Downstream Fractionation

There is significant value in separating and isolating the C3-C4 components. These components may be sold as LPG or serve as a valuable feedstock for other petrochemical processes. The FCC gas plant is responsible for the separation of C3-C4 components and stabilization of gasoline. The stabilization of gasoline refers to controlling the amount C4 components present in the product gasoline. 

The overhead vapor from the main fractionation column enters the wet gas compressor train. The vapor leaving the compressor train then enters a high-pressure flash system. The vapor from the high-pressure flash enters the primary absorber. The C5 components leave with the bottom product from the primary absorber. This bottom product re-enters the high-pressure flash. The overhead vapor product enters a sponge oil absorber where it is contacted with LCO drawn 

Rich Sponge oil (returned to main Overhead naphtha frac.) 

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In the process (Figure 9-12), the feedstock (a vacuum residuum) is mixed with recycle vacuum residue from downstream fractionation, hydrogen-rich recycle gas, and fresh hydrogen. This combined stream is fed into the bottom of the reactor whereby the upward flow expands the catalyst bed. The mixed vapor liquid effluent from the reactor, either goes to flash drum for phase separation or the next reactor. A portion of the hydrogen rich gas is recycled to the reactor. The product oil is cooled and stabilized and the vacuum residue portion is recycled to increase conversion. 

BTX (benzene-toluene-xylene) mixtures are an important petroleum refinery stream that is separated by extractive distillation (Fig. 1) from a hydrocarbon stream, usually a reformate, and followed by downstream fractionation for isolation of the pure materials for further treatment and use (Fig. 2). 

Ushio et al. of Nippon Oil (Japan) have described some of their research work undertaken to produce group III stocks via hydrocracking distillates.12 Their process was similar to BP s, employing a fuels hydrocracker to produce the waxy lubes stream(s) by downstream fractionation of the hydrocracker bottoms. Base oil products were stabilized by furfural extraction. 

Description A typical SED unit mainly consists of an extractive distillation column and a solvent recovery column. The hydrocarbon feed is separated into non-aromatics and aromatics products through extractive distillation with the solvent. For the benzene-recovery case, benzene is directly produced from the SED unit. For the benzene and toluene recovery case, pure benzene and pure toluene are produced from the aromatics product of the SED unit through downstream fractionation. 

Figure 4.37 and Figure 4.38 show the process flow diagrams (PFD) for the FCC unit and downstream fractionation units that we will use to buUd the model in question. We extensively discussed the features and operating issues associated with this type unit in Chapter 2. In the context of this chapter, we also build models for the main fractionator and associated gas plant... 

The FCC configuration requires choosing the riser configuration, number and type of regenerators and catalyst configuration. We may also specify the additional downstream fractionation in the form of a simplified main fractionator for the FCC... 

Figure 4.72 shows the converged FCC unit operation window after Aspen HYSYS has successfully solved the model. We connect an effluent stream by bringing up the Connections section of the Design Tab and typing in Effluent for the Reactor Effluent stream. A stream titled Effluent will appear on the FED and we can use this stream to build further downstream fractionation units. 

This completes the cahbration workshop for the FCC unit. At this point, we can perform case studies and build additional downstream fractionation units. In the next workshop, we will briefly go through some of the issues involved in building a complete downstream fractionation process for this FCC Unit. 

The effluent from the FCC unit is a broad mixture of light gases and liquid products that will be recovered as LPG, Gasoline and Diesel (Light and Heavy Cycle Oil). The downstream fractionation units separate the reactor effluent into the product cut through a series of distillation and absorption columns. The main components of the downstream fractionation are ... 

The most important predictions from the reactor model are the overall yields of all the key products from the unit In case of the reformer, they are the net gas production, LPG (DA301 Ovhd. Liquid) and reformate (DA301 Bttm. Liquid). The yields in the above table are from the rigorous tray-by-tray fractionation section. Therefore, the effect of downstream fractionation is also included in these predictions. We note good agreement with the plant values. The AAD (counting all products) is less than 1.0%. 

The reformer configuration requires choosing the type of reformer, number of reactors and their dimensions and catalyst loadings for each reactor. Additionally, we may also specify additional downstream fractionation equipment such as hydrogen recontactor and stabilizer tower. However, we note that the selecting options produce a simplified model for fractionation that may not be appropriate... 

Additionally, we must also enter the Hydrogen-to-Hydrocarbon ratio for the recycle process in the Reformer model. The typical range of this value for CCR reforming units is 3-4. Reforming plants routinely measure this value and we expect to enter accurate values. The product separator refers to the conditions of the first separator after leaving the last reforming reactor. This value should be accurate if we do not plan to build a downstream fractionation model. 

The next step is to build the downstream fractionation system. The downstream fraction for this CCR reformer has three distinct steps ... 

Methodologies to build downstream fractionation units to expand the scope of models towards integrated refinery models... 

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