Pressure control partial condensation

Second, changes in column pressure have other impacts such as changes in the off-gas rate, the amount of reboiler duty, and hydraulic profile of the plant. In the case of partial condensation, pressure control can interact with the overhead receiver level. While these effects are real, their magnitude is sometimes exaggerated and cited as reasons for not making any changes. 

For partial condenser systems, the pressure can be controlled by manipulating vapor product or a noncondensible vent stream. This gives excellent pressure control. To have a constant top vapor product composition, the condenser outlet temperature also needs to be controlled. For a total condenser system, a butterfly valve in the column overhead vapor line to the condenser has been used. Varying the condenser cooling by various means such as manipulation of coolant flow is also common. 

Figure 3.10. Condensers, (a) Condenser on temperature control of the PF condensate. Throttling of the flow of the HTM may make it too hot. (b) Condenser on pressure control of the HTM flow. Throttling of the flow of the HTM may make it too hot. (c) Flow rate of condensate controlled by pressure of PF vapor. If the pressure rises, the condensate flow rate increases and the amount of unflooded surface increases, thereby increasing the rate of condensation and lowering the pressure to the correct value, (d) Condenser with vapor bypass to the accumulator drum. The condenser and drum become partially flooded with subcooled condensate. When the pressure falls, the vapor valve opens, and the vapor flows directly to the drum and heats up the liquid there. The resulting increase in vapor pressure forces some of the liquid back into the condenser so that the rate of condensation is decreased and the pressure consequently is restored to the preset value. With sufficient subcooling, a difference of 10-15 ft in levels of drum and condenser is sufficient for good control by this method.

Optimization and vacuum control strategies (a) minimizing (floating) pressure by maximizing coolant valve opening, (b) floating pressure control of partial condenser with vapor distillate, (c) floating pressure control when the distillate is both vapor and liquid. 

When a partial condenser is used and the distillate is removed from the column as a vapor, common practice is to use this vapor stream to control column pressure. The reflux drum level is usually controlled by manipulating condenser cooling, and reflux flowrate is fixed or ra-tioed to feed. Heat input is used to control a tray temperature (Fig. 6.9a). 

The bypassed vapor heats up the liquid there, thereby causing the pressure to rise. WTien the bypass is closed, the pressure falls. Sufficient heat transfer surface is provided to subcool the condensate, (f) Vapor bypass between the condenser and the accumulator, with the condenser near ground level for the ease of maintenance When the pressure in the tower falls, the bypass valve opens, and the subcooled liquid in the drum heats up and is forced by its vapor pressure back into the condenser. Because of the smaller surface now exposed to the vapor, the rate of condensation is decreased and consequently the tower pressure increases to the preset value. With normal subcooling, obtained with some excess surface, a difference of 10-15 ft in levels of drum and condenser is sufficient for good control, (g) Cascade control The same system as case (a), but with addition of a TC (or composition controller) that resets the reflux flow rate, (h) Reflux rate on a differential temperature controller. Ensures constant internal reflux rate even when the performance of the condenser fluctuates, (i) Reflux is provided by a separate partial condenser on TC. It may be mounted on top of the column as shown or inside the column or installed with its own accumulator and reflux pump in the usual way. The overhead product is handled by an alter condenser which can be operated with refrigerant if required to handle low boiling components. 

The gas-phase tram-alkylation reaction was performed in an automated micro-flow apparatus containing a quartz fixed-bed reactor (i d. 10 mm) at lO Pa [16 vol% benzene (1, p.a., dried on molsieve), 3.2 vol% diethylbenzene (2, consisting of 25% ortho, 73% meta, 2% para isomers, dried on molsieve), N2 balance (50 mL/min), WHSV =1.5 h ] with 2.0 mL of the tube reactor filled with catalyst particles (500-850 pm sieve fraction, typically 1.4 g). Two separate saturators were connected to the inlet of the reactor for the supply of 1 and 2. The partial vapor pressure of 1 and 2 was controlled by adjusting the temperature of the saturator-condensers and the N2 flow rate. After equilibration for 30 min at the applied reaction temperatures (473 K and 673 K, heating rate 10 K/min) within a dry N2 flow (50 mL/min), benzene (1) and diethylbenzene (2) were passed throu the reactor. To prevent condensation of both reactants and products prior to GC analysis [Hewlet Packard 5710 A, column CP-sil 5CB capillary liquid-phase siloxane polymer (100% methyl) 25 m x 0.25 mm, 323 K, carrier gas N2, FID, sample-loop volume 1.01 pL], tubes were heat-traced (398 K). FID sensitivity factors and retention times were determined using ethene (99.5 %, dried over molsieve) and standard solutions of 1, 2, and ethylbenzene (3, 99%) in methanol (p.a.). The conversion of 2 was measured as a function of time [8]. 

How many variables must be specified in order to define the performance of an existing single-feed, two-product column with a partial condenser (vapor distillate only), a reboiler, and a fixed pressure profile The feed rate, composition, and thermal conditions are also fixed. How would you conceptually control the column operation ... 

A portion of a constant composition stream at a given temperature and pressure is used as the only feed to an existing distillation column. The column has a partial condenser and reboiler, and its only products are the bottoms and vapor distillate. What assumptions must be made to determine the degrees of freedom Describe a conceptual control scheme to control the column pressure, product purities, and reflux rate. 

Each side product provides one additional independent column variable. To define the column performance, the flow rate of each side product must be known. Alternatively, a side product flow rate may be allowed to vary in order to meet a performance specification such as the concentration of a component in that product. The side product flow rate becomes a dependent variable which must be calculated to satisfy the performance specification. It has been established in Chapter 7 that a fixed-feed, fixed-configuration, fixed-pressure column with a partial condenser (having only a vapor distillate) and a reboiler has two degrees of freedom. Two variables, such as the condenser and reboiler duties, may be varied independently. Each side product adds to the column one degree of freedom. Hence, a column as defined above with S side products has 2 + S degrees of freedom. The duties and side product flow rates can each be varied independently, allowing 2 -i- S performance specifications. This conclusion can be reached by applying the description rule since each additional product rate can be controlled independently by external means. 

A stream containing benzene, toluene, and biphenyl is to be separated in a distillation column to produce purified benzene in the distillate. The separation will take place in an existing column with a total condenser, a partial reboiler, and several optional feed locations. The feed stream is of fixed flow rate, composition, and thermal conditions. The entire feed may be introduced at any one of the available feed trays, but may not be split and introduced at more than one feed tray. The condenser pressure is controlled by an inert gas flowing in and out of the reflux drum. Using column modules representation, determine the degrees of freedom for this operation, and recommend a set of specifications to define the column performance. 

For column pressure control there are Ihrea genaial approaches vent bleed (to arraosphere or to vacuum system), hot vapor bypass, and flooded condenser. These approaches are illustrated in Fig. 5.11-2.3 For an atmospheric column, the vent approach is quite simple. The vapor bypass represents a temperature bleading method. Partial flooding of the condenser suiface adjusts the bent transfer capability or the condenser. The schemes are generally self-explanatory. 

The reactor effiuent passes into a feed-product heat exchanger, where it is partially condensed. After washing with dilute caustic soda to neutralize traces of phosphoric acid, it passes into a second exchanger and on to a high-pressure separator tO give a liquid and a vapor stream. The condensate goes to purification and the vapor to recycle. The vapor is cooled by the recyde-gas cooler and scrubbed with water to remove alcohol. The build-up of impurities like methane and ethane is controlled at this point by venting a small stream of the recycle gas. 

Figure 17.8 Condenser and pressure control, two-phase products, (o) PC on inerts stream, superatmospheric (6) PC on inerts stream, vacuum (c) PC on coolant, superatmospheric id) PC on coolant, vacuum (e) flooded partial condenser arrangement.

Condensation in a horizontal in-shell partial condenser with liquid outlet at the bottom and vapor outlets at the top was controlled hy vaijdng liquid level in the condenser. Excessive entrainment was caused by condenser pressure drop building a large hydraulic gradient... 

Calculation of the required condenser surface is not trivial. In contrast to the common applications where saturated vapors are condensed the permeate is a superheated vapor mixture. For design calculations the selection of appropriate heat-transfer coefficients has to consider the cooling to saturation conditions, the presence of noncondensable gases, and the partial condensation of the components along the respective dew lines. Total condensation of the more volatile components of the permeate vapor will often not be possible, but any losses of permeate vapor through the vacuum pump have to cope with the respective emission control regulations. An important factor is the solubility of the components of the permeate in the liquid phase. An additional condenser at the high-pressure side of the vacuum pump is a feasible option. 

The control of partial condenser columns is more complex than total condenser columns because of the interaction among the pressure, reflux-drum level, and tray-temperature control loops. Both pressure and level in the reflux dmm need to be controlled, and there are several manipulated variables available. The obvious are reflux flow, distillate flow, and condenser heat removal, but even reboiler heat input can be used. In this section, we explore three alternative control structures for this type of system, under two different design conditions (1) a large vapor distillate flow rate (moderate RR) and (2) a very small vapor distillate flow rate (high RR). 

Control Structure CS1. Figure 8.9a shows the control structure that is probably most commonly used for distillation columns with partial condensers. The main features of this structure are pressure controlled by manipulating vapor distillate flow rate and reflux drum level controlled by manipulating condenser heat removal. Reflux flow rate is fixed or ratioed to feed. 

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