Distillation topping

Distillation (topping) of the oil product and recycling the residual oil boiling above about 350° C. (662° F.) to the hydrocarbonization reactor. The oil boiling below 350° C. (662° F.) would be washed with aqueous alkali to remove tar acids and subsequently blended with the Diesel oil from the Fischer-Tropsch synthesis. 

Kiss, A.A., Dimian, A.C., Rothenberg, A., The heterogeneous advantage biodiesel by catalytic reactive distillation, Top. Catal., 40, 141-150, 2006... 

It is possible to derive—roughly—the equations underlying the Underwood method (Underwood, 1946) from the above. The variable R represents the reflux ratio defined in terms of the liquid flow at the pinch point relative to the distillate top product flow i.e.,... 

Fig. 2-55. Path of the temperature in the top of a column, which depends on the distillate top product flow, for a three component batch distillation process.

Other factors affecting the design and performance of a distillation column are (a) the feed composition, (b) the specifications of the top and bottom products, (c) the number of stages in the column, and (d) the reflux ratio R. This ratio is the amount of reflux to the amount of distillate (top product). Both quantities are generally expressed in units of moles per unit time and therefore the reflux ratio is dimensionless. 

L/V < (L/y) g point xd at side 1-2 cannot be the distillation top product point. Along with that, at side 1 -2 there is a stable node (Fig. 5.14a). At (L/ E) = (L/ y), the bifurcation goes on. Distillation trajectory tear-off from side 1-2 inside concentration triangle and the distillation process becomes feasible. The stable node at side 1-2 turns into a saddle in point x[. The trajectory bundle appears with saddle point S at side 1-2 and with the stable node A+ inside concentration triangle at reversible distillation trajectory in the vicinity of point x[. At further increase... 

Besides the fresh feed stream data, there are two degrees of freedom for this system needed to be specified. One is an external degree of freedom, which is the makeup stream. The second one is the single operating specification for the stripper. For example, as demonstrated previously in Figure 3.46, the distillate (top vapor) rate can be specified. Alternatively, the bottoms rate can be specified instead of the distillate rate. The overall flowsheet includes a recycle loop between two unit operations (stripper and decanter). 

Some simple regulatory control loops are determined first. The levels of the reflux drums for both columns are controlled by manipulating the distillate. Top pressures of both columns are controlled by the condenser duty. The bottom level of the extractive distillation column is controlled by manipulating the bottoms flow. The entrainer feed temperature is controlled at 72°C by manipulating the cooler duty. The initial control structure tested is one in which the reflux ratios in the two columns are controlled by manipulating the reflux. This was used in the overall control structure in Luyben. ... 

Distillate (Top-Product) Demand Base Level Control Via Feed... 

A feed at its bubble point temperature is fed to a distillation column. The feed contains propane (Cj), M-butane (M-C4), f-butane (1-C4), M-pentane (w-Cg) and i-pentane(f-C5). The mole % of components in the feed is respectively 5%, 15%, 25%, 20%, and 35%. The operating pressure of the colunrn is 405.3 kPa. Ninety-five percent of the i-C is recovered in the distillate (top) and 97% of the m-C5 us recovered in the bottom. Determine the top and bottom flow rats and composition, minimum reflux ratio and minimum number of trays at a reflux ratio of 2.25. 

The reaction of ethylene (C2H4) and hydrogen chloride (HCI) over a copper chloride catalyst supported on silica to produce of ethylene chloride (CjHjCI) is a highly exothermic reaction. In this example, the reaction is assumed to take place in an isothermal conversion reactor. The heat evolved from the reaction is removed from the reactor to keep the reaction at constant temperature. The reactor effluent stream is compressed, cooled, and then separated in a flash unit followed by a distillation column. The flash and distillation top products are collected and then recycled to the reactor after a portion of the stream is purged to avoid accumulation of an inert component (Nj). The recycled stream is depressurized and heated to the fresh feed stream conditions. The liquid from the bottom of the flash enters a distillation column where ethyl chloride is separated from unreacted HCI and ethylene. The entire process is simulated using Hysys/Unisim, PRO/II, Aspen, and SuperPro Designer software packages. 

Vacuum Dbtillation. Vacuum distillation may be employed for the redistillation of pressure distillate, pressed distillate, or bright stock solution. It may be used for distilling topped crude oils for the production of wax distillate and cylinder stock, or it may be used to reduce a residual stock to asphalt or pitch. Vacuum systems are widely used to produce catalytic cracking plant feed stocks of low carbon content. 

---