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Feb-2018

Three ways to effectively monitor total chlorine in liquid hydrocarbons

Corrosion at petroleum refineries is a critical problem all over the world. In the United States alone, NACE International, a professional association dedicated to reducing the economic impact of corrosion, has shown that $3.7 billion in direct costs are realised annually from maintenance, vessel expenditures, and fouling as a result of corrosion.1

Kyle Kuwitzky and Leslie Johnson
XOS
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Article Summary
This huge figure does not take into account the estimated loss of as much as $12 billion in profits due to decreased capacity, unit outages, and premature turnarounds that result from corrosion.2

Challenge
Chlorine in crude oil, if not removed, can hydrolyse during processing to form hydrochloric acid. The crude oil desalter is the first line of defence in the prevention of corrosion, but in order to provide a proper defence, an effective chloride monitoring solution must be implemented. Many refiners rely on semi-periodic testing of inorganic chlorides to get the job done. However, what if a desalter upset occurs in between testing periods? Worse yet, what if an organic chlorine slug is present in the incoming crude? The desalter will remove only the extractable inorganic chlorides, not organic chlorides, and any chlorides that pass through the desalter have the potential of causing fouling and corrosion issues.

Even refiners who don’t rely on semi-periodic testing typically only monitor inorganic chlorine. While this is very important for desalter efficiency, it does not capture all threats.

Introduction
Since its launch in 2007, Clora® Benchtop Analyser has been widely adopted by refineries and test labs for monitoring chlorine in petroleum. With over 200 systems in the field, XOS customers know they can trust Clora for testing products from crude oil to naphtha cuts and VGO.

In this paper, we will discuss the most popular Clora procedures utilised by petroleum laboratories around the world to measure total chlorine for corrosion mitigation:
• Organic Chlorides by ASTM D4929
• Total Chlorine by Modified ASTM D7536 with Accu-flow
• Organic and Inorganic Chlorides Using Water Extraction

Organic chlorides by ASTM D4929
On October 15, 2017, ASTM approved the addition of Procedure C to D4929 Standard Test Method for Determination of Organic Chloride Content in Crude Oil. Procedure C uses X-ray Fluorescence (XRF) to determine the chlorine content in the crude oil naphtha fraction. This is an important step forward, as users will be able to officially use XOS’ Clora, Clora 2XP, and Sindie +Cl analysers for organic chloride analysis in crude oil without ASTM method modification.

Traditionally, users of D4929 distill a crude oil sample to 400°F, wash the resulting naphtha fraction by caustic to remove H2S and then water to remove inorganic chlorides, and then use 
Procedure A or Procedure B to determine chloride content. Chloride content of the crude is then determined by back calculation. Procedure A determines chloride content by sodium biphenyl reduction followed by potentiometry, and Procedure B uses combustion and microcoulometry to determine chloride content in the naphtha fraction.

Procedure C test method precision was determined by an interlaboratory study (ILS) designed to replicate the original D4929 Precision A and B ILS. Eight ILS participants distilled ten crude oil blends of varying nominal organic chloride concentrations in blind duplicate, and the resultant naphtha cuts were washed and analysed by multiple XRF techniques. The Procedure C precision varied by XRF type, and as a result there are separate precision statements for MWDXRF, MEDXRF, and EDXRF. Figures 1 and 2 compare the calculated precision statements obtained from the Procedure C study with the original Procedure A and B precision statements.

Figure 1 illustrates the calculated reproducibility for all D4929 procedures using the published precision statements in D4929. It demonstrates that Clora (MWDXRF) has better reproducibility than the other Procedure 
C XRF techniques and exhibits equivalent or better reproducibility than Procedure B (microcoulometry). In a recent review of the ASTM crude oil proficiency testing program, Procedure B is the most commonly used procedure for organic chloride analysis, though this may change with the addition of Procedure C to D4929. Figure 2 illustrates the calculated repeatability for all of the D4929 procedures, and Procedure C MEDXRF and MWDXRF (Clora) consistently exhibit better repeatability than 
Procedure B.

The addition of Procedure C to D4929 provides a precise, easy to use alternative to Procedures A and B. XOS’ Clora, Clora 2XP, and Sindie +Cl analysers comply with D4929 Procedure C.
 
Total chlorine by modified astm D7536 
with Accu-flow
X-ray Fluorescence (XRF) spectrometry, while widely used for total sulphur testing, has traditionally been an ineffective tool for the direct measurement of total chlorides in crude oil due to the settling effects of inorganic chloride salts during the measurement process. This settling leads to poor precision and an industry preference for other measurement or sample extraction techniques.

To address the effects of particulate settling, XOS introduced a continuous sample flow option to their Monochromatic Wavelength Dispersive XRF (MWDXRF) benchtop chlorine spectrometer: Clora with Accu-flow. The Accu-flow process uses a stepper motor for continuous injection of crude oil during the sample measurement, eliminating the settling of 
inorganic chloride salts. This process allows for the direct measurement of total chlorine without water wash extraction or sample distillation, as in ASTM D4929, and delivers results in just minutes.

The Accu-flow design works for crude oils with a maximum viscosity of 2000 cSt at 70°F. A 60 ml crude oil sample is injected at a 20 ml/min flow rate into the yellow tubing shown in Figure 3. The sample flows from the yellow tubing through the Accu-flow insert assembly, where the sample is exposed to X-rays. The sample then flows through the black drain tube and is collected outside the analyser. Typical measurement time is three minutes.

Depending on the crude viscosity and rate of inorganic chloride settling, the difference between static and Accu-flow measurement results can be quite dramatic. In the following examples, four samples of two crude oils have been run either statically or using Accu-flow for a three minute measurement time. In order to illustrate the behaviour of chlorine during the measurement, 30-
second data points have been taken within the three minute measurement interval and graphed accordingly. Figures 4 and 5 show the static measurement increasing over time, leading to an inaccurate total chlorine measurement.

However, the Accu-flow measurement remains stable throughout the measurement period and is repeatable over multiple measurements, as seen in Table 1.

Organic and inorganic chlorides using 
water extraction
Although all XRF techniques are capable of only total elemental analysis, with some sample preparation, Clora, Clora with Accu-flow, Clora 2XP, and Sindie +Cl can also be used to characterise inorganic and organic chlorides in crude oil. Using a hot water extraction, crude oil may be separated into its organic chloride and inorganic chloride constituents with the organic chlorides staying in the crude oil layer and the inorganic chlorides precipitating into the water layer. Clora can then be used to measure each layer to determine organic and inorganic chlorides. While many laboratories have successfully used this sample preparation technique, unfortunately, this technique has crude-dependent limitations. Not all crude oils are easily extracted and this sample preparation technique has variable within-lab repeatability and poor between-lab reproducibility.
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