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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 design and innovative means to achieve a low cost design are given. The 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 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

as well as $ 3,000 from this project.

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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 It 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.

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Processing Industrial Byproducts to Yield

Ammonium Sulfate Fertilizer

INTRODUCTION

The waste minimization program in Chemical Engineering at North

Carolina State University is aimed at the research, development,

engineering, and implementation for waste minimization in industry. The

program 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 levels in an attempt to clarify the actual

feasibility 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 assessment often vary

among Industrial projects. The level of currently available information

and on-going activity greatly affect the results and level of detail for

such an assessment. However a number of objectives occur routinely i n

these assessments. 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 supplemented with plant visits and appropriate scientific

analysis. 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 until such

time as the manufacturing organization decides to allow specific

Identification. There are a number of benefits to industry when directly

identified with the development of innovative means of reducing wastes

produced. However, technology 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 individually, may be

combined to yield a worthwhile product. Such a case was examined as part

of the waste reduction program at North Carolina State University. The

manufacturing 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 of,

might be combined to yield a saleable product. 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 of economic feasibility. A greater

number of concepts and projects 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 implementing a waste

minimization scheme. 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 amonla 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. In a similar manner the spent sulfuric acid is

presently used 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 medium size chemical

manufacturing plant during this project. 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 in a high

temperature flare. By heating the ammonia to around 2,200 F, it is

decomposed to diatomic nitrogen and hydrogen. The hydrogen then burns as

well, the end result being 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

the sulfuric acid and, particularly, the amnonia production

practical way to store gaseous amnonia) a batch production

would involve

ng the ammon

suitable. One simple approach to the problem

with sulfuric acid solution and introduc

sparger located near the bottom o f the tank.

Sufficient contact to enable reaction could be achieved by bubbling the NH3

gas through the acid solution. Because of the intermittent nature of both

(there being no

mode seems most

filling a tank

a by means o f a

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Because of the rapidity of the neutralization reaction, near total

conversion could probably be achieved without the loss of significant

amounts of NH3 vapor. 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 expensive

materials. Conversely, storage at high (alkaline) pH would minimize

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. There are

two ways of minimizing the heat bui dup: first, through the design of an

external heat exchange system using cooling water or some other heat

transfer medium. The second approach is to remove heat by bubbling air

through the solution. Although the solution would be below the normal

boiling point, the bubbling air would evaporate enough water so that the

latent heat consumed would offset the heat o f neutralization. If the air

bubbling method 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 der ived.

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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. This material is sold on the basis of

containing 7% elemental 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. Since the

facility under consideration is located 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 of the

material about $l/ton. This is due to the fact that this company generates

such a large amount of ammonium sulfate in one location that there is

difficulty in marketing the product. 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. It is possible, of course, that the

distributor might be persuaded 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, therefore the economic

feasibility o f this project is still uncertain.

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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 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 is eliminated. As a feedback to the overall process by

which resources are applied to stimulate waste minimization, this case

study describes that which may not be uncommon. 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 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.