Feed Preheat Control

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). 

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 above swii can be eliminated by controlling the preheater heat duty or, better still, the preheater heat duty per unit feed flow (362), instead of the preheater outlet temperature. For steam (or condensing vapor) preheaters, the duly per unit feed flow equals the ratio of the measured steam (or vapor) flow to the measured feed flow times a constant, the constant being the steam latent heat. For a sensible-heated preheater, the above ratio is multiplied by the measured hot-side temperature difference, and the constant is the average hot-fluid heat capacity. For two or more feed preheaters, it is best to compute their total heat duty on-line and ratio it to the feed (68, 259). The computation can be readily performed using conventional analog instrumentation. Similar techniques cured the above-cited swing problems (239,259). 

When column differential pressure is controlled (see previous section), the preheat can be manipulated to control top section differential pressure, while boilup is manipulated to control bottom section differential pressure (332, 362). This system has the advantage of preventing feed disturbances from interfering with differential pressure control, but is prone to the shortcomings described earlier. 

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While the major control considerations have been discussed in the previous three chapters, several loose ends remain. These loose ends pertain to controls that usually command less attention than others, such as reflux and level controls, and to those controls that are not applied in the majority of columns but are critical to some. The latter group includes side-stream drawoff controls, differential pressure control, and feed preheat control. Poor configuration of any of these loose ends can be just as troublesome as the poor practices described in relation to the major controls. 

Most FCC units use fired heaters for FCC feed final preheat. The feed preheater provides control over the catalyst-to-oil ratio, a key-variable in the process. In units where the air blower is constrained. 

There is a middle steady state, but it is metastable. The reaction will tend toward either the upper or lower steady states, and a control system is needed to maintain operation around the metastable point. For the styrene polymerization, a common industrial practice is to operate at the metastable point, with temperature control through autorefrigeration (cooling by boiling). A combination of feed preheating and jacket heating ensures that the uncontrolled reaction would tend toward the upper, runaway condition. However,... 

Feed temperature is not normally controlled, unless a feed preheater is used. 

The openloop transfer function relating steam flow rate to temperature in a feed preheater has been found to consist of a steadystate gain K, and a flrst-order lag with the time constant T. The lag associated with temperature measurement is t . A proportional-only temperature controller is used. 

This chapter has two alternative structures for feed preheating. Both use a feed effluent heat exchanger, but one also uses a furnace. Steady-state economics favor use of only a heat exchanger. Dynamic controllability favors the use of both a heat exchanger and a furnace. 

Heat energy can be saved by using the hot bottom product to preheat the feed in an economizer (Figure 2.89c). In order to maximize the amount of heat recovered, a VPC is used as the cascade master of the feed temperature controller. The goal of optimization is to keep the bypassed flow at a minimum. Therefore, the VPC is usually set at about 10% of the valve opening. 

Furnace temperature is measured with thermocouples. It is controlled to 1300 K by adjusting fuel combustion rate and air preheat temperature. Feed forward control based on scheduled changes in spent acid composition and feed rate is employed to optimize the process (Rohm and Haas, 2003). 

The production of HIPS begins with the granulating and dissolving of rubber and other additives in styrene monomer (1) and then transferring the rubber solution to a storage tank (2). For general-purpose product, controlled amounts of ingredients are fed directly to the feed preheater (3). 

For example, suppose the temperature of a stream being fed to a distillation column is controlled by manipulating steam flowrate to a feed preheater. And suppose the stream leaving the preheater is partially vaporized. Small changes in composition can result in very large changes in the fraction of the stream that is vaporized (for the same pressure and temperature). The resulting variations in the liquid and vapor rates in the distillation column can produce severe upsets. 

Our first and foremost concern is to dissipate the exothermic heat away from the reactor. This requires that we have the ability to bypass the feed preheaters such that excess heat from the process is not carried back with the reactor feeds. We introduce a set of bypass valves along with temperature controllers as shown in Fig. 5.15, We have elected to install the bypass valves on the cold side to lower the investment cost and facilitate maintenance. We also get better temperature control of the cold stream leaving the preheaters. In Fig. 5.15 we show the stream temperatures at the valve inlets as well as the control points. These data were obtained from the T-H diagram provided by Terrill and Douglas (1987a). Even though we bypass on the cold side, we see that one of the bypass valves has to operate at a temperature over 330°C,... 

Figure 2 Experimental set up (1) 316 stainless steel feed preheater tube (1.3 cm i.d. X 50 cm length) (2) block heater containing a 316 stainless steel fixed bed reactor tube (2.5 cm X 46 cm length) (3) catalyst bed (4) Type J (iron/constantan) thermocouple probe (5) Type J (iron/constantan) thermocouple with temperature controller (6) syringe pump (7) condenser (8) receiving flask (9) gas trap (10) gas collection vessel and (11) nitrogen cylinder.

Reaction it takes place on a feed preheated to around 220°C of ammonia, propylene and compressed air (0.3. 10 Pa absolute) in controlled proportions. It takes place in a multi-tube reactor (catalyst tube dimensions inside diameter 25 to 30 mm, height 3 to 3.5 m), with shell-side circulation of a bath of molten salt intended to remove the heat generated by the reaction, and which is then cooled to produce high-pressure steam. 

Feed preheating system (Figure 23.5b). This block requires one controlled variable (see Table 23.3), which is the temperature 7). The only available manipulated variable is the steam flow rate Ws, thus yielding only one loop configuration (Figure 23.5b). 

Figure 23.5 Control loops for individual segments of plant in Example 23.3 (a) coolant system (b) feed preheating (c) reactor (d) flash drum.

The above simple analysis highlights an important issue in process dynamics the influence of positive and negative feedback on system s stability. Instability can occur in recycle systems due to positive feedback when the gain is larger than unity. We may give as example the recycle of energy developed by an exothermal reaction in an adiabatic PFR for feed preheating. Instability may occur because of the exponential increase in reaction rate with the temperature when this cannot be properly controlled (Bildea Dimian, 1998). Another example is the recycle of impurities in a plant with recycles, whose inventory cannot be kept at equilibrium by the separation system (Dimian et al., 2000). 

Step 3. The reaction is exothermal. After process/process energy saving for feed preheating, the excess energy is rejected to the cooling water. Because the only reason of the furnace is to ensure constant inlet reactor temperature the first control loop is inlet reactor temperature/fliel inflow. To prevent the thermal decomposition of the product, a second loop keeps constant outlet reactor temperature by manipulating the quench stream. 

With schemes 16.4a, h, and d, fluctuations in bottom level cause swings in bottom flow. This may be troublesome if the bottoms product flows into a unit that requires a steady feed, such as a furnace, a reactor, and sometimes even another column. An unsteady bottom flow may also be troublesome when the bottoms product preheats a stream going into such a unit, or even if it preheats the column feed, and preheat control is slow or ineffective. 

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. 

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