Reflux drums holdup

Calculate liquid densities, molar tray and condenser-reflux drum holdups, ana hquor and vapor enthalpies. Determine holdup and enthalpy derivatives with respect to time by forward difference approximations. 

Reflux drum flows (R and D) =2 Reflux drum holdup (M,) = 1... 

Theoretical trays, equimolal overflow, and constant relative volatilities are assumed. The total amount of material charged to the column is M q (moles). This material ean be fresh feed with composition Zj or a mixture of fresh feed and the slop cuts. The composition in the still pot at the begiiming of the batch is Xgoj. The composition in the still pot at any point in time is Xgj. The instantaneous holdup in the still pot is Mg. Tray liquid holdup and reflux drum holdup are assumed constant. The vapor boilup rate is constant at V (moles per hour). The reflux drum, eolumn trays, and still pot are all initially filled with material of eomposition Xg j. 

Reflux drum (holdup Mj> is assumed constant total condenser) ... 

There is some disagreement about optimum reflux drum holdup. Small holdups of liquid are desirable firom the standpoint of reducing time constants in the overhead composition control loop. This permits faster and tighter composition control. 

Determining settings for the reflux drum level controller is, in this case, difficult unless a large reflux drum holdup is available. Preferably one should make 5 minutes level controller tuning will require a dynamic analysis of overall column material balance such as discussed in Chapter 14. If steam flow is metered by an orifice, it should be linearized with a square root extractor. 

Since reflux drum holdups are usually small compared with base holdups, a buffer tank in the top product line (not in the reflux line) is highly recommended. Top product composition may be controlled by trimming the steam/distillate ratio bottom composition may be controlled by trimming the bottom-prod-uct/distillate ratio. 

Next, several single-unit control issues for this plant will be considered—for example, whether the reflux flow rate R for the column will be under flow control or used as the manipulated variable to control the reflux drum holdup/level Hd or the distillate composition Depending on the application, either the bottoms composition xb can be controlled (Luyben, 1993), or both x and xb can be explicitly controlled to their set points (Luyben, 1994). Several alternative... 

Distillate composition, xj) Bottoms composition, xr Relative volatility, a Bottoms holdup. Hr Reflux drum holdup, Hj) Individual stage holdup, Hs... 

Molar holdups in condenser-reflux drum, on trays, and in reboiler ... 

Derivatives or rates of change of tray and condenser-reflux drum hquid holdup with respecl to time are sufficiently small compared with total flow rates that these derivatives can be approximated by incremental changes over the previous time step. Derivatives of liquid enthalpy with respect to time eveiywhere can oe approximated in the same way. The derivative of the liquid holdup in the reboiler can likewise be approximated in the same way except when reflux ratios are low. 

Example 10 Calculation of Multicomponent Batch Distillation A charge of 45.4 kg mol (100 Ih-mol) of 25 mole percent heuzeue, 50 mole percent monochlorohenzene (MCB), and 25 mole percent orthodichloro-henzene (DCB) is to he distilled in a hatch still consisting of a rehoiler, a column containing 10 theoretical stages, a total condenser, a reflux drum, and a distillate accumulator. Condenser-reflux drum and tray holdups are 0.0056 and... 

Reflux drums usually are horizontal, with a liquid holdup of 5 min half full. A takeoff pot for a second liquid phase, such as water in hydrocarbon systems, is sized for a linear velocity of that phase of 0.5 ft/sec, minimum diameter of 16 in. 

Holdup time is 5 min half full for reflux drums, 5-10 min for a product feeding another tower. 

Consider the binary batch distillation column, represented in Fig. 3.58, and based on that of Luyben (1973, 1990). The still contains Mb moles with liquid mole fraction composition xg. The liquid holdup on each plate n of the column is M with liquid composition x and a corresponding vapour phase composition y,. The liquid flow from plate to plate varies along the column with consequent variations in M . Overhead vapours are condensed in a total condenser and the condensate collected in a reflux drum with a liquid holdup volume Mg and liquid composition xq. From here part of the condensate is returned to the top plate of the column as reflux at the rate Lq and composition xq. Product is removed from the reflux drum at a composition xd and rate D which is controlled by a simple proportional controller acting on the reflux drum level and is proportional to Md-... 

Assuming a well-mixed, constant holdup reflux drum, the balance equation for each component i, in the drum, is... 

Molar holdup in still Molar holdup in reflux drum Vapor boil up rate Reflux rate Relative volatilities... 

For the reflux drum and condenser, assuming constant holdup... 

A single feed stream is fed as saturated liquid (at its bubblepoint) onto the feed tray N,. See Fig. 3.12. Feed flow rate is F (mol/min) and composition is z (mole fraction more volatile component). The overhead vapor is totally condensed in a condenser and flows into the reflux drum, whose holdup of hquid is Mj) (moles). The contents of the drum is assumed to be perfectly mixed with composition Xo The liquid in the drum is at its bubblepoint Reflux is pumped back to the top tray (iVj-) of the column at a rate R. Overhead distillate product is removed at a rate D. 

We will assume constant holdups in the reflux drum Aij> and in the column base Mg. Proportional-integral feedback controllers at both ends of the column will change the reflux flow rate and the vapor boilup V to control overhead composition and bottoms composition Xg at setpoint values of 0.98 and 0.02 respectively. 

Volumetric liquid holdups in the reflux drum and column base are held perfectly constant by changing the flow rates of bottoms product B and liquid distillate product. ... 

Weir hei dit and length and column diameter in rectifying section (in) Volumetric holdup in column base and in reflux drum (ft )... 

For example, it is important to have large enough holdups in surge vessels, reflux drums, column bases, etc., to provide effective damping of disturbances (a much-used rule of thumb is 5 to 10 minutes). A sufficient excess of heat transfer area must be available in reboilers, condensers, cooling Jackets, etc., to be able to handle the dynamic changes and upsets during operation. The same is true of flow rates of manipulated variables. Measurements and sensors should be located so that they can be used for eflcctive control. 

A. LIQUID HOLDUPS. The most common and most important trade-off is that of specifying holdup volumes in tanks, column bases, reflux drums, etc. From a steadystate standpoint, these volumes should be kept as small as possible because this will minimize capital investment. The more holdup that is needed in the base of a distillation column, the taller the column must be. In addition, if the material in the base of the column is heat-sensitive, it is very desirable to keep the holdup in the base as small as possible in order to reduce the time that the material is at the high base temperature. Large holdups also increase the potential pollution and safety risks if hazardous or toxic material is being handled. 

The molar holdup in the base of the column is MP and in the overhead accumulator is Mn. These holdups are not constant but vary with time. The total molar and component balances for the base and reflux drum are given below ... 

The liquid holdup in the base MP is 40 kmol and in the reflux drum, 40 kmol. Saturated liquid feed and reflux are assumed, so the liquid flowrate in the rectifying section is the reflux R and in the stripping section is R + F. 

At this time, the coolant valve is opened and the condensed liquid is stored into a reflux drum. The reflux valve is opened when the liquid fills the condenser holdup tank. At this point some product may also be collected simultaneously. 

Three parameters were identified and adjusted to validate the model against the experiments. The parameters are the heat losses, the nominal tray holdup and the Murphree tray efficiency (EM). Figure 4.16 shows how EM is adjusted to match the dynamic model prediction and experimental temperature profile measured on Plate 12. Figure 4.17 shows the comparison between the experimental and model prediction of ethanol composition in the reflux drum, middle vessel and in the bottom of the column. Figures 4.16-17 show a good match between the model prediction and experiments. 

In Policy 1, the feed is distributed equally among the reboiler, vessels and reflux drum. These holdups are kept constant and the column is run under total reflux. Only QR(t) and (/are optimised. 

In Policy 2, the feed is initially charged in the reboiler and holdups in the reflux drum, vessels and reboiler are allowed to vary. Here, Lt is optimised together with Qn(t) and tf. 

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