Proportional Only Controllers

The fixed-gain proportional-only relay, either direct acting or reverse acting, with adjustable bias has become one of the most widely used devices in override circuits. Common values of gain are 2, 3, 4, 6, and 25. Use of the minus sign ( ) with these figures implies reverse action, or negative gains. A proportional-only controller or relay follows a straight-line equation. 90 output signal 6, input signal B bias We have found it convenient to calibrate pneumatic, fixed-gain relays in terms of...

Partially Flooded Reboilers

The partially flooded reboiler is similar in many ways to the partially flooded condenser. Usually, although not always, it is a vertical thermosyphon reboiler. As discussed in Chapter 4, Section 2, it is controlled by throttling the steam condensate, which in turn varies the condensate level in the shell and thereby the heat-transfer area for condensation. That area covered by liquid permits only sensible heat transfer from the condensate this is a small heat load compared with that of the...

Typical Operation and Design

Vertical-thermosyphon reboilers or calandrias are commonly designed for a mass circulation ratio of about 5-20 parts of liquid to one part of vapor. This means that only about 5-25 percent by weight of reboiler effluent is vapor on a volumetric basis, however, the numbers are almost reversed, t The tubes are filled with froth, and pure liquid exists in the tubes only for a few inches above the bottom tube sheet. For most applications, if shell-side steam pressure, tube-side pressure, and...

Base Level Control via Steam Flow Manipulation

In this case th should be 20 minutes or more. In addition, the level controller should be cascaded to a steam-flow controller with a linear flow transmitter or orifice AP transmitter, and square-root extractor . Further, for a thermosyphon reboiler, one should make the volume AB x AHT at least ten times the volume inside the reboiler tubes. For this design wa m input span of steam flow transmitter, pounds per minute that is, maximum steam flow ka steam latent heat of condensation, pcu lb Xp...

Protective circuits for tube bundle chamber in kettletype reboiler

Adequate space for controlling the froth level. There is also the added difficulty of controlling the level of a variable-density froth. In addition, the column must be shut down to clean the tubes. Two basic arrangements for installing and operating internal reboilers are employed 1. Tube bundle without isolating baffle or chamber. 2. Tube bundle with isolating chamber. Tube Bundle Without Isolating Baffle or Chamber This case is illustrated in Figure 4.8. Here the tube bundle is submerged in...

Flooded Thermosyphon Steamside Reboilers

Steam Flow Condensate Pot

As shown by Figure 4.3, a flooded thermosyphon reboiler operates by throttling condensate rather than steam flow. The basic principle is that of varying the heat-transfer surface. The chief advantage is that for a given steam flow in pounds per hour, a condensate valve is much smaller than a vapor supply valve. At a high heat load such that shell-side liquid level is low, condensate subcooling is small and there is some cavitation in or flashing across the condensate valve. Fortunately new...

Steam header configuration

Steam Header

When header pressure variations are small, temperature compensation is sometimes omitted since temperature changes more slowly than pressure. Steam condensate removal is most commonly achieved by traps devices that, in effect, are very simple level controllers. Many plants have experienced the need for high maintenance with these. Sometimes enough steam leaks through to impair the accuracy of steam-consumption estimates based on steam flow metering. In addition, as pointed out by Mathur,9 a...

Figure 226

Minimum Reflux Ratio

McCabe-Thiele diagram for rating problem The minimum reflux ratio for specified product purities and feed conditions occurs when an infinite number of trays are required to make the separation. Figure 2.27 shows the normal minimum reflux ratio situation. It occurs when the operating lines just intersect on the VLE curve. An infinite number of trays are required to step past the feed plate because of the pinch condition the converging operating and VLE lines . The actual reflux ratio used must...

Procedure For Overall Control System Design

We now have enough information to suggest a sequence of design steps to follow that will lead to a quantitative definition of column and hardware performance. 1. After careful discussion with the process engineer and column designer, and after careful review of the overall process flow sheet, prepare a simplified flow sheet that defines the control concepts a. Select the overall material-balance control scheme first, preferably proceeding from final product inventor ' to raw material inventory....

Figures

1.1 Material balance control in direction opposite to flow 7 1.2 Material balance control in direction of flow 7 1.3 Overall material balance control in direction opposite to flow 8 1.4 Overall material balance control with intermediate material balance control in direction of flow 9 1.5 Distillation column with material balance control in direction of 2.1 Nomenclature and conventions for typical distillation column 26 2.2 Control variables for distillation column 27 2.3 Schematic of typical...

Figure 227

Number Trays Versus Reflux Ratio

Column operation at minimum reflux ratio In some unusual VLE systems, the pinch between the VLE curve and an operating line can occur at some point other than the feed point. The minimum number of trays to make a specified separation is found when an infinitely large reflux ratio is used. The L V ratios in both sections of the column become unity and lie on the 45 line Figure 2.29 . This situation actually takes place in a column when it is operated under total reflux conditions. No feed is...

Ls Lr F

Lr liquid flow from top tray, mol min Ls liquid flow from feed tray, mol min Note that if the feed is a liquid at its boiling point, q 1 and VR Fs. If the feed is subcooled liquid, q is greater than 1 and VR is less than Vs. It is also true that Vs F Lf i Lf VR 14.5 From the definition of q above we can write Lf i Lf Fq 14.6 In terms of weight units equation 14.6 becomes wf liquid flow from the feed tray, lbm min From equation 14.7 Since wf jLf 1 , wf F , and wf Lf are constants, we can rewrite...

Functional Layout Of Control Loops

The overall functional layout of a control loop is extremely important. The design must accommodate such diverse requirements as Minimum operator attention for startup and shutdown, and for manual-automatic switching Anti reset-windup for loops with overrides also simplifying manual-automatic switching Feedforward and interaction compensation without interfering with either normal reset or anti reset-windup Use of Smith predictor and other advanced controls without interfering with either...

Newer Methods

Since about 1960 a variety of newer mathematical and computational techniques have been introduced. These are sometimes called, collectively, modern control theory. Their application to industrial process control so far has been limited. This is partly so because few practitioners are familiar with the technology, and partly because it is not really appropriate for many applications. Study of control systems by these methods requires a fairly capable computer on-line implementation usually also...

Design Considerations In Heatrecovery Schemes

Energy recovery in a distillation column means, practically speaking, recovering or reusing heat contained in the column product streams, whether they are liquid or vapor. A number of schemes have appeared in the literature. The two chief ones involve 1 multiple effect distillation, analogous to multiple effect evaporation, and 2 vapor recompression. But, regardless of the scheme, there are five design factors that must be considered 1. Reserve capacities that may be required Extra heating...

Feed Temperature Control

Fluctuations or variations in the column heat balance can be a major factor in interfering with good composition control. One source of heat disturbances is the column feed, particularly if a heat economizer such as that shown in Figure 5.3 is used. The objective to minimize heat consumption is a worthy one, but this system will not, without a bypass, provide constant feed temperature or enthalpy. With a bypass, one can achieve temperature control. Accordingly this arrangement is occasionally...

Differential Vapor Pressure

Another early effort to compensate for pressure variations in binary distillations resulted in the development of the differential vapor pressure cell.7 The version made by Foxboro is known as the DVP Cell. As shown in Figure 10.1, a bulb filled with liquid whose composition is the same as that desired on a particular tray is installed on that tray. It is connected to one side of a differential-pressure transmitter. The other side of the AP transmitter is connected directly to the same tray....

Demand

Drum Level Control

To the restriction that column overrides, particularly high base level, can restrict feed flow. Reflux Drum Level Control Via Distillate Base Level Control Via Bottom Product Figure 6.6 This is one of the most commonly encountered column-control schemes. Both level control systems may be calculated by the method of Chapter 16. Reflux feed and steam feed ratio controls should be provided. Top composition may be controlled by trimming the reflux feed ratio bottom composition by trimming the steam...

Figure 224

Distillation Column Curves

McCabe-Thiele diagram stepping between VLE curve and operating lines to estimate number of trays required same procedure is applied again to the first tray. yy is determined from move vertically to the VLE curve . x2 is determined from yv by moving horizontally to the stripping operating line. This stepping procedure is continued up through the stripping section until the intersection of the operating lines is passed see Figure 2.24 . This determines the number of trays required in the...

Reboiler piping arrangement for preferential boiling of reflux from lowest down comer

Reflux Boiler

Hot-oil reboilers may be like steam-heated reboilers except that there is no phase change on the oil side. Best practice is to estimate boilup wBU from the equation WBU A.BU Wod Cp Tm - Tout 4.8 This requires measuring oil-flow rate and inlet and outlet temperatures. For higher temperatures than available with steam, use is made of other condensing media such as Dowtherm or p-cymene. For even higher temperatures, direct-fired reboilers may be used. 1. Patterson, F. M., Oil Gas J., 118-120 2....

Purge System Errors

When a AP transmitter is used for measuring base level, it is common to locate it above the upper level nozzle and to purge at least the lower impulse line, and often both impulse lines. Generally speaking, purges are intended to accomplish either or both of two functions 1. Sealing, which is simply the isolation of the transmitter from a clean no solids, nonplugging process liquid. Low-velocity laminar flow is usually adequate for sealing, but as we shall see later, may contribute to dynamic...

Reflux Drum Level Control Via Boilup

As shown in Figure 6.3, this case requires reflux bottom-product and distillate bottom-product ratio controls. Determining settings for the reflux drum level controller is, in this case, difficult unless a large reflux drum holdup is available. Preferably one should make rH 5 minutes level controller tuning will require a dynamic analysis of overall column material balance such as discussed in Chapter 14. If steam flow is metered by an orifice, it should be linearized with a square root...

Sidedraw Columns With Large Sidestreams

Figure 7.1 shows a typical control scheme for a column with a vapor side draw. Since the top and bottom products are small, base level is controlled by throttling side draw. When the side draw is a vapor, column base holdup should be generously sized. Changing the side draw changes vapor rate to the top of the column this changes the rate of condensation, which finally changes reflux back down the column. The overhead level control loop that manipulates reflux flow is nested within the base...

Theory for Multicomponent Distillation

This is the same as for the binary case except for calculating dT dP. The equilibrium pressure, PT, at a specified temperature, T, for a constant composition multicomponent mixture is a function of the vapor pressure, mole fraction, and activity coefficient for each component and is defined where n is the number of components and P, is the vapor pressure at T for the ith component. Pt 2 y gt x gt elAi Bi T 273-16 lt i

Reflux Drum Level Control Via Reflux

This case requires, as shown by Figure 6.4, bottom-product top-product and steam top-product ratio controls. The calculation of reflux drum level controller settings follows the method of Chapter 16, Section 4 the calculation of base level controller settings follows that of Chapter 16, Section 6. Since reflux drum holdups are usually small compared with base holdups, a buffer tank in the top product line not in the reflux line is highly recommended. Top product composition may be controlled by...

Calculation Of Distillationcolumn Internal Reflux

Internal Reflux Model

In the design of distillation columns, engineers use internal reflux in their calculations rather than external reflux. Important indices for column separation ability are internal reflux distillate and boilup bottom-product ratios. Increasingly we are controlling columns on the basis of internal reflux rather than external reflux. The most common method of determining internal reflux makes use of external reflux flow measurement and the number of degrees of condensate subcooling see Figure...

Feed Tray Location

For a given feed composition and enthalpy, there is an optimum feed-tray location that permits making the specified separation with the least energy consumption. It is also the tray that will permit maximum feed rate without causing the column to flood. As shown in Figure 5.6, a column should generally be equipped with a number of alternative feed trays to handle changes in operating conditions from those assumed for design. The magnitude of the energy savings to be realized by changing feed...

Arrangements For Materialbalance Control

As shown elsewhere,1 the size and location of tanks and the concept of overall material-balance control used can have a great influence on plant investment and process control. If the design engineer uses the concept of control in the direction opposite to flow, tanks may be smaller and plant fixed investment and working capital can be lower than if control in the direction of flow is used. The meaning of these expressions is illustrated, for simple tanks with level controllers, in Figures 1.1...

Compensation For Various Disturbances

Feed should enter the column with a constant enthalpy. When significant changes are anticipated, a heat exchanger and feed-enthalpy control system should be provided. This is discussed in more detail in Chapters 5 and 11. Changes here can cause changes in boiling rate. The best preventive is a steam-to-feed ratio control system combined with temperature and pressure compensation of steam flow. A high pressure drop across the steam valve favors smooth control but velocity-limiting trim may be...

Fundamentals Of Composition Control

Let us consider briefly what must be done to a column to keep terminal compositions constant on a steady-state basis when the column is subjected to sustained changes in feed flow rate or feed composition. Methods of handling other disturbances will be discussed later. To simplify the analysis, let us limit our attention to an ideal, binary distillation. This is somewhat restrictive, although the results will be applicable in a general way to multicomponent systems, particularly those that may...