CN1388351A - 氮气排除方法 - Google Patents

氮气排除方法 Download PDF

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CN1388351A
CN1388351A CN02125181.9A CN02125181A CN1388351A CN 1388351 A CN1388351 A CN 1388351A CN 02125181 A CN02125181 A CN 02125181A CN 1388351 A CN1388351 A CN 1388351A
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J·D·奥基
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Abstract

从包含甲烷和氮气的原料气流中排除氮气以形成甲烷产品的方法,包括在主热交换器4中冷却原料气流,并在包括一高压精馏塔14和一低压精馏塔16的双精馏塔12中精馏冷却了的原料气流。将一气流从低压精馏塔16循环到高压精馏塔14。循环流的第一部分在压缩机48中被压缩,在一冷凝－再沸器中被冷却、液化,并被引入高压精馏塔14。循环流的第二部分也在压缩机48中被压缩,在主热交换器4中被冷却、液化或通过膨胀而被液化,并以液态形式引入双精馏塔12。两部分循环气的冷却至少有部分是在主热交换器4中进行的。用泵42将甲烷产品以液态形式从低压精馏塔16排出。

Description

氮气排除方法
发明领域
本发明涉及一种从包含甲烷和氮气的原料气流中排除氮气以形成甲烷产品的方法。
技术背景
从地下储气层中生产天然气是公知的。天然气常常被氮气污染。氮气可能部分天然地或完全天然地存在，和/或可被注入到储气层中作为增强油采收率(EOR)或增强气体采收率(EGR)操作的一部分。
US-A-4415345公开了在一个于深冷温度下操作的双精馏塔中从甲烷中排除氮气的方法。双精馏塔包括一个高压精馏塔，一个低压精馏塔，和一个将高压精馏塔的顶部与低压精馏塔的一个区域(通常是底部)布置成间接热交换关系的冷凝-再沸器。在US-A-4415345的方法中，高压下的氮气和甲烷混合流被冷却到适合精馏分离的温度。至少有一部分混合流通过流经与低压精馏塔底部相连的冷凝-再沸器而被液化。因而该塔中分离出的一部分底部液体馏分被再沸，产生一种穿过该塔的向上蒸汽流。剩余的液体被作为供入低压精馏塔的原料，在那里，较纯的液体甲烷产品以底部馏分的形式被分离出来。将所得的液体甲烷流从低压塔中抽出并用泵升压。液体甲烷通过热交换被加热到大约环境温度并因此而汽化。为了增强适合于双精馏塔的液氮回流，从低压精馏塔的顶部抽出氮气流，在一个压缩机中压缩到高压精馏塔的操作压力，并与从高压精馏塔顶部流出的氮气流结合，流入与低压精馏塔热连接的冷凝-再沸器。结果，压缩的氮气被冷凝。一部分冷凝液返回低压精馏塔，而另一部分返回高压精馏塔。流体从低压精馏塔经压缩机到高压精馏塔的流动起到热泵的作用。一个热交换器被结合到该热泵中以便除去压缩热。该热交换器区别于泵送的甲烷产品流被加热的那些。
在与新引入的原料气流进行热交换时发生的液体甲烷产品流汽化加强了该分离方法的低热力学效率问题。本发明的目的是提供一种能分离原料气体混合物并减轻热力学效率低的问题的方法。
发明内容
本发明提供了一种从包含甲烷和氮气的原料气流中排除氮气以形成甲烷产品的方法，包括在一主热交换器中冷却原料气流，在一个包括一高压精馏塔、一低压精馏塔、和一个将高压精馏塔与低压精馏塔布置成热交换关系的冷凝-再沸器的双精馏塔中精馏被冷却的原料气流，将一气流从低压精馏塔循环到高压精馏塔，循环气流的第一部分在冷凝-再沸器中被压缩、冷却、液化，并且至少部分引入到高压精馏塔，从低压精馏塔排出液态的甲烷产品流，提高该液体甲烷产品流的压力，并且在主热交换器中汽化至少部分液体甲烷产品流，其特征在于循环气流的第二部分在主热交换器中被压缩、冷却、液化或通过膨胀而被液化，并且以液态被引入双精馏塔，其特征还在于，两部分循环气体的冷却至少有部分是在主热交换器中进行的。
如果其压力为次临界压力，循环流的第二部分优选在主热交换器中液化。如果其压力等于或高于临界压力，循环流的第二部分优选在主热交换器中被冷却到足够低的温度，使它在通过膨胀而减压到高压精馏塔的操作压力时被液化。
通过在主热交换器中汽化甲烷产品和冷却循环流，与US-A-4415345中阐述的现有方法相比，热交换器中待加热流体的温度-焓图可能更接近热交换器中待冷却流体的。结果，热力学效率低的问题减轻了。
优选地，从循环气流压缩部位上游的循环气流中取出一排出流，并从本发明的方法中除去。优选地，有一个可用来控制被压缩的循环流的量的流量控制阀或其他装置。
形成循环流的第一和第二部分有多种不同的方案。在第一种方案中，所有的循环气在同一压缩机中被压缩到相同的压力。循环气被部分液化，一般采用从超临界压力膨胀的方法，并将液体与剩余蒸汽的混合流引入到高压精馏塔的顶部区域。在第二种方案中，使用一个单独的、多段的压缩机。循环流的第一部分通过压缩机的每一段，同时将循环流的第二部分从压缩机的上游段中取出。第二种方案与第一种方案相比优点在于减少了压缩循环气的工作。也可使用分离的压缩机去压缩循环流的第一和第二部分。
优选将循环流的第二部分全部引入到其液化部位下游的高压精馏塔中。
优选地，为了达到最佳的热力学效率，将新引入原料气流的一部分在主热交换器中液化。如果需要，得到的部分液化的原料流可以经受相分离，将得到的蒸汽相引入高压精馏塔的底部区域，并且将至少一部分液相引入高压精馏塔的中间物质交换区域。优选地，将剩余的液相引入低压精馏塔中的中间物质交换区域。或者，可以将所有的液相通入高压精馏塔的中间物质交换区域，并从高压精馏塔的中间区域取出一液流并引入低压精馏塔。
加压的液体甲烷产品流优选在位于主热交换器中其汽化部位上游的另外一个热交换器中加热，而不被汽化。
优选地，将高压精馏塔中分离出的所有底部馏分从中取出，并送到低压精馏塔。因此高压精馏塔中的这些馏分没有再沸。
附图简述
现在将参考附图通过实施例来阐述本发明的方法，附图是一氮气排除车间的流程示意图。
流程图不是按比例绘制的。
实施本发明的具体方式
用不构成本发明一部分的已知方法从地下储油层或储气层中采收天然气或气态的氮气-甲烷混合气流。该气流的采收压力一般为40bar，并含有10-15％(体积)的氮气。气流可经受初步处理(未示出)以从中除去所有的硫化氢或其他含硫杂质。这样净化天然气是本技术领域中公知的，在此无需更详细地提及。在除去所有这些硫化氢杂质后，高压的甲烷-氮气流一般仍含有水蒸气杂质。水蒸气通过流经一个净化装置2而被除去。净化装置2优选包括多个吸附容器，吸附容器包含能从原料气流中选择性地吸附水蒸气的吸附剂。这种净化装置通常在变压吸附或循环变温吸附中操作，后者通常是首选。如果原料气流还含有二氧化碳杂质，净化装置可另外包含一种对二氧化碳具有选择性的吸附剂，以便除去二氧化碳。
目前基本上由氮气和甲烷组成的所得净化原料气流从主热交换器4的热端6流到其冷端8而通过该主热交换器。主热交换器4包括多个热交换块，优选连接在一起形成一个单一装置。在主热交换器4的下游，原料气流经节流阀23而被膨胀并进入分相器10。取决于原料气流的压力，原料气流或者在主热交换器4中被液化，或者在通过节流阀23而被膨胀时液化。取决于原料气流的组成，通常至少有75％(摩尔)的原料气流被液化。因此，蒸汽流减少了，因而可以使用相比其他需求而言直径更小的高压精馏塔。使蒸汽与分相器10中的液体分离。一蒸汽相流从分相器10顶部经入口20流入高压精馏塔14的底部区域，高压精馏塔14构成双精馏塔12的一部分，双精馏塔12还具有低压精馏塔16以及一个将高压精馏塔14的顶部与低压精馏塔16的底部热连接的冷凝-再沸器18。一液相流从分相器10底部通过另一入口22流入高压精馏塔14的中间物质交换区。通常，原料气流在大大超出高压精馏塔14的操作压力的压力下进入和离开净化装置2。结果，原料流因通过了节流阀23而发生制冷。这种制冷满足了本发明方法的大部分制冷需求，因此一般不需要为此提供任何汽轮式膨胀机。
原料混合气在高压精馏塔14中被分离成为汽化的氮气顶部馏分和液态的富甲烷底部馏分。将富甲烷底部馏分的流体从高压精馏塔14经底部出口24抽出，并通过流经另外一个热交换器26而被低温冷却。得到的经低温冷却的富甲烷液体流流经节流阀28并被引入低压精馏塔16的一个中间物质交换区。另外，将包含甲烷和氮气的液体流从高压精馏塔14的中间物质交换区经出口30抽出，并通过流经另外的热交换器26而被低温冷却，经过节流阀32，并引入到低压精馏塔16的第二中间物质交换区，此区位于第一中间物质交换区之上。
包含甲烷和氮气的流体在低压精馏塔16中被分离，以便形成顶部的氮气汽化馏分和底部的液体甲烷产品馏分。将底部馏分流体经出口40从低压精馏塔16中抽出，并操作泵42使压力升高。使得到的加压液体甲烷产品流穿过另外的热交换器26，穿过方向与在其中被低温冷却的流体呈逆流。给液体甲烷产品流加压的作用是将其压力升高到其饱和压力以上。这样，实际上，当加压液体甲烷产品流进入另外的热交换器26时是处于低温冷却状态。在另外的热交换器26中它被升温以解除低温冷却状态。优选的是在另外的热交换器26中不发生液体甲烷产品流的汽化。出自热交换器26的升温液体甲烷产品流从主热交换器4的冷端6流到其热端8，从而穿过主热交换器4。当它穿过主热交换器4时被汽化。汽化了的甲烷产品在产品压缩机58中被压缩到希望的产品释放压力。
通过将氮蒸汽从高压精馏塔14的顶部取出并在冷凝-再沸器18的冷凝通道中冷凝，形成了高压精馏塔14和低压精馏塔16的回流。一部分得到的冷凝液被作为回流返回高压精馏塔14。其余的通过流经另外的热交换器26而被低温冷却，并通过节流阀44进入低压精馏塔16的顶部，并因此形成了该塔的液体回流。
将汽化氮气流经出口46从低压精馏塔16的顶部抽出，通过流经另外的热交换器26而被加热。使得到的加热氮气流从主热交换器4的冷端8到其热端6而通过主热交换器4，从而将其进一步加热到大约环境温度。加热的氮气流被分成三个子流。一个子流在多段循环压缩机48中被压缩。出自主热交换器4的加热氮气的第二子流被用于净化装置2中吸附剂床的再生。氮气的第三子流被作为废气流经排出管线50排到大气中。循环流的相对量由位于循环压缩机48入口侧的可调节流量控制阀52的位置来决定。
进入压缩机48的循环气流被分为两部分。一部分通过压缩机所有的段，并从主热交换器4的热端6流到其冷端8。得到的冷却氮气流经节流阀54返回高压精馏塔14的上部区域。氮气通常在循环压缩机48中被压缩到超临界压力，并在主热交换器4中被冷却到足够低的温度，以使它在通过节流阀54而被膨胀时能够液化。循环气的这一部分流经主热交换器4有助于使主热交换器4中要被冷却的流体的温度-焓组合图更接近于其中要被加热的流体。
还从压缩机48中抽出一中等压力流体，并通过从主热交换器4的热端6到其冷端8而穿过主热交换器4来冷却。当中压气体从主热交换器4的热端6到其冷端8而通过主热交换器4时，中压气体保持气态。中压氮气经入口56被引入到高压精馏塔14的上部区域。因此中压应选择为基本上是高压精馏塔14的操作压力。
由低压精馏塔16经入口56流到高压精馏塔14的这部分循环气起热泵的作用，增强了精馏塔14和16的液体回流的生成。
在本发明方法的一个典型实施例中，原料气可以在大约40bar的压力下采收，高压塔14可以在其底部压力为16.5-27bar的条件下操作，低压精馏塔16可以在其底部压力为1.3-2.7bar的条件下操作。甲烷产品的压力可为17-28bar，并优选在50-70bar的压力下将液化的氮循环流从循环压缩机48中取出。
高压精馏塔14的操作压力与低压精馏塔16的操作压力之间的大压差，给在热泵循环中获得有效的热交换造成困难，因为在塔14的操作压力下氮气的比热与在塔16的操作压力下氮气的比热差值非常大。在主热交换器4中使氮气冷却减轻了这种困难。所得的提高了的热力学效率可以这样被利用，例如，获得一种提高的产品收率和/或使取出产品时的压力还原。可以获得高的甲烷收率，通常至少达到98.5％。

Claims (10)

1.一种从包含甲烷和氮气的原料气流中排除氮气以形成甲烷产品的方法，包括在一主热交换器中冷却原料气流，在一个包括一高压精馏塔、一低压精馏塔、和一个将高压精馏塔与低压精馏塔布置成热交换关系的冷凝-再沸器的双精馏塔中精馏被冷却的原料气流，将一气流从低压精馏塔循环到高压精馏塔，循环气流的第一部分在冷凝-再沸器中被压缩、冷却、液化，并且至少将一部分引入高压精馏塔，从低压精馏塔排出液态的甲烷产品流，提高该液体甲烷产品流的压力，并且在主热交换器中汽化至少部分液体甲烷产品流，其特征在于循环气流的第二部分在主热交换器中被压缩、冷却、液化或通过膨胀而被液化，并且以液态形式被引入双精馏塔，其特征还在于，两部分循环气体的冷却至少有部分是在主热交换器中进行的。
2.如权利要求1的方法，其中从循环流中取出一排出流并将其排出。
3.如权利要求1或2的方法，还包括控制被排出的循环流的量的步骤。
4.如前述任一权利要求的方法，其中循环气的第二部分被压缩到次临界压力并在主热交换器中被液化。
5.如权利要求1-3的任一方法，其中循环气的第二部分被压缩到其临界压力或高于临界压力，并且循环气的第二部分在主热交换器中被冷却到足够低的温度，使它在膨胀到高压精馏塔的操作压力时被液化。
6.如前述任一权利要求的方法，其中循环气流的第二部分全部被引入高压精馏塔。
7.如前述任一权利要求的方法，其中至少75％的原料气流在高压精馏塔的上游被液化。
8.如权利要求7的方法，其中使部分液化的原料流经受相分离，将至少一部分得到的液相引入高压精馏塔的一个中间物质交换区，并且汽相引入高压精馏塔的底部。
9.如前述任一权利要求的方法，其中加压的液体甲烷产品流在位于所述热交换器中其汽化部位上游的另外一个热交换器中加热，而不被汽化。
10.如前述任一权利要求的方法，其中循环气的两部分在同一个多段压缩机中被压缩。
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