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What Are Spent Caustics?

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An Introduction to Spent Caustic Chemistry, Treatment and Disposal

Spent Caustics are the most difficult of all industrial wastes to dispose properly, with the exception of radionuclide wastes.  Since the dawn of petroleum refining, caustic solutions containing sodium hydroxide (common lye) have been used to wash sulfur and other undesirable compounds out of petroleum.  Its use has been in washing crude oils, intermediate fractions, and finished fuels throughout the refining processes.  The result of this washing is the generation of Spent Caustics, also called Waste Caustics and Toxic Wastes.  Another form of sulfur bearing wash solutions from other refining operations is Sour Water.  Sour Water contains less amounts of sulfur compounds, but is generated in much greater volumes. 


In addition to the refining industry, other industries including chemical manufacturing, LPG (liquefied petroleum gas), Natural Gas (methane), and geothermal energy plants produce huge volumes of Sour Water and Spent Caustics.  Chemical manufacturing produces spent caustics containing specific compounds compared to the wide variety from refining operations.  LPG and Natural Gas plants predominately produce potassium hydroxide spent caustic, whereas, most other sources produce sodium hydroxide spent caustic.

Common to all spent caustics is a high concentration of the extremely deadly and odorous HYDROGEN SULFIDE, an acutely toxic gas most recognized as the odor of rotten eggs when in low ppb (parts per billion) concentrations.  Hydrogen sulfide gas readily dissolves in caustic solutions due to its solubility at high pH or "caustic pH" conditions, typically above pH 10 to 12.  It can be released from solution as a gas easily when the solution is disturbed or the pH is lowered.  It is more toxic than hydrogen cyanide to humans and death can occur in exposures of just 100 to 500 ppmv (parts per million by volume) in the atmosphere. 

Hydrogen sulfide gas is responsible for "multiple sewer-worker's death syndrome".  It starts with 1 worker while working in a sewer suddenly collapses, one by one up to 4 more workers rush in to help the collapsed worker then collapse on-top of each other like a dog-pile.  By now the 6th worker sees the calamity and decides to get help instead of entering the sewer.  Multiple deaths have occurred in cities worldwide since the first wastewater sewers were built.  It was believed that sewer deaths were due to a lack of oxygen before the toxicity of hydrogen sulfide was understood.  Furthermore, hydrogen sulfide contained in an oxygen environment can be explosive when ignited, thus adding to the danger.

Gaseous Hydrogen Sulfide Molecule
Contains 94% Sulfur
(Yellow - Sulfur atom | White - Hydrogen atoms)

Petroleum, LPG and Natural Gas manufacturing plants may also generate spent caustics containing high levels MERCAPTANS.  Naturally occurring mercaptans, also known as organic sulfides or thiols, contain a sulfur atom typically near the end of an organic molecule or chain.  Mercaptans are the most odoriferous compounds known to man and even a minuscule amount in ppb concentration (parts-per-billion), is easily detected by the human nose.  For example, the odor of Skunk is mostly butyl-mercaptan.  Ethyl-mercaptan is intentionally fed into natural gas pipelines in ppb levels to serve as an "odorizer".  When a person "smells a gas leak" they're not smelling the Natural Gas, instead, they're smelling the mercaptan odorizer since LPG and Natural Gas are odorless.

  

Methyl-mercaptan               Ethyl-mercaptan      
(Black - Carbon atoms | Yellow - Sulfur atoms | White - Hydrogen atoms)

The LPG propellants used in aerosol canned products (after CFC's were banned) must be specially deodorized to prevent the odor of mercaptans being detected by the user.  For example, men's aerosol shaving cream is particularly sensitive to discovery if any mercaptans remain in the propellant.  Therefore, LPG aerosol propellants are much more expensive than other  LPG products.


Refinery spent caustics contain several other compounds in high levels including PHENOL together with cresylic  and naphthenic acids that also exhibit toxic and odorous properties.  Phenol is a known biocide used in medicines and for early sanitation in hospitals, etc.  For example, the trademark Champo-Phenique stands for camphor and phenol and is sold over-the-counter to treat bacteriological infections.  It has been established that when as little as 64 square inches (or an area of 8 inches x 8 inches) of skin is covered with pure phenol death will occur.  Other hydroxy-phenols include catechol, resorcinol, and hydroquinone (these compounds, shown with their common names, are also known as hydroxy-benzenes and di-hydroxy-benzenes).

              

             Phenol            Catechol              Resorcinol           Hydroquinone      
(black - Carbon atoms | Red - Oxygen atoms)

Refinery spent caustics often contain high levels of CRESYLATES, a form of phenol containing a methyl group (CH3), sometimes called phenolics as a family group.  In either of the three forms: ortho-cresol, meta-cresol, and para-cresol, they are not as toxic as phenol, however, cresylates are generated in relatively high concentrations, from 1% up to 35%.  Cresylates, or cresylic acids, are often referred to as "Acid Oils" and are solubilized at high pH, but readily separate at lower pH's.  It is not the intent to wash these aromatic compounds from the petroleum since they increase the octane number in the finished fuel, however, when present they will partition into the spent caustic in varying concentrations.

       

     o-Cresol                       m-Cresol                       p-Cresol      
(Black - Carbon atoms | Red - Oxygen atoms | White - Hydrogen atoms)        

Refinery NAPHTHENIC spent caustics are the last of the major chemical classifications that may contained in relatively high concentrations.  Naphthenics generally do not impart a toxic or objectionable odor component to spent caustics.  Naphthenics are partially soluble in caustic and increase the total organic carbon (TOC) content considerably.  Naphthene is a general term for cyclic alkanes, or cyclic non-aromatic hydrocarbons called cycloalkanes.  Refinery naphthenes typically include cyclopentane and cyclohexane as the parent compound.

       

cyclo-Pentane                        cyclo-Hexane
(Black - Carbon atoms | White - Hydrogen atoms)

Many refineries don't discriminate too closely and often include the family of naphthenic acids, or cyclic carboxylic acids, when referring to naphthenics in general terms.  Naphthenic acids contain a cyclic naphthene parent compound bonded to various hydrocarbon chains containing a carboxylic acid end group.  A "catch-all" analytical procedure includes Total Acid Number (TAN) for a rough value of the naphthenic acid content.  Other analytical methods are necessary for concise identification of each naphthenic species. 


GEOTHERMAL spent caustics are in a class by themselves.  Produce spent caustic mostly containing lower levels of hydrogen sulfide, and a lessor amount of mercaptans. 

In the production of today's "LOW SULFUR" fuel requirements, more spent caustics than ever before are being generated.  All of which require proper handling and disposal.

In earlier years, RESOURCE CONSERVATION was a focal point for politicians and environmentalists, some spent caustics were processed with the intent of recovering the remaining caustic value (remaining sodium hydroxide).  The sulfidic types were destined for reuse in the Pulp & Paper industry where caustic sulfides are needed in large quantities.  However, today the cost of reclamation is greater than the value of virgin materials and therefore, most of the reclamation facilities are now reduced to disposal facilities.  A few exceptions include localized Pulp & Paper mills where only a pure stream (hydrocarbon free) of sulfidic spent caustic can be reused.

In the United States,  "GRANDFATHER CLAUSES" in environmental protection laws permit Deepwell Disposal Sites to remain in operation today.  Over 85% of refinery spent caustics are shipped by railcar and truckload to deepwell disposal facilities (TSD's).  Other privately owned Deepwells are also in operation with captive down-hole waste streams.

DEEPWELL disposal is relatively inexpensive and easy for industry to utilize.  It provides "out of sight and out of mind" waste handling and disposal options for the generator.  But, USEPA (United States Environmental Protection Agency) legislation defining "cradle to grave stewardship" will remain a daunting subject for thousands of years yet to come.  The greatest concern of environmental activists is if potable ground water aquifers are breached by nearby disposal wells.

 

Deepwell Disposal Site

The TEXOX^TM PROCESS was developed especially for the effective treatment and environmentally friendly disposal of these troublesome wastewaters.  Furthermore, the TEXOX Process can also transform a wide variety of hazardous wastes into biodegradable products.  The TEXOX Processes have been proven cost competitive with typical deepwell disposal facilities pricing, so what's the delay in replacing your deepwell disposal with a TEXOX Process using environmentally "green" technology?

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