Distillation columns, operation

Distillation capital costs. The classic optimization in distillation is to tradeoff capital cost of the column against energy cost for the distillation, as shown in Fig. 3.7. This wpuld be carried out with distillation columns operating on utilities and not integrated with the rest of the process. Typically, the optimal ratio of actual to minimum reflux ratio lies in the range 1.05 to 1.1. Practical considerations often prevent a ratio of less than 1.1 being used, as discussed in Chap. 3. 

The methanol carbonylation is performed ia the presence of a basic catalyst such as sodium methoxide and the product isolated by distillation. In one continuous commercial process (6) the methyl formate and dimethylamine react at 350 kPa (3.46 atm) and from 110 to 120°C to effect a conversion of about 90%. The reaction mixture is then fed to a reactor—stripper operating at about 275 kPa (2.7 atm), where the reaction is completed and DMF and methanol are separated from the lighter by-products. The cmde material is then purified ia a separate distillation column operating at atmospheric pressure. 

The dominance of distiHation-based methods for the separation of Hquid mixtures makes a number of points about RCM and DRD significant. Residue curves trace the Hquid-phase composition of a simple single-stage batch stiHpot as a function of time. Residue curves also approximate the Hquid composition profiles in continuous staged or packed distillation columns operating at infinite reflux and reboil ratios, and are also indicative of many aspects of the behavior of continuous columns operating at practical reflux ratios (12). 

In the chemical engineering domain, neural nets have been appHed to a variety of problems. Examples include diagnosis (66,67), process modeling (68,69), process control (70,71), and data interpretation (72,73). Industrial appHcation areas include distillation column operation (74), fluidized-bed combustion (75), petroleum refining (76), and composites manufacture (77). 

Many chemical plants, particularly distillation columns, operate at low pressures, and pumps are required to create and maintain the vacuum. Vacuum pumps take in gas at a... 

Distillation is a well-known process and scale-up methods have been well established. Many computer programs for the simulation of continuous distillation columns that are operated at steady state are available. In fine chemicals manufacture, this concerns separations of products in the production of bulk fine chemicals and for solvent recovery/purification. In the past decade, software for modelling of distillation columns operated at non-steady state, including batch distillation, has been developed. In the fine chemicals business, usually batch distillation is applied. 

The distillation column operates at a pressure of 500 mmHg (500 mm of mercury, absolute). The feed point to the column is 12 m above the base of the tank. The tank and column are connected by a 50 mm internal diameter commercial steel pipe, 200 m long. The pipe run from the tank to the column contains the following valves and fittings 20 standard radius 90° elbows two gate valves to isolate the pump (operated fully open) an orifice plate and a flow-control valve. 

Example 9.1 A distillation column operating at 14 bar with a saturated liquid feed of 1000 kmoUr1 with composition given in Table 9.1 is to be separated into an overhead product that recovers 99% of the n-butane overhead and 95% of the i-pentane in the bottoms. Relative volatilities are also given in Table 9.1. 

A distillation column operates with vapor recompression. 

A distillation column operating under constant separation conditions has one fewer degree of freedom, because its energy-to-feed ratio is constant. This means that, for each concentration of the key component in the distillate, a corresponding concentration exists in the bottoms. Therefore, if the concentration of a component in one product stream is held constant, that... 

Inverted, middle vessel and multivessel batch distillation column operations... 

The separation section receives liquid streams from both reactors. For assessment the residue curve map in Figure 5.7 is of help. The first separation step is the removal of lights. This operation can take place in a distillation column operated under vacuum (200mmHg) with a partial condenser. Next, the separation of the ternary mixture cyclohexanone/cyclohexanol/phenol follows. Two columns are necessary. In a direct sequence (Figure 5.15) both cyclohexanone and cyclohexanol are separated as top products. The azeotrope phenol/cyclohexanol to be recycled is the bottoms from the second split In an indirect sequence (Figure 5.16) the azeotropic phenol mixture is a bottom product already from the first split. Then, in the second split cyclohexanone is obtained as the top distillate, while cyclohexanol is taken off as the bottom product The final column separates the phenol from the heavies. 

Distillation columns operating with close to uniform thermodynamic forces are analyzed for separating n-pentane from n-hcptanc (Table 4.14), and ethanol from water (Table 4.15). AsEq. (4.113) shows, chemical separation force (v,V/li,/7) should be uniform throughout the column for thermodynamic optimum. Separation of ethanol from water... 

The simultaneous control of two compositions or temperatures is called dual composition control. This is ideally what we would like to do in a column because it provides the required separation with the minimum energy consumption. However, many distillation columns operate with only one composition controlled, not two. We call this single-end composition control. 

MTBE separation in practice, this section has only one distillation column, operating under pressure so as to use a water-cooled condenser. It separates MTBE at the bottom, and methanol and unconverted C4. at the top. This is because the azeotropes formed by these hydrocarbons with alcohol have boil ing points lower than that of the azeotrope formed by methanol and ether. The latter has a boiling point of 51.6 C at 0.1 10 Pa. Its weight composition for each of its components is 14 and 86 per cent (30 to 70 at 0.8.10 Pa). Raffinate treatment this section comprises two-step water scnibltiag of the raffinate to remove the methanol, followed by fractionation of the water/methanol mixture. The alcohol recovered is recycled to the reactor. 

The first three of these are solely VLE-based approaches, involving a series of simple distillation column operations and recycles. The final approach relies on distillation (VLE), but also exploits another physical phenomenon, hquid-liquid phase formation (phase splitting), to assist in entrainer recovery. This approach is the most powerful and versatile. Examples of industrial uses of azeotropic distillation grouped by method are given in Table 13-20. 

Crude oil is distilled in a distillation column operating near to atmospheric pressure to produce naphtha (b.p. 30°C - 180°C) and gas oil (b.p. 25°C - 360°C). The bottoms of the column, known as atmospheric residua, are passed to a vacuum distillation column which produces vacuum gas oil (b.p. 350°C - 550 C). Any of these distilled liquid feeds can be used to produce petrochemicals. 

A liquid mixture contains 60.0 wt% ethanol (E), 5.0 wt% of a dissolved solute (S), and the balance water. A stream of this mixture is fed to a continuous distillation column operating at steady state. Product streams emerge at the top and bottom of the column. The column design calls for the product streams to have equal mass flow rates and for the top stream to contain 90.0 wt% ethanol and no S. 

Material Balance Constraint For a conventional single-feed, two-product distillation column operating at steady state, the feed composition must lie on a straight line between the compositions for the distillate and bottoms products. 

Distillation and residue lines that we have plotted for three species on a planar triangular diagram remain as lines for all dimensional spaces. Each corresponds to a trajectory of composition points. A distillation curve corresponds to the curve we pass through the liquid compositions that occur on the trays of a distillation column operating at total reflux. A residue curve is the trajectory we get when we solve the differential equations... 

One line we have to think more carefully about is the pinch point line as we move to other dimensions. Each point on the line is the end point of a operating line for a distillation column operating with a fixed distillate (or bottoms) product, varying parametrically with the reflux ratio we use to define that operating line. Thus, a pinch point curve emanating from a fixed distillate composition is a sequence of points. It remains a line. It may bifurcate, as we have shown above, but it remains a line. 

A simple naphtha isomerization process has a feed of 10,000 barrels per day (bpd) of a 50 wt% mixture of n-hexane and methyl pentane. The feed is heated and sent to a reactor, where it is brought to equilibrium at 1300 kPa and 250°C. The reactor products are cooled to the dew point and fed to a distillation column operated at 300 kPa. The bottoms product of the distillation is rich in n-hexane and is recycled to the reactor feed. An overall conversion of n-hexane of 95% is achieved. 

Most experimental studies of distillation are carried out at total reflux to prevent loss of materials. How would the nonequilibrium model of Chapter 14 be used to simulate a distillation column operating under total reflux conditions ... 

A distillation column operating at 500 kPa is used to recover pure ammonia in the distillate from a water-acetone-ammonia solution. The column has 18 theoretical trays, a total condenser, and a reboiler the feed is introduced on the tenth tray from the top. In the same column, crude separation is to be made between the acetone and the water by taking a liquid side stream. The feed, which is superheated vapor at 200°C and 500 kPa before entering the column, has the following component rates ... 

The diameter of a distillation column operating at a specified gas velocity varies with the square root of the volumetric throughput (ft /s) ... 

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