The petrochemical industry generates vast amounts of wastewater or effluent, which must be cleaned before they may be reused or discharged into natural waterways.
Total organic carbon (TOC) and total nitrogen bound (TNb) concentrations are frequently measured because these contaminants contribute to the eutrophication of surface water resources, which affects aquatic life and groundwater supplies.
The European Industrial Emissions Directive requires the implementation of Best Available Techniques (BAT) in the EU for direct wastewater discharges from mineral oil and gas refineries.1,2
The BAT reference document (BREF) states that, among other characteristics, TOC and TNb are becoming increasingly important. They should be monitored daily. TOC is preferred over chemical oxygen demand (COD) because it avoids using very harmful chemicals like dichromate (Cr VI) and mercury.
In many circumstances, the COD and TN contents are measured separately. This is a laborious and time-consuming process that is frequently related to the production of chromium-VI-contaminated trash.
Correlation studies can help create an empirical conversion factor for TOC to COD conversions. As a result, a completely automated analytical procedure for TOC/TNb determination based on DIN EN 1484 and DIN EN 12260 (or DIN EN ISO 20236 for both parameters) can be used to save resources and time.3,4,5
Effluent waters from refining processes typically demand samples for TOC/TNb analyzers to evaluate because the tubing and valve mechanism used to feed the sample into the combustion process is prone to carryover.
Direct injection, as used in the multi N/C 2300 with a septum-free injection port and a wide-bore needle for optimal particle handling, can solve this problem.
Direct injection guarantees particle-free sample transmission and prevents clogs, boosting system uptime while decreasing wear and tear on delicate Teflon parts within the dosing system.
Since the injection needle remains in the hot furnace inlet zone during analysis time, it is thoroughly thermally cleaned before injecting the next sample, ensuring no carry-over occurs. The multi N/C 2300 provides a reliable option for TOC/TNb analysis of particulate or greasy samples.
Materials and Methods
Samples and Reagents
Samples from several process streams and clean-up phases were collected and analyzed alongside a reference standard. 2 M HCl was used to automatically acidify samples to a pH < 2.
Sample Preparation and Measurement
The samples were kept in a refrigerator at 4 °C until analysis. For measurement, they were placed in appropriate autosampler vials. The wastewater samples were directly tested using the NPOC/TN technique.
During the autosampler run, the samples were automatically acidified with 2 M HCl and then purged for five minutes to remove any TIC. The injection volume for these measurement sequences was 250 µL.
The samples were catalytically oxidized at 800 °C in an oxygen-rich environment. A combustion tube containing a platinum catalyst was utilized to completely oxidize the material. The generated nitrogen oxides were identified using a chemiluminescence detector, however, a ChD detector can alternatively be used.
CO2 was detected using focal radiation non-dispersive infrared detection.
Calibration
For TOC determination, the multi-N/C analyzer was calibrated using a potassium hydrogen phthalate reference solution ranging from 1 to 500 mg/L. The NPOC measurement data were evaluated using a multipoint calibration.
The total bound nitrogen was calibrated from 1 to 50 mg/L using an ammonium sulfate and potassium nitrate (50:50 mix) solution under DIN EN 12260.
Within the approach, up to three calibration ranges can be assigned to each parameter, providing a total working range of up to three magnitudes. The detection and quantification limits depend on the working range chosen and can be calculated using the method features listed above.
Figures 1 and 2. Example of NPOC and TNb calibration with method parameters. Image Credit: Analytik Jena US
Method Parameters
TNb contents were determined using the following technique parameters:
Table 1. Method parameters. Source: Analytik Jena US
| Parameter |
multi N/C 2300 |
| Measurement parameter |
NPOC / TNb |
| Digestion |
High temperature digestion at 800° C with platinum catalyst |
| Number of repetitions |
min. 3, max. 4 |
| Rinse with sample before injection |
3 times |
| Sample purge time |
300 sec. |
| Injection volume |
250 μl |
Results and Discussion
The table below displays the mean values of three replicate injections with relative standard deviations for various genuine samples (anonymized) and nicotinic acid recoveries as TOC and TNb reference solutions.
According to the Best Available Techniques (BAT) reference document released under the Industrial Emission Directive 2010/75/EU2, the corresponding average emission levels (BAT-AEL) for direct wastewater discharges from refining processes can be expected to be in the following ranges:
COD: 30–125 mg/L equals to: TOC: 7–32 mg/L, TNb: 1–25 mg/L
Figure 3. Example of a TOC and TNb measurement curve for sample 3. Image Credit: Analytik Jena US
Table 2. Results. Source: Analytik Jena US
| Sample ID |
NPOC
Average [mg/L] |
TOC
RSD [%] |
TNb
[mg/L] |
TNb
RSD [%] |
| Sample 1 |
26.2 |
2.1 |
19.5 |
1.4 |
| Sample 2 |
161 |
0.9 |
41.3 |
1.1 |
| Sample 3 |
12.9 |
1.2 |
5.38 |
0.8 |
Check Standard Nicotinic Acid
(TOC 20.0 / TNb 3.88) |
19.8 |
0.6 |
4.06 |
0.7 |
Summary
The measurements included undiluted wastewater samples collected at various points throughout the wastewater treatment process and samples with varied TOC and TNb levels. All samples were measured with exceptional accuracy and precision.
Nicotinic acid was employed as an analytical quality assurance standard to assess TOC and TNb recoveries. The reference material for organically bound nitrogen showed excellent recovery rates.
The remarkable performance of multi-N/C analyzers for such challenging wastewater matrices is based on an optimized combustion procedure with freely configurable combustion temperatures of up to 950 °C.
Direct injection with a septum-free pneumatic injection head in combination with a wide-bore needle of 0.7 mm inner diameter, as well as proper sample homogenization on the autosampler rack and valve- and tubing-free sample transfer into the combustion system, all contribute to improved performance.
An operation mode that keeps the stainless-steel injection needle in the oven head at elevated temperatures during peak integration time to ensure complete evaporation of TOC components and a clean needle for further sample processing, combined with effective rinsing of the microliter injection syringe, reduces carry-over effects.
The high level of automation provided by the AS 60 autosampler and well-proven Self Check System for trouble-free unattended system operation makes TOC/TNb studies simple, even in tough samples.
The proprietary VITA flow-management technology compensates for flow changes inside the system induced by sample evaporation, ensuring TOC calibration stability for up to a year and saving crucial measurement time.
Figure 4. Multi N/C 2300. Image Credit: Analytik Jena US
References
- DIRECTIVE 2010/75/EU OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 24 November 2010 on industrial emissions (integrated pollution prevention and control)
- Official Journal of the European Union, L 307/38, 28.10.2014, Commission Implementing Decision of October 9, 2014 “Establishing best available techniques (BAT) conclusions, under Directive 2010/75/EU of the European Parliament and the Council on industrial emissions, for the refining of mineral oil and gas”
- DIN EN 1484 Water analysis – Guidelines for the determination of total organic carbon (TOC) and dissolved organic carbon (DOC)
- DIN EN 12260 Water quality – Determination of nitrogen Determination of bound nitrogen (TNb ) following oxidation to nitrogen oxides
- DIN EN ISO 20236: Water quality — Determination of total organic carbon (TOC), dissolved organic carbon (DOC) total bound nitrogen (TNb), and dissolved bound nitrogen (DNb ) after high temperature catalytic oxidative combustion
This information has been sourced, reviewed, and adapted from materials provided by Analytik Jena US.
For more information on this source, please visit Analytik Jena US.