Preflash column

Desalted hot crude oil feed is sent to a preflash column where it is separated into gas, gasoline, naphtha, and topped crude as shown in Figure 9.5. Using column modules representation, determine the number of degrees of freedom for this operation. What specifications may be used to define the column performance The crude feed is of fixed composition and thermal conditions. The stripping steam thermal conditions are fixed, but its flow rate may be varied. The column pressure is independently fixed. 

The preflash column is used when the crude oil reaches a temperature of about 400—450 °F. At this point, some of the crude is above its boiling point, and vaporization occurs. The lightest portion of the crude is flashed (vaporized and separated from the bulk liquid) in the preflash column. The overhead is condensed and added into the atmospheric tower as a reflux, and the noncondensible vapors are added to the atmospheric tower overhead vapors. This step serves to conserve energy and unit investment for preheating the bulk of the crude. 

Occasionally, foaming in the preflash column can limit throughput to the crude unit. Organic surfactants or water carry-over from the desalters can be the cause of the problem. Silicones, usually of 60,000-cSt viscosity, are normally used in this process. However, sometimes the normal silicones are too soluble in the crude to be effective in controlling foam. In... 

We are now ready to proceed with our simulation. We will start by looking at a simple preflash column that is often used in refineries to remove some of the lightest material from the crude before sending it into the pipestill. 

A Design Specs function can be used to achieve the specification. Clicking Design Specs under the PREFLASH column block, clicking New and giving an identification label open the window shown at the top of Figure 11.23. The Type is specified at ASTM D86 temperature (dry, liquid volume basis). The Target is 375 °F at a Liquid % of 95%. Click the Feed/Product Streams and select NAPHTHA as the Selected Stream (see the bottom of... 

The steady-state design is now complete. Figure 11.25a gives the flowsheet of the PREFLASH column with conditions and properties of the various streams. All of the flow rates are given in B/D at standard conditions. 

Because Petroleum English units are not available in Aspen Dynamics, we will switch to metric units when we look at dynamic control. Figure 11.25b gives the flowsheet of the PREFLASH column in metric units (T = 1000 kg). 

It should be noted that there is no claim that the design of the preflash column presented above is the economic optimum. It is presented for purposes of illustration. A modified... 

STEADY-STATE DESIGN OF PREFLASH COLUMN 327 65.9 MMBtu/h... 

The first step is to connect the bottoms from the preflash column to the furnace of the pipestiii. As we did with the preflash furnace, the red input arrow that appears at the bottom of the column (shown at the top of Fig. 11.35) must be dragged over to the furnace (shown at the bottom of Fig. 11.35). 

A petroleum fractionator, such as a pipestill or a cat fractionator, is almost overwhelmingly complex. In addition to the main column, there are strippers that have vapor and liquid streams going back to and coming from the main column. There are a very large number of control loops to set up. Let us enumerate the loops that we will set up, considering both the preflash column and the pipestill. 

The equipment associated with the preflash column has already been sized. The diameters of the pipestill column and the three strippers are sized using the Tray Sizing feature of Aspen Plus for each vessel. The results are... 

Flow controllers are installed on the steam to the base of the two columns. These flow rates are ratioed to the feed flows to the respective column by using multipliers. The molar steam-to-feed ratio in the preflash column is 125.9/2722 = 0.04625. The total crude feed is used (after the summer). The molar steam-to-feed ratio in the pipestiU is 302.1/1654 = 0.1827. 

The 20% increase in feed flow resulted in the saturation of several control valves when the original design size was used (valves 50% open). The valve sizes were doubled to remove these limitations. The valves modified were V14 (LIGHTS from the preflash column), V25 (HNAPH), and V22 (AGO). The steady-state positions of these three valves are now about 25% open, as can be seen on the faceplates of the BChvynaph 95% boiling point controller, the PCI pressure controller in the preflash column, and the FCwash flow controller, which manipulates the AGO to hold constant the Stage 19 liquid flow rate (see Fig. 11.51). 

W. L. Luyben, Design of a petroleum preflash column. Energy Euels 26, 1268-1274 (2012). 

Lately, a popular addition to a crude distillation system has been a preflash column ahead of the two stages shown in Fig. 15.10. The preflash tower strips out the lighter portions of a crude oil before the remainder enters the atmospheric column. It is the lighter portions that set the vapor loading in the atmospheric column, which, in turn, determines the diameter of the upper section of the column. 

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