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) ... 

The control system consists of the following loops fresh feed Fqa is manipulated to control reactor composition za, fresh feed Fqb is manipulated to control reactor holdup Vr, reactor effluent F is How controlled, product flow rate P is manipulated to hold drum holdup M/> constant, and recycle vapor R is manipulated to control product composition xpB- Assume and Mp are held perfectly constant. The molecular weight of component A is 30 the molecular weight of component B is 70. 

M ratio of actual drum holdup to the design holdup p angle subtended by flight at center of the drum... 

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... 

Horizontal Blowdown Drum/Catch Tank This type of drum, shown in Fig. 26-16, combines both the vapor-liquid separation and holdup functions in one vessel. Horizontal drums are commonly used where space is plentiful, such as in petroleum refineries and petrochemical plants. The two-phase mixture usually enters at one end and the vapor exits at the other end. For two-phase streams with very high vapor flow rates, inlets may be provided at each end, with the vapor outlet at the center of the drum, thus minimizing vapor velocities at the inlet and aiding vapor-hquid separation. 

As an alternative to special pressure slop storage, the necessary holdup may be provided in a non-condensible blowdown drum. 

The knock-oul drums or separator tanks/pots can be designed using the techniques offered in the chapter on Mechanical Separadon, and will not be repeated here. API-RP 521 [13] specifies 20-30 minutes holdup liquid capacity from relief devices plus a vapor space for dropout and a drain volume. 

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-... 

CONSTANT MOLAR OVERFLOW AND CONSTANT PLATE HOLDUP CONSTANT HOLDUP IN REBOILER AND SURGE DRUM... 

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. ... 

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