Tower heat balance

A major part of tower heat balance is checking reflux rate and temperature, which determines both the condenser and reboiler duties. It is important to measure reflux temperature as it affects the heat balance significantly when the reflux is subcooled. Example 13.1 demonstrated the effect of reflux subcooling. 

Reflux pumparound rates can usually be calculated from a heat balance around individual towers. This is the best way to account for unrecorded flows. It is not uncommon for a reflux or pumparound duty, calculated from the tower heat balance, to be 10% to 20% higher than the measured flow would indicate. This is probably due to ambient heat losses. For the sake of consistency of the total test report, it is best to stick with the duties from the heat balance calculations. If the difference between a duty calculated in the two ways described above is much more than 20%, there is a significant error in the test data. 

From the viewpoint of the tower heat balance, a low estimate of the exit bottoms temperature will cause the various tower heat removal quantities to be calculated as lower than actual. This applies especially to the cooling requirements for the vacuum residuum. 

Kunesh [126] presents tm overview of the basis for selecting rsuidom packing for a column application. In first deciding between a trayed tower or a packed one, a comparative performance design and its mechanical interpretation should be completed, considering pressure drop, capacity limitations, performance efficiencies (HETP), material/heat balances for each alternate. For one example relating to differences in liquid distribution performance, see Reference 126. 

The decrease in the heat duty of the pumparound heat exchanger would equal the increase in the heat duty of the overhead condenser. Thus, we say that the heat balance of the tower is preserved. Some of the heat that was being recovered to the cold fluid, shown in Fig. 12.2, is now lost to cooling water, in the overhead condenser. This shows the most important function of pumparounds recovering heat to a process stream that would otherwise be lost to the cooling tower. 

The design of large natural draft cooling towers and analysis of their performance are complicated by the effects of variations in ambient air humidity. Often the effluent air from the tower is assumed to be at 100% relative humidity, to simplify calculations for design parameters. This study avoids the simplification, and proposes a procedure for determining the major design parameters for a natural draft tower. The theoretical and empirical relationships applicable to heat balance, heat transfer and transport, and tower draft and air resistance are given. 13 refs, cited. 

A simplified flow diagram for this segment of the process is shown in Figure 4, and thermodynamic analysis in Table III. The refrigerated exchangers and cold box represent about 30% of the lost work of the process. However, the tower itself has a very high percentage of the lost work in the system. Thus the details of the tower heat and material balance were examined in search of ways to improve its efficiency. 

To improve the effectiveness of the process, it is necessary to determine the extent to which the entropy production is characteristic of the process and the extent to which it is a consequence of equipment inefficiencies that could be reduced by increased capital expenditure for either larger or more effective equipment. To do this, we consider an idealized process design for a plant in which the exchangers are so large that the pressure drop is zero and the temperature difference is the minimum consistent with process heat balance. In addition, we assume that the heat leak may be made negligible by elaborate insulation, that only the minimum heat needed to reboil the tower will be used, and that the nitrogen compressor may be made 100 percent efficient and isothermal. ... 

The calculation of the temperature difference between the bottom and the first tray makes use of heat balance at the tower bottom. 

In the 1904 edition there is, for example, a sample calculation of the heat balance on a Glover tower treated as an evaporator, which shows how inefficient it was then ( what a heat waster it is (23)), There is also a discussion on the efficiency of various packings, explaining in terms of surface areas why coke is 1.5 to 2 times more efiBcient than bricks (23, 26), But in general, Davis approach was still empirical the operations are described as procedures of practical utility, and are not based on fundamental physics. Neither the work of Osborne Reynolds nor dimensionless group theory had been assimilated yet into the profession. 

The most precise methods for calculating the performance of absorbers and strippers are based on tray-to-tray heat and material balances, and these usually ate accomplished by computer. One such method was proposed by Sulata." This method makes use of an overall and individual component material balance across each tray which can be adjusted to account for a feed or drawoff stream at the tray. An initially assumed temperature profile is employed for the first iteration and a set of n simultaneous heat balance equations must be solved to provide a correction to the temperature profile and flow rates. Recalculation of the material balance equations and corrections to the temperature profile lead to a converging solution for the entire tower. 

An important aspect of defining the base case is gathering aU the important data for the material and heat balances in one single sheet for a tower of interest. It would be very informative to have important mass flows, temperature, pressure, and composition data in one table so that a snap shot of the tower performance can be seen at a glance. Such an example is a heat pumped C3 splitter shown in Figure 13.1 and Table 13.1. In building such a table, it is a good practice to include the tag number of the instrument for each parameter so that the data can be retrieved readily from the historian to produce the table with snapshots of different... 

Defining a base case is to determine the base case operation of the tower of interest. This requires extracting two kinds of data. One kind is process data in terms of feed and product conditions, such as flows and compositions, while the other is tower operating data including temperature, pressure, and reflux rate. The former defines the mass and composition balances and the latter sets the heat balance around the tower with Table 13.2 giving such an example of the C2 splitter column. 

This assumes that L is essentially constant, since only a small amount is evaporated. The heat capacity of the liquid is assumed constant at 4.187 x 10 J/kg- K (1.00btu/lb °F). When plotted on a chart of Hy versus Tj, this Eq. (10.5-1) is a straight line with a slope of LcJG. Making an overall heat balance over both ends of the tower. 

In industry many of the distillation processes involve the separation of more than two components. The general principles of design of multicomponent distillation towers are the same in many respects as those described for binary systems. There is one mass balance for each component in the multicomponent mixture. Enthalpy or heat balances are made which are similar to those for the binary case. Equilibrium data are used to calculate boiling points and dew points. The concepts of minimum reflux and total reflux as limiting cases are also used. 

Heat balance on the tower is maintained by a device known as a reboiler. Reboilers take suction off the bottom of the tower. The heaviest components of the tower are pulled into the reboiler and... 

Reboiler—a heat exchanger used to maintain the heat balance on a distillation tower. 

At steady state, troubleshooting coker combination tower problems is no different than for a crude unit. Unfortunately, the combination tower heat and material balance are routinely upset due to the switching, steaming, and warm-up of the coke drum. 

The plugged top tray will prevent the reflux from cascading down to the lower trays. The liquid reflux will just overflow into the condensers and circulate back to the reflux drum. The tip-off to this problem is that neither the reboiler duty nor the bottoms temperature is affected in the normal way by raising reflux. The tower s heat balance appears as if the reflux rate had never been increased. This is not much different from the signs of normal tower flooding, except that the AP on all but the top tray is not excessive. 

When I had instructed the crude unit operators to reduce vacuum tower wash oil by 50%, the entrainment of resid or tar into the gas-oil FCCU feed had substantially increased. As shown in the above tabulation of laboratory data, the entrainment had caused an increase in conradson carbon residue of the vacuum gas oil. This was of no consequence. The extra coke made due to the higher conradson carbon was compensated for by cutting the FCCU feed preheat temperature to hold the regenerator in heat balance. Of greater importance was the increase in nickel content from 0.5 ppm in vacuum gas oil to 2.0 ppm. This fourfold multiplication in nickel content was reflected by a concurrent increase of nickel accumulation of the circulating catalyst. As the nickel content of the catalyst increased. 

Manipulation of heat input to control column temperature is not recommended for most applications. Because the temperature of a boiling pure liquid is constant, the sensing element is usually moved up the tower to a tray where the temperature of the saturated liquid is a measure of its composition. Temperature is meant to infer composition-yet because it is temperature, many engineers feel that it is a function of heat input. It has been shown, however, that D/F is 100 times more effective in controlling composition than V/F. The reason that manipulation of heat input affects temperature is beause reflux is on flow control, such that D is dependent on V. If the temperature is too low, additional heat is sent to the reboiler, which ultimately increases distillate flow. With this arrangement, the heat balance is deliberately upset in order to alter the material balance. 

This section outlines procedures for calculating product draw tray temperatures at all points in the tower and for making an overall heat balance around the system. The method is based upon assuming a draw tray temperature and then calculating the internal reflux required by the... 

Heat input to the base section of the tower from feed and bottoms-stripping steam, heat outflow in the bottoms liquid and external heat quantities at the flash zone. This bottoms-section heat balance is shown as Envelope 1 on Figure 2.17. 

Calculate the reflux heat, at Tray Dl. Reflux heat is defined as the apparent heat imbalance between external heat quantities at the point in question in the tower. These external heat quantities are denoted as Q with appropriate subscripts to signify their location. External heat input quantities are defined as the heat contained in the feed plus all heat to the system at product strippers either directly as steam or indirectly throu reboilers. External heat output quantities at a given tray are defined as the heat contained in liquid products leaving the system from points lower in the towier, the heat contained in the internal vapors of products plus steam and the heat contained by a product liquid flowing to the sidestream stripper. If the tray is nbt a sidestream draw tray, this latter quantity does not enter into the heat balance. 

It is an inherent property of a Type U tower and a given material balance that, since internal reflux flows are at their maximum values, column temperatures will also be at their maximum levels. Any modification to the system which removes tower heat will lower tower temperatures. 

The following discussion outlines procedures for calculating the overall heat balance around the flash zone and tower base. These procedures apply for both types of vacuum towers and are considered independently of the rest of the heat and material balance calculations. To this point, it is assumed that the following items have been completed. 

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