This paper describes a study of the possibility of using point efficiencies, measured in a small laboratory column, in the scale-up of distillation efficiencies. The conditions on a large rectangular tray were closely duplicated in a small glass Oldershaw column with one sieve tray having similar hole size and percent free area. Point efficiencies on the large rectangular sieve tray were deduced from the composition profiles and compared with the point efficiencies measured in the small column. A simple relationship between the two columns was found in terms of the froth heights and liquid holdups. The results demonstrate that the slightly lower point efficiencies measured in the Oldershaw column could be used in conjunction with a mathematical mass transfer model based on the eddy diffusion concept, to relate the point efficiencies to the tray efficiences, resulting in a conservative design provided either that allowance is made for other factors, such as nonuniform flow patterns and stagnant zones, or that these are eliminated. A wide range of composition was covered using the two systems MeOH-nPrOH and EtOH-nPrOH operating in the froth region. M. M. Dribika, M. W. BiddulphUniversity of Nottingham Department of Chemical Engineering University Park, Nottingham, England SCOPEThe prediction of the performance of a large-scale distillation tray column is difficult, particularly if a new system is being processed. It is a problem that has attracted a considerable amount of attention over many years. However, the prediction of tray efficiency is still a very uncertain part of the design unless operating data from a similar size column distilling a similar system are available. The problem is that both mass transfer characteristics and tray hydraulic considerations are involved, and so the tray efficiency is a function of system and tray size. This paper is concerned with the mass transfer part of the problem. It describes a study of the relationship betwen point efficiencies, measured in a small glass Oldershaw Column, and the efficiencies inferred at points along a rectangular tray with a liquid flow path length of about 1 m. The tray used is a sieve type with small hole sizes typical of those used in the cryogenic distillation of air. The hole sizes and percent free area in the two trays being compared are similar. CONCLUSIONS AND SIGNIFICANCEThe relationship between point efficiencies measured in a small laboratory column and the point efficiencies that actually exist on a large rectangular tray were studied using two systems, MeOH-nPrOH and EtOH-nPrOH, and covering a wide range of composition. The composition profiles across the tray were measured and used to infer the values of point efficiencies using a mathematical mass transfer model based on the eddy diffusion concept.Similar conditions to those on the large rectangular 1864November 1986 sieve tray were used in a small Oldershaw column with one sieve tray and external downcomers. The results indicate that the point efficiencies obtained on the large rectangu...
The objective of this work was to study the undesirable oscillating behavior which can develop on distillation trays. This takes the form of violent lateral movements of the gadliquid mixture, causing premature flooding and reduced efficiency. It has been demonstrated that entrainment is significantly increased by the presence of oscillations. A simple predictive method by which a designer may evaluate the likelihood of oscillations occurring is proposed. It is concluded that columns of greater than about 1.0 m diam. operating at atmospheric pressure or above are unlikely to oscillate. However, reduced pressure columns of greater diameter may oscillate. A very simple mesh baffle system which completely prevents oscillations developing is described. MICHAEL SCOPEThe objective of this work was to study the violent lateral oscillations of the gadliquid mixture which can occur on distillation trays operating at high vapor loadings. A further objective was to develop a simple method which would be useful for a designer to assess the likelihood of oscillations developing in a column.Oscillations are generally undesirable since they lead to reduced capacity and premature flooding. These symptoms have often been noticed in newly constructed columns, and while direct observation of the cause is usually impossible, it is suggested here that oscillations may have been responsible. It appears likely that columns operating at pressures below atmospheric are more susceptible to oscillations, and some conclusions on this are presented. Since previous observations of oscillation have been on fairly small columns (about 0.3 m diam.), a larger column of diam. of 0.69 m was set up and operated with the system air/water. The column was completely transparent to allow close observation of the operating behavior. All the usual parameters were noted, and in addition the backmixing due to turbulence was measured using the salt injection technique of Barker and Self (1962). Entrainment was measured with oscillations present and then with the same loadings but with oscillations prevented by baffles. Two different sieve trays were studied with a range of air and water rates. The oscillation results were compared with those previously reported. CONCLUSIONS AND SIGNIFICANCEIt was observed that two distinct types of oscillation are possible. In the first type, Full-wave oscillation, the gadliquid biphase moves simultaneously from the walls to meet at the center of the tray. Then the motion reverses and the biphase moves outwards across the direction of liquid flow to strike the walls at the same time. When the biphase comes to a peak in the center and when it hits the walls, spray is hurled upwards and entrainment is increased. This oscillation occurs when a fairly well defined critical vapor velocity is reached. If the vapor velocity is increased, the nature of the biphase becomes somewhat confused, with peaks moving about the tray and occasionally hitting the walls and causing spray to be hurled up towards the tray above. This was still de...
New experimental data presented concern effects of Marangoni surface-tension forces on the mass-transfer eficiency of packed distillation columns. Various random packings of sizes 6 and 25 mm, as well as structured packings of sizes 30 mm diameter and 150 mm, were used with the system n-propanol/water. This system can behave either as a Marangoni positive system at low concentrations of n-propanol or as a negatiue system at high concentrations of n-propanol. It was confirmed that small packings show lower eficiencies for Marangoni negative systems, due to the breakup of the film of liquid on the packing surface. In the larger column, howeuer, the situation is reversed with the negative mixtures often showing better eficiencies, especially at heauier loadings. This is attributed to the observation that spray and droplet formation are much more pronounced for the negative mixtures, providing extra surface for mass transfer and, hence, better mass-transfer eficiency. There are also some indications that the pressure drop is unexpectedly high for these conditions and that premature flooding can occur. This may haue design implications for any mass-transfer deuice in which vapor and liquid phases are contacted in narrow passageways.Packing Suitace Area (sq mlcu m) Figure 4. Effectiveness of some small packings.
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