Distillation capacity parameter

The absolute pressure may have a significant effect on the vapor—Hquid equiHbrium. Generally, the lower the absolute pressure the more favorable the equiHbrium. This effect has been discussed for the styrene—ethylbenzene system (30). In a given column, increasing the pressure can increase the column capacity by increasing the capacity parameter (see eqs. 42 and 43). Selection of the economic pressure can be faciHtated by guidelines (89) that take into consideration the pressure effects on capacity and relative volatiHty. Low pressures are required for distillation involving heat-sensitive material. 

CP Capacity parameter in distillation (m-s-1) or heat capacity flowrate (kW-R, MW-R-1)... 

Packed distillation towers can often be operated over a moderate range of flow rates at nearly constant separation efficiency. Data for isooctane-toluene separation at total reflux are shown in Fig. 22.25. The three Intalox metal (IMTP) packings numbered 25, 40, and 50 correspond to nominal sizes of 1,1.5, and 2 in., respectively. As the capacity parameter increases, both the liquid rate and the vapor rate increase, which explains why HETP is nearly constant. The gas film has the controlling resistance to mass transfer, and Hoy increases with the 0.3 to... 

Operating the column at the minimum pressure minimizes the energy cost of separation. Towering this pressure increases the relative volatility of distillation components and thereby increases the capacity of the reboiler by reducing operating temperature, which also results in reduced fouling. Reducing pressure also affects other parameters, such as tray efficiencies and latent heats of vaporization. 

As discussed in the previous section, the work of Mayur et al. (1970) and Christensen and Jorgensen (1987) on the optimal recycle policy was restricted to binary mixtures. The benefits of recycling were measured in terms of a reduction in batch time although increase in productivity could be a possible alternative. Luyben (1988) considered this productivity measure (as defined as "capacity" which includes both batch time and a constant charging and cleaning time) in a simulation of multicomponent batch distillation with recycle. Luyben (1988), however, showed the effect of different parameters (no of plates, relative volatilities, etc.) on the productivity and did not actually consider the effect of off-cuts recycle on the productivity. 

The calculation of column diameter for distillation and absorption columns %h,6 is usually based on the flooding velocity, which, in turn, requires values of the flooding capacity factor, Cf- Fair s flooding-capacity plot for sieve trays [1] correlates the flooding capacity factor with a flow parameter Flv for each tray-spacing value, t, as shown in Figure 2. The flow parameter involves the liquid mass flow, LMi, and vapor mass flow, F v (both in lb/ s), as well as the densities of the two streams. 

The severity of the process increases inversely with LHSV. A low value of LHSV indicates that less amount of feed is being processed per hour (i.e., more contact time with the catalyst inventory). Usually, distillate HOT is carried out at higher LHSVs (>1.0) than residue HDT and HCR in general (<1.0). LHSV is usually a preestablished design parameter that determines the amount of catalyst and therefore reactor capacity for a required production rate. 

In addition, the experimental data indicate that the maximum value at constant flow parameter varies as the liquid viscosity to the —0.10 to —0.13 power. For liquid viscosities between 0.07 and 1.1 cps, an average of the —0.11 power of liquid viscosity can be used. Therefore, an ethanol/water distillation column would have a 3% lower maximum operational Q in the ethanol-rich rectifying section where the liquid viscosity is 0.39 cps than in the water-rich stripping section with a liquid viscosity of only 0.30 cps. Likewise, the data do not support an increase of more than 9% for the maximum operational Cs, as liquid viscosity is decreased below 0.20 cps. Equation 7-15 is used to calculate the maximum operational capacity for a particular service. 

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