Pressure optimisation

Throttling either the inlet or discharge from an ejector, or throttling the motive fluid, is not generally successful because of the impact they have on ejector performance. However it is common for the ejector spillback to be closed and the pressure controller on manual. This is not a reflection on the performance of the scheme. It can be economically very attractive to operate the column at the lowest possible pressure, even if this means the pressure fluctuating somewhat. We will cover later in the chapter techniques for compensating for such fluctuations so that product composition is not affected. And we will also return later to pressure optimisation. 

Materials suitable as filter aids include diatomaceous earth, expanded perilitic rock, asbestos, ceUulose, nonactivated carbon, ashes, ground chalk, or mixtures of those materials. The amount of body feed is subject to optimisa tion, and the criterion for the optimisa tion depends on the purpose of the filtration. Maximum yield of filtrate per unit mass of filter aid is probably most common but longest cycle, fastest flow, or maximum utilisation of cake space are other criteria that requite a different rate of body feed addition. The tests to be carried out for such optimisation normally use laboratory or pilot-scale filters, and must include variation of the filtration parameters such as pressure or cake thickness in the optimisation. 

Selection of the high pressure steam conditions is an economic optimisation based on energy savings and equipment costs. Heat recovery iato the high pressure system is usually available from the process ia the secondary reformer and ammonia converter effluents, and the flue gas ia the reformer convection section. Recovery is ia the form of latent, superheat, or high pressure boiler feedwater sensible heat. Low level heat recovery is limited by the operating conditions of the deaerator. 

For the ISTIG cycle, Fig. 6.18 shows thermal efficiency plotted against specific work for varying overall pressure ratios and two maximum temperatures of 1250 and 1500°C. Peak efficiency is obtained at high pressure ratios (about 36 and 45, respectively), before the specific work begins to drop sharply. Note that the pressure ratios of the LP and HP compressors were optimised within these calculations. 

Macchi et al. provided a similar comprehensive study of the more complex RWI cycles as illustrated in Fig. 6.19, which shows similar carpet plots of thermal efficiency against specific work for maximum temperatures of 1250 and 1500°C, for surface intercoolers. The division of pressure ratio between LP and HP compressors is again optimised within these calculations, leading to an LP pressure ratio less than that in the HP. For the RWI cycle at 1250°C the optimisation appears to lead to a higher optimum overall pressure ratio (about 20) than that obtained by Horlock [5], who assumed LP and HP pressure ratios to be same in his study of the simplest RWI (EGT) cycle. His estimate of optimum pressure ratio... 

Rice [15] made a comprehensive study of the reheated gas turbine eombined plant. He first analysed the higher (gas turbine) plant with reheat, obtaining (t o)h> turbine exit temperature, and power turbine expansion ratio, all as funetions of plant overall pressure ratio and firing temperatures in the main and reheat burners. (The optimum power turbine expansion ratio is little different from the square root of the overall pressure ratio.) He then pre-seleeted the steam eyele eonditions rather than undertaking a full optimisation. 

Similar considerations apply to the selection of pressure drops where there is freedom of choice, although a full economic analysis is justified only in the case of very expensive units. For liquids, typical values in optimised units are 35 kN/m2 where the viscosity is less than 1 mN s/m2 and 50-70 kN/m2 where the viscosity is 1-10 mN /m2 for gases, 0.4-0.8 kN/m2 for high vacuum operation, 50 per cent of the system pressure at 100- 200 kN/m2, and 0 per cent of the system pressure above 1000 kN/m2. Whatever pressure drop is used, it is important that erosion and flow-induced tube vibration caused by high velocity fluids are avoided. 

While additive analysis of polyamides is usually carried out by dissolution in HFIP and hydrolysis in 6N HC1, polyphthalamides (PPAs) are quite insoluble in many solvents and very resistant to hydrolysis. The highly thermally stable PPAs can be adequately hydrolysed by means of high pressure microwave acid digestion (at 140-180 °C) in 10 mL Teflon vessels. This procedure allows simultaneous analysis of polymer composition and additives [643]. Also the polymer, oligomer and additive composition of polycarbonates can be examined after hydrolysis. However, it is necessary to optimise the reaction conditions in order to avoid degradation of bisphenol A. In the procedures for the analysis of dialkyltin stabilisers in PVC, described by Udris [644], in some instances the methods can be put on a quantitative basis, e.g. the GC determination of alcohols produced by hydrolysis of ester groups. 

On-line SFE-SFC method development for validated quantitative analysis of PP/(Irganox 1010/1076, Tinuvin 327) has been reported [93]. SFE conditions required optimisation of extraction time and pressure, matrix type (particle or film) and matrix parameters (particle size, film thickness, sample weight). About 30% of extracts were lost during collection. Very poor recoveries (20-25 %) were reported from ground samples (particle size 100 p,m dependent recoveries of 45-70% for 30-p.m-thick films. Biicherl... 

Reports of on-line SFE-FIPLC are rare, perhaps because the majority of analytes that have been extracted using SFE can be separated using either GC or SFC. On-line SFE-HPLC is often used to monitor extraction efficiencies. SFE-HPLC optimised for temperature (120 °C), pressure (384 bar), SCF flow and modifier (methanol) has been used for the quantification of Irganox 1010 and Irgafos 168 extracted from PP. In this case Thilen and Shishoo [12] varied three SFE parameters for optimisation of the extraction efficiency, and five parameters for the collection efficiency, see Figures 7.7 and 7.8. Despite these efforts, low recoveries were observed (Table 7.16). This was attributed to problems associated with the compounding process, and not to uncertainties in the extraction and analytical method. 

LC-APCI-MS is a derivative of discharge-assisted thermospray, where the eluent is ionised at atmospheric pressure. In an atmospheric pressure chemical ionisation (APCI) interface, the column effluent is nebulised, e.g. by pneumatic or thermospray nebulisation, into a heated tube, which vaporises nearly all of the solvent. The solvent vapour acts as a reagent gas and enters the APCI source, where ions are generated with the help of electrons from a corona discharge source. The analytes are ionised by common gas-phase ion-molecule reactions, such as proton transfer. This is the second-most common LC-MS interface in use today (despite its recent introduction) and most manufacturers offer a combined ESI/APCI source. LC-APCI-MS interfaces are easy to operate, robust and do not require extensive optimisation of experimental parameters. They can be used with a wide variety of solvent compositions, including pure aqueous solvents, and with liquid flow-rates up to 2mLmin-1. 

If the vessel is a pressure vessel the optimum length to diameter ratio will be even greater, as the thickness of plate required is a direct function of the diameter see Chapter 13. Urbaniec (1986) gives procedures for the optimisation of tanks and vessel, and other process equipment. 

Cost the system and optimise to make the best use of the pressure drop available, or, if a blower is required, to give the lowest operating cost. 

Nie X-R (1998) Optimisation Strategies for Heat Exchanger Network Design Considering Pressure Drop Aspects, PhD Thesis, UMIST, UK. 

Use increased pressures or rotor speed but avoid super-cavitation by operating below a certain optimum value Optimisation needs to be carried out depending on the application. Higher diameters are recommended for applications which require intense cavitation whereas lower diameters with large number of holes should be selected for applications with reduced intensity Lower free areas must be used for producing high intensities of cavitation and hence the desired beneficial effects... 

Although various processes may have been subjected to optimisation in recent years as a result of economic pressures, a survey in the 1980s revealed disparities between different sectors of the industry as summarised in Table 10.29 for batchwise wool bleaching methods. 

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