The Light Hydrocarbon Recovery Biology Essay

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Frames was founded in 1983 as a Dutch company to provide technical solutions for the upstream oil and gas industry. It is specialized in designing, fabrication and supplying complete systems for oil and gas treatment. The company rapidly became larger and now has offices all over the world. The Frames offices are located in Brazil, Germany, India, Malaysia, The Netherlands, Russia, Saudi Arabia, UAE and USA.

Several offices are located in the Netherlands. The main office and the business unit Energy Systems is located in Zoeterwoude. The business unit Separation Technologies has its office in Woerden. NRG Heat Exchangers, which became part of Frames in 2010, is located in Vollenhove. In Alphen aan den Rijn, Frames has two locations. The office located at the Eikenlaan houses the business unit Energy Systems Development and the office located at the Sacharovlaan, seen on Figure , houses the business units of Process Systems and Gas Processing. This internship is conducted for the business unit Process Systems. Currently, Frames wants to include a process for light hydrocarbon recovery in their portfolio. A study to investigate different process option has to be done. The membrane process is one of them and is investigated in this internship.

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Sacharovlaan

Figure : Frames office building in Alphen aan den Rijn, Sacharovlaan. This building houses two business units: Process Systems and Gas Processing.

Light Hydrocarbon Recovery

A product of the oil production is associated gas. Unlike oil, the gas cannot be trucked to the nearest gas processing plant. Since gas transport infrastructure is often not available and building a new pipeline is costly, most of the gas associated with oil production is flared. This gas, mainly consisting of methane and ethane, is saturated with higher hydrocarbons. These higher hydrocarbons consist of propane, butanes, pentanes and higher. The recovery of these higher hydrocarbons, mainly C3+, as condensed liquid could be profitable, because these natural gas liquids (NGL) can be mixed and sold with the oil.

Current techniques to process the flare gas are a sequence of cooling and condensation of the heavier products in the gas stream or removing them by lean oil absorption. The separated hydrocarbons are fractionated by distillation. These methods are very efficient, but usually not at small scales and are often energy intensive.

Membranes are competitive in this area, because they possibly possess the ability to separate the C3+ fraction from methane and ethane with higher efficiency at small scales.

Besides the compressors, the membrane equipment does not have rotational parts, which need a lot of maintenance and the monitoring does not have to be done on location.

Membrane modules are relatively small compared to the alternative equipment and this is an advantage on offshore platforms or other places where optimizing space is crucial.

Membranes can be used in two different ways to recover the condensable hydrocarbons as seen in Figure . In the first case, (a), the organic vapours are rejected by the membrane and the methane and ethane pass through the membrane. The second option, (b), has the organic vapours permeating through the membrane.

This research will go into detail about the use of membranes in the gas separation process of light hydrocarbon recovery. The internship description can be found in Appendix A.

Figure : Difference between glassy and rubbery membranes. (a) Glassy membrane which permeates light components and (b) rubbery membrane which permeates heavier components.

Chapter two describes how membranes can be used to separate components and the different mechanisms of membrane separations are explained. Chapter three gives an overview of the membranes reported in literature and membranes currently commercially available. Chapter four explains how membranes can be implemented in processing equipment and the differences between several frequently used modules. Chapter five shows how the membrane modules are modelled and in chapter six the results for the different process designs are discussed. Chapter seven gives an economic evaluation of the process design alternatives. An example HAZOP study and control scheme are discussed in chapter eight. In chapter nine some other membrane applications than NGL recovery are discussed. Finally, in chapter ten the conclusions are drawn and in chapter eleven the recommendations are given.

Membrane thermodynamics

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This chapter shows the thermodynamics involved in membrane separations. A gas stream consisting of primarily methane and ethane, saturated with organic vapours of higher hydrocarbons enters the membrane module, where the C3+ hydrocarbons are separated from methane and ethane. This is schematically depicted in Figure .

Figure : Schematic working of the membrane.

Membranes can be looked at as an interface that separates two phases and regulates the transport of the components between the phases in a specific manner. Membranes separate a mixture by different transport rates through the membrane for different species in the mixture. Driving forces for membrane separation are concentration, (partial) pressure, temperature and electrical potential gradients. Membranes for gas separation are almost always pressure driven. Therefore, a sweep gas can be used to lower the partial pressure of the permeate in the permeate stream and thus, to increase the driving force, which gives better separation efficiencies. As seen before, membranes can be selective for the condensable or non-condensable gasses. Different membrane types and their transport mechanisms will be discussed.

Porous membranes

Porous membranes consist of a separation layer with defined pores with a pore-size usually less than 0.1 μm. The porous membrane has the so-called Knudsen diffusion as gas transport mechanism. Knudsen diffusion occurs when the mean free path of the molecules, which is in the order of 0.1 μm for a gas, is larger than the pore diameter and can be considered as a type of viscous flow. Figure shows a schematic representation of viscous flow and Knudson diffusion.

Figure : Schematic illustration of viscous flow and Knudsen diffusion Error: Reference source not found.

The gas molecules have more collisions with the wall than with other gas molecules. The Knudsen diffusion coefficient is given by:

\* MERGEFORMAT ()

The Knudsen diffusion coefficient is inverse proportional to the square root of the molecular weight of the component. Thus, the separation is given by the square root of the molecular weights and is consequently low for the separation process of C1 + C2 from C3+. Methane and ethane should permeate faster through the membrane because their molecular weight is lower than for propane or higher hydrocarbons, but the difference in molecular weight between ethane and propane is not very large. Thus large amounts of propane find their way into the permeate stream or large amounts of ethane in the retentate stream. Higher separation efficiencies can be achieved by the use of non-porous membranes.

Non-porous membranes

Non-porous membranes consist of a dense layer of material in which the separation takes place. Transport of the components through non-porous membranes is based on the so-called solution-diffusion mechanism.

The mechanism consists of three steps, depicted in Figure Error: Reference source not found.

Sorption of various components from the feed mixture according to their partition coefficient between the gas and polymer phase.

Diffusion of the individual components within the membrane.

Desorption of the components in the permeate gas stream.

Figure : Three-step transport mechanism in the solution-diffusion model Error: Reference source not found.

The driving force for this mechanism is the activity gradients of the permeating components, which is related to the partial pressures in the feed mixture and permeate. The flux of component k in a membrane without viscous flow and no coupling of fluxes is given by:

\* MERGEFORMAT ()

Where μkm is the chemical potential of the component k and Lk is the phenomenological coefficient. These values are usually unknown. In practice, the diffusion coefficient and solubility coefficient are used to obtain a permeability coefficient for a membrane type. The permeability coefficient for a pure penetrant, P, is often written as:

\* MERGEFORMAT ()

Where S is the apparent solubility coefficient and D is the penetrant diffusion coefficient. The ideal selectivity of a membrane is given by:

(4)

The ratio DA/DB is the ratio of the diffusion coefficients, which represents the diffusion selectivity. This is influenced by the size of the molecules and relative motion of the individual molecules. When methane is compared to larger hydrocarbons, the diffusivity of the smaller methane gas is always higher. The ratio SA/SB is the solubility selectivity and reflects the relative sorption of the gasses. Higher order hydrocarbons such as butane, which is more condensable, have higher sorption coefficients. The ratio between the diffusivity selectivity and solubility selectivity determines whether a membrane is selective for organic vapours or lighter gasses.

Feed conditions

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The temperature of the feed gas has a profound influence on the permeabilities. In the case of membranes based on the solution-diffusion mechanism, lower temperatures favour the permeation of the more condensable component, for example butane for a butane/methane feed. The selectivity of the membrane increases for butane with decreasing temperature as observed by Pinnau Error: Reference source not found. The selectivity for a 2 vol% butane in methane is shown in Figure for different temperatures. However, in all cases the rate of permeation decreases with decreasing temperature, so an optimal temperature has to be found.

Figure : The selectivity for a 2 vol% butane in methane for a non-porous PDMS membrane for a temperature range. With increasing temperature, the selectivity decreases. Error: Reference source not found

The feed pressure and composition have generally also an effect on the permeability and selectivity of the membranes. With increasing total pressure the fluxes become higher, but the selectivity is often compromised. When the concentration of a component becomes higher and thus the partial pressure, the observed selectivity towards that component will increase.

A sweep gas is only used in situations where creating a pressure difference over the membrane is not possible or high purities have to be obtained. The permeate will be contaminated with the sweep gas and is often not used in further process steps.

Membrane materials and structures

Membranes can be made from various materials and different structures are possible. The materials for membrane production are shown in Figure . Other materials, in addition to polymer membranes, are for example glass, metal, ceramic and liquids. With these materials different membrane structures can be made. These structures are shown in Figure .

Figure : Membrane materials

Polymer membranes are the focus of commercialization, because they are a lot easier to produce than for example inorganic membranes. The polymer membranes are typical non-porous and their permeability is described with the solution-diffusion mechanism. Glassy membranes show higher permeability for small non-condensable gasses, because the diffusion or mobility selectivity is determining the permeability for these membranes. However, for rubbery membranes the sorption selectivity is dominant and larger condensable gasses permeate preferably through the membrane.

Figure : Different membrane structures Error: Reference source not found.

The structure of the membrane is also very important for the separating properties. Symmetric membranes have the same structure and properties over the membrane cross-section. Asymmetric membranes have a dense top layer that does the separation and a microporous support. The support is needed for mechanical strength of the membrane and is sometimes also used to prevent swelling of the polymer due the organic vapour adsorption. The mass transfer resistance of the support should be as low as possible, but is often considerable compared to the total resistance of the membrane. In some cases it is difficult to maintain a partial pressure difference as driving force, because the penetrant accumulates in the porous support.

The difference between integral asymmetric and composite asymmetric membranes is that the microporous support in integral asymmetric membranes is made of the same material (polymer) as the selective top layer, while for composite membranes the selective layer is coated onto the microporous support, which could be a different material (see Figure ). The separation function and mechanical support function are separated, allowing for optimization of both materials. This could save costs, because less material of the expensive membrane polymer is needed.

Figure : Typical example of a composite membrane consisting of a microporous support coated with a thin layer of selective material Error: Reference source not found.

Rubbery membranes

Rubbery membranes are known for their preference to permeate larger condensable gasses. As common in membrane systems, the selectivity drops when a mixed gas is used instead of the pure gas, which is used to test the membrane. This occurs due to the swelling of the polymer by sorption of organic vapours, decreasing the chain-chain interaction and thus creating more free volume in the polymer. Because of the increased free volume, the methane can permeate much faster than without swelling of the polymer. Silicone rubbers, such as polydimethyl siloxane (PDMS), have an advantage over other rubbery polymers, because it is cheap and readily available. These polymers combine high permeation rates with reasonable selectivities. However, the selectivity is inadequate to achieve good separation with a single pass. Therefore, multiple stages of membranes with recycle streams are needed. The properties of PDMS are depicted in .

The structure of PDMS was changed to obtain better selectivities, which is the case for polyoctylmethyl siloxane (POMS). Selectivity's for butane/methane are found to be as high as 10 Error: Reference source not found. Even higher values are reported when the POMS membrane is encapsulated in the pores of the support (POMS-SSM). The encapsulation hinders the swelling of the polymer and thus prohibits faster permeation of the non-condensable gasses.

Table : Properties of rubbery membranes at 23 oC, 250 psig and 2 mol% butane in methane Error: Reference source not foundError: Reference source not foundError: Reference source not found.

Tg (oC)

Durability

Pressure resistance

Permeability methane (barrer)

Permeability butane (barrer)

αC4,C1

PDMS

-125

3-5 year

up to 85 bar

1200

7200

6

POMS

~ -100

3-5 year

up to 85 bar

600

6000

10

POMS-SSM

~ -100

3-5 year

up to 85 bar

?

?

12

Polyether block amide (PEBA) polymers are suggested by a membrane supplier as a nowadays better alternative for PDMS membranes, because the permeabilities are close to the values of PDMS, but PEBA have better selectivities towards organic vapours. Since it is not a 'standard' polymer yet, it is more expensive than PDMS.

Glassy membranes

Cellulose acetate, polyimides and perfluoro-polymers (for example PVDF) are three examples of glassy polymer membranes that are used in the field of CO2 removal from gas streams. These membranes are selective for smaller lighter gasses, like methane. Selectivities reported in literature are in the range of 3 to 5 Error: Reference source not found, Error: Reference source not found.

Poly(1-trimethylsilyl-1-propyne), PTMSP, is a glassy polymer with extraordinary properties. This membrane shows a mixed gas feed selectivity higher than the ideal selectivity, measured with pure gas Error: Reference source not found. Selectivity's for butane over methane up to 30 are measured for 2 mol% butane in methane gas feed. This behaviour can be explained by the high free volume fraction of the membrane (up to 34%). Those free volume elements appear to be connected and most of the gas permeation occurs through this network. Condensable gasses sorb onto the wall of the small pores, producing multilayer adsorption and even capillary condensation. These phenomena block the pores partially or completely, reducing the permeation of lighter gasses through the free volume network. The membrane becomes highly permeable for the condensable gasses, but almost no non-condensable gasses can permeate through compared to the pure gas experiments. This explains the observed high selectivity's in mixed gas feeds.

Figure : Schematic gas transport mechanism occurs in PTMSP Error: Reference source not found.

Table : Properties of the PTMSP membrane Error: Reference source not found.

Tg (oC)

Durability

Pressure resistance

Permeability methane (barrer)

Permeability butane (barrer)

αC4,C1

PTMSP

>250

Medium/poor

High

1800

53500

30

Table shows the properties of the PTMSP polymer membrane. Aging of the polymer over a three month period showed that the butane flux decreased, but it is still a viable alternative to rubbery polymers. A selectivity of 27 and a butane flux of 4 m3/m2h bar were still observed. However, the price, availability and varying quality of PTMSP are still a concern and make the use of this membrane polymer unsuitable for industry today. For these reasons, most researchers have abandoned these materials for membrane applications and switched to cheaper more reliable materials Error: Reference source not found.

Zeolites

A type of inorganic material membranes are zeolites. Zeolites show great potential to be used as membrane separators, because they possess good mechanical strength and the long-term stability is very good. A number of zeolite types show high selectivity's for butane/methane separations. The reproducibility is, however, very poor and about 80% of the attempts to create a zeolite membrane fails Error: Reference source not found. This combined with the higher costs over polymeric membranes makes this kind of membranes not yet suitable for use in industry.

Commercial membranes

Membranes that are commercially produced for the industry are shown in this section. Most membranes that are produced today are meant for the purification of water. However, several companies have developed membranes for industrial gas separation use and sell them on the market. The market for C3+ hydrocarbon recovery is an expanding market and not many producers have developed membranes in this area. A number of suppliers and their membranes are listed in Table .

Table : Suppliers of Membrane Gas Separation Systems Error: Reference source not found.

Company

Principal gas separation

Membrane module type

Membrane material

Medal (Air Liquide)

CO2

hollow fiber

polyimide

W.R. Grace

CO2

spiral-wound

cellulose acetate

Separex (UOP)

CO2

spiral-wound

cellulose acetate

Cynara (Natco)

CO2

hollow fiber

cellulose acetate

ABB/MTR

CO2, N2, C3+ hydrocarbons

spiral-wound

perfluoro polymers, silicone rubber

Permea (Air Products)

H2O

hollow fiber

polysulfone

GKSS

C3+ hydrocarbons

spiral-wound

silicone rubber

KNM's Borsig

C3+ hydrocarbons

envelope module

Solsep

Ultra-, Nanofiltration

spiral wound

polyimide, silicone rubber

The list shows three possible suppliers for membranes that are suitable for NGL recovery. KNM's Borsig, GKSS and MTR. GKSS is a polymer research centre and will likely not provide membranes for industrial applications. On the other hand, MTR is a company that can deliver complete membrane system skids and is also not likely to provide only the membrane modules. The same reasoning may also be applied to Borsig.

Membrane stability

The stability of the membrane is of importance when applied in an industrial application. The membrane should have a long life-time and possibly no degradation of the separation efficiency. In practice, this is not possible and the membrane stability is dependent on the chemical, mechanical and toxicological resistance. The membrane feed should be kept free of harmful components that could mechanically or chemically damage the membrane. Solid particles in the feed gas or formed by precipitation could abrade the membrane. The damage done is often irreversible. Chemical damage could be done by oxidizing molecules such as ozone and peroxides, which degrade the crosslinking or packing of the membrane. Aromatic hydrocarbons are known to swell or even dissolve the membrane material. The best way is to filter harmful components before using the membrane module.

The amount of information about the stability of membranes is very limited. It is known that ceramic membranes have very good resistance against aggressive solvent and mechanical damage, while some rubbery membranes have their properties changed when organic compounds adsorb in the membrane structure or worse, dissolve in the organic compound. Rubbery membranes that are applied for a suitable operation last typically between the 3 and 5 years before replacement is needed Error: Reference source not found.

Conclusion

In the field of light hydrocarbon recovery, the non-porous membranes have several advantages over the porous membranes. One of them is the higher selectivities offered by the solution-diffusion mechanism. The material that is most suitable for the recovery of NGL would be PDMS, because it is cheap, readily available and the organic vapours permeate through the membrane. Reasonable selectivities are reported for these membranes. In the remainder of this report the PDMS membrane will be used.

Membrane modules

The performance of the membrane equipment is not only determined by the type and quality of membrane, but also by the design of the equipment and how the membrane is implemented. Several possibilities for implementing a membrane in a module are reported in literature.

The membranes are folded and packed into a housing, which is suitable to use in an industrial plant. Multiple ways of packaging a membrane inside a housing are described in literature and by manufacturers. Some of the most used types are the plate and frame, spiral wound, tubular and hollow fiber module. The modules have different designs, production costs, operation energy requirements and operation modes. Each module has its positive and negative properties and for most applications a suitable module can be found.

Fouling and concentration polarization

To address the performance of a membrane module, two things have to be explained. First, fouling can occur in a membrane module. Viscous or solid materials can adhere to the membrane surface compromising its functionality. To prevent fouling, the feed stream should be filtered before entering the membrane module and if fouling is still an issue, the membrane module could be flushed with a fluid that dissolves the fouling without damaging the membrane. Concentration polarization occurs when the permeating component, permeates faster through the membrane than the bulk of the fluid can supply to the membrane interface. The lower concentration of the component at the interface results in less permeation and a reduced selectivity. To solve this problem, the feed should be well-mixed inside the module. This could be done by using the spacers of the channels as baffles to create turbulence Error: Reference source not found. The four mentioned module types will be discussed briefly.

Frequently used modules

Plate and frame

The plate and frame module finds its origin in the filter press-applications. The membrane is a flat sheet or circle, which is placed between the feed channel spacer and permeate channel spacer as can be seen in Figure . Multiple membranes can be stacked in parallel up to 1,000. Therefore, this module has a relatively high membrane area per unit volume and the membranes can be exchanged easily. The complete system is placed in a housing to maintain the feed pressure and often baffles are installed to increase the path of the feed gas. The need for several sealings and the relative large pressure drop are the main disadvantages for this type of module.

Figure : Schematic representation of the plate and frame membrane module Error: Reference source not found.

Spiral wound

The spiral wound module is very similar to the plate and frame module. This module folds the membrane sheets around a central pipe which has holes to collect the permeate. The feed stream is in the axial direction and the permeate stream flows toward the central collection pipe, which is glued to the permeate flow channel. Small spiral wound modules consist of a single envelope. This, however, limits the membrane area that can be installed, because the length of a single membrane sheet is limited to 2 to 5 meters. Larger values would cause a pressure drop that is not desired in the permeate channel, because this channel is very narrow. To increase the membrane area in a single module, multiple membrane sheets are installed. The module is placed in a pressure vessel to accommodate for the high working pressures involved in membrane separations. This type of module is often used for gas separation processes. The advantages of the spiral wound module are the low production costs and very large membrane area per unit volume. The disadvantage is that the module is quite sensitive to fouling.

MB1111-41[380-ALL]

Figure : Schematic representation of the spiral wound module Error: Reference source not found.

Tubular

The tubular membrane module consists of tubes instead of flat sheet membrane material. The membrane tubes are placed into stainless steel or fiber glass reinforced plastic pipes. The tubes have typically a diameter of 1 to 3 cm. The feed gas flows inside the tubes and the permeate is collected outside the tube. Usually, 10 to 30 tubes are installed inside a larger tube housing. The main advantage is that membrane fouling and concentration polarization effects can easily be controlled, however this is achieved at the cost of the surface area. The low membrane area per unit volume cause very high costs per module. Therefore, tubular membrane modules are only used in situations where the feed has high solid content or high viscosity.

Figure : Schematic representation of the tubular membrane module Error: Reference source not found.

Hollow fiber

The hollow fiber module comprises of hollow fibers with an outer diameter of 50 to 100 μm, which have the selective layer on the outside. Several thousand fibers are bundled to create a very large surface area.

The free ends of the fibers are potted with an epoxy resin to prevent the feed gas from flowing to the permeate exit. Only gases permeating through the membrane can exit at one end by flowing through the hollow core. The remaining gases that do not permeate through the membrane exit at the other end of the vessel. The hollow fiber module has the highest packing density of the modules discussed here. The production costs are relatively low and the module can be operated at 100 bars or higher. The main disadvantage of these modules is the poor resistance to fouling and difficult control of concentration polarization. These modules are used in industry for reverse osmosis and gas separation.

Figure : Schematic representation of the hollow-fiber module Error: Reference source not found.

Conclusion

The modules most suitable and also currently used for gas separation are the spiral wound module and the hollow fiber module, because these have the lowest production cost and high surface area per volume unit. As seen in Table , the suppliers of membrane systems that do NGL separation all choose for the spiral wound module. The poor properties for fouling and concentration polarization are not that important, because these effects are much less for gas separation processes. The cross flow properties of the spiral wound membrane module are ideal for modelling, because a constant permeate concentration can be assumed. The remainder of this document will use the spiral wound module for calculation purposes.

Table : Summary of the membrane modules Error: Reference source not foundError: Reference source not found

Membrane module

Membrane area per unit volume (m2 m-3)

Membrane costs

Control of concentration polarization

Application

Plate-and-frame module

400 - 800

medium

good

MF, UF, RO, D

Spiral-wound module

800 - 1200

low

medium

UF, RO, GS

Tubular module

20 - 100

very high

very good

MF, UF, RO

Hollow fiber module

2000 - 5000

very low

very poor

RO, GS

MF = microfiltration

UF = ultrafiltration

RO = reverse osmosis

D = dialysis

GS = gas separation