Experimental study on diesel fuel haziness

Understanding the exact reasons behind haziness in diesel fuel facilitates quality control strategies.

Dhiraj Gondaliya, Murthy Nelakanti, Kinjal Patel and Shailesh Gadhvi
Nayara Energy Ltd Research & Development Centre

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Article Summary

Nayara Energy Limited operates a 20 million metric tonnes per annum (MMTPA) petroleum refinery and produces valuable fuels, including liquefied petroleum gas (LPG), naphtha, motor spirit (petrol), kerosene, aviation turbine fuel (ATF), and diesel. The refinery can produce around 30 kilo tonne per day (KTPD) ultra low-sulphur diesel (ULSD) products. The diesel hydrodesulphurisation unit (DHDS), diesel hydrotreater (DHDT), and vacuum gasoil mild hydrocracker (VGOMHC), all catalytic conversion processes, are primarily used to produce low-sulphur diesel fuel.Steam is added after hydrotreatment to strip out the hydrogen sulphide (H2S) and ammonia generated during the process. Hence, the saturated moisture will always remain with hydrotreated diesel fuel. The hydrotreatment units are provided with a techno-vacuum dryer or a coalescer facility to remove moisture from diesel and improve haziness.

Test method ASTM D4176, procedure-1 (P-1) was used to measure appearance per the specification for rundown and final diesel fuel certification. It was observed that diesel products from the unit and rundown samples intermittently had a hazy appearance, which could lead to failure of the final diesel product specification during certification. Hence, an experimental study was conducted to evaluate the problem.

Improvement opportunities
The haziness and moisture content were monitored regularly as per scheduled frequency. The detected moisture increased but remained within the acceptable limit of specification, which is a maximum of 200 ppm. The hazy appearance of diesel improved upon increasing retention time, and the product became clear after some time. However, it was an alarming situation that could worsen if the problem was further aggravated, hampering diesel production.

After a period, haziness was observed even after utilising the techno vacuum dryer and coalescer in sequence. An upgrade to the existing system was discussed, which could require high capital investments. Hence, there was an opportunity to study the haziness; accordingly, an upgrade could be planned to address the problem.

Nayara’s Research and Development team conducted comprehensive studies and a literature review to support and resolve the diesel haziness problem. Theoretically, it may be due to the inherent properties of the surfactant type of molecules in the fuel or to moisture present in fine aerosol form.

Steam is used as part of the process; hence, moisture comes from the diesel product, which is removed by the techno vacuum dryer and/or coalescer. The following aspects were studied during experimental work to understand the exact reason behind the haziness:
• Techno vacuum dryer operation simulation study at the laboratory
• Measurement of haziness by various test methods
• Stability of haziness
• Impact of sample storage container on haziness
• Moisture content vs haziness correlation study
• Haze rating correlation between two test methods
• Removal of haziness by alternate techniques.

Techno vacuum dryer operation simulation study
The moisture content and haziness are correlated properties of diesel fuel. The moisture content is controlled by optimising the dryer operational parameters such as vacuum, temperature, and residence time. A bench-top diesel vacuum dryer was developed in the laboratory using a glass assembly with a vacuum pump and heating mantle to study the dryer’s operation performance parameters (see Figure 1). The dryer is operated in batches and can control and study the effect of vacuum, temperature, and residence time.
υ Diesel sample: Since the product diesel sample was not showing enough haziness for study, the techno vacuum dryer inlet samples were collected from the DHDS unit for vacuum dryer simulation study at the laboratory.
ϖ Experiment 1, Analysis of dryer inlet sample:
Aim: To analyse moisture and haziness of dryer inlet samples.
Exp. conditions: The dryer inlet samples were collected from the DHDS unit on different dates and cooled to room temperature. They were then tested for moisture by ISO 12937 and appearance by ASTM D4176, P-1 (haziness visually).
Observations: The dryer inlet diesel samples were always hazy, and moisture content varied widely between 172 and 591 wtppm. The test results were as expected.

ω Experiment 2, Simulation study of vacuum dryer:
Aim: To simulate the vacuum dryer operation and study the effect of vacuum, temperature, and residence time on moisture content and haziness of diesel sample. The test results of this experiment are useful to identify whether haziness is due to moisture or surfactant molecules. Also, having access to operational parameters will be beneficial in order to optimise the performance of the techno vacuum dryer.
Exp. conditions: For this experimental work, hazy diesel samples from the dryer inlet were used for the simulation study, and experiments were conducted using different conditions of vacuum level, temperature, and residence time. The experimental conditions and test results after sample cooling to room temperature are listed in Table 1. Figure 2 shows the feed and product samples of the simulation study. Observations:
• Experiment Sr. No.1 was conducted with various residence times at a constant temperature and vacuum level. The test results of change in residence time indicate that moisture content is reduced upon increased residence time and haziness is improved. The haziness cleared at more than five minutes’ residence time with 100 torr vacuum and 100°C temperature.
• Experiment Sr. No.2 was conducted at various temperatures with constant residence time and vacuum level. The test results of change in temperature indicate that, upon temperature increase, moisture content is reduced and haziness is improved. The haziness cleared at more than 105°C temperature with 100 torr vacuum and two minutes’ residence time.
• Experiment Sr. No.3 was conducted with various vacuum levels, constant temperature, and residence time. The test results of change in vacuum level indicate that, upon increased vacuum level, moisture content is reduced and haziness is improved. The haziness cleared at less than 90 torr with 100°C temperature and two minutes’ residence time.
• Experiments Sr. No.1, 2, and 3 confirm that haziness in diesel study samples is due to free water molecules. Theoretically, if surfactant molecules are present, the haziness of the diesel sample may not clear by changing these physical parameters.

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