阅读说明：本技术 分离卤代硅烷的方法 (Method for separating halosilanes ) 是由 S·蓬努斯瓦迈 N·R·科塔 S·布萨拉普 P·古普塔 于 2015-09-01 设计创作，主要内容包括：公开了分离卤代硅烷的方法,其涉及使用具有将塔分成用于产生三个产物馏分的部分的隔板的蒸馏塔。还公开了使用这样的塔的通过卤代硅烷的歧化生产硅烷的方法和系统和生产多晶硅的方法。(A process for separating halosilanes is disclosed that involves the use of a distillation column having a partition dividing the column into portions for producing three product fractions. Also disclosed are methods and systems for producing silane by disproportionation of halosilanes and methods for producing polycrystalline silicon using such columns.)

1. A method of isolating a halosilane, comprising:

introducing a first halosilane, a second halosilane, and a third halosilane into a halosilane distillation column, the first halosilane having a boiling point less than that of the second halosilane, the second halosilane having a boiling point less than that of the third halosilane, the distillation column comprising a partition dividing the column into a main portion and a side portion;

withdrawing an overhead fraction enriched in the first halosilane relative to the collection of feeds introduced into the distillation column;

withdrawing from the side of the column as a side draw enriched in the second halosilane relative to the collection of feeds introduced into the distillation column; and

a bottoms fraction enriched in the third halosilane relative to the collection of feeds introduced into the distillation column is withdrawn.

2. The method as set forth in claim 1 wherein the first halosilane is dihalosilane, the second halosilane is trihalosilane, and the third halosilane is silicon tetrahalide.

3. The process as set forth in claim 2 wherein monohalosilane is introduced into the distillation column, and the overhead fraction is enriched in monohalosilane relative to the collection of feeds introduced into the distillation column.

4. A process as set forth in any one of claims 1 to 3 wherein the distillation column is operated at a pressure as measured at the top of the column of from about 200kPa gauge to about 2000kPa gauge or from about 200kPa gauge to about 1500kPa gauge, from about 200kPa gauge to about 1000kPa gauge, from about 400kPa gauge to about 2000kPa gauge or from about 800kPa gauge to about 2000kPa gauge.

5. The process as set forth in any one of claims 1 to 4 wherein the distillation column comprises a condenser having a temperature of from about 20 ℃ to about 120 ℃ or from about 40 ℃ to about 120 ℃, from about 60 ℃ to about 120 ℃, from about 20 ℃ to about 100 ℃, or from about 20 ℃ to about 80 ℃.

6. The process as set forth in any one of claims 1 to 5 wherein the distillation column has a reboiler having a temperature of from about 90 ℃ to about 200 ℃ or from about 90 ℃ to about 180 ℃, from about 90 ℃ to about 150 ℃, from about 110 ℃ to about 200 ℃, or from about 140 ℃ to about 200 ℃.

7. A process as set forth in any one of claims 1 to 6 wherein the halosilane is introduced into the distillation column facing the partition and a second halosilane-enriched side-draw is withdrawn facing the partition.

8. The process as set forth in claim 1 wherein the distillation column comprises a condenser and a reboiler, the distillation column operating at a pressure of from about 200kPa gauge to about 2000kPa gauge, the condenser temperature being from about 20 ℃ to about 120 ℃, the reboiler temperature being from about 90 ℃ to about 200 ℃, the first halosilane-enriched overhead fraction having a purity of 80%, the second halosilane-enriched side fraction having a purity of 80%, and the third halosilane-enriched bottoms fraction having a purity of 95%.

9. A method for producing silane by disproportionation of halosilanes, the method comprising:

isolating a halosilane according to the method of any one of claims 1 to 8, the first halosilane being a dihalosilane, the second halosilane being a trihalosilane, and the third halosilane being a silicon tetrahalide;

introducing a side-draw produced from the distillation column into a first disproportionation reactor to produce a first disproportionation reactor product stream comprising dihalosilane and silicon tetrahalide, the first disproportionation reactor product stream being introduced into the distillation column;

introducing a first disproportionation reactor product stream into the distillation column; and

introducing the overhead fraction produced by the distillation column into a second disproportionation reactor to produce a second disproportionation reactor product stream comprising silane and trihalosilane.

10. The method as recited in claim 9, further comprising:

introducing the second disproportionation reactor product stream into a silane separation system to separate silane and trihalosilane; and

introducing the separated trihalosilane to the distillation column.

11. The process as set forth in claim 10 wherein the distillation column is a first distillation column and the silane separation system is a second distillation column, the second separation column producing a silane-enriched overhead fraction and a trihalosilane-enriched bottoms fraction, the process comprising introducing the bottoms fraction produced by the second distillation column into the first distillation column.

12. The method as set forth in any one of claims 9 to 11 wherein dihalosilane is introduced into the distillation column by recycling dihalosilane produced in the disproportionation process to the distillation column.

13. The method as set forth in claim 12 further comprising introducing a disproportionation system feed to the distillation column, the disproportionation system feed comprising dihalosilane.

14. The method as set forth in claim 12 further comprising introducing a disproportionation system feed to the distillation column, the disproportionation system feed not comprising dihalosilane.

15. The method as set forth in any one of claims 9 to 14 wherein the halogen component of the halogen-containing compound is chlorine.

16. A method of producing polycrystalline silicon, the method comprising:

producing a silane according to the method of claim 10; and

the silane is introduced into a fluidized bed reactor or a Siemens reactor to produce polycrystalline silicon.

17. The process as set forth in claim 16 wherein the distillation column is a first distillation column and the silane separation system is a second distillation column, the second separation column producing a silane-enriched overhead fraction and a trihalosilane-enriched bottoms fraction, the process comprising introducing the overhead fraction produced by the second distillation column into a fluidized bed reactor or a Siemens reactor to produce polycrystalline silicon.

18. A system for producing silane by disproportionation of halosilanes, the system comprising:

a distillation column comprising a partition dividing the column into a main portion and a side portion to produce an overhead fraction enriched in a first halosilane relative to a collection of feeds introduced into the distillation column, a side fraction enriched in a second halosilane relative to a collection of feeds introduced into the distillation column, and a bottoms fraction enriched in a third halosilane relative to a collection of feeds introduced into the distillation column;

a first disproportionation reactor for producing a first disproportionation reactor product stream from a side-draw produced by the distillation column, the disproportionation reactor product stream comprising dihalosilane and silicon tetrahalide;

a second disproportionation reactor for producing a second disproportionation reactor product stream from the overhead produced by the distillation column, the second disproportionation reactor product stream comprising silane and trihalosilane; and

a silane separation system for separating silane and trihalosilane.

19. The system as set forth in claim 18 wherein the separation system is a second distillation column.

20. The system as set forth in claim 19 wherein the system for producing silane comprises no more than two distillation columns.

21. A system as claimed in any one of claims 18 to 20 in combination with:

an overhead fraction comprising a first halosilane;

a side-draw comprising a second halosilane; and

a bottoms fraction comprising the third halosilane.

22. The system as set forth in claim 21 wherein the first halosilane is dihalosilane, the second halosilane is trihalosilane, and the third halosilane is silicon tetrahalide.

23. A system for producing polycrystalline silicon, the system comprising:

a system for producing silane by disproportionation of halosilane as recited in any one of claims 18 to 22; and

a silane reactor for producing polycrystalline silicon by thermal decomposition of silane.

24. The system as set forth in claim 23 wherein the silane reactor is a fluidized bed reactor or a Siemens reactor.

Brief Description of Drawings

FIG. 1 is a flow diagram of a dividing wall distillation column for separating halosilanes;

FIG. 2 is a flow diagram of a disproportionation system for converting halosilanes to silanes;

FIG. 3 is a flow chart of a system for producing polycrystalline silicon; and is

FIG. 4 is a flow diagram of a conventional disproportionation system for converting halosilanes to silanes.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

Detailed description of the invention

The present disclosure provides a distillation column for separating halosilanes. The distillation column includes baffles to separate the halosilane-containing stream into three fractions. Such an arrangement reduces the capital cost and energy requirements for separating three or more halosilanes relative to prior art arrangements that use two distillation columns to achieve the same separation. Also provided are methods and systems for producing silane by disproportionation of halosilanes and methods for producing polycrystalline silicon comprising such distillation columns.

Distillation column for separating halosilanes

In accordance with embodiments of the present disclosure, components of halosilane-containing stream 26 are separated using distillation column 65 (FIG. 1) having partition 5 therein. Partition 5 separates feed 26 and side-draw 9 and produces main column section 70A and side column section 70B. The partition 5 extends only partially up and down in the housing 11. This configuration enables the separation of three or more halosilane components using a single column (which acts as two separate columns).

Halosilane-containing stream 26 includes a first halosilane, a second halosilane, and a third halosilane. These halosilanes have different boiling points, and as used herein, the first halosilane has a boiling point less than the second halosilane, and the second halosilane has a boiling point less than the third halosilane. In some embodiments, the first halosilane is a dihalosilane, the second halosilane is a trihalosilane, and the third halosilane is a silicon tetrahalide. In some embodiments, the halosilane-containing stream comprises a fourth halosilane (e.g., monohalosilane). It should be noted that the halosilane-containing stream may also include other components and/or various impurities.

Although halosilane-containing stream 26 is shown as being fed to column 65 as a single feed, it should be noted that feed stream 26 may be comprised of a number of feed streams (including halosilane-free streams) that are introduced at the same or different vertical locations in the column. These feed streams (not shown) collectively form halosilane-containing stream 26. Halosilane-containing stream 26 can also include a stream recycled downstream from column 65. Unless otherwise indicated, reference herein to a "halosilane-containing stream" refers to the collection of feed streams that excludes reboiler and condenser recycles.

In this regard, in embodiments where two or more feed streams are introduced into the distillation column, the feed stream containing the relatively heavier components compared to the other streams may be introduced relatively lower in the column and the stream having the relatively lighter components may be introduced higher in the column. For example and with reference to fig. 3 discussed below, the relatively heavier feed 20 may be introduced below the feeds 94, 97, and the relatively lighter feed 97 may be introduced above the feeds 20, 94.

The halogen component of the halosilane in the halosilane-containing stream may be fluorine, chlorine, bromine, iodine, or even a combination of these compounds. In some embodiments, the halogen component is chlorine (e.g., the first halosilane is dichlorosilane, the second halosilane is trichlorosilane, the third halosilane is silicon tetrachloride, and the optional fourth halosilane is monochlorosilane).

A divided or "dividing wall" distillation column 65 separates halosilane-containing stream 26 into three fractions — a first halosilane-enriched overhead fraction 69, a second halosilane-enriched side fraction 9, and a third halosilane-enriched bottoms fraction 57. In this regard, as used herein, "first halosilane-enriched overhead" 69 refers to the distillation overhead or "overhead" withdrawn from column 65 that is enriched in first halosilane relative to halosilane-containing stream 26. "second halosilane-enriched sidedraw" 9 refers to a sidedraw of distillation column 65 that is enriched in second halosilane relative to halosilane-containing stream 26. "third halosilane-enriched bottoms" 57 refers to a bottoms stream withdrawn from distillation column 65 that is enriched in third halosilane relative to halosilane-containing stream 26. In embodiments where the first halosilane is dichlorosilane, the second halosilane is trichlorosilane, and the third halosilane is silicon tetrachloride, and where the halosilane-containing stream includes monochlorosilane as the fourth halosilane, the first halosilane-enriched overhead fraction 69 may also be enriched in monochlorosilane.

In embodiments where the first halosilane is dihalosilane, the second halosilane is trihalosilane, the third halosilane is silicon tetrahalide, and optionally contains monohalosilane as the fourth halosilane, the first halosilane-enriched overhead fraction 69 preferably has a purity of at least about 80 mole percent (i.e., the purity of dihalosilane and monohalosilane, if present in feed 26), the second halosilane-enriched sidedraw fraction 9 preferably has a purity of at least about 80 mole percent (i.e., the purity of trihalosilane), and the halosilane-enriched bottoms fraction 57 preferably has a purity of at least about 90 mole percent (i.e., the purity of silicon tetrahalide). In other embodiments, the purity of the first halosilane-enriched overhead fraction 69 is at least about 90%, the purity of the second halosilane-enriched sidedraw fraction is at least about 90%, and the purity of the halosilane-enriched bottoms fraction 57 is at least about 95%. In still other embodiments, the purity of the first halosilane-enriched overhead fraction 69 is at least about 95%, the purity of the second halosilane-enriched sidedraw fraction is at least about 95%, and the purity of the halosilane-enriched bottoms fraction 57 is at least about 95%.

Distillation column 65 also includes reboiler 17 for recycling bottoms in vapor form and condenser 19 for returning a portion of the overheads in liquid form.

Dividing wall distillation column 65 may be operated at various pressures, temperatures, reflux ratios, column loadings, and feed and side draw locations as a function of the total loading of the column, the composition of the silane-containing feed, and the desired purity of the overheads, side draw, and bottoms fractions. Typically, column 65 may be operated at a pressure of about 200kPa gauge to about 2000kPa gauge, as measured at the top of the column (about 200kPa gauge to about 1500kPa gauge, about 200kPa gauge to about 1000kPa gauge, about 400kPa gauge to about 2000kPa gauge, or about 800kPa gauge to about 2000kPa gauge, as measured at the top of the column). The temperature of the column condenser can be from about 20 ℃ to about 120 ℃ (e.g., from about 40 ℃ to about 120 ℃, from about 60 ℃ to about 120 ℃, from about 20 ℃ to about 100 ℃, or from about 20 ℃ to about 80 ℃). The temperature of the reboiler can be from about 90 ℃ to about 200 ℃ (e.g., from about 90 ℃ to about 180 ℃, from about 90 ℃ to about 150 ℃, from about 110 ℃ to about 200 ℃, or from about 140 ℃ to about 200 ℃).

Feed 26 is introduced into (across) column 65 generally vertically across from baffle 5 of the column. If more than one feed stream is added to the column (e.g., various recycle streams are added to the column), the feed stream containing the relatively heavier components compared to the other streams may be introduced relatively lower in the column and the stream having the relatively lighter components may be introduced higher in the column. The second halosilane-enriched side-draw 9 is typically withdrawn at a vertical point (e.g., the middle 1/3 of the partition) facing partition 5 of column 65.

It should be noted that the column 65 may include various internal recycles that may affect the loading of the column. In some embodiments, about 10% to about 90% of the liquid above partition 5 is recycled to major side 70A (rather than being recycled down to side 70B). Preferably, about 20% to about 60% of the liquid above the baffle 5 is recycled to the major side 70A. Alternatively or additionally, from about 10% to about 90% of the vapor below the baffle 5 can be recycled to the major side 70A of the column 65, or as in other embodiments, from about 20% to about 60% of the vapor below the baffle 5 can be recycled to the major side 70A of the column 65. It should be noted that the liquid and vapor split streams may affect the loading of the column.

In some embodiments of the present disclosure, the first halosilane-enriched overhead fraction 69 having a purity of about 80%, the second halosilane-enriched side fraction 9 having a purity of about 80%, and the third halosilane-enriched bottoms fraction 57 having a purity of about 95% may be achieved by operating the column at a pressure of about 200kPa to about 2000kPa, a column condenser temperature of about 20 ℃ to about 120 ℃, and a reboiler temperature of about 90 ℃ to about 200 ℃. In such embodiments, the reflux ratio of column 65 may vary from about 1 to about 50, and the column loading may be from about 75kcal/kg feed to 125kcal/kg feed.

Method and system for producing silane by disproportionation of halosilanes

The dividing wall distillation column 65 described above (which may also be referred to as a "first" distillation column) may be incorporated into a system for producing silane by disproportionation of halosilane, such as the exemplary system shown in fig. 2. The disproportionation system 76 may include any unit operation conventional in disproportionation operations as understood by those skilled in the art, particularly equipment suitable for converting trihalosilanes to silanes as disclosed in U.S. patent No.4,676,967, which is incorporated herein by reference for all relevant and compatible purposes. Typically, the disproportionation process includes a dihalosilane as the first halosilane, a trihalosilane as the second halosilane, a silicon tetrahalide as the third halosilane, and optionally further includes a monohalosilane as the fourth halosilane.

The disproportionation system 76 includes a dividing wall distillation column 65, a first disproportionation reactor 50, a second disproportionation reactor 52, and a silane separation system 56 (e.g., a second distillation column). Halosilane-containing stream 26 introduced into distillation column 65 includes system feed 20, dihalosilane and silicon tetrahalide 94 produced by first disproportionation reactor 50 described below, and trihalosilane-containing fraction 97 discharged from silane separation system 56 described below to separate dihalosilane (and optionally monohalosilane, if present in the feed) into overhead fraction 69, trihalosilane into sidedraw fraction 9, and silicon tetrahalide into bottoms fraction 57. The system feed stream 20, which forms part of the halosilane-containing stream 26 introduced into the distillation column, includes trihalosilane and silicon tetrahalide, and may also include other halosilanes (e.g., dihalosilane or monohalosilanes) and various impurities.

The trihalosilane-enriched side-cut 9 produced from distillation column 65 is introduced into a first disproportionation reactor 50 to produce a first disproportionation reactor product stream 94 containing dihalosilane and silicon tetrahalide according to the following reaction

2SiHX₃→SiH₂X₂+SiX₄ (1)

Wherein X is halogen. One or more catalysts may be included in reactor 60 to promote reaction (1), including, for example, a polymeric resin (e.g., AMBERLYST a 21). The first disproportionation reactor product stream 94 containing dihalosilane, silicon tetrahalide, and unreacted trihalosilane is recycled back to the distillation column 65.

The dihalosilane-enriched overhead fraction 69 produced by distillation column 65 is introduced into second disproportionation reactor 52 to produce a second disproportionation reactor product stream 98 containing trihalosilane and silane according to the reaction shown below,

2SiH₂X₂→SiH₃X+SiHX₃ (2)

2SiH₃X→SiH₂X₂+SiH₄ (3)

in this regard, it should be understood that reactions (1) - (3) do not represent a complete set of reactions that may occur during disproportionation, and that other reactions may occur such that other intermediates and byproducts are produced, including, for example, monohalosilanes. One or more catalysts may be included within reactor 52 to facilitate this reaction, including, for example, a polymeric resin (e.g., AMBERLYST a 21).

The second disproportionation reactor product stream 98 is introduced into the silane separation system 56 to separate silane and trihalosilane. In some embodiments, the silane separation system 56 is a second distillation column (typically without separation). The second distillation column separates silane into an overhead fraction 29 and trihalosilane into a bottoms fraction 97. The second distillation column can be operated at a pressure of at least about 10 bar (e.g., about 10 bar to about 35 bar or about 20 bar to about 25 bar) and at an overhead pressure of at least about-75 ℃, at least about-50 ℃, or at least about-25 ℃ (e.g., about-75 ℃ to about 100 ℃ or about-50 ℃ to about 50 ℃). The trihalosilane-containing bottoms fraction 97 is introduced into the dividing wall distillation column 65. In this regard, it should be understood that systems and methods for producing silane silanes other than that shown in FIG. 2, including the rearrangement, addition, or elimination of reactors and/or columns shown therein, may be used without limitation. Further, in some embodiments, the systems of the present disclosure include various process mixtures and/or various inlet and outlet process streams as described herein within units of the system (e.g., process mixtures and/or process streams that are present when the system is operating at steady state).

It should be understood that although the substantially closed-loop processes and systems described herein are generally described with reference to the production and thermal decomposition of silane, the disproportionation system 76 may be modified to produce dihalosilane rather than silane. For example, the system 76 shown in FIG. 2 may be operated without the second disproportionation reactor and silane separation system 56. The dihalosilane-containing overhead fraction 69 produced by dividing wall distillation column 65 can be vaporized and introduced into silane reactor 30 (fig. 3) to produce polycrystalline silicon 70 as described above.

Method for producing polycrystalline silicon

In some embodiments of the present disclosure, polysilicon is produced using silane 29 (or dihalosilane as described above) produced in a disproportionation system 76 containing a dividing wall distillation column 65. Silane 29 produced by the disproportionation system 76 (or dichlorosilane as described above) is introduced into the silane reactor 30 (fig. 3) to produce polycrystalline silicon, which may be withdrawn from the reactor 30 as a polycrystalline silicon product 70. The reactor 30 may be a fluidized bed reactor in which silane fluidizes growing silicon seed particles to produce polycrystalline silicon, or may be a Siemens reactor in which polycrystalline silicon is deposited onto electrically heated silicon rods in a bell jar reactor. Polycrystalline silicon 70 is produced from silane 29 according to the following pyrolysis reaction and the formation of hydrogen byproduct

SiH₄→Si+2H₂ (5)

In embodiments where the reactor 30 is a fluidized bed reactor, the polycrystalline silicon 70 may be intermittently or continuously withdrawn from the reactor 30 via a product withdrawal line, and an effluent gas 75 comprising hydrogen, unreacted silane (or dihalosilane), and silicon dust may be withdrawn from an upper portion of the reactor 30. The vent gas 75 may contain up to about 15 weight percent silicon dust and up to about 5 weight percent unreacted silane. A particulate separator (not shown) may be used to remove dust from the exhaust gas. Suitable particulate separators include, for example, bag filters, cyclones, and liquid scrubbers. The silicon dust can be recycled for use in reactor 30 as disclosed in U.S. patent publication No.2009/0324819, which is incorporated herein by reference for all relevant and compatible purposes. Alternatively, the silicon dust may be discarded or even collected as a product when it contains a low metal impurity content (e.g., when the particulate separator system includes a ceramic, quartz, or silicon carbide surface). The de-dusted vent gas may be compressed and a portion of the vent gas 75 may be reintroduced into the reactor 30 as a carrier for the silane 29.

In embodiments where reactor 30 is a fluidized bed reactor, reactor 30 can be operated at an overhead pressure of about 3 bar to about 15 bar, and the feed gas can be preheated to a temperature of at least about 200 ℃ (e.g., about 200 ℃ to about 500 ℃, or about 200 ℃ to about 350 ℃). Reactor 30 can be maintained at a temperature of at least about 600 ℃ (e.g., 600 ℃ to about 900 ℃ or about 600 ℃ to about 750 ℃) using external heating means, such as induction heating or using resistive heating elements. The velocity of the gas through the fluidized bed reactor 30 can generally be maintained at a velocity of from about 1 to about 8 times the minimum fluidization velocity necessary to fluidize the particles within the fluidized bed. The average diameter of the particulate polysilicon withdrawn from the reactor 30 can be at least about 600 microns (e.g., about 600 microns to about 1500 microns or about 800 microns to about 1200 microns). The silicon seed particles introduced into the reactor may have an average diameter of less than about 600 microns (e.g., about 100 microns to about 600 microns).

Quench gas may be introduced into the reactor 30 (e.g., in the freeboard zone of the reactor) to reduce the temperature of the effluent gas 75 prior to its discharge from the reactor to inhibit the formation of silica fume. The fluidized bed reactor may include an enclosure in which the inert gas is maintained at a pressure above the pressure of the process gas (e.g., a pressure differential in the range of about 0.005 bar to about 0.2 bar) to ensure that the process gas does not flow through cracks and pores within the reaction chamber. Silane may be introduced into the core region of the reactor as disclosed in U.S. patent publication No.2009/0324479 and U.S. patent publication No.2011/0158888, both of which are incorporated herein by reference for all purposes related and compatible, and a carrier gas (e.g., hydrogen) may be introduced into the peripheral portion of the reactor near the reactor walls to reduce deposition of silicon on the reactor walls. In some embodiments of the present disclosure, the silane conversion in the fluidized bed reactor may be at least about 70%, at least about 80%, at least about 90%, or even at least about 95% (e.g., from about 70% to about 99% or from about 90% to about 99%).

Polycrystalline silicon production using dividing wall distillation column 65 may be incorporated into a closed loop process, such as the process described in U.S. patent publication No.2012/0189527, which is incorporated herein by reference for all relevant and compatible purposes.

All equipment used in the disproportionation system of halosilanes can withstand corrosion including exposure to the environment of the compounds used and produced within the system. Suitable materials of construction are conventional and well known in the art of the present disclosure and include, for example, carbon steel, stainless steel, MONEL alloy, INCONEL alloy, HASTELLOY alloy, nickel, and non-metallic materials such as quartz (i.e., glass) and fluorinated polymers such as TEFLON, KEL-F, VITON, KALREZ, and AFLAS.

It is to be understood that the above-described methods and systems may include more than one of any of the listed units (e.g., reactors, columns, and/or separation units) and that multiple units may be operated in series and/or in parallel without departing from the scope of this disclosure. In this regard, it should also be understood that the methods and systems are exemplary and that these methods and systems may include, without limitation, additional elements that implement additional functionality.

The dividing wall distillation column enables equivalent separation with fewer trays and at lower reboiler loadings than conventional two column systems for separating halosilanes. These advantages can be seen from the following simulation of example 1.

Examples

The process of the present disclosure is further illustrated by the following examples. These examples should not be considered in a limiting sense.