Overhead level control

By breaking the main fractionator overhead level control loop, the vicious cycle described above can be avoided, because many of the heat integration imbalances simply show up as fluctuations in the overhead accumulator level. MPC will do a very good job of controlling overhead drum level so as to keep the gas plant stable. 

Level control in condensate receivers or reflux drums is commonly achieved by manipulating either top product flow or reflux flow. Less commonly, overhead level control is accomplished by adjusting boilup or by adjusting condenser cooling water. For the first two cases, a relatively simple control system can be used. 

For overhead level control via boilup, a dynamic analysis should be made to determine proper holdup and controller type. If level is cascaded to flow control, the flow transmitter should have a linear output with flow. If an orifice AP transmitter is used, this should be followed by a square root extractor. 

In calculating reflux drum level controller settings, whether proportional only or PI, one should take care to account for reflux subcooling (see Chapter 16, Section 4). Overhead composition may be controlled by trimming the distillate/bottom-produrt ratio with perhaps a feedforward compensator connected into the overhead level control loop. Base composition may be controlled by trimming the steam/bottom-product ratio control. 

Bottom product demand, overhead level control via top product, base level via feed... 

This is also a commonly encountered control scheme. Both level controls may be calculated by the method of Chapter 16 care should be taken to include the subcooled reflux effect on overhead level control. 

Overhead level control may be calculated simply by the method of Chapter 16, Section 3, but base level control by boilup is very difficult. It is normally used only when the average bottom-produa flow is very small. The characteristic time constant th should be at least 15 minutes and other design factors should be as indicated in Chapter 16, Section 7. In most cases base level control by boilup requires a dynamic analysis, and perhaps supplementary plant tests. If steam flow is measured with an orifice, a square root extractor should be used. 

If the distillate is the demand stream, then base level control via bottom product and overhead level control via boilup will not control the column properly. This scheme is analogous to the preceding sdieme. 

The manufacturing cost consists of direct, indirect, distribution, and fixed costs. Direct costs are raw materials, operating labor, production supervision, utihties, suppHes, repair, and maintenance. Typical indirect costs include payroll overhead, quaHty control, storage, royalties, and plant overhead, eg, safety, protection, personnel, services, yard, waste, environmental control, and other plant categories. However, environmental control costs are frequendy set up as a separate account and calculated direcdy. The principal distribution costs are packaging and shipping. Fixed costs, which are insensitive to production level, include depreciation, property taxes, rents, insurance, and, in some cases, interest expense. 

Our example system has a flow-controlled feed, and the reboiler heat is controlled by cascade from a stripping section tray temperature. Steam is the heating medium, with the condensate pumped to condensate recovery. Bottom product is pumped to storage on column level control overhead pressure is controlled by varying level in the overhead condenser the balancing line assures sufficient receiver pressure at all times overhead product is pumped to storage on receiver level control and reflux is on flow control. 

Remember that the stripping column has no reflux. These levels are controlled by proportional level controllers that manipulate P and D. Tray holdup is 0.3 kmol, and the steady-state holdups in the base and overhead accumulator are each 75 kmol. 

The scheme for the reactor/stripper process uses a PI controller to hold product composition (xB) by manipulating vapor boilup in the stripper. The same analyzer deadtime is used. Proportional level controllers are used for the stripper base (manipulating bottoms flow ), the overhead receiver (manipulating recycle flow), and the reactor (manipulating reactor effluent flow) with gains of 2. 

Step 7. Methane is purged from the gas recycle loop to prevent it from accumulating, and its composition can be controlled with purge flow. Diphenyl is removed in the bottoms stream from the recycle column, where steam flow controls base level. Here we control composition (or temperature with the bottoms flow. The inventory of benzene is accounted for via temperature and overhead receiver level control in the product column. Toluene inventory is accounted for via level control in the recycle column overhead receiver. Purge flow and gas-loop pressure control account for hydrogen inventory. 

In both of these schemes, the feed is flow controlled and the overhead vapor flow is adjusted for pressure control. The arrangement shown in Figure 3.14(A) is more common, with the level controller adjusting the bottom flow and the temperature controller adjusting the steam flow. Note that the temperature is an inferred measure of composition. This inferred composition control is achieved by adjusting the steam flow such that the material balance has more or less vapor removed overhead. 

Scheme 3 (Figure 3.17) indirectly adjusts the material balance through the two level loops. If the steam flow is increased, then the sump level controller decreases the bottom flow. As the additional vapors go overhead and condense, the reflux accumulator level control increases the distillate flow a like amount. The separation is held constant by manually setting the reflux flow to maintain a relatively constant energy per unit feed. This scheme is recommended for columns with a small energy per unit feed (VfF < 2). This scheme also offers the fastest dynamics. 

For example, assume that you want to perform tests on the plant, represented by Figure 15.74. The plant is a simple distillation column with overhead accumulator pressure controlled by moving the hot vapor bypass, bottoms level maintained by bottoms product draw rate, and the overhead accumulator level controlled by adjusting the overhead product draw rate. Reflux is on flow control, and the reboiler is on temperature control. Typical move sizes for this plant are shown in Table 15.12. 

Fig. 15.11 Colorimetric phenol analyser, (a) Details of the distillation unit, condenser and heat exchanger, (b) General scheme of the instrument. A, sampler B, heating bath with heating rod C, distillation head D, condenser E, heat exchanger F, strip-chart recorder G, colorimeter H, coll J, single bead-string reactor K, peristaltic pump (1) wash water (2) sample (3), air (4) phosphoric acid (5) overhead condensate (6) 4-AAP reagent (7) Fe(III) reagent (8) air (9) level control (10) bottoms draw (11) debubbler draw. (Reproduced from [39] with permission of Elsevier).

Both the level indicator and level controller friiled on the overhead receiver, whidi separated liquid HF from liquid hydrocatixBia HF overflowed into the hydrocarbon product route, which included a bed of solid KOH. Violent reaction between KOH and HF overpressured the vessel, causing multiple eiqilosions and rupture of the vessel... 

Based upon the analysis of the previous SBCR runs (in 1995-96), several more design changes were carried out to the SBCR system to increase the conversion stability. An automatic level controller was added to the overhead slurry/gas separation tank. This insured a constant inventory of catalyst particles was being maintained in the reactor vessel if the superficial gas velocity within the column was constant. 

Several of the control loops in Figure 21.35 are provided for inventory control, in three level-control loops and two pressure-control loops. Note, however, that the pressure in V-100 is assumed to be constant and loop PC-1 is not simulated by HYSYS.Plant. In contrast, pressure control is crucial to maintain stable internal flows in the column. Finally, because the feed flow rate and temperature controllers are decoupled from the rest of the process, they are not included in the C R analysis. Consequently, the interactions to be analyzed involve the four valves V-7, V-9, V-10, and V-12, and four controlled variables Xdj, JC/u (mole fractions of benzene in the distillate, MCB in the bottoms, and HCl in the absorber overhead stream, respectively) and Tg, the recycle temperature. Note that to improve the dynamic performance, the temperature of tray 4 is controlled rather than the distillate benzene mole fraction. 

Store water in overhead tanks (with automatic level controller to start and stop the water pump) for use in many sections of the plant. This will reduce continuous running of the main water pump. 

In many cases, shortcut calculations can fill in the gaps. An example used in Kenney s book (Kenny, 1984) gives good illustration for how to do it Consider the tower in Figure 13.2. As for many plants, cooling water rates are not measured and overhead product comes off on level control. However, since feed rate and composition and overhead product composition are known, much of the missing data can be derived by energy and mass balances. 

The manipulated bottoms flow rate scheme is not used very frequently because of problems with the column base level control using steam. When a thermosiphon reboller Is used and the steam flow Is Increased, there Is usually a reversal in the column base level response. The column level first rises and then falls. The usual case for using this scheme is when the feed concentration is 95% light key or more. In other words, most of the feed is distilled overhead from a few percent heavies or tars. The MRT point is usually in the reboiler. 

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