chemical engineering - facts at your fingertips 2007 april to december

Department Editor: Rebekkah Marshall

Preventing Runaway Reactions

general considerations [1]A process is considered to be thermally safe only if the reactions can easily be controlled, and if the raw material, the products, the intermediates and the re-action masses are thermally stable under the considered process conditions. Check into the process equipment, its design, its sequence of operation and the control strategies. In addition to the engineer-ing aspects, get detailed information on thermodynamic and kinetic properties of the substances involved, such as the reaction rates or heat-release rates as a function of process conditions. Deter-mine the physical and chemical proper-ties, as well.

Understanding of thermal-hazard po-tential requires knowledge of various skills and disciplines [3]. These include:Operating mode: The mode of opera-tion is an important factor. For instance, a batch reaction, where all the reactants are charged initially, is more difficult to control than a semi-batch operation in which one of the reactants is charged progressively as the reaction proceeds (for more, see Design Options).Engineering: Design and layout of the plant and equipment and its built-in con-trols impact the entire process. The ca-pacity of the heating or cooling system is important in this context. Process en-gineering is used to understand the con-trol of the chemical processes on a plant scale. It determines which equipment should be used and how the chemical processes should be performed. In ad-dition, take into account technical failure of equipment, human errors (deviations from operating instructions), unclear operating instructions, interruption of energy supply, and external influences, such as frost or rain (for more, see De-sign Options). Chemistry: The nature of the process and the behavior of products must be known, not only under reaction conditions, but also in case of unexpected deviations (for example, side reactions, instability of intermediates). Chemistry is used to gain information regarding the reaction pathways that the materials in question follow.Physical chemistry and reaction kinetics: The thermophysical properties of the reac-tion masses and the kinetics of the chemi-cal reaction are of primary importance. Physical chemistry is used to describe the reaction pathways quantitatively.

data collectionThe following data are especially rel-evant in avoiding runaway reactions:• Physical and chemical properties, ig-

nition and burning behavior, electro-static properties, explosion behavior and properties, and drying, milling, and toxicological properties

• Interactions among the chemicals• Interactions between the chemicals

and the materials of construction• Thermal data for reactions and de-

composition reactions• Cooling-failure scenarios

design options [2]If a reaction is has the potential for runaway, the following design changes should be considered:

• Batch to continuous. Batch reactors require a larger inventory of reac-tants than continuous reactors do, so the potential for runaway in continu-ous systems is less by comparison

• Batch to semi-batch. In a semi-batch reaction, one or more of the reactants is added over a period of time. There-fore, in the event of a temperature or pressure excursion, the feed can be switched off, thereby minimizing the chemical energy stored up for a sub-sequent exothermic release

• Continuous, well-mixed reactors to plug flow designs. Plug-flow reactors require comparatively smaller volumes and therefore smaller (less dangerous) inventories for the same conversion

• Reduction of reaction inventory via increased temperature or pressure, changing catalyst or better mix-ing. A very small reactor operating at a high temperature and pressure may be inherently safer than one operating as less extreme conditions because it contains a much lower in-ventory [3]. Note that while extreme conditions often result in improved reaction rates, they also present their own safety challenges. Meanwhile, a compromise solution employing mod-erate pressure and temperature and medium inventory may combine the worst features of the extremes [3].

• Less-hazardous solvent

• Externally heated or cooled to inter-nally heated or cooled

thermal stability criteria [1, 4]As a guideline, three levels are sufficient to characterize the severity and prob-ability of a runaway reaction, as shown in the Table.

Defining high, meDium anD low risk [1]Severity Probability

High ΔTad > 200K TMRad < 8 hMedium 50K < ΔTad < 200K 8 h < TMRad <

24 hLow ΔTad < 50K and

the boiling point cannot be sur-passed

TMRad > 24 h

adiabatic temperature riseThe adiabatic temperature rise is calculated by dividing the energy of reaction by the specific heat capacity as shown in Equation (1).ΔTad = 1,000Qr/Cp (1)where:ΔTad = adiabatic temperature rise, KQr = energy of reaction, kJ/kgCp = heat capacity, J/(kg)(K)

time to maximum rate (tmr)TMRad (the time to maximum rate, adiabatic) is a semiquantitative indicator of the probability of a runaway reaction. Equation (2), defining TMRad in hours, is derived for zero-order reaction kinetics:TMRad = CpRTo2/3,600qoEa (2)where:R = gas constant, 8.314 J/molKTo = absolute initial temperature, Kqo = specific heat output at To, W/kgEa = activation energy, J/mol

The TMR value provides operating personnel with a measure of response time. Knowledge of the TMR allows decisions to be based on an understanding of the time-frame available for corrective measures in case heat transfer is lost during processing.

References1. Venugopal, Bob, Avoiding Runaway Reac-

tions, Chem. Eng., June 2002, pp. 54–58.2. Smith, Robin, ”Chemical Process Design,”

McGraw-Hill, New York, 1995.3. Kletz, T. A., “Cheaper, Safer Plants,”

IChemE Hazard Workshop, 2d., IChemE, Rugby, U.K., 1984.

4. Gygax, R., Reaction Engineering Safety, Chem. Eng. Sci., 43, 8, pp. 1759–71, Au-gust 1998.


Hazardous Area Classification

Guidelines by locationOver the years, hazardous area clas-sification requirements for the U.S. have evolved around a single area-clas-sification system known as the Class/Division system. Today, the system addresses establishment of boundaries of hazardous areas and the equipment and wiring used in them. Meanwhile, European countries, as well as some other countries around the world, have developed their own area classification systems to address hazardous locations safety issues. This independent develop-ment has resulted in systems for these countries or groups of countries based on the International Electrotechnical Commission (IEC) Zone system, with de-viations to meet each country’s national codes. While other countries do accept and use the Division system (most nota-bly Canada and Mexico), the majority of the world’s hazardous locations are classified using the concepts of the IEC Zone system. The U.S. National Electri-cal Code (NEC; NFPA 70) also recog-nizes the Zone system and allows its use in the U.S. under article 505 of the NEC. ATEX requires the use of IEC-type hazardous area classifications.

defininG hazardous areas A hazardous area is designated as any location in which a combustible material is or may be present in the atmosphere in sufficient concentration to produce an ignitable mixture. The North American method identifies these areas by Class, Division and Group or optionally by Class, Zone and Group, while the IEC and CENELEC designate these areas by Gas/Dust, Zone and Group. The likeli-hood that the explosive atmospheres are present when the equipment is operating are designated in Tables 1, 2 and 5.

equipment selection For equipment selection purposes, haz-ardous area classifications also consider:• The maximum surface temperature of

the equipment under normal operat-ing conditions (see the Temperature Code designations in Table 3)

• The ignition-related properties of the explosive atmosphere (see the Group designations in Table 4)

• The protection method(s) used by the equipment to prevent ignition of the surrounding atmosphere (see the Protection Method designations in Table 6)

Acknowledgement and referencesWe would like to thank Vladimir Stetsovsky of Chilworth Technology, Inc. for his contributions to this page1. National Electrical Code-2005-NFPA 70, National

Fire Protection Association.2. NFPA 497-2004, Recommended Practice for the

Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.

3. NFPA 499-2004, Recommended Practice for the Classification of Combustible Dusts and of Hazard-ous (Classified) Locations for Electrical Installations in Chemical Process Areas.

4. IEC 60079-10-2002 Electrical apparatus for explo-sive gas atmospheres — Part 10: Classification of hazardous areas.

5. IEC 61241-3-2005 Electrical apparatus for use in the presence of combustible dust — Part 3: Classifi-cation of areas where combustible dusts are or may be present.

Table 3. Temperature CodesThe Temperature class defines the maximum surface temperature of the device. Ratings are given with reference to 40°C ambientT1 450°C T3A 180°CT2 300°C T3B 165°CT2A 280°C T3C 160°CT2B 260°C T4 135°C

T2C 230°C T4A 120°C

T2D 215°C T5 100°CT3 200°C T6 85°C

The additional temperature classifications high-lighted above are for USA and Canada only

Table 4. Gas and Dust GroupsHazardous locations are grouped according to their ignition propertiesTypical gas

IEC gas group

North Ameri-can group

Minimum ignition energy

Acetylene IIC A 20µJHydrogen IIC + H2 B 20µJEthylene IIB C 60µJ

Propane IIA D 100µJ

*Methane I —Metal dust — E

Coal dust — F

Grain dust — G

Fibers — —*Mining application under jurisdiction of U.S. Mine Safety and Health Administration (MSHA)

Table 2. Relationship Between Divisions and ZonesNorth America Europe

Division method

Zone method IEC standard

Ignitable mixture present continuously (long periods) Division

1

Zone 0

Zone 0 (Zone 20-Dust)

Ignitable mixture present intermittently

Zone 1


Ignitable mixture is not normally present

Division 2

Zone 2


Table 5. Information Required For Establishing Extent of Hazardous Area

Gas/Vapors Dust• Flash point• Flammability limits• Auto-ignition temperature• Minimum ignition energy,

MIC or MESG – for equipment selection purposes

• Gas/Vapor group• Vapor/Gas density• Area ventilation conditions• Location of gas/vapor release

points. Frequency and rate of release

• A/B classification• Minimum explosible dust

concentration• Minimum ignition energy• Minimum ignition tempera-

ture (cloud/layer)• Electrical resistivity• Dust group• Area ventilation conditions• Location of dust release

points. Frequency and rate of release

Table 6. Types of Protection for Electri-cal Equipment (IEC/ATEX and NEC)

TechniqueIEC Des-

cription

Permitted Zone

Permit-ted Di-vision

Oil immersion Ex o 1 & 2 —Pressurization Ex p 1 & 2 1 & 2

Powder filling Ex q 1 & 2 —Flameproof Ex d 1 & 2 —

Explosion Proof — — 1 & 2

Increased safety — — —

Intrinsic safety Ex ia 0,1 & 2 1 & 2

Intrinsic safety Ex ib 1 & 2 —

Encapsulation Ex m 1 & 2 —

Special protection Ex s 0,1 & 2 —

Nonincendive — — 2

Nonsparking Ex nA 2 —

Enclosed break Ex nC 2 —

Energy limited Ex nL 2 —Simplified pressurization Ex nP 2 —

Restricted breathing Ex nR 2 —

Table 7. Types of Ignition Protection for Mechanical Equipment (ATEX)Method DescriptionTo ensure that ignition sources cannot arise Construction safety “c”, Inherent safety “g”,To ensure that ignition sources cannot become active Control of ignition sources “b”To prevent the explosive atmosphere from reaching the ignition source

Inert liquid immersion “k”, Inert gas pressuriza-tion “p”, Flow restricting enclosure “fr”

To contain the explosion and prevent flame propagation Flame proof enclosures “d”, Flame arresters

Table 1. Hazardous Areas*North America IEC (Europe)Class — Division Zones

Class I — Gas or vapor

Class II — Dust

Class III — Fiber or flying (no group designation)

Division 1: Pres-ent or likely to be present in normal operation

Division 2: Not present in normal operation, could be present in ab-normal operation

Gas/Vapor or Dust

Zone 0 (Gas) / Zone 20 (Dust)



An area in which an explosive atmosphere is continually present or present for long periods or frequently

An area in which an explosive atmosphere is likely to occur in normal operation

An area in which an explosive atmosphere is not likely to occur in normal operations and, if it does occur, will exist for only a short time

* This table represents a corrected version from that in the original printing

Department Editor: Rita L. D'Aquino

Solvent SelectionMethodology

A STEPWISE ProcEdurE

Organic solvents have been used in many industries for centu-ries, but the methods and tools to select optimal solvents while minimizing their adverse environmental, health, safety and op-erational concerns are still evolving. The appropriate selection of solvents depends to a large extent on the application — more specifically on what needs to be dissolved, and under what conditions. This article presents a four-step approach to solvent selection based upon Ref. 1*, where the reader will find a list of additional resources on this topic.

Identify the challenge and solvent characteristics. The first two steps are: 1) identifying the actual problem and technology or unit operation required to solve it; and 2) defining the requirements that must be met by the solvent, using criteria related to its physical and chemical properties (e.g., pure-solvent properties, such as normal boiling point, the Hildebrand solubility parameter at 300 K, the Hansen solubility parameters; solvent-solute properties, such as the solubility of the solute as a function of the composition of the mixture; and functional constraints, such as solute loss in solute).

Obtain reliable values of solvent properties and narrow down selection. There are several alternatives for this third step. For example, one can measure the required properties, use a database of properties of chemicals (or solvents), or, use property models to estimate them. For solvent-selection problems not involving chemical reactions, the pattern of the desired solvent is established through analysis of the solute, application type, and other constraints. Once this is established, a database of known solvents can be used to identify the solvents that match the necessary pattern (Table 1). On the other hand, when chemical reactions are involved, the approach is based on transition-state theory and requires consideration of the solvation energies of the reactants, products and transition states, and thus, knowledge of the reac-tion mechanism.

When the crucial values have been found, the solvent search could be such that first, solvent-pure properties are used, followed by solvent-EHS, then solvent-solute, and finally solvent-function. Narrow down the list by removing the compounds that do not match desired properties.

A protocol derived by Britest Ltd. (www.britest.co.uk) seeks to use mechanistic principles to guide solvent selection (Figure). The objective is to follow the arrows according to the problem defini-tion and a search criterion until an end-point is reached, thereby obtaining the characteristics of the candidate solvents. These characteristics are used to identify the group to which the solvents belong using solvents database (see Table 2). The corresponding group-types are evaluated and a final selection is made.

Verify selection. The fourth step is to verify that the solvent works as expected by performing a computational validation by simulation. Experimental validation of a solvent candidate is required at all stages of process development.

Table 2. Well-known solvents together with their related properties

Solvent Name Molecule type Group type Charge NBP NMP Sol. Par.(K) (K)

1-Methyl-2-pyrrolidinone Amide 1 NE/EPD 475.15 249.15 23.16Acetonitrile Nitrile 1 E/NPG 354.75 229.35 24.05Dimethyl sulphoxide S-oxide 1 E/NPG 462.15 291.65 26.75Dimethyl formamide Amide 1 NE/NPG 426.15 212.75 23.95Dimethylacetamide Amide 1 NE/NPG 438.15 253.15 22.35Diisopropyl ether Ether 2 NE/EPD 341.65 181.35 14.45Dimethyl ether Ether 2 NE/EPD 248.35 131.65 15.12Methyl tertbutyl ether Ether 2 NE/EPD 328.35 164.55 15.07Tetrahydrofuran Ether 2 NE/EPD 338.15 164.85 18.97Chlorobenzene Chloride 3 NE/P 632.35 404.9 19.35m-xylene (also o-; p-) Aromatic HC 3 NE/P 412.27 225.3 18.05Toluene Aromatic HC 3 NE/P 383.95 178.25 18.32Acetic acid Acid 4 PG 391.05 289.81 19.01Propionic acid Acid 4 E/PG 414.25 252.45 19.41Sulfuric acid Acid 4 E/PG 610

ordered283.46 28.41

Propanol Alcohol 5 E/N 370.35 147.05 24.45Ethanol Alcohol 5 E/N 351.35 159.05 26.13Butanol Alcohol 5 E/N 390.81 183.85 23.35Ethylene glycol Alcohol 5 E/N 470.45 260.15 33.7Dichloromethane Chloride 6 NE/EPD 313.15 178.05 20.37Heptane Alkane 7 NE/I 371.65 182.55 15.2Hexane Alkane 7 NE/I 341.85 177.85 14.9Pentane Alkane 7 NE/I 309.22 143.42 14.4Methanol Alcohol 4, 5 E/N 337.85 175.47 29.59Water Aqueous 4, 5 E/N 373.15 273.15 47.81

NE = non-electrolytic solvent; E = electrolytic solvent; P = polarizable; EPD = electron-pair donor; I = inert; PG = protogenic (proton donor); N= neutral (donor & acceptor); NPG = non-protogenic (proton acceptor); NBP = normal boiling point; NMP = normal melting point; Sol. Par. = Hildebrand solubility parameter at 300 K (MPa1/2)

Table 1. Some well-known databases and solvent selection tools

Databases Address and commentsChemFinder Searchable data and hyperlink index: http://chemfinder.cambridgesoft.comSolvents Databases Solvent substitution data systems at http://es.epa.gov/ssds/ssds.html;

“Handbook of Solvents” from www.chemtec.org/cd/ct_23.html; andSOLVDB at http://solvdb.ncms.org/index.html

NIST WebbookDIPPR and TAPP

Source of physical and chemical data at http://webbook.nist.govwww.aiche.org/TechnicalSocieties/DIPPR/About/Mission.aspx; and www.chempute.com/tapp.htm

CAPEC Database Pure as well as mixture properties data, including solvent-solute database:www.capec.kt.dtu.dk/Software/ICAS-and-its-Tools

Selection Tools Address and comments

SMSwin A specialized software for property estimation and solvent classification: www.capec.kt.dtu.dk/documents/software/SMSWIN.htm

NRTL-SAC and eNRTL-SAC

Activity coefficient method based on segment contributions. Predictive based on a small set of solubility data. Useful for crystallization solvent selection and extends to LLE and VLE: www.aspentech.com

Stability, solubility of reactants, products

Single phase or solid-liquid

Homogeous catalysis by Pt group complexes

Moderate polarity

DPA ethers, aromatics

Dipolaraprotic

Fast, low temp, but recoverydifficult

Slow, high- temperature, easy recovery

Aromatic hydrocarbon(xylene)

High polarity

Water, carboxylic acids, inorganic acids, lower alcohols

Condensation SN1/E1 SN2/E2

Substrate/product hydroxyl sensitive

NoYes

Water,alcohols

Dipolar aproticethers

Considersolvation

Two-phase or liquid-liquid (polar phase is water)

Water, immiscible solvent

Choose ‘polarity’ based on substrateand reagent solubility. May need phase-transfer catalyst

Group 7Group 5Group 2Group 4Group 3Group 1Group 3

Group 6Group 1Group 2

Group 3 Group 1

*Reference: 1. Gani, R., et al., A Modern Approach to Solvent Selection, Chem. Eng., Vol. 113, No. 3, pp. 30–43, Mar. 2006. Author E-mail: [email protected]


The formation of crystals requires the birth of new particles, also called nucleation, and the growth of these particles to the final

product size. The driving force for both rates is the degree of supersaturation, or the numerical difference between the concentration of solute in the supersaturated solution in which nucleation and growth occurs vs. concentration of solute in a solution that is theoretically in equilibrium with the crystals.

In a batch crystallizer, the crystal size distribution (CSD) is controlled by first seeding the initially supersaturated batch with a known number and size distribution of crystals, and then controlling the rate of evaporation or cooling (i.e., rate of energy transfer) so as to achieve a level of supersaturation that supports adequate crystal growth and an acceptable rate of nucleation. The relationship between supersaturation and growth is linear, but that between nucleation and growth is raised to a power that is usually greater than one, making it difficult to grow large crystals when nucleation is occurring. The following procedure describes how to achieve the optimal growth rate:1. Screen the seeds at the beginning of the ex-

periment to determine the cumulative number of crystals that are greater than a given size N’. Estimate NLi, the number of crystals of a given size (Lav) obtained from the screening:

(1) N W

L kLi

av v ci

= ∆3 ρ

The parameters are defined in the table

of nomenclature. To convert from µm to ft, multiply by 3.28 x 10–6.

2. Continue to measure the number and size of crystals as the cooling or evaporation program is in progress. Prepare an inverse cumulative plot of the number of crystals greater than a given size vs. size of the crystal (Figure 1). The crystal growth rate de-pends on the energy transfer rate, so modify the rate of energy transfer until a desirable product is obtained.

3. Repeat the first two steps at intervals throughout the batch cycle and plot the results as shown in Figure 1. The family of curves resulting from data plotted under the selected conditions indicates that the number of crystals is not increasing with time. Thus, no additional nucleation is occurring yet.

4. Proceed to collect crystal samples, an-ticipating the onset of nucleation. Figure 2 indicates that the number of crystals is significantly increasing with time. In this figure, t1 (not to be confused with t1 in Figure

1) represents the start of this new set of batch dynamics. It is safe to assume that significant nucleation is now occurring and that the rate of energy transfer is too high.

5. By taking the slope of the curve represent-ing the estimated number of nuclei present at the measured point in time (Nti) vs. time (ti), one can determine the nucleation rate. Using your representation of Figure 3, create a dashed, horizontal line across the lower portion of the graph depicting the selected, cumulative number of crystals (Ni’), and their sizes (L1–L4) over time (t1–t4).

6. For a selected cumulative number of crystals (Ni’), plot the crystal size (L) vs. time (t), as demonstrated in Figure 3. The slopes repre-sent the crystal growth rate (G). If the level of supersaturation changes during the run, the growth rate also changes. Non-parallel lines would indicate that the larger crystals are growing at a faster rate, due to a reduced diffusional resistance [layer] at the crystal surface. With larger particles, the resistance layer may be smaller, allowing the solute to more readily reach the crystal surface and incorporate itself into the lattice. These fac-tors collectively contribute to the accelerated growth rate of the larger particles. Parallel lines indicate that the growth rate is not dependent on crystal size.

7. Increase the rate of cooling or evaporation until additional nucleation occurs, upon which you can safely assume that the growth rate is too high.

8. Develop a seeding and evaporation profile that will yield a growth rate that is lower than the value found in Step 6.

When determining the growth rate, keep in mind the difference in mixing characteristics between a laboratory-scale vessel and a com-mercial configuration. A small tank generally offers a higher relative pumping capacity, shorter blend time, and higher average shear rates within a narrower range.

UsefUl observations

• Most processors will agree that when it comes to crystals, the larger, the better. Large crystals are easier to handle in downstream operations, such as washing, centrifugation and drying.

• As previously mentioned, it is desirable for the seeds’ size distribution to reflect a nar-row cut of particles. In this cut, the weight of crystals with sizes finer than Ls should be minimal because these tiny particles add enormously to the number of crystals that

compete for supersaturation and growth. • Studies show that milled seeds may not grow

as well as unmilled seeds. Furthermore, not all crystals of a given size grow at the same constant rate. This is sometimes attributed to the differences in the surface characteristics of particles that have equal dimensions.

• Fines destruction in a batch system can greatly reduce the effects of secondary nucle-ation on the CSD, and significantly increase crystal size while narrowing the CSD.

• In practice, not all additional nucleation can be suppressed. Crystallizations carried out at low levels of supersaturation near the meta-stable zone (i.e., the conditions under which crystals grow, but do not typically nucleate) will display some secondary nucleation, due to crystal-crystal interactions and contact between the crystals and the impeller. Nev-ertheless, the mean crystal size, shape and distribution are dramatically improved when seeding is followed by a programmed rate of energy transfer.

Reference: Genck, W., Better Growth in Batch Crystal-lizers, Chem. Eng., Vol. 106, No. 8, pp. 90–95, Aug. 2000. E-mail: [email protected]

Size, ( m) = smallest measurable size

1

1

1 2 3

2 3 4

num

ber o

f cry

stal

s la

rger

than

= smallest measurable size

12

3

4

12 3

4

=

1

2

3

1 2 3 4

Time, (min)

Controlling Crystal Growth

NomeNclature A Crystalsurfacearea,ft2

B° Nucleationrate,(numberofnuclei)/ft3/s

G Crystalgrowthrate,µm/s kv Crystal-volumeshapefactor,

dimensionless L Crystalsize,µm L’ Smallest-measurablesize,µm Lav Sizeofcrystalfraction,µm Lf Finalsizeofcrystal,µm Ls Seedsize,µm N Numberofseeds Ni’ Constant,cumulativenumber ofcrystalsincrystallizer NLi Numberofcrystalsofagivensize,

Lav

Nti Numberofcrystalnucleiatanytime S Rateofsupersaturation S* Maximumallowablesupersatura-

tion,lb/ft3solvent t, ti, tf Time,h ∆Wi Weightofcrystalsonscreen c Crystaldensity,lb/ft3

FiGure 1. FiGure 2. FiGure 3.


B

C

A

FIGURE 1. This nomograph is used to estimate annual cost savings from reducing combustible losses due to unburned carbon

This article has been drawn from the work of Wayne Turner and Steve Doty, “Boilers and Fired Systems,” Energy Management Handbook, 6th Ed., Ch. 5, The Fairmont Press, Lilburn, Ga., 2006.

Fuel Selection Considerations

The selection and application of fuels to various combustors are complex. Most existing units have limited flexibility in their ability to fire alternative

fuels. New units must be carefully planned to assure the lowest first costs without jeopardizing the future capability to switch to a different fuel.

Natural gasNatural gas has traditionally been the most attrac-tive fuel type for combustors because of the limited need for fuel-handling equipment (e.g., pipelines, metering, a liquid-knockout drum, and appropriate controls) and the freedom from pollution-control equipment. Drawbacks include rising fuel costs, in-adequate gas supplies and lower boiler efficiencies that result from firing natural gas, particularly when compared to the firing efficiencies of oil or coal.

Fuel oilFuel oils are graded as No. 1, No. 2, No. 4, No. 5 (light), No. 5 (heavy), and No. 6. Distillates are Nos. 1 and 2, and residual oils are Nos. 4, 5, and 6. Oils are classified according to their physical characteristics by the American Society for Testing and Materials (ASTM) per Standard D-396. No. 2 oil is suitable for industrial use and for home heating. The primary advantage of using a distillate oil rather than a residual oil is that it is easier to handle, requiring no heating to transport and no temperature control to lower the viscosity for proper atomization and combustion. However, consider-able purchase cost penalties exist between residual and distillate.

Distillates can be divided into two classes: straight-run, which is produced from crude oil by heating it and then condensing the vapors; and cracked, which involves refining at a high tempera-ture and pressure, or refining with catalysts to pro-duce the required oil from heavier crudes. Cracked oils contain substantially more aromatic and olefinic hydrocarbons, which are more difficult to burn than the paraffinic and naphthenic hydrocarbons obtained from the straight-run process. Sometimes a cracked distillate, called industrial No. 2, is used in fuel-burning installations of medium size (small package boiler or ceramic kilns, for example).

Because of the viscosity range permitted by ASTM, No. 4 and No. 5 oil can be produced in a variety of ways: blending of No. 2 and No. 6, mixing refinery by-products, utilization of off-specification products, and so on. Because of the potential variations in characteristics, it is important to monitor combustion performance routinely to obtain optimum results. Burner modifications may be required to switch from, say, a No. 4 blend to a No. 4 distillate.

Light (or cold) and heavy (or hot) No. 5 fuel oil are distinguished primarily by their viscosity rang-es: 150 to 300 SUS (Saybolt Universal Seconds) at 100°F and 350 to 750 SUS at 100°F, respectively. The (No.) classes normally delineate the need for preheating for proper atomization.

The No. 6 fuel oil is also referred to as residual, Bunker C, and reduced- or vacuum bottoms. Because of its high viscosity, 900 to 9,000 SUS at 100°F, it can only be used in systems designed with heated storage and a high enough temperature (to achieve proper viscosity) at the burner for atomization.

Notable fuel oil properties include the following: 1) Viscosity indicates the time required in seconds for 60 cm3 of oil to flow through a standard-size orifice at a specific temperature. In the U.S., it is normally determined with a Saybolt viscosimeter, which comes in Universal and Furol variants. The differences between them are the orifice size and the sample temperature. Thus, when stating an oil’s

viscosity, the type of instrument and temperature must also be stated. The Universal has the small-est opening and is used for lighter oils. 2) The flash point is the temperature at which oil vapors are ignited by an external flame. As heating continues above this point, sufficient vapors are driven off to produce continuous combustion. The flash point is also an indication of the maximum temperature for safe handling. Distillate oils have flash points of 145–200°F; heavier oils have flash points up to 250°F. 3) The pour point is the lowest temperature at which an oil flows under standard conditions, and is roughly 5°F above the solidification temperature. Knowledge of the pour point helps determine the need for heated storage, the storage temperature, and the need for heating and pour-point depressant.

Coal

The selection of coal as fuel involves higher capital investments because of the need for handling equipment, coal preparation (crush-ing, conveying, pulverizing, etc.) and storage; ash handling and storage; pollution-abatement equipment; and maintenance. The operating cost savings at current (2007) fuel prices of coal over oil or gas justifies a great portion, if not all, of the significantly higher capital invest-ments required for coal.

Coal-fired steam generators and vessels inherently suffer efficiency losses due to a failure to burn all the available fuel. The unburned fuel is the remaining carbon in the leftover ash. The nomograph (Figure 1) may be used to assess how a reduction in unburned carbon translates into energy and cost savings. A sample calcula-tion follows.

Example: The system is a coal-fired steam generator with a continuous rating of 145,000 lb/h; average (avg.) boiler load = 125,000 lb/h; existing combustibles in ash = 40% (measured); obtainable combustibles in ash = 5%; actual oper-ating time = 8,500 h/yr; design-unit heat output = 150 × 106 Btu/h; avg. heat output = 129 × 106 Btu/h; avg. fuel cost = $1.50/106 Btu.

Analysis: In Figure 1A, the percent of existing combustibles (measured) are shown on the horizontal axis. The curves above it represent the proposed improvement in percent of unburned carbon in ash. From the coordinates in Figure 1A draw a horizontal line to the curve in Figure 1B that represents the design-unit heat output. Drop the line to the appropriate fuel-cost curve in Figure 1C. Extend the line from that point to the left to obtain the corresponding annual fuel sav-ings, assuming continuous operation at full boiler design output. To calculate actual annual fuel

savings, a correction factor (CF) is required that considers actual boiler load and actual run time:

Actual savings, $ = Savings from chart x CFwhere CF = operating avg. heat output/design heat output × [(actual operating h/yr)/(8,760 h/yr)]

Savings for this example =$210,000/yr × [(129 × 106 Btu/h)/(150 × 106 Btu/h)] × [(8,500 h/yr)/(8,760 h/yr)]= $175,200/yr.Note: If the heat output of the unit or the average fuel cost exceeds the limit of the figures, use half of the particular value and double the savings obtained from Figure 1C.

It is probable that pulverized-coal-fired instal-lations suffer from high UCL whenever any of the following are experienced: a change in the raw-fuel quality from the original design basis; deterioration of the fuel burners, burner throats, or burner swirl plates or impellers; increased frequency of soot blowing to maintain heat-transfer surface cleanliness; a noted increase in stack gas opacity; uneven flame patterns characterized by a particularly bright spot in one section of the flame and a notably dark spot in another; CO formation as determined from a flue-gas analysis; frequent smoking observed in the combustion zone; increases in refuse quantities in collection devices; neglect of critical pulverizer internals and classifier assemblies; a high incidence of coal “hang-up” in the distribution piping to the burners; and frequent manipulation of the air/coal primary and second-ary air registers.

Techniques used successfully to reduce high UCL and/or high-excess-air operation include: modifying or replacing the pulverizer internals to increase the coal fineness; installing additional or new combustion controls to maintain consistent burner performance; purchasing new coal feeders that are compatible with and responsive to unit demand fluctuations; calibrating air flow and metering devices to ensure correct air/coal mixtures and velocities at the burner throats; installing turning vanes or air foils in the secondary air-supply duct or air plenum to ensure even distribution and proper air/fuel mixing at each burner; replacing worn and abraded burner impeller plates; installing new classifiers to ensure that proper coal fines reach the burners for combustion; rerouting or modifying air/coal distribution piping to avoid coal hang-up; increasing the air/coal mixture temperature exiting the pulverizers to ensure good ignition without coking; and cleaning deposits from burner throats. ■


Materials of Construction

Low-temperature appLications [1]

One key engineering consideration is the choice of materials of construction for frigid applications. Nickel-chromium (Ni-Cr) type stainless steels are notably versatile at low or cryogenic temperatures. They offer a combi-nation of high impact strength (IS) and corro-sion resistance. In the austenitic phase, with face-centered-cubic crystals, the combination of Cr and Ni in the material improves IS and toughness down to temperatures as low as –250°C. For good IS at temperatures down to –45°C, C-Mn-Si steels are recommended. The most preferred grades are fine-grained steels of pressure-vessel quality, such as ASTM A 516 and ASTM A 537 (in all grades). For temperatures between –45 and –100°C (for example, for liquid-ethylene storage), steels containing 2.5–9% Ni are useful. Between –150 and –250°C, the Ni-Cr austenitic steels (300 series, of 18/8 varieties), are highly recommended. In the nonferrous category, Al has excellent properties for temperatures as low as –250°C. Also attractive are Cu and some of its alloys, which can withstand tem-peratures down to –195°C.

chemicaL resistance

CPVC [2]. Many nonmetals do not have the tensile strength to meet the pressure require-ments of various process applications, espe-cially at elevated temperatures. But years of testing and actual field performance prove that chlorinated polyvinyl chloride (CPVC) systems can be pressure rated for operation as high as 200°F. CPVC’s high heat-distor-tion temperature and resistance to corrosion make it suitable for applications such as metal processing, pulp and paper, and in-dustrial wastewater treatment, where harsh and corrosive chemicals are commonly used (see Figure 1). Another advantage of CPVC is that it is lighter than metal, and therefore less expensive to install, from both a mate-rial cost and labor perspective. CPVC is not recommended where aromatic solvents and

esters are present in high concentrations. FRP pipe [3]. Composite fiberglass-rein-forced plastic (FRP) pipe has been replac-ing conventional pipe material, such as steel and concrete, in numerous applica-tions because of its corrosion resistance, low design weight (25% of concrete pipe and 10% of steel pipe), high fatigue en-durance, and adaptability to numerous composite blends (Table 5, Ref. 3) and manufacturing methods. FRP pipe may be divided into two broad categories: gravity pipe (dia. from 8 to 144 in.) and pressure pipe (dia. from 1 to 16 in.). It is not unusual to see FRP pressure pipe han-dling pressures as high as 2,000–5,000 psi during chemical processing, with the higher-pressure pipe at the lower end of the diameter scale.

heat transfer properties [4]

Metals, including specialty materials, are the best choice in terms of good heat trans-fer. In the lined category, glass is used ex-tensively for process equipment where good heat transfer is required. Lined materials, however, often have the problem of uneven thermal expansion, which may weaken the bonding of the lining in due course. While fluoropolymers have excellent compatibility with various chemicals and special surface and physical chemistries, they are gener-ally not used for reaction vessels because of their poor heat-transfer properties. Ther-mal conductivities for various materials are listed in the Table, and typical applications are shown in Figure 2.

THERMAL CONDUCTIVITY OF VARIOUS MATERIALS OF CONSTRUCTION [4]

Material Thermal conduc-tivity, W/(m)(K)

Carbon Steel (CS) 60.59SS 304 40.71SS 316 14.23SS 316 L 14.23Hastelloy B2 9.12Hastelloy C2 10.21Tantalum2 57.5Titanium2 21.67Zirconium2 20.77Graphite 121.15Hexoloy 125.65Glass1 1.00Lead 35.30Inconel2 12.00CPVC 0.14PTFE (Polytetra- fluoroethylene)1

0.25

PFA (Perfluoro- alkoxy resin)1

0.19

ETFE (Ethylene tetrafluoroethylene)1

0.24

PVDF (Polyvinylidene fluoride)1

0.23

ECTFE (Ethylene chlo-rotrifluoroethylene)1

0.16

1. Common choice for lining material2. Exotic metals

Excellent

Good

Fair

Poor

Weak acids Weak bases Salts Strong acids

Aliphatics Strong bases

Strong oxidants Halogens Aromatic solvents Esters and ketones

Figure 1. CPVC offers resistance to a variety of harsh chemicals

Exotic300

250

200

150

100

50

0

-50

-100

Tem

pera

ture

, °C

Exotic Exotic

Exotic

Fluoropolymer,glass lined,

exotic

Fluoropolymer,glass lined,

exotic

Glass lined,*exotic

Glass lined,*exotic

Exotic

Exotic Exotic

Exotic

Exotic

ExoticApplication:

Typical equipment:

Tanks, vessels

Pipelines,valves, �owmeters

Mixers Reactors

Storage Transport Agitation (Agitation + heat transfer)

Exotic Exotic Exotic

Figure 2. When looking beyond steel for materials of construction, it is impor-tant to consider the intended application and temperature range. Exotic (specialty) metals (see Table) are shown here to serve well in all applications. Another mate-rial, equally suited to a specific requirement, however, may be chosen as the more cost-effective option

References1. Nalli, K., Materials of Construction For Low-Tem-

perature and Cryogenic Processes, Chem. Eng. July 2006, pp. 44–47.

2. Newby, R. and Knight, M., Specifying CPVC In Chemical Process Environments, Chem. Eng., Oc-tober 2006, pp. 34–38.

3. Beckwith, S., and Greenwood, M., Don’t Over-look Composite FRP Pipe, Chem. Eng., May 2006, pp. 42-48.

4. Robert, J., Selecting Materials of Construction, Chem. Eng., September 2005, pp. 60–62.


Heat Transfer Fluids and Systems

STARTUP1.Verifycontrolandsafetysystems:Itisvitallyim-

portant toverifyallcontrolandsafetysystemsarecalibratedandreadyforoperationandarefunctioningproperly

2.Checkforleakage3.Remove moisture from the system, using dry,

compressedairorothersuitablemeans.Fillthesystemwithheattransferfluid

4.Systemfilling a. Fillthesystemtodesiredlevelavoidingany

unnecessaryaerationofthefluid b. Openallvalves,thenstartthemaincircula-

tionpumpinaccordancewiththemanufac-turer’s recommendations.Allow for thermalexpansion of fluid in determining the coldchargelevel

c. Circulate theheat transfer fluid through thesystemforabout3to4hourstoeliminateairpockets, and to assure complete system fillbeforefiringtheheater

5.Starttheheater a. Bringthesystemuptotemperatureslowlyto

helpprevent thermalshock toheater tubes,tube/heater jointsandrefractorymaterials;and allow operators to check the function-ing of instruments and controls. The slowheat-upwillalsoallowmoisture trapped inallsectionsofthesystemtoescapeasvapor.Inertgasshouldsweeptheexpansiontanktoremovenoncondensablesandresidualmois-turetoasafelocation.Holdthetemperaturestableabove100°C(212°F)untilnosignsof moisture remain (knocking or rattling ofpipes,nomoisturefromvents,andsoon)

b. Bring the system to operating temperature,putthe“users”online,andplacetheexpan-sion-tankinertingsystemintooperation

c.Thefluidshouldgenerallybeanalyzedwithin24hofplantstartupandannuallythereafter

d.CheckandcleanstartupstrainersasneededThe system should be heated and cooled for atleast two cycles with the filter in place since theresultingexpansionandcontractionwillloosenmillscale. Reinsulate any surfaces left bare for leak-checkingpurposes.

OPERATIONSHeaters: Proper fluid-heater operation will helpensurelonglifeofthefluid.Commonheaterprob-lems include flame impingement, excessive heatflux,controlovershoot,lowfluidflow,andinterlockmalfunctions.Piping and pumps: Aleak-freesystemwillhelptoensuresafeandreliableoperation.Somekeyfea-turesofaleak-freedesignareasfollows:• Maintainvalvesandpumppackingandseals• Avoid the use of threaded fittings (welded or

flangedconnectionsarepreferred)• Realignpumpsandretorqueflangesoncesys-

temachievesoperatingtemperatureafterinitialsystemstartup

• Confirmwithyourfluidsupplierwhattheproperelastomersare.Notallelastomersarecompat-iblewithallheattransferfluids

FLUID ANALYSISFluid testing helps detect system malfunction, fluid contamination, moisture, thermaldegradation,aswellasotherfactors that impactsystemperformance(seeTable).Forsystemsoperatingnearthefluid’smaximumtemperature,annualanalysisissuggested.

Possible actions1. Filtration: Small diameter particles sus-

pended inheat transfer fluid canbeef-fectivelyremovedviafiltration.Filterswith100-mmorlessnominal-particle-removalratings should be considered for initialsystem treatment. Continuous filtrationthrough10-mmratedfilterscanmaintainsystemcleanliness

2. Venting: Iflowboilerconcentrationand/ormoistureisallowedtoreachexcessivelevelsinthefluid,problemssuchaspumpcavitation,increasedsystempressureandflash-pointdepressioncanoccur.Intermit-tent,controlledventingtoasafelocationisacommonsolutiontominimizethepo-tential forproblemscausedbyexcessivelowboilerormoistureconcentration

3. Inerting: Aneffectivemethodofminimiz-ing fluid oxidation is to blanket the ex-pansiontankwithaclean,dry,inertgas,suchasnitrogen,CO2,ornaturalgas

4. Dilution/replacement: Canbeusedtore-movesomefraction(orall)ofthefluidandreplacewithvirgin fluid tomaintain fluidpropertieswithinnormalranges.Cautionis advised when using reclaimed fluid,which can return degradation productsand/orcontaminantsintothesystem

5. Cleaning: Ifa system flush isnecessary,several different methods are available.Specialty-engineered, heat-transfer flushfluidsmaybeused to removesludgeortar from piping/equipment. Hard car-bondepositsonheatersurfaces(“coke”)generally require the use ofmechanicalcleaning techniques like sand or bead

Test result Potential effects Possible cause Possible actions*

Viscosity increase

Poor heat-transfer rate, de-posits, high vapor pressure, pump cavitation

• Contamination• Thermal degradation• Fluid oxidation

4, 54, 53, 4

Total acid number increase

System corrosion, deposits

• Severe oxidation• Acidic contamination

3, 44, 5

Moisture increase

Corrosion, excess system pressure, pump cavitation, mechanical knocking

• System leaks• Residual moisture in

new or cleaned unit• Unprotected vent or

storage

22

2, 3

Insoluble solids increase

Poor heat transfer, wear of pump seals, plugging in narrow passages

• Contamination• Dirt• Corrosion• Oxidation• Thermal stress

1, 4, 51, 41, 3, 51, 31, 4

Low- and high-boiler increase**

Pump cavitation, poor heat transfer, excess system pressure, deposits

• Low boilers• High boilers• Contamination

244, 5

* For detailed guidance on actions, please consult with your fluid engineering specialist.** For an excellent discussion on low and high boilers, please consult Ref. [4].

blasting,wirebrushing,orhigh-pressurewaterjetting.Forprocesscontamination,consult with your fluid supplier for sug-gestedcleaningmethods

SHUTDOWNPreventoverheatingoffluidduetoresidualheatintheheater.1. Shutoffburnercompletelywiththecircu-

lating pump still operating. Continue torunthepumpatfullcapacitytodissipateresidualheatintheheater

2.Whentheheaterhascooledtothemanu-facturer’srecommendedlowtemperature,shutoff thecirculatingpumpandswitchoffrequiredheaterelectricalcontrols

3.Cautionmustbeexercisedduringshut-downtoensurethatnoareainthesys-tempipingistotallyandcompletelyiso-lated. This will prevent a vacuum fromforming,whichcoulddamage(implode)equipment

4.Operateheattracing,ifneeded

References and further reading1.Gamble, C.E., Cost Management in Heat

TransferSystems,Chem. Eng. Prog.,July2006pp.22–26.

2. Gamble,C.E.,CleaningOrganicHeatTransferFluidSystems,Process Heating,Oct.2002.

3. Beain, others, Properly Clean Out Your Or-ganicHeatTransferFluidSystem,Chem. Eng. Prog.,May2001.

4. Spurlin,others,DefiningThermalStability,Pro-cess Heating,Nov.2000.

5. “LiquidPhaseDesignGuide,”Pub#7239128C,Solutia,Inc.,1999.

Facts at Your Fingertips SponSored by

Department Editor: Kate Torzewski

Pristine Processing Equipment

Processes in the pharmaceutical, biotechnology, food and semiconductor industries must meet a high set of standards to ensure high product purity. Equipment criteria specific to high-purity processes are established to minimize contamination and maintain prod-

uct integrity. In designing a pristine process, material and equipment style are of upmost importance. Bacteria is the main cause of contamination and is prone to growing in the dead cavities of equipment created by sharp corners, crevices, seams and rough surfaces. Another source of contamination is leaking, which allows undesirable chemicals to compromise the quality of the process ingredients, by causing contamination, rusting and particle generation.

MATERIALS OF CONSTRUCTION

Many factors must be taken into consideration when selecting materials of construction for use in pristine process applications where high-purity and sanitation are paramount. All sur-faces should be constructed of a smooth material that will not corrode, generate particles or harbor dead cavities. These criteria can be met with three standard materials: 316L stainless steel (SS), polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). The advantages and disadvantages of these materials are summarized below to facilitate the material selection process for a given application with consideration of chemical compat-ibility, cost, and temperature stability.

ValvesValves should not harbor contaminants and must be easy to clean. By these criteria, diaphragm and pinch valves are excellent choices for ultrapure processes, as they have smooth, gently curved surfaces that will not harbor contaminants. Ball check, full-port plug and full-port ball valves are good choices as well, while butterfly, spring check, gate and swing check valves are all unacceptable, since contamination can col-lect in the corners that are essential to their design. Though several valves are appropri-ate for pristine processes, certain valves are better suited for particular applications. Diaphragm valves are the most widely used in high-purity systems for their resistance to contamination and ability to be used as a control valve. Ball and plug valves, on the other hand, are less costly and are not limited by temperature and pressure. Also, in applications using sterile steam and freeze-drying, ball valves are preferred over diaphragm valves because they eliminate the risk of catastrophic seat failure.

SealsAs with all pristine processing equipment, high-purity seals should not have any cavities where contaminants can breed. By choosing a seal with gland rings that do not need to be threaded or ported, the areas where bacteria can breed are minimized. In choosing a seal material, it is important to find a compound that will not swell, crack, pit or flake, thus reducing seal failure and contamination. To ensure the success of seals, fluroelastomers are a top choice in pristine processing applications for their ex-cellent thermal stability, chemical resistance and mechanical durability.

PipingThe surface of piping, as well as any wet-ted equipment parts, should have a very smooth surface. When 316 SS is being used, electropolishing is a good method for achieving an ultra-smooth finish. Joining methods should minimize crevices and dead cavities, and all materials should be free of biological degradable substances, leachable substances, and glues and solvents that may migrate into the product stream.

References1. Smith, B., What Makes a Pump for High-Purity

Fluids?, Chem. Eng., pp. 87–89, April 2002.

2. Schmidt, M., Selecting Clean Valves, Chem. Eng., pp. 107–111, June 2001.

3. Wulf, B., Pristine Processing: Designing Sanitary Systems, Chem. Eng., pp. 76–79, Nov. 1996.

4. Weeks, D. T. and Bennett, T., Specifying Equip-ment for High-Purity Process Flow, Chem. Eng., pp. 27–30, Aug. 2006.

EQUIPMENT STYLE SELECTION

Critical factors in high-purity equipment selection include cleanability, cost, flow capabili-ties and product compatibility. With these considerations in mind, criteria useful for choos-ing pumps, valves, seals and piping are described in this section.

PumpsA fundamental requirement of pristine processing pumps is the ability to clean a pump in place without disassembly. Pump seals, gaskets and internal surfaces should eliminate the buildup of material and should clean out easily during wash cycles. The most common pump styles for high-purity processes are centrifugal, lobe-style and peristaltic pumps, which are outlined below.

MATERIALS OF CONSTRUCTIONMaterial Advantages Disadvantages

Stainless Steel

• Mechanical strength • Functions at 121°C (steam-sterilization temperature)

Vulnerability to corrosion by certain chemicals, which increases with temperature

PVDF

• Chemically inert • Resistant to corrosion and leaching • Durable and long-lasting • Retains circumferential strength

Functions only intermittently at 121°C

PTFE

• The most chemically inert plastic• Resistant to corrosion and leaching• Avoids leaching • Suitable for coating equipment

Complex shapes are difficult to form

PUMPSPump style Advantages Disadvantages Applications best

suited for this style

Centrifugal• Low cost • Easy cleanability

Efficiency and flow de-crease with increasing pressure and volume

• Handling low- viscosity products • Handling high flowrates (40–1,500 gal/min)

Peristaltics

• Low cost • Easy cleanability • No mechanical seals • Non-damaging to delicate products

The need for hoses may cause issues in elastomeric compatibility, temperature and pressure limitations, and a need to change hose regularly

• Small, batch-oriented applications • Laboratory or pilot- scale plants

Rotary Lobes

• Higher pressure and flow capabilities • Unaffected by pressure variations

High cost

• Large, continuous duty applications • Steaming and high pressure applications


Pump Selection and Specification

PUMP SELECTION

In choosing a pump, it is important to match a pump’s capabili-ties with system requirements and the characteristics of the liquid being processed. These factors include the inlet conditions,

required flowrate, differential pressure and liquid characteristics. Generally, the quality of the liquid should remain unchanged after passage through a pump. Therefore, material compatibility, viscos-ity, shear sensitivity and the presence of particulate matter in a liquid are important considerations in pump selection. Most engineering applications employ either centrifugal or positive displacement (PD) pumps for fluid handling. These pumps function in very different ways, so pump selection should be based on the unique conditions of a process.

Centrifugal pumpsThe most widely used pump in the chemical process industries for liquid transfer is the centrifugal pump. Available in a wide range of sizes and capacities, these pumps are suitable for a wide range of applications. Advantages of the centrifugal style include: simplic-ity, low initial cost, uniform flow, small footprint, low maintenance expense and quiet operation.

Positive displacement pumpsThough engineers may be first inclined to install centrifugal pumps, many applications dictate the need for PD pumps. Because of their mechanical design and ability to create flow from a pressure input, PD pumps provide a high efficiency under most conditions, thus reducing energy use and operation costs.

Choosing centrifugal versus positive displacementThese two main pump styles respond very differently to various operating conditions, so it is essential to evaluate the requirements of a process prior to choosing an appropriate pump. Table 1 il-lustrates the mechanical differences between these pumps, as well as the effects of pressure, viscosity and inlet conditions on flowrate and pump efficiency.

Range of operationPump styles range far beyond simply PD and centrifugal pumps. PD pumps encompass many specific styles, including a variety of reciprocating, rotary and blow-cover pumps. Likewise, centrifugal pumps encompass radial, mixed, and axial flow styles, which all

belong to a greater category of kinetic pumps. A simple way to narrow down pump styles is to determine the required capacity that your pump must handle. Based upon a required capacity in gal/min. and a pressure in lbf/in.2, the pump coverage chart below can help engineers focus their selection to a just a few pump styles.

PUMP SPECIfICaTIONS

Based on the application in which a pump will be used, the pump type, and service and operating conditions, the specifications of a pump can be determined.• Casting connection: Volute casing efficiently converts velocity en-

ergy impacted to the liquid from the impeller into pressure energy. A casing with guide vanes reduce loses and improve efficiency over a wide range of capacities, and are best for multistage high-head pumps

• Impeller details: Closed-type impellers are most efficient. Open-type impellers are best for viscous liquids, liquids containing solid matter, and general purposes

• Sealings: Rotating shafts must have proper sealing methods to prevent leakage without affecting process efficiency negatively. Seals can be grouped into the categories of noncontacting seals and mechanical face seals. Noncontacting seals are often used for gas service in high-speed rotating equipment. Mechanical face seals provide excellent sealing for high leakage protection

• Bearings: Factors to take into consideration while choosing a bearing type include shaft-speed range, maximum tolerable shaft misalignment, critical-speed analysis, loading of compressor impellers, and more. Bearing styles include: cylindrical bore; cy-lindrical bore with dammed groove; lemon bore; three lobe; offset halves; tilting pad; plain washer; and taper land

• Materials: Pump material is often stainless steel. Material should be chosen to reduce costs and maintain personnel safety while avoiding materials that will react with the process liquid to create corrosion, erosion or liquid contamination

References1. “Perry’s Chemical Engineers’ Handbook,” 7th ed. New York: McGraw

Hill, 1997.2. Petersen, J. and Jacoby, Rodger. Selecting a Positive Displacement Pump,

Chem. Eng. August 2007, pp. 42–46.

PumP ComParison ChartCentrifugal Pump

Positive displacement pump

Mechanics

The pump imparts a velocity to the liquid, resulting in a pressure at the outlet. Pressure is created and flow results

The pump captures confined amounts of liquid and transfers them from the suction to discharge port. Flow is created and pressure results

Performance Flow varies with changing pressure

Flow is constant with changing pressure

Viscosity Efficiency decreases with increasing viscosity

Efficiency increases with increasing viscosity

Efficiency

Efficiency peaks at the best-of-efficiency point. At higher or lower pressures, efficiency decreases

Efficiency increases with increasing pressure

Inlet conditions

Liquid must be in the pump to create a pres-sure differential. A dry pump will not prime on its own

Negative pressure is created at the inlet port. A dry pump will prime on its own

Adapted from Perry’s Chemical Engineers’ Handbook


Avoiding Seal Failure

Seals are assemblies of elements that prevent the passage of a solid, liquid, gas or vapor from one system to another. When a seal

allows leakage of material, failure has occurred. This guide provides an overview of common seal types and a discussion of seal failure to aid in choosing the most effective seal and avoiding future failure.

seal types

Seals types can be classified within two broad categories: static and dynamic. Static seals have no relative motion between mating surfaces, while dynamic seals do have relative motion between a moving surface and a stationary surface. Seals do not have to fit into one category or the other; rather, seal types can fall anywhere on a spectrum between static and dynamic, and few seals are strictly one type or the other. Table 1 describes the applications and requirements of several common seal types.

seal failure

Seal failure is caused by a wide variety of circumstances, including improper in-stallation and environmental factors such as temperature, pressure, fluid incompat-ibilities, time and human factors.

Most causes of failure can be described as mechanical difficulties or system operations problems. Examples of mechanical difficulties include strain on the seal face caused by improper installation and vibration caused by improper net positive suction head. Meanwhile, system operating problems can include condi-tions that are outside of a pump’s best performance envelope, such as upsets, dry running, and pressure or temperature fluctuations. Changes in the fluid being pro-cessed can cause problems as well, especially with fluids that flash or carbonize.

Common visual indicators of failure include short cuts, V-shaped notches in the seal, skinned surface in localized areas, or thin, peeled-away area on the seal. Table 2 describes causes of some of the most prevalent types of seal failure with recommended methods of action.

In some cases, the cause of failure may be difficult to determine due to the com-plexity of the seal construction. These unique failure modes can result in flaking or peeling of the seal face, corrosion, flaking or pitting of the carbon faces, degrada-tion of the elastomer energizer seals, and spring or bellows breakage. It is likely that these rapid degradations are a result of contamination, which can be avoided with careful installation or using pre-assembled, cartridge-type mechanical seals.

References1. Ashby, D. M. Diagnos-

ing Common Causes of Sealing Failure, Chem. Eng. June 2005, pp. 41–45.

2. Netzel, J., Volden, D., Crane, J. Suitable Seals Lower the Cost of Ownership, Chem. Eng. December 1998, pp. 92–96.

TABLE 1. COMPARISON OF COMMON SEAL TYPESType Applications Periodic

Adjustment Required?

Moving friction

Tolerances required (mov-ing seals)

Gland adapters required?

Space require-mentsStatic Dynamic

O-ring X X No Medium Close No SmallT-seal X X No Medium Fairly close No SmallU-packing — X No Low Close No SmallV-packing — X Yes Medium Fairly close Yes SmallCup-type pack-ing

— X No Medium Close Yes Medium

Flat gasket X — Yes — — No LargeCompression or jam packing

X X Yes High Fairly close Yes Large

TABLE 2. SOLUTIONS TO COMMON CAUSES OF SEAL FAILUREFailure type Definition Causes SolutionsCompression set

A lost of resiliency caused by the failure of a seal to rebound after it has been deformed for some period of time. The seal will exhibit a flattened surface corresponding to the contours of the mating hardware

Exposure to excessive tempera-ture or incompatible fluids Excessive deformation of the elastomer at installation An incompletely vulcanized seal

Choose proper deflection for the seal Choose appropriate elastomer mate-rial for the application in terms of thermal stability and compression set resistance

Nibbling and extrusion

A seal starts to appear to be torn away in little pieces until it loses its overall shape and flows into whatever void area is available

Excessive clearance gaps Improper seal material Excessive volume-to-void ratio Inconsistent clearance gaps

Increase bulk hardness of the sealing element Decrease clearance gaps Redesign volume-to-void ratio Add anti-extrusion devices

Spiral failure A seal rolls within its gland, resulting in cuts or marks that spiral around the circumference of the seal

Applications where a seal is used in a slow, reciprocating fashion Irregular surface over the mating parts causing the seal to grip to certain contact points

Use an elastomer with a higher bulk hardness For male-type installation, increase the installed stretch on the seal Specify a smoother, more uniform fin-ish on mating hardware Change the type of seal to a lip-type configuration

Explosive decompres-sion

Seal exhibits blisters, fissure, pock marks or pits, both externally and internally

Gas entrapment within the elastomer during high-pres-sure cycling, followed by rapid depressurization

Use an elastomer material that is more resilient to explosive decompression Use polymeric or metal seals if 0possible

Wear Smooth burnishing of a sealing surface

Relative motion of the seal against the mating surface

Use a harder material Use a polymeric solution

chemical engineering - facts at your fingertips 2007 april to december

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