An important process carried out globally in refineries is fluid catalytic cracking (FCC), which involves converting high molecular weight and low-value feedstocks such as coker gas oil, tar sands oil, and shale oil into high-value products by “cracking” the C-C bonds.
Such feedstocks may include organic nitrogen compounds such as pyridine, indole, quinoline, and carbozole (Figure 1) at higher levels. These compounds form cyanide and ammonia in the FCC unit reactors.
Normally, crude petroleum has a nitrogen content of 0.1%-0.9%, but certain types of crude petroleum may include nitrogen up to 2%. If the oil is more asphaltic, then the nitrogen content is higher.
Figure 1. Common organic nitrogen compounds in petroleum.
Hydrocracking is a thermal process (>350°C) that involves performing hydrogenation at the same time as catalytic cracking to transform high-boiling feedstocks into low-boiling products through cracking of hydrocarbons and subsequent hydrogenation of the resultant unsaturated reaction products.
Prior to cracking of the aromatic nucleus, polycyclic aromatics are partially hydrogenated. Consequently, nitrogen and sulfur atoms are transformed into cyanide, ammonia, and hydrogen sulfide.
Dual-function catalysts are used to catalyze hydrocracking reactions. Silica-alumina, or zeolite, catalysts are used to carry out cracking, and platinum or nickel catalysts are used for hydrogenation. Hydrogenation is performed to reduce the poisoning of catalyst by oxygen, nitrogen, and sulfur compounds in feedstocks. Figure 2 illustrates certain reactions employed for nitrogen removal.
Figure 2. Hydrogenation reactions involving nitrogen compounds in petroleum.
Cyanide Generation and Corrosion
Ammonia, hydrogen cyanide (HCN), and other nitrogen compounds are discharged during the cracking of organic nitrogen compounds in petroleum feedstocks. A principal area of concern related to the hydrocracking and FCC processes is the formation of cyanide as well as its downstream effects.
Gas phase reaction products such as H2S, HCN, NH3, and hydrocarbons that are formed in the FCC reactor are conveyed to a distillation column. In the distillation column, steam is introduced into the overhead stream for the purpose of reducing the partial vapor pressure of hydrocarbon to allow the operation to be carried out under lower temperatures. The sour water formed in this step combines with HCN and NH3.
The ammonia cyanide (NH4CN) formed by the reaction of the HCN and NH3 is ionized in the sour water and discharges ammonium ions (NH4+) and cyanide ions (CN-). These cyanide ions form a soluble ferrocyanide complex upon reacting with insoluble iron sulfide.
Such a reaction affects the protective iron sulfide film on metal surfaces, exposing the fresh metal. The corrosion consequently discharges hydrogen atoms, which penetrate into the steel surfaces, leading to hydrogen blistering and subsequent stress corrosion cracking.
Cyanide Analysis of NPDES Wastewater Samples
Large volumes of wastewater are generated by petroleum refineries - from 0.4 to 1.6 times the volume of oil processed. The U.S. EPA conducted a water use survey at 27 refineries and found that sour water stripping contributed 19.6% of the total amount of wastewater released from the refineries.
The main constituents of the sour wastewater resulting from hydrocracking and FCC processes are cyanide, sulfides, emulsified oils, mercaptans, phenols, and ammonia. The measurement of the cyanide species is made difficult by the chemical composition of sour wastewater as all of the other constituents are known matrix interferences. As indicated by the U.S. EPA Solutions to Analytical Chemistry Problems with Clean Water Act Methods (“Pumpkin Guide”), “Next to oil and grease, cyanide is the pollutant for which the most matrix interferences have been reported.”
Sulfides are a specific concern in sour wastewater samples, as they are known to cause low or high cyanide recoveries with respect to the analytical methodology and technique used for testing. On average, sour wastewater was found to comprise 15 to 23 mg/L of sulfide, 98 to 128 mg/L of phenol, and 5.1 to 21.1 mg/L of ammonia.
The U.S. refineries have National Pollution Discharge Elimination System (NPDES) permits, indicating limits for the amount of cyanide in wastewater. U.S. EPA approved methods can be employed to obtain data submitted for NPDES compliance reporting.
The earlier U.S. EPA cyanide analysis methods followed from the period of 1970s involve an acid distillation process for dissociating cyanide from metal-cyanide complexes and separating cyanide from the matrix.
However, this process was found to create either positive or negative analytical biases based the chemical composition of the sample matrix under test. Sulfide was found to cause interference during the processes of sampling, sample preparation, and measurement in the U.S. EPA methods such as 335.4.
In the process of sampling, sulfide combines with cyanide and forms thiocyanate, decreasing the concentration of cyanide. This reaction specifically takes place at a fast pace in the presence of metal sulfides such as lead sulfide.
While carrying out acid distillation, sulfide gets distilled over into the basic absorber solution, and cyanide in the solution reacts with sulfur to form thiocyanate, decreasing its cyanide concentration.
In methods employing colorimetry, sulfide reacts with the color reagents and totally consumes the reagents, resulting in a negative bias and consequently leading to underreporting of the actual cyanide concentration.
Sulfide Interference Demonstration
In order to demonstrate the impact of sulfide interference on different cyanide analysis methods, test solutions with sulfide concentrations up to 100 ppm and containing 100 ppb CN (as KCN) were prepared and analyzed by employing ASTM methods D 7284-08, D 7511-09e2, and colorimetry.
Figure 3 illustrates the recovery of cyanide through the analysis of solutions with sulfide concentrations up to 100 ppm by employing ASTM methods D 7284, D 7511, and colorimetry.
Figure 3. Cyanide recoveries obtained from solutions containing sulfide concentrations up to 100 ppm by three cyanide analysis methods.
Reliable results for samples containing sulfide were only produced by ASTM D 7511. About 60 to 70% cyanide was recovered by employing ASTM D 7284, whereas no cyanide was recovered by the colorimetric method. Samples are distilled before analysis in the ASTM D 7284 method as well as colorimetric methods such as U.S. EPA 335.4.
However, both of the methods differ in the determination step where automated gas diffusion amperometry is employed in ASTM D 7284 in contrast to the automated colorimetry employed in U.S. EPA 335.4.
Manual cyanide determination through a colorimetric test is depicted in Figure 4, in which the center vial contains 200 ppm sulfide and 100 ppb CN. There is only turbidity and no color development. An automated colorimetric method like the EPA 335.4 can detect the turbidity as a small amount of cyanide, i.e. ~30 ppb. However, the vial test obviously exhibits no color development.
Figure 4. Colorimetric test for cyanide showing turbidity and no color development indicating no cyanide is present in center vial containing 100 ppb CN and 200 ppm sulfide.
Chloramine T is consumed by sulfide. In colorimetric cyanide methods, cyanogen chlorine is formed when cyanide reacts with chloramine T. Subsequently, the cyanogen chlorine reacts with pyridine or a pyridine analog such as isonicotinic acid and forms a colored product having absorption that is proportional to the concentration of cyanide.
The lack of color development demonstrates that sulfide prevents the reaction of cyanide with chloramine T leading to a negative bias that causes under- reporting of the actual cyanide concentration.
ASTM Method D 7511-09e2 for Total Cyanide Analysis
An automated method defined by ASTM D 7511-09e2 to determine total cyanide in aqueous samples such as industrial effluents can be applied to cyanide mass concentration range of 3 to 500 µg/L. Even higher concentrations can be determined by adjusting operating conditions or diluting the samples.
ASTM D 7511-09e2 was specifically devised to prevent the interference normally experienced with other methods that involve a high-temperature acid distillation step. A sulfide concentration lower than 50 mg/L does not interfere the ASTM D 7511-09e2 analysis. Sulfide concentrations that are more than 50 mg/L can be detected by testing the samples with a lead acetate test strip.
To ensure interference-free analysis, samples with sulfide concentrations of more than 50 mg/L can be treated with a sulfide scrubber reagent or diluted to minimize the sulfide level.
In April 2012, a Methods Update Rule (MUR) was signed by the U.S. EPA Administrator. This rule approves the application of novel analytical techniques to test the pollutants in wastewater under the Clean Water Act.
The final MUR published in the Federal Register in May 2012 comprised six ASTM methods that included the preservation, sampling, and analysis of available, free, and total cyanide species. ASTM D 7511-09e2 was also included in those six methods.
ASTM D 7511-09e2 can now be used by refineries to test the wastewater samples for the purpose of NPDES regulatory compliance reporting.
Principle of Operation
Figure 5 depicts a flow diagram for carrying out total cyanide analysis by using ASTM D 7511-09e2. Here, a sample injected into a carrier stream is segmented and acidified.
(1) The weak acid dissociable cyanide complexes are transformed into HCN and (2) the strong metal-cyanide complexes are irradiated using UV light in a UV digestion module under acidic conditions. This results in break down and emission of HCN.
(3) The HCN gas liberated from all the cyanide species in the sample is diffused through a hydrophobic membrane into a basic acceptor solution. The cyanide is converted back into CN- and conveyed to a flow cell of an amperometric detector.
(4) Here, the cyanide ions react with a silver electrode to generate current that is proportional to the concentration of cyanide ions. The detector response is exhibited as a peak. The height of this peak is proportional to the concentration of cyanide in the sample.
Figure 5. Flow diagram for total cyanide analysis by ASTM D 7511- 09e2.
Instrumentation for Cyanide Analysis by ASTM D 7511-09e2
The design and performance attributes required for a flow injection analysis (FIA) instrument to carry out the in-line UV digestion, gas-diffusion amperometry method are defined by ASTM D 7511-09e2.
The design attributes for this method require the following: an amperometric detector provided with a silver working electrode, an AgCl reference electrode, a gas diffusion manifold including a hydrophobic membrane, a UV digestion module including a 312 nm lamp, and a stainless steel or platinum counter electrode.
All the requirements specified in ASTM D 7511-09e2 are satisfied by OI Analytical’s Flow Solution™ 3700 cyanide analyzer. As shown in Figure 6, the Flow Solution™ 3700 is a modular and compact laboratory instrument.
Figure 6. OI Analytical Flow Solution™ 3700 cyanide analyzer for ASTM D 7511- 09e2.
Request more information about the Flow Solution™ 3700 cyanide analyzer
Due to the occurrence of matrix interference such as sulfide, the chemical composition of sour wastewater makes cyanide analysis rather difficult. Analytical methods involving colorimetry and an acid distillation step are often affected by this sulfide interference.
ASTM D 7511-09e2 is a method approved by the U.S. EPA for the total cyanide analysis of NPDES wastewater samples. ASTM D 7511-09e2 inhibits the interference usually experienced in other methods while analyzing sour wastewater samples having sulfide concentrations of < 50 mg/L for cyanide content.
This information has been sourced, reviewed and adapted from materials provided by OI Analytical.
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