Vapor load

To calculate the vapor load for a single column of a sequence, start by assuming a feed condition such that q can be fixed. Initially assume saturated liquid feed (i.e., q = 1). Equation (5.1) can be written for all NC components of the feed and solved for the necessary values of 0. There are (JVC - 1) real positive values of 0 which satisfy Eq. (5.1), and each lies between the a values of the... 

Having computed actual vapor load can be calculated... 

The problem with this approach is obvious. It involves a considerable amount of work to generate a measure of the quality of the sequence, the total vapor load, which is only a guideline. There are many other factors to be considered. Indeed, as we shall see later, when variables such as reactor conversion are optimized, the sequence might well need readdressing. 

Equation (5.8) tends to predict vapor loads slightly higher than those predicted by the full multicomponent form of the Underwood equation. The important thing, however, is not the absolute value but the relative values of the alternative sequences. Porter and Momoh have demonstrated that the rank order of total vapor load follows the rank order of total cost. 

The best few nonintegrated sequences can be identified most simply using the total vapor load as a criterion. If this is not satisfactory, then the alternative sequences can be sized and costed using shortcut techniques. 

Establish simple sequences. Using methods described in Chap. 5, sequences of simple columns with low overall vapor load are established. Consideration should not be restricted to the single sequence with the lowest overall vapor load, since many factors need to be considered in finally arriving at the best design. 

Pressure. Within limits, pressure may have Htfle effect in air-sparged LPO reactors. Consider the case where the pressure is high enough to supply oxygen to the Hquid at a reasonable rate and to maintain the gas holdup relatively low. If pressure is doubled, the concentration of oxygen in the bubbles is approximately doubled and the rate of oxygen deHvery from each bubble is also approximately doubled in the mass-transfer rate-limited zone. The total number of bubbles, however, is approximately halved. The overall effect, therefore, can be small. The optimum pressure is likely to be determined by the permissible maximum gas holdup and/or the desirable maximum vapor load in the vent gas. 

At higher vapor loads, the kinetic energy of the vapor rather than the bubble burst supphes the thrust for jets and sheets of hquid that are thrown up as well as the energy from breakup into spray. This yields much higher levels of entrainment. In distillation trays it is the most common limit to capacity. 

Will the facility be subject to the marine vapor loading MACT rule (Explain.)... 

VDdsg = Downcomer design velocity, GPM/fT Vioad = Column vapor load factor WFP = Width of tray flow path, in. pL = Liquid density, Ibs/ft pv = Vapor density, Ibs/ft ... 

N,n = Minimum theoretical stages at total reflux Q = Heat transferred, Btu/hr U - Overall heat transfer coefficient, Btu/hrfP"F u = Vapor velocity, ft/sec U d = Velocity under downcomer, ft/sec VD(js = Downcomer design velocity, GPM/fL Vioad = Column vapor load factor W = Condensate rate, Ibs/hr Xhk = Mol fraction of heavy key component Xlk = Mol fraction of the light key component a, = Relative volatility of component i versus the heavy key component... 

Fig. 15. Effect of vapor loading rate on gasoline working capacity...

Also, the vapor load at the time of relief may be reduced below the normal design rate, due to the higher pressure, which may suppress vaporization at the time of the overpressure. Pinchout of a reboiler is such a situation. In such a case, steam pressure design conditions may be used, rather than the maximum steam pressure which could exist under pressure relieving conditions of the steam system. These changes can be taken into account, where appropriate, both for the equipment involved and for downstream equipment. 

Loss of Heat in Series Fractionation System - In series fractionation, i.e., where the bottoms from the first column feeds into the second column and the bottoms from the second feeds into the third, it is possible for the loss of heat input to a column to overpressure the following column. Loss of heat results in some of the light ends remaining with the bottoms and being transferred to the next column as feed. Under this circumstance, the overhead load of the second column may consist of its normal vapor load, plus the light ends from the first column. If the second column does not have the condensing capacity for the additional vapor load, excessive pressure could occur. 

Determining Vapor Loads from Liquid-Containing Vessels Exposed to Fire... 

The single contingency which results in the largest vapor load regardless of any associated liquid load, is used to determine the maximum required vapor space C-F, and a high level alarm is placed at point C. 

Vapor load considerations must include all safety valve, emergency vapor blowdown and vapor stream diversion sources which release as a result of a single contingency. 

The maximum vapor load on the drum is based on the largest release from safety valves discharging as a result of a single contingency. Vapor velocities in the drum are based on 100% of critical velocity (refer back to Equation 1). However, a velocity of 175% of critical is permitted when one is applying the 1.5... 

After the total number of flare headers has been established, it may be necessary to recheck the vapor load in individual headers since introduction of a separate header may allow subtraction of the flow quantity from the low-pressure header to which it was added initially. 

The sucdon pressure of an ejector is expressed in absolute units. If it is given as inches of vacuum it must be converted to absolute units by using the local or reference barometer. The suction pressure follows the ejector capacity curve, varying with the non-condensable and vapor load to the unit. 

The four stage exhaust box Includes the oil box separator, the demister pad, the oil mist eliminator, and the synthetic oil baffle. Additional features include ein automotive type spin-on oil filter, a built-in inlet anti-suckback valve that prevents oil from being drawn into the system when the pump is stopped, eind a built-in gas ballast, available on the RA version, which permits pumping with high water vapor loads. 

Figure 8-101. Qualitative effect of liquid and vapor loads on bubble cap tray performance. Used by permission, Bolles, W. L., Pet. Processing, Feb. thm May (1956).

Figure 8-101 presents a generalized representation of the form useful for specific tray capacity analysis. Instead of plotting actual vapor load versus liquid load, a similar form of plot will result if actual vapor load per cap (here the cap row relative to inlet or outlet of tray is significant) versus the liquid load per inch or foot of outlet weir length. 

A tray is flexible when it operates with acceptable efficiency under conditions which deviate significandy from those established for design. The usual changes affecting flexibility are vapor and/or liquid loading. A tray may operate down to 50% and up to 120% of vapor load, and down to 15% and up to 130% of liquid load and still be efficient. Beyond these points its efficiency may fall off, and the flexible limits of the tray would be established. 

When how values exceed 1% to 2 in., consider special downcomers or down pipes to conserve cap area for high vapor loads. 

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