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Apr-2008

Thermodynamic model of sediment deposition in the LC-Fining process

The LC-Fining ebullated bed hydrocracking process is used to hydrocrack residuum. In this process, sediment deposition in the downstream equipment sometimes affects the overall economics by limiting the operating conversion, even though the upstream reactor system is capable of achieving much higher conversion levels

K M Sundaram, Lummus Technology
U Mukherjee and M Baldassari, Chevron Lummus Global
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Article Summary
The sediment is measured by SHFT value (Shell Hot Filtration Test method), which mainly measures that portion of the asphaltene that is insoluble in heavy oil at specific laboratory test conditions. A solubility-theory-based thermodynamic model was developed to predict the SHFT value. According to this model, asphaltenes are in equilibrium (in solution) with the surrounding fluid. The model parameters are asphaltene molecular weight and its solubility parameter. Both are related. Solubility parameter is modelled as a function of the hydrogen-to-carbon (H/C) atomic ratio of the heavy oil. The molecular weights are estimated from pilot plant data by minimising the sum of squares of the residuals between the observed SHFT value and the calculated value for all runs. Asphaltene molecular weight decreases with increasing process severity, indicating that the asphaltene becomes more aromatic by losing side chains. The simple model is able to explain the observed trends with respect to different LC-Fining catalysts, feed sources and diluent effects. Commercial data are compared with the pilot plant data and the model prediction validates the model.

Introduction
With ever-increasing demand for low-sulphur middle distillates and with crude oil prices hovering around $90/barrel, refiners have taken a keen interest in converting vacuum residuum to distillates. The search for Best Available Technology (BAT) has intensified over the last few years because of diminishing supplies of sweet crudes and incremental supplies coming predominantly from heavy sour crudes and synthetic crudes. The traditional outlet for vacuum residue was high-sulphur fuel oil (HSFO), but HSFO demands in most regions have diminished over the last ten years, giving further impetus to residue conversion processes. Various catalytic residue-upgrading technologies are available from Chevron Lummus Global (CLG), including residue desulphurisation technologies atmospheric residue desulphurisation (ARDS), vacuum residue desulphurisation (VRDS), upflow reactor (UFR), online catalyst replacement (OCR) and LC-Fining process (Dahlberg et al, 2007; Mukherjee et al, 2007; Gupta and Brossard, 2007). Lummus Technology also offers delayed coking, shell soaker visbreaking, and RFCC processes (Soni et al, 2003). LC-Fining process integrated with the Isocracking process (CLG’s hydrocracking process) offers a proven high-conversion option (Spieler et al, 2006). The combined process is especially attractive in situations requiring high conversion of residuum with high metals content and where diesel demand is higher than gasoline demand.

In the LC-Fining process a series of ebullated- bed reactors are used. A suitable catalyst such as Ni/Mo extrudate catalyst particles, suspended in the liquid and in the presence of high hydrogen partial pressure (100-180 bar) and at moderate temperature (400-440°C) removes the heteroatoms and the metals while converting 60-80% of the residue. Subsequently, further upgrading can be done with integrated hydrocracking. Figure 1 is a simplified flow diagram of the LC-Fining process. Typically, anywhere from one to three reactors are used for the LC-Fining process and one or two additional fixed-bed reactors for the integrated hydrocracking segment. To keep the catalyst activity high at all times, a small amount of catalyst is removed from the reactors daily and an equivalent quantity of fresh catalyst is added. Usually the feed contains the residuum and a small quantity of an aromatic diluent. Some internal recycle streams are sometimes used.

In its natural state, residua has no or a very low SHFT value. However, it is the conversion process that creates the sediments by disturbing the SARA (saturates, aromatics, resins, asphaltenes) balance. The ebullated-bed reaction system emulates a CSTR. The nature of the feedstock is such that high operating temperatures are required for conversion and a significant amount of thermal cracking occurs in addition to normal hydroconversion. Hydrogen partial pressure and residence time are major variables in aiding hydrogenation and ensuring that the conversion of residuum occurs in an environment where the resins-to-asphaltenes ratio remains in a domain where sedimentation does not occur.

After the reaction section, the products are recovered in the separation section. Since considerable research and development, backed by commercial feedback, has been spent in understanding the variables within the reactor system this article focuses on the other parts of the process that are impacted by sediment deposits. Unless addressed by process parameters and equipment design, the sediment issue can reduce on-stream factor and conversion targets significantly.

The sections most often impacted by sediment deposits are the bottom of the atmospheric and vacuum towers, and in the vacuum tower bottoms product rundown circuit. One such area is highlighted in Figure 1. Unlike the reactor system, where sediment formation can be addressed by increased catalyst addition, sediment formation in equipment such as the vacuum bottoms product rundown exchangers cannot be addressed or even predicted easily.

In commercial application, sediment deposition affects heat transfer and pressure drop. Unfortunately, the physical and chemical properties of the deposits are rarely analysed. Sediment comes in different forms: it can be hard like coke, or in gelatinous, gummy form. In commercial units, CLG has observed different types of sediments at different points in the residue rundown circuit, with the nature of the deposit being a strong function of operating temperature. It has also observed that these deposits can be rendered back into solution at reactor operating temperatures. Currently, the prevalent mechanism for removing sediment deposits from exchanger circuits is by mechanical cleaning or hydrojetting.

Data generation for solubility model
To design the LC-Fining unit for a specific feed, CLG performs detailed pilot plant work and collects the necessary design data. Figure 2 is a simplified sketch of pilot plant. As shown, there are two reactors in series. There is no continuous withdrawal or addition of catalyst in the pilot plant unlike the commercial unit. After a specified decline in activity, a new batch of catalyst is used. The spent catalyst is analysed for metals, coke and other physical properties, and the data are used for kinetic calculations. In a typical pilot plant campaign, all feed and product quantities are measured along with the temperatures and pressures, which permit the calculation of the kinetic parameters for hydroprocessing with a specific catalyst.

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