Major Unit Operations and Processes: An Overview
Authors:Lipton, Sydney
in77. Chemical Processing, McCann, Michael,Stellman, Jeanne M.,
Editor,Encyclopedia of Occupational Health and Safety, Jeanne Mager
Stellman, Editor-in-Chief. International Labor Organization,
Geneva. 2011.
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This article presents information on basic process equipment,
storage, plant layout and operations considerations in chemical
process industries, including major items and concepts that are
broadly applicable throughout the chemical industry. However, much
of the equipment required in chemical processing is highly
specialized and cannot be broadly generalized. More detailed
information on toxicity and hazardous materials and process safety
are reviewed elsewhere in thisEncyclopaedia.
There are two basic categories of layout in chemical processing
industries: plant layout, which covers all process units,
utilities, storage areas, loading/unloading areas, buildings, shops
and warehousing, and unit or process layout, which covers only
equipment placement for a specific process, also termed a process
block.
Plant Layout
Siting
Locating or siting an overall plant is based upon a number of
general factors, as shown in table 1 (CCPS 1993). These factors
vary considerably with locations, governments and economic
policies. Of these various factors, safety considerations are an
extremely important concern, and in some locations they can be the
major factor that governs plant siting.
Table 1. Some general site selection factors
Population density around the site
Natural disaster occurrence (earthquake, flood, etc.)
Prevailing winds and meteorological data
Availability of power, steam and water
Safety considerations
Air, water and waste regulations and their complexity
Accessibility to raw materials and markets
Transportation
Siting permits and complexity of obtaining them
Interaction requirements in industrial developments
Labour availability and costs
Investment incentives
One important aspect of plant safety in siting is defining a
buffer zone between a plant with hazardous processes and nearby
plants, dwellings, schools, hospitals, highways, waterways and
airplane corridors. Some overall safety considerations are
presented in table 2. The buffer zone is important because distance
tends to reduce or mitigate potential exposures from various
accidents. The distance necessary to reduce toxic concentrations to
acceptable levels through atmospheric interaction and the
dispersion of toxic materials from an accidental release can be
defined. Moreover, the time lag between a toxic release and public
exposure created by a buffer zone can be used to warn the
population through pre-planned emergency response programmes. Since
plants have various types of facilities containing toxic materials,
dispersion analyses should be conducted on the potentially
hazardous systems to ensure the buffer zone is adequate in each
area surrounding the plant perimeter.
Table 2. Plant siting safety considerations
Buffer zone
Location of other hazardous installations in vicinity
Inventory of toxic and hazardous materials
Adequacy of firefighting water supply
Emergency equipment access
Availability of emergency response support from adjacent
industries and the community
Weather extremes and prevailing winds
Location of highways, waterways, railroad and airplane
corridors
Environmental and waste disposal restrictions during
emergencies
Draining and grade slope
Maintenance and inspection
Fire is a potential hazard in process plants and facilities.
Large fires can be a source of thermal radiation which can also be
mitigated by distance. Elevated flares can also be a source of
thermal radiation during an emergency or startup/shutdown
operation. A flare is a device that automatically burns exhaust
gases or emergency vapour releases at elevated positions or special
ground locations. These should be sited away from the plant
perimeter (for community protection) and an area at the flare base
should be prohibited to workers. If not operated properly, liquid
carryover into the flare can result in burning liquid droplets. In
addition to fire, there can be explosions within equipment or a
vapour cloud that produces blast waves. Although distance will
reduce the blast intensity somewhat over the buffer zone, the blast
will still have an effect on the nearby community.
The potential of accidental releases or fires from existing
facilities that may be near the proposed site should also be
considered. Potential incidents should be modelled and evaluated to
determine the possible effect on the proposed plant layout.
Emergency responses to an external event should be evaluated and
responses coordinated with other plants and affected
communities.
Other considerations
Dow Chemical Company has developed another approach to plant
layout based on an acceptable level of Maximum Probable Property
Damage (MPPD) and Business Interruption Risk (B1) (Dow Chemical
Company 1994a). These considerations are important for both new and
existing plants. The Dow Fire and Explosion Index is useful in new
plant layouts or in the addition of equipment to existing plants.
If risks calculated from the Index are found to be unacceptable,
the separation distances should be increased. Alternatively, layout
changes may also reduce the risk potential.
Overall layout
In an overall plant layout, the prevailing winds are an
important consideration. Ignition sources should be located upwind
of potential leak sources. Fired heaters, boilers, incinerators and
flares are in this category (CCPS 1993). The location of storage
tanks downwind of process units and utilities is another
recommendation (CCPS 1993). Environmental regulations have led to
significantly reduced leakage from tankage (Lipton and Lynch
1994).
Minimum separation distances have been outlined in various
publications for process units, equipment and different plant
functions (CCPS 1993; Dow Chemical Company 1994a; IRI 1991).
General facilities that normally have recommended distance
separations in overall plant layouts are shown in table 3. Actual
distance recommendations should be carefully defined. While fired
heaters and process furnaces are not shown in table 3, they are an
important item and recommended distance separations must be
included in a unit process layout.
Table 3. Facilities generally separated in overall plant
layouts
Process units
Tank farms
Loading and unloading facilities
Flares
Power, boilers and incinerators
Cooling towers
Substations, large electrical switch yards
Central control houses
Warehouses
Analytical laboratories
Incoming utility metering and block systems
Fire hoses, fixed monitors, reservoirs and emergency fire
pumps
Waste treatment areas
Maintenance buildings and areas
Administrative buildings
In addition, roads are necessary for emergency and maintenance
vehicle or equipment access and require careful placement between
process units and throughout the various sections of the plant.
Acceptable clearances for overhead pipe racks and other overhead
equipment should be established along with lateral clearances at
cross-roads and entrances to all facilities.
The layout requirements can be based on recommended minimum
separation distances (CCPS 1993; NFPA 1990; IRI 1991; Mecklenburgh
1985) or determined through a hazard analysis (Dow Chemical Company
1994a).
Process Unit Layout
Table 3 presents an overall plant separations layout summary.
The process units are contained within the specific block shown in
the general layout. The chemical process is generally shown in
detail in process and implementation diagrams (P&IDs). A
process layout requires considerations beyond specific equipment
separation distances, some of which are shown in table 4.
Table 4. General considerations in a process unit layout
Area definition for future expansion and unit accessibility
Repair equipment accessibility for frequent maintenance
Space requirements for individual equipment repair (e.g., area
needed for pulling heat exchanger bundle or accessibility for
control valve)
Barriers for high pressure equipment or reactors with explosion
potential
Mechanical and space requirements for loading/unloading
solids-filled reactors or towers
Space for venting dust explosions
Separation of frequently opened or maintained equipment from
high temperature piping, vessels, etc.
Special buildings or structures and necessary clearance (e.g., a
compressor house withan internal bridge crane or external
crane)
The assemblage of equipment in any particular process unit will
vary considerably, depending on the process. The toxicity and
hazardous characteristics of the streams and materials within the
units also vary widely. Despite these differences, minimum distance
standards have been developed for many equipment items (CCPS 1993;
NFPA 1990; IRI 1991; Mecklenburgh 1985). Procedures for calculating
potential leakage and toxic exposures from process equipment that
can also affect separation distance are available (Dow Chemical
Company 1994b). In addition, dispersion analysis can be applied
when leakage estimates have been calculated.
Equipment and separation distance
A matrix technique can be used to calculate the space needed for
separating equipment (CCPS 1993; IRI 1991). Calculations based upon
specific processing conditions and an equipment hazard evaluation
may result in separation distances that differ from a standard
matrix guide.
Extensive lists for a matrix can be developed by refinement of
individual categories and by the addition of equipment. For
example, compressors may be split into several types, such as those
handling inert gas, air and hazardous gases. Separation distances
for engine-driven compressors may differ from motor- or
steam-driven machines. Separation distances in storage facilities
that house liquefied gases should be analysed on the basis of
whether the gas is inert.
The process battery limits should be carefully defined. They are
the boundary lines or plot limits for a process unit (the name
derives from the early use of a battery of ovens in processing).
Other units, roads, utilities, pipeways, runoff ditches and so on
are plotted based upon battery limits. While unit equipment
location does not extend to the battery limits, separation
distances of equipment from battery limits should be defined.
Control rooms or control houses
In the past each process unit was designed with a control room
that provided operational control of the process. With the advent
of electronic instrumentation and computer-controlled processing,
individual control rooms have been replaced by a central control
room that controls a number of process units in many operations.
The centralized control room is economically advantageous because
of process optimization and increases in efficiency of personnel.
Individual process units still exist and, in some specialized
units, older control houses which have been supplanted by
centralized control rooms may still be used for local process
monitoring and for emergency control. Although control room
functions and locations are generally determined by process
economics, the design of the control room or control house is very
important for maintaining emergency control and for worker
protection. Some considerations for both central and local control
houses include:
pressurizing the control house to prevent the entrance of toxic
and hazardous vapours
designing the control house for blast and explosion
resistance
establishing a location that is at minimal risk (based upon
separation distance and probability of gas releases)
purifying all inlet air and installing an inlet stack location
that minimizes the intake of toxic or hazardous vapours
sealing all sewer outlets from the control house
installing a fire suppression system.
Inventory reduction
An important consideration in process and plant layouts is the
quantity of toxic and hazardous material in the overall inventory,
including the equipment. The consequences of a leak are more severe
as the volume of material increases. Consequently, the inventory
should be minimized wherever possible. Improved processing that
reduces the number and size of pieces of equipment reduces the
inventory, lowers the risk and also results in lower investment and
improved operating efficiencies.
Some potential inventory reduction considerations are shown in
table 6. Where a new process facility will be installed, processing
should be optimized by taking into consideration some of the
objectives shown in table 5.
Table 5. Steps for limiting inventory
Reducing storage tank inventory reduction through improved
process control, operation and just-in-time inventory control
Eliminating or minimizing onsite tank inventory through process
integration
Using reaction variable analysis and development for reactor
volume reduction
Replacing batch reactors with continuous reactors, which also
reduces downstream holdup
Lowering distillation column holdup through bottoms-volume
reductions and tray holdup with either more advanced trays or
packings
Replacing kettle reboilers with thermosyphon reboilers
Minimizing overhead drum and bottoms surge drum volumes
Improving pipe layout and sizing to minimize holdup
Where toxic materials are produced, minimizing the toxic section
holdup
Storage Facilities
The storage facilities in a chemical processing plant can house
liquid and solid feed, intermediate chemicals, by-products and
process products. Products stored in many facilities serve as
intermediates or precursors for other processes. Storage may also
be required for diluents, solvents or other process materials. All
of these materials are generally stored in above-ground storage
tankage (AST). Underground tankage is still used in some locations,
but use is generally limited due to access problems and limited
capacity. In addition, potential leakage of such underground
storage tanks (USTs) presents environmental problems when leaks
contaminate ground water. General earth contamination can lead to
potential atmospheric exposures with higher vapour-pressure
materials leaks. Leaked materials can be a potential exposure
problem during ground remediation efforts. UST leakage has resulted
in stringent environmental regulations in many countries, such as
the requirements for double-walled tanks and underground
monitoring.
Typical above-ground storage tanks are shown in figure 1.
Vertical ASTs are cone or domed roof tanks, floating roof tanks
that are covered or non-covered floating roof or external floating
roof tanks (EFRTs). Converted or closed roof tanks are EFRTs with
covers installed on the tanks that are frequently geodesic type
domes. Since EFRTs over time do not maintain a perfectly circular
shape, sealing the floating roof is difficult and a covering is
installed on the tank. A geodesic dome design eliminates roof
trusses needed for cone roof tanks (FRTs). The geodesic dome is
more economical than a cone roof and, in addition, the dome reduces
losses of materials to the environment.
Figure 1. Typical above-ground storage tanks
Normally, the tanks are limited to liquid storage where the
liquid vapour pressure does not exceed 77 kPa. Where the pressure
exceeds this value, spheroids or spheres are used since both are
designed for pressure operation. Spheroids can be quite large but
are not installed where the pressure may exceed certain limits
defined by the mechanical design. For most higher vapour-pressure
storage applications, spheres are normally the storage container
and are equipped with pressure relief valves to prevent over
pressuring. A safety concern that has developed with spheres is
rollover, which generates excessive vapour and results in relief
valve discharges or in more extreme situations such as sphere wall
rupture (CCPS 1993). In general, the liquid contents stratify and
if warm (less dense) material is loaded into the sphere bottom, the
warm material rises to the surface with the cooler, higher density
surface material rolled over to the bottom. The warm surface
material vaporizes, raising the pressure, which may result in
relief valve discharge or sphere overpressuring.
Tank layout
Tankage layout requires careful planning. There are
recommendations for tank separation distances and other
considerations (CCPS 1988; 1993). In many locations, separation
distances are not specified by code, but minimum distances (OSHA
1994) can be a result of various decisions applicable to separation
distances and locations. Some of these considerations are presented
in table 6. In addition, tank service is a factor in tank
separation for pressurized, refrigerated and atmospheric tanks
(CCPS 1993).
Table 6. Tank separation and location considerations
Separation based on shell to shell distances can be based on
references and subject to calculating the thermal radiation
distance in the event of fire in an adjacent tank.
Tanks should be separated from process units.
A tank location, preferably downwind from other areas, minimizes
ignition problems in the event of a tank releasing a significant
vapour quantity.
Storage tanks should have dykes, which are also required by law
in most regions.
Tanks can be grouped for utilization of common dykes and
firefighting equipment.
Dykes should have isolation capability in an emergency.
Dykes are required and are nominally sized volumetrically to
hold the contents of a tank. Where multiple tanks are within a
dyke, the minimum volumetric dyke capacity is equivalent to the
capacity of the largest tank (OSHA 1994). The dyke walls can be
constructed of earth, steel, concrete or solid masonry. However,
the earth dykes should be impenetrable and have a flat top with a
minimum width of 0.61 m. In addition, the soil within the dyked
area should also have an impenetrable layer to prevent any chemical
or oil leakage into the soil.
Tank leakage
A problem that has been developing through the years is tank
leakage as a result of corrosion in the tank bottom. Frequently,
tanks have water layers in the tank bottom that can contribute to
corrosion, and electrolytic corrosion may occur due to contact with
the earth. As a result, regulatory requirements have been
instituted in various regions to control tank bottom leakage and
underground soil and water contamination from contaminants in the
water. A variety of design procedures have been developed to
control and monitor leakage (Hagen and Rials 1994). In addition,
double bottoms have also been installed. In some installations,
cathodic protection has been installed to further control metal
deterioration (Barletta, Bayle and Kennelley 1995).
Water draw off
Manually discharging water periodically from the tank bottom can
result in exposure. Visual observation to determine the interface
through open manual draining can result in worker exposure. A
closed discharge can be installed with an interface sensor and
control valve minimizing potential worker exposures (Lipton and
Lynch 1994). A variety of sensors are commercially available for
this service.
Overfilling tanks
Frequently, tanks are overfilled, creating potential safety and
worker exposure hazards. This can be prevented with redundant or
dual-level instruments controlling inlet block valves or feed pumps
(Bahner 1996). For many years, overflow lines were installed on
chemical tanks, but they terminated a short distance above a drain
opening to permit visual observation of the overflow discharge.
Moreover, the drain had to be sized for greater than the maximum
fill rate to ensure proper drainage. However, such a system is a
potential exposure source. This can be eliminated by connecting the
overflow line directly to the drain with a flow indicator in the
line to show the overflow. Although this will function
satisfactorily, this results in overloading the drain system with a
very large contaminant volume and potential health and safety
problems.
Tank inspection and cleaning
Periodically, tanks are removed from service for inspection and/
or cleaning. These procedures must be carefully controlled to
prevent worker exposure and minimize potential safety hazards.
Following draining, tanks are frequently flushed with water to
remove process liquid traces. Historically, the tanks have then
been cleaned manually or mechanically where necessary. When tanks
are drained, they are filled with vapour that may be toxic and can
be within a combustible range. Water flushing may not significantly
affect vapour toxicity, but it may reduce potential combustion
problems. With floating roofs, the material below the floating roof
can be flushed and drained, but some tanks may still have material
in the sump. This bottom material must be removed manually and may
present potential exposure concerns. Personnel may be required to
wear personal protective equipment (PPE).
Normally, enclosed tanks and any volume below the floating roofs
are purged with air until a specified oxygen concentration level is
achieved before entry is permitted. However, concentration
measurements should be continually obtained to ensure toxic
concentration levels are satisfactory and do not change.
Vapour venting and emission control
For fixed roof or converted floating roof tanks (CFRTs), venting
to the atmosphere may not be acceptable in many locations. The
pressure-vacuum (PV) vent (shown in figure 2 these tanks are
removed and the vapours flow through a closed duct to a control
device where the contaminants are destroyed or recovered. For both
tanks, an inert purge (e.g., nitrogen) can be injected to eliminate
the diurnal vacuum effect and maintain a positive pressure for the
recovery device. In the CFRT tank, the nitrogen eliminates the
diurnal effect and reduces any vapours to the atmosphere through a
PV vent. However, vapour emissions are not eliminated. A large
number of control devices and techniques are available including
combustion, absorbers, condensers and absorption (Moretti and
Mukhopadhyay 1993; Carroll and Ruddy 1993; Basta 1994; Pennington
1996; Siegall 1996). Selection of a control system is a function of
final emission targets and operating and investment costs.
In floating roof tanks, both external and internal, seals and
auxiliary fitting controls effectively minimize vapour losses.
Safety hazards
Flammability is a major concern in tankage and fire-fighting
systems are required to aid in control and prevention of expanded
fire zones. Firewater systems and installation recommendations are
available (CCPS 1993; Dow Chemical Company 1994a; NFPA 1990). Water
can be sprayed directly on a fire under certain conditions and is
essential in cooling adjacent tankage or equipment to prevent
overheating. In addition, foam is an effective fire-fighting agent
and permanent foam equipment can be installed on tanks. The
installation of foam equipment on mobile fire-fighting equipment
should be reviewed with a manufacturer. Environmentally acceptable
and low toxicity foams are now available that are effective and
comparable to other foams in quickly extinguishing fires.
Processing Equipment
A wide variety of process equipment is required in chemicals
processing as a result of the numerous processes, specialized
process requirements and variations in products. Consequently, all
of the chemical equipment in use today cannot be reviewed; this
section will concentrate on the more widely applied equipment found
in processing sequences.
Reactors
There are a large number of reactor types in the chemical
industry. The basis for reactor selection is a function of a number
of variables, beginning with classifying whether the reaction is a
batch or continuous reaction. Frequently, batch reactions are
converted to continuous operations as experience with the reaction
increases and some modifications, such as improved catalysts,
become available. Continuous reaction processing is generally more
efficient and produces a more consistent product, which is
desirable in meeting product quality targets. However, there are
still a large number of batch operations.
Reaction
In all reactions, the classifications of a reaction as
exothermic or endothermic (producing heat or requiring heat) is
necessary in order to define the heating or cooling requirements
necessary to control the reaction. In addition, runaway reaction
criteria must be established to install instrument sensors and
controls that can prevent a reaction from becoming out of control.
Prior to full-scale operation of a reactor, emergency procedures
must be investigated and developed to ensure the runaway reaction
is safely contained. Some of the various potential solutions are
emergency control equipment that is automatically activated,
injection of a chemical that stops the reaction and vent facilities
that can accommodate and contain the reactor contents. Safety valve
and vent operation are extremely important requiring
well-maintained and functioning equipment at all times.
Consequently, multiple interlocked safety valves are frequently
installed to ensure that maintenance on one valve will not reduce
the required relief capacity.
Should a safety valve or vent discharge due to malfunction, the
discharge effluent must be contained in practically all
circumstances to minimize potential safety and health hazards. As a
result, the method of containing the emergency discharge through
piping along with final disposition of the reactor discharge should
be carefully analysed. In general, liquid and vapour should be
separated with the vapour sent to a flare or recovery and liquid
recycled where possible. Solids removal may require some study.
Batch
In reactors involving exothermic reactions, an important
consideration is fouling on the walls or internal tubing by the
cooling media used to maintain the temperature. Removal of fouled
material varies considerably and the method of removal is a
function of the fouled material characteristics. Fouled material
can be removed with a solvent, a high-pressure jet nozzle stream
or, in some cases, manually. In all these procedures, safety and
exposure must be carefully controlled. Movement of material in and
out of the reactor must not permit the entrance of air, which may
result in a flammable vapour mixture. Vacuums should be broken with
an inert gas (e.g., nitrogen). Vessel entry for inspection or work
can be classified as entry into a confined space and the rules for
this procedure should be observed. Vapour and dermal toxicity
should be understood and technicians must be knowledgeable about
health hazards.
Continuous
Flow-through reactors can be filled with liquid or a vapour and
liquid. Some reactions produce slurries in the reactors. Also,
there are reactors that contain solid catalysts. The reaction fluid
may be liquid, vapour or a combination of vapour and liquid. Solid
catalysts, which promote a reaction without participating in it,
are normally contained within grids and are termed fixed beds. The
fixed-bed reactors may have single or multiple beds and can have
exotherinic or endothermic reactions, with most reactions requiring
a constant temperature (isothermal) through each bed. This
frequently requires the injection of feed streams or a diluent at
various locations between beds to control the temperature. With
these reaction systems, temperature indication and sensor location
through the beds are extremely important to prevent a reaction
runaway and product yield or quality changes.
Fixed beds generally lose their activity and must be regenerated
or replaced. For regeneration, deposits on the bed may be burned
off, dissolved in a solvent or, in some cases, regenerated through
the injection of a chemical in an inert fluid into the bed, thereby
restoring catalyst activity. Depending on the catalyst, one of
these techniques may be applied. Where beds are burned, the reactor
is emptied and purged of all process fluids then filled with an
inert gas (usually nitrogen), which is heated and recirculated,
raising the bed to a specified temperature level. At this point, a
very small volume of oxygen is added to the inert stream to
initiate a flame front that gradually moves through the bed and
controls the temperature rise. Excessive oxygen quantities have a
deleterious effect on the catalyst.
Fixed-bed catalyst removal
Removal of fixed-bed catalysts must be carefully controlled. The
reactors are drained of process fluid and then the remaining fluid
is displaced with a flushing fluid or purged with a vapour until
all of the process fluid has been removed. Final purging may
require other techniques before the vessel can be purged with an
inert gas or air prior to opening the vessel or discharging the
catalyst from the vessel under an inert blanket. Should water be
used in this process, the water is drained through closed piping to
a process sewer. Some catalysts are sensitive to air or oxygen,
becoming pyrophoric or toxic. These require special procedures to
eliminate air during filling or emptying the vessels. Personal
protection along with handling procedures must be carefully defined
to minimize potential exposures and protect personnel.
Spent catalyst disposal may require further treating before it
is sent to a catalyst manufacturer for recycling or into an
environmentally acceptable disposal procedure.
Other catalyst systems
Gas flowing through a loose solid catalyst bed expands the bed
and forms a suspension that is similar to a liquid and termed a
fluid bed. This type of reaction is used in various processes.
Spent catalysts are removed as a gas-solids side stream for
regeneration and then returned to the process through an enclosed
system. In other reactions, catalyst activity may be very high and,
although catalyst is discharged in the product, the concentration
is extremely low and does not pose a problem. Where a high
concentration of catalyst solids in the product vapour is
undesirable, solids carryover must be removed before purification.
However, traces of solids will remain. These are removed for
disposal in one of the by-product streams, which in turn must be
clarified.
In situations where spent catalyst is regenerated through
burning, extensive solids recovery facilities are required in
fluid-bed systems to meet environmental restrictions. Recovery may
consist of various combinations of cyclones, electric
precipitators, bag filters) and/ or scrubbers. Where burning occurs
in fixed beds, the basic concern is temperature control.
Since fluid-bed catalysts are frequently within the respiratory
range, care must be exercised during solids handling to ensure
worker protection with either fresh or recovered catalysts.
In some instances a vacuum may be used to remove various
components from a fixed bed. In these situations, a steam-driven
vacuum jet is frequently the vacuum producer. This produces a steam
discharge that frequently contains toxic materials although in very
low concentration in the jet stream. However, the discharge of a
steam jet should be carefully reviewed to determine contaminant
quantities, toxicity and potential dispersion if it is discharged
directly to the atmosphere. Should this be unsatisfactory, the jet
discharge may require condensing in a sump where all vapours are
controlled and the water is sent to the closed sewer system. A
rotary vacuum pump will perform in this service. The discharge from
a reciprocating vacuum pump may not be permitted to discharge
directly to the atmosphere, but can in some instances discharge
into a flare line, incinerator or process heater.
Safety
In all reactors, pressure increases are a major concern since
the vessel pressure rating must not be exceeded. These pressure
increases may be a result of poor process control, malfunction or a
runaway reaction. Consequently, pressure relief systems are
required to maintain vessel integrity by preventing reactor
overpressuring. Relief valve discharges must be carefully designed
to maintain adequate relief under all conditions, including
relief-valve maintenance. Multiple valves may be required. Should a
relief valve be designed to discharge into the atmosphere, the
discharge point should be elevated above all nearby structures and
a dispersion analysis should be conducted to ensure adequate
protection for workers and nearby communities.
If a rupture disk is installed with a safety valve, the
discharge should also be enclosed and the final discharge location
designated as described above. Since a disk rupture will not
reseat, a disk without a safety valve will probably release most of
the reactor contents and air may enter the reactor at the end of
the release. This requires a careful analysis to ensure that a
flammable situation is not created and that highly undesirable
reactions do not occur. Moreover, the discharge from a disk may
release liquid and the vent system must be designed to contain all
liquids with vapour discharged, as described above. Atmospheric
emergency releases must be approved by regulatory authorities
before installation.
Mixer agitators installed in reactors are sealed. Leaks may be
hazardous and if they occur the seal must be repaired which
requires a reactor shutdown. The reactor contents may require
special handling or precautions and an emergency shutdown procedure
should include reaction termination and disposition of the reactor
contents. Flammability and exposure control must be carefully
reviewed for each step including final disposition of the reactor
mix. Since a shutdown can be expensive and involve production loss,
magnetic driven mixers and newer seal systems have been introduced
to reduce maintenance and reactor shutdowns.
Entrance to all reactors requires compliance with safe
confined-space entry procedures.
Fractionation or distillation towers
Distillation is a process whereby chemical substances are
separated through methods which take advantage of differences in
boiling points. The familiar towers in chemical plants and
refineries are distillation towers.
Distillation in various forms is a processing step found in the
great majority of chemical processes. Fractionation or distillation
can be found in purification, separation, stripping, azeotropic and
extractive process steps. These applications now include reactive
distillation, where a reaction occurs in a separate section of the
distillation tower.
Distillation is conducted with a series of trays in a tower, or
it can be conducted in a tower filled with packing. The packings
have special configurations that readily permit the passage of
vapour and liquid, but provide sufficient surface area for
vapour-liquid contact and efficient fractionation.
Operation
Heat is normally supplied to a tower with a reboiler, although
the heat content of specific streams may be sufficient to eliminate
the reboiler. With reboiler heat, multiple step vapour-liquid
separation occurs on the trays and lighter materials ascend through
the tower. Vapours from the top tray are fully or partially
condensed in the overhead condenser. The condensed liquid is
collected in the distillate recovery drum, where part of the liquid
is recycled to the tower and the other portion is withdrawn and
sent to a specific location. Non-condensed vapours may be recovered
elsewhere or sent to a control device which can be a combustor or
recovery system.
Pressure
Towers typically operate at pressures higher than atmospheric
pressure. However, towers are frequently operated under vacuum to
minimize liquid temperatures that may affect product quality or in
situations where tower materials become a mechanical and economic
concern due to the temperature level that may be difficult to
achieve. Also, high temperatures may affect the fluid. In heavy
petroleum fractions, very high tower bottoms temperatures
frequently result in coking problems.
Vacuums are typically obtained with ejectors or vacuum pumps. In
process units, vacuum loadings consist of some light vapour
materials, inerts that may have been in the tower feed stream and
air from leakage. Normally the vacuum system is installed after a
condenser to reduce the organic loading to the vacuum system. The
vacuum system is sized based upon the estimated vapour loading,
with ejectors handling larger vapour loadings. In certain systems a
vacuum machine may be directly connected to a condenser outlet. A
typical ejector system operation is a combination of ejectors and
direct barometric condensers where the ejector vapours have direct
contact with the cooling water. Barometric condensers are very
large consumers of water and the steam-water mixture results in
high water outlet temperatures that tend to vaporize any organic
compound traces in the atmospheric barometric sump, potentially
increasing workplace exposures. In addition, a large effluent load
is added to the waste-water system.
A large water reduction is achieved along with a substantial
reduction in steam consumption in modified vacuum systems. Since
the vacuum pump will not handle a large vapour load, a steam
ejector is used in the first stage in combination with a surface
condenser to reduce the vacuum pump load. In addition, a sump drum
is installed for above-ground operation. The simpler system reduces
waste-water loading and maintains a closed system that eliminates
potential vapour exposures.
Safety
All towers and drums must be protected from overpressure that
may result from malfunction, fire (Mowrer 1995) or utility failure.
A hazard review is necessary and is required by law in some
countries. A general process safety management approach that is
applicable to process and plant operation improves safety,
minimizes losses and protects worker health (Auger 1995; Murphy
1994; Sutton 1995). Protection is provided by pressure relief
valves (PRVs) that discharge to the atmosphere or to a closed
system. The PRV is generally mounted at the tower top to relieve
the large vapour load, although some installations locate the PRV
in other tower locations. The PRV can also be located on the
distillate overhead recovery drum as long as valves are not placed
between the PRV and the tower top. If block valves are installed in
the process lines to the condenser then the PRV must be installed
on the tower.
When distillation tower overpressure is relieved, under certain
emergency scenarios, the PRV discharge may be exceedingly large.
Very high loading in a closed system discharge vent line may be the
largest load in the system. Since a PRV discharge can be sudden and
the overall relieving time may be quite short (less than 15
minutes), this extremely large vapour load must be carefully
analysed (Bewanger and Krecter 1995; Boicourt 1995). Since this
short, large peak load is difficult to process in control devices
such as absorbers, adsorbers, furnaces and so on, the preferable
control device in most situations is a flare for vapour
destruction. Normally, a number of PRVs are connected to a flare
line header that in turn is connected to a single flare. However,
the flare and overall system must be carefully designed to cover a
large group of potential contingencies (Boicourt 1995).
Health hazards
For direct relief to the atmosphere, a detailed dispersion
analysis of the relief valve discharge vapours should be conducted
to ensure that workers are not exposed and that community
concentrations are well within allowable concentration guidelines.
In controlling dispersion, atmospheric relief valve discharge lines
may have to be raised to prevent excessive concentrations on nearby
structures. A very tall flare-like stack may be necessary to
control dispersion.
Another area of concern is entering a tower for maintenance or
mechanical changes during a shutdown. This entails entering a
confined space and exposes workers to the associated hazards. The
flushing and purging method prior to opening must be carefully
conducted to ensure minimal exposures by reducing any toxic
concentrations below recommended levels. Before commencing with
flushing and purging operations, the tower pressure must be reduced
and all piping connections to the tower must be blinded (i.e., flat
metal disks must be placed between the tower flanges and the
connecting pipe flanges). This step should be carefully managed to
ensure minimum exposures. In different processes, the methods of
clearing the tower of toxic fluids vary. Frequently, the tower
fluid is displaced with a fluid that has very low toxicity
characteristics. This displacement fluid is then drained and pumped
to a selected location. The remaining liquid film and droplets can
be steamed to the atmosphere through a top flange that has a
special stand-off blind with an opening between the blind and tower
flange. Following steaming, air enters the tower through the
special blind opening as the tower cools. A manhole at the tower
bottom and one at the tower top are opened permitting the blowing
of air through the tower. When the internal tower concentration
reaches a predetermined level, the tower can be entered.
Heat exchangers
There are a wide variety of heat exchangers in the chemical
process industry. Heat exchangers are mechanical devices for the
transfer of heat to or from a process stream. They are selected in
accordance with process conditions and exchanger designs. A few of
the common exchanger types are shown in figure 2. Selection of the
optimum exchanger for a process service is somewhat complicated and
requires a detailed investigation (Woods 1995). In many situations,
certain types are not suitable because of pressure, temperature,
solids concentration, viscosity, flow quantity and other factors.
Moreover, an individual heat exchanger design can vary
considerably; several types of floating head tube and sheet
exchangers are available (Green, Maloney and Perry 1984). The
floating head is normally selected where the temperatures may cause
excessive tube expansion that otherwise could not maintain
integrity in a fixed tube sheet exchanger. In the simplified
floating head exchanger in figure 2, the floating head is contained
completely within the exchanger and does not have any connection
with the shell cover. In other floating head designs, there may be
packing around the floating tubesheet (Green, Maloney and Perry
1984).
Figure 2. Typical heat exchangers
Leakage
The packing on floating tubesheets is in contact with the
atmosphere and may be a source of leakage and potential exposure.
Other exchangers may also have potential leakage sources and should
be examined carefully. As a result of their heat transfer
characteristics, plate and frame exchangers are often installed in
the chemical industry. The plates have various corrugations and
configurations. Plates are separated by gaskets that prevent mixing
of the streams and provide an external seal. However, the seals
limit temperature applications to about 180 C, although seal
improvements may overcome this limitation. Since there are a number
of plates, the plates must be compressed properly to ensure proper
sealing between them. Consequently, careful mechanical installation
is necessary to prevent leakage and potential hazards. Since there
are a large number of seals, careful seal monitoring is important
to minimize potential exposures.
Air cooled exchangers are attractive economically and have been
installed in a wide number of process applications and in various
locations within process units. To save space, these exchangers are
often installed over pipe runs and are frequently stacked. Since
tube material selection is important, a variety of materials is
used in the chemical industry. These tubes are connected to the
tube sheet. This requires use of compatible materials. Leakage
through a tube crack or at the tube sheet is a concern since the
fan will circulate vapours from the leak and dispersion may result
in potential exposures. Air dilution may significantly reduce the
potential exposure hazard. However, fans are frequently shut down
under some weather conditions and in these circumstances leak
concentrations can increase thereby increasing potential exposures.
Moreover, if leaking tubes are not repaired, the crack may worsen.
With toxic liquids that do not readily vaporize, dripping can occur
and result in potential dermal exposure.
Shell and tube heat exchangers may develop leaks through any of
the various flanges (Green, Maloney and Perry 1984). Since shell
and tube heat exchangers vary in size from small to very large
surface areas, the diameter of outer flanges is generally much
larger than typical pipe flanges. With these large flanges, the
gaskets must not only withstand process conditions, but provide a
seal under bolt load variations. Various gasket designs are used.
Maintaining constant bolt load stresses on all of the flange bolts
is difficult, resulting in leakage in many exchangers. The flange
leakage can be controlled with flange sealing rings (Lipton and
Lynch 1994).
Tube leakage may occur in any of the available exchanger types,
with the exception of plate exchangers and a few other specialty
exchangers. However, these latter exchangers have other potential
problems. Where tubes leak into a cooling water system, the cooling
water discharges the contaminant into a cooling tower which can be
an exposure source to both workers and a nearby community.
Consequently, the cooling water should be monitored.
The dispersion of cooling tower vapours can be widespread as a
result of the fans in forced and induced draft cooling towers. In
addition, natural convection towers discharge vapours to the
atmosphere which then disperse. However, dispersion varies
considerably based upon both weather conditions and the discharge
elevation. Less volatile toxic materials remain in the cooling
water and the cooling tower blowdown stream, which should have
sufficient treatment capability to destroy contaminants. The
cooling tower and tower basin must be cleaned periodically and
contaminants add to the potential hazards in the basin and in the
tower fill. Personal protection is necessary for much of this
work.
Exchanger cleaning
A problem with tubes in cooling water service is the build-up of
material in the tubes resulting from corrosion, biological
organisms and solids deposition. As described above, tubes may also
leak through cracks, or leakage may occur where tubes are rolled
into striations in the tube sheet. When any of these conditions
occur, exchanger repair is required and the process fluids must be
removed from the exchanger. This requires a completely contained
operation, which is necessary to meet environmental, safety and
health exposure objectives.
Generally, the process fluid is drained to a receiver and the
remaining material is flushed out of the exchanger with a solvent
or inert material. The latter material is also sent to a receiver
for the contaminated material by draining or pressuring with
nitrogen. Where toxic material was in the exchanger, the exchanger
should be monitored for any traces of toxic material. If testing
results are unsatisfactory, the exchanger can be steamed to
vaporize and remove all traces of material. However, the steam vent
should be connected to a closed system to prevent vapour escape
into the atmosphere. While the closed vent may not be absolutely
necessary, at times there may be more contaminant material in the
exchanger, requiring closed steam venting at all times to control
potential hazards. Following steaming, a vent to the atmosphere
admits air. This general procedure is applicable to the exchanger
side or sides containing toxic material.
Chemicals then used for cleaning the tubes or the shell side
should be circulated in a closed system. Normally, the cleaning
solution is recirculated from a tank truck system and the
contaminated solution in the system is drained to a truck for
disposition.
Pumps
One of the most important process functions is the movement of
liquids and in the chemical industry all types of liquid materials
are moved with a wide variety of pumps. Canned and magnetic pumps
are sealless centrifugal pumps. Magnetic pump drivers are available
for installation on other pump types to prevent leakage. Types of
pumps used in the chemical process industry are listed in table
7.
Table 7. Pumps in the chemicals process industry
Centrifugal
Reciprocating (plunger)
Canned
Magnetic
Turbine
Gear
Diaphragm
Axial flow
Screw
Moving cavity
Lobe
Vane
Sealing
From a health and safety standpoint, sealing and repairing
centrifugal pumps are major concerns. Mechanical seals, which
constitute the prevalent shaft sealing system, can leak and at
times have blown out. However, there have been major advances in
seal technology since the 1970s that have resulted in significant
leakage reductions and extended pump service life. Some of these
improvements are bellows seals, cartridge seals, improved face
designs, better face materials and improvements in pump variable
monitoring. Moreover, continuing research in seal technology should
result in further technology improvements.
Where process fluids are highly toxic, leakless or sealless
canned or magnetic pumps are frequently installed. Operating
service periods or the mean time between maintenance (MTBM) has
improved markedly and generally varies between three and five
years. In these pumps, the process fluid is the lubricating fluid
for the rotor bearings. Vaporization of the internal fluid
adversely affects the bearings and often makes bearing replacement
necessary. Liquid conditions in the pumps can be maintained by
ensuring the internal pressure in the bearing system is always
greater than the liquid vapour pressure at the operating
temperature. When repairing a sealless pump, completely draining a
relatively low volatility material is important and should be
carefully reviewed with the supplier.
In typical centrifugal process pumps, packing has essentially
been replaced with mechanical seals. These seals are generally
classified as single or dual mechanical seals, with the latter term
covering tandem or double mechanical seals. There are other dual
seal combinations, but they are not as widely used. In general,
tandem or double mechanical seals with liquid buffer fluids between
the seals are installed to reduce seal leakage. Pump mechanical
seal standards for both centrifugal and rotary pumps covering
single and dual mechanical seal specification and installation were
issued by the American Petroleum Institute (API 1994). A mechanical
seal application guide is now available to aid in the evaluation of
seal types (STLE 1994).
To prevent excessive leakage or blow-out from a failed seal, a
gland plate is installed following the seal. It may have a gland
flush fluid to move the leakage into a closed drain system (API
1994). Since the gland system is not a complete seal, auxiliary
seal systems, such as throttle bushings are available.They are
installed in the gland that controls excessive leakage to the
atmosphere or seal blow-out (Lipton and Lynch 1994). These seals
are not designed for continuous operation; after activation they
will operate for up to two weeks before failure, thereby providing
time for operations to switch pumps or make process
adjustments.
A newer mechanical seal system is available that essentially
reduces emissions to the nil level. This is a double mechanical
seal system with a gas buffer system that replaces the liquid
buffer in the standard dual mechanical seal system (Fone 1995;
Netzel 1996; Adams, Dingman and Parker 1995). In the liquid buffer
systems, the seal faces are separated by an extremely thin
lubricating film of buffer fluid that also cools the seal faces.
Although separated slightly, a certain amount of face contact
exists which results in seal wear and seal face heating. The gas
seals are called non-contact seals since one seal face with curved
indentations pumps gas through the seal faces and builds a gas
layer or dam that completely separates the seal faces. This lack of
contact results in a very long seal life and also reduces the seal
friction loss, thereby noticeably decreasing power consumption.
Since the seal pumps gas there is a very small flow into the
process and to the atmosphere.
Health hazards
A major concern with pumps is draining and flushing to prepare
the pump for maintenance or repair. Draining and removal covers
both process fluid and buffer fluids. Procedures should require
discharge of all fluids into a closed connection drain system. In
the pump stuffing box where a throat bushing separates the impeller
from the stuffing box, the bushing acts as a weir in holding some
liquid in the stuffing box. Weep holes in the bushing or a drain in
the stuffing box will permit complete process liquid removal
through draining and flushing. For buffer fluids, there should be a
method of draining all fluid from the dual seal area. Maintenance
requires seal removal and if the seal volume is not completely
drained and flushed, the seals are a potential source of exposure
during repair.
Dust and powders
Handling of dusts and powders in solids processing equipment is
a concern due to the potential for fire or explosion. An explosion
within equipment may burst through a wall or enclosure as a result
of explosion-generated pressure sending a combined pressure and
fire wave into the workplace area. Workers can be at risk, and
adjacent equipment can be severely impacted with drastic effects.
Dusts or powders suspended in air or in a gas with oxygen present
and in a confined space are susceptible to explosion when a source
of ignition with sufficient energy is present. Some typical
explosive equipment environments are shown in table 8.
Table 8. Potential explosion sources in equipment
Conveying equipment
Storage
Pneumatic ducts
Bins
Mechanical conveyors
Hoppers
Rotary valves
Processing equipment
Filter dust collectors
Grinders
Fluid bed dryers
Ball mills
Transfer line dryers
Powder mixing
Screening
Cyclones
An explosion produces heat and rapid gas expansion (pressure
increase) and generally results in deflagration, which is a flame
front that moves rapidly but at less than the sound velocity for
these conditions. When the flame front velocity is greater than the
sound velocity or is at supersonic velocity the condition is termed
detonation, which is more destructive than deflagration. Explosion
and flame front expansion occur in milliseconds and do not provide
sufficient time for standard process responses. Consequently, the
potential fire and explosion characteristics of the powder must be
defined to determine the potential hazards that may exist in the
various processing steps (CCPS 1993; Ebadat 1994; Bartknecht 1989;
Cesana and Siwek 1995). This information can then provide a basis
for the installation of controls and the prevention of
explosions.
Explosion hazard quantification
Since the explosions generally occur in enclosed equipment,
various tests are conducted in specially-designed laboratory
equipment. While powders may appear similar, published results
should not be used since small differences in the powders can have
very different explosion characteristics.
A variety of tests conducted on powder can define the explosion
hazard and the test series should encompass the following.
The classification test determines whether a powder dust cloud
can initiate and propagate flames (Ebadat 1994). Powders that have
these characteristics are considered Class A powders. Those powders
that do not ignite are termed Class B. The Class A powders then
require a further series of tests to evaluate their explosion and
hazard potential.
The minimum ignition energy test defines the minimum spark
energy necessary for ignition of a powder cloud (Bartknecht
1989).
In explosion severity and analysis Group A powders are then
tested as a dust cloud in a sphere where the pressure is measured
during a test explosion based on minimum ignition energy. The
maximum explosion pressure is defined along with the rate of change
in pressure per unit time. From this information, the explosion
specific characteristic value (Kst) in bar metres per second is
determined and the explosion class is defined (Bartknecht 1989;
Garzia and Senecal 1996):
Kst(barm/s) Dust explosion class Relative strength
1-200 St 1 Somewhat weaker
201-300 St 2 Strong
300+ St 3 Very strong
A large number of powders have been tested and the majority were
in the St 1 class (Bartknecht 1989; Garzia and Senecal 1996).
In assessment of non-cloud powders, powders are tested to
determine safe operating procedures and conditions.
Explosion prevention tests
Explosion prevention tests can be helpful where explosion
suppression systems cannot be installed. They provide some
information on desirable operating conditions (Ebadat 1994).
The minimum oxygen test defines the oxygen level below which the
dust will not ignite (Fone 1995). Inert gas in the process will
prevent ignition if the gas is acceptable.
The minimum dust concentration is determined in order to
establish the operating level below which ignition will not
occur.
Electrostatic hazard tests
Many explosions are a result of electrostatic ignitions and
various tests indicate the potential hazards. Some of the tests
cover the minimum ignition energy, powder electric charge
characteristics and volume resistivity. From the test results,
certain steps can be taken to prevent explosions. Steps include
increasing humidity, modifying construction materials, proper
grounding, controlling certain aspects of equipment design and
preventing sparks (Bartknecht 1989; Cesana and Siwek 1995).
Explosion control
There are basically two methods of controlling explosions or
fronts from propagating from one location and another or containing
an explosion within a piece of equipment. These two methods are
chemical suppressants and isolation valves (Bartknecht 1989; Cesana
and Siwek 1995; Garzia and Senecal 1996). Based upon the explosion
pressure data from the explosion severity tests, rapid response
sensors are available that will trigger a chemical suppressant and/
or rapidly close isolation barrier valves. Suppressants are
commercially available, but suppressant injector design is very
important.
Explosion vents
In equipment where a potential explosion may occur, explosion
vents that rupture at specific pressures are frequently installed.
These must be carefully designed and the exhaust path from the
equipment must be defined to prevent a worker presence in this path
area. Moreover, impingement on equipment in the explosion path
should be analysed to ensure equipment safety. A barrier may be
required.
Loading and Unloading
Products, intermediates and by-products are loaded into tank
trucks and railcars. (In some cases, depending on location of
facilities and dockage requirements, tankers and barges are used.)
Location of the loading and unloading facilities are important.
While the materials loaded and unloaded usually are liquids and
gases, solids are also loaded and unloaded at preferred locations
based upon the type of solids moved, potential explosion hazard and
the degree of transfer difficulty.
Open hatches
In loading tank trucks or railcars through top opening hatches,
a very important consideration is minimizing splashing as the
container is filled. If the fill pipe is located well above the
bottom of the container, filling results in splashing and
generation of vapour or mixed liquid-vapour evolvement. Splashing
and vapour generation can be minimized by locating the fill pipe
outlet well below the liquid level. The fill pipe is normally
extended through the container a minimum distance above the
container bottom. Since liquid filling also displaces vapour, toxic
vapours can be a potential health hazard and also present safety
concerns. Consequently, the vapours should be collected. Fill arms
are commercially available that have deep fill pipes and extend
through a special cover that closes the hatch opening (Lipton and
Lynch 1994). In addition, a vapour collection pipe extends a short
distance below the special hatch cover. At the upstream end of the
arm, the vapour outlet is connected to a recovery device (e.g., an
absorber or condenser), or the vapour can be returned to the
storage tank as a vapour balance transfer (Lipton and Lynch
1994).
In the tank truck open hatch system, the arm is raised to permit
draining into the tank truck and some of the liquid in the arm can
be pressured with nitrogen as the arm is withdrawn, but the fill
pipes during this operation should remain within the hatch opening.
As the fill arm clears the hatch, a bucket should be placed over
the outlet to catch arm drippings.
Railcars
Many railcars have closed hatches with deep fill legs very close
to the bottom of the container and a separate vapour collection
outlet. Through an arm that extends to the closed hatch, liquid is
loaded and vapour collected in a fashion similar to the open hatch
arm method. In railcar loading systems, following valve shut off at
the arm inlet, nitrogen is injected into the container side of the
arms to blow the liquid remaining in the arm into the railcar
before the fill valve on the railcar is closed (Lipton and Lynch
1994).
Tank trucks
Many tank trucks are filled through the bottom to minimize
vapour generation (Lipton and Lynch 1994). The fill lines can be
special hoses or manoeuvrable arms. Dry break couplers are placed
on the hose or arm ends and on the tank truck bottom connections.
When the tank truck is filled and the line is automatically
blocked, the arm or hose is disconnected at the drybreak coupling,
which automatically closes as the couplings are separated. Newer
couplings have been designed to disconnect with almost zero
leakage.
In bottom loading, vapour is collected through a top vapour vent
and the vapour is conducted through an external line that
terminates near the bottom of the container (Lipton and Lynch
1994). This permits worker access to the vapour coupling
connections. The collected vapour, which is at a pressure slightly
above atmospheric, must be collected and sent to a recovery device
(Lipton and Lynch 1994). These devices are selected based upon
initial cost, effectiveness, maintenance and operability.
Generally, the recovery system is preferable to a flare, which
destroys the recovered vapours.
Loading control
In tank trucks, level sensors are permanently installed within
the truck body to indicate when the fill level has been reached and
signal a remote control block valve that stops flow to the truck.
(Lipton and Lynch 1994). There may be more than one sensor in the
tank truck as backup to ensure that the truck is not overfilled.
Overfilling can result in serious safety and health exposure
problems.
Railcars in dedicated chemical service may have level sensors
mounted internally in the car. For non-dedicated cars, a flow
totalizer controls the amount of liquid sent to the railcar and
automatically shuts the remote control block valve at a
predetermined setting (Lipton and Lynch 1994). Both container types
should be investigated to determine whether liquid remains in the
container prior to filling. Many railcars have manual level
indicators that can be used for this service. However, where level
is shown by opening a small level stick vent to the atmosphere,
this procedure should only be performed under properly controlled
and approved conditions due to the toxicity of some of the loaded
chemicals.
Unloading
Where chemicals have a very high vapour pressure and the railcar
or tank truck has a relatively high pressure, the chemical is
unloaded under its own vapour pressure. Should the vapour pressure
fall to a level that will interfere with the unloading procedure,
nitrogen gas can be injected to maintain a satisfactory pressure.
Vapour from a tank of the same chemical can also be compressed and
injected to raise the pressure.
For toxic chemicals that have a relatively low vapour pressure,
such as benzene, the liquid is unloaded under nitrogen pressure,
which eliminates pumping and simplifies the system (Lipton and
Lynch 1994). Tank trucks and railcars for this service have design
pressures capable of handling the pressures and variations
encountered. However, lower pressures after unloading a container
are maintained until the tank truck or railcar is refilled; the
pressure rebuilds during loading. Nitrogen can be added if
sufficient pressure has not been attained during loading.
One of the problems in loading and unloading operations is
draining and purging lines and equipment in the loading/unloading
facilities. Closed drains and particularly low point drains are
necessary with nitrogen purges to remove all traces of the toxic
chemicals. These materials can be collected in a drum and returned
to a receiving or recovery facility (Lipton and Lynch 1994).