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Effectively Control Column Pressure Andrew W. Sloley, Sloley, The Distillation Group, Inc.
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Dist
Maintaining proper pressure can play a crucial role the stable operation of a distillation column. Here’s
to select the most appropriate pressure-control sch
You're Reading a Preview Unlock full access with a free trial.
Download With Free Trial
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ost distillation-column control
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flashing feed. Two-phase feed in a colum
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Distillation
Figure 1. Net vapor rate > 0, direct control of product rate
PC
Table 1. Common methods for pressure control of towers. Method Type (Figure No.)
Description Net vapor rate always > zero
1
1A
Direct control of vapor product rate
2
1A
Control recycle vapor rate to compressor
3
1A
Control recycle vapor rate to ejector: ejector discharge recycle
4
1A
Control makeup ejector load
5
1A
Control ejector motive-steam pressure
6
1A
Control recycle vapor rate to ejector: ejector discharge recycle Net vapor rate ≥ zero
7
2A+
Control vapor product rate in conjunction with a secondary method
8
2A
Control makeup vapor supply Net vapor rate = zero
Method: Vary net vapor rate, no makeup gas. Process: Must always have vapor product. Advantages: Simple and direct. Disadvantages: Generally none. Response may be slow when Application: Usually the best choice when vapor rate is a Variants: Condenser outlet may enter bottom of drum (but necessary and adds extra pressure drop to the overhead Configuration notes: Works best mounting condenser abo allowing gravity flow of condensate to condensate drum. outlet nozzle should allow for free draining of two-phase o Operation: Controller directly affects column vapor invent pressure. Warnings: —
9
3B
Control flow to condenser — condenser at low pressure
10
3B
Control flow from condenser — condenser at high You're Reading pressure
11
3B
12
3B
Control bypass flow to condensate receiver — Unlock full access with a free trial. “hot vapor bypass” Figure 2. Net vapor rate > 0, control recycle vapor rate to Direct control of liquid product rate
13
3B
Dual pressure control of bypass andDownload condensate
14
3D
Control vaporizing coolant level
15
3D
Control vaporizing coolant pressure
16
3E
Control condenser inlet coolant temperature — “ tempered water”
17
3E
Control coolant rate
18
3E
Control of air-cooler coolant rate
19
3C
Control condensation temperature or pressure
The bases for control
a Preview
With Free Trial
PC
Alternatives
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Figure 3. Net vapor rate > 0, control recycle vapor rate to ejector.
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Figure 4. Net vapor rate > 0, control makeup ejector load PC
PC
Steam
Vapor Product
To Hot Well
Method: Vary net vapor rate, makeup vapor (steam). Method: Vary net vapor rate, no makeup gas. Process: Vacuum systems, always must have vapor produ Process: Vacuum systems, always must have vapor product. Advantages: Easily added to systems after construction, c Advantages: All ejector discharge available for recycle, often gives the piping are small and relatively cheap. most stable operation. You're Reading a Preview Increases steam use and hot-well water m Disadvantages: Disadvantages: Difficult to implement in systems with the ejector directly es cooling water load. attached to its condenser, relatively large recycle line and control valve Application: Often added after-the-fact to vacuum system Unlock full access with a free trial. frequently are required. control, and frequently used in batch distillation systems. A Application: The best ejector pressure-control system for ejectors that are batch systems may have nearly zero load from the tower t not stable to zero load, often the most economical system for stable ejecInstead of steam, air or inert gas may be used to Download With Variants: Free Trial tor control. suction. Variants: Recycle to upstream of condenser: This requires a process comConfiguration notes: Using air or inert gas instead of steam patible with water and water removal in condensate drum. Most of the reinert gas load to the condenser and may affect condenser cycle may be condensed in the condenser — if this happens, recycle of multistage ejector systems, using noncondensable gas (ai off-gas from the ejector condenser to upstream of the tower condenser is load up all ejectors in the system. Steam will load the ejec recommended instead. steam injection directly upstream. Recycle ejector condenser vent to upstream of ejector: This variant is not Operation: Added load moves the vacuum jet ejector along recommended. Noncondensable recycle often can have a limited operatcurve — the higher the load to the ejector, the higher the e ing range due to low noncondensable flow rates. In tight batch systems, pressure.Sign up to vote on this title noncondensable load after startup can approach zero. This mandates use Warnings: Wet steam may cause ejector erosion from wat Not useful Useful tracing, of condensable recycle immediately downstream of the ejector. combination of electrical superheated steam, loca Configuration notes: Vapor product may be either a true vapor product or a before addition to the system, or other measures may be r nonproduct material to be evacuated from the system. In multistage syserosion to allowable levels. tems, the recycle stream must loop only around the initial ejector. Recycle
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Distillation
Figure 5. Net vapor rate > 0, control ejector motive steam.
Figure 6. Net vapor rate > 0, control recycle vapor rate to
Steam
PC Steam PC
Vacu
To Hot Well
Not Recommended
Method: Vary ejector driving steam pressure — not normally recommended . Process: Vacuum systems, always must have vapor product. Method: Vary net vapor rate, no makeup gas. Advantages: Minimizes total steam demand, hot-well water generation, and Process: Vacuum systems, always must have vapor produ condenser cooling needs; requires a small control valve but minimum piping You're Reading ashown Preview is commonly referred to as a “dry” or “damp” syst changes. upon the application, because there is no condenser befo Disadvantages: Often only a very small control range available. ejector. Unlock full access with a free trial. Application: Added to vacuum ejector systems after construction — never Advantages: All ejector discharge available for recycle, of should be selected as the primary control system in preconstruction design. most stable operation. Variants: — Download With Disadvantages: Free Trial Difficult to implement in systems with the Configuration notes: — attached to its condenser. Relatively large recycle line and Operation: Lower-pressure supply steam reduces the mass rate of steam to frequently are required. Overhead product must be compa the ejector. This lowers the pressure ratio across the ejector and the ejector Application: The best ejector pressure-control scheme for suction pressure rises. nected directly to the tower that are not stable to zero loa Warnings: Ejectors have limited motive-steam-mass and volume operability most economical system for stable ejector control. ranges. Sometimes this method works, other times it does not. Improper Variants: System frequently is used without re flux return t ejector operation can result in rapid and unstable shifts in vacuum pressure. ternal liquid feed or pumparound reflux generation). The author has seen sudden swings from 10 torr (10 mm Hg) vacuum to 40 Configuration notes: In multistage systems, the recycle str torr (40 mm Hg) vacuum and back resulting from minute changes in motive Signtheupinitial to vote onRecycle this title only around ejector. from the last ejecto steam pressure. In other systems, motive steam pressure could be varied by primary ejector can cause unstable operation due to incre Useful load Not useful as much as 10–20% with reasonable control still achieved. densable on the intermediate or final ejector. Operation: Recycle gas moves the vacuum jet ejector curve — the higher the load to the ejector, the higher the
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Figure 7. Net vapor rate secondary method.
≥
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0, control vapor product rate combined with a
Simulacion Proceso
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Figure 8. Net vapor rate
≥
0, control makeup vapor suppl
Normal Set Point PC
To Zero-Vapor-Rate Control Scheme Type
PC PC
High Set Point
Method: Vary blanketing vapor above drum. Method: Various. Process: Net vapor rate positive or zero. Process: Net vapor rate positive or zero. Advantages: Simple, fast response. Advantages: — Disadvantages: Net consumption of pressurizing gas, requ Disadvantages: — You're Reading trol a Preview valves, and may pose tuning problems. Application: Effective choice for systems that require inert venting. Application: Pressure towers. Variants: Many. Variants: Adding makeup gas upstream of the condenser m Unlock full access with a free trial. Configuration notes: Can be used with methods that allow for variable congas rate required, due to partial vapor blanketing of the co denser duty. See Figures 9–11 and 13–19 for examples. The flooded drum sponse time will be slightly slower, however. Another varia (Figure 12) is not suitable for addition to this basic method. drum is open to the atmosphere provides partial exchange Download Free but Trial Operation: Direct control of overhead pressure. Differential pressure (DP) With keting, is rarely used today due to potential emission o is manipulated to control condensation temperature in condenser — this Configuration notes: Pressurizing gas must be compatible varies the condenser’s log-mean temperature difference (LMTD). When cess. Some pressurizing gas may enter the distillate produ pressure rises above the high set point on the receiver, the high-set-point Equalizing line is not required. Works best if the control se controller opens and gas leaves the system. slightly overlapping range: a small operating band is prese Warnings: — is inert gas coming in and some vented gas going out.
water boots on condensate drums, and other equipment. To keep the diagrams clear and concise, they only include information required for discussion of the pressure control problem. In addition to the P&ID, each figure includes the following descriptive information and guidelines:
Operation: When pressure drops below the desired set po added to the receiver. When pressure rises above the des up to on this title the vaporSign product linevote is opened. Warnings: —
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with reflux provided by pumparounds, internal co
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Distillation
Figure 9. Net vapor rate = 0, control flow to condenser at low pressure.
PC
Figure 10. Net vapor rate = 0, control flow from condense pressure.
Equalizing Line
Equalizing Line
PC
NC
NC = Normally Closed Valve Method: Vary condensing area. Method: Vary condensing area. Process: Net vapor rate zero. Process: Net vapor rate zero. Advantages: Simple. Advantages: Simple, requires relatively small control valve Disadvantages: Liquid level control in drum affects pressure balance, reDisadvantages: Condensate from condenser to drum must quires relatively large control valve, as well as extra exchanger area. ity flow of liquid. Great attention must be paid to layout an Application: Generally a stable and effective control scheme for many sizing. Extra exchanger area is necessary. You're Reading aApplication: PreviewGenerally a stable and effective control sche processes. Variants: — processes. Configuration notes: Pressure in the drum and column overhead are the Unlock full access with a free trial. Variants: Liquid may enter through top of condensate drum same. Equalizing line is needed for stable control. Liquid from condenser Configuration notes: Pressure in the drum and column must enter the drum in the liquid layer so that control valve changes can same. Equalizing line is needed for stable control. If airaffect liquid level in the condenser. Preferred application is liquid entry the condenser, Download With Free Trial they should be angled to allow for gradual into drum from below (shown). If air-fins are used for the condenser, they changes with liquid level changes. should be angled to allow f or gradual surface-area changes with liquid Operation: Control valve imposes a variable DP in main level changes. Cooling is done at the lowest possible utility temperature condenser. Liquid level in control line up to exchanger var (potential problem for cooling water, potential advantage for heat-integratchanges. This exposes or submerges tubes, shifting area f ed cryogenic systems). tion to subcooling. Operation: Control valve imposes a variable DP in main flow line through Warnings: — condenser. Liquid level in control line up to exchanger varies with DP changes. This exposes or submerges tubes, shifting area from condensa Sign up to vote on this title tion to subcooling. Warnings: —
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Heavy exchangers, exchangers requiring fre ing, cooling b Method 13 is recommended f
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Figure 11. Net vapor rate = 0, control bypass flow to condenser receiver. PC
Figure 12. Net vapor rate = 0, direct control of liquid prod
PC
NC
NC Flooded Drum
Method: Vary condensing area. Method: Vary condensing surface area vs. subcooling surface area. Process: Net vapor rate zero. Process: Net vapor rate zero. Advantages: No liquid control system for condensate drum Advantages: Condenser may be mounted below condensate drum. Disadvantages: Control action in changing surface area Disadvantages: Concept is not straightforward, and method requires subproduct rate. This may cause problems due to unsteady fe cooling area in condenser. stream units. It may be difficult to vent inert gas or noncon Application: Often used with very large and heavy condensers or with terial from upsets. equipment requiring recurring cleaning or maintenance (exchangers at Application: Often used with very large and heavy conden You're Reading aequipment Preview grade). requiring recurring cleaning or maintenance (e Variants: Condensate drum pressure instead of tower overhead pressure grade). may be controlled. Works for units with continuous vapor products well. Unlockasfull access with a free trial. Variants: No condensate drum, reflux rate may be control In this case, vapor product composition is tower overhead composition distillate product. and the condenser does not count as a separation stage. Configuration notes: A liquid level must be maintained in t Configuration notes: General method often is referred to as “hot vapor byat all times. The purpose of the drum, where present, ofte Download With Free Trial pass.” A liquid level must be maintained in the exchanger at all times. second phase from the overhead, for example, water from Careful attention to piping is required. Liquid must enter the drum without system — this has not been shown in the figure. mixing with the drum’s vapor space (7) . This mandates either having the Operation: To maintain pressure on the t ower overhead, t liquid enter the drum from the bottom or having an internal pipe down from control valve in the condenser line is manipulated. As the the top. The author prefers bottom entry as it creates less liquid turbuDP changes, the pressure balance between the bypass a lence. The control valve and bypass line must be sized so that the bypass denser flows varies the liquid level in the condenser. This flow-rate changes allow for a DP change that corresponds to the liquid allocation of condensing vs. subcooling surface available level range available in the condenser. drum pressure, the control valve in the bypass line varies Sign up to vote on this title Operation: To maintain pressure, the control valve DP is manipulated. As the bypass. the control valve DP changes, the pressure balance between the bypass Warnings: Mounting below the drum requi Not useful Useful thecondenser and the condenser flows varies the liquid level in the condenser. This in the condenser for stable operation. changes the allocation of condensing vs. subcooling surface available. The liquid and vapor are not in equilibrium. A thin layer of hot liquid separates the bulk liquid pool from the vapor. This essentially allows for maintaining situation depends upon the needs of the individu
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Distillation Figure 13. Net vapor rate = 0, dual pressure control of bypass and condensate.
PC
Figure 15. Net vapor rate = 0, control vaporizing coolant NC
PC
PC
NC
Method: Vary condensing area. Process: Net vapor rate zero, condensate drum runs at a lower pressure than tower pressure. Advantages: Condenser may be mounted below condensate drum. Disadvantages: Requires two control valves and subcooling area in condenser. Application: Often used with very large and heavy condensers or with equipment requiring recurring cleaning or maintenance (exchangers at grade). It is the preferred system to use with cooling boxes as condensers, as cooling boxes have too high an internal heat capacitance Method: Vary heat flux, vary vaporizing temperature (coola on the cooling water side to allow for rapid changes in cooling water Process: Net vapor rate zero, heat recovery into vaporizin level. Advantages: Blowdown, if required, can be easily made Variants: — You're Reading aDisadvantages: Preview Extra drum required. Configuration notes: A liquid level must be maintained in the exchanger Application: Used with heat recovery by having condense at all times. tower or vaporize a utility stream (water to steam). Most o Unlock full access with a free trial. Operation: Variable product rate changes the level in the condenser. systems with multiple parallel shells. Controlling drum pres Warnings: — the condensers simultaneously. Variants: —
Download With Configuration Free Trial notes: Unusual. Figure 14. Net vapor rate = 0, control vaporizing coolant level.
PC
Operation: Control valve varies the pressure of the vaporiz the utility side of the condenser. This, in turn, controls the LMTD. Warnings: —
Vapor LC Liquid NC
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Figure 16. Net vapor rate = 0, control condenser inlet-coolant temperature. PC
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Figure 17. Net vapor rate = 0, control coolant rate. PC
PC
TC
NC
Method: Vary heat flux to vary cooling medium rate. Process: Net vapor rate zero. Advantages: Simple. Disadvantages: Coolant return temperature may be high. Method: Vary heat flux, vary utility supply t emperature. Application: Frequently used in older plants (8,9) , but no lo Process: Net vapor rate zero. called for in the design of new plants. Often added to exis Advantages: Cooling stream leaves unit at maximum temperature, and conoriginal control schemes prove inadequate. Generally denser film temperature is at maximum. You're Reading the a Preview form of this control scheme is used. Block valves on the c Disadvantages: Local pump required, and response may be slow due to pinched to reduce cooling water flow on a seasonal basis system liquid inventory. Unlock full access with a free trial. turndown is necessary. Then, a more conventional pressu Application: Used for heat recovery to liquid streams from condenser. Conscheme handles day-to-day control to the set point. denser coolant always runs at maximum temperature consistent with duty Variants: — removal. This can be useful to prevent localized solidification in some Download With Configuration Free Trial notes: Cooling water fouls more rapidly with processes. perature and decreasing velocity. A minimum rate or retur Variants: Pump may be installed at exchanger outlet. over-ride with a secondary pressure control system may b Configuration notes: When used with cooling water, this often is referred to Operation: Varying coolant rate causes the LMTD to chan as a “ tempered water” system. total heat flux. Operation: Variable recycle changes the temperature of the cooling stream. Warnings: Low velocity and high temperature cooling wat This, in turn, changes the exchanger LMTD. probability and severity of condenser fouling. Warnings: —
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Distillation
Figure 18. Net vapor rate = 0, control of coolant rate with air cooler.
Figure 19. Net vapor rate = 0, control condensation temp pressure.
PC
PC
NC
Method: Vary heat flux to vary cooling medium rate. Process: Net vapor rate zero. Method: Vary heat flux. Advantages: Simple. Process: Net vapor rate zero. Disadvantages: High maintenance for louvers and variable-pitch con figAdvantages: Simple, fast response. urations. Variable speed rarely used due to cost. Disadvantages: Control valve is bigger than for other optio Application: Most common method of control with air-fins. and condensate drum pressure are variable. This may cre Variants: Fan pitch, louver position, or motor speed can be manipulated. You're Reading awith Preview condensate pump head and drum level control. Configuration notes: Variable position louvers tend to be troublesome. Application: Pressure towers — good system for towers w Louvers must be designed for automatic control. Do not refi t an automatquantities of noncondensable gas in overhead vapor (8) Unlock full access with a free trial. ic positioner to a set designed for manual operation. Variable fan pitch Variants: — also is subject to maintenance problems and fans have to be shut down Configuration notes: Cooling is done at lowest possible uti before the pitch mechanism can be worked on. Controlling speed using ture (9) (potential problem for cooling water, potential adv Download With integrated Free Trial a variable frequency motor is most reliable, but most expensive and, so, cryogenic systems). is rarely used. Systems can be combined in multiple bay units. In such Operation: Direct control of overhead pressure, DP is man units, gross control often is achieved by shutting down entire fans on control the condensation temperature in the condenser units, then achieving fine control with louvers, variable-pitch, or variable condenser LMTD. speed fans on a few units. Warnings: In vacuum systems, the minimum pressure dro Operation: Variable flow rate controls the air outlet temperature; varicontrol valve may have significant utility and equipment c able LMTD controls heat removal. plies to a lesser extent to any system with downstream co Warnings: — vapor product. In liquid-ring vacuum pump systems, suctio
Sign to votefor onvaporizing this titlethe seal fluid. can create theup conditions
Literature Cited 1. Li le J. “New, Simpler Equations Calculate Pressure-Compensat-
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