The present invention relates to a system and method for removing particulate matter from a flow stream. More particularly the invention relates to removal of particulate matter from a liquid hydrate inhibitor flow stream.

Hydrate inhibitors such as mono-ethylene glycol (MEG) are used in hydrocarbon gas and/or condensate pipelines to absorb moisture and prevent hydrate forming in the pipe, which can lead to blockage and corrosion. Typically the MEG (or other inhibitor) is injected into the upstream end of the pipeline, and is separated from the hydrocarbon fluids at a receiving facility at the downstream end. The separated MEG (known as rich MEG), which carries absorbed water, is regenerated by a water removal process to produce "lean MEG" for re-use. However, hydrate inhibitors such as MEG also tend to become polluted by other components in the pipeline. Some of the pollutants, for example pipeline corrosion products and scale are present as particles in the flow stream. Others, such as hydrocarbons, salts from formation water or production chemicals are present in solution, but may precipitate as small particles during the MEG regeneration process.

Removal of these particles is important for the performance of the MEG regeneration process, because the particles tend to accumulate in the regeneration process, and to clog process equipment. One known solution to this problem is to separate the particles by introducing a solids separation unit and a desalination unit (reclaimer) into the process. A problem with this approach is that significant quantities of the particles are very fine and difficult to separate. Separation processes and equipment for handling these particles are large, expensive, operator intensive and prone to failure due to clogging. In some applications MEG reclamation may be carried out on off¬ shore platforms where space is at a premium. Also, depending on operating conditions, which may change over the operational life of the plant, the distribution of particle size may vary significantly. This makes it difficult to design a system that can perform reliably for all operating conditions over the life of the plant.

Another process, which requires solids removal from a flow stream is in the recovery of amines used for the removal of acid components from natural gas, a process known as gas sweetening.

It is an aim of the present invention to provide a system and method that alleviate the aforementioned problems.

According to a first aspect of the present invention there is provided an apparatus for removing particulate matter from a flow stream, the apparatus including: an evaporator for creating a concentrated phase from said flow stream, said concentrated phase containing particulate matter; a separator including means for introducing a flocculant into said concentrated phase so as to agglomerate particulate matter by flocculation, and means for separating the agglomerated particulate matter from the concentrated phase; means for returning said concentrated phase, absent said separated particulate matter, to said evaporator; and means for removing said flow stream absent said particulate matter from said evaporator.

The flow stream may be a side-stream of a main process flow stream, the apparatus further including: means for removing said side-stream from said main process flow stream; and means for returning said side-stream absent said particulate matter from said evaporator to said main process flow stream.

Preferably, the side-stream is a desalination process side-stream.

It is an advantage that, by using flocculation, even the smallest particles agglomerate to form larger particles, which can then easily be separated. Furthermore, the flocculant may be introduced and the particles removed at any required location in the process (in the main stream and/or a sidestream), allowing flexibility for designing a system that can operate reliably over a wide range of conditions.

The apparatus may be employed in a hydrate inhibitor recovery process. In a preferred embodiment, the fluid comprises mono-ethylene glycol (MEG). The hydrate inhibitor fluid may comprise other fluids such as other glycols or amines.

Alternatively the flow stream may comprise is an amine solution used for gas sweetening (removal of acid components from natural gas).

The composition of the flocculant may be adjusted to suit the composition of the flow stream.

In a preferred embodiment, the means for separating the agglomerated particles comprises a settling tank. It is an advantage that most known MEG reclamation plants already employ storage tanks for both rich and lean MEG, and so the provision of a settling tank in place of the storage tank represents an insignificant increase in equipment size/complexity. This means, for example, that the system is ideal for use on off-shore platform installations.

Alternatively, or additionally, the means for separating the agglomerated particles may be a centrifuge. It is an advantage that many MEG reclamation plants already employ a centrifuge as part of a desalination process, so that the use of a flocculant at this stage of the process does not represent a significant increase in plant size/complexity.

Preferably, the means for introducing the flocculant is situated upstream of the separating means such that the flocculation process agglomerates the particles before entering the separating means.

The means for introducing the flocculant may be an injection quill that penetrates the flow stream at a suitably adapted injection location in a pipeline.

According to a second aspect of the present invention there is provided a method for removing particulate matter from a flow stream, the method comprising: creating a concentrated phase from said flow stream in an evaporator, said concentrated phase containing particulate matter; introducing a flocculant into the concentrated phase so as to agglomerate particulate matter by flocculation; separating the agglomerated particulate matter from said concentrated phase; returning said concentrated phase, absent said separated particulate matter, to said evaporator; and removing said flow stream, absent said separated particulate matter, from said evaporator. The flow stream may be a side-stream of a main process flow stream, the method including: removing said side-stream from a main process flow stream; and returning to said side-stream, absent said separated particulate matter, from said evaporator to said main process flow stream.

The method may also comprise adjusting the composition of the flocculant to suit the composition of the flow stream.

Embodiments of the present invention will now be described by way of example referring to the accompanying drawings, in which:

Figure 1 is a simplified schematic flow diagram of a known MEG reclamation process;

Figure 2 is a schematic flow diagram of part of the MEG reclamation process of Figure 1 adapted in accordance with an embodiment of the present invention;

Figure 3 is a schematic flow diagram of part of the MEG reclamation process of Figure 1 adapted in accordance with another embodiment of the present invention.

Referring to Figure 1 , a known MEG reclamation process receives a flow from a pipeline into a separation process unit 12. The flow contains a hydrocarbon gas and may also include a condensate. The flow also contains a hydrate inhibitor in the form of MEG. The separation process unit 12 separates MEG from the hydrocarbon gas and condensate, which leave via gas and condensate outlet lines 14 and 15 respectively. The separated MEG contains moisture (water) absorbed from the gas stream in the pipeline and is referred to as rich MEG. This is fed via a further outlet through a feed line 16 to a rich MEG storage tank 18. MEG to be regenerated is fed via a line 20 from the rich MEG storage tank to a regeneration unit 22, where a substantial portion of the moisture is removed from the MEG. After removal of the moisture, the MEG is referred to as lean MEG and is fed via a connecting line 24 to a lean MEG storage tank 26.

The lean MEG leaving the regeneration unit 22 may still contain dissolved salts and contaminants in suspension such as precipitated salt crystals. In the process shown in figure 1, a portion of the lean MEG is taken out of the connecting line 24 into a side-stream line 28 and fed to a desalination unit 32, which separates and removes suspended and dissolved contaminants. The desalinated MEG is then fed to the lean MEG tank 26. The operation of the desalination unit 32 as a side-stream off the connecting line allows this process to be operated and controlled independently of the main regeneration unit according to the degree of separation and desalination required. However, very small particles, especially those of precipitated carbonates are too small to be easily removed by the centrifuge of the desalination unit 32.

Lean MEG in the lean MEG storage tank 26 is ready for re-use. A pump 34 pumps lean MEG back into a. hydrocarbon gas pipeline via a return line 36.

In the known regeneration and reclamation plant 10 shown in figure 1, if no means are provided for removing particulate materials from the rich MEG, then these may be carried through the regeneration unit 22 and still be present in the lean MEG tank 26. The particulate matter is liable to clog process equipment, especially if it is allowed to accumulate. One method that has been used to alleviate this problem is a two-stage particle and salt removal process 21, provided as shown within the broken lines in figure 1. The first stage uses a filter in the main process flow stream. The second stage uses a reclaimer that removes pollutants from the MEG by evaporation of the MEG and forced precipitation of the pollutants. A problem with this approach is that significant quantities of the particles are very fine and difficult to separate. Particularly difficult are certain carbonates (e.g. Na2CO3, CaCO3, FeCO3) and iron oxide (Fe3θ4). Separation processes and equipment for these particles are large, expensive, operator intensive and prone to failure due to clogging. Also, depending on operating conditions, which may change over the operational life of the plant, the distribution of particle size may vary significantly. This makes it difficult to design a system that can perform reliably for all operating conditions over the life of the plant.

Figure 2 shows a system for separating particles in the process of Figure 1. An injector 50 is inserted into the feed line 16 upstream of the rich MEG storage tank 18" . The injector is supplied with a flocculant (also known as a flocculating agent) by means of a pump 52 from a flocculant storage tank 54. The flocculant mixes with the rich MEG flow stream in the downstream portion of the feed line 16' between the injector 50 and the rich MEG storage tank 18\ The injector 50 employs an injection quill of known design to ensure good distribution of the flocculant in the rich MEG flow stream.

Suitable flocculants include solutions of anionic and cationic polymers and currently preferred flocculants include "Hydro Sepco DC 4002" and "Hydro Sepco AE6230 H" available from Hydro Gas and Chemicals, Postboks 23, Haugenstua, 0915 Oslo, Norway. The flocculant causes flocculation whereby the particles carried by the rich MEG are agglomerated. Once the flow stream enters the storage tank, the flocculation process ensures that the agglomerated particles are large enough to settle due to gravity and the particulate material gathers at the bottom 56 of the rich MEG storage tank 18\ Provided that the injector 50 is situated at a sufficient distance upstream of the rich MEG tank 18s , the reaction time for flocculation is sufficient without any need to provide additional reaction/retention vessels. However, as settling time for particles is dependent on viscosity, this may be speeded up by heating the rich MEG in the rich MEG storage tank 18\

The particulate matter that collects at the bottom 56 of the rich MEG storage tank 18 v does not typically occupy a large volume, and so can be removed at relatively infrequent intervals, for example as a routine (e.g. bi¬ annual) maintenance operation. After removal, the particles are treated as a special waste.

The rich MEG storage tank 18" therefore has the dual purpose of storing rich MEG and acting as a settling tank for separating the agglomerated particles from the rich MEG. This means that the filter process described above for the system of Figure 1 is not required.

Figure 3 shows another arrangement in which fine particles are removed using flocculation together with a separation process. In this arrangement, fine particles are removed from the side-stream 28 that flows through the desalination unit 32 (as shown in figure 1). The process includes a flash evaporator 29 and a centrifuge 30. Separation of the MEG in the flash evaporator 29 leaves the salts and solids to crystallise and accumulate in a concentrated phase. A stream 31 from this phase is fed to the centrifuge 30 for separation of the solids. The system for separating fine particles is inserted into this stream 31. The particles at this stage of the process may be predominantly carbonates that have precipitated out of the MEG. Flocculant from a storage tank 64 is injected by means of an injector 60 and a pump 62 into the stream 31 to the centrifuge 30. The flocculation process agglomerates the particles as the MEG flows between the injector 60 and the centrifuge 30. On entering the centrifuge 30, the agglomerated particles are separated from the MEG due to their greater density. The separated particles may be fed to a dissolver (not shown), to be mixed with water and sent to a waste water treatment plant. The recovered MEG from the centrifuge is fed back to the flash evaporator 29. The MEG leaving the flash evaporator is "clean" lean MEG which is fed back into the connecting line 24 to the lean MEG storage tank 26 (as shown in figure 1).

It may also be possible, depending on the efficiency of the flocculation process , to replace the centrifuge 30 with a simpler and less costly separating device such as a settling tank.

Although the process shown in Figure 3 is carried out on a side- stream, it will be appreciated that this need not be the case. Instead of removing the side stream 28, the evaporator 29 could be inserted directly into the main process flow stream in the connecting line 24. This would mean that the entire lean MEG flow from the regeneration unit 22, shown in Figure 1 , would pass through the solids removal apparatus.

The systems described above relate to MEG reclamation. However, the same principles of the apparatus and method may be applied to other processes that require solids removal. One example is in the reclamation of amines used for gas sweetening. Gas sweetening is the removal of acids from natural gas.

The systems described above for removing particulate matter by flocculation and separation of agglomerated particles have numerous advantages over known particulate removal systems. In particular the size of the plant required is smaller and simpler to construct and operate. Less energy is required than for comparable filtration/removal systems and very limited operator attendance is required. This provides significant benefits in terms of space, capital cost and operating cost savings. The systems are ideally suited for inclusion on off-shore platform installations as well as at on-shore treatment plants. Furthermore, the flocculation process provides a reliable method of removing even the smallest particles, and so can be used to provide reliable performance over a wide range of operating conditions, and over the entire life cycle of the plant. The flocculation process can also be operated intermittently if so required - providing greater flexibility in controlling the removal of particulate matter.