Archive for June, 2013

Dead Band in Process Control

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Dead Band in Process Control

The range through which an input signal can be varied, upon reversal of direction, without initiating an observable change in the output signal. Dead band is the name given to a general phenomenon that can apply to any device. For the valve assembly, the controller output (CO) is the input to the valve assembly and …

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Vapor-Liquid Equilibria

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What we know about vapor-liquid equilibria is mainly based on experimental data. There are of course predictive methods, but they are based on the results of measurements for the determination of the parameters they involve and the control of their validity. Accordingly, this aspect must be emphasized. However, we will not develop here the experimental …

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Membrane Separations

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Membrane separation operations are still seldom used in refining today. The potential applications of the different ultrafiltration, pervaporation and gaseous permeation technologies are described in the other section. Since selectiv­ity is kinetic, a membrane separation module separates a mixture into prod­ucts whose composition may be very far from thermodynamic equilibrium. Provided the membrane is sufficiently …

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Fixed Bed Adsorption Operations

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Fixed Bed Adsorption Operations

In an adsorption operation the movement of the solid phase gives rise to seri­ous problems (solid phase displacement devices, mechanical resistance of the adsorbent, etc.). As a result, the adsorbent is generally implemented in a fixed bed. The adsorption-desorption operation is carried out batch-wise as shown in the simplified flowsheet in Figure 1.11. The unit …

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Thermodynamic Equilibrium and Kinetic Factors

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Thermodynamic Equilibrium and Kinetic Factors

Table 1.1 shows that the most common separation operations are based on equilibrium selectivity, also termed thermodynamic selectivity. This selectivity is due to the fact that at thermodynamic equilibrium, the distribution of the component(s) to be separated out is different in the exist­ing phases. The difference is related either to the thermodynamic properties of the …

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Membrane Separation

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Membrane Separation

Separation can also be based on kinetic selectivity, i.e. on a difference in migration velocity of the relevant components. This principle is used in a membrane separation operation as represented schematically in Figure 1.5 for a binary mixture of two components A and B. Separation is based on the difference in diffusion velocity of components …

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Equilibrium Based Separation

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Separation operations based on equilibrium selectivity can be classified according to the separating agent that is used (matter or energy) and the nature of the phases in contact: Distillation is the most commonly used basic operation. The feed is nor­mally liquid, but refrigerated distillation can also be used to fractionate a gaseous mixture. The second …

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Function of Separation Operations in Refining

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Function of Separation Operations in Refining

A crude oil is made up of a complex mixture of a very large number of compo­nents, most of which are hydrocarbons. In order to get petroleum products that meet given specifications, it is first necessary to sep­arate out the different fractions or cuts, which then need to be purified or con­verted, especially when they …

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Differential Pressure in Distillation Tower

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The tower differential pressure is the difference between the pressures at the bottom of the tower and the pressures at top of the tower. This difference in pressure is caused by the flow of vapor in the tower. Without vapor flow there is no different pressure. Generally if the rate which the vapor moves up …

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Temperature Control in Distillation Tower

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The temperature decreases as the material move higher in the tower. The gradual decrease in temperature from the bottom of the distillation tower to the top is called the temperature gradient. The temperature gradient is measured in term of difference between the temperature at the bottom of the tower and the temperature at the top …

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