I
CAP
Processing Industrial Byproducts to Yield
Amnonium Sulfate Fertlliter
Michael R. Overcash, Professor
Ken Wood, Research Assistant
Department of Chemical Engineering
North Carolina State University
Raleigh, North Carolina 27695-7905
EXECUTIVE SUMMARY
The waste minimization program in Chemical Engineering at North
Carolina State University has developed a screening and selection process
to identify industrial situations with favorable probabilities for
prevention of pollution emissions.
The potential utilization of spent
sulfuric acid and b y p r Q d q amnonia to yield amnonlum sulfate fertilizer
from gkhemical manufacturjr was identified by this screening process as a
candidate for a' prel-hnary engineering assessment.
The magnitude o f
ammonium sulfate production would be approximately 2,000 tons per year with
a annual value between $2,000 and $34,000. In addition, the geographic
location o f this industrial plant in western North Carolina matches the
regional need for sulfate-based fertilizer.
In this report the objectives of the overall industry screening
process and of the typical preliminary engineering assessment are given.
The pollution prevention situation illustrated by this case study is not
one of direct reduction of environmental emission since the two byproduct
streams are currently treated or sold completely.
Rather the issue
involved herein is one of potential increased value by the combination of
two byproduct streams.
Process calculations and overall stoichiometric determinations were
made on the reaction of ammonia gas and sulfuric acid solutions.
Considerations of the pH range to be used and the need to cool the reaction
in a full-scale operation were evaluated. Some suggestions of reactor
The
design and innovative means to achieve a low cost design are given.
direction o f future work is described for both engineering pilot studies as
well as the agricultural evaluations which would be appropriate.
The industrial concern has expended approximately f 10,000 on this
concept as well as $ 3,000 from this project. However factors outside the
waste minimization considerations have dictated that the ammonium sulfate
will not be produced. As is often the case, market conditions change and
in this case the need for the product from which ammonia gas was a
byproduct has been reduced such that further production was eliminated.
The industry has switched to other products. Thus the opportunity for this
waste minimization i s eliminated. As a feedback to the overall process by
I
whlch resources are applled to stlmulate waste ninlmizatlon, thls case
study descrlbes that which may not be uncommon. Not all concepts developed
as means of pollution preventlon can be implemented since factors outside
the technical aspects for such waste reduction are also very Important.
Thus I t I s important to undertake as much screening and continual
re-evaluation to assess both the technical factors and the broader
industrial setting in which all process modifications or recycle/reuse must
be placed.
I
Processing Industrial Byproducts to Yield
Ammonium Sulfate Fertilizer
INTRODUCTION
The waste minimization program
Carolina
State
University
in Chemical Engineering at North
is aimed
at the research,
development,
engineering, and implementation for waste minimization in industry.
program
The
is thus focused on the technical and economic stages necessary to
actually implement the lowering of waste emissions from industry.
In order
to make significant contributions to the waste minimization field, a
selection process has been developed to screen concepts or interests o f
industry through several
feasibility
levels in an attempt to clarify the actual
of pollution prevention
(Overcash 1986).
This iterative
screening process leads to a subset of industrial circumstances in which
a) the emission magnitude or value is within a reasonable range
for probable recovery or elimination
b) the industry comnitment to considering a waste minimization
scheme is evident
c) there exist critical unavailable information which the N.C.S.U.
program could generate by laboratory and pilot-scale studies or
by detailed engineering analysis and design.
.
An intermediate stage in this overall identification and screening
process
for waste minimization situations i s a preliminary engineering assessment
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of a project.
The objectives of a preliminary engineering
among Industrial projects.
assessment often vary
The level of currently available information
and on-going activity greatly affect the results and level of detail
such an assessment.
these assessments.
for
However a number of objectives occur routinely i n
These are
a) to determine the extent of tangible industry interest in
undertaking the various stages toward the elimination or
reduction of a particular waste stream
b) to begin quantification o f the extent by which chemicals may
be eliminated at individual manufacturing facilities and
the corresponding preliminary economics
c) to explore the nature of potential solutions such that
several alternative approaches are available to allow for
changes in manufacturing and transfer to a wide range of
similar manufacturing facilities
d) to identify the appropriate next steps toward implementing
a potential waste minimization scheme, usually the initiation
of laboratory or pilot-scale tests aimed at critical missing
information.
The preliminary engineering assessment usually involves currently available
information
analysis.
supplemented
with plant visits and appropriate scientific
These investigations are aimed at relating prior experience with
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Industry and waste mlnlmlzatlon technology to a new circumstance In which
there appears to be potential and Interest In reducing waste emisslons.
In
this process, the technology group at North Carolina State University
provides Industry the opportunity to maintain confidentiallty
time
as
until
such
the
manufacturing organization decides to allow specific
Identification.
There are a number of benefits to industry when directly
identified with the development of
produced.
However, technology
innovative means of reducing wastes
implementation group at N.C.S.U.
does
extract and disseminate the generic and developmental facets of these
pollution prevention activities in order to more broadly
and
rapidly
advance the field of waste minimization.
At chemical manufacturing facilities,
the
situations
sometimes
encountered indicate the possible uses of chemical process wastes, in which
two or more waste streams,
although not useful
combined to yield a worthwhile product.
individually, may be
Such a case was examined as part
of the waste reduction program at North Carolina State University.
manufacturing
The
facility under consideration i s engaged in the production of
specialty chemicals from a primarily batch operational basis.
There are
two primary waste streams (spent sulfuric acid and gaseous amnonia)
resulting from this plant operation which, although currently disposed
might be combined to yield
a saleable product.
of,
Thus this industrial
situation emerged as a candidate for further technical
evaluation.
The
waste minimization opportunity represented a series of generic situations
in which two byproducts are necessary to develop a favorable alternative.
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The prellmlnary englneeriq assessment of thls industrial sltuatlon as a
candidate for waste mlnlmization is the subject of the following report.
It should be noted that projects reaching this level in the screening
process have a higher probability
number of concepts and projects
of economic feasibility.
A
greater
have been excluded due to a lack of
economic feasibility, thus not every waste or industry can demonstrate that
pollution prevention pays.
In these circumstances treatment and discharge
is generally the most cost-effective approach.
WASTE EMISSION AND ENVIRONMENTAL ENDPOINT
In any waste minimization situation it is important to have
a clear
definition of the environmental receiver system or potential impact which
will be ameliorated by the reduction of an emission.
This definition
clarifies potential environmental benefits as well as the direct comp iance
savings which might accrue to a particular plant
minimization scheme.
implementing a waste
The latter benefit (expenditure savings) is the usual
primary driving mechanism for adopting manufacturing changes, although the
magnitude of savings may not have to be large.
The former benefit is
rarely quantifiable for any given industrial facility and thus only affects
the perception of the magnitude o f direct economic savings which might be
acceptable.
The indirect environmental
benefits accrue to industry or
society as a whole and are often net financial gains (Royston, 1979)
associated with improved waste treatment or source control.
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The environmental receiver affected by the combining and sale of the
two byproduct stream is the atmosphere f o r the gaseous a m o n l a stream.
However since the amnonia byproduct i s presently treated in compliance with
all regulations, the environmental benefits due to an alternate use of this
material are minimal.
presently
used
In a similar manner the spent sulfuric acid
is
commercially on a local basis and hence no change in
environment is expected.
This case study demonstrates the potential
increase in value and lowering of present treatment cost associated with
combining two byproduct waste streams, rather than any
substantive
environmental improvement.
PROCESS AND WASTE MINIMIZATION ASSESSMENT
The waste minimization group visited a
manufacturing
).
plant during this project.
medium
size
chemical
The plant operations and
synthesis unit processes were observed in detail.
The first waste stream
of interest is a sulfuric acid stream of about 25% strength.
Sulfuric acid
is used in this facility in the sulfonation and solvation o f esters, fats,
oils,
etc.,
hence this waste stream contains trace amounts of organic
impurities.
It is believed, however, that these impurities are not present
in sufficient quantity
nor are these sufficiently toxic to necessitate
removal from the acid stream, in the event that the stream is to be
utilized.
At present this facility i s generating about 30,000 gallons per
month of this waste acid, which is then used commercially.
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The other waste stream under consideratlon consists of gaseous
ammonia,
generated
as a byproduct I n several dlfferent reactions.
Currently, this amnonia i s decomposed to nonhazardous products
temperature flare.
By
in a high
heating the ammonia to around 2,200 F, it is
decomposed to diatomic nitrogen and hydrogen.
well, the end result being
The hydrogen then burns
as
that the ammonia is disposed of without the
formation of any nitrogen oxides.
Based on our assessment of the waste streams and the application of
chemical
reaction and thermodynamic principles, it was concluded that an
innovative Bhigher value product could be achieved for this manufacturing
operation.
The reaction o f ammonia with an aqueous solution of sulfuric
acid is a simple acid-base neutralization:
2NH3 + H2SO4
---- (NH4)2SO4
Sufficient contact to enable reaction could be achieved by bubbling the NH3
gas through the acid solution.
Because of the intermittent nature of both
the sulfuric acid and, particularly, the amnonia production (there being no
practical way to store gaseous amnonia) a batch production mode seems most
suitable.
with
One simple approach to the problem would involve filling a tank
sulfuric acid solution and introduc ng
sparger located near the bottom o f the tank.
the ammon a by means o f a
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Because of the rapidity of the neutralization
conversion could probably
amounts of NH3 vapor.
reaction,
near total
be achieved without the loss of significant
Since the pH of the solution would be on the acid
side throughout the course of the reaction, ammonia volatilization would be
minimized.
The progress of the reaction could be tracked by monitoring the
solution pH.
Introduction of NH3 would be stopped when the solution
reached the desired pH.
The actual value of this target pH will
involve
some compromise, depending on the mode and length of storage of the product
amnonium sulfate solution.
Maintaining the solution at low pH would,
as
stated, minimize the problem of ammonia volatilization, but would tend to
accelerate corrosion, thus necessitating
the use of more
materials.
(alkaline) pH would minimize
Conversely,
storage at
high
expensive
corrosion but would lead to more noticeable NH3 fuming.
It would also be necessary in this production scheme to provide for
the removal o f the large amount o heat generated by the neutralization
process, since high temperatures lead to accelerated corrosion.
two ways of minimizing the heat bui dup:
external heat exchange system using
transfer medium.
first, through the design of an
cooling water or some other heat
The second approach is to remove heat by bubbling air
through the solution. Although the solution would
boiling
There are
be below the normal
point, the bubbling air would evaporate enough water so that the
latent heat consumed would offset the heat o f neutralization.
bubbling method
If the air
proves feasible it will clearly be much cheaper (hence
preferable) than external heat exchange.
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This method of production could be Implemented easily by installing a
reactor with an air feed line and an amnonia feed line bled o f f the already
existing amnonia flare feed line.
At this stage, it appears technically feasible to generate amnonium
sulfate although a number of important engineering design parameters will
have to be specified and possibly some pilot tests conducted.
This would
be the next stage In development of this overall scheme to better utilize
waste byproducts.
In addition, the market for ammonium
sulfate fertilizer in the
vicinity of the plant needs to be gauged with respect to whether customers
exist with the capability of handling liquid fertilizer products,
and
if
so, the possible selling price of the ammonium sulfate needs to be
ascertained.
In the event that the project appears worthwhile after these questions
are answered, additional factors will need to be dealt with, such as how to
deal with the seasonal fluctuations in the fertilizer market, how to handle
bulk storage problems, and how to ensure the purity of the product in view
of the many different processes from which the waste material is derived.
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ECONOMICS
Most of the comnercial amnonium sulfate ferti 1 izer presently
produced
is sold as a crystallized, dry solid. This is mainly due to the fact that,
unlike some other fertllizer products (amnonium nitrate, etc.)
amnonium
sulfate is only soluable in water up to about 42% by weight.
This fact
makes it generally uneconomical to sell amnonium sulfate in liquid form due
to the high costs involved in shipping dilute solutions.
However, it has been found that there is in fact a North Carolina
market f o r
ammonium sulfate in solution form. An aqueous solution o f
ammonium sulfate (also a byproduct from an industrial process) is currently
being marketed in fairly large quantities (10,000 tons/yr.) throughout the
eastern portion of the state.
containing
7% elemental
This material
is sold on the basis of
nitrogen, essentially identical to the projected
product o f the process under study, and is not marketed extensively in
western North
Carolina because of the geographical source of the product,
thus making it too expensive for shipment to this region.
facility under consideration is located
Since the
in the western portion of the
state, there should be a ready market for any amnonium sulfate produced.
There are two possible ways of distributing such a product.
The first
is through a commercial distributor, the second, by dealing directly with
local farmers.
Although dealing with a distributor would involve much less
effort on the part of the company, there is serious doubt as to whether it
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would be profitable to operate in this manner.
Although the anmonium
sulfate solutlon currently available In the eastern part of the state is
sold to fanners for $17/ton, the distributor only pays the producer o f the
material about $l/ton.
This is due to the fact that this company generates
such a large amount of ammonium sulfate in one
difficulty
in marketing the product.
location that there is
With this oversupply of product, the
producer i s only able to charge just enough to cover expenses.
While this
may be satisfactory to this producer, which generates a large volume o f
waste and has no other market outlet, it would not be acceptable to the
company under study, which generates a much smaller amount of waste and has
alternate methods of disposal.
distributor
might be persuaded
It is possible,
of course, that the
to pay a higher price f o r the ammonium
sulfate from this new source, but prospects are doubtful.
A more likely possibility for obtaining a higher selling price for the
m o n i u m sulfate is direct dealing with local farmers.
Since the volume of
fertilizer produced would be fairly small, this might be a manageable
alternative.
There may
be other nonfarm markets as well, such as DOT
right-of ways, golf courses, institutional uses, etc.
Operating in this manner would allow more o f a p r o f i t to be made than
working with a distributor, however, the exact price that could be obtained
for the material i s still unknown at this time,
feasibility o f this project is still uncertain.
therefore the economic
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At this stage, the technical factors appeared feasible for generating
aamonium sulfate from chemical byproducts.
However factors outside the
waste minimization considerations have dictated that the amnonium sulfate
will not be produced.
As is often the case, market conditions change and
in this case the need for the product from which
byproduct has been reduced
ammonia gas was a
such that further production was eliminated.
The industry has switched to other products.
Thus the opportunity for this
waste minimization is eliminated. As a feedback to the overall process by
which resources are applied to stimulate waste minimization,
study describes that which may not be uncommon.
this case
Not all concepts developed
as means o f pollution prevention can be implemented since factors outside
the technical aspects for such waste reduction are also very important.
Thus it
is
important to undertake as much
re-evaluation to assess both the technical
screening and continual
factors and the broader
industrial setting in which all process modifications or recycle/reuse must
be placed.