Process design

8.1 Introduction

Chemical engineers are responsible for the design, construction, and operation of chemical plants and processes. Design engineers are constantly searching for information that will aid them in these tasks.

Engineering publications, process data from existing equipment, laboratory and pilot-plant studies are just a few of the many sources of information that design engineers must use. It is important for students to learn the difference between ‘theoretical’ designs and ‘practical designs’. To reflect economic, safety, construction, and maintenance realities that will affect the design, engineers modify design calculations. For example:

Design calculation for a reactor might show that the optimum pipe diameter is D = 3.43 inches. A survey of supplier catalogs will quickly show that schedule-40 steel pipe is not manufactured with this diameter. The design engineer must then choose between both the 3.07 and 3.55-inch diameter pipe that can be easily obtained from the vendor.

Design calculations for a distillation column might show that a 600 ft tower is required to achieve the specified product separation. The maximum height of towers is generally limited to about 175 ft, however, because of wind-loading and construction considerations. A 600 ft tower would therefore need to be built in several different sections if alternative designs were not available.

Typical Specifications for Utilities in a process plant:

• Steam (Low Pressure): 15–30 psig
• Steam (High Pressure): 100 psig
• Cooling Water: Supplied at 90°F from a cooling tower, and returned at 110–120°F
• Cooling Air: Supplied at 80–100°F and returned at 100–140°F
• Compressed Air: 45 or 150 psig
• Instrument Air: 45 psig, 0°F dew point
• Electricity: 1–100 hp, 220–550 V

8.2 Process design considerations

The rapid development in technology has warranted comprehensive review of both technical and economic evaluations. A modern chemical process involves a series of operations, which are run round the clock throughout the year .It demands equipment of exceptional robustness, ingenuity and reliability. Also, these are to be achieved at an optimum cost.

8.2.1 General design procedure

It is difficult to suggest a standard design procedure. Such a procedure, however, would involve the following steps:

• Specifying the problem
• Analyzing the probable solution
• Applying chemical process principles and theories of mechanics satisfying the conditions of the problem
• Selecting materials and stresses to suit processing conditions
• Evaluating and optimizing the design
• Preparing the drawings and specifications

Each step is to be checked both in terms of mathematical calculations and engineering feasibility. It is necessary to ascertain whether the results are consistent with experience and feasibility. It may take several iterations before the satisfactory solution is obtained.

8.3 Equipment design factors

A variety of equipment is required for storage, handling and processing of chemicals. Each piece of equipment is expected to serve a specific function, although in some cases, it can be suitably modified to perform a different function. Conditions such as temperature, pressure, etc under which the equipment is expected to perform, are stipulated by the process requirements. Although the maximum capacity or size of the equipment may be specified, it is necessary to assure satisfactory performance even with a certain amount of overloads.

The overall satisfactory performance and reliability of the equipment will depend on the following factors

• Optimum processing conditions
• Appropriate materials of construction
• Strength and rigidity of components
• Satisfactory performance of mechanisms
• Reliable methods of fabrication
• Ease of maintenance and repairs
• Ease of operation and control
• Safety requirements

8.3.1 Equipment classification

The classification of chemical equipments is normally based on the specific type of unit operation. Equipments may also be defined to emphasize certain common features, which require similar design procedures. These result in the following classification:

Pressure vessels

This group of equipment has a cylinder or spherical vessel as the main component, which has a cylinder or vessel as the principal component and has to withstand variations in pressures and temperatures in addition to other loading conditions.

Structural group

This essentially comprises of equipments or components, which are stationary and have to sustain only dead loads. They are generally made up of structural sections and must satisfy conditions of elastic and structural stability.

Rotary equipments

This section covers equipments or components where a rotational motion is necessary to satisfy process requirements. Considerations of torque, dynamic stresses apart from other loading conditions

8.4 A look at common industrial chemicals

Sulphuric acid – probably the most common industrial acid. Used widely in mineral leaching and gas scrubbing (removing dangerous substances). Also used to neutralize alkaline substances.

Nitrogen – Most common inert substance used in industry. Used for everything from tank blanketing (so vapors don’t combine with oxygen to form explosive mixtures) to control reaction temperatures in exothermic reactions. Also used as a solid conveying gas carrier when air cannot be used due to explosion threats (ex-fertilizers).

Oxygen – The ultimate oxidizer. Used in any application where the introduction of oxygen to the reaction mixture is necessary.

Ethylene – Probably the most popular industrial precursor to polymer manufacturing (ex/ polyethylene).

Ammonia – Very popular scrubbing solvent to remove pollutants from fossil fuel combustion streams before they can be released to the atmosphere. Also a popular refrigerant.

Phosphoric Acid – Main use is in fertilizer production, other uses include soft drinks and other food products.

Sodium Hydroxide – The most popular alkaline substance in industry. Widely used in dyes and soap manufacturing. Also a good cleaning agent and can be used to neutralize acids. Also known as lye.

Propylene –Industrial polymer precursor (polypropylene).

Chlorine – Used in the manufacture of bleaching agents and titanium dioxide. Many of the bleaching agents based on chlorine are being replaced by hydrogen peroxide due to environmental restrictions placed on chlorine.

Sodium Carbonate – Most commonly known as soda ash, sodium carbonate is used in many cleaning agents and in glass making. Most soda ash is mined from trona ore, but it can be manufactured by reacting salt and sulfuric acid.

Sodium Silicate – Perhaps the most widely used industrial insulation.

Cyclohexane – While cyclohexane is a common organic solvent, its crowning achievement is its use as a reactant in the production of a nylon precursor (later).

Adipic Acid – This is the chemical that is made from cyclohexane and in turn is polymerized to nylon.

Nitrobenzene – Primary use is in the manufacture of aniline, which is in turn used as a rubber additive to prevent oxidation (antioxidant).

Butyraldehyde – Used to manufacture 2-ethylhexanol, which is then used to manufacture hydraulic oils or synthetic lubricants.

Aluminum Sulfate – Widely used in the paper and wastewater treatment industries as a pH buffer.

Methyl tert-butyl ether – Also known as MTBE, it is most famous for its role as a gasoline additive (oxygenate). Due to its toxic affect on mammals, the EPA has been ordering a decrease in its use and an increase in the use of ethanol as a replacement.

Ethylene Dichloride – Nearly all ethylene dichloride produced is used to produce vinyl chloride which is then polymerized to polyvinyl chloride (PVC).

Nitric Acid – Most common application is its reaction with ammonia to form the solid fertilizer ammonium nitrate.

Ammonium Nitrate – Probably the most widely used solid fertilizer

Benzene – The two largest uses for benzene are as reactants to produce ethylbenzene (used to produce styrene) and cumene (used to produce phenols). Also a very common organic solvent as well as a precursor to cyclohexane.

Urea – The majority of urea is used in fertilizer production. Some is also used in the manufacture of livestock feed.

Vinyl Chloride – As previously mentioned, this is the monomer form of polyvinyl chloride (PVC), which finds uses as a building material and other durable plastics.

Ethylbenzene – Used almost exclusively as a reactant for the production of styrene

Styrene – Monomer form of polystyrene. Polystyrene is used in pure form and expanded. Styrene can also be used in mixed forms or copolymers such as ABS (acrylonitrile-butadiene-styrene).

Methanol – Used as a reactant to make methyl tertbutyl ether (MTBE), formaldehyde, and acetic acid. Typically produced from synthesis gases, namely carbon monoxide and hydrogen.

Xylene – o-xylene (ortho) is used primarily to manufacture phthalic anhydride, which is in turn used to make a variety of plasticizers and polymers. p-xylene is used to manufacture terephthalic acid, a polyester feedstock.

Formaldehyde – Commonly used as part of a copolymer series (Urea-formaldehyde resins) or as another polymer additive used to bring out desired characteristics.

Terephthalic Acid – Almost exclusively used in the manufacture of polyethylene terephthalate (PET) or polyester.

Ethylene Oxide – Majority of ethylene oxide is used to manufacture ethylene glycol, which is described later.

Hydrochloric Acid – Two main uses include production of other chemicals (by acting as a reactant or a catalyst) and the pickling of steel. Also widely used in the pharmaceutical industry.

Toluene – Used primarily to manufacture benzoic acid. Benzoic acid is a precursor to phenol (later), various dyes, and rubber products.

Cumene – Nearly all cumene produced is oxidized to cumene hydroperoxide, then cleaved (splitting a molecule) to form phenol and acetone.

Ethylene Gylcol – Most common use is as a reactant to form polyethylene terephthalate (PET). Also used a primary ingredient in antifreeze.

Acetic Acid – Used primarily to manufacture vinyl acetate monomer (later) and acetic anhydride which is used to manufacture cellulose acetate.

Phenol – Used to manufacture Bispenol-A (later) as well as phenolic resins and caprolacturm.

Propylene Oxide – Two primary uses include urethane polyether polyols (both flexible and rigid foams) and propylene glycol, which is used as a polymer additive as well as a common, refrigerate.

Butadiene – Uses include styrene-butadiene rubber, polybutadiene, and other copolymers.

Carbon Black – Most common use is a rubber additive

Isobutylene – Most production is used to make butyl rubbers.

Potash – Used in agriculture as a crop fertilizer.

Acrylonitrile – Used as a reactant to form various copolymers along with acrylic fibers.

Vinyl Acetate – Monomer form polyvinyl acetate, a common emulsion polymer and resin. PVA is the ‘sticky’ agent in ordinary white glue.

Titanium Dioxide – Used as a white pigment for many products ranging from paints and polymers to pharmaceuticals and food items. In short, if it’s white, it probably has titanium dioxide in it.

Acetone – Used primarily to manufacture methyl methacrylate and Bisphenol-A

Bisphenol-A – Used as the main feedstock for polycarbonate resins and epoxy resins.

8.5 Materials of construction

The basic properties involved in material selection are:

• Composition
• Structure
• Specific weight
• Thermal conductivity
• Expandability
• Resistance to corrosion.

The mechanical properties that are to be kept in mind during material specification are:

• Strength
• Elastic limit
• Moduli of elasticity
• Endurance limit, resilience
• Toughness, ductility
• Brittleness
• Hardness

From the point of view of fabrication, machinability, weld ability and malleability might be considered as relevant properties.

8.5.1 Choice of material

Choice of the material cannot be made merely by choosing a suitable material having the requisite mechanical behavior and anticorrosive properties, but must be based on a sound economic analysis of competing materials.

8.6 Types of corrosion

8.6.1 Uniform attack

Uniform attack is a form of electrochemical corrosion that occurs with equal intensity of the entire surface of the metal. Iron rusts when exposed to air and water, and silver tarnishes due to exposure to air. Potentially very risky, this type of corrosion is very easy to predict and is usually associated with ‘common sense’ when making material decisions.

8.6.2 Galvanic corrosion

Galvanic corrosion is a little more difficult to keep track of in the industrial world. Galvanic corrosion occurs when two metals having different composition are electrically coupled in the presence of an electrolyte. The more reactive metal will experience severe corrosion while the more noble metal will be quite well protected. Perhaps the most infamous examples of this type of corrosion are combinations such as steel and brass or copper and steel. Typically the steel will corrode the area near the brass or copper, even in a water environment and especially in a seawater environment. Probably the most common way of avoiding galvanic corrosion is to electrically attach a third, anodic metal to the other two. This is referred to as cathodic protection.

8.6.3 Crevice corrosion

Another form of electrochemical corrosion is crevice corrosion. Crevice corrosion is a consequence of concentration differences of ions or dissolved gases in an electrolytic solution. A solution became trapped between a pipe and the flange on the left. The stagnant liquid in the crevice eventually had a lowered dissolved oxygen concentration and crevice corrosion took over and destroyed the flange. In the absence of oxygen, the metal and/or it’s passive layer begin to oxidize. To prevent crevice corrosion, it is recommended to use welds rather than rivets or bolted joints whenever possible. Also consider nonabsorbing gaskets. Remove accumulated deposits frequently and design containment vessels to avoid stagnant areas as much as possible.

8.6.4 Pitting

Pitting, just as it sounds, is used to describe the formation of small pits on the surface of a metal or alloy. Pitting is suspected to occur in much the same way crevice corrosion does, but on a flat surface. A small imperfection in the metal is thought to begin the process, and then a ‘snowball’ effect takes place. Pitting can go on undetected for extended periods of time, until a failure occurs. A textbook example of pitting would be to subject stainless steel to a chloride-containing stream such as seawater. Pitting would overrun the stainless steel in a matter of weeks due to its very poor resistance to chlorides, which are notorious for their ability to initiate pitting corrosion. Alloy blends with more than 2% Molybdenum show better resistance to pitting attack. Titanium is usually the material of choice if chlorides are the main corrosion concern. (Pd stabilized forms of Ti are also used for more extreme cases).

8.6.5 Intergranular corrosion

Occurring along grain boundaries for some alloys, intergranular corrosion can be a real danger in the right environment. On the left, a piece of stainless steel (especially susceptible to intergranular corrosion) has seen severe corrosion just an inch from a weld. The heating of some materials causes chromium carbide to form from the chromium and the carbon in the metals. This leaves a chromium deficient boundary just shy of the where the metal was heated for welding. To avoid this problem, the material can be subjected to high temperatures to redissolve the chromium carbide particles. Low carbon materials can also be used to minimize the formation of chromium carbide. Finally, the material can be alloyed with another material such as Titanium, which forms carbides more readily so that the chromium remains in place.

8.6.6 Selective leaching

When one element or constituent of a metal is selectively corroded out of a material it is referred to as selective leaching. The most common example is the dezincification of brass. On the right, nickel has been corroded out of a copper-nickel alloy exposed to stagnant seawater. After leaching has occurred, the mechanical properties of the metal are obviously impaired and some metal will begin to crack.

8.6.7 Erosion corrosion

Erosion-corrosion arises from a combination of chemical attack and the physical abrasion as a consequence of the fluid motion. Virtually all alloy or metals are susceptible to some type of erosion-corrosion as this type of corrosion is very dependent on the fluid. Materials that rely on a passive layer are especially sensitive to erosion-corrosion. Once the passive layer has been removed, the bare metal surface is exposed to the corrosive material. If the passive layer cannot be regenerated quickly enough, significant damage can be seen. Fluids that contain suspended solids are often times responsible for erosion-corrosion. The best way to limit erosion-corrosion is to design systems that will maintain a low fluid velocity and to minimize sudden line size changes and elbows. The photo above shows erosion-corrosion of a copper-nickel tube in a seawater surface. An imperfection on the tube surface probably causes an eddy current which provided a perfect location for erosion-corrosion.

8.6.8 Stress corrosion

Stress corrosion can result from the combination of an applied tensile stress and a corrosive environment. In fact, some materials only become susceptible to corrosion in a given environment once a tensile stress is applied. Once the stress cracks begin, they easily propagate throughout the material, which in turn allows additional corrosion and cracking to take place. Tensile stress is usually the result of expansions and contractions that are caused by violent temperature changes or thermal cycles. The best defense against stress corrosion is to limit the magnitude and/or frequency of the tensile stress.

8.7 Linings for chemical plants and equipment

Storage tanks, reaction vessels, pipes, ducting, etc are covered with linings in order to:

• Give the underlined structure protection against chemical attack
• Prevent contamination of the materials being processed
• Minimize the effect of abrasion

The various materials commonly used for lining are as follows:

• Rubber
• Lead
• Glass
• Plastic

8.8 Rules of thumb

Although experienced engineers know where to find information and how to make accurate computations, they also keep a minimum body of information in mind on the ready, made up largely of shortcuts and rules of thumb. The present compilation may fit into such a minimum body of information, as a boost to the memory or extension in some instances into less often encountered areas.

An Engineering Rule of Thumb is an outright statement regarding suitable sizes or performance of equipment that obviates all need for extended calculation. Because any brief statements are subject to varying degrees of qualification, they are most safely applied by engineers who are substantially familiar with the topics.

8.8.1 Compressors and vacuum pumps

• Fans are used to raise the pressure about 3% (12 in. water). Blowers raise to less than 40 psig, and compressors to higher pressures, although blower range commonly is included in the compressor range
• Vacuum pumps: reciprocating piston type decrease the pressure to 1 Torr; rotary piston down to 0.001 Torr, two-lobe rotary down to 0.0001 Torr; steam jet ejectors, one stage down to 100 Torr, three stages down to 1 Torr, five stages down to 0.05 Torr
• A three-stage ejector needs 100 lb steam/lb air to maintain a pressure of 1 Torr
• Efficiencies of reciprocating compressors: 65% at compression ratio of 1.5, 75% at 2.0. And 80-85% at 3-6
• Efficiencies of large centrifugal compressors, 6000-100,0000 ACFM at suction, are 76-78%
• Rotary compressors have efficiencies of 70%, except liquid liner type which have 50%

8.8.2 Conveyors for particulate solids

• Screw conveyors are suited to transport of even sticky and abrasive solids up inclines of 20 or so. They are limited to distances of 150 ft or so because of shaft torque strength. A 12 in. dia conveyor can handle 100-3000 cu, ft/hr, at speeds ranging from 40 to 60 rpm
• Belt conveyors are for high capacity and long distances a mile or more, but only several hundred feet in a plant), up inclines of 30 maximum. A 24-inch wide belt can carry 300 cu. ft/hr at a speed of 100 ft/min, but speeds up to 600 ft/min are suited to some materials. Power consumption is relatively low
• Bucket elevators are suited to vertical transport of sticky and abrasive materials. With buckets 20 × 20 inches, capacity can reach 100 cu. ft/hr at a speed of 100 ft/min, but speeds to 300 ft/min are used
• Drag-type conveyors (Redler) are suited to short distances in any direction and are completely enclosed. Units range in size from 3 in, square to 19 in square and may travel from 30ft/min (fly ash to 250ft min (grains). Power requirements are high
• Pneumatic conveyors are for high capacity, short distance (400ft) transport simultaneously from several sources to several destinations. Either vacuum or low pressure (6–12 psig) is employed with a range of air velocities from 35 to 120 ft/sec depending on the material and pressure, air requirements from 1 to 7 cu. ft/cu. ft of solid transferred

8.8.3 Cooling towers

• Water in contact with air under adiabatic conditions eventually cools to the wet bulb temperature
• In commercial units 90% of saturation of the air is feasible
• Relative cooling tower size is sensitive to the difference between the exit and wet bulb temperatures:
• T (F) 5 15 25
• Relative volume 2.4 1.0 0.55
• Tower fill is of a highly open structure so as to minimize pressure drop, which is in standard practice a maximum of 2 inches of water
• Water circulation rate is 1–4 gpm/sqft and air rates are 1300–1800 lb/(hr)(sq. ft or 300–400 ft/min)
• Chimney-assisted natural draft towers are of hyperboloidal shapes because they have greater strength for a given thickness; a tower 250 ft high has concrete walls 5–6in. thick. The enlarged cross section at the top aids in dispersion of exit humid air into the atmosphere
• Countercurrent induced draft towers are the most common in process industries. They are able to cool water within 2 deg F of the wet bulb
• Evaporation losses are 1% of the circulation for every 10 deg F of cooling range. Windage or drift losses of mechanical draft towers are 0.1-0.3%. Blow down of 2.5-3.0% of the circulation is necessary to prevent excessive salt buildup

8.8.4 Crystallization from solution

• Complete recovery of dissolved solids is obtainable by evaporation but only to the eutectic composition by chilling. Recovery by melt crystallization is also limited by the eutectic composition
• Growth rates and ultimate sizes of crystals are controlled by limiting the extent of super saturation at any time
• In crystallization by chilling, the temperature of the solution is kept at the most at 1–2 deg F below the saturation temperature at the prevailing concentration
• Rate of crystals under satisfactory conditions are in the range of 0.1–0.8 mm/hr. The growth rates are approximately the same in all directions
• Growth rates are influenced greatly by the presence of impurities and of certain specific additives that vary from case to case

8.8.5 Disintegration

• Percentages of material greater than 50% of the maximum size are about 50% from rolls, 15% from tumbling mills, and 5% from closed circuit ball mills
• Close circuit grinding employs external size classification and return of oversize for regrinding. The rules of pneumatic conveying are applied to design of air classifiers. Closed circuit is most common with ball and roller mills
• Jaw crushers take lumps of several feet in diameter down to 4 in stroke rates of 100–300/ min. The average feed is subjected to 9–10 strokes before small enough to escape. Gyratory crushers are suited to shabby feeds and make a more rounded product
• Roll crushers are made either smooth or with teeth. A 24 in. toothed roll can accept lumps 14in. dia. Smooth rolls affect reduction ratios up to about 4. Speeds are 50–900 rpm. Capacity is about 25% of the maximum corresponding to a continuous ribbon of material passing through the rolls
• Road mills are capable of taking feed as large as 50 mm and reducing it to 300 meshes, but normally the product ranges is 8-65 mesh. Rods are 25-150 mm dia. Ratio of rod length to mill diameter is about 1.5. About 45% of the mill rods occupy volume. Rotation is at 50-65% of critical
• Ball mills are better suited than rod mills to fine grinding. The charge is of equal weights of 1.5, 2 and 3in ball for the finest grinding. Volume occupied by the balls is 50% of the mill volume. Rotation speed is 70-80% of critical. Ball mills have a length to diameter ratio in the range 1–1.5. Tube mills have a ratio of 4–5 and are capable of very fine grinding. Pebble mills have ceramic grind elements, used when contamination with metal is to be avoided
• Roller mills comply with cylindrical or tapered surfaces that roll along flatter surfaces and crush nipped particles. Products of 20–200 meshes are made

8.8.6 Distillation and gas absorption

• Distillation usually is the most economical method of separating liquids, superior to extraction, adsorption, crystallization, or others
• Tower operating pressure is determined most often by the temperature of the available condensing medium, 100-120 F if cooling water; or by the maximum allowable reboiler temperature, 150 psig steam, 366 F
• Sequencing of columns for separating multicomponent mixtures: (a) perform the easiest separation first, that is, the one least demanding of trays and reflux, and leave the most difficult to the last; (b) when neither relative volatility nor feed concentration vary widely, remove the components one by one as overhead products; (c) When the adjacent ordered components in the feed vary widely in relative volatility; (d) when the concentrations in the feed vary widely but the relative volatilities do not, remove the components in the order of decreasing concentration in the feed
• Economically optimum reflux ratio is about 1.2 times the minimum reflux ratio
• The economically optimum number of trays is near twice the minimum value
• A safety factor of 10% of the number of trays calculated by the best means is advisable

8.8.7 Drivers and power recovery equipment

• Efficiency is greater for larger machines. Motors are 85–95%; steam turbines are 42–78%; gas engines and turbines are 28–38%
• For under 100 HP, electric motors are used almost exclusively; they are made for up to 20,000 HP
• Induction motors are most popular. Synchronous motors are made for speeds as low as 150 rpm and are thus suited, for example, for low speed reciprocating compressors, but are not made smaller than 50 HP. A variety of enclosures is available, e.g. weatherproof
• Steam turbines are competitive above 100 HP. They are speed controllable. Frequently they are employed as spares in case of power failure
• Combustion engines and turbines are restricted to mobile and remote locations
• Gas expanders for power recovery may be justified at capacities of several hundred HP; otherwise any required pressure reduction in process is effected with throttling valves

8.8.8 Drying of solids

• Drying times range from a few seconds in spray dryers to 1 hr or less in rotary dryers and up to several hours or even days in tunnel shelf or belt dryers
• Continuous tray and belt dryers for granular material of natural size or pelleted to 3–15 mm have drying times in the range of 10–200 min
• Rotary cylindrical dryers operate with superficial air velocities of 5–10 ft/sec, sometimes up to 35 ft/sec when the material is coarse. Residence times are 50–90 min. Holdup of solid is 7–8%; an 85% free cross section is taken for design purposes. In countercurrent flow, the exit gas is 10–20 deg C above the solid; in parallel flow, the temperature of the exit solid is 100 deg C. Rotation speeds of about 4 rpm are used, but the product of rpm and diameter in feet is typically between 15 and 25
• Drum dryers for pastes and slurries operate with contact times of 3–12 sec, produce flakes 1–3 mm thick with evaporation rates of 15–30 kg/m2hr. Diameters are 1.5–5.0 ft; the rotation rate is 2–10 rpm. The greatest evaporative capacity is to the order of 3000 lb/hr in commercial units
• Pneumatic conveying dryers normally take particles 1–3 mm dia but up to 10mm when the moisture is mostly on the surface. Air velocities are 10–30 m/sec. Single pass residence times are 0.5-3.0 sec but with normal recycling the average residence time is brought up to 60 sec. Units in use range from 0.2 m dia by 1 m high to 0.3 m dia by 38 m long. Air requirement is several SCFM/lb of dry product/hr
• Fluidized bed dryers work best on particles of a few tenths of an mm dia, but up to 4 mm dia have been processed. Gas velocities of twice the minimum fluidization velocity are a safe prescription. In continuous operation, drying times of 1–2 min are enough, but batch drying of some pharmaceutical products employs drying times of 2–3 hr
• Spray dryer: Surface moisture is removed in about 5sec, and most drying is completed in less than 60 sec. Parallel flow of air and stock is most common. Atomizing nozzles have openings 0.012–0.15in. and operate at pressures of 300–4000 psi.
  Atomizing spray wheels rotate at speeds to 20,000 rpm with peripheral speeds of 250–600 ft/sec. With nozzles, the ratio is 0.5–1.0. For the final design, the experts say pilot tests in a unit of 2m dia should be made

8.8.9 Evaporators

• Long tube vertical evaporators with either natural or force circulation are most popular. Tubes are 19–63 mm dia and 12–30 ft long
• In forced circulation, linear velocities in the tubes are 15–20 ft/sec
• Elevation of boiling point by dissolved solids results in differences of 3–10 deg F between solution and saturated vapor
• When the boiling point rise is appreciable, the economic number of effects in series with forward feed is 4–6
• When the boiling point rise is small, minimum cost is obtained with 8–10 effects in series
• In backward feed the more concentrated solution is heated with the highest temperature steam so that heating surface is lessened, but the solution must be pumped between stages
• The steam economy of an N-stage battery is approximately 0.8 N lb evaporation/lb of outside steam
• Inter stage steam pressures can be boosted with steam jet compressors of 20–30% efficiency or with mechanical compressors of 70–75% efficiency

8.8.10 Filtration

• Processes are classified by their rate of cake buildup in a laboratory vacuum leaf filter: rapid 0.1–10.0 cm/scc; medium, 0.1–10.0 cm/min; slow, 0.l–10.0 cm/hr
• Continuous filtration should not be attempted if 1/8 inch cake thickness cannot be formed in less than 5 min
• Rapid filtering is accomplished with belts, top feed drums, or pusher-type centrifuges
• Medium rate filtering is accomplished with vacuum drums or disks or peeler-type centrifuges
• Slow filtering slurries are handled in pressure filters or sediment centrifuges
• Clarification with negligible cake buildup is accomplished with cartridges, precoat drums, or sand filters
• Laboratory tests are advisable when the filtering surface is expected to be more than a few square meters; when cake washing is critical; when cake drying may be a problem; or when pre-coating may be needed

8.8.11 Heat exchangers

• Take true countercurrent flow in a shell-and-tube exchanger as a basis
• Standard tubes are 3/4 in. OD, an inch triangular spacing, 16 ft long; a shell 1 ft dia accommodates 100 sq. ft; 2ft dia, 400 sq. ft, 3ft dia, 1100sqft
• Tube side is for corrosive, folding, scaling, and high pressure fluids
• Shell side is for viscous and condensing fluids
• Pressure drops are 1.5 psi for boiling and 3–9 psi for other services
• Minimum temperature approach is 20 deg F with normal coolants. 10 deg F or less with refrigerants
• Water inlet temperature is 90 deg F, maximum outlet 120 F
• Double-pipe exchanger is competitive at duties requiring 100–200 sq. ft
• Compact (plate and fin) exchangers have 350sqft/cuft about 4 times the heat transfer per cuf. t of shell and tube
• Plate and frame exchangers are suited to high sanitation service, and are 25–50% cheaper in stainless construction than shell-and-tube units

8.8.12 Mixing and agitation

• Mild agitation is obtained by circulating the liquid with an impeller at superficial velocities of 0.1-0.2ft/sec, and intense agitation at 0.7-1,0ft/sec
• Intensities of agitation with impellers in baffled tanks are measured by power input, HP/1000 gal and impeller tip speed

Operation	HP/1000 gal	Tip speed (ft/min)
Blending	0.2-0.5	75-10
Homogeneous reaction	0.5-1.5	10-15
Reaction with heat transfer	1.5-5.0	15-20
Liquid-liquid mixtures	5	15-20
Liquid-gas mixtures	5-10
Slurries	10

• Proportions of a stirred tank relative to the diameter D: liquid level=D; turbine impeller diameter=D/3; impeller level above bottom=D/3; impeller blade width=D/15; four vertical baffles with width=E/10
• Propellers are made a maximum of 18in., turbine impellers to 9ft. Gas bubbles sparged at the bottom of the vessel will result in mind agitation at a superficial gas velocity of 1ft/min, severe agitation at 4ft/min
• Suspension of solids with a settling velocity of 0.03 t/sec is accomplished with either turbine or propeller impellers, but when the settling velocity is above 0.15ft/sec intense agitation with a propeller is needed
• In-line blenders are adequate when a second or two contact time is sufficient, with power inputs of 0.1-0.2 2 HP/gal