D. L. A L L E N
BEWARE OF TWO-PHASE FLOW Vapor-liquid flow causes many of the common d&culties in start-up of new plants, in mod$ing processes, and in process areas striving for increased capacities
n certain unexpected situations, the occurrence ot
Itwo-phase flow can result in intolerable conditions. High pressure drops, intermittent flow, flow stoppage, surging, and inaccurate metering may be the symptoms, any of which can cause process upsets. The process trouble shooter should be acutely aware of the conditions which can produce a second phase. In fact, if enough attention is paid to this possibility in the design of the equipment, the expense of trouble-shooting will be saved.
Mixed-phase flow caused all of the process problems in the plant examples which follow. These show the insidious nature of this phenomenon, and demonstrate several satisfactory solutions. Obviously, many of these cases could have been avoided in thedesignstage. In some cases, relatively cheap revisions of the system corrected the problem. But where gravity is the only driving force for flow, correction may be expensive, or impossible. Gravity flow systems must therefore be designed with particular care and foresight.
LOOK FOR MIXED-PHASE FLOW UNDER THESE CONDITIONS 1 If liquid under equilibrium conditions is required to flow either uphill, or through long lines by pressure difference
2 If saturated vapor at elevated'pressure goes through a pressure drop. Near the critical point, liquid will be formed
3 If there is any sudden, appreciable flow restriction of saturated liquid. Large amounts of flashing will limit system capacity, while even small amounts will ,reduce instrument accuracy
4 If a vessel overflow system is used to maintain liquid level. For smooth flow, these systems should be designed using tray downcomer criteria
5 If gravity is the sole driving force for flow. Only minimal corrections are possible if elevations are not sufficient
6 If liquid lines are under vacuum. Gas leakage into the line can cause a two-phase problem V O L 5 4 ' NO. 2
FEUR'UARY 1 9 6 2
is
1.
HEAT EXCHANGER ECONOMIZER
liquid at Equilibrium Conditions Flows Uphill
SUBCOOLED
A 3wO-hp. propane refrigeration u$ was installed in a new plant. Ajter start-up, the actual delivered refrigeration was only about 507,of design.
LYI PSIG. 27 f
iROM RRST ECONOMlZfR
, ?WQANEV M O R TO INlERSTAGE 3 WG. 17-F
,
The system had been installed so that saturated cold propane liquid had to flow not only considerable distances but also uphill. Preliminary pressure surveys and other equipment checkouts showed that the excessive resistances to flow resulted from vapor generation and mixed-phase flow. Uphill flow of the saturated liquid was the chiefculprit. In this instance, rather than revise all piping and control valves, plus perhaps all disengaging facilities at each refrigerated exchanger, it was decided to assure capacity conditions by revising the system to transport subcooled liquid. The 6nal flash drum economizer reducing the liquid propane to low temperature prior to flow to the process was removed and replaced with an available exchanger-typeeconomizer. The 20" F. liquid propane to the process was now produced close to the original temperature, but 41 p s i . over saturation pressure higher. The revised systemwas satisfactory at full capacity.
2.
Saturated Vapor Goes Through a Pressure Drop
The 4-inch ethane feed line to a pyrolysis plant was quite long, about ZOO0 j t . Ethane gas at saturation conditions of 410 p.s.i.g. and 47' F. was pressured from the separation plant to the mer. Under former cond.tions oj lower production rates the pressure drop was nominal and about expected. At higher rates this pressure drop increased beyond normal, and some liquid was observed at the delivery end. Inspection of the enthalpy diagram revealed that under these conditions of throttling due to line resistance, the ethane passes down through the vapor-liquid region of the enthalpy chart. Thus, again a mixed-phase flow is generated with resultant higher pressure drop coupled with the problem of liquid separation at the terminus. This particular problem does not need correction at this time. When it does, one solution would be to apply a small exchanger or some l i e tracing to bring the saturated vapor slightlyiout into the superheated region prior to throttling.' No liquid phase would then appear. Throttling of many saturated vapors relatively near the critical point can produce liquid. This should be recognized in any design work PafliCularlY for 10% plant interconnecting piping. 46
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
= Lu
3 2
k
ENTHALPY Whn ethanc ir throttltd fm470 p . ~ . i . g . ,47' F. 27' F., about 7% liquid if jorrncd
60
290 p.i.i.g.
3. Flow Restriction of Saturated liquid A tower bottoms meter on an ethylenc stream hod been used for years as a guide. A material balance showed it to be 30% in error due toflashing. Metering problems with saturated liquids are severe, no vapor generation through orifices can be tolerated. The meter was probably satisfactory originally, but as production rates increased over the years, its orifice drop exceeded the available excess head and flashing began. It is not always satisfactory just to change a d/p cell at higher rates, an orifice change to prevent flashing may well be necessary.
A change in procersins concept resulted in eliminating two control values and running on full open bypass. A large
4.
Vessel Overflow System Design
A reactor ouerflowed into a decanter. The ouerJlow line necessarily entered the decanter at the liquid-liquid interface. Flow between uessels sursed, as vapor was carried down and could not immediately disengage in the decanter. Level in the reactor surged in consequence, and the whole systemflow stability was disturbed. Overflow systems from one vessel to another are commonly used for level control in the first vessel. Of necessity, these systems require liquid flow from the first vessel at its vapor-liquid interface. Inevitable consequence is entrainment of vapor along with the liquid.
restriction in Jlow which limited production was apparent. In tracing the system pressure drop, it was found that the greater part was due to mixed-phase flow downstream of the former control valve manifold caused by flashing in the bypass valve. This valve was found to be a globe valve (in the author’s opinion, a gate valve should always be used). Spooling out the control valve eliminated the problem with no process interruption. If the proper by pass valve had been installed initially, the problem would not have occurred. In addition a good general principle to use in controlling a pressured bottoms flow from one distillation column to another is to place the control valve as close to the second as possible.
This is not much of a problem if the vapor-liquid stream has a relatively unrestricted path, and the vessels can be equalized on the vapor side. Many times, however, the line to the second vessel must be submerged, or the path is tortuous. In this case, a line was sized based on typical tray downcomer disengaging principles, with an adequate vent from the top of this line to the first vessel. No nozzle changes were required. The system performed satisfactorily after installation, and decanter separation was also improved since no vapor disturbed the interface. (Continued on page 48)
FEEDS
INCH
U I
A
REACTOR
REACTOR
L
DECANTER
DECA N1E R
Ongin< System
Revise System
SUGGESTED READING
“Flow of Gas-Liquid Mixtures” 0. P. Bereelin. Chem. Ene. Flow of Fluids Reoort. Mav 1949. pp. 104-186. ’ “New Correlation for 2-Phase Pipeline Flow” J. A. Chavea, 0.& G. Journal, Aug. 24, 1959, Val. 57, No. 35, pp. 100-102.
“What You Need to Design Therrnosiphon Reboiled’ J. A. Fair, Pet. Refiner,Vol. 39, No. 2,1960, pp. 105-122. “Multi Phase Flow in Pipelines” Ovid Baker, 0. & G . Journal, No”. 10,1958, pp. 156-167.
Y
VOL. 5 4 NO. 2
I
,
I
FEBRUARY 1962
A7
5.
Gravity Driving Force for Flow
The most trying problems occur when a mixed-phase flow is produced by inadequate elevations of equipment, where elevation is the sole driving force for flow. While gravity is an enviable means of transport, and is always there, it is almost impossible to correct for errors in judgment or adapt the system to changed conditions when elevation is the only variable. Increasing elevation of a distillation column or equipment structure is a major undertaking, and almost always would involve intolerable downtime and cost. Consequently, great care should be exercised in all instances where gravity is used to transport saturated liquids.
A common example of this situation involved the bottoms pump of a distillation tower. The j u i d pumped was impure dimethyl ether at 33p.s.i.g. and 40’ F. The required pump head was 700 feet and in consequence an expensive, multistage pump had been installed. Inadequate care was taken to provide su$cient suction head for the pump and cauitation and frequent loss of suctron resulted. Since this tower and pump were part of a series in a solucut separation train, the results of these process upsets were serious in lost production and product quality.
Changing elevations was impossible. Modifying suction lines, pump impeller, suction nozzle, etc., on the pump would have helped but not with sufficient assurance. It was decided to subcool the suction line to the pump by jacketing a section of line with liquid propane refrigerant and thus provide the needed net positive suction head. Normal production rates were achieved with little further difficulty.
I n another example, adequate elevation was prouided for a CAIculatzng pump on a flash drum containing ethyl chloride saturated with hydrogen chloride, but j o w was mixed-phase and the pump cavitated. It was believed the tangential pump discharge entry to the drum produced a sufficiently deep vortex to entrain vapor into the bottom outlet even with a %foot liquid level in the vessel. Installation of four side wall baffles the height of the liquid, plus a typical “cross” vortex breaker over the suction nozzle eliminated this problem.
TANGENTIAL
t
-39’ F PROPANE VAPOR TO SUCTION D R W IN0 CONTROU
L’--
/ IIQUID PROPANE IHAND CONTROllEDi
EXCHANGER
37’ F
ADDED lACI(E1 O h IO FOO S€ClO14 OF SUCllOh UNt
Dimethyl Ethcr Pumping Problem
AUTHOR Douglas L. Allen is the Chief Technologist at U. S.Industrial Chmical Co.’s complex of chemical plants at Tuscola, Ill. 40
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
Ethyl Chloride Flarh Drum Syrtern
6. liquid lines Under Vacuum
Liquidflow out of vacuum s y s t q c a n be seriously disrupted by leakage into the lrquid lines. In vacuum systems, it is common practice to provide flow out of the system to storage or atmospheric surge by elevating equipment the equivalent of one atmosphere or more and sealing the lines in the receivers. The most common example is vacuum jet barometric systems, where the tailpipes are sealed in a hot-well about 34 feet below. Vacuum systems are frequently used to permit processing heat sensitive materials at lower temperatures and thus reduce degradation. Since these materials are frequently heat labile because of their more complex molecular structure, they frequently have high boiling points even under vacuum conditions, are often sensitive to oxygen, and usually have high freezing points. The
typical method of handling, then, is in steam-jacketed lines. In jacketed and barometric lines having relatively small flows of these materials, the problem of in-leakage is severe. The expansion ratio of the jacket steam or of air can be easily 500 to 1, going from the jacket steam to the hot liquid under vacuum. Thus mixed-phase flow occurs, and if the vapor cannot disengage (usually the case) flow is intermittent or completely stopped. Finding and correcting the minute leaks is very difficult since not only are they inward, but hidden under the jacket of the line. It is highly desirable to reduce these leakage effects by providing, where feasible, a positive driving force for flow other than gravity. Thus instead of a wholly gravity operated reflux and draw system on a vacuum tower, a reflux drum, sealed pump, and storage transfer line will be superior.
I
w CONDENSER
ro
VACUUM
SYSTEM
VACUUM ._i__ TOWEE
SEALED PUMP
Mixed-phase JIow occurred zn a string of three HLL Jalling film absorbers under medium uacuum. Disengaging of liquid was solely in thc bottom heads. The bottom linesjotncd into a nngle transfer line which was sealed in a storage uessel. Ltquid surging w a severe and resulted in gas surging and system t pressurepulses whzch were intolerable.
It was believed vapor was being entrained in the bottoms liquid and that flange leaks and even a certain
degree or line porosity contributea 10 the prolmui. The solution was to run larger individual bottom lines, venting them back to the absorber. Thus a certain amount of disengaging space was provided, plus a route for vapor venting. The intolerable pressure pulses in the system were eliminated. The major contributor to the mixed-phase flow was undoubtedly the small bottom nozzle. Vapor entrainment at this point was similar to that which occurs in overflow systems.
g"Ag
0 Rctised System V O L 5 4 NO. 2
FEBRUARY 1 9 6 2
49
