Stripper columns system

K.Y. Chang and W.L. Luyben. Design and control of coupled reactor/column systems- Part 3. A reactor/stripper with two colnmns and recycle. Comput. Chem. Eng., 21(l) 69-86, 1997. 

Description The TAC9 process consists of a fixed-bed reactor and product separation section. The feed is combined with hydrogen-rich recycle gas, preheated in a combined feed exchanger (1) and heated in a fired heater (2). The hot feed vapor goes to a reactor (3). The reactor effluent is cooled in a combined feed exchanger and sent to a product separator (4). Hydrogen-rich gas is taken off the top of the separator, mixed with makeup hydrogen gas, and recycled back to the reactor. Liquid from the bottom of the separator is sent to a stripper column (5). The stripper overhead gas is exported to the fuel gas system. The overhead liquid may be sent to a debutanizer column or a stabilizer. The stabilized product is sent to the product fractionation section of the UOP aromatics complex. 

This configuration adds two degrees of freedom to the conventional column (the liquid flowrate to the stripper and the heat input to the stripper reboiler), so two compositions in the sidestream product from the stripper can theoretically be controlled. The control system shown in Fig. 6.23a uses stripper reboiler heat input to control the impurity of A in the sidestream product. The impurity of C in the sidestream product is controlled by manipulating the flowrate of liquid to the stripper. This system presents a highly interacting 4X4 multivariable control problem. Therefore in practice it may be more effective to control only one composition (or temperature) in the stripper and one temperature in the main column, with the flowrate of reflux and liquid to the stripper flow controlled. 

Rigorous distillation models can be used to model absorber columns, stripper columns, refluxed absorbers, three-phase systems such as extractive distillation columns, many possible complex column conflgurations, and columns that include... 

At steady state, and aside from inevitable small losses, almost all of component A in the fresh feed is recovered as azeotropic column bottoms and B as stripper column bottoms. The entrainer, once introduced into the system, circulates within it indefinitely, and any losses are replaced by fresh makeup. The reflux rate and composition are controlled to maintain the ternary azeotrope composition in the overhead. The reflux composition is controlled by the inventory of the entrainer in the receiver and to some degree by the receiver temperature. 

Modern ion chromatography was in a sense bom in 1975 when Small, Stevens and Bauman [3] devised a new system that made it possible to use a conductivity detector. In this paper, 0.01 to 0.02 M hydrochloric acid was used as the eluent in the separation of alkali metal cations. A stripper column (later called a suppressor column) below the separator column, containing an anion-exchange resin in the -OH form, was used to convert the H Cl" to water. 

The treatment presented below is restricted to systems of distillation columns. Systems containing both absorbers, strippers, or reboiled absorbers and distillation columns may also be solved by use of the capital 0 method of convergence as demonstrated in Chap. 4. 

LPG Lean oil stripper Column was pressured up throu a connection in the overhead system while liquid circulated throu its valve tr. The gas could not travel downward, causing mechanical damage to top 12 trays. This later resulted in premature flooding. Always pressure columns fn>m the bottom up, espedaUy when column contains valve trays. 

If steam is injected directly into the bottom of the stripper column, it may bring with it chemicals added to the boiler to guard against corrosion of the steam system (e.g. cyclohexylamine). There is therefore a danger that with direct steam injection an impurity can reach the solvent circuit and it may be necessary to use a heat exchanger to prevent this. 

One example of a single variable control strategy is applied to a stripper in a hydrocracking unit. The main purpose of the stripper is to remove H2S and noncondensable components from the bottom product. One of the key indicators identified is the water dew point at the top of the stripper column. As a matter of fact, the dew point is a function of column overhead vapor composition and the amount of water. There was no monitoring capability available for the dew point temperature. If the column top temperature is lower than the dew point, the hydrogen sulfide will dissolve in the condensed water and cause corrosion to the column overhead system. 

Figure 7.5 shows the Aspen Plus process flowsheet of the two-column system. Each column is a stripper with its individual reboiler. The bottoms of the first column Cl is 99.9 mol% water. The bottoms of the second column C2 is 99.9 mol% n-butanol. The overhead vapors from the two strippers are fed to a single condenser HXl. The compositions of the two vapor streams are almost the same, with values that are on opposite sides of the azeotropic composition. 

In both cases, the carbonate ion concentration increases and eventually equiUbrates in the system, releasing carbon dioxide in the stripping column and thereby reducing product purity. Hence, a small caustic wash tower is employed to remove any carbon dioxide that is Hberated in the stripper. 

Distillation Columns. Distillation is by far the most common separation technique in the chemical process industries. Tray and packed columns are employed as strippers, absorbers, and their combinations in a wide range of diverse appHcations. Although the components to be separated and distillation equipment may be different, the mathematical model of the material and energy balances and of the vapor—Hquid equiUbria are similar and equally appHcable to all distillation operations. Computation of multicomponent systems are extremely complex. Computers, right from their eadiest avadabihties, have been used for making plate-to-plate calculations. 

The column is designed as an ammonia rectifier-stripper using fundamental design techniques. A 48-in. diameter column will handle at least 500 tons of refrigeration system load for the above temperature range, using 10 bubble cap trays with 32, 4-in. pressed steel caps per tray (slot area = 7.81 in. /cap riser area 4.83 in. /cap 3 ft 0 in. weir length). Tray... 

A Develop the equations describing an inverted batch distillation column. This system has a large reflux drum into which the feed is charged. This material is fed to the top of the distillation column (which acts like a stripper). Vapor is generated in a reboiler in the base. Heavy material is withdrawn from the bottom of the column. 

Figure 18 Cyanide generation and disposal in a typical refinery. Cyanide and other gases are formed in FCC or coker units during cracking of organics and go overhead on the fractionating column. Wash water dissolves these gases and becomes sour water. Part of the cyanide is removed by the sour water stripper and the rest goes to the sewer and eventually to the wastewater treatment system. (From Ref. 48.)...

In ammonium phosphate production and mixed and blend fertilizer manufacturing, one possibility is the integration of an ammonia process condensate steam stripping column into the condensate-boiler feedwater systems of an ammonia plant, with or without further stripper bottoms treatment depending on the boiler quality makeup needed. 

When compared to conventional systems (such as strippers, scrubbers, distillation columns, packed towers, bubble columns, evaporators, etc.), membrane contactors present several advantages, as reported in Figure 20.3. However, some drawbacks have also to be taken into account, as shown in Figure 20.4. 

The SR method is suitable for modeling absorbers and strippers. For some extremely wide boiling systems, especially those with noncondensables, it is the best method. It has been found to work very well for the side strippers of a refinery fractionator. Absorbers typically have a rich gas bottom stage feed and a lean oil top stage feed. The equations of the SR method do not allow its direct use for reboiled absorbers, absorbers with condensers, or distillation columns. For these columns, other methods like that of Tomich (Sec. 4.2.8) or Russell Sec. 4.2.10) can be used. 

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