Pressure Control with Vapor Distillate Product

2 Pressure control with vapor distillate product 

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 swings in the vapor flowrate are typically quite significant to achieve tight pressure control. In an isolated column environment or when this vapor stream simply flows into a large vapor surge vessel or into a large pipeline (header), the variations in the flow cause no problem. But, if this vapor stream is the feed to a downstream unit, the flow variability can significantly disturb this unit and can result in poor plantwide control performance. 

So what can we do in this case If column operation requires that we stick to this control structure, feedforward control will help to reduce the swings in distillate flowrate. However, better plantwide performance can be achieved if we can switch the control structure to one in which the vapor distillate is not used to control pressure. One possible alternative is shown in Fig. 6.27. Condenser cooling is used to control pressure, reflux flowrate controls reflux drum level (with P-only control), and the flowrate of the vapor distillate is ratioed to reflux flowrate. With this structure we allow the disturbances that the column energy 

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Pressure is often considered the prime distillation control variable, Pressure affects condensation, vaporization, temperatures, compositions, volatilities, and almost any process that takes place in the column. An unsatisfactory pressure control often implies poor column control. Pressure is therefore paired with a manipulated stream that is most effective for providing tight pressure control. When the top product is liquid, this stream is almost always the condensation rate when the top product is vapor, this stream is almost always the top product rate (see Sec. 17.2.). 

The distillation column used in this study is designed to separate a binary mixture of methanol and water, which enters as a feed stream with flow rate F oi and composition Xp between the rectifying and the stripping section, obtaining both a distillate product stream D oi with composition Ad and a bottom product stream 5vo/ with composition Ab. The column consists of 40 bubble cap trays. The overhead vapor is totally condensed in a water cooled condenser (tray 41) which is open at atmospheric pressure. The process inputs that are available for control purposes are the heat input to the boiler Q and the reflux flow rate L oi. Liquid heights in the column bottom and the receiver drum (tray 1) dynamics are not considered for control since flow dynamics are significantly faster than composition dynamics and pressure control is not necessary since the condenser is opened to atmospheric pressure. 

The catalytic reactions were carried out at atmospheric pressure in a conventional flow reactor using a U-shaped quartz tube (0 10 mm) with a fixed bed catalyst. The catalyst was diluted with quartz beads. CH4 and H2O were mixed with N2 in the ratio of 1/2/2. The flow rate of CH4 and N2 was controlled with a mass flow controller (STEC SEC-400 Mark3). Distilled water was fed into the reactor with a liquid pump (Shimadzu LC-lOADvp) through a vaporizer. The space velocity changed from 6.0 x 10 to 3.0 x 10 ml h g-cat . The products were analyzed by three on-line TCD gas chromatographs with Porapak-Q and Molecular Sieve 5A columns. 

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 ... 

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. 

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. 

If a smooth flow to the next step in the process is needed, a reflux drum, with averaging level control of distillate, should be employed. If the column top product is a vapor, takeoff should be by averaging pressure control. As an alternative, vapor may be taken off on flow control cascaded from top composition control while column pressure is controlled by heat input. 

The stabilizer reboiler boils the bottom produet from the column as in other distillation processes. The reboiler is the source for all the heat used to generate vapor in a crude stabilizer. When we control the heat input with the reboiler, the boiling point of the bottom product can be controlled. Together with the stabilizer operating pressure, this action controls the vapor pressure of the bottom product. 

General. With simple instrumentation discussed here, it is not possible to satisfactorily control the temperature at both ends of a fractionation column. Therefore, the temperature is controlled either in the top or bottom section, depending upon which product specification is the most important. For refinery or gas plant distillation where extremely sharp cut points are probably not required, the temperature on the top of the column or the bottom is often controlled. For high purity operation, the temperature will possibly be controlled at an intermediate point in the column. The point where AT/AC is maximum is generally the best place to control temperature. Here, AT/AC is the rate of change of temperature with concentration of a key component. Control of temperature or vapor pressure is essentially the same. Manual set point adjustments are then made to hold the product at the other end of the column within a desired purity range. The technology does exist, however, to automatically control the purity of both products. 

The sketch below shows a distillation column that is heat-integrated with an evaporator. Draw a conirot concept diagram which accomplishes the following Directives (a) In the evaporator, temperature is controlled by steam, level by liquid product, and pressure by auxiliary cooling or vapor to the rcboiler. Level in the condensate receiver is controUed by condensate. 

The stream defined below is heated to 100°C to be partially vaporized in a flash drum before entering a distillation column. The fraction vaporized is controlled by the flash drum pressure. Calculate the required pressure at 100°C to have 20% mole vaporization, assuming Raoult s law applies. What are the products flow rates and compositions The constants for the Antoine Equation 2.19 are given for each component, with the pressure in kPa and the temperature in K. 

To form both a distillate of lower boiling point as well as such oxidation products as acids, ketones, aldehydes, and alcohols, hydrocarbon oil is treated by a stepwise process.25 The oil is first vaporized under pressure and passed over metallic oxide catalysts such as copper oxide or barium peroxide. The heavier of the evolved vapors are condensed and treated with air, oxygen, ozone, or nitrogen oxides and are then returned to the main stream of oil undergoing treatment. It is also possible to induce restricted oxidation by the use of metallic oxides capable of reduction at the temperature of the reaction. The process may be more readily controlled than when air or oxygen is used for the oxidation. 

For instance, Swann, Howard, and Reid27b passed air into petroleum hydrocarbons held in a still at about 750° F. (398.9° C.) under a pressure of 300 pounds per sq. in. (21.09 kgs. per sq. cm.) and analyzed the products contained in the aqueous layer. The apparatus used in the work consisted of a cylindrical steel still twenty-six feet high and four feet in diameter with a capacity of about 1000 gallons of oil. Heat exchangers wore provided to preheat the feed and water coils were used to control the reflux. The vapor product passed through a condenser to a receiver. The gas oil used in the work was a Midcontinent distillate of 38.3° B density at 60° F. (15.6° C.). The oil had an initial boiling point of 552° F. (288.9° C.) and an 80 per cent over point of 701° F. (371.7° G). A complete distillation curve for the oil up to 80 per cent is given. 

An illustration of the use of chromatography in this industry is in the control of distillation towers. Distillation uses the difference in composition between a liquid and the vapor formed from that liquid as the basis for separation. The efficiency of the process is affected by temperature, pressure, feed composition, and feed flow-rate. Chromatography is used to monitor the composition of the feedstock and to apply feedforward control of the heat input (temperature) to the tower, or to monitor and control the composition of the product. In this latter case, the chromatograph output is simply compared with a set point, and the controller (using feedback) manipulates the temperature, pressure, or feed flow-rate by activating the appropriate final operator. Both types of distillation control are widely employed in petroleum refining. 

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