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  1. 1 point
    Solvent Criteria One of the most important steps in developing a successful (economical) extractive distillation sequence is selecting a good solvent. Approaches to the selection of an extractive distillation solvent are discussed by Berg, Ewell et al. ,and Tassions. In general, selection criteria include the following : 1. Should enhance significantly the natural relative volatility of the key component. 2. Should not require an excessive ratio of solvent to nonsolvent (because of cost of handling in the column and auxiliary equipment. 3. Should remain soluble in the feed components and should not lead to the formation of two phase. 4. Should be easily separable from the bottom product. 5. Should be inexpensive and readily available. 6. Should be stable at the temperature of the distillation and solvent separation. 7. Should be nonreactive with the components in the feed mixture. 8. Should have a low latent heat. 9. Should be noncorrosive and nontoxic Naturally no single solvent or solvent mixture satisfy all the criteria, and compromises must be reached. Solvent Screening Perry's handbook serve as a good reference for the solvent selection procedure, which can be thought of as a two step process, i.e.: Broad screening by functional group or chemical family Homologous series : Select candidate solvent from the high boiling homologous series of both light and heavy key components. Robins Chart: Select candidate solvents from groups in the Robbins Chart that tend to give positive (or no) deviations from Raoult's law for the key component desire in the distillate and negative (or no) deviations for the other key. Hydrogen-bonding characteristic: are likely to cause the formation of hydrogen bonds with the key component to be removed in the bottoms, or disruption of hydrogen bonds with the key to be removed in the distillate. Formation and disruption of hydrogen bonds are often associated with strong negative and positive deviations, respectively from Raoult's Law. Polarity characteristic: Select candidate solvents from chemical groups that tend to show higher polarity than one key component or lower polarity than the other key. Identification of individual candidate solvents Boiling point characteristic: Select only candidate solvents that boil at least 30-40C above the key components to ensure that the solvent is relatively nonvolatile and remains largely in the liquid phase. With this boiling point difference, the solvent should also not form azeotropes with the other components. Selectivity at the infinite dilution: Rank the candidate solvents according to their selectivity at infinite dilution. Experimental measurement of relative volatility: Rank the candidate solvents by the increase in relative volatility caused by the addition of the solvent. Residue curve maps are of limited usefulness at the preliminary screening stage because there is usually insufficient information available to sketch the them, but they are valuable and should be sketched or calculated as part of the second stage of the solvent selection.
  2. 1 point
    The most basic model for insulation on a pipe is shown above. R1 and R2 show the inside and outside radius of the pipe respectively. R3 shows the radius of the insulation. Typically, when dealing with insulations, engineers must be concerned with linear heat loss or heat loss per unit length. Generally, the heat transfer coefficient of ambient air is 40 W/m2K. This coefficient can of course increase with wind velocity if the pipe is outside. A good estimate for an outdoor air coefficient in warm climates with wind speeds under 15 mph is around 50 W/m2K . Eqn(1) The total heat loss per unit length is calculated by: Eqn(2) Figure 3: Heat Loss vs. InsulationThickness Since heat loss through insulation is a conductive heat transfer, there are instances when adding insulation actually increases heat loss. The thickness at which insulation begins to decrease heat loss is described as the critical thickness. Since the critical thickness is almost always a few millimeters, it is seldom (if ever) an issue for piping. Critical thickness is a concern however in insulating wires. Figure 3 shows the heat loss vs. insulation thickness for a typical insulation. It's easy to see why wire insulation is kept to a minimum as adding insulation would increase the heat transfer.
  3. 1 point
    For our example which deals with the azeotropic mixture formed between benzene and cyclohexane, we have chosen extractive distillation (one of the homogeneous azeotropic distillation methods). The reason of choosing this method is due to the availability of information regarding this separation technique and its tendency to operate more efficiently, i.e. in separating and recycling the separating agent. A brief discussion of the process is given below. After the mixture exited as the bottom product of the flash unit, it contains mostly our desire product of cyclohexane and also a significant amount of unreacted benzene, which is to be recycled back to the reactor for further conversion. Our main goal now is to further separate the remaining components in the mixture. As cyclohexane and benzene have been encounter most of the remaining composition with the mole % of 44.86 and 54.848 respectively (Table 1), we will consider this to be a binary mixture in our further discussion. From the process flowsheeting, we would like to operate the distillation column at the pressure of 150 kPa. At this condition, cyclohexane and benzene will have boiling points of 94.34 °C and 93.49 °C respectively (Figure 3). This is a typical case where conventional distillation would struggle to perform the separation of this type of close boiling mixture. Thus, a special type of distillation technique, i.e. extractive distillation has been chosen in order to purify the desire product, i.e. cyclohexane to our desired purity of 99.3%. As can be shown from Figure 3, this binary composition will form a minimum boiling, homogeneous azeotrope at the temperature of 91oC and the corresponding composition at this point will be 45.5 mole % for cyclohexane and 55.5 mole % for benzene (Figure 4). Temperature 5.4279 °C Pressure 1376.6 kPa Molar Flow Rate (kg mol/h)  Hydrogen 0.914 Cyclohexane 159.447 Benzene 194.944 n-Hexane 0.127 Total 355.430 Mole % Composition  Hydrogen 0.256 Cyclohexane 44.861 Benzene 54.846 n-Hexane 0.036 Total 1.000 Figure 3: T-xy Plot for Benzene and Cyclohexane at 150 kPa Figure 4: x-y Plot for Benzeneand Cyclohexane Solvent Selection for the Benzene-Cyclohexane Binary Mixture In order to perform a successive extractive distillation, a solvent needs to be chosen to "break" the azeotrope that forms at the operating pressure of the distillation column. Recommended solvent for the benzene-cyclohaxane mixture from the literature,,,is aniline, with a solvent to feed ratio (S/F) of 4, which will shift the azeotropic point toward the corner of the high-boiling component cyclohexane, and the equilibrium curve of the original components fall below the diagonal (Figure 5). Figure 5: Elimination of Azeotropic Point with the Addition of a Separating Agent As was stated in the above section, the primary goal of solvent selection is to identify a group of feasible solvents to perform a good separation. The desired product, i.e. cyclohexane should have a purity of above 99% to meet the market standard. Aniline was the first solvent that had been put to the simulator to be tried out, as it is of the same homologous group as benzene. As can be shown from the result in Table 2, this solvent will produce the desire production rate of 150 with the solvent flow rate of 3500, i.e. a S/F ratio of 9.85. However, the product purity can only reach 70.08% and this does not meet our product specification. As a result, other solvent may have to be researched to perform the desire separation. We will have to perform the solvent selection criteria as stated in the preceding section. At the column pressure of 150 kPa, cyclohexane and benzene boil at 94.34 °C and 93.49 °C respectively and form a minimum-boiling azeotrope at 91 °C. The natural volatility of the system is benzene > cyclohexane, so the favored solvents most likely will be those that cause the benzene to be recovered in the distillate. However, in order to get a better quality of product, we would like to recover cyclohexane as the distillate rather than from the bottom stream. Thus, solvent to be chosen should give positive deviations from Raoult's law for cyclohexane and negative (or zero) deviation for benzene. Feed Stream Molar Flow Rate (kg mol/h)  Hydrogen 0.914 Cyclohexane 159.447 Benzene 194.944 n-Hexane 0.127 Total 355.43 Solvent Stream (Aniline) molar flow rate (kg mol/h) 3500 Solvent/Feed (S/F) ratio 9.85 Distillate Product Molar Flow Rate (kg mol/hr)  Hydrogen 0.914 Cyclohexane 150 Benzene 62.956 n-Hexane 0.127 Aninline 0.058 Total 214.05 Mole % Composition in the Distillate Product  Hydrogen 0.427 Cyclohexane 70.08 Benzene 29.41 n-Hexane 0.059 Aniline 0.027  Total 1.000   Solvent Class Solute Class Group 1 2 3 4 5 6 7 8 9 10 11 12 11 Aromatic, olefin, halogen aromatic, + + + 0 + 0 0 - 0 + 0 0 multihola paraffine without active H, monohalo paraffine 12 Paraffin, carbon disulfide + + + + + 0 + + + + 0 + Solvent/solut eclass Group Solvent/soluteclass Group 4 Active -H in multihalo paraffin 8 Primanry amine, ammonia, amide with 2H or N Ether, oxide, sulfoxide 7 Secamine 9 Turning to the Robbins Chart(Table 3), we note that solvents that may cause the positive deviation for cyclohexane (Class 12) and negative (or zero) to benzene (Class 11) came from the groups of 4, 7, 8 and 9, which consist of polyol, amine and ether. We further consider the solubility, the hydrogen bonding effect, and also the homologous characteristic of the solvent with the corresponding components in the feed mixture. As few candidate solvents that had been put to the computer simulation, included phenol (homologous to benzene), 1,2-benzenediol (homologous to benzene, with -OH group that will produce hydrogen bonding), 1,3-butanediol (with -OH group that will produce hydrogen bonding), and also 1,2-propanediol (same characteristic as with 1,3-butanediol). 1,2-propanediol (often known as propylene glycol), seem to give the most promising results compared to the other solvents. This result may be caused from the high solubility of benzene in this solvent and the hydrogen bonding that were formed between the two constituents. Simulation result of this solvent can be view in Table 4.
  4. 1 point
    Distillation is the most widely used separation technique in the chemical and petroleum industry. However, not all liquid mixture are amenable to ordinary fractional distillation. When the components of the system have low relative volatilities (1.00 < a < 1.05), separation becomes difficult and expensive because a large number of trays are required and, usually, a high reflux ratio as well. Both equipment and utilities costs increase markedly and the operation can become uneconomical. If the system forms azeotropes, as in a benzene and cyclohexane system, a different problem arises, - the azeotropic composition limit the separation, and for a better separation this azeotrope must be bypassed in some way. At low to moderate pressure, with the assumption of ideal-gas model for the vapor phase, the vapor-liquid phase equilibrium (VLE) of many mixture can be adequately describe by the following Modified Raoult's Law: Eq .(1)  where: yi = mole fraction of component i in vapor phase xi = mole fraction of component i in liquid phase P = system pressure Psat = vapor pressure of component i ?i = liquid phase activity coefficient of component i When ?i = 1, the mixture is said to be ideal Equation 1 simplifies to Raoult's Law. Nonideal mixtures  (?i ? 1) can exhibit either positive (?i > 1) or negative deviations (γi < 1) from Raoult's Law. In many highly nonideal mixtures these deviations become so large that the pressure-composition (P-x, y) and temperature-composition (T-x, y) phase diagrams exhibit a minimum or maximum azeotrope point. In the context of the T-x, y phase diagram, these points are called the minimum boiling azeotrope(where the boiling temperature of the azeotrope is less than that of the pure component) or maximum boiling azeotrope (the boiling temperature of the azeotrope is higher than that of the pure components). About 90% of the known azeotropes are of the minimum variety. At these minimum and maximum boiling azeotrope, the liquid phase and its equilibrium vapor phase have the same composition, i.e., xi = yi for i = 1, ..., c Eq. (2) Two main types of azeotropes exist, i.e. the homogeneous azeotrope, where a single liquid phase is in the equilibrium with a vapor phase; and the heterogeneous azeotropes, where the overall liquid composition which form two liquid phases, is identical to the vapor composition. Most methods of distilling azeotropes and low relative volatility mixtures rely on the addition of specially chosen chemicals to facilitate the separation. The selection of the separating agent will be discussed later. The five methods for separating azeotropic mixtures are: Extractive distillation and homogeneous azeotropic distillation where the liquid separating agent is completely miscible. Heterogeneous azeotropic distillation, or more commonly, azeotropic distillation where the liquid separating agent, called the entrainer, forms one or more azeotropes with the other components in the mixture and causes two liquid phases to exist over a wide range of compositions. This immiscibility is the key to making the distillation sequence work. Distillation using ionic salts. The salts dissociates in the liquid mixture and alters the relative volatilities sufficiently that the separation become possible. Pressure-swing distillation where a series of column operating at different pressures are used to separate binary azeotropes which change appreciably in composition over a moderate pressure range or where a separating agent which forms a pressure-sensitive azeotrope is added to separate a pressure-insensitive azeotrope. Reactive distillation where the separating agent reacts preferentially and reversibly with one of the azeotropic constitutes. The reaction product is then distilled from the nonreacting components and the reaction is reversed to recover the initial component.

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