Feed enthalpy control

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. 

Column feed enthalpy control with economizer and preheater... 

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. 

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. 

One of the key requirements of the basic column controllers is to maintain the energy balance. Energy enters the column as feed enthalpy and in the reboiler. It leaves as product enthalpy and in the condenser. If we neglect losses these inputs and outputs must balance. 

While we may have some limited control over feed enthalpy, and maybe some control over product enthalpy, the main source of energy is the reboiler and the main sink is the condenser. If the input energy is greater than the output then more vapour will be produced than condensed and the column pressure will rise. By controlling column pressure we therefore maintain the energy balance. 

Alternatively, we could attempt to obtain the feedforward gains (AO empirically by plant testing, providing that we can introduce a disturbance into feed enthalpy. We may be able to determine K from analysis of historical data but if these were collected while tray temperature (or some other composition) control was in service then it will only be possible to model steady state behaviour. Similarly we could identify K from steady state simulation. Dynamic compensation would then have to be tuned by trial and error. 

Once the column has schemes which provide effective energy and material balance and composition control there may still remain a number of variables which can be manipulated to improve profitability. It may be possible to adjust feed rate, feed composition or feed enthalpy. There is usually scope to adjust column pressure. And, if there is a large difference in product prices, compositions can be adjusted to be better than specification. 

Feedback control of product quality from a column is not always satisfactory even when the control loops are properly arranged. Proper arrangement only protects the process from upsets in heat input, feed enthalpy, and reflux flow and enthalpy. The most significant disturbances to quality control are generally variations in feed rate and composition. 

As wiU be shown later (Chapters 5 and 20), a properly designed column feed system can play a very important role in filtering out disturbances in feed rate, feed composition, and feed enthalpy, thereby making composition control much easier. 

Feed should enter the column with a constant enthalpy. When significant changes are anticipated, a heat exchanger and feed-enthdpy control system should be provided. This is discussed in more detail in Chapters 5 and 11. 

If the feed is to enter at its bubble point, or if it is partially vaporized, then we need an enthalpy control scheme such as that shown in Figure 5.5. Since... 

In addition to the recovery of the latent heat of vapor streams, in many cases it is practical to recover part of the sensible heat in the column bottom product and steam condensate by exchange with column feed. Such schemes have been used in the chemical and petroleum industries for years. Since feed flow is typically set by level controllers or flow-ratio controllers, its flow rate will not be constant. The feed enthalpy or temperature, therefore, is apt to be variable. This may make column-composition control difficult unless one employs either feedforward compensation or a trim heater with control for constant temperature or enthalpy. (See Chapters 5 and 11.)... 

Our use of feedforward compensation for distillation columns curroitiy is based almost entirely on feed-rate changes. Thus we have reflux-to-feed, steam-to-feed, and sometimes other ratio-control systems. But there are at least three other variables that can be important feed composition, feed enthalpy or q, and column pressure. 

Consider Figure 19.2 where top-product flow is set by flow control, reflux flow is set by condensate receiver level control, boilup is fixed by flow control of steam or other heating medium, and bottom-product flow is determined by column-base level control. As shown by the dotted line, we wish eventually to control column top composition by manipulating distillate flow. Let us assume that feed rate, feed composition, feed enthalpy, and boilup are fixed and that we wish to find the changes (i.e., gains ) of top and bottom compositions in response to a change in D, the top-product rate. 

For the control system of Figure 19.4, we may wish to determine overhead and base composition responses to changes in feed enthalpy factor, q. The following prep equations are needed ... 

The reactor is fitted with a cooling coil for temperature control. A heat exchanger (preheater) is provided to heat the feed stream to the flash unit to ensure that the feed enthalpy is sufficient to provide a complete separation of B and D (vapor) from A and C (liquid). Several dynamic models of the primary process units in this plant are presented in Appendix 1.2. For simplicity, the flash unit is modeled as a splitter rather than by a more complex flash model. 

If the substitute fuel is of the same general type, eg, propane for methane, the problem reduces to control of the primary equivalence ratio. For nonaspiring burners, ie, those in which the air and fuel suppHes are essentially independent, it is further reduced to control of the fuel dow, since the air dow usually constitutes most of the mass dow and this is fixed. For a given fuel supply pressure and fixed dow resistance of the feed system, the volume dow rate of the fuel is inversely proportional to. ypJ. The same total heat input rate or enthalpy dow to the dame simply requires satisfactory reproduction of the product of the lower heating value of the fuel and its dow rate, so that WI = l- / remains the same. WI is the Wobbe Index of the fuel gas, and... 

The temperature gap (AT) between the two flows is chosen as the controlling parameter it determines both the enthalpy feed and the Carnot efficiency of the thermoelectric element. The value of AT is related to the heat exchanger efficiency Tiexc or e-NTU (normal thermal unit), the ratio of the heat exchanged to the total exchangeable heat. This relationship comes from the definition of e-NTU for the exchanger efficiency and, in this specific case, it has the following form [16] ... 

An extension of the use of RF plasma for particle heating is the spheroidization of solids. By careful control of plasma enthalpy, particle size, feed rate, and feed position, it is possible to melt each particle as it passes through the plasma. The liquid droplet forms a sphere due to surface tension and, on cooling, retains its spherical shape. Spheroidized particles are commercially useful because they will flow easily. 

The main variables associated with phase relationships include the overall composition, Z , temperature, pressure, liquid composition, X , vapor composition, F, vapor mole fraction, /, and heat transferred, Q. A process in which Z, and two other independent variables are set, and equilibrium separation of the phases is allowed to take place, is called a flash operation. A general flash operation is shown in Figure 2.4. A feed stream initially at conditions T, and P, is controlled so that its final conditions satisfy two specifications. The feed is of fixed rate and composition, F and Z . A heat duty, Q, may be added to or removed from the system as required. The feed is flashed to generate a vapor product with flow rate Ft r and a liquid product with flow rate F(1 -1 /), where / is the vapor mole fraction at flash conditions and P. In general, tj/ may be equal to zero or one or any value in between. The enthalpies of the vapor and liquid products are H2 and /Z2> respectively. The type of flash operation... 

The objective of the preheat control system is to supply the column with a feed of consistent specific enthalpy (enthalpy per unit mass). With a single-phase feed, this translates into a constant feed temperature with a partially vaporized feed, this translates into a constant fractional vaporization. Maximizing feed temperature (if desired) is usually performed manually, by an advanced control system, or by a valve position controller similar to that used in floating pressure control (Sec. 17.2.4). 

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