Gas Sep. Purif: Vol. 9, No. 2, pp. 81-86, 1995 0 1995ElsevierScienceLtd Copyright Printed in Great Britain. All rights 095&4214,'95 A study of foaming and carry-over reserved $10.00 + 0.00 problems in oil and gas separators Habib. Department I. Shaban of Chemical Engineering, Kuwait University, PO Box 5969, 13060 Safat, Kuwait In oil fields, separators are used to separate oil and gas contained in the crude oil pumped from the wells before processing. Although there are many factors influencing the performance of these separators, one of the crucial problems is the formation of foam due to the impurities present in the crude. Another operating problem is carry-over, which occurs when free liquid escapes with the gas phase. Of the several methods used to control foam, chemical control by use of antifoam agents is very important. In this work, the main objective is to study the poor separation of oil and gas due to foaming and the carry-over problem in separators observed in one of the oil fields in Kuwait. Studies were also conducted on the effect of a silicone antifoam agent used to control foaming in order to increase the separation efficiency and thereby increase the production capacity. Keywords: oil and gas separators; foaming; Introduction The oil and gas composing well fluid are usually separated in separators. A separator is essentially a vessel whose interior is kept at a prescribed separation pressure and temperature. The three principles used to achieve physical separation are momentum, gravity settling and coalescing. Any separator may employ one or more of these principles, but the three phases must be immiscible and have different densities for separation to occur. Some oil and gas separations are simple while others are very complex’. Separators may be vertical cylindrical, monotube and dual-tube horizontal, or spherical, depending on their design’. There are several factors which affect the performance of a vapour-liquid separator. The temperature and pressure are two important factors which affect the separation for a feed of given composition. With increasing temperature and/or decreasing pressure, the volume of vapour increases and the volume of liquid decreases. In addition to this, there is a decrease in the densities of both the vapour and liquid phase. Compared with the other changes involved, this density change is more significant ‘. Some systems may contain small amounts of surfactants, which cause the formation of foam in vapour-liquid systems and emulsions in liquid-liquid systems. The major cause of foam is impurities (other than water) in the crude oil which it is not practical to remove before the stream reaches the separator. The formation of foam and emulsions adversely affects the carry-over; antifoam agents performance of separators, especially when they are stable. The presence of surfactants is usually not known in advance, so separators are designed presuming that they are not present. Foam creates no problem within the separator if the internal design ensures adequate time or a sufficient coalescing surface for the foam to break. In addition to this, the size of the entrained particles is one of the important properties affecting separation efficiency. Other factors influencing separation are gas velocity, surface tension and viscosity3. Foaming in the separator creates the following problems4: a. Mechanical control of the liquid level is difficult in the presence of foam, as any control device has to deal with three phases instead of two. b. Foam has a large volume and will occupy much of the vessel space, which would otherwise be available for liquid collecting or gravity settling. c. In an uncontrolled foam bank, it is impossible to remove the separated gas or liquid oil from the separator without entraining some of the material in either the liquid or gas outlets. Of the various methods used to control foam, such as removing the foam stabilizing agent by filtration and using mechanical devices or employing new equipment designs to break foam, the best method to accomplish foam removal is the use of an antifoam agent. Chemical control of foam using such an ‘antifoam’ is very important. Gas Separation & Purification 1995 Volume 9 Number 2 81 Foaming and carry-over Components problems: affecting H. 1. Shaban separation To have efficient and stable operation over a wide range of conditions, a gas and liquid separator must have the features described below1*2. 1. A primary separation section for removing the bulk of the liquid from the inlet stream containing liquid and gas, mainly by centrifugal force. It is essential to remove slugs and large droplets of liquid from the gas stream to minimize gas turbulence and re-entrainment of liquid particles. This is usually accomplished by a change in direction of fluid flow. Centrifugal force from a tangential inlet on a vertical vessel quickly removes large volumes of liquid and allows redistribution of gas velocity. Properly shaped and positioned deflection plates are usually used in horizontal and spherical vessels to accomplish this effect with a minimum of re-entrainment. 2. A secondary separation section for removing a maximum of smaller liquid droplets is enhanced by gravity settling. The major separation principle in this section primarily depends on gas turbulence. The turbulence factor is often minimized by a suitable inlet arrangement and properly designed and positioned baffles. 3. In order to separate mist not settled out by centrifugal force or gravity, a mist extractor is placed in the way of the gas stream. The mist extractor may be of the vane or coalescing-pack type, or it might be a hydrocyclone. The usual oil field separator employs the impingement principle as the primary mechanism of mist extraction. The tiny liquid droplets are collected on a surface, where they are drained away from the gas stream or form large droplets that can fall back into the primary separation section. 4. An oil collecting section for receiving and disposing of collected liquid. The liquid separated from the well stream and condensed in the separator collects in the bottom part of the separator. This section should be arranged so that the separated liquid has a minimum level of disturbance from the flowing gas stream. The liquid level is maintained within given limits by a liquid level control device, to prevent gas from entering the oil outlet and liquid from rising to the elements in the separator, and to ensure sufficient retention time for the gas bubbles to break out of the liquid. Foaming problems Foam can seriously impair the operation of petroleum processes, but it is not often recognized as the major cause of a particular problem. Many problems, such as reduced capacity of equipment, large overhead losses, overhead fouling or poor separation efficiency, may occur because of foam. Foam occurs when a surfactant, which is either added intentionally or is naturally present during a process, dissolves in a liquid. For example, crude oil may not foam, but once a corrosion inhibitor is added 82 Gas Separation & Purification it may foam to such an extent that the crude oil cannot be processed effectively without the aid of an antifoam’. Foam control One way of controlling foam is to prevent it from forming in the first place. This can either be accomplished by removing the foam stabilizing agent by either changing or discontinuing the use of a problem chemical. A simple filtration may selectively remove the problem chemical and reduce or eliminate the foaming problem. This approach is not always a possible alternative. Surface whips, thermal shock and ultrasonic techniques are various mechanical devices that have been proposed to control foam. New equipment such as a cyclone-style gas/oil separator may be used to minimize the amount of foam. Using the above methods, a reduction in foaming tendency is usually accomplished and the use of antifoam is still required. Chemical control of foam by the use of antifoams is very important but is often not well understood. Distinction is made between antifoams (chemicals added before the foam has formed to inhibit foam formation) and defoamers (chemicals added after foam has formed to knock down the foam). An antifoam is usually a good defoamer; it functions in the bulk of the liquid to control foam as the bubbles are nucleating, but will also act on the bubble surface to destroy the foam. Many organic materials are good defoamers but poor antifoams’. Foam depressants will often increase the capacity of a given separator. However, in sizing a separator to handle a particular crude, use of an effective depressant should not be assumed, because the crude and foam characteristics may change during the life of any field. Sufficient capacity should be provided in the separator to handle anticipated production without the use of a foam depressant or inhibitor. Once in operation, use of a foam depressant may allow more throughput than the designed capacity’. Discussion carry-over of poor problem separation and The carry-over problem has been observed in separators due to foaming. Such foaming has resulted in the carry-over of crude into the gas stream and, in addition, the separators have to operate at their full design capacity to meet the production target rate set for the gathering centre. To reduce foaming and the carry-over problem, the oil level in the separators is lowered and the total span of the level controllers set at 10%. By doing this, the oil and gas separation becomes inefficient in the first stage and causes slippage of gas to the second stage, exceeding its design capacity and thus resulting in poor performance. Furthermore, this operating condition results in an increased amount of gas reaching the high-pressure (HP) flare, as the low-pressure (LP) gas compressor at the gas booster station is already operating at full design capacity and cannot process any additional LP gas. Moreover, the scrubbers in the LP 1995 Volume 9 Number 2 Foaming CHEMICAL TANK and carry-over problems: H. 1. Shaban li- CHEhllCAL PUMP PRV HP HEADER 1 ~‘1, ,,.n.,r ALAnM q AnHt HP GAS TO BOOSTER STATION \I FLOW LINES (HP WELLS) I LPHEADER CARRY OVER Al ARM q ADDCL v.-..... “,,I,,,L, 1 LP GAS TO BOOSTER I STATION LP SEPARATOR FLOW LINES (LP WELLS ) CRUDE OIL TANK Figure 1 Process flow diagram and HP gas systems at the gas booster station have to be drained out many times a day due to the excessive carry-over of crude oil in the gas stream. Process description The dry crude oil from the wells flows through valves and headers into separators and then to oil tanks, as shown in Figure 1. The separators and tanks degas the crude oil. The separators in this particular oil field are of a horizontal type and are equipped with foam breakers to improve performance. The mechanical foam breakers are divided into two parts (see Figure 2). First, the inlet performs degassing by gently agitating the feed fluid. This will assist in removing gas from the oil and breaking foam bubbles as they flow through the inlet element. 2 /4 for HP and LP crude oil separation The second mechanical foam breaker is defoaming plates, which extend from near the inlet to the outlet of the separator. The plates that are immersed in the oil assist in removing non-solution gas from the oil and in breaking foam in the oil. The plates that are above the oil/gas interface in the gas section of the separator remove oil mist from the gas and assist in breaking foam that may exist in the gas section of the vesse17. Excessive foaming was observed in day-to-day operations and it became evident that this mechanical means of breaking down foam is not efficient and an alternative method is therefore required. Table 1 gives the design specifications of the HP and LP separators. Chemical control of foaming Silicone fluids have been used for many years to suppress foaming of crude in separators and to prevent carry-over of liquid with the gas. To increase the capacity, a silicone antifoam injection was introduced into the separators in the unit under study. A silicone injection installation provided on site was connected to the HP separator inlet to ensure that the chemical was adequately mixed, as 6 1 Table 1 HP and LP separator operating Description (1) (2) (3) (4) (5) (6) Well fluid inlet Inlet degassing element Defoaming plates Gas outlet Oil outlet Oil level control Dimensions (internal Dimensions (length) Flow rate (BPD) Pressure (psig) Temperature (‘F) Retention Figure 2 Horizontal diameter) (m) (m) (min) design specifications HP LP 2.5 13 50 000 470 190 4.5 2.5 13 50 000 85 180 4.5 I separator Gas Separation & Purification 1995 Volume 9 Number 2 83 Foaming and carry-over problems: H. 1. Shaban shown in Figure I. The antifoam is continuously injected into the feed and stays in the separated oil through the next processing stages. The ability of the antifoam to enter the liquid/gas interface and spread within a developing foam system is determined by a combination of surface and interfacial tension. The silicone, with its low surface tension and low interfacial tension with oils (2.8 to 4.8 dyne cm ‘) is specially suited to enter and spread in non-aqueous systems. Polydimethyl siloxane (PDMS) is non-polar, non-ionic, insoluble in water and hydrophobic in nature. Silicone fluid against water has a relatively high interfacial tension and therefore has a poor or reduced spreading tendency. However, if compounded with silica, it makes an efficient antifoam for aqueous foams by entering the film and bursting the bubbles using the hydrophobic silica via a dewetting mechanism5. A silicone antifoam may also disrupt the other foam stabilizing mechanisms. Surface viscosity will be reduced if the foam stabilizing surfactant is displaced by a silicone that does not exhibit hydrogen bonding. Displacement of ionic surfactants by non-ionic silicone destroys electrical double-layer effects. Similarly, displacement of the surfactant by gas-permeable silicone will remove the resistance to gas diffusion between bubbles and allow this bubble-breaking mechanism to operate. Table 4 Oil flow rate amounts of antifoam and (GPD) LCV Antifoam injection Table open Table 0 5 10 15 20 25 30 68 71 78 80 81 81 81 55 57 60 67 69 70 68 injection on foam Foam collapsing time (min) (GPD) 0 5 10 15 20 Gas Separation 7 4.2 2 1.3 0.9 0.9 1 & Purification 1995 Volume Oil flow rate HP and LP ( x 1000 BPD) 50 52.4 54.8 57 57.6 57.7 57.7 5 Effect of antifoam on gas production Gas production rate ( x 1000 MSCFD) Antifoam injection (GPD) HP LP 0 5 10 15 20 25 30 16 23 29 30.5 35 37 34 12 11 9 5.5 5 5.1 5 6 Effect Antifoam injection of antifoam on oil level Oil level separators in separators in (%) and throughput Separator throughput (GPD) HP LP 0 23.7 20 20 8 13.9 15 15.2 10.2 12.6 15 17.8 Figures Antifoam injection 84 (%) LP of antifoam varying (BPD) 52 52 55 63 724 724 400 000 achieved without carrying-over. All the results are summarized in Tables 24 and the data are plotted in HP Effect with prior (GPD) 3 LP separators 0 5 10 15 20 25 30 Results for the HP and LP systems described previously are discussed as a function of antifoam addition and its effects on separation characteristics. The feed was initially set at the design rate of 50000 BPD for each stage (HP and LP) and gradually increased to the maximum possible throughput rate that could be valves days and through separators discussion Table 2 Status of HP and LP separator level control to and after antifoam injection over a period of seven HP Antifoam injection Table Results through 3-5. As shown in Figure 3, the collapse time curve consists of two regions. The first part is linear, where collapse time decreases with increase in antifoam injection rate. The second region has parabolic behaviour, with a levelling trend at high antifoam rates. This behaviour is expected, since, at low antifoam concentrations, the collapse rate can be assumed to be first order with respect to antifoam concentration. Similarly, at higher antifoam concentrations, the rate becomes zero order with respect to antifoam concentration. In Figure 4, the curve shows clearly that the oil flow rate has two regions. The first region is linear, where the oil production rate increases because foam concentration decreases with the addition of antifoam. The second region is parabolic in nature with a levelling trend at high antifoam injection rates. Also, it can be seen that there is an optimum region for foam addition, where levelling of oil flow rate occurs. Figure 5 shows the effect of antifoam on gas production. It shows that the antifoam enhances gas 9 Number 2 Foaming and carry-over problems: H. 1. Shaban HP LP 2 0 IS 20 Anli-foam Figure 0 3 Effect of antifoam 10 Anti-foam Figure varying 30 injection 20 injection 4 Plot for oil flow rate through amounts of antifoam 40 0 10 Anti-loam ( GPO ) injection on foam Figure 30 (GPO) HP and LP separators with 5 Effect 20 inisction of antifoam 30 (GPD) on gas 40 production release in the first stage (HP) separator, accompanied by a decrease in gas production in the second stage (LP) separator, to conserve the total gas mass. Due to this, the flaring quality also improves in both the HP and LP flares. Table 6 shows that adding the antifoam agent while keeping the throughput constant results in an increasing oil level in the separators, as shown in the first two runs in Table 6. Also, keeping the antifoam injection rate high (20 gpm) enables the operator to increase the throughput flow rate and, consequently, increases the oil level in the separators, as shown in the last two runs in Table 6. The test results in this study indicate that there is a marked difference in functioning in terms of separator throughput when silicone antifoam is injected into the crude stream. The handling capacity of the separators increased far in excess of their rated nominal capacity. Operating conditions and separation efficiency were improved tremendously. Flaring quality improved in the HP and LP flares and the light ends were retained in the crude stream. There was no carry-over in the gas to the booster station. Moreover, the results indicate that the increased throughput did not cause an inconvenience in terms of plant operation, as all level control valves (LCVs) and pressure control valves (PCVs) were well within their designed operating flow range, and no abnormal vibration was detected during the test period. The maximum capacity attained before silicone injection was 50000 BPD with the operating level set as low as 10% of the total control span of the level controllers. Gas Separation & Purification 1995 Volume 9 Number 2 85 Foaming and carry-over problems: H. I. Shaban Above this level, excessive carry-over occurs due to foaming. After silicone injection, the feed capacity could be increased to a maximum of 57 500 BPD and the separator level could be increased to 15% of the total control span. remarkably, reducing wastage, and the available LP gas was processed at the LP compressor within the design limit. Efficient separation permitted processing of extra crude. The processing capacity increased to 57 700 BPD using the silicone antifoam. References Conclusions Even though various mechanical devices have been proposed to control foam formation, which is one of the most significant factors affecting oil and gas separation efficiency, the chemical control of foam by the use of antifoam is very important. Using silicone, the operating level increased to W 15% of the total span of the level controllers. Efficient oil/gas separation was achieved with the antifoam injection set at optimum dosage; more HP gas was separated/produced in the first stage separator and processed at the HP compressor at the booster station. Simultaneously, LP gas production was decreased 86 Gas Separation & Purification 1995 Volume 9 Number Chilingarian, G.V., Robertson, J.O. and Kumar, S. Surface operations in petroleum production, in Decelopmrnts in Petroleum Science, Vol 19A, Elsevier, New York, USA (1987), Ch 3, 59-69 Szilas, A.P. Production and transport of oil and gas, in Developments in Petroleum Science, Vol 18B, Part B (Gathering and Transport), Elsevier, New York, USA (1986), 68-69 Amoid, K. and Stewart, M. Designing oil and gas production systems: How to size and select two phase separators J World Oil (NovjDec 1984) Worely, MS. and Laurence, L.L. Oil and gas separation is a science J Petrol Technol (April 1957) Callaghan, I.C., Gould, C.M., Reid, A.J. and Seaton, H.D. Crude oil foaming problems at the Sullom Voe terminal J Petrol Technol (Dee 1985) Engineering Data Book 10th Edn, Vol 1, Section 1. Gas Processors Suppliers Association (1987) Bradely, H.B. Petroleum Engineering Hand Book, Richardson, USA, Ch 12, I-40 2