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Analytical Instrumentation
Determination of Mercury in Crude Oil
Oliver Büttel, Sören Joester, Analytik Jena AG Jena/ Germany,
www.analytik-jena.com,
info@analytik-jena.com
Besides other heavy metal, crude oil also contains mercury in varying mass fractions. When refining the crude oil, the mercury must be removed to prevent contamination of the resulting products. A precise knowledge of the heavy metal content is decisive for the control of the refinement processes. In addition to the crude oil, the waste products of oil refinement, such as sludge, also contain mercury. Prior to disposing the refinement waste, it must also be examined for mercury.
The mercury concentration in crude oil is usually in the lower mg/kg range or below, therefore a highly sensitive measuring method is required. Refinement waste, on the other hand, can contain several percentage points of mercury, making a flexible analysis system necessary, which can analyse both very small and clearly increased concentrations, without significant carry-over effects. Usually cold vapor AAS or AFS is used, with atomic fluorescence (AFS) being characterised by lower detection limits and a clearly larger dynamic range. Both methods can be combined with an enrichment of the mercury by amalgamation in a gold net to further increase the measuring sensitivity. Due to the wider analysable concentration range, cold vapor AFS is the preferred method.
Common methods of heavy metal determination, such as AAS, AFS and ICP OES, usually process samples as aqueous solutions, oil samples must be mineralized prior to analysis. A simple dilution of the samples in a low viscosity solvent is usually not possible because of the heterogeneous nature of the samples. The particle- bonded element parts would be lost by sedimentation. Various standardized methods are available for mineralising oil samples; e.g., by ASTM. These are usually dry or wet incineration methods carried out in open crucibles. They are either carried out on a heating plate or in the muffle furnace. Alternatively, the samples can be mineralised in a microwave- assisted pressure digestion.
Whilst open digestion methods provide satisfactory results for numerous elements, they are unsuitable for volatile mercury. There is also the danger that the sample is contaminated by dust and thus distorts the measurements. Therefore, digestion in a sealed pressure vessel is the most reliable method of sample preparation.
Sample Preparation
Both the loss of volatile species and the contamination of the sample are largely precluded when using sealed pressure vessels. This is evident in table 1 where the recovery rates for mercury in certified reference
Table 1: Recovery rate (WFR) for mercury using different digestion methods ASTM D 5863-00a
(muffle furnace) WFR [%]
Kerosene (approx. 10 mg/kg)
Paraffin-base oil (approx. 10 mg/kg)
Refined oil 20 cSt (approx. 10 mg/kg)
Conostan standard (100 mg/kg) Naphtha 1 Naphtha 2 Naphtha 3 Heavy oil 1 Heavy oil 2
2,0 2,2 2,1
- - - - - -
materials or spiked real samples are compared by various digestion methods. Both open methods apparently lead to significant analyte losses. Only in the pressure vessel can the samples be digested without significant losses.
Microwave-heated pressure digestion also allows a precise reaction control through power output, controlled by temperature and pressure. Oil samples can contain very reactive components that can already react exothermically at a low temperature. To keep such reactions under control, a precise monitoring of the individual digestion vessels is useful to adjust the heating output in accordance with the progression of the reaction. The digestion duration is also reduced significantly compared to open methods.
These and other samples were digested in the microwave system TOPwave (figure 1), equipped with the high performance vessels CX 100 (100 mL, max. 300°C, max. 100 bar). For this a mixture of 6 mL nitric acid, 2 mL hydrochloric acid and 2 mL hydrogen peroxide was used together with the temperature program shown in table 2. The program contains two steps at low temperature to digest reactive sample components. The stable components are then mineralised at high temperature. The sample weight was 100 - 200 mg in each case.
Table 2: Temperature program for microwave digestion Step
1
Temperature [°C] Max. pressure [bar] Max. output [%] Heat ramp [min] Hold time [min]
Figure 1: Microwave digestion system TOPwave
130 80 70 10 10
2
160 80 70 10 10
3
230 100 100 10 30
The system features a contact-free temperature and - - -
54 - - - - -
98 91 99 -
88 91 92 94 98
Open wet incineration (heating plate) WFR [%]
Microwave
pressure digestion WFR [%]
Figure 2: Cold vapor atomic fluorescence spectrometer mercur
pressure measurement individually for each digestion vessel. Because every sample can be monitored separately, comprehensive reaction control is possible. Because the measuring methods work using optical methods and without sensors in the digestion vessel, handling the vessels is very easy and due to the use of optical measuring principles there are also not wearing parts.
When using digestion vessels from PTFE there is the occasional risk of adsorption of mercury in the pores of the material. The use of quartz vessels is therefore recommended. This is not necessary for digestion vessels of Analytik Jena, because the PTFE-TFM material is processed using a patented method drastically reducing the porosity and related analyte adsorption. This is evidenced by the very good recovery rates for mercury.
June/July 2010
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