Method to recover LPG and condensates from refineries fuel gas streams

Abstract

A method to recover olefins and C.sub.2.sup.+ fractions from refineries gas streams. The traditional recovery methods employed at refineries are absorption with solvents and cryogenic technology using compression and expansion aided by external refrigeration systems. In contrast to known methods, there is provided first a pre-cooling heat exchanger on a feed line feeding the gas stream to a in-line mixer, secondly by injecting and mixing a stream of LNG to condense the C.sub.2.sup.+ fractions upstream of the fractionator. The temperature of the gas stream entering the fractionator is monitored downstream of the in-line mixer. A LNG stream is temperature controlled to flow through the injection inlet and mix with the feed gas at a temperature which results in the condensation of the C.sub.2.sup.+ fractions before entering the fractionator. A LNG reflux stream is temperature controlled to maintain fractionator overhead temperature. The fractionator bottoms temperature is controlled by a circulating reboiler stream.

Claims

1. A method of recovering C.sub.2.sup.+ fractions from a refinery gas stream, the method comprising the steps of: obtaining the refinery gas stream, the refinery gas stream being in a vapor phase and comprising hydrogen, methane, C.sub.2+ fractions, and olefins; injecting the refinery gas stream into a fractionator having a plurality of trays that enable heat exchange and fractionation within the fractionator, the fractionator having an overhead temperature and a bottoms temperature; using a liquid pump, pressurizing liquid natural gas (LNG) from a supplemental source of methane that is separate and distinct from the refinery gas stream, the liquid pump pressurizing LNG from a storage pressure of the supplemental source of methane to a refinery gas line pressure, the supplemental source of methane being in a liquid form and comprising a cryogenic energy source relative to the refinery gas stream; injecting an injection stream of the LNG at the refinery gas line pressure into the refinery gas stream via an inline gas mixer upstream of the fractionator such that the injection stream increases a methane content in the refinery gas stream, the injection of the injection stream being controlled to condense the C.sub.2.sup.+ fractions present in the refinery gas stream entering the fractionator; feeding a reflux stream of the LNG from the liquid pump to a top tray of the fractionator, the reflux stream being controlled to control the overhead temperature; and controlling the bottoms temperature by circulating a fluid stream between a bottom tray of the fractionator and a reboiler.

2. The method of claim 1, wherein the injection stream of the LNG is injected from the supplemental source of methane using a cryogenic feed pump.

3. The method of claim 1, wherein the bottoms temperature is controlled to achieve a predetermined fractionation of the C.sub.2.sup.+ fractions.

4. The method of claim 1, wherein the injection of the LNG into the refinery gas stream is controlled to maintain the refinery gas stream at a constant temperature immediately upstream of the fractionator.

5. The method of claim 1, further comprising a step of recovering H.sub.2 from the refinery gas stream by: connecting a fractionator overhead line between a top of the fractionator and a separator, the fractionator overhead line comprising a gas heat exchanger and a second inline gas mixer upstream of the separator; removing a gas stream from the top of the fractionator through the fractionator overhead line, the fractionator overhead line carrying the gas stream to the separator; using the heat exchanger, pre-cooling the gas stream upstream of the second inline gas mixer; and using the second inline gas mixer, injecting a further stream of LNG from the the supplemental source of methane into the gas stream upstream of the separator, the injection of the further stream of LNG being controlled to condense and separate the methane and the C.sub.2+ fractions from the gas stream at the separator to obtain the H.sub.2.

6. The method of claim 1, wherein the refinery gas stream is pre-cooled upstream of the inline gas mixer in a heat exchanger.

7. The method of claim 6, wherein the refinery gas stream is pre-cooled in the heat exchanger by a vapor fraction of the fractionator.

8. The method of claim 6, wherein the reboiler comprises the heat exchanger.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other features will become more apparent from the following description in which reference is made to the appended drawings, the drawings are for the purpose of illustration only and are not intended to in any way limit the scope of the invention to the particular embodiment or embodiments shown, wherein:

(2) FIG. 1 is a schematic diagram of a gas liquids recovery facility equipped with a heat exchangers, a in-line mixer, a LNG storage bullet, pumps and a fractionator. The LNG is supplied to two locations: the in-line mixer and a reflux stream to the fractionator.

(3) FIG. 2 is a schematic diagram of a gas liquids recovery facility equipped with a variation in the process whereas LNG is added only as a reflux stream.

(4) FIG. 3 is a schematic diagram of a gas liquids recovery facility equipped with a variation in the process whereas LNG is added only to the feed gas and mixed before fractionation.

(5) FIG. 4 is a schematic diagram of a gas liquids recovery facility with a variation in the process to recover liquids and also hydrogen from refinery fuel gas streams.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(6) The method will now be described with reference to FIG. 1.

(7) As set forth above, this method was developed with a view for cryogenic recovery of C.sub.2.sup.+ fractions from typical refinery fuel gas streams. The description of application of the method should, therefore, be considered as an example.

(8) Referring to FIG. 1, a refinery feed gas stream 1 is cooled to ambient temperature in a fin-fan air heat exchanger 2. The ambient cooled refinery feed gas stream 3 enters a heat exchanger (cold box) 4. The heat exchanger (cold box) 4 houses the reboiler coils 10 and the overhead condenser coils 13. The stream 3 is first pre-cooled by a circulating reboiler stream 9 in a counter-current flow through coil 10, this counter-current heat exchange provides the heat required to fractionate the bottoms stream while cooling the inlet refinery gas stream. The reboiler re-circulation stream 9 feed rate is controlled to meet fractionator bottoms needs. The refinery feed gas stream is further cooled by a stripped fractionator overhead stream 12 in a counter-current flow through coil 13. This counter current heat exchange substantially cools the refinery feed gas stream. The pre-cooled refinery feed gas stream 5 exits heat exchanger (cold box) 4 and flows through in-line mixer 6 where LNG stream 21 is added and mixed as required to meet a selected stream temperature in stream 7. The two-phase temperature controlled stream 7 enters fractionator 8 to produce a vapour and a liquid stream. In this mode of operation, the fractionator 8 overhead vapour stream 12 is primarily a C.sub.1.sup.− fraction. The fractionator 8 overhead temperature is controlled by a LNG reflux stream 22. The trays in the fractionator 8 provide additional fractionation and heat exchange thus facilitating the separation. The bottoms temperature in fractionator 8 is controlled by a circulating liquid stream 9 that gains heat through coil 10 in heat exchanger (cold box) 4. The heated circulating bottoms stream 11 is returned to the upper bottom section of fractionator 8 to be stripped of its light fraction. The fractionated liquid stream 16 is primarily a C.sub.2.sup.+ fraction. It exits fractionator 8 as its bottoms stream for further fractionation ie: a de-ethanizer, de-propanizer, etc.

(9) The refrigerant used in the process is LNG which is stored in bullet 17. The LNG is added to the process through LNG feed line 18 to LNG pump 19. The pressurized LNG stream 20 supplies LNG through stream 21 to in-line mixer 6. The LNG stream 21 flowrate is controlled to meet a selected two-phase stream 7 temperature. Stream 21 is added and mixed with pre-cooled refinery gas stream 5 at in-line mixer 6 to produce a desired temperature two-phase stream 7. The LNG pressurized stream 21 also supplies LNG to reflux stream 22 that enters the top tray in fractionator 8. LNG reflux stream 22 controls the temperature at the top of fractionator 8.

(10) A main feature of the process is the simplicity of the process which eliminates the use of compression and expansion and or external refrigeration systems. Another feature is the flexibility of the process to meet various operating conditions since only LNG is added on demand to meet process operations parameters. The process also provides for a significant savings in energy when compared to other processes since no compression or external refrigeration facilities are employed as in conventional cryogenic processes. The proposed process can be applied at any refinery fuel gas plant size.

(11) Referring to FIG. 2, the main difference from FIG. 1 is the removal of in-line mixer 6. In this process mode, LNG is added only as a reflux stream to the top tray of fractionator 8. The temperature of the refinery feed gas stream 5 into fractionator 8 is fully dependent on the pre-cooling at heat exchanger (cold box) 4. The operation of the fractionator 8 bottoms is controlled by a circulating reboiler flowrate stream 9, through coil 10. The heated stream 11 flows back to the upper bottom section of fractionator 8, just as in FIG. 1. The temperature at the top of fractionator 8 is controlled by a LNG reflux stream 22. The trays in the fractionator 8 provide additional fractionation and heat exchange, thus facilitating the separation. This process orientation provides an alternative method to fractionate refinery feed gas at albeit less efficiency than when using an in-line mixer 6 as shown in FIG. 1.

(12) Referring to FIG. 3, the main difference from FIGS. 1 and 2 is the removal of the reflux stream to the top of fractionator 8 and the removal of a circulating reboiler stream from the bottoms of fractionator 8. In this mode of operation, fractionator 8 becomes a simple gas/liquid separator where both vapour and liquid streams are not fractionated. This simple mode of operation allows for the recovery of LPG and reduction of the dew point in refinery fuel gas for combustion.

(13) Referring to FIG. 4, the main difference from FIGS. 1-3 is the addition of an heat exchanger (cold box) 23 that houses coils 24, 32, and 36, in-line mixer 27, LNG addition line 26, and separator 29. In this process mode, LNG is also supplied through in-line mixer 27 to condense the C.sub.2.sup.− fractions and separate it from the H.sub.2 fraction in vessel 29. The objective is to recover an H.sub.2 rich stream 31 that can be re-used in the refinery or sent to a PSA unit for further purification and use in the refinery. The addition of heat exchanger (cold box) 23 provides the ability to recover the cold energy from streams 30 and 31 and transfer this cold energy to stream 12 through coils 32, 36, and 24. The energy required to cool stream 25 is provided by adding LNG to mixer 27 through stream 26. The now-cooled stream 28 enters separator 29 to separate the H.sub.2 fraction from the C.sub.1.sup.+ fractions. The condensed fraction stream 30 leaves separator 29 and enters heat exchanger 23 giving up some of its cold energy through coil 36, continuing onto heat exchanger 4, giving up its remaining cold energy through coil 37, and through stream 38 to fuel gas header 15. The gaseous fraction stream 31 leaves separator 29 and enters heat exchanger 23 giving up some of its cold energy through coil 32, continuing onto heat exchanger 4 giving up its remaining cold energy through coil 34, and to hydrogen recovery header 35. This process orientation provides an alternative method to recover and fractionate valuable components in the refinery feed gas, primarily; H.sub.2, C.sub.2.sup.+ and C.sub.2.sup.− fractions, the main difference being the additional recovery of H.sub.2.

(14) In this patent document, the word “comprising” is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements.

(15) The following claims are to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, and what can be obviously substituted. The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.