Pump-around rates

El. A system known as a punp-around is shown below. Saturated liquid is withdrawn from stage 2 above the partial reboiler, and the liquid is returned to stage 3 (assume it is still a saturated liquid). Pump-around rate is P = 40.0 kmol/h. The column is separating methanol and water at 101.3 kPa. The feed flow rate is 100.0 kmol/h. The feed is 60.0 mol% methanol and 40.0 mol% water. The feed is saturated liquid. We desire a bottoms product that is 2.5 mol% methanol. The distillate product should be 95.0 mol% methanol. The column has a total condenser and the reflux is a saturated liquid. Assume CMO. Use (L/D) = 2.0 x (L/D)Tnin- Data are given in Table 2-7. Find Xp, the optimum feed stage and total number of equilibrium stages required. 

Pump-Around Many fractionation towers have pump-arounds to remove excess heat in the key sections of the tower. The effect of increasing pump-around rate is reduced internal reflux rate in the trays above the pump-around, but increased internal reflux rate below the pump-around. Thus, change in pumparound duty affects fractionation. On the other hand, pump-around rates and return temperature have effects on heat recovery via the heat exchanger network. It is not straightforward in optimizing pump-around duties and temperamres since the effects on both fractionation and heat recovery can only be assessed in a simulation model. An APC application incorporated with process simulation should be able to handle this optimization. 

As shown in Fig. 13-92, methods of providing column reflux include (a) conventional top-tray reflux, (b) pump-back reflux from side-cut strippers, and (c) pump-around reflux. The latter two methods essentially function as intercondenser schemes that reduce the top-tray-refliix requirement. As shown in Fig. 13-93 for the example being considered, the internal-reflux flow rate decreases rapidly from the top tray to the feed-flash zone for case a. The other two cases, particularly case c, result in better balancing of the column-refliix traffic. Because of this and the opportunity provided to recover energy at a moderate- to high-temperature level, pump-around reflirx is the most commonly used technique. However, not indicated in Fig. 13-93 is the fact that in cases h and c the smaller quantity of reflux present in the upper portion of the column increases the tray requirements. Furthermore, the pump-around circuits, which extend over three trays each, are believed to be equivalent for mass-transfer purposes to only one tray each. Bepresentative tray requirements for the three cases are included in Fig. 13-92. In case c heat-transfer rates associated with the two pump-around circuits account for approximately 40 percent of the total heat removed in the overhead condenser and from the two pump-around circuits combined. 

HD relies on good vascular access to the circulation of the patient to enable blood to be pumped around the extracorporeal circuit at a rate in excess of 300 mL per minute. This ahflity was not introduced until the 1960s, Although Kolff in Groningen Hospital in the Netherlands performed the first dialysis experiments in humans in 1943, the problem of dialysis support with long-term vascular access was not solved until Scribner developed the arteriovenous cannula in 1960, This was followed by the development of the surgically... 

The large amount of seed presents a high-growth area from the outset of the operation. The initial presence of the seed avoids the issue of when to add it to prevent seed dissolution and homogeneous nucleation. Note that for this process, the size of the particles does not need to decrease, as steady-state particle size distribution is reached at the low but finite natural attrition rate achieved in the pump-around and agitation systems. 

McNulty, K. J. and Chatterjee, S. G., Simulation of Atmospheric Crude Towers Including Packed Bed Pump-around Zones using a Rate-based Simulator, The Institution of Chemical Engineers Symposium Series No. 128, Distillation and Absorption 1992, A329-A344 (1992). 

The pulse to the molecular valve is repeated at a rate from one to ten Hz, depending on the carrier gas, back pressure, and nozzle size. Typically, 4 x particles are released per gas pulse. We use a ten-inch diffusion pump that can pump around 4 x lol particles per second, hence the 10 Hz repetition rate. 

Column Component Flow Column Component Fraction Column Component Ratio Column Component Recovery Column Cut Point Column Draw Rate Column DT [Heater/Cooler) Spei Column Dt Spec Column Duty Column Duty Ratio Column Feed Ratio Column Gap Cut Point Column Liquid Flow Column Physical Properties Spec Column Pump Around Column Reboil Ratio Spec Column Recovery... 

Side columns are used, for instance, in the most important distillation processes worldwide, the fractionation of air (see Fig. 11.2-18) and the distillation of cmde oil (Meyers 1996). The atmospheric tower of oil refineries consists of a main column and four stripping side columns (Fig. 11.2-12). In this tower the crude oil is split into six fractions which are processed further in several subsequent columns. Oil refineries also have some other interesting features. Steam is fed into the bottom of the main column and most of the side columns. This causes a stripping effect and reduces the temperatures in the columns (steam distillation). The overhead fractions of all side columns are fed into the main colunrn thus increasing the vapor flow there. So-called pump arounds effect a partial condensation of the vapor in the main column and, in turn, a reduction of the vapor flow rates in the upper sections. 

To test alternative control strategies, Prett and Morari (1986) provide a linearized model, referred to as the Shell process, of a distillation tower to separate crude oil into fractions in a refinery. Part of the model describes the dynamics of the two top compositions as a function of the manipulated variables (the two top draw rates) and two key disturbances (the heat removal loads in pump-around streams used to remove heat and create intermediate reflux). For this example, it is sufficient to examine die matrices specific to the nominal model ... 

It is essential to avoid flooding and dumping, which could severely affect fractionation and thus energy efficiency. Fractionation efficiency can be monitored by column internal V/F ratios. Desired V/F can be achieved jointly by optimizing feed heater outlet temperature, fractionation stripping steam, and overhead reflux rate together with pump-around heat duties, which are used to control excess heat in the column. 

For a simple tower that does not have any pump-arounds or side draws, there are two sections, namely, rectification section, which is above the feed tray, and stripping section below the feed tray. The rectification section has a vapor rate higher than the liquid rate, whereas it is reversed for the stripping section. The UV ratio is the indication of distillation that could take place. For most towers, the L/V ratio is 0.3-3.0. L/V ratios outside this range may give sloppy or too easy distillation. Determination of UV ratio is a major part of energy optimization, which will be discussed in detail. 

For a fractionation column, the key operating conditions are the temperature and pressure of operation, vapor-to-liquid ratios, feed temperature, reflux rate, and pump-around duties. These parameters have significant effects on energy use and fractionation efficiency. 

Sample Problem Calculate the flow rate (FR) of the following pump-around system. 

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