Downstream fractionation units

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... 

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 ... 

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

In this configuration, the hydrogen and the methane from the demethanizer column are spUt into their component streams. The hydrogen is for use in various downstream processes and the methane is used as a fuel-gas stream. Bottoms from the de-ethaniser are further split into C3 and C4. stream. The C3 is treated similarly to the C2 to produce polymer grade propylene. After removing the C4 fraction, which is passed to downstream separation units, the heavy components form pyrolysis-gasoUne. The latter may be further separated to produce benzene, toluene and xylene. 

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. 

The profitability of the DIP column was determined based on the value of separating 1C5 for direct blending to gasoline and nC to be used as feed for the Cs/Cg isomerization unit, less the utility and downstream isomerization unit opportunity costs incurred to do so. Lighter feed eomponents, such as n-butane, were assumed to always be fractionated into the DIP overhead, and components heavier than nCs were assumed to always be found in the DIP bottoms stream. Thus, only the disposition of iCs and nCs components were considered in the profitability calculation. Therefore, the objective function for optimizing the DIP tower is defined as... 

The process layout consists of two consecutive static mixers (Fig. 28). To the first mixer, the olefin feedstock is cofed with a recycled isobutane/alkylate stream. The stream coming out the first static mixer is then combined with the recycled IL-based composite catalyst and fed into the second static mixer where the alkylation reaction takes place at a reaction temperature around 15°C and a total pressure of 0.4 MPa. The reaction products are then sent to a settler where the composite catalyst is collected from the bottom, due to its higher density, and recycled. The supernatant is later split into a recycle (isobutane -I- alkylate) to the first static mixer upstream and a product effluent, which constitutes the incoming to the fractionation unit downstream. Total reaction time, considering residence times in the second static mixer and in the settler, is 10 min while the overall I/O ratio in the reactor is set to a value as high as 500. No details on catalyst regeneration or replacement have been disclosed (257). 

The manufacture of distillates either directly for fuels or for feed to downstream processing units ordinarily does not require any particular degree of fractionation between cuts. Also, wide cuts are usually acceptable. For these reasons, the distillates can be condensed by cooled pumparound reflux, grid type contacting sections and chimney draw trays. For all practical purposes, the operation of the main condensing sections can be described as a single-stage equilibrium condensation. 

There are two major types of VDU ojjerations in a modem refinery -feedstock preparation and lubricant production. Feedstock preparation is the most common ojjeration that recovers gas oU from the atmospheric residue as a feed to the downstream conversion units (e.g. FCC and hydrocracking units), which converts the gas oil into more valuable liquid products such as gasoline and diesel. Lubricant production is designed to extract petroleum fractions from the atmospheric residue to produce luboil with desirable viscosity and other related properties. 

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%. 

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. 

Fractionation of petroleum in the refinery, to obtain streams with specific boiling ranges for various downstream processes, is performed by distillation in a crude unit. To determine how Ni and V compounds are distributed as a function of boiling point is, therefore, useful for evaluating their impact in the refinery. Petroleum may also be fractionated by solvent separation and chromatography to obtain more detailed information on the distribution of Ni and V compounds. This section will review the available literature on how metals are distributed in petroleum by boiling point and solubility class. It will also include some discussion of the structure of heavy oil in general and asphaltenes in particular. Vercier etal. (1981) have provided an excellent review of methods and procedures involved in petroleum fractionations. 

The desalting and distillation units are shown in Figs. 18.8-18.10 along with the crude fractions from the crude distillation column. The relationships between some finished products and downstream processing steps will be expanded upon later in the chapter. 

One of the issues that concern liquid feedstock cracking operations is a higher rate of fouling. This is not only a consequence of heavier coke forming precursors, but also as a consequence of long lived free radicals which act as agents for the formation of a polymer (often referred to as pop-corn polymer) in the primary fractionator and downstream units. For instance, free radicals based on styrene or indene have sufficiently long half-lives to pass from the pyrolysis section into the primary fractionator. These can concentrate in this unit and produce polymer (free radical polymerisation) when sufficient amounts of suitable olefins are present, in particular styrene itself and di-olefins such as cyclo-pentadiene or butadiene. 

The objective in analyzing these units is to calculate the temperature, the conqjosition, and the flow rates of the vapor and hquid exit streams, given the properties of the entering streams. First, write the mole balances. For two components, we write two component balances and a mole fraction summation for each unknown stream as given by Equations 3.3.1 to 3.3.4 in Table 3.3.1. There are two phases in equilibriiun leaving the valve, condenser and vaporizer, although the phases have not, as yet, been separated. A phase separator will separate the phases. For a vaporizer, both component and phase separation occur in the same process unit. As stated before, the first numerical subscript is the line number and the second the component number. We also identify the phases by an additional subscript, V for vapor and L for liquid. Because we are assuming equilibrium between the vapor and liquid for each component downstream of the valve, we can... 

Another aspect of heating soybeans in particular is the impact on the phospholipase enzyme. The phospholipase enzyme is activated at approximately 55°C and remains activated up to approximately 100°C. In this temperature range, and with sufficient exposed surface area and time, the phospholipase enzyme modifies a portion of the phospatides in the oil fraction by splitting off the non-fatty acid moiety (16). The resultant calcium and magnesium salts of phosphatidic acids that are formed tend to be more oil-soluble than water-soluble, thereby converting phospatides from a hydratable form to a nonhydratable form (16). This has a resultant impact on the quantities of acid, caustic and silica needed to reduce the phosphorus content of the soybean oil in the downstream degumming and refining unit operations. 

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