CN100416197C - 排除氮气的方法和设备 - Google Patents

排除氮气的方法和设备 Download PDF

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CN100416197C
CN100416197C CNB031550053A CN03155005A CN100416197C CN 100416197 C CN100416197 C CN 100416197C CN B031550053 A CNB031550053 A CN B031550053A CN 03155005 A CN03155005 A CN 03155005A CN 100416197 C CN100416197 C CN 100416197C
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J·D·奥尔基
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Abstract

从含有甲烷和氮气的供气流中排除氮以得到一种甲烷产物。供给的气体在一个双塔精馏塔中被分离，该精馏塔包括一个高压精馏塔(14)，一个低压精馏塔(16)，和一个冷凝再沸器(18)。甲烷产物通过泵(42)从塔(16)中回收，并被汽化。气流从塔(16)向塔(14)循环，在主热交换器(4)中被加热。第一部分热循环气在压缩机(48)中被压缩到第一压力，并且被引入到高压精馏塔(14)中。第二部分热循环气在压缩机(48)中被压缩，并且在一个与低压精馏塔(16)的中间主要交换区域相联系的第二冷凝再沸器中凝结，然后返回到高压精馏塔(14)中。

Description

排除氮气的方法和设备
技术领域
本发明涉及一种从含有甲烷和氮气的供气流中除去氮以获得甲烷产物的方法。
背景技术
已知从地下储池中可以生产天然气。但天然气总是被氮气污染。该氮气可能是完全自然存在的。替代地，至少一部分氮气可能是被引入储池中作为提高油回收(EOR)或提高气体回收(EGR)勘探的一部分。
US-A-4415345公开了一种在低温下从双精馏塔中除去氮气的方法。双精馏塔包括一个高压精馏塔，一个低压精馏塔，以及一个位于高压精馏塔顶部的冷凝再沸器，它与通常位于低压精馏塔底部的区域处于间接热交换关系。根据US-A-4415345中所述的方法，氮气和甲烷的混合气流在高压下冷却到适合精馏分离的温度。通过流经与低压精馏塔底部连接的冷凝再沸器该气流至少被部分液化。塔内分离出的一部分底部液体馏分于是再沸产生通过塔身向上的蒸汽流。剩余液体供给低压精馏塔，而相对纯的液体甲烷作为底部馏分被从该塔分离出来。产生的液体甲烷从低压塔抽出，并用泵升高压力。然后通过热交换加热到接近环境温度，从而汽化。为了增加可用于在双精馏塔中的液氮回流量，氮气流从低压精馏塔的顶部回收，并在压缩机中压缩达到高压精馏塔的操作压力，然后又与从高压精馏塔顶部流出的氮蒸汽流汇合到与低压精馏塔热连接的冷凝再沸器中。结果压缩氮气被凝结。一部分冷凝液返回到低压精馏塔，而另一部分进入到高压精馏塔。从低压精馏塔经压缩机到高压精馏塔的液体流起热泵的作用。在热泵中加入热交换器以除去压缩产生的热量。该热交换器与加热被压送甲烷流产物的热交换器是分开的。
在一定的氮气浓度下，典型地在占供气的体积比为15-30％范围内，低压精馏塔中会出现(平衡一操作线)收聚现象。我们发现收聚点的位置与氮气浓度有关。当氮气浓度接近30％时，高压精馏塔就会成为收聚的位置。这给分离过程增加了难度，并导致甲烷回收量减少或比能量消耗增加。
发明内容
本发明提供了一种从含有甲烷和氮的供气流中除去氮气以获得甲烷产物的方法，包括在一个主热交换器中冷却供气流，在一个双精馏塔中精馏冷却的供气流，该双精馏塔包括一个高压精馏塔，一个低压精馏塔，以及一个第一冷凝再沸器，第一冷凝再沸器位于高压精馏塔的顶部，与低压精馏塔的底部区域可以进行热交换，从低压精馏塔向高压精馏塔循环一股气流，循环气流被加热，第一部分变热的循环气流被压缩至第一压力，冷却，并至少被部分引入至高压精馏塔中，从低压精馏塔中抽出液态甲烷流产物，升高该液态甲烷流产物的压力，并汽化液态甲烷产物流，其中第二部分变热的循环气流被压缩至低于第一压力的第二压力，在第二冷凝再沸器中冷却，凝结，其中该第二冷凝再沸器与低压精馏塔的一个中间物质交换区域相连。
本发明还提供了一种从含有甲烷和氮的供气流中除去氮气以获得产物甲烷的设备，包括一个用来冷却供气流的主热交换器，一个用来精馏冷却了的供气流的双精馏塔，该双精馏塔由一个高压精馏塔，一个低压精馏塔，以及一个可操作地位于高压精馏塔顶部区域并与低压精馏塔底部有热交换关系的第一冷凝再沸器组成，多条用来从低压精馏塔向高压精馏塔循环蒸汽的路径，这些路径包括第一高压路径，依次包括加热通道，压缩手段，冷却通道，还包括第二低压路径，该路径依次包括加热通道，压缩手段和冷却通道，以及一个第二冷凝再沸器，该第二冷凝再沸器与低压精馏塔的中间物质交换区有热交换关系，一个从低压精馏塔底部回收液体甲烷产物的泵，以及一个汽化甲烷产物的热交换手段。
本发明的方法和设备使得热循环气体在第一循环气流和第二循环气流之间按照低压精馏塔的收聚点位置进行分配成为可能。如果氮气浓度达到15％的体积含量，则大部分或者全部的循环气体(氮气)被作为第一部分循环气流。当氮浓度达到30％的体积含量时，低压精馏塔的收聚点倾向于向上移动。在这种情况下，被送到第二冷凝再沸器的循环气流(因此形成第二部分循环气流)的相对比例会增加，以便增加通过收聚点的蒸汽流。这样，可以维持低压精馏塔进行相对有效的操作。优选地，第一部分循环气流被冷却到液化温度与产物甲烷流的汽化在同一热交换器中进行。
优选地，在主热交换器中使循环气流变热。更优选地，所述的两部分循环流都在主热交换器中被冷却。
如果第一压力是亚临界压力，第一部分循环气流优选部分地在所述的热交换器中进行液化。如果第一压力高于临界压力，第一部分循环气流优选地在主热交换器中冷却到足够低的温度，使其膨胀到高压精馏塔的操作压力时能部分液化。
通过优选地使产物甲烷的汽化和循环气流的冷却在同一热交换器中进行，该热交换器中被加热液流的温度-焓的曲线与在其中被冷却的气流的曲线较US-A-4415345中描述的现有方法中的更接近。结果，降低了该方法中热力学的无效性。
优选地，放空气流在压缩前从循环气流中取出，并按照本发明所述的方法和设备放空。优选地，还有一个能够操作地控制被压缩循环气流大小的气控阀。
循环气流优选地在主热交换器中被加热。第一和第二路径优选共用加热通道。
形成第一和第二部分加热循环气流有不同的方式。典型地，它们可以在同一压缩机中被压缩，该压缩机备有多个段，以便使第一循环气流以高于第二气流的压力产生。或者，还可以有压缩这两部分循环气流的独立压缩机。
第二部分循环流优选地通过第二冷凝再沸器被全部引入位于通道下游的高压精馏塔。
优选地，为达到最佳热力学效率，一部分引入的供气流被液化。如果需要，可以对得到的部分液化的供气流进行相分离，所得的汽相被引入到高压精馏塔的底部区域，并且至少一部分液相被引入到高压精馏塔的中间物质交换区域。优选地，残留的液相被引入到低压精馏塔中间物质交换区域。选择性地，所有的液相可以进入高压精馏塔的中间物质交换区域，并且一部分液流从高压精馏塔的中间区域回收并引入到低压精馏塔中。
优选地，在高压精馏塔中分离的所有底部馏分被从中抽出，并且被送到低压精馏塔中。因而在高压精馏塔中这部分馏分不再沸。
附图说明
图1是氮排除设备的示意流程图。
发明的具体实施方式
在此将通过实施例并参照氮排除设备的示意流程附图来描述本发明所述的方法。
附图不按比例。
天然气流或者气态的氮气-甲烷混合物通过已知方法从地下油藏或气藏回收不构成本发明的部分。气流典型地在大约40巴的压力下回收，并含有15-30体积％的氮。为除去气流中的任何硫化氢或其它含硫杂质，需对气流进行预处理(没有显示)。天然气的这种纯化方法是本领域公知的，不需要在此做进一步详细的描述。除去上述任何硫化氢杂质后，压力升高的甲烷-氮的混合气流中典型地含有水蒸气杂质。水蒸气通过净化设备2除去。净化设备2优选包括很多含有吸附剂的吸附容器，所述吸附剂可以从供气流中选择性地吸附水蒸气。这样的净化设备典型地依靠高压吸附或高温吸附循环工作，后者通常是优选的。如果供气流也含有二氧化碳杂质，净化设备2还可另外包括能够选择吸收二氧化碳杂质的吸附剂以便除去二氧化碳。
此时得到的基本上由氮气和甲烷组成的净化供气流通过一个主热交换器4，从它的热端6流向冷端8。主热交换器4包括许多热交换块，优选地这些块接合到一起形成一个单独的装置。在主热交换器4的下游，供气流通过节流阀23膨胀到相分离器10中。供气流或者在主热交换器4中，或者经过节流阀23膨胀后变成液体，具体取决于它的压力。典型地，按照它的组成，至少75体积％的供气流液化。结果，相分离器10中出来的蒸汽流较流入设备中的汽流相对减少了，这就使得使用直径比不这样的话要小的高压精馏塔成为可能。在相分离器10中，蒸汽从液体中分离出来。蒸汽流从相分离器10的顶部通过一个入口20流到高压精馏塔14的底部区域，塔14形成双精馏塔12的一部分，该双精馏塔还包括低压精馏塔16，而第一冷凝再沸器18将高压精馏塔14顶部与低压精馏塔16底部热连接。液相流从相分离器10的底部通过另外一个入口22流到高压精馏塔14的一个中间物质交换区域。典型地，该供气流以大大高于高压精馏塔14操作压力的压力进入和离开净化设备。结果，供气流从节流阀23流过而实现冷却。这种冷却满足本发明所述方法所需冷量的大部分，因此不必再设涡轮扩张机了。
高压精馏塔14中的供气混合物被分为气态氮气顶部馏分和液态富含甲烷的底部馏分。富含甲烷的底部馏分气流通过底部出口24从高压精馏塔14中抽出并且通过另一热交换器26而被过冷。过冷的富含甲烷的液流流过一个节流阀28，并且被引入到低压精馏塔16的中间物质交换区域。另外，包含甲烷和氮气的液流通过一个出口30从高压精馏塔14的中间物质交换区域被抽出，并且通过另一热交换器26而被过冷，然后通过节流阀32被引入到位于第一中间物质交换区域上部的低压精馏塔16的第二中间物质交换区域。
含有甲烷和氮气的流体在低压精馏塔16中分离以便形成顶部氮蒸汽馏分和底部液体甲烷产物馏分。底部馏分流通过一个出口40从低压精馏塔16中抽出，并通过泵42的操作进行加压。加压后的产物甲烷液流被送到另一热交换器26中在其中与被过冷的流呈逆流流过。产物甲烷液流的加压的作用是将其压力提高到高于它的饱和压力。因而事实上，加压甲烷产物液流在它进入另一热交换器26时已经处于过冷状态。在另一热交换器26中，它被加热以消除过冷。优选地，在另一热交换器26中，没有甲烷产物液流被汽化。被加热的甲烷产物液流从热交换器26中从主热交换器4的冷端8流向热端6。在经过主热交换器4的过程中它被汽化。汽化的甲烷产物在产物压缩机58中被压缩到所需的产物输出压力。
通过从高压精馏塔14的顶部收集氮蒸汽并将其在第一冷凝再沸器18中凝结，形成高压精馏塔14和低压精馏塔16的回流。将一部分得到的冷凝物作为回流返回到高压精馏塔14中。其余的通过另一热交换器26被过冷，并通过节流阀44送到低压精馏塔16的顶部，进而将液体回流供给该塔。
从低压精馏塔16的顶部通过出口46抽出氮气，并通过另一热交换器26将其加热。得到的热氮气流再从主热交换器4的冷端8流到热端6从而被进一步加热到接近环境温度。接近环境温度的热氮气流被分成三支细流。其中一支细流在循环压缩机48经过多段被压缩。第二支加热的氮气细流从主热交换器4流出被应用到净化设备2的吸附床的再生中。第三支氮气细流通过一个排气管道作为废气流被放空到大气中去。循环气流的相对大小由一个在循环压缩机入口端48处可调气流控制阀52的位置来确定。
进入压缩机48的循环气流在其中被分成两部分。第一部分气流经过压缩机的所有段，并沿着第一条循环路经从主热交换器4的热端6流向冷端8。得到的冷却氮气流通过节流阀54返回到高压精馏塔14的上部区域。典型地，第一部分循环氮气流在循环压缩机48中被压缩到超临界压力，并且在主热交换器4中被冷却到足够低的温度，以使它通过节流阀54进行膨胀而被部分液化。从节流阀23和54流出液体的相对流量应调整到能够获得适宜的高压精馏塔14的蒸汽负载量。与第一部分循环气流通过单独的热交换器相比，通过主热交换器4的第一部分循环气流有助于使得在主热交换器4种被冷却的循环气流与在主热交换器4中被加热的气流的复合物的温度-焓曲线更匹配。
从压缩器48中还抽出一种中间压力氮气流，并且经过循环气流路径的第二分支。因此中间压力气体流从主热交换器4的热端6流向冷端8。在这一过程中，中间压力气流保持在气态。中间压力氮气被引入到第二冷凝再沸器60的冷凝通道中，此过程与选择的低压精馏塔16的中间物质交换水平相关。该水平典型地是在液体从高压精馏塔14的中间区域经过节流阀32流向低压精馏塔16的水平。第二冷凝压缩器60具有增加低压精馏塔16上部区域蒸汽流量的效果，而与此同时增强液体氮回流到精馏塔14和16的能力。从低压精馏塔16上部区域通过的额外的气流尤其能有效增强当塔的上部区域有了一个收聚点时塔的操作效率。在第二冷凝再沸器60中形成的液氮冷凝物典型地与节流阀54下游的第一循环气流重新结合。从循环压缩器48中抽出第二部分循环气流的中间压力典型地选择为基本上等于高压精馏塔14的操作压力。当供给的气体中氮气含量升高时，通过阀54的作为液体流出的液流比例减少。
两部分循环气流都起了热泵的作用，从而提高精馏塔14和16的液体回流产量。
在本发明方法典型的实施例中，供给气体的压力大约为40巴，高压精馏塔14可以在底部压力为16.5至27巴的压力下操作，低压精馏塔16可以在底部压力为1.3至2.7巴的压力下操作。产物甲烷的压力可以在17至28巴之间，循环气流的高压部分优选地以50至70巴的压力从循环压缩器48中抽出。高压精馏塔14和低压精馏塔16的操作压力之间的大的压力差会导致很难在热泵循环中达到有效的热交换，因为在精馏塔14和精馏塔16的操作压力下的氮的比热差异太大。在主热交换器4中冷却氮能够减少这种难度。更进一步，在低压精馏塔16的下部和上部的相对气体流率可以根据供给气体的精确组成进行调节，以使低压精馏塔16的操作效率达到最大，并且抵消塔16中的收聚点的操作效率的负面影响，不论收聚点的位置如何。
能够获得甲烷的高回收率，典型地达到98.5％。

Claims (11)

1. 一种从含有甲烷和氮气的供气流中排除氮从而制备甲烷产物的方法，包括在主热交换器中冷却供气流，在一个双精馏塔中精馏冷却的供气流，所述双塔精馏塔包括一个高压精馏塔，一个低压精馏塔，和一个第一冷凝再沸器，第一冷凝再沸器位于高压精馏塔的顶部区域，并与低压精馏塔的底部区域处于热交换关系，从低压精馏塔向高压精馏塔循环一股气流，该循环气流被加热，第一部分被加热的循环气流被压缩到第一压力，进行冷却，并至少部分被引进到高压精馏塔中，从低压精馏塔中抽出液态甲烷流产物，升高液体甲烷流产物的压力，并且汽化液体甲烷流产物，第二部分被加热的循环气流被压缩到低于第一压力的第二压力，在第二冷凝再沸器中进行冷却，并进行冷凝，所述第二冷凝再沸器与低压精馏塔的中间物质交换区域相联系。
2. 根据权利要求1所述的方法，其中所述方法还包括将放空流从循环气流抽出并将所述放空流放空的步骤。
3. 根据权利要求1或2所述的方法，其中第一部分循环气流全部引入到高压精馏塔中。
4. 根据权利要求1或2所述的方法，其中至少75％体积的供气流在高压精馏塔的上游被液化。
5. 根据权利要求4所述的方法，其中部分液化的供气流进行相分离，至少部分得到的液相被引入到高压精馏塔的中间物质交换区域，而汽相被引入到高压精馏塔的底部。
6. 根据权利要求1或2所述的方法，其中加压的液体甲烷流产物在主热交换器中被气化之前不在另一热交换器中进行汽化。
7. 根据权利要求1或2所述的方法，其中两部分循环气流都在同一多段压缩机中被压缩。
8. 一种从含有甲烷和氮气的供气流中排除氮从而制备甲烷产物的设备，包括一个用来冷却供气流的主热交换器，一个精馏冷却的供气流的双精馏塔，所述双精馏塔包含一个高压精馏塔、一个低压精馏塔，和一个位于高压精馏塔顶部区域并与低压精馏塔底部区域处于热交换关系的第一冷凝再沸器，从低压精馏塔向高压精馏塔中循环气体的多条途径，所述途径包含第一高压途径，依次包括加热通道，压缩装置，以及冷却通道，和第二低压途径，依次包含加热通道，压缩装置，冷却通道和一个第二冷凝再沸器，所述第二冷凝再沸器与低压精馏塔的中间物质交换区域处于热交换关系，一个从低压精馏塔底部抽出液态甲烷产物的泵，以及一个用于汽化甲烷产物的热交换装置。
9. 如权利要求8所述的设备，其中所述的用于汽化甲烷产物的热交换装置是主热交换器本身。
10. 如权利要求8或9所述的设备，其中第一和第二途径中的冷却和加热通道由主热交换器提供。
11. 如权利要求10所述的设备，其中第一和第二途径共用通用的加热通道。
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