Distillation sidestream

As in distillation, sidestreams can be introduced or sidestream products can be withdrawn at different points or stages. In distillation practice at least, it is preferable that the sidestreams introduced be approximately of the same composition as at the point or stage introduced. Again by analogy, the membrane section above the feed location may be referred to as the rectifying section, and the section below as the stripping section or vice versa. The former is the accepted convention in distillation practice, where the more-volatile component(s) move toward the top of the column, the less-volatile toward the bottom. 

The system studied in this paper is a ternary separation of propane-butane-pentane. The separation is carried out in two columns operating at different pressures (See Figure 1). The first column is a high pressure (HP) prefractionator, which performs the propane/pentane split. Both the distillate and the bottom products from the HP column is fed to the second, low pressure (LP) column. Here propane, butane and pentane are the products from the distillate, sidestream and bottom stream, respectively. In the LP column the top part of the column (above sidestream) performs the propane/butane split while the bottom part (below the sidestream) performs the butane/pentane split. 

As shown in Fig. 10.6, the vapor from the reactor flows into the bottom of a distillation column, and high-purity dichloroethane is withdrawn as a sidestream several trays from the column top. The design shown in Fig. 10.6 is elegant in that the heat of reaction is conserved to run the separation and no washing of the reactor... 

Catalyst recovery is a major operational problem because rhodium is a cosdy noble metal and every trace must be recovered for an economic process. Several methods have been patented (44—46). The catalyst is often reactivated by heating in the presence of an alcohol. In another technique, water is added to the homogeneous catalyst solution so that the rhodium compounds precipitate. Another way to separate rhodium involves a two-phase Hquid such as the immiscible mixture of octane or cyclohexane and aliphatic alcohols having 4—8 carbon atoms. In a typical instance, the carbonylation reactor is operated so the desired products and other low boiling materials are flash-distilled. The reacting mixture itself may be boiled, or a sidestream can be distilled, returning the heavy ends to the reactor. In either case, the heavier materials tend to accumulate. A part of these materials is separated, then concentrated to leave only the heaviest residues, and treated with the immiscible Hquid pair. The rhodium precipitates and is taken up in anhydride for recycling. 

The first step in CTO distillation is depitching. A relatively small distillation column is used as a pitch stripper. The vapor from the pitch stripper is fed directiy into the rosin column, where rosin and fatty acids are separated. Rosin is taken from the bottoms of the column and fatty acids as a sidestream near the top. Palmitic acid and light neutrals are removed in the rosin column as heads. The operation is designed to minimize holdup and product decomposition. Care is taken to prevent carryover of some of the heavier neutrals, such as the sterols, from the depitcher to the rosin column (24). 

The carbon monoxide-rich, Hquid condensate from the primary separator is expanded and exchanged against the incoming feed and is then sent to a distillation column where the carbon monoxide is purified. The bottoms Hquor from the methane wash column is expanded, heat-exchanged, and sent to the bottom section of the distillation column for methane rectification and carbon monoxide recovery. The methane bottom stream is recompressed and recycled to the top of the wash column after subcooling. A sidestream of methane is withdrawn to avoid a buildup of impurities in the system. 

Where is naphthenic acid corrosion found Naphthenic acid corrosion occurs primarily in crude and vacuum distillation units, and less frequently in thermal and catalytic cracking operations. It usually occurs in furnace coils, transfer lines, vacuum columns and their overhead condensers, sidestream coolers, and pumps. 

In light ends fractionation it is usually just as important to remove light material from the heavier cut as it is to keep heavy material out of the lighter cut sidestreams are seldom withdrawn. The desired purity (expressed as per cent of impurity) of the overhead and bottoms is determined by product specifications or by the requirements of subsequent processing units. To meet these purity requirements, higher reflux ratios and greater numbers of plates between cuts are required than in crude distillation units. 

Steam used in the sidestream strippers and in the stripping section of the main column is condensed in the overhead condenser. This water settles to the bottom of the distillate dmm and is drawn off through a small water pot in the bottom. In most installations, ammonia gas is injected into the overhead line to raise the pH of this water and reduce corrosion. 

The heated oil is flashed into the VPS flash zone where vapor and liquid separate. Split between distillate and bottoms depends on flash zone temperature and pressure. Separation of vapor and liquid in the flash zone is incomplete, since some lower boiliug sidestream components are present in the liquid while bottoms components are entrained in the vapor. The liquid from the flash zone is steam stripped in the bottom section of the VPS to remove the lower boiling components. 

A single-stage pipe still used for distilling whole crude is shown in Figure 1. The unit shown is designed to separate six products gas, overhead distillate, three sidestream distillates, and undistilled residue, or bottoms. Modem pipe stills usually have several sidestreams. 

It would be possible to remove all of the heat by pumping cold reflux from the distillate drum to the top of the tower and thus eliminate the cost of the pumparound circuit. Where more than one sidestream is withdrawn, however, it is usually economical to withdraw part of the heat in a pumparoimd reflux system farther down the tower. The following economic factors affect the choice ... 

Rerunning operations are characterized by large volumes of distillate products and relatively small residue yields. Frequently, the product is withdrawn as a sidestream with undesirable light fractions passing overhead and polymers being withdrawn from the bottom of the tower. Lube rerun stills usually have several sidestreams which permit close control of flash point and viscosity while producing a wide range of stocks. 

For systems with one sidestream drawoff, either above or below the feed, Tsuo et al. [102] propose a method for recognizing that the minimum reflux ratio is greater for a column with sidestream drawoff. At the sidestream the operating line has an inflection. For multifeed distillation systems, the minimum reflux is determined by factoring together the separate effect of each feed [103]. 

UK. = Light key component in volatile mixture L/V = Internal reflux ratio L/D = Actual external reflux ratio (L/D) ,in = Minimum external reflux ratio M = Molecular weight of compound Mg = Total mols steam required m = Number of sidestreams above feed, n N = Number of theoretical trays in distillation tower (not including reboiler) at operating finite reflux. For partial condenser system N includes condenser or number theoretical trays or transfer units for a packed tower (VOC calculations) Nb = Number of trays from tray, m, to bottom tray, but not including still or reboiler Nrain = Minimum number of theoretical trays in distillation tower (not including reboiler) at total or infinite reflux. For partial condenser system,... 

Yn + 1 = Mol fraction VOC component in the incoming fresh air (equals zero for fresh air) yj = Mol fraction solvent component in vapor Ys = Mol fraction steam in vapor YjB = Percent recovery of, i, in the bottoms YiD = Percent recovery of, i, in the distillate Z = Compressibility factor z F = Mol fraction component, i, in feed Zj Fj = Mol fraction component, i, in feed, j z s = Mol fraction component, i, in sidestream Zi sk = Mol fraction component, i, in sidestream, k... 

Example 8-25 Scheibel-Montross Minimum Reflux, 80 Minimum Number of Trays Total Reflux — Constant Volatility, 80 Chou and Yaws Method, 81 Example 8-26 Distillation with Two Sidestream Feeds, 82 Theoretical Trays at Operating Reflux, 83 Example 8-27 Operating Reflux Ratio, 84 Estimating Multicomponent Recoveries,... 

The rate of withdrawal of the sidestream is 10 per cent of the column feed rate and the external reflux ratio is 2.5. Using the enthalpy composition method, determine the number of theoretical stages required, and the amounts of bottom product and distillate as percentages of the feed rate. 

If we listed all the variables in this system and subtracted all the equations describing it and all the parameters that are fixed (all feeds), we would find that the degrees of freedom would be equal to the number of sidestreams plus two. Thus if we have no sidestreams, there are only two degrees of freedom in this multicomponent system. This is the same number that we found in the simple binary colunrn. T q)ically we would want to control the amount of heavy key impurity in the distillate and the amount of light key impurity in the... 

The distillation column sketched in Fig. P8.ll has an intermediate leboiler and a vapor sidestream. Sketch a control concept diagram showing the following control objectives ... 

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