6.2 Separation processes 6.2.1 Characteristics kerosene, which is an oily combustible material used for of separation processes heating plants and as a propellant for jet planes, condenses between 250°C and 160°C; naphtha, which is The separation of solutions and mixtures into their single used as a fuel and as a raw material for pesticides, components is an operation of great importance for the fertilizers, and plastic materials, condenses between chemical, petrochemical, and oil industries. Almost all 160°C and 70°C. Gasoline, mainly used as a fuel for chemical processes need preliminary raw material planes and cars, condenses between 70°C and 20°C. At purification or the separation of primary from secondary 20°C only gaseous products like methane, ethane, products. These operations go against the tendency of propane, and butane remain. Butane and propane, in substances to intimately and spontaneously mix, which is, as particular, form the fuel called LPG (Liquefied is well-known, a consequence of the second law of Petroleum Gas). thermodynamics. For instance, salt mixed in water dissolves The above example shows how a separation process to give a homogeneous solution and the separation of the makes it possible to transform a mixture of substances components in the mixture requires the use of energy. In this into two or more products with different compositions. A case separation can be performed in one of the following separation process is fed by one or more streams, ways: whereas streams of products of different compositions • By heating the solution to make the water evaporate, and leave the separation equipment. Separation is caused by subsequently condensing it at a lower temperature. a separation agent that can be another mass stream, or an • By cooling the solution in order to separate the water in energy flux, or both. Often, separation processes the form of ice. generate the formation of a further phase, different from • By exploiting the selective properties of a membrane; that of the feed. For instance, by feeding a liquid stream, water passes through this membrane more easily than products can be made up of two streams, one liquid and salt. one vapour. Separation processes are of paramount importance Based on the above, it is possible to formulate a general in oil plants. Crude oil, in fact, contains a very high classification of the most commonly used separation number of hydrocarbons, which go from light gases to processes in industry. This is shown in Table 1, which heavy fractions which are difficult to distill even in a summarizes their essential characteristics. vacuum. The most important classes are alkanes and It is convenient to characterize separation processes by cycloalkanes (naphtenes), in particular, and aromatic means of a separation factor, defined as follows: compounds in various proportions. In refineries, the xx various fractions are separated by distillation and then [1] α s = ij11 ij xx treated further in order to supply different products of ij22 specific interest. Usually, first crude oil undergoes where xi indicates the molar fraction of component i and xj washing with water to remove salts and possible indicates the molar fraction of component j, whereas suspended particles, and then is evaporated in an oven, indexes 1 and 2 indicate the two streams of separation s which takes it to a temperature of about 400°C. Crude products. Therefore, aij represents the ratio between the oil vapours are then sent to a distillation column, or molar fractions of the two components i and j in the two sϭ refining tower, where the separation of the different streams 1 and 2. Therefore, if aij 1, the process does not sϾ hydrocarbon fractions is obtained: in the lower point of allow any separation of components i and j. If aij 1, the column combustion oils are condensed, together component i tends to concentrate in stream 1, whereas if sϽ with lubricating oils, paraffins, waxes, and bitumens; aij 1, this behaviour is manifested in component j. s gas oil condenses between 350°C and 250°C and is used Conventionally, the two components are chosen so that aij as a fuel in heating plants and in Diesel motors; is always greater than one. VOLUME V / INSTRUMENTS 319 PROCESS ENGINEERING ASPECTS Table 1. Classification of separation processes Type of process Feed Separating agent Products Separation principle Evapouration Liquid Heat Liquid and vapour Volatility difference Distillation Liquid Heat Liquid and vapour Volatility difference Absorption Gas Non volatile liquid Liquid and gas Preferential solubility Extraction Liquid Immiscible liquid Two liquids Different solubilities Difference in crystallization Crystallization Liquid Heat (heating or cooling) Liquid and solid temperature Difference in adsorption Adsorption Gas or liquid Adsorbing solid Fluid and solid characteristics Ionic exchange Liquid Solid resin Liquid and solid Adsorption equilibrium Solid-liquid extraction Solid Liquid Liquid and solid Diffusion and osmosis Drying Solid Heat Solid and vapour Volatility difference Sedimentation Slurry, dispersion Gravitational force Solid and liquid Density difference and centrifugation Filtration Suspension Filter Solid and liquid Dimensional difference Difference in dimensions Membrane processes Gas or liquid Membrane Gas or liquid or difference in membrane solubility Flotation Suspension Collector agents Solid and liquid Surface tension 6.2.2 Mass and energy balances +[]generation inside the system − in separation equipment −[]consumption inside thhe system A continuous separation plant can be considered as a thermodynamic system open to mass and energy exchange. It The balance can be applied to all the equipment or to is possible to associate to a separation equipment a series of any portion of it arbitrarily chosen. In the following, it will mass fluxes corresponding to feed streams and separation be assumed that no chemical reaction takes place inside the products as well as a series of energy fluxes necessary for the separation equipment and therefore mass generation and separation to take place. In normal conditions, continuous consumption terms are equal to zero. The balance equation, equipment works in stationary regime, so that the values of the therefore, assumes the following simplified form: intensive parameters of the system (the pressure, temperature [accumulation]ϭ[mass in]Ϫ[mass out]. and concentrations of the different components) are not time- In order to formulate the above in quantitative terms, dependent but vary according to position. Their gradients, in the mass of component i contained in the system will be fact, determine the rates at which mass and energy transfers indicated with mi and the mass flow rates of the same (e) take place in the different regions of the system. component in and out of the equipment with Fi and (u) Fi , respectively. Material balance can be therefore be Mass balances written as: The mass conservation principle should be applied to dm () () separation equipment, which in general terms [2] i =−∑FFe ∑ u dt i i (non-stationary conditions) can be expressed as follows: e u Summations at the second member are to be mass accumulation = performed on all in and out streams. The preceding in the system equation expresses the mass balance on component i. By summing balance equations relative to the different components, the following total mass balance equation = transpport inwards through − is obtained: the system surface bounndary dm () () [3] =−∑∑FFe u dt e u − transport outwards through + the system surface boundary In stationary regime, mi and m do not vary with time and therefore [2] and [3] become: 320 ENCYCLOPAEDIA OF HYDROCARBONS SEPARATION PROCESSES ()e = ()u when it moves from section 1 to section 2; dW is, on the ∑FFi ∑ i s [4] e u other hand, the work performed on the system by mechanical () () ∑∑FFe = u equipment, or subtracted by a turbine. Equation [8] can e u therefore be written in the following form: [9] ∆∆∆∆UKPVdmWQ+++Φδδ( ) =+ Similarly, it is possible to write molar balance equations s referring to species i and to total moles. If the number of By recalling the definition of the enthalpy function moles of component i contained in the system is indicated HϭUϩPV, and dividing all terms of the equation by dt one ˜ with ni and the molar flow rate of the component with Fi one obtains: obtains: [10] ()∆∆∆HKmWQ++Φ =+ dn s i =−(e) (u) и и ∑FFi ∑ i и dt e u where m is the mass flow rate and Ws and Q respectively [5] represent the quantities of mechanical and thermal energy dn (e) (u) =−∑∑FF delivered to the system in the unit time. By dividing dt и e u equation [10] by m one obtains: which in stationary regime becomes: ∆∆∆++=+Φ [11] HKWQs () () ∑FFe =∑ u i i where W˜ and Q˜ are the work and heat exchanged per unit e u s [6] mass of flowing fluid respectively. ()e = ()u ∑∑FF Equation [11] can be extended to systems with several e u inlet outlet streams and in this case it is necessary to calculate the difference between the sum of the values of the Energy balances variables for all the outlet streams and the sum of the values A characteristic of chemical equipment is the presence of the variables for all the inlet streams. Often, when of movements of fluid streams in which transformations can analysing chemical equipment, the potential and kinetic take place. In order to write energy balance equations for the energy terms are neglected and [11] simply becomes: separation equipment it is therefore necessary to combine [12] ∆HW=+ Q fluid mechanics and thermodynamics. Consider, for s instance, a system where a fluid flows continuously in a tube Expressed in this way, the energy balance is called between two sections 1 and 2, at different heights with enthalpy or thermal balance. respect to a reference plane. A pump supplies work, W, and a heat exchanger delivers (or subtracts) heat quantity Q.