Constant-pressure operation

In the filtration of a slurry, the resistance of the filter cake progressively increases and consequently, in a constant pressure operation, the rate of filtration falls. If the build-up of solids can be reduced, the effective cake thickness will be less and the rate of flow of filtrate will be increased. 

This kind of reactor can be used for isothermal constant pressure operations, of reactions having a single stoichiometry. For such systems the volume is linearly related to the conversion, or... 

Constant filtrate flow rate operation Constant pressure operation Constant filtrate flow rate operation Constant pressure operation... 

Another thing to consider with gradient elution is changes in the eluent viscosity. When gradient elution with a hydro-organic mobile phase is used (e.g., methanol-water), systematic variations in the flow rate are expected under conditions of constant-pressure operation, and systematic variations in the operating pressure will be found when a constant flow rate is used [1]. The compressibility of the solvent is species-specific. 

Discuss the simplifications of the nonequilibrium model that are possible when (a) constant pressure operation is assumed and (b) the resistance to mass transfer in the liquid phase is ignored. 

A high pressure syringe pump operated under constant pressure mode Is often used for low flow rate operation In narrow-bore packed column HFLC. Flow rates as low as 0.05 Mi /nln can be obtained. The drawback of constant pressure operation Is the potential flow rate variation due to a flow resistance change In the column. 

Membrane fouling may result in a significant increase in filtration resistance, leading to unstable filtration behavior. The pressure-driven membrane processes can be operated either with constant feed pressure or in constant flux mode. For constant pressure operation where the transmembrane pressure (TMP) is maintained at a constant value during the filtration, the flux will decline with time due to the... 

FIGURE 19.8 Conceptual stages of flux decline in UF at constant-pressure operation stage I, concentration polarization stage II, membrane fouling (usually due to protein deposition) stage III, further particle deposition or consolidation of the fouling layer. (From Marshall, A.D. et al.. Desalination, 91, 65, 1993. With permission.)... 

Constant rate followed by constant pressure operation... 

Figure 9.8 Plot of Ap = f(f) for constant rate followed by constant pressure operation (incompressible cakes)...

The same filter cloth material is to be used in a plate and frame filter press, each frame having dimensions 0.5 m x 0.5 m x 0.08 m, to filter the same slurry. The flow rate of slurry per unit area of cloth during the initial constant rate period is to be the same as that used in the preliminary experiment and the constant rate period is to be followed by constant pressure operation once the pressure reaches 8 x lO Nm". If the volume of cake formed per unit volume of filtrate, v is 0.02, calculate the time required to fill the frame. 

Another method is to use pilot test data from constant pressure operations carried out at different pressures. This yields a family of lines on a plot of t/V against V Figure 9.5) with varying slopes, from which the cake resistances a can be determined. 

Method 1— by calculating the filtration rate and using the solution for constant pressure operation. 

Consider reaction 1.1 (with first-order kinetics) conducted in a nonisothermal PFR. The mass balance for the PFR given by Equation 1.58 must now be supplemented by the energy balance. Restricting our attention to adiabatic constant-pressure operation, the energy balance may be written as... 

An optimum chamber pressure for turkey meat has been found experimentally by Sandall et al. [62] and has also been established through theoretical analysis [12]. Litchfield et al. [61] compared cyclic pressure and near-optimal constant pressure freeze drying processes in a situation in which operation at a pressure that would minimize drying time was not possible because of an interface temperature constraint at no time during the entire run did the cycled pressure process prove superior. In view of these considerations, it may be inferred that for materials exhibiting an attainable optimum with respect to pressure, there will be no advantage in cycling the chamber pressure when compared with near-optimal constant pressure operation. 

In order to measure the particulate fouling potential of feed water, Schippers and Verdouw [48] proposed a modified fouling index (MFI) by using a microfiltration membrane as a quick test of the feed water quality. They further improved the accuracy of the index and developed a new index of MFI-UF by using ultrafiltration membranes to increase the index sensitivity to the presence of colloidal particles for constant pressure operation [49, 50] and constant flux operation [51]. 

TABLE 7 Rheological data for constant pressure operation of LHDPE Marlex 6009, at 186°C ... 

Operating Constant pressure operation (70 kPa) Constant flow operation (5 m /m /... 

Since sliding pressure plants are operated at subcritical pressure at partial load, they achieve higher thermal efficiency than constant pressure operation at supercritical pressure. To improve the thermal efficiency at rated power, the high pressure plant, Kawagoe No. 2 started operation with conditions of 31 MPa and 566°C in 1989. This was followed by the high temperature plant, Tachibanawan No. 1, with conditions of 25 MPa and 610 C. 

Once-Through Boilers (UP Universal Pressure Boiler for Constant Pressure Operation)... 

By the sliding pressure, thermal efficiency of the turbine is improved in partial operating loads though with decreasing thermodynamic efficiency, as follows, in comparison with constant pressure operation. 

In comparison to constant pressure operations, a sliding type enables much Improvement in plant efficiency under partial load operations. 

Under the isobaric (constant pressure) operation of a 1 bar tank, the heat flow into the liquid is removed from the tank continuously by the latent heat of vaporisation of the boil-off vapour, apart from the portion entering the lower layer when there is two level stratification. 

---