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SPE 50685 A NEW APPROACH TO GAS-LIQUID SEPARATION 5
SPE 50685
A New Approach to Gas-Liquid Separation
A.C. Stewart, N.P. Chamberlain and M.丨rshad, Kvaerner Paladon Ltd.
Copyright 1998, Society of Petroleum Engineers Inc.
This paper was prepared for presentation at the 1998 SPE European Petroleum Conference held in The Hague, The Netherlands, 20-22 October 1998.
This paper was selected for presentation by an SPE Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Papers presented at SPE meetings are subject to publication review by Editorial Committees of the Society of Petroleum Engineers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented. Write Librarian, SPE, P.O. Box 833836, Richardson, TX 75083-3836, U.S.A., fax 01-972-952-9435.
Abstract
Effective gas-liquid separation is important not only to ensure that the required export quality is achieved but also to prevent problems in downstream process equipment and compressors. Once the bulk liquid has been knocked out, which can be achieved in many ways, the remaining liquid droplets are separated from by a demisting device. Until recently the main technologies used for this application were reverse-flow cyclones, mesh pads and vane packs. More recently new devices with higher gas-handling have been developed which have enabled potential reduction in the scrubber vessel size. This paper will present some of the recent developments in scrubbing technology which have been aided by the use of computational fluid dynamics.
In addition, this paper will review some of the latest compact separation systems which have also enabled a whole new approach to gas-liquid separation to be considered. There are several new concepts currently under development in which the fluids are degassed upstream of the primary separator. These systems are based on centrifugal and turbine technology and have additional advantages in that they are compact and motion insensitive, hence ideal for floating production facilities
Introduction
To avoid problems in compression or downstream processing, the normal criteria for the liquid content of processed gas is
0. 1 U.S.gal/mmscf, thus the gas must be passed through a scrubber. In order to cope with slugging conditions, this is usually a two stage separator with an inlet device for free liquid knockout and a gas demisting device for removal of the entrained droplets.
Inlet devices range from simple deflector plates to cyclonic separators. The more effective the inlet device the lower the liquid load to the demisting device which can result in more compact scrubbers, or in some cases reduce the number of separation stages required.
The demisting devices most commonly used at present are vane packs, sometimes known as wave plates, wire mesh pads and reverse flow cyclones. More recently an alternative device, the axial flow cyclone, has also been re-introduced into the market. This device has the advantage of being able to handle a high gas throughput with a relatively small pressure drop and thus again offers the potential for smaller, lighter vessels.
The drive to more compact technology has also had impact on the way in which gas-liquid separation is being addressed in three phase flows. Primary three phase separators are usually the largest process vessels on the facility and the restrictions on velocities in the gas phase play an important part in their sizing. The space reduction possible with the introduction of high technology demisting devices, are small compared to that which could be achieved if the gas flow through the vessel could be reduced or even eliminated. A number of initiatives are thus underway to develop efficient methods of preseparation of the gas, with the objective of substantially reducing the size of, or even eliminating the need for a three phase separator.
Advantages of a Well Designed Inlet Device
Most demisting devices are designed to handle liquid loadings of no more than 1% by volume. The fluid entering a scrubber may contain in the order of ten times this value, so it is imperative that the inlet device will knock out sufficient liquid to ensure the demister is not flooded.
The exact nature of fluid entering the vessel is often unknown, although and indication of the flow structure in the inlet pipe can be estimated from a flow map of the type presented by Taitel el al (1). As the fluid enters the vessel it will be subjected to some degree of turbulent shear which will result in the formation of droplets, even if the flow is not fully dispersed upstream. The degree of shear will depend on inlet velocity, liquid characteristics and the type of inlet device used. An estimation of the maximum drop size entering the separator or scrubber can be made using the Hinze equation (Ref. 2):
G
.■⑴
々0.6 ”0.4
P 8
..(2)
The biggest error in this calculation is in determining the value of 8, the energy dissipation. Computational fluid dynamics (CFD) can be used to estimated the turbulent dissipation in the inlet region but of course it is possible that there could be droplet break-up from shear imparted upstream. Despite this such correlation still provide a useful starting point for a comparative study of different technology. CFD can also be used to determine the percentage of particles in different size bands which will reach the demisting elements. For example figure 1 shows the comparison in performance between a simple deflector plate and a vane type inlet. This calculation has not assumed a maximum droplet size in either case. It shows that with the deflector plate the liquid knockout is very inefficient with droplets of up to 3500 ^m reaching the demister. By replacing this with the vaned inlet diffuser the liquid knock is dramatically increased and with a maximum drop size reaching the demister reduced to 650 |im and a significant percentage of smaller droplets being separated. This is partly due to increased coalescense and gravity knockout by the inlet device and partly due to a better flow distribution downstream. Figure 2 shows the flow distribution in the vessel in each of these cases as predicted by CFD. It can be seen with the vane-type inlet the velocity profile across the vessel is much flatter, with none of high velocity gas streams present in the previous case. The maximum drop size determined from equation 1 can be then be inputted into a probability function (3) to predict the size distribution at the vessel inlet. If information of type presented in figure 1 is then used to determine the percentage knockout by the inlet device, the liquid loading to the demister can be estimated.
Even more effective liquid knockout can be achieved through the use of a cyclonic inlet device. These devices which were initially developed to reduce foaming in three phase separators (4,5) have now been adapted to treat the high GORs typical of scrubber applications, and have been successfully installed and operated North Sea gas platforms. A typical scrubber layout with a cyclone inlet is shown in figure 3. Laboratory studies have shown that the liquid removal from these device is typically around 98%, compared to around 60% from a vane type device and 30% from a deflector plate. Thus the liquid loading on the demisting device is significantly decreased and in some cases there may no longer be the need for a secondary device. In addition to the improved liquid knockout these devices can handle much higher inlet momentums than other inlet devices and hence are ideal for debottlenecking.
Advances in Demisting Technology
The devices used for the removal of liquid mist from gas can be divided into two main categories. The first of these are the direct interception devices which includes filters and mesh pads. While these devices maintain high efficiencies down to small droplet sizes, they are prone to fouling problems, giving high maintenance costs, and hence are generally restricted to clean, solid free applications. The second category of gas- liquid separator is the inertial devices which cyclones and vane packs. Conventional reverse flow cyclones have been used for over 40 years and clean both solids and liquids from gas streams. They also have high efficiencies for small droplet sizes but at high liquid loadings a two stage design with secondary drainage is required requiring additional control expenditure. The high pressure drop characteristics also makes them unfavourable in some cases. Vane packs by comparison have a relatively low pressure drop. The efficiency of these devices is dependant on the geometry of the device and some of the more sophisticated designs compete favourably with mesh pads.
The performance of vane packs is dependant on two independent factors. The first of these is its drop removal efficiency. The second is liquid strip-off characteristics. Prediction of drop removal efficiency has been given a lot of attention over the last decade. Assuming a uniform flow field Burkholz (6) derived the following expression for separation efficiency for one bend:
apd ud2 18^s
This can be extended by assuming partial remixing of the droplet laden gas, to give the following expression for total efficiency:
nt = 1 - (1 -nP)n
where n is the number of bends.
Figure 4 shows a comparison between experimental data (7) and that predicted by the Burkholz formula. It shows the theory gives a good indication of the performance however it does slightly over predict the measured efficiency. More recently computational fluid dynamics, CFD, has been used to gain a better understanding of the flow field with the wave plates and to give an alternative method of predicting efficiency. Drop removal efficiency data obtained using CFD code CFX is also shown in figure 4. In this case the model under predicts the efficiency, particularly for the small drop sizes, but gives a very accurate prediction of the cut-off size. The advantage of this technique is that it gives also gives a good indication of the flow distribution of the channel. Typical gas flow distribution patterns for two simple vane channels are shown in figure 5. In the channel without the hook, it can be seen that large recirculation zones exist at each bend, this is potentially dangerous as liquid collected in these zones would be re-entrained into the gas. The presence of such a hook, as shown in the lower pictiure, improves performance by providing a drainage channel for the separated
..(3)
liquid but it also reduces the size of these recirculation zones.
K
When the velocity exceeds a certain critical value it strips liquid off the separated film on the vane wall and re- entrains it into the gas phase. In terms of bulk liquid, the carry-over from this source is often more than that from the presence of small droplets which have not been removed. This critical velocity can be determined from the following expression (Ref. 1):
.(4)
2
pg
Where the K is the Kutateladze number, which depends on the characteristics of the vane. For a simple vane geometry, K has been reported to have a value of 2.46 (8). New vane types have been developed however, with a construction designed to lower the exit velocity and hence minimise strip-off, in these devices higher values of K can be obtained. These new vanes also incorporate a coalescing section which increases their drop removal efficiency.
Over the last few year another type of demisting device has also introduced successfully into the oil and gas market, the axial flow cyclone. The axial flow concept, shown in figure 6, as been around for many years, however, it has only been through recent development work that the concept has been developed into a viable separator for demisting applications. In contrast to a conventional reverse flow cyclone, the gas stream in an axial flow cyclone does not change direction but takes a path straight through the device. The tangential velocity in this case is developed by passing the gas over a vaned section which forces the droplets to the cyclone wall. The film is then drained either through a series of slots and/or an annular gap, assisted by a small gas flow which is recycled through the central body. The processed gas then exits via the vortex finder.
In work carried out at Delft University (8,9), a number of different geometries were tested at atmospheric conditions in the vertical orientation. Further development work has recently been carried out (10) in which tests were carried out in both the horizontal and vertical orientation. It was found that the axial flow cyclone was efficient over a wide flow range and that the droplet cut-off sizes, were lower than those obtained for a vane pack. Typical performance curves at different air flow rates are given in figure 7. It was also found that at high liquid loadings, the design of the drainage system is crucial to the cyclone’s performance and the optimum design has been shown to depend on the orientation of the device. A slot system has been found to be more effective for vertical operation and an annular system for horizontal installations.
Axial flow cyclones have now been installed in a number of separators and scrubbers in North Sea fields. Their high gas handling capacity means that they can be retrofitted to help debottlenecking in fields with increasing gas flows and in new fields they offer the potential of smaller vessel sizes. Particularly if used in conjunction with an effective inlet device which produces a good flow distribution at the inlet to the demister. CFD can be a useful tool to help optimise the design of internals for specific systems and to show if a good flow distribution will be obtained (see figure 2).
Comparison of the Performance of Demisting Devices
Despite the advantages of axial flow cyclone, it does not offer the best solution for all applications. In order to chose the best demisting device for a particular service a number of factors have to be taken into account,. these are summarised in Table
1. The four devices are rated from best (1) to worst (4) in the first five categories. and the in the last case the approximate cut-off size is given.
The turndown of the cyclonic devices tends to be better that other demisters. In independent tests (11) multicyclones were found to have a turndown of around 1:6 and still maintain particle removal efficiency. This is much higher than could be achieved for any of the other demisters. In terms of gas handling capacity vane packs still come out top at low pressures but at higher operating pressures axial flow cyclones are better as the velocity through the device does not have to be significantly reduced to prevent strip-off. While mesh pad have a limited turndown and gas handling capacity their relatively low costs means they are often used as a coalescer upstream of other demisters in cases where there is a concern over carryover of small drops. In this situation they operate under flooded conditions but the liquid re-entrained into the outlet gas has a larger drop size distribution than that at the inlet.
The values given for drop removal are for a generic device of each type but it should be remembered that in each case there are a wide range of products of each type on the market and the performance of individual devices may vary. These figures also depend very much on the process conditions. It has been shown that efficiency in terms of particle/droplet removal is directly proportional to pressure drop. Figure 8 shows how the cut-off size varies with pressure drop for a range of devices. Thus if a high performance is required it is necessary to accept a pressure drop penalty.
Preseparation of Gas
The size vessel reduction which can be achieved by the introduction of high performance demisting devices is relatively small compared to the saving that could be made if the gas was taken out of the vessel completely. This has prompted a whole new approach to the problem of gas liquid separation. There are a number of different concepts being developed to remove the gas from the process fluid upstream of the primary gravity two or three phase separator or electrostatic coalescers. Most of these make us a centrifugal separators which in addition to being compact have the added advantage of being insensitive to motion and hence could be of particular value on FPSO installations. Figure 9 shows how a cyclonic pre-degasser which could be used in conjunction with a two phase separator to replace a three phase separator, with substantial space and weight reduction. The main component of the device is a bulk gas liquid cyclone similar to that used as an inlet device for two and three phase separators. The cyclonic inlet, which has been developed over the last five years, has been installed successfully in a number of three phase separators in the field. From field an laboratory tests it has been shown that this device reduces gas carry-under into the vessel to less than 1% and hence eliminates foaming and in addition, the outlet gas contains less than 2% volume liquid. In the well-head degasser, the outlet gas then passes into a primary scrubber stage in which a vaned swirl element imparts further spin on the gas throwing most of the entrained liquid to the vessel walls where it drain down to the liquid drainage section at the bottom of the vessel. In order to ensure the required gas quality of 0.1 USgal/mmscf the gas is then passed through a second scrubber stage in a second chamber which contains axial flow cyclones of the type described above.
Another new separator concept currently under development is separators illustrated in figure 10. It is a compact centrifugal separator based on a turbine concept with the added advantage of power recovery. The technology, which has already been used successfully in geothermal and refrigeration applications (12), is currently being developed for two and three phase separation and well-head energy recovery. A field test of the prototype two phase separator is currently underway at Texaco’s Humble test facility. The fluid is introduced at high velocity through a contoured two phase nozzle and impinged tangentially onto the inner surface of the rotating cylindrical rotor. The high centrifugal force produces an immediate separation of the gas and the liquid. The separated liquid forms a layer on the cylindrical surface where most of its kinetic energy is transferred to the rotor by shear forces, before it is removed through a specially designed scoop or internally cast reject channels. The separated gas flows through the vanes attached to the turbine supplying further kinetic energy.
The main limitations with these new compact technologies is that by their very nature there is only limited fluid volume available in the system. This has obvious implications for control particularly under slugging conditions. These problems are being addressed and it seems certain that novel compact technologies will play a large role in offshore separation in the future, with the centrifugal devices described above, being of particular value in the development of separation trains for floaters.
Conclusions
Recent developments in technology have made it possible to reduce the size of gas-liquid separation vessels or to increa