Feed enthalpy

Feed enthalpy condition Slope of q line q Line coord... 

Fig. 9. McCabe-Thiele lines for various feed enthalpy conditions. Terms are defined in text.

Binary minimum reflux so calculated implies feed enthalpy just equal to the above started vapor V and liquid L. Any increase or decrease in that enthalpy must be matehed by inerease or decrease in total heat content of overhead reflux. Note that the Underwood" binary reflux equation essentially computes the flash versus specifi-eation composition relationship along with enthalpy correction. 

Next, note that in Table 1.10 this molecular weight is between those of heptane (100 MW) and octane (114 MW). Also please note the close enthalpy values at the various temperatures of these two components. Here it is expedient and reasonable per our findings to set heptane as the means of reference to determine the debutanizer feed enthalpies. The following calculations are made using the heptane source in Table 1.10 as a reference for all the enthalpy points. 

Controlled variables include product compositions (x,y), column temperatures, column pressure, and the levels in the tower and accumulator. Manipulated variables include reflux flow (L), coolant flow (QT), heating medium flow (Qb or V), and product flows (D,B) and the ratios L/D or V/B. Load and disturbance variables include feed flow rate (F), feed composition (2), steam header pressure, feed enthalpy, environmental conditions (e.g., rain, barometric pressure, and ambient temperature), and coolant temperature. These five single loops can theoretically be configured in 120 different combinations, and selecting the right one is a prerequisite to stability and efficiency. 

Feeds enthalpy + heat added = products enthalpy + heat removed... 

H feed = enthalpy at the 1st catalyst bed feed gas temperature Hr bed = enthalpy at a temperature part way down the catalyst bed. 

Input/Output Performance Parameters for Furnace Operation The term firing density is typically used to define the basic operational input parameter for fuel-fired furnaces. In practice, firing density is often defined as the input fuel feed rate per unit area (or volume) of furnace heat-transfer surface. Thus defined, the firing density is a dimensional quantity. Since the feed enthalpy rate Hf is... 

For low firing rates, the exit temperature of the furnace gases approaches that of the sink i.e., sufficient residence time is provided for nearly complete heat removal from the gases. When the combustion chamber is overfired, only a small fraction of the available feed enthalpy heat is removed within the furnace. The exit gas temperature then remains essentially that of the inlet temperature, and the furnace efficiency tends asymptotically to zero. 

The asymptotic behavior of Eq. (5-189) mirrors that of the LPFF model. Here, however, for low firing densities, the exit temperature of the furnace exit gases approaches 0e = 0i - A rather than the sink temperature. Moreover, for Deff 1 the reduced furnace efficiency adopts the constant value rig = 1 — 0 = 1 + A — 0f. Again at very high firing rates, only a very small fraction of the available feed enthalpy heat is recovered within the furnace. Thus the exit gas temperature remains nearly unchanged from the pseudoadiabatic flame temperature [Te Tf,] and the gas-side efficiency necessarily approaches zero. 

H and G are vectors containing the specific enthalpies of liquid and vapor phases in each stage. Q is the vector of stage heat duties, and Qf is the feed enthalpy vector. Q and Qf are assumed to be given in the problem statement. 

Calculate XHp, the difference between the molar feed enthalpy at initial conditions Tp and Pp and the enthalpy of a mole of saturated vapor at 7 and Pf. 

Liquid and vapor enthalpies are designated by h and H, respectively, and a feed enthalpy is expressed as Hf. 

Number of stages Feed stage number Feed rate Feed composition Feed enthalpy Reflux ratio Distillate to feed ratio Pressure Distillate composition Bottoms composition... 

Distillate composition Bottoms composition Feed tale Feed composition Feed enthalpy Desiga/minimum reflux ratio Optimum feed stage Pressure Number of stages Feed stage Reflux ratio Distillate rate... 

Specific heat at constant pressure, J/g-°C or Btu/Ib- F of feed Enthalpy of thick liquor, J/g or Btu/lb H, of condensate of feed Hj, of saturated steam H , of vapor or superheated steam Mass flow rate, kg/h or Ib/h of liquor leaving single-effect evaporator rhj, of feed ifi, of steam and steam condensate Number of evaporator effects... 

It is required to design a fractionation tower to operate at 101.3 kPa to obtain a distillate consisting of 95 mole% acetone (A) and 5 mole% water, and a residue containing 1 mole% A. The feed liquid is at 125°C and 687 kPa and contains 57 mole% A. The feed is introduced to the column through an expansion valve so that it enters the column partially vaporized at 60°C. Construct an H-x-y diagram and determine the molar ratio of liquid to vapor in the partially vaporized feed. Enthalpy and equilibrium data are as follows. 

Is feed enthalpy based on temperature measurement of a 2-phase flow ... 

When the bottoms stream provides the bulk of the column preheat, bottom flow swings may cause fluctuations in feed enthalpy. Unless the feed temperature controller can suppress these rapidly and effectively, the disturbances will reenter the column and interact with the composition controller. In one column controlled with scheme 16.4a, this resulted in severe oscillations of the composition controller. 

The ability of analyzer control to improve product purity depends on the performance of the rest of the control system. In one case (259, 309), adding analyzer control to an unstable column experiencing frequent upsets in feed enthalpy did little to improve column control in fact, the analyzer could be operated on automatic less than half the time. Once the instability was eliminated, the expected control improvement was achieved. In another case (378), an analyzer controller responded far worse to feed step changes than a temperatvu e controller it has been recommended (378) to maintain extremely stable feed flow and feed composition when using analyzer control. In a third case (309), however, an analyzer control system was demonstrated to tolerate a reasonable degree of feed fluctuations. 

The feed enthalpy is normally inferred from a temperature measurement of the feed leaving the preheater, and preheat is manipulated to control this temperature. This is satisfactory when the feed is a single-phase fluid, and often also with partially vaporized wide-boiling mixtures at superatmospheric pressures, but not with partially vaporized narrow-boiling mixtures. In the latter case, fractional... 

With partially vaporized feeds vmder vacuum, feed temperature varies largely with pressure Jis well as fractional vaporization and will not provide a reliable measure of feed enthalpy, l e consequences of preheater outlet temperature control will be similar to those described above. 

The column feed was preheated by the bottoms then a steam preheater. Preheater steam was controlled by the feed temperature downstream. The feed entha fluctuated with fluctuations in column bottom flow. This interfered with the column product anal3rzer control Problem was cured by a feed enthalpy controller which regulated preheater steam flow. 

Anal5 zer control may fall short of achieving its objectives if column is unstable. A feed enthalpy control may be needed if heat input to the feed fluctuates. 

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