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OCR for page 15
Approaches to Obtaining Mass Balance Information
INTRODUCTION
Two general approaches are used to obtain
mass balance information: the traditional
EMB and the MA. Both approaches are
described and compared in this chapter as a
basis for discussing the potential of mass
balance information for the applications
described in SARA Section 313~1~. A quali-
tative comparison is made from the perspec-
tive of a reporting facility between the two
approaches concerning amounts of data,
technical skills of personnel to obtain and
evaluate the data, and the resulting costs.
(Only anecdotal quantitative cost data were
available to the committee for its analysis.)
The components of the SARA mass bal-
ance definition fit closely with the tradi-
tional EMB approach. However, the goals of
a mass balance analysis also could be some-
times met through information collection
using MA.
ENGINEERING MASS BALANCE
The EMB concept is more than a century
old. It has been the subject of numerous
books (e.g., Felder and Rousseau, 1978) and
is an established tool in engineering practice
(Austin, 1984; Perry et al., 1984~. The basic
15
goal of EMB is closure when all chemical
inputs to a manufacturing facility, outputs
from that facility, and accumulations within
have been identified and the masses have
been measured. The mass of inputs should
equalizer closely approximate the mass of
outputs plus accumulations (i.e., mass
balance requires "closure"~. Lack of closure
indicates that errors have been made in
quantifying one or more of the mass balance
components or that additional information is
needed. However, the errors inherent in any
sampling and analysis procedure make
attainment of complete closure an unrealistic
expectation. All routes or streams for each
process step (e.g., distillation) within a facil-
ity must be identified and evaluated in-
dependently to account for all chemicals
entering or leaving a facility. This is a rig-
orous evaluation procedure requiring ac-
curate and precise measurements, and expert
knowledge is needed to use it.
Standard Theory and Practice
of EMB
Although EMB does not necessarily track
specific chemicals, the committee evaluated
chemical-specific applications of EMB,
because SARA Section 313 requires report
OCR for page 16
16
ing releases of specific chemicals. These
chemicals enter and exit processes (e.g.,
blending) within a facility through material
streams conducted by pipes, air ducts, bar-
rels, and so on. The chemicals within
streams can become partitioned into gas,
liquid, or solid phases (and phase mixtures),
depending on the thermodynamic conditions
within the streams. In addition, in many
processes, reactions occur that convert input
chemicals to entirely different output chemi-
cals. In such cases, knowledge of the chemi-
cal transformations is needed to relate the
reacted inputs to the resulting outputs. A
further discussion of the implications of
such reactive systems for performing a mass
balance evaluation is provided in Chapter 5.
EMB uses measurement data and judg-
ment (McMichael, 1988~. The measurements
require knowledge of conservation of mass
and of the standard units for expressing mass
data as well as flow rates, determinations of
chemical mass, accumulation, and the com-
positions and concentrations of specific
chemicals within streams flowing through a
facility. Streams can be multiphase and
chemically unstable, and they can contain
components that interfere with analytical
procedures (Rohlik, 1986~.
Engineering judgment must be exer-
cised in selecting the methods for sampling
and analysis and in choosing the frequency
and duration of data acquisitions on the mass
flows within and across facility boundaries.
Sampling procedures can range from in situ
continuous stream sampling and analysis to
instantaneous grab samples at periodic inter-
vals. Judgment is needed to select the ap-
propriate analytical procedure and to collect
samples in a manner that adequately ac-
counts for daily and longer-period variabil-
ity in the concentration of a specific chemi-
cal discharged from a facility. Temporal
variation in waste-water concentration, for
example, can be attributed to normal process
variations in raw material content and reac-
tion efficiency (Tischler, 1988~.
An annual average EMB requires simul-
taneous evaluation of the variations in
chemical concentration and flow rate for the
entire year. For a simple EMB on one
chemical, such as purchased chlorofluoro-
carbons used as blowing agents to manu-
facture open-cell polymeric foams, variabil-
ity of mass flow through the facility is
manageable. In this case, essentially the
M45S BALANCE INFORMA TION
entire amount of chlorofluorocarbon is re-
leased into the atmosphere within several
hours after the manufacturing process.
Therefore, the mass of chemical input to this
facility would be equivalent to the amount
released into the atmosphere (assuming in-
significant amounts of the chemical remain
in the foam product).
EMB generally is difficult to conduct,
however, because the number of necessary
judgments tends to increase with the com-
plexity of the facility, and because many
manufacturing facilities have multiple unit-
process components with multiple-process
streams. EMB requires an evaluation of each
stream, and the total flow rate and the con-
centration of specific chemicals can vary
significantly with the inevitable fluctuations
in facility operation. The cost of analytical
work required to obtain this information
usually is very high, and the paperwork bur-
den in collecting the information would be
formidable.
A simplified example of an EMB applied
to all material entering and exiting an adipic
acid manufacturing facility is shown in Fig-
ure 2.1 (Nickolaus, 1988~. This EMB ex-
ample is not chemical-specific and probably
involves less-complex analytical methods
than would be required if the analysis were
performed for a specific chemical. This
schematic also does not depict the composi-
tions of streams flowing into and out of pro-
cess components within the facility. In
Figure 2.1, no data were available for the
amount of reaction water generated from
this process; it was therefore assumed that all
of the water theoretically predicted was gen-
erated as a byproduct from the reactor.
Measurement Error in
EMB Applications
Several factors limit the certainty of data
acquired through EMB, including facility
size and complexity, instrument accuracy
and application, system variability, errors
inherent in sampling and analytical proce-
cures, recorOkeeping errors, and waste ac-
countability (McMichael, 1988~. The mag-
nitude of errors in this information can vary
significantly as a function of the types of
industry, facility, and technology. For ex-
ample, analysis to determine the annual
quantity of TRI-listed contaminants within
OCR for page 17
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18
the copper ore fed to a smelter facility is
more prone to error than is determining the
annual quantity of chlorofluorocarbon
shipped to a foam-blowing facility, because
the copper ore composition is much more
heterogeneous and variable than that of the
chiorofluorocarbon feed.
The errors in the data acquired through
EMB make it virtually impossible to achieve
closure of EMB for a manufacturing facility
other than one composed of a single small
production unit. Even if 99% closure were
reached, a large amount of chemical mass
would remain unaccounted for, especially at
facilities where millions of pounds of chemi-
cals per day are produced or used.
The mass of a chemical flowing through
a facility often is not measured directly but
instead is calculated from independent
measurements, such as total flow rate and
chemical concentration within each stream.
Mass data thus are obtained by multiplying
the total flow rate (e.g., pounds of stream
per day) by the concentration of each chem-
ica1 within the stream (e.g., pounds of ben-
zene per million pounds of stream). Errors
can be introduced into the mass balance
results from flow rate measurement error
and as a result of the precision and accuracy
limits of the analytical procedures employed
to measure chemical concentrations (Klusek
et al., 1983~. Sampling methods and analyti-
cal procedures have greatly improved in the
past decade for many toxic chemicals of
concern and could help reduce measurement
error. However, these advances can be very
costly. For example, the use of gas chrom-
atography for EMB measurements has been
estimated to require an annual cost of
$15,000 per stream at a multistream adipic
acid manufacturing facility (Nickolaus,
1988).
For petroleum refineries and some other
industries, material flow rates are measured
by volume instead of by mass. This adds
uncertainty to EMB data because the mea-
surement units must be converted. For ex-
ample, crude oil is purchased by volume; to
convert the volume to mass requires that the
crude oil gravity and temperature at delivery
be determined accurately. Furthermore, the
amount of sediment and water in suspension
must be known. The combined error in
measuring volume and converting it to mass
can be 0.3-0.4%. The magnitude of this
error approaches that of the overall quantity
MASS BALANCE INFORMATION
of releases from a refinery, estimated to be
0.5 weight-percent of the crude oil input (T.
Yosie, American Petroleum Institute, per-
sonal communication, August 29, 1988~.
The following example illustrates the
significance of how an apparently slight de-
gree of uncertainty affects EMB. An
ethylene production facility that incorporates
more than 200 state-of-the-art monitoring
points in its system reports measurement
accuracies that range from +0.25% to +1.0%,
and closure to within 1% for large volumes
of ethylene. When these overall
uncertainties are based on daily balances and
applied to the facility's current feed rates,
they represent an imbalance of +50,000 lb of
ethylene per day. This figure stands in
striking contrast to the emissions from this
facility, which are estimated to be 191 lb of
ethylene per day. A monitoring program
was used to confirm that emissions are not
underestimated by the calculation procedure
(Chlapek, 1988~.
A high degree of closure could also be
the result of two or more errors negating one
another on EMB results. For example, errors
in measuring the total amount of a volatile
chemical in a stack emission coupled with an
error in determining the total amount of that
same chemical in the feedstock could result
in an apparently accurate balance (Kaakinen
and Jorden, 1973~.
EMB is limited by analytical measure-
ment error and lack of precise knowledge of
chemical process science. For a facility in
which a specific chemical undergoes no
chemical change and is transferred to only
one stream, EMB results are reasonably ac-
curate. However, the processes in many
facilities required to report under SARA
Section 313 are not that simple. Many listed
chemicals flowing through the reporting
facilities occur in multiple phases and are
routed through multiple process streams.
MATERIALS ACCOUNTING PRACTICES
OF POTENTIAL UTILITY
MA is a means of obtaining mass bal-
ance information that uses readily available
information on material flow across facility
boundaries. Unlike EMB, MA does not
require closure and therefore does not re-
quire detailed measurements of all process
streams within facility boundaries. How
OCR for page 19
APPROACHES TO OBTAINING MASS BAL4NCE INFORMATION
ever, MA can include measured data; it can
include any or all of the SARA mass balance
data that are obtained for EMB. Because
MA and EMB can obtain similar types of
information, the prime distinction between
the two approaches is the method by which
the information is obtained.
MA draws on the basic accounting and
production information of a facility. Such
information may include purchase and pro-
duction records; sales and shipping records;
inventories of raw materials, intermediates
(materials in process), and products; permit
records of waste streams, and manifest and
waste facility records.
In comparison with EMB, MA is less ac-
curate and less precise; however, this does
not adversely affect MA's potential utility
MA provides an approximation of the quan-
tities of chemicals flowing through a facility
for any or all of the same mass balance com-
ponents that are required for an EMB.
Although the MA approach generally uses
information about the quantities crossing
facility boundaries, its potential application
to assess waste-reduction efficiency (Chapter
5) would require MA data collected from
production units within ~ facility. A facility
that produces several products might contain
several production units each dedicated to a
single product, or it might contain one pro-
duction unit used to produce multiple prod-
ucts. Therefore, MA for multiproduction
unit facilities uses data on chemical quan-
tities crossing the production unit boundaries
within a facility.
An illustration of points in a manu-
facturing facility that can be represented by
MA data is shown in Figure 2.2, which was
adapted from a case study used in an indus-
trial waste audit and reduction manual
(Ontario Waste Management Corp., 1987~.
This steel-pickling operation is used within a
facility that manufactures nuts and bolts for
the automobile industry. Before the steel
can be drawn into wire for manufacturing,
oxidized iron scale first must be removed
from the steel with sulfuric acid. (Sulfuric
acid is a TRI-listed chemical, and no quan-
tities of the acid released in waste water
should be reported if the acid is neutralized
to pH 6-9, or if the acid is not transformed
into another reportable substance.) Because
the sulfuric acid is transformed into sulfate
compounds as it flows through the pickling
operation, MA can be applied more effec
19
tively by an approximate accounting of the
sulfate flowing through the pickling opera-
tion based on purchase and waste disposal
and treatment records.
The facility's purchase records provide
the annual usage of sulfuric acid from
which the sulfate input can be calculated.
Waste disposal records indicate the amounts
of spent pickle liquor and sludge (ferrous
sulfate) found in the bottom of the pickling
tanks. An analysis was performed to charac-
terize the iron and sulfate concentrations in
both wastes to determine waste-reduction or
treatment options. The disposal records and
the analytical results allow annual sulfate
outputs to be estimated via spent liquor and
sludge. Sulfate is also contained in the
scrubber waste water and rinse water that
are neutralized before discharge to the
sewer. Sulfate releases to the atmosphere
from the scrubber can be estimated from
measurement data used to design the scrub-
ber and the calculated removal efficiency of
the scrubber. The amount of lime purchased
and the portion judged to be used for
neutralization during the year can be used to
calculate the annual amount of sulfate output
to the sewer using stoichiometric relation-
ships of the sulfate-lime neutralization
reaction.
SUMMARY
Table 2.1 summarizes the differences
between EMB and MA approaches for
several key parameters. The goal of closure
for EMB requires extensive data and person-
nel with the technical skills to obtain and
evaluate the data; therefore, EMB is expen-
sive. The great diversity in the facility
operations and in the characteristics of the
chemicals handled, however, makes cost
generalizations extremely difficult.
MA is less costly and complex because
fewer data and less technical expertise are
needed. MA is less accurate and precise
than EMB, but material that cannot be ac-
counted for does not necessarily reflect
releases to the environment. Figure 2.3
presents an example of the difference in
data requirements between EMB and MA for
arsenic flowing through a primary copper
smelting facility and fed into the smelter
as a contaminant of the copper ore.
EPA presents the mass balance approach
OCR for page 20
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TABLE 2.1 A Companson of Engineenng Characteristics of EMB and MA Approaches
Approach
Characteristic EMB MA
M4SS BALAbJCE INFORMA TION
Closure as objective Mandatory Not required
Accurapy of data High Variable
Level of detail of data Each process step Vanable (facility level acceptable)
Data requirements Extensive new data Readily available
Skill requirements Highly technical Moderately technical
Additional cost High Low to moderate
(including the use of MA data) as a poten-
tial method for estimating environmental
releases by taking the difference between
input and product quantities (EPA, 1987~.
However, EPA cautions that significant re-
lease estimate errors can result from this
procedure due to relatively small errors in
the quantities used to estimate the releases.
A properly designed and executed EMB or
MA discourages and avoids the practice of
assigning uncertain or erroneous data as be
ing equivalences of environmental releases.
-
Representative terms from entire chapter:
manufacturing facility
