CN1161308C - 乙烯装置的冷冻系统 - Google Patents

乙烯装置的冷冻系统 Download PDF

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CN1161308C
CN1161308C CNB988138603A CN98813860A CN1161308C CN 1161308 C CN1161308 C CN 1161308C CN B988138603 A CNB988138603 A CN B988138603A CN 98813860 A CN98813860 A CN 98813860A CN 1161308 C CN1161308 C CN 1161308C
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methane
binary refrigeration
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heat exchanger
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C·苏穆纳
��Τ����˹
V·T·韦
J·J·克劳福德
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S·J·斯坦利
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R·J·麦纳布
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Abstract

一种乙烯装置冷冻系统，采用低压脱甲烷塔(12)和含甲烷-乙烯或甲烷-乙烷混合物的二元冷冻剂(20)。该冷冻组成可沿整个系统保持恒定，或者可采用分离器(88、108、120、128)将冷冻剂分为富甲烷二元冷冻剂(140)和富乙烯或富乙烷二元冷冻剂(142)。

Description

乙烯装置的冷冻系统
发明背景
本发明涉及提供乙烯装置冷却之需的冷冻系统。更具体地说，本发明涉及采用包含甲烷-乙烯混合物的二元冷冻剂来冷却乙烯装置。
乙烯装置需要利用冷冻从裂化炉流出物中分离出需要的产物。通常，采用C3冷冻剂，一般为丙烯，以及C2冷冻剂，典型的为乙烯。一般而言，尤其是在采用要求较低温度的低压脱甲烷塔的系统中，还常常使用单独的甲烷冷冻系统。于是，需要3个单独的冷冻系统，从最低温度到最高温度串联布置。需要3台压缩机和驱动系统，再配上吸入罐、单独的交换器、管线等等。还有，甲烷冷冻循环常常需要往复压缩机，这又会部分地抵消因采用低压脱甲烷塔而带来的任何基本投资上的节省。
混合的冷冻剂系统在工业上已问世几十年了。在这样的系统中，设置采用多组分的单一冷冻系统，以便在较宽温度范围提供致冷，因此，一套混合冷冻系统就能代替多个纯组分串联的冷冻系统。这样的冷冻系统已在基本负荷液态天然气装置中得到广泛应用。已见到有关在乙烯装置设计中应用混合冷冻系统的文章，但是由于冷冻剂中存在多种组分，它们在操作上都很复杂。而且，当丙烯冷冻压缩机循环温度等于或高于-40℃时它们的效率较低。
发明概述
因此，本发明的目的是提供一种乙烯装置用的简化冷冻系统，具有以甲烷和乙烯，或者替代地甲烷和乙烷，的混合物作为二元冷冻剂的低压脱甲烷塔，以串联配合丙烯，或替代地丙烷，冷冻系统一起工作。该系统可代替传统装置中配合丙烯冷冻系统使用的单独的甲烷和乙烯冷冻系统。该冷冻剂的组成可沿整个系统保持恒定，或者可采用分离器将二元冷冻剂部分闪蒸，从而分为富甲烷流和富乙烯流，用于在一个或多个热交换器中的独立的循环。本发明冷冻系统的目的、布置和优点在研读了下面的描述之后将变得更加清楚。
附图简述
图1是部分乙烯装置的示意流程图，说明本发明冷冻系统的一种
实施方案。
图2是类似于图1的示意流程图，但用于说明本发明的一种替代
实施方案。
图3是说明图2实施方案的一种变换方案的示意流程图。
优选实施方案描述
本发明涉及一种乙烯装置，其中裂解气体首先接受脱甲烷和氢气处理，随后按已知方式加工以生产和分离出乙烯和丙烯以及一些其他副产物。乙烯装置通过低温冷凝和分馏实现气体分离的过程要求宽温度范围致冷。涉及乙烯装置冷冻系统的基本投资可占到整个装置投资的相当大一部分。因此，在冷冻系统的基本投资上的节约将显著影响整个装置的成本。
带高压脱甲烷塔的乙烯装置操作在高于2.758MPa(400psi)的压力下，并可通过纯组分乙烯的致冷作用而冷凝，从而产生塔顶回流。此种系统的脱甲烷塔顶部温度一般介于-85℃～-100℃。大约-101℃的乙烯致冷作用通常用于对塔顶冷凝器的冷却。在低于2.758MPa的压力下，塔顶温度通常都太低，以致不宜使用乙烯冷冻，除非采用真空。然而，这是不理想的，因为基本投资将增加而且空气漏入到系统中的潜在危险将导致安全隐患。
本发明涉及低压脱甲烷塔和二元冷冻剂体系的应用。为实现本发明的目的，低压脱甲烷塔操作在低于约2.41MPa(350psi)，一般介于0.345～1.034MPa(50～150psi)，其间塔顶温度介于-200～-235℃。低压脱甲烷塔的优点是，装置总功率要求较少，装置总基本投资较少，而缺点是，要求的冷冻温度较低，因此迄今为止需要单独的甲烷冷冻压缩机。
本发明的二元冷冻剂包含甲烷与乙烯的混合物。甲烷对乙烯的比例将取决于乙烯装置的裂解原料、裂解深度、冷却系列压力(chillingtrain pressure)，以及冷冻剂的性质等因素，然而一般介于10∶90～50∶50，更可能介于20∶80～40∶60。甲烷与乙烯或甲烷与乙烷二元冷冻剂配合丙烯或丙烷冷冻系统的使用，为带有低压脱甲烷塔的乙烯装置提供所要求的冷冻负荷及温度，同时避免对3种单独的甲烷、乙烯和丙烯冷冻剂的需要。
二元冷冻剂将不用在高压脱甲烷塔中，因为不需要提供这样深度的冷冻。没有必要用二元冷冻系统作为纯组分乙烯冷冻系统的简单替代物。这只能导致成本更高、更复杂。已有人建议用混合的冷冻系统代替乙烯和丙烯这2个冷冻系统，然而，它们要求至少1种比乙烯轻的组分，例如甲烷。因此，它将至少是三元体系。通常更经济的做法是再使用一种比丙烯重的组分，例如C4组分，这样一来系统通常将成为四元冷冻剂系统了。
本发明的目的是为从进料气体(裂解气体)中大致分离出氢气和甲烷提供所需要的冷冻，并提供脱甲烷塔的进料。参见图1所示本发明的实施方案，进料气体2，即按要求进行调节的并冷却的裂解气体，其具有介于约-35～-37℃的典型温度以及约3.45MPa(500psi)典型压力，此时一般将已经出现部分液化。
进料气体2在热交换器4、6、8和10中由本发明的冷冻系统逐步冷却并正如下面将解释的，发生分离，生成脱甲烷塔进料。热交换器4、6、8和10一般为铜焊的铝质交换器，亦称为板翅(platefin)或芯式交换器，并可物理地组合成较少的单元，或者扩展为较多的单元数目。在脱甲烷塔12中，C1和更轻的组分，主要是甲烷和氢气，与C2和更重的组分彼此分离。从脱甲烷塔12出来的净塔顶物14用作冷冻系统中的冷却流，正如下文将解释的。从脱甲烷塔出来的塔底物16也可用作冷冻系统另一部分中的冷却流，下文也将解释。
现在转向冷冻系统本身，二元冷冻剂，如同前面所提到的，以甲烷与乙烯混合物的形式由冷冻压缩机18压缩到高达约3.0～4.0MPa的压力。下面给出的表格中，列出本发明一种实施例的具体温度和压力。压缩的二元冷冻剂20在22和24中，受到例如冷却水或其它冷物流的冷却，并进一步在26，例如由丙烯冷冻剂冷却到约-30～-40℃的温度。液态二元冷却剂被收集在接受器或收集器28中。
冷却剂30从受槽28出来，可在32中与来自脱甲烷塔12的塔底物16或者其他在此受到加热的冷物流进行热交换从而进一步降温。脱甲烷塔塔底物从热交换器32的34处出来，送往传统生产和分离乙烯、丙烯及其他副产物的脱乙烷塔。
从热交换器32出来的二元冷冻剂36，随后送往一系列热交换器4、6、8、10以及11当中的第1个。热交换器4～10对来自裂解炉的进料气体实施冷却。热交换器11向脱甲烷塔提供回流。
首先来看热交换器4，二元冷冻剂36流过热交换器盘管46从而受到冷却。随后，部分二元冷冻剂作为48排出，其温度通过膨胀阀50的减压作用而下降。该冷却的二元冷冻剂部分，随后再反向通过热交换器盘管52。膨胀阀50根据在热交换器4中受到冷却的进料气流54的温度来控制，从而控制热交换器盘管52中冷冻剂的温度。热交换器盘管52中的二元冷冻剂吸热并蒸发，进而过热到比进来的流36低1～5℃的温度。蒸发的二元冷冻剂56从盘管52出来，进入到吸入罐58中，由此，冷冻剂蒸汽流60喂入到二元冷冻剂压缩机18中。吸入罐58以及下面将提到的其他吸入罐84、102和130，仅用来分离出在失常状态下可能存在的任何液体，以防止可能对压缩机造成的损坏。在系统正常操作期间不需要它们。
二元冷冻剂之所以首先通过热交换器4接受冷却，然后再在50中闪蒸，是为了降低固定闪蒸压力下闪蒸出的蒸汽百分率。这样，经闪蒸后的液体将变得更冷，故可提供在更冷的温度下的更强的致冷作用。在纯组分冷冻剂的情况下，对任何给定闪蒸液体压力而言，闪蒸液体温度是固定的，因此，闪蒸前进行冷却将得不到任何净收益。同样的原理适用于其他热交换器6、8、10和11。
在热交换器4以及其他热交换器6、8及10中的进一步冷却是由物流62、64和66提供的，它们分别是氢气、低压甲烷和高压甲烷的低温流。这些低温流62、64和66来自低温氢/甲烷分离系统68以及脱甲烷塔12的塔顶物14。净塔顶物66还可为当作脱甲烷塔回流冷凝器的热交换器11提供冷却。
冷却后的进料气体54可进一步在70中冷却并喂入到下一个热交换器6中。交换器70中的冷却可以是脱甲烷塔12的再沸和中间再沸(interboiling)作用。从热交换器4出来的其余冷却的二元冷冻剂72还喂入到下一个热交换器6中。该热交换器6的操作方式与热交换器4相同，只是这时所有相关的温度都更低，包括进入的二元冷冻剂流72、流出的二元冷冻剂流74、膨胀阀78以后的二元冷冻剂流76、从盘管81出来的蒸发二元冷冻剂流80以及流出的进料气体流82的温度。蒸发的二元冷冻剂80喂入到吸入罐84，再作为86喂入到二元冷冻压缩机18。
进料气体流82喂入到分离器88中，在其中，冷却的进料气体分离成挥发性较小的脱甲烷塔进料流90，以及挥发性较大的顶部流92，后者这时已含有进一步浓缩的甲烷和氢。顶部物92和二元冷冻剂74流入下一个热交换器8，在此，继续相同方式的冷却过程，从而产生进一步冷却的进料气体94和二元冷冻剂96。一部分二元冷冻剂再次流过膨胀阀98和盘管100，进入到吸入罐102中。然后，蒸汽104喂入到二元冷冻剂压缩机18中。热交换器8还可利用来自热交换器10的蒸发二元冷冻剂流106进一步冷却。
从热交换器8出来的进料气体94喂入到分离器108中，在此，挥发性较大的组分作为110从顶部排出，并喂入到热交换器10中。此时，该顶部物中所含氢气和甲烷已得到更进一步浓缩。从分离器108出来的底部物作为112喂入到脱甲烷塔12中。
在热交换器10中，冷却过程依靠又一部分二元冷冻剂经由膨胀阀114的膨胀以及盘管116中的蒸发而继续进行，从而产生前面所提到的二元冷冻剂流106。流出的进料气体118喂入到分离器120中，现在，分离器中的顶部物122已经主要是氢气和甲烷了。顶部物122喂入到氢气/甲烷分离系统68中，在此，氢气与甲烷经过低温分离产出氢气流62和低压甲烷流64。从分离器120出来的底部物作为124喂入到脱甲烷塔12中。这时，剩余的二元冷冻剂流126在热交换器11中被脱甲烷塔净塔顶物66进一步冷却。二元冷冻剂流126在133中膨胀并反向通过热交换器11中的盘管135，准备与来自阀114的冷冻剂混合。
从脱甲烷塔12出来的总塔顶流14进入到热交换器11中，在此，它部分地冷凝。该部分冷凝的物流127流入到分离器128中。从分离器128出来的液体129返回到脱甲烷塔12中作为回流。从分离器128出来的顶部物66，这时已是净脱甲烷塔塔顶物，主要含甲烷，通过反向流经热交换器11、10、8、6及4而被再加热。脱甲烷塔12备有典型的再沸器和塔板间的中间再沸器(未表示出)。脱甲烷塔的塔底物16是C2以及更重组分。再沸以及中间再沸一般依靠进料气体的冷却作用，例如通过热交换器70实现。
流106进入到吸入罐130，然后作为132进入到二元冷冻剂压缩机18中。虽然图1表示出4台热交换器4、6、8和10，然而，这些热交换器的数目可根据任何具体乙烯工艺，特别是具体进料气体情况作出变更。下表给出作为本发明一个具体实施方案，在图1所示工艺流程中的，各个点处的，包括脱甲烷塔系统的二元冷冻剂以及进料气体(工艺气体)的温度和某些压力：
  二元冷冻剂     部位     T-℃     压力-MPa
28 -37     4.5
    58     -48     1.6
    84     -65     0.8
    102     -75     0.5
    130     -106     0.02
工艺气体     66     -133     0.6
    2     -37     3.5
    62     -135     ---
    92     -72     ---
    110     -91     ---
    122     -132     ---
本发明二元冷冻剂系统的某些优点已在上面提到，包括压缩机系统数目减少，以及能采用任何离心或轴流压缩机来代替甲烷往复压缩机。另一个优点是，二元冷冻剂的组成比包含3或更多组分的更复杂的混合冷冻剂更容易维持。这在系统发生差错或受到干扰，导致冷冻剂放空的情况下表现得尤其明显。放空过程造成冷冻剂中轻组分比重组分损失得更多。这使组分比例改变，一旦重新开车必须予以矫正。冷冻剂组成越复杂，比例矫正起来越困难。
在图1表示的本发明方法中，冷冻剂组成沿全过程维持恒定。然而在图2表示的本发明替代方案中，将出现二元冷冻剂分离为二元富甲烷流和二元富乙烯流。
图2仅大致表示出图1中修改过的部分，图中有一个膨胀阀136设置在管线36中。二元冷冻剂压力下降，于是一部分蒸发。液体部分和蒸汽部分在闪蒸罐138中分离，借此，蒸汽部分140将富含甲烷，而液体部分142将富含乙烯或乙烷。在该图2的实施方案中，富甲烷流140流过全部热交换器4、6、8和10，然后在144处，一部分膨胀并作为流146反向流过全部热交换器10、8、6和4。离开热交换器10的流140的另一部分126在交换器11中冷却，在133中膨胀并反向流过交换器11，然后在交换器10的进口与流146汇合。流出的富甲烷二元冷冻剂流146，随后将返回到压缩机18的第1级。富乙烯流142的处理与图2中二元冷冻剂流有些相似，即，在流过头3个热交换器中每一个之后，分别排出一部分--148、150和152并在154、156和158中膨胀。膨胀的部分随后分别反向流过这些热交换器中一个或多个，从而产出流出的富乙烯或富乙烷二元冷冻剂流160、162和164，它们再返回喂到适当的压缩机级中。
图2中二元冷冻剂发生分离的方案的优点是，对压缩机出口处的任何给定二元冷冻剂组成来说，压缩机能够以较高的压力吸入。吸入压力之所以较高，是因为冷冻剂组成中富含甲烷，因此在固定冷冻剂温度的条件下，压力将比较高。这就是说，压缩机的压缩比比较低，这将导致压缩机成本的降低。
图2的一个变换方案是在营线36中不设阀门136。这时，管线36中的压力将降低，以致该物流不完全液化，蒸汽部分依然保留。分离器138将冷凝液部分与富甲烷蒸汽部分分开。这一变换方案允许压缩机18在流36中的任何给定甲烷-乙烯(或甲烷-乙烷)组成条件下具有较低排出压力。压缩机18的总压缩比降低了。流36的流率增加，以补偿任何给定流36的组成。然而，压缩机成本却得以降低。这一方案对于小规模乙烯装置，即，压缩机18出口的实际压缩机体积流量接近离心压缩机的设计允许下限的情况，尤其有用。
图3是类似于图2所示的本发明另一种修改方案，但是它增加了一个二元冷冻剂的分离步骤。如图所示，恰似图2的方案，在138处设有一个第1分离。富甲烷二元冷冻剂蒸汽流140流经热交换器4而部分地液化，然后流经管线166进入到附加冷冻剂分离器170，在此再次分离为第2富甲烷蒸汽流172和第2富乙烯或富乙烷液体流174。富甲烷流172将比流174和流140更富甲烷。富乙烯或富乙烷流142恰如图2中的方案一样流经热交换器。类似地，第2富甲烷流172流经第2热交换器6，然后如同其他方案一样流入到较低温度热交换器中，在此它膨胀并反向流过热交换器。第2富乙烯或富乙烷流174流经第2热交换器，在178膨胀并反向流过热交换器。该图3为简单计仅表示出2台热交换器，然而实际上也可有另外的热交换器和类似于分离器170的另外的分离器。
该图3工艺方案的优点在于，在任何给定冷冻温度条件下，二元冷冻剂压力比较高。这将降低二元冷冻剂压缩机的压缩比并可降低压缩机基本投资。

Claims (9)

1.在由含氢、甲烷、乙烯和其他C2以及更重烃类的进料气体生产乙烯的方法中，其中所述方法包括在低于2.41MPa的压力下操作的脱甲烷塔且其中所述进料气体由一种冷冻系统冷却，一种在所述冷冻系统中采用二元冷冻剂来冷却所述进料气体的方法，包括下列步骤：甲烷与乙烯或甲烷与乙烷的混合物经过压缩产生一种二元冷冻剂，所述二元冷冻剂通过一系列热交换器逐步膨胀和冷却，所述逐步冷却的二元冷冻剂与所述进料气体在所述热交换器中逐步进行热交换接触从而冷却，并借此分离出所述氢和一部分所述甲烷，并产生浓集了所述乙烯和其他C2以及较重烃类的液态脱甲烷塔进料流，将所述液态脱甲烷塔进料流喂入到所述低压脱甲烷塔中，并产出主要由甲烷组成的总脱甲烷塔塔顶流，所述总脱甲烷塔塔顶流与所述逐步冷却的二元冷冻剂进行接触，并分离出脱甲烷塔回流流，以及净脱甲烷塔塔顶流，将所述脱甲烷塔回流流返回到所述脱甲烷塔中。
2.在权利要求1的方法中，其中所述净脱甲烷塔塔顶流与所述进料气体在所述热交换器中进行热交换接触。
3.在权利要求1的方法中，其中从通过在所述热交换器中冷却的所述进料气体中分离出来的所述氢与所述甲烷的所述部分，进行低温分离，产出氢气流和甲烷流，其中所述氢气流和甲烷流各自在所述热交换器中与所述进料气体进行热交换接触。
4.在权利要求1的方法中，其中所述二元冷冻剂通过一系列热交换器逐步膨胀和冷却的过程包括下列步骤：所述二元冷冻剂流经所述热交换器之一，所述二元冷冻剂的一部分在通过所述一个热交换器之后进行膨胀，所述膨胀部分反向流过所述一个热交换器，而所述二元冷冻剂的其余部分则送入并流经所述热交换器中的下一个，继而重复下述所述步骤，即，使又一部分膨胀，然后所述又一部分反向流过所述热交换器。
5.在权利要求4的方法中，其中所述二元冷冻剂的诸所述部分在反向流过所述热交换器之后，返回到所述二元冷冻剂的压缩步骤中。
6.在权利要求1的方法中，其中从每台所述热交换器分离出来的所述脱甲烷塔进料流各自喂入到所述脱甲烷塔的不同级中。
7.在权利要求1的方法中，还包括所述二元冷冻剂分离为富甲烷二元冷冻剂和富乙烯或富乙烷二元冷冻剂的步骤，其中所述逐步冷却的二元冷冻剂与所述进料气体之间逐步接触的所述步骤包括下列步骤：所述进料气体与所述富甲烷二元冷冻剂和所述富乙烯或富乙烷二元冷冻剂各自的流在所述热交换器中进行接触。
8.在权利要求7的方法中，还包括所述富甲烷二元冷冻剂分离为第2富甲烷二元冷冻剂和第2富乙烯或富乙烷二元冷冻剂的步骤，且其中所述的所述进料气体的接触步骤还包括下列步骤：所述进料气体与所述第2富甲烷二元冷冻剂和所述第2富乙烯或富乙烷二元冷冻剂各自的流也进行接触。
9.在权利要求7的方法中，其中甲烷与乙烯或甲烷与乙烷的所述混合物的压缩步骤包括压缩成液-汽混合物的步骤，且其中所述液-汽混合物又被分离成所述富甲烷二元冷冻剂和所述富乙烯或富乙烷二元冷冻剂。
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US5979177A (en) 1999-11-09
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CA2317534C (en) 2005-06-14
DE69806815D1 (de) 2002-08-29
WO1999035110A1 (en) 1999-07-15
EP1045820B1 (en) 2002-07-24
AU2017899A (en) 1999-07-26
CA2317534A1 (en) 1999-07-15
EP1045820A1 (en) 2000-10-25
CN1286671A (zh) 2001-03-07
DE69806815T2 (de) 2003-02-06
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ATE221036T1 (de) 2002-08-15
RU2190169C2 (ru) 2002-09-27

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