Jul-2013

Combating green oil formation in a CCR reformer

The addition of a chloride adsorber guard bed solved a refiner’s issues with contamination affecting a CCR reformer’s rich gas compressor

OSMAN KUBILAY KARAN, MEHMET ASIM AY and KORAY KAHRAMAN, Tüpra¸s Kirikkale refinery
ARNAUD SELMEN, Axens Technology & Technical Services

Article Summary

CCR reformer units need chloride injection during regeneration to promote platinum dispersion and to restore catalyst acidity. This leads to hydrogen chloride formation, and a highly viscous green oil may also be formed at some locations in the unit. This is one of the main reasons why the operation of a CCR reformer’s rich gas compressors can affect unit reliability. This article looks at a real-world example of green oil formation, and shows how the main cause was identified and then overcome.

When looking for a high rate of unit utilisation in a refinery, the availability and reliability of all equipment is of utmost importance to prevent loss of production and negative effects on profitability. So a refiner should either perform proper maintenance or find a way to deal with any kind of problem that threatens unit reliability.

Catalyst deactivation
It has been known for many years that acid gases are present in the petroleum industries in liquid or gas streams. These gases include hydrogen halides such as HCl, HF, HBr, HI and mixtures thereof. From an acid gas point of view, one of the key processes of the petroleum industry are reforming reactions such as those in CCR reformer units. In the catalytic reforming process, sweet heavy naphtha is processed in contact with a platinium-based catalyst to produce a high-octane product. Hydrogen is a byproduct of the catalytic reforming process and some of this product is recycled to the reaction section to maintain catalyst stability. This reforming catalyst is promoted with chloride in the presence of water, resulting in the production of hydrogen chloride. Thus, the gas that is not recycled but sent to downstream catalytic processes and known as net gas contains hydrogen chloride. As a result, this chloride-containing gas can deactivate downstream catalysis because it can poison catalysts and cause undesired reactions.

Even the presence of a small amount of HCl in the net hydrogen gas can seriously interfere with the operation of downstream processes that use hydrogen. It can also cause corrosion problems in equipment such as pipes, valves and compressors. In addition, the formation of polymerised long-chain hydrocarbons, generally called green oils, is a common problem in CCR reformer units.

Green oils are actually the end products of undesirable polymerisation reactions taking place over the catalyst surface area, in which the reaction of HCl with hydrocarbons leads to chlorinated hydrocarbons. The presence of HCl will promote olefin polymerisation reactions with green oil downstream of the reaction section. These reactions are mainly chemical combinations of relatively small molecules with huge chain-like or network-structured molecules. Polymerised molecules formed in this fashion have complex multi-chain chemistries and high boiling points, and are typically waxy in nature. These molecules are green or red in colour and contain mainly C6-C18 hydrocarbons, with a potential tail above C40, and are believed to be oligomers of light olefinic hydrocarbons, with some aromatic nuclei included in the structures. HCl in gas or liquid hydrocarbon streams must be removed, since it may cause undesired catalytic reactions and poison the catalyst systems of the downstream units. Moreover, HCl is considered a hazardous material, so the release of this substance to the environment must be avoided.

Chlorine source
For the time being, the exact mechanism of green oil formation is unknown, but it is believed that it is formed by the catalytic reaction of HCl with hydrocarbons, which leads to chlorinated hydrocarbons. Classically, a chlorination agent is injected during catalyst regeneration in the oxychlorination part of the regenerator to restore the optimal metallic phase dispersion of the platinum-based catalyst and to restore a normal chlorine content of 0.9-1.1 wt% on the catalyst. This leads typically to a recycle gas chlorine content of 1 ppmwt with a water content of less than 30 ppm. The HCl content of the recycle gas is kept under control, but hydrogen gas from the reduction section is also a chlorine source and both streams enter the net gas booster compressor section. The main chlorine contributor in the net gas comes from the reduction gas, which leads to exacerbated issues of green oil formation.

Tüpra¸s Kirikkale refinery’s CCR reformer unit is licensed by Axens and was put into operation in 2008. The basic flow scheme is shown in Figure 1. Shortly after unit startup, the CCR reformer’s H2-rich gas compressor was encountering frequent emergency shutdowns. These shutdowns were initiated by serious vibrations threatening the operation and reliability of the compressor. When it was dismantled for investigation, some deposits were found (see Figures 2a-e).

The findings of this investigation included:
• Dark green oil was adhering to the bottom of the first suction snubber
• Liquid oil was found in the third-stage cylinder, and the valve plate was covered with coked hydrocarbon. Part of the valve ring was damaged
• When draining the suction line of the third stage, a small amount of green and yellow oil was found. This oil was very sticky and gum-like.

For analysis purposes, the sticky oil samples were sent to an accredited laboratory. Inductively coupled analysis (ICP) was carried out because there was not enough sample available for X-ray fluorescence (XRF) analysis. The dominant metal in the fouling material is iron at 8 wt% content. This suggests that the residue of the particulate analysis is mainly iron oxides. The iron can be present in the sample as metal particles or as Fe2O3. During analysis, metallic iron is oxidised so a distinction cannot be made.

Laboratory analysis
The laboratory reported that “the sample from the CCR reformer compressor contains 73 wt % C and 10 wt % H. No sulphur or nitrogen is detected. Next to that, several metals were detected, with iron being the most dominant at 8 wt%. The molar H/C ratio is 1.64, indicating very likely an olefin structure together with an aromatic nucleus. There are no signs of oxygen present in the sample. Approximately 8 wt% of the elemental contents cannot be explained, but based on the process this is possibly organically bonded chloride. However, this needs to be confirmed with a different analysis, because the current results are not conclusive. Further structural detailed information can be obtained by means of GC-MS.”

The same kind of liquid was found in a Korean refinery complex designed by Axens, and the problem was solved by the installation of a chloride adsorber at the outlet line of the reduction chamber.