Majari Magazine

Heuristics Rules for Process Equipment

by Marthin Winner on 18/03/08 at 1:55 am | 9 Comments | Print article | Email article

Storage tanks

• For less than 100 gal, it is common practice to use vertical tanks on legs
• For between 100 and 10,000 gal, horizontal tanks on concrete supports are commonly used
• For beyond 10,000 gal, consider vertical tanks on concrete foundations
• Liquids that are subject to breathing losses may conveniently be stored in tanks with floating or expansion roofs, for conservation
• Although the amount of material inventoried is highly plant-specific, many process plants specify 30 days worth of capacity, for raw materials and products alike

Drums

• Liquid drums usually are horizontal
• Drums for gas-liquid separation are vertical
• A length-to-diameter ratio of 3 is considered optimal; but in practice, the ratio for drums commonly falls between 2.5 and 5.0
• In liquid-liquid separation, reflux drums are usually kept about half full, with holdup time of about 5 min. (or 5 to 10 min if the drum liquid is fed to a downstream separation tower)
• For entrainment removal, mesh pads of 4 to 12 in. thickness can achieve 99% removal; a thickness of 6 in. is widely used

Reactors

• In stirred tank reactors, it is preferable to maintain a liquid level that is approximately equal to the tank diameter
• Common motives for conducting batch reactions, in stirred-tank reactors, are: the daily production rate is relatively low; reaction times are relatively long; particular process parameters, such as the feed rate or the vessel temperature, must be programmed during the course of the reaction
• An array of continuous stirred-tank reactors in series (four or five, for instance) is in many cases the system of choice for slow reactions of liquids and slurries
• Tubular reactors are attractive for short-residence-time reactions (seconds or minutes), high throughputs, and reactions that require a relatively large amount of heat transfer

Distillation and gas absorption

• Generally speaking, distillation tends to be the most economical method for liquid-liquid separation; more so, for instance, than liquid-liquid extraction or crystallization. Flashing can be more economical than distillation, but is more limited by physical properties of the mixture
• The well-known simple equation for relative volatility — relative volatility = (vapor pressure of more volatile component)/(vapor pressure of less-volatile component) — is valid only for ideal mixtures
• If the system is ideal and there are only two components, the McCabe-Thiele method offers a good approximation to the number of equilibrium stages
• The most common determinant of the column operating pressure is either the temperature of the available condensing medium (in many cases, cooling water at about 100 to 120°F) or the maximum allowable reboiler temperature (for instance, 366°F for 150-psig steam)
• For many separations, the optimal reflux ratio is 1.2 times the minimum reflux ratio
• In many cases, the economically optimal number of trays equals twice the minimum number of trays
• Reflux pumps should be oversized by about 25%
• From a maintenance standpoint, tray spacings of about 20 to 24 in. are attractive
• Typical pressure drop per tray is of the order of 3 in. of water or 0.1 psi
• For separation of light hydrocarbons and aqueous solutions, the tray efficiencies are typically 60 to 90% for distillation, and 10 to 20% for gas absorption and stripping
• For a typical sieve tray, the holes are 0.25-0.50 in diameter, and the hole area is about one-tenth of the active cross-section area
• For a typical valve tray, the holes are about 1.5 in. diameter, each outfitted with a liftable cap; there are typically 12 to 14 caps per square foot of active tray cross-section
• The typical height of a column weir is 2 in.; the weir length is usually about 75% of the tray diameter; maximum liquid rate is about 8 gal/ min. per inch of weir; for high liquid rates, multipass arrangement are often the choice for towers of less than 3 ft diameter and where low pressure drop through the tower is desirable, packings (random or structured) are commonly preferred over trays. If the packing is initially distributed with care and is periodically redistributed, the volumetric efficiency can be greater than that of a comparable tray tower
• Most reflux drums are horizontal, kept about half full, and have a liquid holdup of 5 min
• For 3-ft-diameter towers, about 4 ft of column height should be added at the top for vapor disengagement, and 6 ft at the bottom for liquid level and reboiler return
• Due to wind-loading and other structural considerations, towers should be no higher than about 175 ft. Furthermore, the ratio of tower height to diameter should be less than 30

Liquid-liquid extraction

• Ordinarily, the phase with the greater volumetric flowrate should be the dispersed phase; however, in extractors subject to back mixing, the phase with the lower flowrate should instead be dispersed. It is also preferable that the dispersed phase be the one that wets the equipment less well. Finally, because the holdup of continuous phase is usually the greater, it is desirable that that phase consist of the less expensive and/or less hazardous material
• For separations achievable in relatively few stages (5 to 10 for instance), packed extraction towers offer advantages, unless the surface tension exceeds 10 dynes/cm. It is possible to achieve attractive HETS values (5 to 10 ft, for example). Dispersed- phase loadings should not exceed 25 gal/(ft2)(min), and the dispersed phase should be redistributed every 5 to 7 ft
• Sieve tray on extraction columns typically have holes of only 3- to 8- mm diameter. Velocities through the holes should kept below about 0.8 ft/s to minimize formation of excessively small drops. Typical tray spacings are 6 to 24 in.; typical tray efficiencies are in the range of 20 to 30%

Crystallization from solution

• Whether melt crystallization or crystallization from solution is employed, the maximum recovery of solids is limited by the eutectic composition
• Crystal growth rates and the final crystal size are both controlled by limiting the extent of supersaturation in the liquid
• It is good operating practice to hold the liquid temperature at a few Fahrenheit degrees below the saturation temperature for the prevailing concentration