Application Note · multi N/C® 2100S, multi N/C® 3100

TOC Determination in Brine Samples Comingfrom Desalting Process of Crude OilChallenge
To improve stability of resultsand prolong maintenance cycles
for TOC analysis in high salinesamples.

Solution
Reduction of maintenanceissues by optimization of thecombustion process, TOC method
settings and sequence setup.

Introduction
Crude oil always contains salt, either dissolved in water droplets or in form ofcrystalline salt. Since salt contents can cause problems by forming precipitates,
accelerating corrosion processes or catalyst degradation during the refining
process, crude oil is undergoing a desalting process. By-product of this process iswater of high salt concentrations up to 26 % of mostly NaCl (smaller amounts of
CaCL2 and MgCL2), so-called brine. Brine, as a product of petrochemical industry,is a starting material in other industries, e.g. as refrigerating fluid, in water
purification or de-icing processes. One of the quality criteria for the further useof brine is its TOC content. Analysis of brine for TOC content is an important,however challenging task. Quartz components of the TOC analyzer as well as
the expensive platinum catalyst are exposed to the destructive influence of thisheavily salt loaded water sample. The common TOC analysis technique uses anoxidation process at 800 °C. At this temperature the melting of NaCl takes place.
Salt deposits inside the combustion tube lead to a devitrification of quartz, a fast
consumption of the platinum catalyst and gas flow blockages, thus causing highmaintenance costs.
To overcome these difficulties the common technique and method have been
improved. An optimized construction of the combustion tube, the use of a special
catalyst filling and a reduced combustion temperature (750 °C) proved to deliver
reliable analysis results, improve long-term stability and prolong the lifetime ofthe components.

2Total TOC Determination in Brine Samples Coming from Desalting Process of Crude Oil

Instrumentation
The analysis has been run with the multi N/C® 3100 TOC analyzer using the non purgeable organic carbon mode (NPOC).
The samples have been diluted 1:10 to achieve best analytical performance for this sample matrix.
Table 1: Analyzer specifications: multi N/C® 2100S and mutli N/C® 3100
Parameter

Specification Analyzer

Parameter

TC, TOC, NPOC, POC, TIC and TNbNorm

DIN EN 1484, DIN EN 12260

Detection

Focus Radiation NDIR, Chemolumineszenz Detection (CLD) or electrochemical detection (ChD)

Digestion

Thermocatalytic up to 950 ° C

Sample feeding

Direct injection/ flow injection

User confidence and comfort

multiWin® control and evaluation software, VITA® technology, self-check system, etc.

Measuring range

multi N/C® 2100S: Carbon: 50 ppb-30000 ppm,multi N/C® 3100 Carbon: 4 ppb-30000 ppmNitrogen: 5 ppb-200 ppm (CLD) or 50 ppb-100 ppm (ChD)

Injection volume

multi N/C® 2100: 50- 500 µl
multi N/C® 3100: 100-1000 µl

Measuring timeappr. 3-5 minAutosampler

AS 60 (8 ml or 2 ml vials) or AS Vario (40 ml, 20 ml or 12 ml vials)

Gas supply

Oxygen 4.5 or better, alternatively: HC- and CO2-free synthetic air

Method settings
The following method settings were used to determine the TOC content:
Table 2: Method settings
Parameter

multi N/C® 2100S, multi N/C® 3100

Measurement parameters

NPOCDigestion

High temperature digestion at 750 ° C with platinum catalyst

Number of single repetitions

min.2, max. 3

Rinse with sample before injection

3 times

Injektion volume200 µlDilution

1:10

Total TOC Determination in Brine Samples Coming from Desalting Process of Crude Oil

Samples and Reagents
Prior to analysis the sample is automatically acidified with 2 M HCl and subsequently purged with synthetic air/pureoxygen. This ensures a complete removal of the TIC. The subsequent NPOC measurement is carried out with the aid of anautosampler, as described in table 1.
The method setup was optimized by reduction of the sample injection volume to max. 200 µl (instead of 500 µl for standardapplications). This reduces the salt load onto the combustion tube siginificantly, while maintaining a very good TOC
measurement sensitivity. During the oxidation process all carbon compounds are quantitatively converted to CO2.
The wide-range Focus Radiation NDIR detector is used for the determination of the carbon content.
The measurement sequence was supported with the auto sampler AS Vario, which provides parallel purge and analyzing forhighest sample throughput and operation comfort.
This application can be performed in the same way using a multi N/C® 2100S with autosampler AS 60 in sequential purge
and analyzing mode.
Calibration
The multi N/C® analyzer was calibrated for NPOC measurement in the range 0.5 - 10 mg/l C with standard solutions ofpotassium hydrogen phthalate. A multi-point calibration type was used. The calibration curve and its characteristics arepresented in the figure 1 below.

Calibration Curves

Figure 1: NPOC calibration curve and characteristics,
range 0.5-10 mg/L

3

4Total TOC Determination in Brine Samples Coming from Desalting Process of Crude Oil

Results and Discussion
Two samples and one NPOC standard were measured for carbon content. The sequence was set up to have an ultra purewater sample after every 5th sample measured to regularly clean the combustion tube from excessive salt deposits. Eachsample has been measured at minimum by triplicate injection. The analysis results are summarized in table 3. Examples ofthe measured curves are shown in figure 2 and 3 below.
Table 3: Results
Sample ID

Dilution ratio

Mean value NPOC [mg/L]

NPOC RSD [%]Sample A1:10

23.7

1.3

Sample B

1:10

7.9

1.8

TOC control 2.0 mg/l

no2.1

0.7

Figure 2: Sample A – Example of analysis curve

Figure 3: Sample B – Example of analysis curve

The long term stability for the method applied – crucial for extreme matrices with high salt loads – was evaluated as well.
A synthetic brine solution, prepared from sodium chloride and ultra-pure water, was spiked with an NPOC standard to have a2.0 mg/l TOC concentration in the 1:10 dilution. After every 5th brine sample ultra-pure water was inserted into the sequence
for rinsing purpose. Almost 800 injections have been run without loss in recovery rates, see figure 4, and without any
maintenance. The results have been within the normally expected range of ± 5 % deviation from the nominal concentration.

Total TOC Determination in Brine Samples Coming from Desalting Process of Crude Oil

Figure 4: Recovery rates of the 2.0 mg/l NPOC standard material

Figure 5: New filling of thecombustion tube (hightemperature mat, Pt-catalyst andPt-grid on the bottom)

Figure 6: Combustion tube after800 brine injections (saltdeposits on the colder exit partof the tube)

Figure 7: Re-dissolving the salt
crystals by immersion into
ultra-pure water

The above pictures 5-7 show the salt throughput capability of the combustion tube used. Salt aerosols are effectivelytransported through the combustion tube filling to be condensed and washed off partially at the colder exit part of thecombustion tube. This effectively avoids gas blockages due to salt deposits piling up inside the filling, thus providing long
maintenance cycles.

Conclusion
The multi N/C® 3100 and multi N/C® 2100S with specially optimized method settings and sequence setup can be applied for
the TOC measurement of most challenging brine samples. This application successfully demonstrates that high salt loads do
not compromise analysis results or the long term stability.
The use of an autosampler, AS Vario enables a high sample throughput and provides comfortable and fast measurements in
NPOC mode with automatic acidification and sample purge in “parallel purge & analysis mode”.

Headquarters

Analytik Jena AG
Konrad-Zuse-Strasse 107745 Jena · Germany

PhoneFax+49 36 41 77 70+49 36 41 77 9279

info@analytik-jena.com
www.analytik-jena.com

en · 08/2017© Analytik Jena AG | Picture p.1 © Istock/Baton725