the dispersion process itself. Methods to calculate power draw, gas holdup and mass-transfer rate are illustrated with examples. It is shown that concave-blade turbines provide better performance than the traditional flat-blade units.

Advanced impeller geometry boosts liquid agitation

Abstract: A traditional agitator impeller often functions as a rather inefficient pump because of the way it produces fluid motion and pressure head. However, one can improve the amount of flow or shear generated by an impeller at constant power consumption and torque by changing its design. For example, a high-efficiency, axial-flow impeller produces more fluid motion per unit of power at constant torque than an otherwise similar pitched-blade turbine. The more-vigorous fluid motion cuts blend time and enhances heat-transfer in various flow-controlled mixing operations, such as blending of miscible fluids. For most applications, a higher degree of agitation intensity can be achieved on the same machine by substituting a high-efficiency impeller for a conventional pitched-blade unit. The high-efficiency impeller features a larger geometric pitch angle (30--60 deg) at the hub than at the tip (10--30 deg). Results from recently conducted controlled experiments indicate the beneficial effects of the high-efficiency impeller on blend time and heat-transfer coefficients in liquid-liquid mixing as well as solids suspension. This articles focuses on liquid agitation, with discussions of solids suspension set aside for a forthcoming piece in this series of articles on mixing.

Removal of Water Pollutants by Lemna gibba

Abstract: Duckweed can be used in water treatment systems to remove various pollutants such as ammonia and phosphate. In this work the influence of temperature and concentration of pollutants on the rates of removal of ammonia, phosphate, and organic load was investigated experimentally. From the results of such experiments, a few mathematical relations were deduced, which allow prediction of the rate of pollutant removal. These relations represent the first step in constructing a mathematical model able to simulate duckweed-based aquatic systems.

A model solution for tracking pollution

Abstract: In the US, operators throughout the chemical process industries are being pressed by a strong environmental force--the 1990 Clean Air Act Amendments (CAAA). Far broader than the narrowly focused Clean Air Act of 1970, the latest amendments have been designed to sharply reduce smog, acid rain and various environmental pollutants in the earth's atmosphere. Combined with individual regional and state regulations, the CAAA declare that all major sources of air pollutants (plants emitting 10 tons/yr of any listed pollutant, or 25 tons/yr overall) will be compared against 12% of their competitors' most tightly controlled plants and will be expected to perform at that benchmark level. New plants will be required to beat the best-controlled plants in the industry, from an emissions standpoint. To ensure compliance with the new and more-stringent standards, process facilities must know the quantity of air emissions continuously, especially sulfur dioxide (SO[sub 2]), nitrogen oxides (NO[sub x]), carbon monoxide (CO) and total reduced sulfur (TRS).

Process simulation: The art and science of modeling

Abstract: Process simulation has gone from a small, if clannish, phenomenon to being part of the everyday tasks carried out by engineers in the chemical process industries (CPI). Numerous software packages are now enabling process operators, technicians and supervisors to make moment-to-moment decisions about a process on the basis of simulation results graphically displayed on a computer screen. Indeed, process simulation has fundamentally changed not only how engineering is practiced on the plant floor, but what is expected of today's design and operation teams. Traditionally, process modeling has been used to design flowsheets and to specify only the important parameters of equipment, such as the number of trays in a distillation column and the diameter of a vessel. Meanwhile, other application areas have been growing in importance that deal with a process at all stages of its life cycle, from synthesis to design, construction, startup, modification and retirement. The paper discusses variables to optimize, paradigm shifts in simulation, having realistic expectations, problem abstraction, setting the right objectives, getting at thermodynamic data, collecting kinetic data, evaluating process alternatives, designing a new process, and training for process operators.

Catalytic reactor design.Don't overlook static-mixer reactors

Abstract: We emphasize teamwork between reactor and catalyst designers and highlight the benefits of static mixers as reactors

Bioremediation: optimizing results

Abstract: As it is now practiced, bioremediation involves stimulating naturally occurring bacteria to degrade hazardous waste in soils and groundwater. While it can be employed as a stand-alone treatment method, today, bioremediation is seen more as part of an integrated treatment system. Combined with different technologies--particularly air spraying and bioventing--it can help achieve target cleanup goals at the lowest possible cost. While bioremediation cannot handle metals, and some chlorinated organics still elude it, the technology can destroy many hazardous compounds, including some that resist other forms of treatment. Microbial treatment is more expensive than such techniques as soil vapor extraction, but cheaper--in many cases--than offsite treatment and faster than many other remediation methods. It can also be used as a polishing'' treatment, to further reduce contaminant levels after another type of treatment has been used. The paper describes how it works and factors governing its success.

Getting the most from advanced process control

Abstract: We present an overview of advanced control engineering, with tips on evaluating existing equipment and choosing vendors; pratical examples of CPI (chemical process industry) applications; and advice on estimating the costs and benefits of advanced control, with CPI case studies

Effective design for absorption and stripping

Abstract: Absorption and stripping transfer one or more constituents from a gas stream to a liquid stream (absorption) or from a liquid stream to a gas stream (strippling). Both are widely used in the process industries, for product manufacture and environmental protection alike. They are most often carried out counter currently in packed towers. The aim here is not to reproduce absorption and stripping theory, adequately given elsewhere. Instead, the authors offer practical, often overlooked guidelines and key equations for effective design of packed absorption or stripping towers. The design task consists mainly of two interrelated parts: the hydraulic design and the mass transfer design. Hydraulic design determines the tower diameter; the mass transfer design sets the packed height within the tower. A third basic consideration is the type and size of packing. If these are not given beforehand, the designer should select a packing for which hydraulic and mass-transfer data are available, and which falls in the 1-to-2-in. nominal size range for random packing (which includes over 90% of all applications) or has nominal 1/2-in. crimp height for structured packing.

Removing the uncertainty in solids mixer selection

Abstract: A step-by-step guide will smooth things out. It discusses options, and provides a flowchart for matching a mixer and mixing process to the application at hand

Biology boosts waste treatment

Abstract: While biotreatment is also being developed as a primary remediation technique, it is now coming into its own as a booster process. Combined with such methods as air injection or vapor extraction to treat soil, or with reverse osmosis or activated carbon adsorption to treat wastewater, bioremediation is successfully treating petroleum-based contaminants and even chlorinated organics. This paper describes several biotreatment systems currently in operation.

Solvents: Know your options

Abstract: The use of solvents, integral to many CPI operations, is changing. Industry and regulatory players are rethinking solvent processes, compounds and equipment. In the past, hydrocarbons were used as cleaning agents. These were replaced by chlorofluorocarbons (CFCs) and chlorinated solvents, which are nonflammable, have high solvency and are compatible with most substrate materials. Unfortunately, these replacements have a high ozone-depletion potential (ODP) and are health hazards. The costs of using these solvents have grown significantly because of increasingly strict management requirements. These include engineering to minimize solvent release, regulatory reporting requirements, worker-training requirements, secondary containment, and disposal restrictions. Most use of CFCs and some chlorinated hydrocarbons has been curtailed strongly. However, there are many ways to further reduce the dependency on these compounds. These are discussed here. They include retrofitting a process instead of changing it, reusing waste solvents, using replacement solvents, and making a process solventless.

How to choose the best evaporator

Abstract: Part I is a wide-ranging look at the options available for carrying out an evaporation task. Part II focuses on agitated thin-film units

Hot tips to rejuvenate old boilers

Abstract: Many steam boilers built before 1960 were constructed so well that they not only remain in service today, but give little indication that they might be nearing the end of their useful life. In these boilers, the drop in efficiency primarily results from air leaking into the furnace and heat leaking out of the furnace. In turn, both of these leaks are related to the condition of the refractories inside the furnace. The deterioration of refractories over time is not often viewed as a factor in a resulting drop in furnace efficiency. In fact, even when engineers are aware of the link, the losses in efficiency tend to creep up so gradually that they seldom seem to warrant immediate attention or to draw funding from other types of plant improvement that are more directly related to increasing production. Fortunately, the efficiency losses of older boilers can be corrected at a much lower cost than by replacing the units with newer, more-efficient models. In fact, by correcting age-related boiler problems engineers can often raise furnace efficiency to the point that emissions of certain gases are also minimized. At the same time, this rejuvenation restores the boiler's original steam-generating capacity, enabling itmore » to support further production increases without requiring any expansion of the power plant. The paper describes the role of excess air in combustion and the repair or installation of refractory linings.« less