Abstract:
A process for separation of particularly propane, methane or ethane from natural gas includes providing a distillation tower arrangement for separating the heavier components discharged at the bottom from lighter gas discharged at the top with at least two separate towers at different pressures such that the heavier product from the higher pressure tower is expanded and fed to the lower pressure tower. A feed gas containing the components under a first pressure is separated into a first proportion and a second proportion, where neither proportion is zero. The first proportion is fed to the tower. The second proportion is compressed to a pressure higher than the first pressure, heat is extracted from the compressed second proportion to effect condensation thereof, the compressed condensed second proportion is sub-cooled, expanded to the first pressure and supplied after expansion to the distillation tower arrangement at a position thereon adjacent the top of the distillation tower arrangement so as to cause cooling of the materials in the distillation tower arrangement. The method is particularly advantageous in a low pressure supply system in which the lighter gas discharged at the top of the tower arrangement is supplied at a pressure less than 100 psi and more preferably less than 75 psi.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the separation of hydrocarbon gases into components of differing boiling points. The invention relates more specifically to a method and an apparatus especially suited for separating propane, methane or ethane from natural gas. 
     The applicant&#39;s prior U.S. Pat. No. 4,770,683, issued Sep. 13, 1988, describes a process and an apparatus for distillation of two materials of differing boiling points. A process for distillation of two materials of differing boiling points particularly propane, ethane or carbon dioxide from natural gas is described in which the conventional distillation tower is divided into a first tower at higher pressure than a conventional tower and a second tower at lower pressure. Liquid drawn from the first is expanded to the lower pressure through two or more stages with cool extracted at each stage and used to cool gas withdrawn from the top of the first tower to keep the top tray at a required temperature. Gas withdrawn from the second tower is compressed and cooled for return to the first tower as a reflux. The use of the cool from the expanded liquid and the use of the two towers provides an improved thermodynamic efficiency and avoids the use of costly turbo-expanders. 
     In addition, a further arrangement by the present applicant is disclosed in PCT published application WO95/10011 of Apr. 13, 1995. This discloses an improvement to the above patent in which efficiency is enhanced by the provision of a third tower and an arrangement by which additional cool is supplied to the top of the high pressure tower as a reflux. 
     Traditionally natural gas at less than 100 psig has been ignored for lpg recovery. Whenever such gas is processed, it is first compressed to above 300 psig before processing. However the process of the present invention, used for separation of various materials of close boiling points generally uses a distillation tower arrangement. 
     This invention is particularly concerned with separation of heavier components from natural gas. 
     Ethane recovery is similar to lpg recovery in concept except that more energy is required for refrigeration and reflux compression. This process also applies to situations where the low pressure gas is sold at higher pressures but the benefits compared to other processes are much less than that described in the first paragraph where, essentially there are no other processes that are ever considered unless the desired residue gas pressure for the sales pipeline is above 200 psig. The use of this technology for the recovery of ethylene in ethylene plants, will reduce the power requirements and capital cost of the Demethanizer portion of these plants. The above U.S. patent of the applicant was described as being very applicable to the separation of ethane and ethylene. That patent could also be used for the Demethanizer in an Ethylene Plant but it is believed that this patent will be an improvement when combined with that patent. 
     This invention relates to distillation processes for the separation of close boiling point materials. Such a process is used in the extraction of various materials generally using a distillation tower. Examples of such separations are: 
     1. Recovering ethane from natural gas 
     2. Recovering propane from natural gas 
     3. Recovering carbon dioxide from natural gas 
     4. Recovering helium from natural gas 
     5. Rejecting nitrogen from natural gas 
     6. Recovering ethylene in ethylene plants. 
     This patent has optimal advantage when utilised in conjunction with a two tower or multi-tower process described in the above United States patent. It may also be used to advantage with other distillation patents for example the various arrangements described in patents held by the Ortloff Corporation. 
     SUMMARY OF THE INVENTION 
     It is one object of the present invention to provide an improved method for separation of residue gases from natural gas which provides improved efficiencies particularly for processing gases where the residue gases are supplied at low pressure. 
     It is one object of the present invention, therefore, to provide an improved distillation process which obtains as good or better separation recoveries but with an improved thermodynamic efficiency and in many cases reduced equipment cost. It is expected that this patent will improve the Demethanizer portion of ethylene production plants. 
     It is another object of the present invention to provide economical means of recovering ethane and/or lpg from low pressure natural gas that is otherwise ignored for liquid recovery. 
     According to the first aspect of the invention it is provided a method of separating heavier components from natural gas comprising: 
     providing a distillation tower arrangement for separating the heavier components discharged at the bottom of the tower arrangement from lighter gas discharged at the top of the tower arrangement; 
     providing a feed gas containing the components under a first pressure sufficient to supply the gas to the distillation tower arrangement; 
     separating the feed gas into a first proportion and a second proportion, where neither proportion is zero 
     feeding the first proportion to the distillation tower arrangement at a feed position thereon between the top and bottom thereof for separation within the distillation tower arrangement; 
     compressing the second proportion to a pressure higher than the first pressure, extracting heat from the compressed second proportion to effect condensation thereof, sub-cooling the compressed condensed second proportion, expanding the compressed condensed second proportion to the first pressure and supplying the second proportion after expansion to the distillation tower arrangement at a position thereon adjacent the top of the distillation tower arrangement so as to cause cooling of the materials in the distillation tower arrangement. 
     The lighter gas discharged at the top of the tower arrangement is supplied at a pressure which is selected depending the requirement of the supply. In many cases this is a high pressure requirement greater than 100 psig and often of the order of 500 psig. This invention however has particular applicability and advantage when the supply pressure is less than 100 psig thus leading to a low operating pressure. 
     Preferably the second proportion is sub-cooled by cool from the lighter gas. 
     Preferably the second proportion is further sub-cooled by a refrigerant. 
     Preferably the feed gas is dehydrated prior to separation. 
     Preferably the feed gas is dehydrated by a molecular sieve. 
     Preferably the first proportion is cooled by cool from a re-boiler of the tower arrangement. 
     In one example the tower arrangement includes at least two separate towers at different pressures such that the heavier product from the higher pressure tower is expanded and fed to the lower pressure tower. In one arrangement of this example, the lighter gas from the top of the lower pressure tower is fed back to the feed gas for reprocessing. In an alternative arrangement, the gas is flared. 
     Preferably the lighter gas from the top of the tower arrangement is supplied to a low pressure pipe line system having a supply pressure of the order of 75 psi. 
     Preferably the lighter gas from the top of the lower pressure tower is added to the second proportion for processing therewith and supply to the top of the higher pressure tower. 
     Preferably the supply gas is separated from a crude oil supply and wherein the separated heavier components are returned to the crude oil as a supplement thereto. 
     Preferably the separated lighter gas is flared. 
     According to a second aspect of the invention there is provided an apparatus for separating heavier components from natural gas comprising: 
     a distillation tower arrangement for separating the heavier components discharged at the bottom of the tower arrangement from lighter gas discharged at the top of the tower arrangement; 
     a feed gas supply line for a feed gas containing the components under a first pressure sufficient to supply the gas to the distillation tower arrangement; 
     means for separating the feed gas into a first proportion and a second proportion, where neither proportion is zero 
     a supply duct for feeding the first proportion to the distillation tower arrangement at a feed position thereon between the top and bottom thereof for separation within the distillation tower arrangement; 
     a compressor for compressing the second proportion to a pressure higher than the first pressure; 
     means for extracting heat from the compressed second proportion to effect condensation thereof 
     a heat exchanger for sub-cooling the compressed condensed second proportion; 
     means for expanding the compressed condensed second proportion to the first pressure; 
     and a second supply duct for supplying the second proportion after expansion to the distillation tower arrangement at a position thereon adjacent the top of the distillation tower arrangement so as to cause cooling of the materials in the distillation tower arrangement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One embodiment of the invention will be described hereinafter in conjunction with the accompanying drawing in which: 
     FIG. 1 is a schematic illustration of the elements of a first process according to the present invention using the two tower system of the above prior patent which is particularly but not exclusively designed for supplying the residue gas at a low pressure. 
     FIG. 2 is a schematic illustration of the elements of a second process according to the present invention using the a single tower. 
     FIG. 3 is a schematic illustration of the elements of a third process according to the present invention using the two tower system of the above prior patent and which is particularly but not exclusively designed for returning the extracted components to a crude oil processing plant to supplement the crude oil and for flaring the residue gases. 
     FIG. 4 is a sketch indicating the lowest propane plus content recovery. 
     FIG. 5 is a schematic illustration of the elements of a fourth process according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     Turning firstly to FIG. 1 there is shown an arrangement for separating lpg+ products from a feed of natural gas leaving a residue sales gas for sale at low pressure that is less than 100 psig. 
     The arrangement provides a feed supply line  10  which feeds to an inlet separator  11  which acts to separate gas from any incoming liquid. The liquid can be handled in a number of different ways including supply to the free water knock out system of the crude oil processing plant in arrangements where such is available. Alternatively, the liquid can be passed through a dehydrator and fed to the de-ethanizer. 
     The inlet gas from the inlet separator  11  is supplied to an inlet compressor  12  having an after-cooler  13 . The gas is compressed to a sufficient pressure in the compressor  12  so that after the compressor the gas can be dehydrated in a molecular sieve  14  and processed in the lpg recovery plant and then has sufficient pressure for the gas entering the sales pipeline  15 . 
     Prior to entering the dehydrator  14  in the form of a molecular sieve, a further liquid separator  16  is provided for recycling a liquid through a return line  17  having a let down valve  18 . 
     As stated above, the arrangement described herein is particularly designed for low pressure residue gas. However if the desired pipeline pressure in the residue gas is intended to be above 600 psig, it is preferred that compression be added to the residue gas downstream of the recovery plant so that the tower assembly described hereinafter can operate at approximately 400 psig. 
     After the inlet gas is compressed, after-cooled and the liquids extracted in the separator  16 , the gas is dehydrated in the dehydrator  14  which is preferably a molecular sieve as described above or can possibly be a “Dryso”™ process which is a tri-ethylene glycol process. In such an arrangement a sophisticated regeneration system as shown can be provided using extractive distillation to reduce the water content of ethylene glycol. The extracted material from the regeneration system is returned to the feed as indicated in the supply line  20 . 
     Downstream of the dehydrator  14 , there is provided a supply line  21  which is divided into two supply lines  22  and  23  acting to effect a proportional division of the feed in the supply line  21 . Each line includes a flow control valve  22 A and  23 A which are controlled using conventional flow control systems well known to one skilled in the art to maintain the required proportions depending upon the measurement of various parameters of the process. 
     The process further includes a processing tower arrangement generally indicated at  30  including a high pressure tower  31  and a low pressure tower  32 . These two towers are generally as described in the above United States patent and the disclosure of that document is incorporated herein by reference. The two towers each comprise a distillation tower section for effecting separation of the components in the feed so that the high pressure tower section  31  discharges lighter gas components at an upper discharge  33  and heavier components at a lower discharge  34 . The low pressure tower  32  has an upper discharge  35  and a lower discharge  36 . The upper gas discharge  33  provides the residue sales gas  15  while the bottom discharge  36  of the low pressure tower provides the heavier lpg+ product  37 . 
     The first portion of the feed gas divided into the supply line  23  is supplied as a feed to the lower part of the high pressure tower  31 . Prior to supply to the tower arrangement, the gas in the supply line  23  is passed through a heat exchanger R which includes a component  38 A on the line  23  and second component  38 B forming a reboiler for material at the bottom of the low pressure tower component  32 . Thus the heat exchanger R extracts cool into the component  38 A to cool the supply on the line  23  and applies heat to the component  38 B acting as a reboiler to return the material as a side feed to the lower part of the lower pressure component  32 . 
     The supply on the line  23  is further passed through a second heat exchanger S having a first component  39 A and a second component  39 B which again acts to extract cool for the material in the line  23  and acts as a heat supply for a side reboiler on the lower pressure tower component  32 . 
     Gas from the top discharge  35  of the low pressure tower  32  is returned to the feed through a supply duct  40 . Prior to return to the feed, cool is extracted from the return gas in a further heat exchanger  41  and that cool is applied to the feed on the line  23 . 
     Finally a refrigerator unit  42  is used to apply external cooling to the feed prior to injection into the high pressure tower component  31  at a feed position  43 . 
     The second proportion on the line  22  is passed to a compressor system  44  including a compressor  45  and a heat extractor  46 . The second proportion of the gas is compressed to a pressure in the range 500 to 1400 psig so that it can be cooled and condensed and used for injecting into the tower arrangement as a cooling top supply. 
     The prior patent and the prior published application of the present inventor disclose the use of liquid injection at the top of the high pressure tower for maintaining a cool temperature in the high pressure tower. In the prior application this is termed as “reflux”. However in the present invention the compressed material includes a component of the original supply from the feed  10  and in addition includes a component from the discharge gas from the discharge outlet  35  of the low pressure tower. 
     The second proportion is thus compressed in the compressor system  44  and cooled by the cooling arrangement  46 . It is then passed through a heat exchanger  47  which extracts cool from the residue gas and supply line  48 . Further cooling is effected in a further heat exchanger L which includes first component  49 A on the line  22  and a second component  49 B extracting cool from the product  37 . Further refrigeration cooler  50  is provided using external refrigerant. Downstream of the refrigerator  50  is provided a further heat exchanger  51  extracting cool from the residue gas on the supply line  48 . 
     After the passage through the heat exchangers, the second proportion of the feed is usually totally condensed and sub-cool is provided by the heat exchanger  51 . The second proportion of the feed is then passed through a let down valve  52  before injection into the high pressure tower  31  at a feed entry  53 . 
     The compression of the second proportion only provides significant advantages in economical recoveries. In the past, all processes considered compressing all of the inlet gas to the high pressure before processing. In the present invention only the proportion in the line  22  is compressed thus avoiding the necessary power requirements for compression and also reducing capital cost. 
     In some situations a phase envelope of the second proportion gas has to be considered so that an optimum pressure is chosen which provides optimum cool recovery by the gas/liquid and thus the most economical system. The above optimum cool recovery is usually at a pressure that is close to the maximum cool recovery. 
     Turning now to FIG. 2 there is shown substantially the same arrangement having the same first and second proportions divided into the first and second feed systems. In this arrangement, however, the two tower process is replaced by a more conventional single tower process as indicated in the single tower  55  as is well known from the processes of Ortloff. 
     Turning now to FIG. 3, there is shown a similar system to that of FIG. 1 which utilises the two tower process of FIG. 1 including the high pressure tower  31  and the low pressure tower  32 . 
     This process operates similar to that previously described and is used for enhancing or supplementing crude oil processing from a crude oil supply  60 . In this arrangement the residue gas is supplied to a flare  61  so that it is effectively at zero pressure. 
     The crude oil is supplied to a separator  62  where the liquid is withdrawn on a line  63  and supplied to a dehydrator and stabilisation system schematically indicated at  64 . This can be of the type known as a “feed water knockout” but other processing systems can be used. From the processing system the crude supply is indicated at  65 . 
     The discharge gas from the top of the high pressure tower is discharged on a line  66  and is fed to the flare  61 . The discharged liquid at the bottom of the high pressure tower is fed through a supply line  67  and a let down valve  68  to provide a feed to the top of the low pressure tower  32 . 
     The gas separated from the crude supply in the separator  62  is supplied to a molecular sieve  69  for dehydration of the gas. The gas is passed through the first heat exchanger  70  and a refrigeration unit  71 . The proportional separation is effected between the lines  72  from the refrigerator  71  so that the first portion is supplied on the line  73  to a feed location  74  on the high pressure tower. The second proportion is fed on a line  74  through a let down valve  75  so that the feed is lowered in pressure to the same pressure as the discharge  76  from the top of the low pressure tower. The feed on the line  74  is thus added to the gas discharge from the outlet  76  and this combined flow is passed through a line  78  to an inlet  79  at the top of the high pressure tower  31 . The gas in the line  78  is passed through a two-stage compressor including compressor components  80  and  81  and cooling components  82  and  83 . 
     A heat exchanger R including a first component  84 A and a second component  84 B extracts cool from the reboiler at the bottom of the low pressure tower  32 . Further heat exchangers  85 ,  86 ,  87  and  88  act to extract cool from the discharge gas from the discharge  66 . A further heat exchanger A includes a component  89 A and a second component  89 B so as to extract cool from the gas upstream of the compressor components. A refrigerant system  90  corresponds to the refrigeration system  50  of FIG. 1. A let down valve  91  corresponds to the let down valve  52 . The compressed, condensed and sub-cooled supply is expanded back to the pressure of the high pressure tower and is injected as a reflux-cooled supply into the top of the high pressure tower previously described. 
     The discharge from the top of the high pressure tower through the line  66  is divided into two sections passing to the flare  61 . One proportion passes through the heat exchangers  85 ,  86 ,  87  and  88 . A second proportion passes through the heat exchanger  70 , the heat exchanger  85  and to the molecular sieve regeneration system generally indicated at  92 . Two valves  93  and  94  let down the pressure from the pressure of the high pressure tower to the flare pressure of approximately zero. 
     Again therefore in the arrangement of FIG. 3, the second proportion of the divided supply is compressed for injection into the high pressure tower at the cooling feed at the upper end. The second part of the feed on the line  73  is not compressed thus providing significant processing economies. 
     The liquid from the bottom of the low pressure tower extracted from the otherwise waste or flare gas is returned through a line  95  as a supplement to the feed thus enhancing the supply crude  65 . 
     When the residue gas goes to flare, recovery of C 3 + is similar in concept to the arrangement shown in FIG. 3 for the recovery of C 4 +. There will be some change in heat exchanger arrangement and the temperatures will be much colder. Similarly, the recovery of C 2 + will also be similar but colder with a different heat exchanger arrangement. One big difference in the arrangement for C 2 + and C 3 + in comparison with FIG. 3 is that these will be produced as a separate product rather than recycling the liquid into the inlet crude stream. 
     The arrangement of FIG. 3 could also be modified so that the C 4 + could also be recovered as a separate product. However normally if a separate product is desired, recovery of C 3 + is desirable also. The effects of recycling the recovered C 4 + to the inlet crude stream is to reduce the content of C 3  and allows the components in the stabilised crude. 
     When treating gases at the low pressure similar to that of FIG. 3, there is some incentive to mount the high pressure tower above the low pressure tower so that there does not have to be such a large pressure drop between the towers. This raises the suction pressure for the compressor  80  but also raise the operating temperature of the reboiler  84 B. 
     All of these arrangements have the advantage that the overhead from the low pressure tower is recycled to the high pressure tower. This provides an effective reflux supply for the high pressure tower. For example in the case of propane recovery, the low pressure tower overhead is rich in ethane which makes very good reflux for separating propane from natural gas. 
     The flow split in the feed to the bottom of the high pressure tower does not need to be controlled by a ratio flow control. The split stream of dehydrated feed to the recycle compressor is controlled to maintain the suction pressure of that compressor. Thus the compressor  80  at constant speed will deliver a constant flow rate to the high pressure tower thus compensating for the volume of gas exiting from the discharge  76  by taking a portion of the feed on the line  72 . This is also has the effect that when the plant is turned down in flow rate or composition, the percentage recovery of liquid product will increase. 
     The power requirement for the Feed Compressor is minimized since the gas is only compressed as much as required considering pressure drops in the dehydrator and lpg recovery process. When the desired Residue Gas pressure is the same or less than the Feed Pressure, very little Feed Compression is required, resulting in much less power requirement for this process than any other process. 
     Typical propane recoveries from this process are 90% using the two towers as shown in FIG.  1 . When this process is configured with a three tower process (our United States patents 1988 and 1997 patents), 95% propane recoveries can be easily achieved. 
     In addition to saving energy, the lower power requirement results in a smaller compressor installation and a reduced capital cost compared to other processes. 
     FIG. 4 is a sketch indicating the lowest propane plus content recovery. Note that processes using the Reflux Compressor can recover lpg from much lower Feed Pressures as long as the lpg concentration in the gas is high enough. The Reflux Compressor adds a considerable number of applications for economical lpg recovery compared to other processes that do not have a Reflux Compressor. 
     Turning now to FIG. 5 there is shown a further modified arrangement in which there is a three tower system including towers  100 ,  101  and  102 . In this arrangement the feed is again split to provide a proportional flow at the location  103  and a portion of the feed is compressed through the system  104  as previously described and fed at the reflux location  105  to the tower  100 . In this arrangement the gas from the top of the second tower  101  is sent to flare  106 . Also in this arrangement the gas from the top of the tower  100  is sent to flair  7 . In this arrangement there is provided a water separation at a condenser  108  which is located upstream of a dehydrator  109 . 
     Note that the dehydrator  109  is located after 90° of the water has been condensed out of the gas at 38 F. The refrigerant temperature in the chiller is 33 F so there is no danger of freezing and the chiller assures a maximum temperature into the dehydrator. Location of the dehydrator after most of the water has been removed greatly reduces it&#39;s size and regeneration heat requirement. Note also that the overhead from the de-ethanizer is not recycled, it is sent to flare along with the other residue gas from the process. Heat for the de-ethanizer reboiler is obtained from the process as we normally do in our other designs. This means that heating medium is not required for this tower, but it is required for the debutanizer. There may be situations where having this extra tower is not warranted, trays could be added below the bottom feed on the gas fractionator and the reboiler added to that tower. That would be the conventional Ortloff patent. 
     All the metallurgy is carbon steel except for the top feed to the gas fractionator. For this design, it would likely be wise to have the top reflux meet the gas fractionator overhead in a small stainless vessel having one or two trays. This stainless steel vessel would be mounted on top of the main gas fractionator column which would have a −50 F design temperature so could be Charpy-tested carbon steel. 
     Since various modifications can be made in my invention as herein above described, and many apparently widely different embodiments of same made within the spirit and scope of the claims without departing from such spirit and scope, it is intended that all matter contained in the accompanying specification shall be interpreted as illustrative only and not in a limiting sense.