Extracto Cuentas Manual

The following three typical examples are given to illustrate that certain very simple process concepts, which are almost taken for granted, can become quite complicated to design and operate in the plant, and require careful attention to many details to be applied successfully.

Figure 9.1 illustrates a very simple statement in a process description: “the overflow (stream 6507) from stage 1 is cooled to 45°C and transferred to stage 2.” In the plant, this statement translates into three pieces of equip- ment and three control loops, plus pipes and valves and local instruments. Of course, most chemical engineers know this but quite a few process devel- opers have no definite idea of the translation of such a simple statement (“the engineers will take care of that”). This means that the overflow process stream PI-6507 is collected in a buffer tank TK-1054 and pumped by P-1054

Figure 9.1 P&ID example of overflow cooling and transfer duty.

LT 06 FEM 09 TT 08 TC08 TY 08 TV 08 FV 09 FY 09 FT 09 FC 09 LC 06 TT 07 TI 07 1" 2" 2" TK-1054 3"-N-4367 8"-V-4320 6'-PI-6507 P-1054 Cooler E-1054 3"-CWS 3"-CWR 3"-PI-6508

through a plate heat exchanger E-1054. The flow is measured by FEM-09 and the liquid level in TK-1054 is kept constant at a desired level by LC-06, which cascades on FC-09, which operates the control valve FV-09 on the outgoing stream to stage 2. The temperature is monitored in TK-1054 by TI- 07 and the final cooler temperature is measured and controlled by TC-08, through the control valve TV-08 on the return flow of the cooling water circuit. Of course, TK-1054 has to be vented and (in this particular process) kept under nitrogen blanketing. A bypass is provided for the process stream around the plate heat exchanger to be able to continue operation while maintenance operations are done in this cooler. Sampling valves and other stand-by valves are also provided.

Figure 9.2 represents a portion of a P&ID for a process making pure dry hydrofluoric acid (HF), by distillation from an intermediate process stream containing HF, water, and a third component (needed to decrease the vapor pressure of the water). In principle, this is a very simple strip- per/rectification column, with a reboiler, a condenser, and condensate reflux, and extensive physical data has been published on this system. The bottom stream is recycled to the process backwards. But in fact, there are quite a few complications that require experienced decisions. First, the atmospheric boiling point of HF is about 20°C. Operating the condenser

Figure 9.2 P&ID example of a distillation section for dry HF.

P-101 P-102 P-103 TK-105 E-102 Refrig. system HS-101 freon vapors freon liquid reflux HF vapors bottom to recycle CWR CWS M mixed solution HPS D-101 to vent to vent to vent product storage from to PI LT LIC TE TRC PCV E-101 TE TRC TE TRC TI PI PI TI PC PI

with cooling water would require maintaining the whole system under pressure and raising the temperature in the reboiler, thus requiring a very high steam pressure and mostly very expensive materials of construction. This was ruled out and an operating pressure around atmospheric was opted for; therefore, the condenser was designed with a dedicated mechanical refrigeration unit. The column is operated with a temperature gradient; its upper section is kept quite cool by the reflux of cold HF, which serves also as a direct contact cooling medium, as a large part of the reflux is just evaporated and returned to the condenser. This means that the column cannot be operated, or even started, without a significant amount of reflux, and therefore a stock of HF must always be kept in the receiver TK-105 to bridge temporary interruptions in operation. After longer stoppages, HF may have to be brought back from the product tank into TK-105 to restart this unit.

The ultimate irony is that this plant cannot be started for the first time without buying some product from the competition! Of course, the whole unit must be close-vented and all the noncondensable gases sent back to the scrubbers operating in another section of the plant. Thus the amount of instrumen- tation and control shown in Figure 9.2 is in fact only the starting minimum for review, and careful designers may decide to add more means of oper- ational flexibility and safety. These problems are typical in many cases of new process design and development.

Figure 9.3 illustrates some typical complications that have to be taken into account in the P&ID for such a simple operation like a thermal evapo- rator for large-scale preconcentration of a relatively diluted aqueous solution going to crystallization. Significant amounts of water have to be evaporated at the lowest cost. Mechanical recompression is generally one of the best choices in connection with a falling-film evaporator operating under vac- uum, which seems to be simple enough and there would be a number of specialized suppliers always eager to make an offer. In principle, the solution is circulated and distributed as a film on the inner wall of the vertical tubes; it falls down while it is heated by the condensing steam in the chest outside the tubes; part of the water is evaporated, separated in a side vessel. After that, the water vapors are compressed and sent into the chest. A centrifugal compressor is generally the best choice for such duty (compression ratio), energy-wise. But such a compressor is very sensitive to the presence of solid particles, or even liquid drops in the vapor, considering the very high shear- ing forces. Thus the vapors from the evaporation have to pass through a series of treatments:

1. Separation from the main concentrated liquid, which may be froth- ing, into a side vessel.

2. Passed through a mesh entrainment separator, equipped with a peri- odical washing system, actuated by a differential pressure controller. 3. Mixed with recycled hotter vapors to “dry” any possible microscopic

droplets remaining, before entering the compressor. This recycle is set by a down-stream temperature controller.

4. After the compressor, the vapors are desuperheated by a spray of condensate water in excess, since for a change, such excess is not detrimental.

5. Then, the make-up stream of low-pressure steam is mixed in. There could be different control schemes, according to the characteristics of the system and the quality of the thermal insulation. The mixed vapors are distributed in the chest and flow downwards while con- densing on the tubes.

6. The chest is a closed vessel and any noncondensable gases present would accumulate and prevent further condensation. Thus these gases have to be continuously removed by a side-vacuum system, under the control of a pressure-control system. The standard vacuum system requires a direct condenser stage, discharging through a “barometric leg” or “hot well” and a water-ring vacuum pump with a cold water stream.

Figure 9.3 P&ID example of a falling-film, vapor-recompression evaporator.

M M M condensate feed hot well cold water vacuum pump condensate concent. soln. CW DC TI TI TI TI TIC PE PIC PDI compress. LC LIC FR LP steam makeup TE TI PI LE LE LE

7. The amount of noncondensable gases in a system under vacuum depends mostly on leaks inward of air, due to faulty installation or maintenance, and many plants experience difficulty as a result. Thus, an oversized vacuum system could help in many cases.

One can therefore see that the smooth operation of such a conceptually simple evaporator can depend on many detailed issues requiring decisions, and the developers cannot just relegate these details to the expertise of the suppliers. Instead, they should at least understand exactly what is involved in the new process. We have not discussed proprietary technology that every supplier is claiming for himself such as, for instance, the exact distribution of the liquid films inside the tubes, the eventual cleaning of these tubes, various internal baffles and vapor routes, etc.