CN85103384B - 用于烃分离的两级精馏 - Google Patents

用于烃分离的两级精馏 Download PDF

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CN85103384B
CN85103384B CN85103384A CN85103384A CN85103384B CN 85103384 B CN85103384 B CN 85103384B CN 85103384 A CN85103384 A CN 85103384A CN 85103384 A CN85103384 A CN 85103384A CN 85103384 B CN85103384 B CN 85103384B
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Abstract

为了用分级冷凝和分级精馏的方法从含烃的气态混合物中分离Ｃ2+Ｃ3+，精馏在两个塔中进行，第二塔的压力比第一塔至少低５巴。为获得最大冷却，冷凝物之一在送往高压力塔上部之前经过再冷却，这样实现了显著的节能。

Description

用于烃分离的两级精馏
本发明是关于在加压下从含烃原料气中分离C2+或C3+烃的方法。特别是，在本方法中，原料气经冷却和膨胀，生成的冷凝物被导入精馏塔里，其中C2+和C3+烃与低沸点组分分离。
在受让人的德国专利申请P3332943.5和在1984年9月13日由Kumman等人提交的美国相关专利申请序号№650016中，涉及到制备C2+烃为特定目的的这个流程，仅作为参考编入本文。在先前的专利申请里，送进塔里的气体以及在某些情况下从塔里排出的气体经受了不同的压力级，这样的压力级有助于各气流之间依序进行热量传递。在具体设计中，甲烷馏分是在冷却主要经过串联分凝阶段的原料气时产生的。在C2+烃中该甲烷馏分十分贫集，以致没有必要把它送进精馏级。而该馏分没有被膨胀到精馏压力，因此，在排入高压甲烷管道前几乎不需要压缩。
本发明的目的之一是提供一种特别从能量观点上对上述类型流程进行了改进的方法。
还有另一个目的是提供本发明的新颖的而又可以获得专利权的分系统流程。
通过进一步研究本说明书和所附的权利要求书，对于熟悉该技术的人们来说，本发明的其它目的和优点将变得十分清楚。
为了达到这些目的，至少在两个不同的压力级中进行蒸馏。在原料气冷却和膨胀时产生的馏分被送进处于较高压力下操作的第一精馏塔里。这样得到：（1）含有沸点低于C2+或C3+烃的组分的塔顶产物;（2）富集了C2+或C3+烃的塔底馏分。塔底馏分在膨胀后被送进处于较低压力下操作的第二精馏塔里，在该塔获得作为塔底产物的C2+或C3+烃馏分。将第二精馏塔塔顶得到的塔顶馏分进行再循环，在压缩后送进第一精馏塔。
两级精馏得到的一个好处是由于从高压力级得到的塔顶气体，即不含C2+或C3+馏分的气体，所处的压力比利用单级精馏时高。这种甲烷产物馏分通常排到高压管道里，因此可以省掉或至少减少对它的压缩。这与在受让人的早先申请序列号№650016中获得的优点是一样的。然而，本发明还提供了如下所述的更多的优点。
在本发明的一个优先选用的具体设计中，让来自第二精馏塔的塔顶产物通过一个开式致冷循环，为此，需将它加热、压缩并经再冷却后膨胀进入第一精馏塔。一个择优的技术是，冷却的塔顶馏分不是直接导入第一精馏塔，而是再循环进入待冷却的原料气中，或者是进入已除去冷凝物的原料气中。这种开式循环也包含溶于第一精馏塔塔底产物里的低沸点组分。在本发明的流程中，现在只需要再压缩这一小部分低沸点组份，例如通常，在主要组成甲烷的低沸点组分里占不到15%，特别是不到8%。例如，在一个用于从天然气中分离C2+烃的流程中，进入第二精馏塔的低沸点组分的比例仅约占原料气中低沸点组分全部数量的4%。需要再压缩的数量愈少，能量节省愈多。同样很重要的一点是，开式致冷循环能在相当高的温度下操作。例如大约在210到300K，特别是在230到290K操作。这样减少了对外部致冷的要求，否则将不得不利用以C3烃作冷却剂的多级封闭式致冷循环来达到这一要求。
在本发明的流程中，由于最大部分的低沸点组分在如此之高的压力下分离，以致不需要或至少大大减少了对压缩能量的需要。所以，在常规叶轮机膨胀这种冷原料气时所获得的能量除用于该馏分的常规再压缩外也可作它用，特别是用来提供象本流程中所需要的这种致冷。
通过操作本发明的具有两个压力段的流程，得到的另一个好处是，在相当低的压力下操作的第二精馏塔，能使该塔处于特别有利的温度水平，从而进一步达到该流程的致冷要求和/或能以特别有利的方式达到这一要求。这种特别有利的温度水平使得第二塔顶部和底部温度在230和300K之间。因此，由于冷却原料气导致再沸腾和中间再沸腾。
在两个精馏塔里需维持的最佳流程压力在每种情况下取决于原料气的组成、具体的分离对象、气态低沸点组分所要求的卸料压力以及在某些情况下还取决于其他工艺参数。在个别情况下利用常规最佳工程计算可以很容易确定该压力。通常，第一精馏塔的压力范围在20和40巴之间，第二精馏塔的压力范围在8和25巴之间，特别是在10和18巴之间。就此而论，两塔间的压力差至少应是5巴，最好在5到20巴左右。虽然本发明的流程也能在低于5巴的两塔压力差下操作，但是在这种情况下可能获得的积极效果将比较小。
当联系附图进行讨论时，由于理解的深入，本发明的其他各种目的、特点以及附带的优点将会得到更全面的评价。附图里同一参考标志在几个图里表示同样或类似的部件。其中：
图1是从原料气混合物中分离C2+烃的流程示意图。
图2是图1流程的最佳改进流程图。
图3是从原料气流中分离C3+烃的最优具体设计的流程图。
在图1所示的具体设计中，含有84.78克分子%甲烷、11.96克分子%乙烷和3.26克分子%丙烷的天然气，在57巴的压力和320K温度下经过导管1供料。在间接热交换器3中，通过待加热的流程物流和致冷循环2，天然气在3中冷却至234K。在出口相分离器4里分离在该步中冷凝的组分，经过导管5排出，在膨胀阀6里膨胀到22巴的压力，然后送进在该压力下操作的第一精馏塔7的较下部。
天然气中未冷凝的部分经过导管8从相分离器4卸料，再与待加热的流程物流以间接热交换的方式在热交换器9中冷至214K，然后导入另一冷凝物相分离器10中。在第二冷却阶段冷凝出来的组分要比相分离器4中得到的冷凝物包含更高含量的甲烷和乙烷。该组分经过导管11卸料，在膨胀阀12中膨胀到精馏塔7的压力，并在与精馏塔7中的平衡曲线相应的某一位置送进塔里。
在分离器10里，天然气中未冷凝的部分经过导管13卸料，在叶轮机14中发动机膨胀到精馏塔7的压力，然后经过导管15引入该塔的塔顶。经过发动机膨胀阶段实现的冷却在精馏塔7的塔顶产生180.7K的最大冷却。
在22巴压力下工作的精馏塔7在塔顶为180.7K和槽区为230K的温度范围内操作。含有97.4克分子%甲烷、2.55克分子%乙烷和0.05克分子%丙烷的甲烷馏分在该塔的塔顶排出。该气体经过导管16，先在热交换器9中，然后在热交换器3中与待冷却的天然气换热被加热。最后在环境温度和在精馏塔7的压力减去导管中存在的少量压力损失的压力下卸料到气体产品线17。
在精馏塔7的槽区，通过将冷凝物经导管18送入热交换器9，在部分加热后又送回精馏塔7的槽区来提供所需的热量。塔7的塔底产物中含有的甲烷仅占待卸料到线17的甲烷量的4%左右。该塔底产物在230K经过导管19排出，在阀20中膨胀到16巴的压力，并将之导入在该压力下操作的第二精馏塔21的上部。
塔21在塔顶为230K和槽区为265K之间的温度范围下操作，该塔从要制取的C2+烃中分离出从精馏塔7的槽区产物中产生的残余甲烷成份。
在阀20中发生的膨胀造成部分脱气，例如占塔7槽区产物的8%到25%。
精馏塔21受到中间加热和槽区加热。对于中间加热，液体经导管22排出，在热交换器3里部分加热，并再循环进入精馏塔。为加热槽区，塔底液体经导管23排出，在热交换器24中加热并作为两相混合物回到槽区。液体馏分约为75%，这主要取决于中间加热的程度。
含有0.7克分子%甲烷，74.65克分子%乙烷和24.65克分子%丙烷的塔底产物流经导管25从塔21排出。在塔21塔顶得到的甲烷、乙烷和丙烷混合物经导管26排出，并在一个开式循环中循环。在热交换器3里与待冷却的天然气换热而被加热后，该馏分在压缩机27中压缩（压缩机例如可由叶轮机14驱动），在再冷却器28中再冷却到环境温度，然后经导管29导入热交换器24，最后进入精馏塔7的较下部。另一种可行的方法，该馏分在热交换器24中冷却后可以经过导管30送入已在热交换器3中冷却过的天然气中，或再用另一种可行的方法，该馏分在热交换器24中冷却后可经导管31引送进第一分离器4中。
图2是图1所示流程运行的一个改进方案，它具有附带的节能优点。该流程的这种改进方案规定，在分离器10中得到的冷凝物先在热交换器9中过冷却，然后在阀12中减压，以带有约10%蒸汽相的蒸汽-液体混合物形式入精馏塔7。在该过程中，经减压的过冷冷凝物产生最大冷却并在经导管15进塔的气相之上进料到塔7，该气相来自分离器10和膨胀叶轮机14。为清楚起见，精馏塔7和21用的中间加热器不在图中说明。导管29和23经过热交换器3。
在图3给出的关于从天然气中分离C3+馏分的具体设计中，一种含有82.4克分子%甲烷，6.1克分子%乙烷，3克分子%丙烷，2.7克分子%C4+烃，4.9克分子%二氧化碳，0.8克分子%氮和0.1克分子%硫化氢的天然气在75巴压力和322k温度下经导管32供料。在用待加热的流程物流冷却的间接热交换器33里，该天然气被冷却到229.5K。主要由在天然气中所含的较高级烃组成的组分在这一步被冷凝，然后在下游的相分离器34中分离出来，经导管35排出并在阀36中膨胀到精馏塔37的压力。在热交换器33中与待冷却的天然气换热而被加热后，这种膨胀过的加热过的冷凝物被送进第一精馏塔37的下部。
相反地，天然气中未冷凝的部分经导管38从分离器34排出，并在叶轮机39中经发动机膨胀。经发动机膨胀将之达到精馏塔37的压力后，天然气经导管40卸料，并在冷凝物分离器41中进行相分离。来自分离器41的液相经导管42导入精馏塔37的中部，而来自分离器41的气相经导管43送进精馏塔37的塔顶冷凝器44里。此时基本上只含甲烷的该馏分通过间接热交换产生最大冷却。随后该馏分经导管45进入热交换器33，在那里加热后经导管46作为产品流卸料。
天然气中比C3烃容易沸腾的组分是从冷凝物分离器41的气体空间以及在精馏塔37的塔顶得到的。在32巴压力下操作的塔37的塔顶温度为220K，槽部温度为286K。塔顶产物经导管47卸料，在热交换器33中加热，然后与来自分离器41经导管45排出的气体馏分相混合。经导管46排出的气体馏分在混合阶段后其组成为：87.3克分子%甲烷，6.4克分子%乙烷，0.2克分子%C3+烃，5.2克分子%二氧化碳，0.8克分子%氮和0.1克分子硫化氢。该产物流是在319.4K温度和31.8巴压力下得到的。
在286k的温度下，在精馏塔37的槽区得到一种含少量挥发性大于C3烃组分的液体。底部残留液体按导管49的路线卸料，在阀50膨胀到10巴并送进在该压力下操作的第二精馏塔51中。
在塔顶为278K和槽区320K之间的温度下操作的精馏塔51，从烃由精馏塔的槽产物得到的残留轻组分出C3+中分离。与单级精馏相比，本流程的这种方案除了在塔37里得到了有相对高压力的塔顶产物这一优点外，还具有另一重要优点，即能在足够低的温度下得到塔51的塔底产物，这使得这样产生的C3+烃不用专门再冷却就可以储存。塔51槽区的这种有利的温度条件也可能节省大量加热介质，并且允许用简单的办法来加热槽内液体，例如用热水就行。为此，通过导管53从塔底产物分岔出一股支流，再使之通过管道52，在热水交换器54中加热并再循环进入塔51。经导管55排出的C3+馏分仅含有0.78克分子%的较轻组分，即0.76克分子%乙烷和0.02克分子%硫化氢。在该馏分中较重组分的含量为：50.9克分子%的丙烷和48.32克分子%的较高级烃。
从塔51的塔顶得到一种含有已经过塔51的轻组分的塔顶馏分，该馏分经导管56卸料。该馏分在热交换器33中与待冷却的天然气换热而加热后，在压缩机57中经压缩（压缩机57例如可用在叶轮机39中获得的能量开动），然后在再冷却器58中再冷却到环境温度，并且最后经导管59导入精馏塔37的较下部。
在该流程中，产品流包括：在导管46中处于31.7巴压力和319K温度的气体馏分和在导管55中含有处于10巴压力和320K温度下的C3+烃的物流。
与以前的美国专利申请650016中提出的流程相比较，对于解决从天然气中分离C2+烃时产生外部致冷的动力需要方面，本发明的流程（图1的具体设计）能够节省能量。在以前的专利申请中，需要0.72MW的电能产生外部冷却。在按图1的本发明流程中，这种动力需要减少到0.53MW。用按图2的最佳具体设计，该需要进一步减小到0.24MW。
通过用本发明的一般或具体反应物和操作条件来替换前面例子中所用过的反应物和操作条件，能够同样成功地重复以前的例子。
熟悉本技术的人们从以上描述很容易确定该发明的实质性特征，如果不偏离本发明的精神和范围，就能对本发明进行各种修改以适合不同的用途和条件。

Claims (3)

1、用精馏法在加压下从含烃原料气中分离C2+或C3+烃的方法，其中原料气经冷却和膨胀，得到的冷凝物导入精馏塔，在塔中C2+或C3+烃分别与低沸点组分分离，其特征在于该改进方法包括，在两个不同压力级进行精馏，在原料气冷却和膨胀时生成的馏分送进在较高压力下操作的第一精馏塔里，其压力为20到40巴以及至少比第二精馏塔的压力高5巴，以产生含有沸点低于C2+或C3+烃的组分的塔顶产物和富集了C2+或C3+烃的塔底馏分，塔底馏分经膨胀后送进比在第一塔低的压力下操作的第二精馏塔中，其压力为8到25巴，以产生作为塔底产物的C2+或C3+烃和塔顶产物，塔顶产物经压缩和再循环，进入第一精馏塔或进入待冷却的原料气或进入冷却原料气的冷凝馏分中。
2、根据权利要求1的方法，其中第二精馏塔的塔顶馏分经压缩和再循环进入第一精馏塔，而在压缩前加热，在再循环前再冷却。
3、根据权利要求1的方法，其中第二精馏塔在10和18巴之间的某一压力下操作，而第一精馏塔的操作压力比第二精馏塔的操作压力至少高5巴。
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