Distillation product composition

Example 1.5. For a binary distillation column (see Fig. 1.6), load disturbance variables might include feed flow rate and feed composition. Reflux, steam, cooling water, distillate, and bottoms flow rates might be the manipulated variables. Controlled variables might be distillate product composition, bottoms product composition, column pressure, base liquid level, and reflux drum liquid level. The uncontrolled variables would include the compositions and temperatures on aU the trays. Note that one physical stream may be considered to contain many variables ... 

For a rectifying column, the number of plates required to enrich from the feed composition, Zp, to the distillate (product) composition, x, at fixed reflux ratio, R, is required by... 

Tray temperatures correlate very well with product compositions for many distillation columns therefore, inferential control of distillation product composition is a widely used form of inferential control. Figure 15.58 shows the arrangement for inferential temperature control of the bottoms product composition for this column. Note that the tray temperature controller is cascaded to a flow controller. 

The task of determining distillation product compositions of ideal mixtures in infinite column at minimum reflux is discussed in the previous section. The Underwood equation system solves this task for set composition xf and thermal state of feeding q at two set parameters (e.g., R and D/F or d, and d,). 

Petlyuk, F. B., Kievskii, V. Ya, Serafimov, L. A. (1979). Determination of Distillation Product Composition of Polyazeotropic Mixtures. Theor. Found. Chem. Eng, 13,643-649. 

Fig. 4. Product compositions as a function of carbon number for the Shell middle distillate synthesis process (a) the Fischer-Tropsch product following...

The conversion of fatty alcohols is approximately 99%. The reaction product is then condensed and sent to a distillation column to remove water and high boilers. Typically, a-olefin carbon-number distribution is controlled by the alcohol composition of the reactor feed. The process is currentiy used to produce a-olefins from fatty alcohols. A typical product composition is at <5%, at 50—70%, C g at 30—50%, C2Q at <2%,... 

Camphene Manufacture. Camphene (13) is produced by the reaction of a-pinene (8) with a Ti02 catalyst (80). Preparation of the catalyst has a great influence on the product composition and yield. Tricydene (14) is formed as a coproduct but it undergoes the same reactions as camphene thus the product is generally used as a mixture. They -menthadienes and dimers produced as by-products are easily removed by fractional distillation and the camphene has a melting poiat range of 36—52°C, depending on its purity. Camphene is shipped ia tank cars, deck tanks, and dmms. 

Even though the simple distillation process has no practical use as a method for separating mixtures, simple distillation residue curve maps have extremely usehil appHcations. These maps can be used to test the consistency of experimental azeotropic data (16,17,19) to predict the order and content of the cuts in batch distillation (20—22) and, in continuous distillation, to determine whether a given mixture is separable by distillation, identify feasible entrainers/solvents, predict the attainable product compositions, quaHtatively predict the composition profile shape, and synthesize the corresponding distillation sequences (16,23—30). By identifying the limited separations achievable by distillation, residue curve maps are also usehil in synthesizing separation sequences combining distillation with other methods. 

An azeotrope limits the separation that can be obtained between components by simple distillation. For the system described by cui ve B, the maximum overhead-product concentration that could be obtained from a feed with X = 0.25 is the azeotropic composition. Similarly, a feed with X = 0.9 could produce a bottom-product composition no lower than the azeotrope. 

The Smith-Brinkley Method uses two sets of separation factors for the top and bottom parts of the column, in contrast to a single relative volatility for the Underwood Method. The Underwood Method requires knowing the distillate and bottoms compositions to determine the required reflux. The Smith-Brinkley Method starts with the column parameters and calculates the product compositions. This is a great advantage in building a model for hand or small computer calculations. Starting with a base case, the Smith-Brinkley Method can be used to calculate the effect of parameter changes on the product compositions. 

Calculate (L/D)nun by substituting the final 9 value in Equation 8-130 solving for (L/D)niin- Note that this requires evaluating the functions associated with 9 at the composition of the distillate product. The a values are the constant values previously used above. 

Determine top tray temperature for use in relative volatility calculations by running a dew point on the overhead rapor. For total condenser its composition is same as distillate product. For a partial condenser, run a dew point on the column overhead vapor composition as determined by a material balance around the partial condenser, reflux, and product. 

After an initial distillation to split the coproducts phenol and acetone, each is purified in separate distillation and treating trains. An acetone finishing column distills product acetone from an acetone/water/oil mixture. The oil, which is mostly unreacted cumene, is sent to cumene recovery. Acidic impurities, such as acetic acid and phenol, are neutralized hy caustic injection. Figure 10-7 is a simplified flow diagram of an acetone finishing column, and Table 10-1 shows the feed composition to the acetone finishing column. 

Distillation at constant reflux ratio but varying top product composition. 

Distillation at constant top product composition but varying reflux ratio. 

A distillation column divides the feed stream components between the top and bottom streams, and any side streams. The product compositions are often known they may be specified, or fixed by process constraints, such as product specifications, effluent limits or an azeotropic composition. For a particular stream, 5 , the split-fraction coefficient is given by ... 

A distillation column uses a partial condenser as shown in Figure 9.19. Assume that the reflux ratio and the overhead product composition and flowrate and the operating pressure are known and that the behavior of the liquid and vapor phases in the column is ideal (i.e. Raoult s Law holds). How can the flowrate and composition of the vapor feed to the condenser and its liquid products be estimated, given the vapor pressure data for the pure components. Set up the equations that need to be solved. 

The rectifying or stripping section of a column must operate somewhere between total reflux and minimum reflux conditions. The range of feasible operation of a column section can thus be defined for a given product composition. It can be seen in Figure 12.19 that these section profiles are bounded for a stage column by the distillation line and the pinch point curve. As noted previously, the pinch point curve provides a minimum reflux boundary for both staged and packed columns,... 

Also shown in Figure 12.19 is the residue curve projected from the same product composition. The area enclosed within the residue curve and the pinch point curve thus provides the feasible compositions that can be obtained by a packed column section from a given product composition. For any given product composition, the operation leaf of feasible operation for a column section can be defined by plotting the distillation line (or residue curve) and the pinch point curve1314. The column section must operate somewhere between the total and minimum reflux conditions. 

For a simple distillation column separating a ternary system, once the feed composition has been fixed, three-product component compositions can be specified, with at least one for each product. The remaining compositions will be determined by colinearity in the ternary diagram. For a binary distillation only two product compositions can be specified independently, one in each product. Once the mass balance has been specified, the column pressure, reflux (or reboil ratio) and feed condition must also be specified. 

Wahnschafft OM, Koehler JW, Blass E and Westerberg AW (1992) The Product Composition Regions of Single-Feed Azeotropic Distillation Columns, Ind Eng Chem Res, 31 2345. 

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