Column section side draw columns

Differential temperature as well as differential pressure can be used as a primary control variable. In one instance, it was hard to meet purity on a product in a column having close boiling components. The differential temperature across several bottom section trays was found to be the key to maintaining purity control. So a column side draw flow higher in the column was put on control by the critical temperature differential. This controlled the liquid reflux running down to the critical zone by varying the liquid drawn off at the side draw. This novel scheme solved the control problem. 

As will be shown later, the trays in the main column are mostly rectifying sections for the side products some of them are pumparound contact condensers. There is a wide variation in the industry of tray count between side draws, from 4 to 15, depending on the degree of rectification desired. 

Two binary streams each containing component 1 (the more volatile) and component 2 are to be separated in a single column equipped with a total condenser and a reboiler. Feed Fl enters the column as saturated vapor and feed F2 enters the column as saturated liquid. A vapor side draw Sl and a liquid side draw S2 are taken from the column. Using the data below, determine the correct relative locations of the feeds and products, the distillate and bottoms flow rates, and the L/V ratio in each column section. The column uses a reflux ratio of 1.8. 

One possible set of specifications would include the flow rates of three of the products and the column reflux ratio. The way the components are split among the products, the identity of the key components in each section are determined by the product rates, and the locations of the feed and products. In this example the products are specified at flow rates that correspond to the component flow rates in the feed. Thus, the distillate is specified at 12 kmol/h, the upper side draw at 48 kmol/h, and the lower side draw at 25 kmol/h. By overall material balance, the resulting bottoms flow rate is 15 kmol/h. 

The fourth column specification is the reflux ratio, which controls the quality of separation between the products. The reflux ratio sets the L/V ratios in the column. These ratios tend to be fairly constant within each section, depending on the distillation characteristics of the mixture. They are different, though, in each section because the side draws alter the fluid flow in the column. If, for instance, a side draw is a liquid, the liquid flow on the trays below the draw tray is smaller than that on the trays above it. In the feed section, the L/V ratio is generally different above and below the feed tray, depending on the feed thermal conditions. 

An intermediate product such as the upper side draw, which is mostly propane, may contain impurities from components both lighter and heavier than the main component. The upper side draw is at the same time the bottom product of the top column section and the top product of the second section. The fractionation in the upper section determines to what extent ethane is stripped off from the propane product, and the fractionation in the second section determines to what extent butane is removed from the propane product. Again, in this situation since the number of stages in each section and the reflux ratio are all fixed, the fractionation is fixed. The propane recovery and purity depend mostly on its flow rate and on the flow rates of the adjacent products above and below it. If the propane product contains too much ethane, its flow rate should be cut back and the overhead rate increased. If the propane product contains too much butane, its flow rate should be cut back and the lower side draw rate increased. Table 9.14 summarizes the purities of components in different products at different flow rates. The recoveries can also be calculated from Table 9.14. The dependence of the other products compositions on their rates may be analyzed in a similar manner. 

If a condenser or side cooler is placed on a draw tray or right below it, a portion of the rising vapor condenses. This causes the liquid flow, and consequently the vapor flow, below the side draw to rise, which tends to equalize the profiles above and below the draw tray. The side cooler duty may be adjusted to obtain the desired profiles. In this manner, maintaining the desired liquid and vapor flows in the lower sections of the column does not require unnecessarily high flows in the upper sections. 

Equation 12.24 is applicable to all column sections that are bounded by side products on both sides. The uppermost and lowest sections are bounded by the overhead or bottoms on one side and a side draw on the other. If a partial condenser is used, Equation 12.22 should be applied to the uppermost section. Expressed in terms of stream and component flow rates, the equation for the uppermost section takes the form... 

A distillation column is designed for the separation of a mixture of benzene, toluene, and biphenyl into a distillate (mostly benzene), a side draw (mostly toluene), and a bottoms product (mostly biphenyl). The column will operate at a pressure of about 175 kPa and a temperature ranging from about 90°C in the condenser to about 240°C in the reboiler. The three products define two column sections, and in each section a A -value for biphenyl, designated as the reference component, and relative volatilities for the other components are estimated based on the average temperature and pressure in the section. The feed stream component flow rates, relative volatilities, reference K-values, and product rates are given below ... 

Module 2 models the draw tray and consists of a mixer and an equilibrium stage. Module 3 is a splitter that takes the liquid from the draw tray and splits it into side draw SD and the remaining liquid flowing down to the bottom column section. The side-stripper and the upper column section are modeled with column sections, modules 4 and 5, and the condenser is modeled with an equilibrium stage, module 6. Using computational sequence 1,2, 3,4, 5, 6 requires initialization of streams L3, L5, OH, and R. 

In many side-drawoff columns, the side draw is very small compared to the other product streams. This is a common situation when the side stream serves to remove an intermediate impurity (Sec. 13.7). In many other side-drawoff columns, distillate is withdrawn as a side product a few trays below the top of the column, leaving an upper "pasteurizing section for separating light ends as a small vent stream. Common examples are an ethylene plant Cg splitter, where small quantities of methane are "pasteurized out of the ethylene product, and an alcohol still, where "heads are pasteurized out of the alcohol product. The reverse situation has the bottom product drawn as a vapor side product a few trays above the bottom, leaving a small heavy end stream to exit from the bottom. 

When vapor flow up the column is small compared to the side draw rate, configuration 19.6c (with any of the Fig. 16.4 control schemes) can induce large swings in column vapor traffic above the side draw, and a severe interference with the pressure controller. This is analogous to the problem described with configuration 19.66. The vapor swings are most pronounced with schemes 16.46 and d, because the top section temperature is more sensitive to the feed than to the changes in the small vapor flow. 

Subject to availability of an external recycle stream, as well as spare capacity in the column section below the side draw, configuration 19.6c can also be used. However, this scheme is energy-wasteful and will usually accomplish little more than the IVC. 

The expander discharge stream enters the bottom of the upper (rectification) section of the deethanizer column. The upper section contains packed bed and lower section contains trays. The SCORE process include two side draw loop on the deethanizer that result in to significant improved NGL recovery compared to conventional processes. Heat supplied to deethanizer column through reboiler and reboiler is design to operate using LP and MP steams depending on the mode of SCORE process either TT or Turbo expander. 

If a product of intermediate composition is required, a vapor or liquid side stream may be withdrawn. This is commonly done in petroleum refineries and is illustrated in Figure 4-22A for a liquid side stream Three additional variables such as flow rate, S, type of side draw (liquid or vapor), and location or composition Xg or yg, must be specified. The operating equation for the middle section can be derived from mass balances around the top or bottom of the column. For the situation shown in Figure 4-22A. the middle operating equation is... 

Understand Process Characteristics H2, H2S, and NH3 and light ends are removed from reaction effluents through a series of separation and flashes, resulting in the reaction products in a liquid form, which goes to the stripper, the feed heater, and then to the main product fractionator. The task of the product fractionation is to separate different products based on their product specihcations such as distillation endpoint, ASTM D-86 T90% or T95% point, and so on. Side draws from the column go to the product strippers where kerosene and diesel products are made. The net draw from the column bottom is called unconverted oil (UCO), which is recycled back to the reaction section for nearly complete conversion. There are two pump-arounds, namely, kerosene and diesel pumparounds, as a main feature of heat recovery from the main fractionation column. 

The base-case condition in Section 9.3 with no side-draw and no feed impurity is compared with the ones in this section. Figure 9.38 displays the column composition profiles for the base case in Section 9.3 and the optimum TAC case in this section with a side-draw flowrate of 50 kg/h. 

For the purpose of discussion in this chapter, a column section is defined as any part of a distillation or other multistage separation process that includes no side feeds or draws and no heaters or coolers. A column section does necessarily have a liquid feed at the top and a vapor feed at the bottom. It also has a vapor product at the top and a liquid product at the bottom. It operates adiabatically, i.e., no heat is transferred across the system boundary to or from the surroundings, although enthalpy is transferred with the streams themselves. 

The column is expected to recover most of the ethane in the distillate, the propane in the upper side draw, the butane in the lower side draw, and the pentane in the bottoms. Ethane is the light key in the upper section, propane is the heavy key in the upper section and the light key in the middle section, butane is the heavy key in the middle section and the light key in the lower section, and pentane is the heavy key in the lower section. The distinction between rectification and stripping is relative in a multiproduct situation. In the upper section, for instance, ethane is stripped from the upper side draw and propane is separated by reflux from the distillate. Similarly, propane is stripped from the lower side draw, butane is separated from the upper side draw by liquid reflux, butaue is stripped from the bottoms, and pentane is separated from the lower side draw by reflux. 

As mentioned in Section 1.9, a distillation column sometimes tends to collect intermediate boilers, compounds that are heavier than the light key but lighter than the heavy key. In this case a small sidestream is required. As shown in Figure 7.3, the column-control scheme is very similar to that of a conventional two-product column with the addition of sidestream controls. Ideally the side draw should be ratioed to the feed, but in practice, if it is very smidl, simple flow control is sometimes used. 

Figure 8.10 shows the flowsheet configuration of a column with a rectifying section liquid side draw. The multi-component feed comes from an upstream unit in the process. The benzene liquid side draw is the product stream, and has a purity specification in terms of benzene. The distillation removes n-pentane from the feed mixture and the heavies (toluene, naphthalene and biphenyl) are purged from the reboiler. A small overhead purge stream is coimected to the condenser for pressure rehef. 

Finally, a temperature controller can be added to provide a method of controlling the conposition of the liquid side draw. This controller can have its temperature sensor on the bottom tray of the main tray section and use the bottoms flow rate as a manipulated variable. Temperature sensitivity analysis can be performed using the steady-state model to ascertain the proper location for the tenperature sensor. Using the bottoms flow rate allows a method for excess heavies to be removed from the system in the event of a disturbance while retaining the target composition of the liquid side draw. The resulting control scheme for the liquid side draw benzene column is shown in Figure 8.11. 

We first enter the column environment as shown in Figure 2.73. We can then double click on the column environment to bring up the advanced configuration for this column. From this interface, we can add non-standard units such as fhermosiphon reboilers, etc. ffowever for this example, we will focus on adding a water draw. We select the Side Draws section and create an additional WATER DRAW stream at tray 27 (the bottom tray) as shown in Figure 2.74. 

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