Tài liệu Guidelines for Engineering Design for Process Safety P2 pdf

As an example Koestler 1992, an organic material was originally chlorinated in a glass-lined batch stirred tank reactor, with chlorine fed through a dip pipe.. Table 2-3 Effect of Reacto

elements to replace the batch process The new reactor was so much smaller that when a group of people who had seen the original plant toured the new manufacturing facility, they looked for a large reactor and finally mistook the final product storage tank for the reactor Paul (1988) emphasizes the impor-tance of a thorough study of the chemical reaction mechanisms and kinetics

in several examples from the pharmaceutical industry, allowing the process designers to identify optimal reactor configurations using novel designs including tubular reactors with static mixing elements

2.2.1.3 Gas-Liquid Reactions

Mass transfer is often the rate limiting step in gas-liquid reactions, and novel reactor designs that increase mass transfer can reduce reactor size and also improve process yields As an example (Koestler 1992), an organic material was originally chlorinated in a glass-lined batch stirred tank reactor, with chlorine fed through a dip pipe Replacement of the stirred tank reactor with

a loop reactor, with chlorine fed to the recirculating liquid stream through an eductor, reduced reactor size, increased productivity and reduced chlorine usage as summarized in Table 2-3

RAW

MATERIALS

STATIC MIXER

REACTOR (SEVERAL THOUSAND GALLONS)

STORAGE TANK (SEVERAL THOUSAND GALLONS)

Figure 2-2 A large batch reactor to

manufacture a product

Figure 2-3 A tubular reactor to

manufac-ture the product of Figure 2-2.

Table 2-3 Effect of Reactor Design on Size and Productivity for a Gas-Liquid Reaction8

Reactor Type

Reactor Size (liters)

Chlorination Time (hr)

Productivity (kg/hr)

Chlorine Usage (kg/ 100 kg product)

Caustic Usage in Vent Scrubber

a Koestler!992

Batch Stirred Tank Reactor 8000 16 370 33 31

Loop Reactor

2500 4 530 22 5

2.2.1A Some Additional Examples of Intensification

Nitroglycerine formerly was manufactured in batch reactors containing more than one ton of material Newer CSTR processes significantly reduce the inventory, and the Nobel AB process uses a mixing eductor reactor to reduce inventory to about 1 kg (Dale 1987, Kletz 1984,199Id) Some ethylene oxide derivatives can be manufactured in a continuous tubular reactor rather than

a batch reactor containing a potentially flammable vapor space (Kletz 199Id) Adipic acid can be manufactured in an internally cooled plug flow reactor rather than an externally cooled CSTR (Kletz 1984) Kletz (1984,199Id) pro-vides additional examples of intensification through improved reactor design.

2.2.2 Storage and Material Transfer

Raw material and in-process storage tanks often represent a major portion of the risk of a chemical plant Hazardous material transfer lines can also be a significant hazard Attention to the design of storage and transfer equipment can reduce hazardous material inventory.

2.2.2.1 Storage

Storage tanks for raw material and intermediates are often much larger than really necessary, usually because this makes it "easier" to operate the plant The operating staff can pay less attention to ordering raw materials on time,

or can accept downtime in a downstream processing unit, because upstream production can be kept in storage until the downstream unit is back on line This convenience in operation can come at a significant cost in risk of loss of containment of the hazardous materials being stored The process design engineers must question the need for all intermediate hazardous material storage, and minimize quantities where such storage is really needed

Similar-Iy, hazardous raw material storage should also be minimized, with greater attention being given to "just in time" supply Inventory reduction can also result in lower inventory costs, as well as increasing the inherent safety of the manufacturing facility

The reduction in inventory resulting from greater attention to plant opera-tions and design of unit interacopera-tions can be extremely large Wade (1987) gives several excellent examples:

• An acrylonitrile plant eliminated 500,000 pounds (-277,000 kg) of in-process storage of hydrogen cyanide by accepting a shutdown of the entire unit when the product purification area shut down This applied pressure to the plant to solve the problems that caused shutdown of the purification area

• Another acrylonitrile plant supplied by-product hydrogen cyanide to various other units An inventory of 350,000 pounds (-159,000 kg) of hydrogen cyanide was eliminated by having the other units draw directly from the acrylonitrile plant This required considerable work to resolve many issues related to acrylonitrile purity and unit scheduling

• A central bulk chlorine system with large storage tanks and extensive piping was replaced with a number of small cylinder facilities local to the individual chlorine users Total inventory of chlorine was reduced by over 100,000 pounds (~45,360 kg)

2.2.2.2Transfer Piping

Inventory in transfer lines can be a major factor in overall facility risk For example, a quantitative risk analysis of a chlorine storage and supply system identified the pipeline from the storage area to the manufacturing area as the most important contributor to total risk (Hendershot 199Ib) To minimize the risk associated with transfer lines, their length should be minimized by careful attention to unit location and pipe routing Pipe size should be sufficient to convey the required amount of material and no larger However, it is impor-tant to remember that small bore piping is less robust and less tolerant of abuse when compared to large piping, and that additional attention to proper support and installation will be required (IChemE 1987) In some cases, for example, chlorine for water treatment applications, it may be possible to transfer material as a gas rather than a liquid with a large reduction of inventory in the transfer line

Options to reduce the inventory in a pipeline will reduce the downwind distance to a particular concentration of concern of a toxic or flammable material For example, Table 2-4 compares the downwind distance to a 25 ppm chlorine concentration as a result of the rupture of various size liquid and vapor chlorine pipes

2.2.3 Distillation

Some suggestions for inventory reduction in conventional distillation systems include:

• Minimize the size of reflux accumulators and reboilers (Dale 1987)

• Use internal reflux condensers and reboilers where practical (Dale 1987)

• Use column internals that minimize holdup without sacrificing operation efficiency pale 1987)

• Reduce the amount of material in the base of the column by reducing the diameter of the base (Kletz 199Id)

• Remove toxic, corrosive, or otherwise hazardous materials early in a distillation sequence, reducing the spread of such materials throughout

a process (Wells and Rose 1986)

Low-inventory distillation equipment, such as the thin film evaporator, is also available and should be considered for hazardous materials This equip-ment offers the additional advantage of short residence time and is

particular-ly useful for reactive or unstable materials

The use of Higee rotating distillation equipment, invented by Imperial Chemical Industries (ICI), can reduce inventory by a factor of 1000 The distillation occurs in a rapidly rotating bed containing a packing with a high specific surface area Vapor is fed to the outside and moves to the center, contacting liquid fed at the center and moving outward Extremely effective separations are possible with a small in-process inventory and very short residence time This technology is described in more detail by Kletz (199Id)

Table 2-4 Effect of Various Options to Reduce Inventory on the Hazard Zone Resulting from the Rupture of a 500-Foot Chlorine Transfer Pipea

Pipe Diameter (in)

2

1

1

Chlorine State Liquid Liquid Vapor

Inventory (kg) 430 110 2

Downwind Distance

to Atmospheric Chlorine Concentration of

25 ppm (m) 2400 1700 650

a Henderehot 1991a

a

2.2.4 Heat Transfer

Heat transfer equipment has a great variation in heat transfer area per unit of material volume Table 2-5 compares the surface compactness of a variety of heat exchanger types Process inventory can be minimized by using heat exchangers with the minimum volume of hazardous process fluid for the heat transfer area required

2,3 SUBSTITUTION

2.3.1 Chemistry

Inherent safety of the manufacturing process for a material can be greatly increased by development of alternate chemistry using less hazardous raw material or intermediates, reducing inventories of hazardous materials, or operating at less severe processing conditions Identification of catalysts to enhance reaction selectivity or allow desired reactions to be carried out at a lower temperature or pressure is often a key to development of inherently safer chemical synthesis routes The following are some specific examples of innovations in process chemistry that result in inherently safer processes Halogenated polymers can be manufactured by conducting the polym-erization step first, followed by halogenation of the polymer This avoids

Table 2-5 Surface Compactness of Heat Exchangers8

Type of Exchanger

Shell and tube

Plate

Spiral plate

Shell & finned tube

Plate fin

Printed circuit

Regenerative-rotary

Regenerative-fixed

Twin screw extruder

Human lung

Surface Compactness (rr^/m 3 ) 70-500

120-225 up to 1,000

Up to 185 65-270 up to 3,300 150-450 up to 5,900 1,000-5,000

Up to 6,600

Up to 15,000*

"High"

20,000

* Kletz 1991 d

Some types have a compactness as low as 25m /m

Halogenated polymers can be manufactured by conducting the polym-erization step first, followed by halogenation of the polymer This avoids manufacture and handling of hazardous halogenated monomers (Burch 1986; Kharbanda and Stallworthy 1988)

The insecticide carbaryl, the product manufactured at Bhopal, can be produced by several routes, some of which do not use methyl isocyanate, the material that was released in the Bhopal accident, or that generate only small quantities of methyl isocyanate as an in-process intermediate (Kletz 199Id) DuPont has developed a proprietary process for manufacture of carbamate insecticides which generates and immediately consumes methyl isocyanate Total methyl isocyanate inventory in the process is no more than 10 kilograms (Kharbanda and Stallworthy 1988)

Acrylonitrile can be manufactured by reacting acetylene with hydrogen cyanide:

CHSCH + HCN -4 CH2=CHCN

A newer ammoxidation process uses less hazardous raw materials (propyl-ene and ammonia) (Dale 1987; Puranik et al 1990):

CH2=CHCH3 = NH3 + |o2 -* CH2=CHCN + 3H2O

2t

The Reppe process for manufacture of acrylic esters uses hazardous raw materials, acetylene and carbon monoxide, and a catalyst with high acute toxicity, nickel carbonyl, to react with an alcohol to make the corresponding acrylic ester

CH^CH + CO = ROH Uj^l?*4 CH2=CHCO2R

rid

The newer propylene oxidation process uses less hazardous materials to first manufacture acrylic acid followed by esterification with the appropriate alcohol (Hochheiser 1986)

CH2=CHCH3 + |o2 cataI H st> CH2CHCO2 + H2O

TJ+

CH2CHCO2H + ROH -£L-» CH2=CHCO2R + H2O

Polymer supported reagents, catalysts, protecting groups and mediators can be used in place of the corresponding small molecule materials (Sher-rington 1991) The reactive species is tightly bound to a macromolecular support which immobilizes it This generally makes toxic, noxious or cor-rosive material much safer The use of polystyrene sulfonic acid catalyst for

CH3OH + CH2=C(CHO2 Pd ^ rem Sul f mic Ad4 CH3OC(CHs)3

Sherrington (1991) provides several additional examples and suggestions for future development

Chemistry of side reactions and by-products may also offer opportunities for increasing the inherent safety of a process For example, a process involv-ing a caustic hydrolysis step uses ethylene dichloride as a solvent Under the reaction conditions a side reaction between sodium hydroxide and ethylene dichloride produces small but hazardous quantities of vinyl chloride:

C2H4Cl2 + NaOH -» C2H3Cl + NaCl = H2O

An alternative nonreactive solvent has been identified which eliminates the hazard (Hendershot 1987)

Phase transfer catalysis ("Phase Transfer" 1990; Starks 1987; Starks and Liotta 1978) processes for the synthesis of many organic materials use less, or sometimes no, organic solvents, may use less toxic solvent, may allow use of less hazardous raw materials (for example, aqueous HCl instead of anhydrous HCl), and operate at milder conditions Some types of reactions where phase transfer catalysis has been applied include:

• esterification

• nucleophilic aromatic substitution

• etherification

• dehydrohalogenation

• oxidations

• alkylation

• aldol condensations

Rogers and Hallam (1991) provide a number of additional examples of chemical approaches to inherent safety, involving synthesis routes, reagents, catalysts and solvents

2.3.2 Solvents

Replacement of volatile organic solvents with aqueous systems or less haz-ardous organic materials improves safety of many processing operations and final products Some examples include:

• Water based paints and adhesives in place of solvent based products

• Aqueous or dry flowable formulations for agricultural chemicals instead

of organic solvent formulations

• British computer manufacturer ICL has eliminated chlorofluorocarbons from its manufacturing processes, replacing them with aqueous cleaning systems for flux removal ("Technology" 1991) In the United States, IBM

Table 2-6 Some Examples of Solvent Substitutions

Chloroform -+ Acetone —* Ethyl Acetate -* Ethanol

Dichloromethane -4 Ethanol

Trichloroethylene -* Aqueous System

Acetic Acid -4 Aqueous System

Propanol -* 1,2-Propanediol —> Aqueous System

a Adapted from Goldschmidt and Filskov 1990

has reduced or eliminated chlorofluorocarbons, chloroform, methylene chloride, and other hazardous solvents, replacing them with nonhazar-dous materials (Kelley 1992) Apple Computer reports the elimination of all chlorofluorocarbons for cleaning electronic assemblies and has

con-verted to water based processes (Chemical WeekNewswire 1992).

• The United States Air Force is evaluating a process called Coldjet which removes paint from airplanes using a jet of frozen carbon dioxide pellets

in place of hazardous paint removal solvents (Welter 1991)

• Consumer paint removal products based on less volatile organic esters are now available as substitutes for products based on hazardous solvents such as methanol, toluene, acetone and methylene chloride ("Paint Re-movers" 1991)

• A Danish survey (Goldschmidt and Filskov 1990) confirms the feasibility

of solvent substitution as a way of reducing workplace exposure to hazardous materials, particularly organic degreasing solvents Table 2-6 lists some of the substitutions identified by this industrial survey

2.3.3 Utility Systems

Utility and plant services systems must also be examined for options to increase the inherent safety of a plant or process For example:

• Use water or steam as a heat transfer medium rather than flammable or combustible oils (Kharbanda and Stallworthy 1988; Kletz 199Id)

• Use high flash point oils or molten salt if water or steam is not feasible (Dale 1987; Kletz 199Id)

• Chlorofluorocarbon refrigerants have been cited as inherently safer alter-natives to refrigerants such as ammonia and propane Many chloro-fluorocarbons are now being phased out because of suspected adverse environmental impact This creates new challenges for industry in iden-tifying new refrigerants that have the low acute toxicity and fire hazards

of chlorofluorocarbons but that do not have long term adverse environ-mental impacts

• Alternatives to chlorine are available for water treatment and disinfection applications For example, sodium hypochlorite has been used both in industrial and municipal water treatment applications (Governale 1989; Somerville 1990), and calcium hypochlorite is another possible alterna-tive

• Use magnesium hydroxide slurry to control pH, rather than concentrated sodium hydroxide (Englund 199Ia)

2.4 ATTENUATION

Attenuation means using materials under less hazardous conditions This can

be accomplished by strategies that are either physical (e.g., lower tempera-tures, dilution) or chemical (e.g., development of a reaction chemistry that operates at less severe conditions)

2.4.1 Dilution

Dilution reduces the intrinsic hazards associated with storage of a low-boiling hazardous material in two ways: by reducing the storage pressure and by reducing the initial atmospheric concentration in the event of a release Materials that boil below normal ambient temperature have often been stored

in pressurized systems under their ambient temperature vapor pressure The pressure in such a storage system can be lowered by diluting the material with

a higher boiling solvent This reduces the driving force (the pressure difference between the storage system and the outside environment) in case of a leak in the system, reducing the rate of release As an example, Table 2-7 shows the effect of water dilution on the vapor pressure of ammonia and of mono-methylamine solutions Handling of these materials as a sufficiently dilute aqueous solution allows them to be stored at atmospheric pressure rather than

in a pressurized system

A distinct benefit of storage in the diluted form is the reduced partial pressure of the hazardous component in the solution In the event of a loss of containment accident, the atmospheric concentration of the hazardous mate-rial at the spill location will be reduced The reduced atmospheric concentra-tion at the source results in a smaller hazard zone downwind of the spill

The effect of water dilution of monomethylamine, a flammable and toxic material, on the vapor cloud resulting from a loss of containment incident is shown in Figure 2-4 Monomethylamine boils at -6.70C and has a vapor pressure of about 50 psig at 250C Figure 2-4 shows the relative hazard zones, defined as the distance from the source within which the monomethylamine vapor concentration will exceed a specified value The loss of containment event in this example is the complete failure of a 1-inch liquid pipe under a specific atmospheric condition for (A) anhydrous monomethylamine and (B)

a 40% aqueous monomethylamine solution The hazard zone extends to a much greater distance in the case of ambient storage of anhydrous mono-methylamine

Many materials can be handled in a dilute form to reduce the risk of handling and storage Some other examples include:

• muriatic acid in place of anhydrous HCl

• dilute nitric acid in place of concentrated fuming nitric acid

• sulfuric acid in place of oleum (SOs solution in sulfuric acid) for sulfona-tion reacsulfona-tions

If a chemical process requires the concentrated form of a material, it may

be feasible to store it as a more dilute form and concentrate the material, by distillation or some other technique in the plant prior to introduction to the process This reduces the inventory of material with greater intrinsic hazard

to the minimum amount required to operate the process

Table 2-7 Vapor Pressure of Aqueous Ammonia and Monomethylamine

Solutions 8

Ammonia (21 0 C)

Concentration

(Wt %)

100.0

48.6

33.7

28.8

19.1

Vapor Pressure (atm) 8.80 3.00 1.10 0.75 0.31

Monomethylamine (2O 0 C) Concentration

(Wt %)

100.0 50.0 40.0

Vapor Pressure (atm)

2.80

0.62

0.37

a Henderehot 1991a