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4.4 FRACTIONATOR CONTROL
4.4 FRACTIONATOR CONTROL
The fractionators operate by using a controlled temperature gradient from top to bottom. The composition of the top product is fixed by its bubble point or dew point. The bottom product is controlled by its bubble point.
The fractionators always must operate so that the material and energy balances around it are satisfied on a steady-state basis. Any momentary upsets will be reflected by internal unstable operation which causes intolerable "upsets." Furthermore, it is a "sluggish" device. Liquid "hold-up time" is fairly large since flow rates are relatively low compared to its capacity. Therefore, an inherent time lag occurs when controlling at the tower extremities.
The fractionators must process that feed inlet rate, condition and composition that comes to it. Attempts will be made to control these but the success for this will vary. The fractionator's control system must, therefore, be fairly flexible.
A demethanizer and/or deethanizer normally is used to remove noncondensibles that are prohibited in the saleable products. The problem is to keep these noncondensibles from passing out the bottom (fairly easy) with only minimum loss of saleable products out the top (more difficult). Therefore, the overhead product is the more critical of the two, although both are important.
Usually, a depropanizer and/or debutanizer is producing a commercial overhead product that must meet certain specifications. At this point (hopefully) no noncondensibles are in the system. In the usual situation the propane, butane or LPG mix are less valuable per unit volume than the heaviest product (natural gasoline, condensate, etc.). This latter product should contain all of the propane and butane that the sales specifications allow. Still...the most sensitive control problem is to keep the methane and ethane levels low (from previous fractionation) to meet vapour pressure specs and maintain the heavier ends at a concentration to satisfy weathering tests.
There are several rules that should be followed in fractionator's control:
1.The lesser of the two streams should be manipulated (control wise) to obtain the greatest sensitivity in product quality.
2.Separation should be manipulated to control the purity of the purest product; the material balance should be manipulated to control the quality of the less pure product.
Older control systems attempt to accomplish these functions by the use of pressure, temperature, level and flow controls on each stream independently. The next plateau was to recognize that these streams were not really independent and to address the interaction between them by means of control loops. The next level of sophistication is to add a chromatograph to sense directly those composition changes that are critical and transmit the proper signal to the controls. A simple analog system may be used to accomplish this. The final plateau is reached by "marrying" all of these to a computer which has been properly programmed. All streams being sensed feed their information into this computer or programmable logic controller which runs through a dynamic simulation and then tells the controls what to do.
A computer does not solve the control problem; it can only react within the limits imposed on it by its creator. It also represents an expensive control system.
As a guide in this endeavour, a series of control systems will be shown. These systems should be viewed as examples to illustrate the principles involved.
2.4.1 Feed Surge Control
Regardless of the process used to recover liquid, both flow rate and composition will vary to the first fractionator. A combination surge drum and vessel to flash-off a portion of the methane and ethane might be used ahead of this fractionator. The level must fluctuate in this vessel. One simple approach is to use a liquid level control with a long displacement type float. By setting it on 100-200 % proportional control, large level fluctuations will dampen out rate changes.
Figure 2.14 (A) to (D) show several possible arrangements. The pump would be eliminated in the first three if the tank is at a high enough pressure. If the tank is large, (A) could be used. A level indicator with level alarms would be required on the tank to guard against low and high levels. Method shown (B) would use the wide band (and maybe long float) with or without the pump. Method (C) is a further addition that might be necessary when the pressure on the accumulator is not constant. The LC resets the flow recording controller (FRC). In each of these systems, any pump used would have to be of the centrifugal type subject to back pressure control.
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| Figure 2.14 Feed Surge Control |
Figure 2.14 (D) shows a steam pump layout. The level controller sets the control point for flow. The flow controller actuate; the valve on the steam line. Few steam pumps are now used, but this diagram illustrate; a simple interlock system.
2.4.2 Feed Temperature (Thermal Condition)
For efficient separation; it is usually desirable to have the feed at its bubble point when it enters the tower, unless the feed comes directly from some preceding udistillation step, an outside source of heat is required.
Steam may be used to heat the feed, any change in feed temperature, a corrective adjustment to the supply of steam into the exchanger. To maintain feed temperature; usually a three mode controller is used. The use of cascade loop (Figure 2.15) cans Provide Superior Temperature Control.
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| Figure 2.15 Improved Column Feed Temperature Control |
Constant temperature feed does not necessarily mean constant feed composition (quality). If the feed composition varies, its bubble point varies. It is common practice to set the temperature control at a point which is equivalent to the bubble point of the heaviest feed.
2.4.3 Column Pressure Control
Regardless of the column control system, it must contain some provision for pressure control. Column pressure can be controlled by manipulating the material balance (rate of distillate product) or by manipulating the condensing temperature (bubble/dew point pressure of distillate). Figure 2.16 (A) to (E) show several arrangement.
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| Figure 2.16 Column Pressure Control |
Figure 2.16 (A) shows a simple back pressure control on the vapour from the partial condenser. In this case, only enough liquid is condensed to provide reflux. The pressure tap could be on the tower, as shown, or on the reflux accumulator. A proportional response plus reset might be used, although a narrow band proportional control alone could be adequate since pressure offsets are often not critical.
Figure 2.16 (B) shows a system for a total condenser that has proven suitable for a narrow boiling range product. The disadvantage is that a large control valve must be placed in the overhead line.
Figure 2.16 (C) shows a flooded condenser system for a total condenser. In this system the accumulator runs completely full of liquid and pressure is controlled by manipulating the heat transfer area in the condenser. This method is commonly used in NGL fractionators.
Figure 2.16 (D) is an example of one type of hot vapours bypass to control tower pressure. The condenser is partially flooded. The vapours bypass changes the surface temperature of the liquid in the accumulator, hence controlling tower pressure.
The temperature of the condensed product in the accumulator can also be controlled by controlling the cooling medium. This is shown schematically in Figure 4.16 (E). This method is not recommended if the cooling medium is cooling water as it induces fouling and scaling in the condenser. If the cooling medium is air, louvers or variable pitch for blades can be used to control air flow. Induced draft coolers are preferred because the tube bundle is not exposed to precipitation.
Most pressure control systems are based on manipulating the cooling rate at the condenser. If the condenser is allowed to operate without restriction, the column pressure will be as low as possible given the cooling medium and operating conditions. This is called "floating pressure control" and has the benefit of reducing the difficulty of separation, as relative volatilities tend to increase with decreasing pressure for most hydrocarbon separations.
With a total condenser, the rate of reflux would be controlled by a flow control set manually or representing a ratio to some stream.
2.4.4 Reboiler Control
Boil-up rate is controlled by setting the flow of heat to the reboiler. A flow controller is placed in the line carrying the heating medium to the reboiler.
The amount of lighter boiling material in the bottom product is determined by the set point of the steam rate controller, A setting which permits a greater amount of steam into the reboiler will cause more of the lighter material to be driven back into the column as vapours.
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| Figure 2.17 Kettle Reboiler Control |
Other types include thermosyphon reboilers and forced-circulation reboilers. For them, the bottom product is withdrawn from the column (Figure 2.18)
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Figure 2.18 Therosyphon Reboiler Control |
2.4.5 Variable Feed Tower
Suppose the distillations product is to be fed into a second column. Then any inadvertent changes in the first column would be reflected in the quantity and composition of the feed to the second.
One way to iron out temperature variations caused by liquid level changes, in the first column, is to add flow controllers to the product lines with variable feed rates and compositions, cascade controls are justified. If the feed rate and composition are relatively constant, hand reset of the major control loop is sometimes adequate, in other cases the flow set point is continuously adjusted by the level controller in cascade arrangement Figure 2.19.
When composition of the bottom product is the important consideration, it is desirable to maintain a constant temperature in the lower section. This can be done by letting the temperature measurement set the control point of the reboiler steam supply as shown in Figure 2.20.
When compensation of the distillation product is the important consideration it is desirable to maintain a constant temperature in the upper section as shown in Figure 2.21.
Measuring temperature in a column usually requires that the sensing device be in the liquid on the tray. Heat transfer from a liquid medium to the sensing device is much greater than the heat transfer from a gas medium.
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Figure 2.19 Cascade Control of Feed to Second Column |
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Figure 2.20 Temperature Cascaded Heat Addition to the Reboiler |
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Figure 2.21 Temperature Cascaded Reflux Flow for Improved Overhead Composition Control |
2.5 OPERATING DIFFICULTIES
2.5.1 Fouling
Sometimes upsets in operation are caused in sections of a column due, for example, to local fouling. Differential pressure measurements are helpful in locating the sections that are causing the difficulties. Careful and repeated tests have to be made to determine the sections at which the increased pressure drop first occurs, as vapours below a flooded section may lift the liquid upward and lead to misleading results.
2.5.2 Temperature Profile
A second method of locating trouble spots is by the use of temperature profile data. In fractionating columns the temperature of any given plate is dependent on the pressure and composition of the material on the tray. A plot of temperature versus plates for the column is called the temperature profile. When a column is operated to yield a product consisting mainly of one component only (e.g. deisopentanizer with a top product of 95%, pure isopentane) the temperature gradient over the trays near the product outlet will be very small. When both top and bottom product are very narrow boiling range products only one steep column temperature gradient will be located near the feed inlet where all components to be separated are present in appreciable quantities.
When wide boiling range products are manufactured to flattening down of the temperature profile will occur over the trays near the product outlets.
Comparison of the known profile for normal operation with that when operation is poor may help to locate the source of the trouble. In a flooded area the composition of the liquid on all trays will be the same and hence the temperature will not change over this section.
2.5.3 Operation near Critical Conditions
When a column is operated under conditions of temperature and pressure which are very near to the critical values of the hydrocarbons to be processed, fractionation can be poor owing to the fact that a column section contains only one phase and is consequently flooded. Fractionation of light hydrocarbons carried out under very high pressure in order to avoid refrigeration systems, is conducive to this condition.
2.5.4 Using of Grid Trays
Because of the low liquid hold-up, grid trays offer difficulties at starting up of the unit. These difficulties can also be experienced when during operation the vapour load is momentarily much reduced resulting in a tray condition below the dispersion point. Consequently all liquid will then fall through the trays. The installation of a distributor tray above each section containing about 10 grid trays will increase the liquid holdup and will, therefore, make the column less sensitive to outside disturbances.
2.5.5 Loads in Rectifying Section
The highest load in the rectifying section of a column normally occurs at the top plate. Sometimes this tray is flooding and further increases in reflux are ineffective because it merely goes overhead as liquid. A heat balance around the overhead condenser shows if much liquid is entrained, which is an indication of overloading of at least the top part of the column.
2.5.6 Way of Introducing Feed
The way of introducing the feed into the column may also be a source of trouble, as the vapour or liquid entering may upset the flows in that part of the column. Furthermore attention must be given to the velocity of the vapour and liquid feed in the inlet. Too high a vapour velocity may cause atomization of the liquid feed resulting in considerable entrainment.
2.5.7 Reboiler
The reboiler may be the cause of operating troubles in the bottom section. This is especially the case with reboilers of the thermosyphon type. Here fluid is driven through the reboiler by the static head driving force created by vaporization in the reboiler tubes. Frictional losses oppose the flow of fluid through the circuit. As a result the flow will adjust itself in such a manner that a balance is obtained between the static head driving force and the flow resistance. If the resistance increases owing to fouling in the pipes or if the lines are too small, the balancing liquid level in the column may build up and rise even as high as the bottom tray, which causes flooding.
2.6 TROUBLESHOOTING OPERATING PROBLEMS
This part deals with common operating problems. It is not apropos to try to cover all problem situations. The following are discussed:
1. Flooding
2. Dry trays
3. Damaged trays
4. Water in a hydrocarbon column
5. Foaming
6. Condenser fogging
7. Suspect laboratory analyses
2.6.1 Flooding
Design
Flooding is a common operating problem. Companies naturally wish to obtain, maximum capacity out of fractionation equipment and thus often run routinely close to flooding conditions. New columns are typically designed for around 80% of flood. Clearly, the column needs some flexibility for varying operating conditions. Vendors, state that their modern methods for determining percentage of flood represent very closely the true 100% flood point The designer, therefore, shouldn't expect to design for 100% of flood and be able to accommodate variations In operating conditions. Designers recommend a percentage of flood of not more than 77% for vacuum towers or 82% for other services, except that for columns under 36" diameter, 65-75% is recommended.
Down comer Backup Flood
If the down comer backup for valve trays exceeds 40% of tray spacing for high vapor density systems (3.0 lbs/ft3), 50% for medium vapor densities, and 60% for vapor densities under 1.0 Ibs/ft3, flooding may occur prior to the rate calculated by jet flood. Another good rule of thumb is that the down comer area should not be less than 10% of the column area, except at unusually low liquid rates. If a down comer area of less than 10% of column area is used at low liquid rate, it should still be at least double the calculated minimum down comer area.
Tower Operations
The tower operator can quickly determine which type of flooding will tend to be the limiting one for a particular system. If a rigorous computer run is available for the anticipated or actual operation, the operator can quickly calculate the expected limiting column section. The operator can then provide DP cell recording for the entire column and limiting section (s). As mentioned previously, a DP cell is the best measure of internal traffic and flooding tendency.
Many plants non technical operators do not understand that high vapor rates as well as high liquid rates can cause down comer backup flooding. It is well to explain to the plant operators the mechanism of down comer backup flooding and show them with a diagram how the head of liquid in the down comer must balance the tray pressure drop. Then it can be explained how vapor flow is a major contributor to this pressure drop.
Flooding across a column section reflects itself in an increase in pressure drop and a decrease in temperature difference across the affected section. Product quality is also impaired, but it is hoped that the other indicators will allow correction of the situation before major change in product quality. When a column floods, the levels in the accumulator and bottom often change. It can occur that the accumulator fills with liquid carried over while the reboiler runs dry.
Also, in a flooded column, the pressure will often tend to fluctuate. This may help to differentiate between flooding and a high column bottom level, if the bottom level indicator reading is suspect. The high bottom level will give higher than normal pressure drop but often not the magnitude of pressure fluctuations associated with flooding.
Here is a tip for possible capacity Increase for towers with sloped down comers. Usually, the tray vendor doesn't use the dead area next to the bottom part of the sloped down comer as active area if the trays are multipass, since he would require a different design for alternate trays. This area could be used for additional vapor capacity in an existing column.
2.6.2 Dry Trays
This problem, as with flooding, also impairs product quality. No fractionation occurs in the dry section, so the temperature difference decreases. However, unlike flooding, the pressure drop decreases and stays very steady at the ultimate minimum value. This problem is usually easier to handle than flooding. The problem is caused by either insufficient liquid entering the section or too much liquid boiling away. The problem is solved by reversing the action that caused the dry trays.
Since the changes usually occur close to the source of the problem, the source can usually be quickly found with proper instrumentation. Too little reflux or too much side stream withdrawal are two examples of insufficient liquid entering a section. Too hot a feed or too much reboilling are examples of excessive liquid boiloff.
2.6.3 Damaged Trays
Effects
Trays can become damaged several ways. A pressure surge can cause damage. A slug of water entering heavy hydrocarbon fractionators will produce copious amounts of vapor. The author is aware of one example where all the trays were blown out of a crude distillation column. If the bottom liquid level is allowed to reach the reboiler outlet line, the wave action can damage some bottom trays.
Whatever the cause of the tray damage, however, it is often hard to prove tray damage without column shutdown and inspection, especially if damage is slight. Besides poorer fractionation, a damaged tray section will experience a decrease in temperature difference because of the poorer fractionation. An increase in pressure difference may also result since the damage is often to down comers or other liquid handling parts. However, a decrease in pressure difference could also occur.
Bottom Level
Trays are particularly vulnerable to damage during shutdown and startup operations. Several good tips to minimize the possibility of tray damage during such periods are provided.
First it is important to avoid high bottoms liquid level as mentioned-previously. Initial design should provide sufficient spacing above and below the reboiler return vapor line. A distance equal to at least tray spacing above the line to the bottom tray or better tray spacing plus, say, 12" and a distance of at least tray spacing below the line to the high liquid level is absolutely necessary. Probably more tray problems occur in this area of the column than any other. In spite of good initial design, however, the bottoms liquid level needs to be watched closely during startup.
Steam/Water Operations
Steam/water operations during shutdown have high potential for tray damage if not handled correctly. If a high level of water is built up in the tower and then quickly drained, as by pulling off a bottom man way, extensive tray damage can result, similar to pumping out hydrocarbons too fast during operation.
Steam and water added together to a tower can be a risky operation. If the water is added first at the top, for instance, and is raining down from the trays when steam is introduced, the steam can condense and impose a downward acting differential pressure. This can result in considerable damage. If steam and water must be added together, start the steam first. Then slowly add water, not to the point of condensing all the steam. When finished, the water is removed first. One vendor estimates that he sees about six instances of tray failure per year resulting from mishandled steam/water operations.
Depressuring
Depressuring a tower too fast can also damage the trays by putting excessive vapor flow through them. A bottom relief valve on a pressure tower would make matters worse since the vapor flow would be forced across the trays in the wrong direction. The equivalent for a vacuum tower would be a top vent valve which would suck in inerts or air and again induce flow in the wrong direction. Such a top vent should not be designed too large.
Phase Change
Overlooking change of phase during the design stage can also cause tray damage. An example is absorber liquid going to a lower pressure stripper and producing a two-phase mixture. In one case, the absorber stream entered the stripper in a line that was cited down onto the stripper tray. The two phase mixture beet out a section of trays. A protection plate was provided and this had a hole cut in it in two years.
2.6.4 Water in Hydrocarbon Column
Here small amounts of water are meant rather than large slugs which could damage the trays. Often the water will boil overhead and be drawn off in the overhead accumulator bootleg (water drawoff pot). However, if the column top temperature is too low, the water is prevented from coming overhead. This plus too hot a bottom temperature for water to remain a liquid will trap and accumulate water within the column. The water can often make the tower appear to be in flood.
Many columns have water removal trays designed into the column. Top or bottom temperatures may have to be changed to expel the water if the column isn't provided with water removal trays. In some instances, the water can be expelled by venting the column through the safety relief system.
It should be remembered that water present in a hydrocarbon system, being immiscible, will add its full vapor pressure to that of the hydrocarbons. The author once wondered why the pressure was so high on a certain overhead accumulator until he noticed the installed bootleg.
A small steady supply of water entering a column through solubility or entrainment can, in some cases, cause severe cycling at constant intervals during which time the water is expelled. After expulsion of water, the column lines out until enough is built up for another cycle.
Besides water, other extraneous substances can leak into a column with varied effects. One example was a column separating two components and using the light component as seal flush for the reboiler pump. When excessive lights leaked into the tower system, the bottoms product went off specification. It took a long time to solve the problem since at first the operators suspected loss of tray efficiency.
2.6.5 Foaming
The mechanism of foaming is little understood. During the design phase, foaming is provided for in both the tray down comer and active areas. "System factors" are applied that derate the trays for foaming. Oil absorbers are listed as moderate foamers. Heavy oil mixed with light gases often tends to foam. The higher the pressure, the more foaming tendency, since the heavy oil will contain more dissolved gases at higher pressures. Liquids with low surface tension foam easily. Also, suspended solids will stabilize foam. Foaming is often not a problem when a stabilizer is not present.
No laboratory test has been developed to adequately predict foaming. An oil that doesn't foam in the laboratory or at low column pressure might well foam heavily at high column pressure. In general, aside from adding antifoam, there seems to be no better solution to foaming than providing adequate tray spacing, and column down comer area. One designer solved a down comer foaming problem by filling the down comer with Raschig rings to provide coalescing area.
For troubleshooting suspected foam problems, vaporize samples of feed and bottoms to look for suspended solids. Also, one can look for the Tyndall effect as described in the section on condenser fogging.
In investigating foaming problems. It is helpful to have an estimate of foam density.
2.6.6 Condenser Fogging
Fogging occurs in a condenser when the mass transfer doesn't keep up with the heat transfer. The design must provide sufficient time for the mass transfer to occur. A higher temperature differential (∆T) with noncondensibles present or a wide range of molecular weights can produce a fog. The high ∆T gives a high driving force for heat transfer. The driving force for mass transfer, however, is limited to the concentration driving force (∆Y) between the composition of the condensable component in the gas phase and the composition in equilibrium with the liquid at the tube wall temperature. The mass transfer driving force (∆Y) thus has a limit. The ∆T driving force can, under certain conditions, increase to the point where heat transfer completely outstrips mass transfer, which produces fogging.
Nature of a Fog
Fog, like smoke, is a colloid. Once a fog is formed. It is very difficult to knock down. It will go right through packed columns, mist eliminators, or other such devices. Special devices are required to overcome a fog, such as an electric precipitator with charged plates. This can overcome the zeta potential of the charged particles and make them coalesce.
A colloid fog will scatter a beam of light. This is called the "Tyndall Effect" and can be used as a troubleshooting tool.
Review
1. Quality control over distillation products is maintained by setting specifications for these products.
2. An initial boiling point test identifies the presence of light hydrocarbons.
3. An end point test identifies heavy hydrocarbons.
4. The temperature at which a petroleum product generates ignitable vapors is called the flash point.
5. Light hydrocarbon products have relatively high API gravity readings. Reduced crude has a relatively low API gravity reading.
6. Cut point changes are made on distillation products by adjusting the heat balance inside a tower.
7. Whatever material enters a tower as feed leaves the tower as products.
8. You can increase the temperature in a crude column by increasing the feed temperature or decreasing the reflux rate.
9. A temperature increase means that products get heavier.
10. You can decrease the temperature in a crude column by decreasing the feed temperature or increasing the reflux rate.
11. A temperature decrease means that products get lighter.
12. When the tower temperature increases, the amount of overhead product produced increases and the amount of bottom product formed decreases.
13. When the tower temperature decreases, the amount of overhead product produced decreases and the amount of bottom product formed increases.
14. When we close a stripper draw on the side of a crude unit more reflux flows to the trays below the draw-off tray.
15. Closing a stripper draw makes this product and the products below this tray lighter.
16. To change the composition of one side draw product without affecting the composition of products below this point, you must close one stripper draw and open another.
17. The temperature on the top tray of a tower should be just high enough to completely vaporize the overhead product.
18. In order to store the overhead product in liquid form, the pressure in a condenser or accumulator must be slightly higher than the vapor pressure of the product at the temperature it is being stored.
19. The ideal temperature at the bottom of the tower is the temperature at which the vapor pressure of the bottom product is slightly below the operating pressure of the tower.
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