Predicting priority pollutants from petrochemical processes The wastewater effluents of 172 productlprocesses of commercial importance were characterized by in-plant sampling at 40 manufacturing sites of the organic chemical and plasticslsynthetic fibers industries. Predictably, priority pollutants were found to be primarily associated with those productlprocesses that employ certain precursorlgeneric process combinations
Hugh E. Wise, Jr. Paul D. Fahrenthold Environmental Protection Agency Washington,D.C. 20460 ~
The 1972 Clean Water Act (CWA) mandated control of potentially toxic organic chemicals in wastewater being discharged to the environment via regulation of its collective organic chemical content. The environmentally significant organic loading of wastewater was monitored as BOD5 (biochemical oxygen demand) and TSS (total suspended solids), two of four parameters that were measured and reported a t mg/L (ppm) concentrations. The 1977 CWAdistinguished three categories of pollutants-toxic, conventional, and nonconventional. Conventionals remained essentially the same, toxics were defined, and all other pollutants were assigned to the nonconventional category. The toxic pollutant category was defined by the 1976 EPA Consent Decree, drawn a t the conclusion of a lawsuit filed against the agency by several private environmental organizations. The resulting list of toxic pollutants, known synonymously as “priority pollutants” and limited to 129 in number, included I16 organic chemicals, 88 of which were named specifically. Henceforth, control of the priority pollutants in wastewater discharges was to rely on guidelines (best available technology, BAT) separate from those for conventional pollutants (by either best conventional or practicable technology). With sensitive G C methods of instrumental analysis offering the capability of measuring priority pollutants a t p g / L (ppb) concentrations, the 1977 C W A potentially added to the original regula1292
Environmental Science a Technology
tory burden an expanded list of pollutants that may be monitored at concentration levels a t least two orders of magnitude lower. In the summer of 1977 theorganic Chemical Branch of EPA’s Effluent Guidelines Division began an investigation of the occurrence of priority pollutants in the process contact wastewaters of plants manufacturing organic chemicals and plastics/synthetic fibers. The information was to serve as a portion of the data base supporting BAT guidelines for regulation of these two industrial categories. Since the high-volume organic chemicals are by and large monomeric precursors for manufacture of the polymeric plastics and synthetic fibers, and since integrated organics-plastics plants are not uncommon in the petrochemicals industry, the two indus-
trial categories were amenable to joint study. This article summarizes the priority pollutants that were observed and proposes a rationale for their occurrence based on feedstock source and process chemistry. Synthetic routes to the priority pollutants were developed to show the intrinsic potential certain feedstock-generic process combinations have for the generation of priority pollutants. Such combinations have predictive value in anticipating the priority pollutants likely to be associated with the manufacture of other petrochemical products not yet explicitly investigated.
I
winated allphatics:
I
:2, c3,c 4 Chloroalkyl elkrs Phthalate esters
Chlorophanols Nitrophenols
Aromatics
Precursor(s)
Polyarcinatics Chloroaromatlcs
ntimon
Coppel
cadmium
Lead Mercuy
Ckomlr
Nickel
Pr
DBmm(
blenium Silver Zinc
ollular . lorn (or never) observed
Nitrosamines Pesticides
lridines
PCBs
Generic
Product(s)
nrocess
Served Arsenic
Generic character of processes All chemical products are manufactured from some precursor by exposure to definite process conditions that dictate the chemistry of the transformation, and are designed to favor a desired product over others capable of being formed under the same conditions. The chemistry and conditions of the process give it a generic character. Generic processes link products to their precursors, and this combination has predictive value.
Beryl1 Thalli
Thousands of products are being manufactured in plants throughout the organic chemicals industry. In fact, industry representatives have long argued that this diversity of products makes it extremely difficult, some say impossible, to draw up guidelines for effluent standards that would apply to the industry as a whole. These arguments conveniently ignore the fact that, despite an apparent diversity, most of the products derive from a few primary feedstocks via a limited number of synthetic routes that em-
This article not subject to U S . Copyrighi. Published 1981 American Chemical Society
-
‘LOW CHART 1
Primary refinery and petrochemical plant operations are the source of eight major feedstocks..
.
Petrochemical plant operations
Refinery operations
Crude 1 Refinerygas LPgas
Synthesisgas(C0. H2)
3
Hydrogen
2 4
Naphtha 1
Cyclohexane
1
Gasoil
9
Ethylbenzene 11
4
Naphthalene
Kerosene 5 Waxy 5 distillates Heavy
fuel oil
Pyrolysis gasoline
Vacuum bottoms
8
Cyclopenladiene
Reformate
Isoprene 7
Process code
1. Distillation 2.
Steam cracking (pyrolysis)
3. Steam reforming 4. Catalytic reforming 5. Hyrocracking
6. Hydrodealkylation 7. Liquid-liquid extraclion
1
Extractive distillation Dehydrogenation 10. Alkylation 11. Hydrogenation 8.
9.
ploy processes with generic character. An earlier EPA study ( I ) attempted to simplify the organic chemicals industry by compiling these diverse products under the generic processes employed in their manufacture. While this approach did condense thousands of products into relatively few classes, its merit for assessing the potential coproduction of other chemicals is limited because it failed toconsider the fundamental interdependence, of feedstock and generic process in determining what products will be formed. The product/process unit The SIC codes define the organic chemicals and plastics/synthetic fibers industrial categories by products manufactured. Rather than simply product alone, however, the term “product/process” is a more appropriate fundamental unit with which to study these two industrial categories. As used in our studies, product/process names the product, its precursor(s), and gives a generic process descriptor for at least one of the following: a coreactant (for example, chlorination and oxidation)
Polvaromatics
the art the essential details of how precursor gets converted to product.
a coproduct (for example, dehydrochlorination and dehydration) nature of the transformation (for example, rearrangement and steam pyrolysis) unit operations required for product recovery (for example, distillation and solvent extraction). The product/process concept describes a chemical reaction in words. This avoids the mystifying effect chemical symbols seem to have on those who are unfamiliar with them, while preserving for those trained in
Selection of product/processes An econometric model of the two industrial categories as well as annual production figures show that the major organic chemicals are those involved in the manufacture of the monomeric precursors of the commercially important plastics and synthetic fibers. It was also apparent that these organic chemicals derive from only eight feedstocks-benzene, toluene, o-xy-
Examples of product/proc:esses and the chemical reactions impliemd: 1.2dichloroethane from ethy lene by oxychlorination CH*=CH2
+ HCI +
CUCI; 02
-
+
L
- CICH~CH~CI H ~ O 300 C
vinyl chloride from 1.2dichlalroethane by dehydrochlorination CICHzCH2CI
CH,=CI
450 ‘C
+CI
+ HCI
ethylene from naphtha by ste‘am pyrolysis
+ H2 + olefins + pyrolysis pasoline ~
aromatics (BTX) from pyrolys,isgasoline by solvent extraction.
Volume 15. Number 11, November 1981 1293
FLOW CHART 2
...which are used via synthetic routesto monomeric precursors
of the plastics and synthetic fibers industry
-2
X y Ienes
3
Toluenediamine
TDI Terephthalic acid
p-Xylene
4
m-Xylene
4
o-Xylene
4
lsophthaiic acid
4
Phthalic anhydride
5
7
Aniline
3
Polymeric MDA
DMT
Polymeric MDI
Formaldehyde Maleic anhydride Cyclohexane
4
Cyclohexanone 8
Caprolactam
Cyclohexanol
Adipic acid
4
11.12
10 13
Cumene
2
Adiponitrile 9.8
Phenol
7
Hexamethylene diamine Bisphenol A Acetone Propylene oxide
15
12
Propylene
13
9 Ethylene
Methyl styrene Styrene
16 16
Generic processes 1. 2.
Nitration Hydrogenation 3. Phosgenation 4. Oxidation
5. 6. 7. 8.
Esterification Reduction Condensation Oximationlrearrangement
lene, p-xylene, ethylene, propylene, butane/butene. and methane. These feedstocks in turn principally derive from crude oil fractions. as illustrated in Flow Chart I ; however, natural gas and coal tar distillates are also important sources. A synthetic route is essentially a sequence of product/processes. Synthetic routes from the primary feedstocks to the monomeric precursors were devised by combining product/ processes in the same series or parallel sequences employed by the petrochemical industry. Flow charts representing product/process profiles of the two industrial categories were constructed by compositing the major synthetic routes from crude oil fractions, natural gas (methane), and coal tar distillates to the plastics and synthetic fibers of commercial impor1294 Environmental Science 8 Technology
9.
Dehydrogenation
10. Hvdrodimerization 11. 12.
Amidification Dehydration
13. Alkylation 14. Peroxidation 15. Epoxidation 16. Chlorination
tance. Flow Chart 2, for example, depicts the composited synthetic routes from the primary aromatic feedstocks lo some monomeric precursors. Other flow charts (not shown here) addressed the monomeric combinations and generic polymerization processes used in the manufacture of plastics and synthetic fibers. The overall sequence of the product/process flow charts may be sum-
.~rioritypoiiutant.
marized as shown below. The occurrence of priority pollutants was investigated by characterizing the wastewater effluent of individual product/ processes, which were selected for study from among those that occur along the major synthetic routes of the two industry categories. Other product/processes with somewhat lower production volumes were also included in this study if the
productlprocesses
Crude o i l fractions coal tar distillates
Major plastics
products were priority pollutants. Examples of such products are the chloromethanes. chlorinated C2's, chlorophenols, and chlorobenzenes. Synthetic routes to these product/processes were integrated with those of the high-volume products. Product/process effluent characteristics that were investigated ineluded: the identification of orioritv oollutants associated with t i e pro%ct/ process a determination of the concentration (ppb) of each of the priority pollutants identified effluent flow rate (gpd), for calculation of the unit loading (Ib/d) of each characteristic priority pollutant. These product/process effluent loadings were ultimately used in various combinations to estimate treatment costs for BAT. Sampling and analysis program
T h e wastewater collection and treatment system typically found a t many plants is illustrated by Flow Chart 3. Water is brought in from a supply source (river, wells, or municipal system) and, if necessary, upgraded to the quality required for
to reduce the analytical burden (and cost) of the subsequent stage of the program by limiting the number (from 129) of priority pollutants that needed to be addressed. Screening samples were analyzed by the still unvalidated EPA G C / M S protocol. In verification we went upstream to examine thc effluents of individual product/processes, as well as to resample the combined untreated wastewater and the treated final effluent. Samples taken around the wastewater treatment facility provided a measure of the priority pollutant reduction afforded by existing technology. A three-day average loading of priority pollutants was obtained for each product/process effluent. We analyzed verification samples by G C / C D (conventional detectors), confirming identification when necessary by GC/MS. Characteristic priority pollutants were more accurately quantified by employing an adequate QA/QC program that allowed apparent concentrations to be adjusted for recovery. The G C / C D analytical procedures employed were largely innovated by analysts in our contracting laboratories during the course of the investigation (2-5). Representative samples were taken
process contact use. The wastewater effluent from the individual product/ processes is combined, treated, and discharged as treated effluent. The effluent from some product/ processes was readily isolable, such as for product/processes I and 2. In other cases, however, the effluent had to be sampled at two (or more) points within the production area and combined, in proportion to the respective flows, to obtain a representative sample of the product/process effluent, such as product/process 3. Sometimes the product/process effluent was inaccessible until after it became confounded with other effluents such as product/process 4. In such cases, the effluent from this product/process was characterized at another plant. Our program to acquire a credible analytical data base to support BAT consisted of two stages: screening and verification. Screening focused on the priority pollutants in the combined untreated effluents of all product/ processes operating at a plant, although process water supply and treated effluent were also routinely sampled. The objective of screening was to identify, with approximate quantification, the priority pollutants that were present. This was intended
FLOW CHART 3
Typicalwastewater collectionand treatment system with sampling locations
,--------------------------------------,
: Reactant(s) : Impurities :
Catalyst
Spent catalyst
Chemical process
Product(s)
Solvent
Derivatives
L______________________________________! 01 impurities
Generic chemical process
Coproducts
Equipment cleaning
By-products
Miscellaneous' resinous materials Material losses
Productl process
Combined influent to
Productl
Process water 0 SUPPlY
process 2 Productl 3a 0 process 3b Productl process 4
Equalization basin
treatment
Treated
Aeration basin
Sampling locations
effluent
Clarifier
Receiving waters
.s!,iibottomr. reactorcoke.eic.
Volume 15. Number
11.
November 1981
1295
TABLE 1
Groups of priority pollutants Metals Antimony Arsenic Beryllium
-
Cadmium Chromium
Copper Lead Mercury * Nickel Selenium
Chloroalkyl ethers Bis(chloromethylbther Bis(2-chloroelhyl)ether Bis(2-chloroisopropylbther 2-Chloroethylvinyl ether * Bis(2chloroethoxy) methane Pesticldes Aldrin
Endrin Endrin aldehyde Heptachlor Heptachlor epoxide a-BHC
Halogenated methanes (Cl's) Methyl bromide Methyl chloride Methylene chloride (dichloromelhane) Bromoform (tribromomethane)
Chloroethylene (vinyl chloride) 1.2Dichloroethane (ethylene dichloride) 1.1-Dichloroethane 1.2-!raN-Dichlorcelhylene 1.lOichloroethylene (vinylidene chloride)
* lIl,2-Trichloroettnne 1.1.1-Trichloroethane (methyl chloroform) Trichlwoethylene Tetrachloroethylene * 1.1.2.2-Tetrachloroethane Hexachloroethane
-.
Chlorlnated CS'r 1.2-Dichioropropane 1,3-Dichlwopropylene
Chlorlnated C4 Hexachlorobutsdiene Chlorlnated C5
Hexachlorocyclopentadiene
* Chlorobenzene @Dichlorobenzene
* pDichlorobenzene Michlorobenzene 1.2.4-Trichlorobenzene Hexachlorobenzene Chlorlnated potyaromatlc 2-Chloronaphthalene Polychlwinated biphenyls
Seven listed Phthalate esters Bis(2-ethylhexyl)
4.4'DDT
-
NHroramlnes NnitrosOdimethylamine Nnitroscdiphenyl amine NnitrosOdi-+propyl amine
Miscellaneous Acrolein Acrylonitrile lsophorone Cyanide
--
Ammatles Benzene Toluene Emylbenzene Polyaromatlcs Naphthalene Acenaphthene Acenaphthytene Anthracene * Benzo[a]anthracene (1.2benzanthracene) &nzo[a]pyrene (3.4benzopyrene) 3,kBenzolluoranthene Benzo(k)flUoranthene (11.12benzolluoranthene) Benzo(ghi)perylene (1,tZbenzoperylene) Chrysene * Dibenzda. h)anthracene (1.2.5,€dibenzanthracene) Fluorene Fluoranthene
Note: Byllets rater to llrlwity wllutants found in verification.
1298 Environmental Science 8 Technology
Chloroaromatics
8-BHC y-BHC (Lindane) 6-BHC 4.4'ODE (p,d-WX) 4.4'DDD (p.p'-TDE) Toxaphene
Chloroform (trichloromethane) Bromodichloromettnne DibromOchloromethane
Chlorlnated C2's Chlwoethane (ethyl chloride)
Fyrene
Dieldrin
Zinc
Carbon tetrachloride (tetrachloromethane)
phenylene pyrene)
* Phenanthrene
Chlordane aIndosulfan
Silver Thallium
DichlorOdifluOrOmethne TrichloroflUoromettnne
* Indendl.2.3-c%yrene (2,3-@
Butylbenzyl Di-+butyl Di-Roctyl Diethyl
* Dimethyl Nltroaromatles * Nitrobenzene 2,dDinitrotoluene 2.6Dinitrotoluene Benrldlnes Benzidine 3,3'-Dichlorobenzidine 1.2-Diphenylhydrazine Phenols Phenol 2.4-Dimethylphenol Nitrophenols * 2-Nitrophenol 4-Nitrophenol * 2.4-Dinitrophenol
* 4.6Dinitro-wresol Chlorophenols 2-Chlorophenol 4-Chlwc-mcresol 2.4-Dichlorophenol 2.4,6-TrichlorophenoI Pentachlorophenol TCDD (2.3.7.EtetrachlorOdibenz~ pdioxin) Haloaryl ethers 4-Chlorophenylphenyl ether CBromophenylphenyl ether
from the effluents of 147 product/ processes manufacturing organic chemicals and 25 product/processes manufacturing plasticsJsynthetic fibers. These I12 product/processes included virtually all of the high-volume organic chemicals, plastics and synthetic fibers. Forty plants ultimately became involved in the verification effort as a consequence of being appropriate sites at which to study cost-effectively the product/processes of interest. Occurrence of priority pollutants The occurrence of priority pollutants can be more conveniently discussed by condensing the standard alphabetical listing to a generic classification that is based on a similarity of functional group, or structure (isomers, homologs, and analogs). As a consequence of these similarities, members of such genera share precursors and/or have a common response to generic process chemistry. Table 1 defines the priority pollutant classes to be used in the following discussion. In reviewing the analyses of product/process effluents, we noted that extraneous priority pollutants were often reported. Unrelated to the chemistry or feedstock of the process. and typically reported at concentrations less than 0.1 ppm, these anomalies could usually be attributed to one or more of the following sources: extraction solvent (methylene chloride), or its associated impurities false-positives (misidentification of organic chemicals that are not priority pollutants) plasticizers (phthalates) from auto-sampler tubing, process water supply, pump seals, and gaskets sample contamination. either during sampling or during sample prep at the laboratory in-situ generation in sewer. When irreconcilable with feedstock or process chemistry, extraneous priority pollutants were generally ignored in determining product/process effluent characteristics. A review of the data from both industrial categories revealed that a number of individual priority pollutants, as well as whole classes, were seldom (or never) observed. Only about 75 of the 116 organic priority pollutants were verified by analysis, as summarized in Table la. No analysis was performed for T C D D or asbestos during the screening and verification efforts. Priority pollutants found at concentrations greater than 0.5 ppm in the effluents of plastics/synthetic fibers
product/processes are summarized in Table 2, and are generally related to the monomer, or process solvent. There are three priority pollutants that are commercially significant monomers: acrylonitrile, phenol, and vinyl chloride. Another priority pollutant monomer, vinylidene chloride, is involved in the manufacture of lowervolume plastic products that were not investigated. Less than half of the organic chemicals productJprocess effluents characterized were found to contain concentrations of priority pollutants greater than 0.5 ppm (see Table 3). This is readily explained; priority pol-
lutants can only be derived from specific precursors, or more correctly. from certain precursor/generic process combinations. Thus a knowledge of these critical combinations is the key to prediction of priority pollutants in productJprocess effluents. Sources of priority pollutants In any product/process (see Flow Chart No. 3) if any one of the reactant(s), solvent, catalyst system, or product(s) is a priority pollutant, then the priority pollutants likely to be found in that process wastewater effluent are obvious. Equally obvious are metallic priority pollutants, which are
TABLE 2
Plastlcslsynthetlc fibers effluents wlth concentrations greater than 0.5 ppm of priority pollutants Pmduu
ABS resins
Acrylic fibers
Acryllc resins (Latex)
A..OClMt.d PIOW pOllvlann
)1-. Aaylmitrlle Styrene Polybutedlene Acrylonitrile Comonomer (variable): Vinyl chloride Acrylonitrlle Acrylate ester MethvlmelhacNlate
Acryloniblle Aromatics AaylonHrlle Chlwlnated C2's AcryloniRlle Acrolein
Acrylic resins Alkyd resins
Methylmethacrylate Glycerin lsophthnllc acid Phthalic anhydride
Cyanide Acrolein Aromatics Polyaromatlcs
Cellulose acetate
Diketerm (acetylating agent) Bisphenol A Epichlwohydrin
lsophorone
Dlcyclopentadlene
Aromatics
Phenol Formaldehyde Bisphenol A Phosgene
Phenol Aromatics (Not investigated) Predicted:phenol Chloroaromatics Halomethanes Phenol Aromatics
Epoxy resins
Pelroleurn hydrocarbon resins Phenolic resins POlycarbCmatWS
Polyester
Terephthalic acldl dimethylterephthalate Ethylene glycol
HD polyethylene resin Polypropylene resin Polystyrene Polyvinyl chloride resln
Ethylene Propylene Styrene Vinyl chlwlde
SAN resln
Styrene
Styrene-Butadiema resin (Latex)
Acrylonitrile Styrene(50%) polybutadiene
Unsaturatedpolyester resin
Maleic anhydride Phthalic anhydride Propylene glycol (Styrene-added later)
Phenol Chlorinated C3's Aromatics
Aromatics Aromatics Aromatics ChlorinatedCZ's Aromatics Acrylonitrile Aromatics Phenol Aromatics
Volume 15. Number 11, November 1981
1297
I
c chemical effluents with concentrations greate
.
&.mk pot-
f. -
Acetone Acetylene Acrolein
Alkylation. peroxidation Dehydrogenation Oxidation
Benzen Methan
Acrylic acid Adiponitrile
Oxidation Ammonolysis. dehydration Hydrodimerization Cyanation. hydrogenation Alkylation
.-L l .=l
Propyle Propyle Adipic i
Reduction (by alkoxide)
Acrylor c12-c Phenol, Acrolei
Aniline
Hydrogenation
Nitroba
Benzene
Hydrodealkylation
Toluem
BTX extraction BTX extraction
CatalyI Coal la
Benzyl chloride Bisphenol A
BTX extraction Chlorination Condensation
Toluen,
Butadiene
Extractive distillation
Butenes Butylbenzyl phthalate
Esterification
BButa Phthaii
Caprolactam
Oxidation. oximatlon Dehydrogemtlon.oximatlon
CYClOt Phenol
Carbon tetrachloride
Chlorination
Methai
Chiwination
Ethylei
Chlwlnatlon Chlorination
Benzei
Chloroform mChlwonltrobenzene
Chlorination
Nitroh
Creosote
Distillation
coal 1;
Cmne Cyctohexanoll-one
Alkylation Oxidation
Benze CVCIOt
1,2Dichlwmthane Dicyclopentadiene Diethylphthalate
Oxychlorlnation Extraction. dimerization Esterification
Ethylei
Diketene Dimethyl terephthalate Dinitrotoiuenes
Dehydration Esterification
Diphenylisodecyl phosphate ester
Esterification
Phem Pa&
Epichlwohydrln
Chlwohydriniation
Ethoxylates-alkylphenol Ethylbenzene
Etbxylation Alkylation
Allyl c Alkylp
Alkyl (C13, C19) amines Alkyl (C8. C9) phenols Allyl alcohol
Chlorobenzenes
Nitration
PYrOlYS
Phenol
Methai
c 5 PYI Ethan( Acetic Mema Toluer
Benze
Extraction fr()m BTX
BTX e
Ethylene
Steam pyrol)rsis
LPG. r
Ethylene amines Ethylene diamine
Amnmnation Ammonation
1.2-Di 1,201
Ethylene oxide
Oxidation
Ethyle
certainly not transformed to another metal (transmutation) by exposure to process conditions. As suggested by Tables 2 and 3 , however, there are more subtle sources of the organic priority pollutants. They may be introduced into the process as impurities in the feedstock or solvent. They may also be generated by the process chemistry and appear as minor byproducts, or derivatives of the impurities. Commercial grades of primary feedstocks and solvents commonly contain 0.5% or more of impurities. While 99.5% purity approaches laboratory reagent quality, 0.5% is nevertheless equal to 5 X lo3 ppm. Not surprisingly, then, water coming into direct contact with these process streams will acquire up to 1 ppm (or more) of the impurities. Priority pollutants representing impurities were, in fact, routinely observed in product/process effluents in the concentration range 0.1- 1.O ppm. Specifications or assays a t these trace levels are seldom available, nor previously (before BAT) of any interest, since even 0.5% impurity in the feedstock and/or solvent would typically have a negligible effect on process efficiency or product quality. Exceptions include impurities like sulfur, which can poison sensitive catalyst surfaces. In lieu of specific information, probable feedstock or solvent impurities may be anticipated by considering their source. While preceding product/processes are principally responsible for impurities in intermediate feedstocks further along the synthetic routes, potential impurities in the primary feedstocks may be identified in Flow Chart 1. This chart offers no more than general guidance, however, because of the practice of purchasing primary feedstocks from several suppliers, who often draw from different sources. In Flow Chart 1, steam pyrolysis of LPG, naphtha, or gas oil feedstocks (purchased from refihery) is used principally to manufacture ethylene and propylene (see process code 2). Pyrolysis gasoline is a copyrolysate and is recovered for its aromatic and C4/C5 olefinic content. Since ethylene and propylene are gases at ambient temperature and pressure, they are relatively easy to separate from the pyrolysis gasoline, and are typically subjected to further purification before being used as a feedstock in any process. Consequently, priority pollutants found in wastewater effluents from generic processes using ethylene or propylene are not likely to be derived 1300
Environmental Science & Technology
from impurities in these primary feedstocks. In contrast, the aromatics-benzene, toluene, and the xylenes (BTX)-are separated from pyrolysis gasoline, or catalytic reformate, by solvent extraction. Joint extraction explains why benzene and toluene are mutual impurities, and why ethylbenzene is a typical impurity in toluene. When gas oil is the feedstock to the steam cracker, the BTX extract is higher in polyaromatic content than when LPG is the feedstock. BTX from coal tar distillates is even richer in polyaromatics. Although not a primary feedstock, phenol from coal tar distillates would likely contain cresols, aromatics, and polyaromatics. In contrast, phenol from cumene typically has less cresol or polyaromatic content.
Routes to priority pollutants Not all of the product/process effluents had significant loadings of priority pollutants; some of the priority pollutants were seldom (or never) observed. This is a direct consequence of the fact that organic priority pollutants require specific precursors and appropriate process conditions for their formation. The manufacture of commercial organic chemicals only become coincident with the generation of priority pollutants when there is a commonality of reactants and process chemistry. To provide a systematic examination of those products of coincidence, we developed synthetic routes from the primary feedstocks to the priority pollutants in reverse, Le., by starting with the priority pollutants and working backwards. At each intermediate product/process along the routes we employed well-established organic reaction mechanisms to predict reaction products; these included any priority pollutants that might conceivably be formed under the process conditions from appropriate precursors likely to be present in the reaction mixture. Except for major commercial intermediate products required to maintain the continuity of the routes, all nonpriority pollutant organic chemicals anticipated from these reactions were edited out. Priority pollutants predicted by the principal routes initially developed were compared with those actually observed in analyses of product/process effluents. Predictions were found to be preponderantly consistent with observations, demonstrating among other things that chemistry is indeed a science. At times, the analytical data offered
unpredicted priority pollutants, some of which were recognized as legitimate minor reaction products and/or derivatives of probable feedstock impurities. Alternate routes devised to rationalize these results contributed to the design of the flow charts by enhancing the principal routes. Perhaps more importantly, the alternate routes illustrated minor equilibria normally operative in each product/process that are the more subtle sources of priority pollutants. The synthetic routes that ultimately evolved provided a separate scheme for each of the following five groups of priority pollutants (6): nitroaromatics, nitrophenols, phenols, benzidines, and nitrosamine chlorophenols, chloroaromatics, chloropolyaromatics, haloaryl ethers, and PCBs chlorinated C2’s, C4, and chloroalkyl ethers chlorinated C 3 3 , acrolein, acrylonitrile, isophorone, and chloroalkyl ethers halogenated methanes. Flow Chart 4 illustrates the scheme for Group 1.
Predictability of priority pollutants By superimposing the synthetic routes to the priority pollutants on the synthetic routes employed by the industry to manufacture organic chemicals, we can identify the products of coincidence. Praduct/processes that generate priority pollutants lie no more than one step removed from these products. Flow Chart 5 represents such a superimposition by compositing the synthetic routes to important commercial products with those leading to priority pollutants in order to tie priority pollutants back to crude oil fractions, natural gas, and coal tar distillates via the coincident products. This, in effect, creates a priority pollutant profile of the industry. Priority pollutants (or precursors) typically introduced into a product/ process as impurities in primary feedstocks have been discussed previously. For product/processes in the synthetic sequence subsequent to those employing the primary feedstocks, however, the reaction products of one become the feedstock to the next. Therefore, some understanding of the process chemistry taught by these synthetic routes is essential in order to anticipate impurities in the intermediate feedstocks. Insofar as the occurrence of potentially toxic organic chemicals is concerned, the abridged consideration of all organic molecules represented by
I.----L
4
Phenyl esters 3
....
Phenol.
..
4
.. .
4
me 2
4-
I
.
..
I
0-Cresol
Maiw-;'
'
4
4.6-Dinitro-0-cresol
. :.
..:.
I
2
m-Dinitrobenzene
Generic processes 1. Chlorination (FeCI, 2. Diazotization 3. Hydrolysis 4. Nitration
>a,.. ':. =Prlnc#3.:'r M
