CN101351680A - 低温空气分离法 - Google Patents

低温空气分离法 Download PDF

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CN101351680A
CN101351680A CNA2006800502675A CN200680050267A CN101351680A CN 101351680 A CN101351680 A CN 101351680A CN A2006800502675 A CNA2006800502675 A CN A2006800502675A CN 200680050267 A CN200680050267 A CN 200680050267A CN 101351680 A CN101351680 A CN 101351680A
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CN101351680B (zh
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P·J·兰金
N·M·普罗塞尔
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Praxair Technology Inc
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Abstract

一种将低温空气分离的方法，其中液氧(30)经过增压(112)并逆着冷凝进料空气(5)蒸发以产生氧气产品(42)，其中产生过量装置冷冻，使得所述方法的总热端温差超过所述主换热器(101)的最低内部温差至少2K。

Description

低温空气分离法
技术领域
[0001]总地来说，本发明涉及低温空气分离，更具体地讲，涉及低温空气分离来制备氧产品。
背景技术
[0002]通过蒸馏分离空气成其各种组分在低温下进行并需要一定量的冷冻。这种冷冻通常使加工气体通过涡轮膨胀获得。设计空气分离法时，通常将膨胀产生的冷冻量保持在最低，因为所有形式的冷冻都不利于这种方法，或者降低分离效率或需要比装置蒸馏塔最低需要更多的压缩能。用于装置的冷冻效率通过进、出装置的流之间的温度差反映。这种温度差称为总热端温差(aggregate warm end temperaturedifference，WEDT)。当WEDT为极端最低值时，0开尔文的WEDT表示仅产生了驱动空气分离所需冷冻。
[0003]在液氧泵送低温空气分离装置中，将产品氧作为液体从低压蒸馏塔的底部移走，在此将其泵送到较高压力，在主换热器或产品蒸发器中逆着冷凝空气流沸腾，使所得蒸气在主换热器中过热形成气态氧产品。如果液氧被泵送到其最终输送压力，则将气态氧产物直接送到最终用户，否则需要进一步压缩。氧逆着冷凝空气沸腾引起内部窄点温差(internal pinch temperature difference)。换句话说，其在主换热器(，primary heat exchanger，PHX)中引起冷却流和加热流之间的最低总温差。PHX内部窄点的大小取决于可用换热器表面积。PHX越大，窄点越靠近。通常在液氧泵送空气分离装置中，PHX窄点温差(“DT”)为约1-2K。
[0004]在冷凝空气流进入PHX之前必须将其压缩到比输送至装置的主空气进料压力更高的压力。这种压缩通常采用单独的增压空气压缩机进行。冷凝空气流的压力通常高于沸腾氧气流的压力。这样，当要求高压氧作为产品时，增压空气压缩机消耗大量能量。因为增加能量消耗，需要使用较少总能量的改进冷冻空气分离法。本发明目标是通过降低冷凝空气流的压缩需求来减少总能量消耗。
发明概述
[0005]一种将进料空气低温分离的方法，其中进料空气在主换热器中冷却，通过在至少一个塔中低温精馏分离来制备富氧液体和富氮蒸气，将富氧液体压力提高并将该加压的富氧液体与至少部分进料空气进行间接热交换蒸发来制备产品氧，改进之处包括产生超过进行低温精馏所需的足够过量冷冻，使得所述方法的总热端温差超过主换热器的最低内部温差至少2K。
[0006]本文中所用的术语“总热端温差”是指进入主换热器的流与离开主换热器的流的总温度之差。
[0007]本文中所用的术语：“主换热器的最低内部温差”是指主换热器内加热流和冷却流的总温度之间的最小差异。
[0008]本文中所用的术语“塔”是指蒸馏或分馏塔或区，即接触塔或区，其中液相和气相逆流接触以进行流体混合物的分离，例如通过使气相和液相在安装于所述塔内的一系列垂直间隔的塔盘或塔板和/或填料元件(如规整填料或散堆填料)上接触。蒸馏塔的进一步讨论参见theChemical Engineer’s Handbook(化学工程师手册)，第五版，R.H.Perry和C.H.Chilton编辑，McGraw-Hill Book Company(McGraw-Hill图书公司)，纽约，第13节，The Continuous Distillation Process(连续蒸馏 法)。双塔包括高压塔，所述高压塔上端与低压塔下端具有热交换关系。
[0009]蒸气和液体接触分离法取决于各组分的蒸气压差。较高蒸气压(或挥发性较高或低沸点)组分将在气相中浓缩而较低蒸气压(或挥发性较低或高沸点)组分将在液相中浓缩。部分冷凝为其中蒸气混合物的冷却可用于浓缩气相中的挥发组分以及液相中的低挥发性组分的分离法。精馏或连续蒸馏是结合了连续部分汽化和浓缩的分离法，如通过气相和液相的逆流处理获得。气相和液相的逆流处理通常为绝热的且可包括各相之间的积分(多极)或微分(连续)接触。利用精馏原理来分离混合物的分离法装置经常可互换称为精馏塔、蒸馏塔或分馏塔。低温分离为至少部分在150开氏度(K)以下进行的精馏法。
[0010]本文中所用的术语“间接热交换”是指引起两种流体热交换而没有任何物理接触或流体互相混合。
[0011]本文中所用的术语“进料空气”是指主要包含氧气和氮气的混合物，如环境空气。
[0012]本文中所用的术语塔的“上部”和“下部”分别是指所述塔中点以上和以下的部分。
[0013]本文中所用的术语“涡轮膨胀”和“涡轮膨胀机”分别是高压流体流过涡轮以降低其压力和温度，以产生冷冻的方法和仪器。
[0014]本文中所用的术语“低温空气分离装置”是指塔或多个塔(其中进料空气通过低温精馏产生氮气、氧气和/或氩气)及连接管道、阀、换热器等。
[0015]本文中所用的术语“压缩机”是指通过做功来提高气体压力的机器。
附图简述
[0016]图1是低温空气分离法的示意图，所述方法可与本发明一起使用且可通过应用本发明方法得益。
[0017]图2表示图1所示方法当以常规方式进行时主换热器中复合热和冷流之间温度差与换热器负荷之间的函数关系图。
[0018]图3是图1所示装置和方法当以本发明方式进行时主换热器中复合热和冷流之间温度差与换热器负荷之间的函数关系图。
发明详述
[0019]总地来说，本发明液氧泵送低温空气分离法的特征在于：总热端温差(WEDT)比主换热器最低内部温差(PHX窄点DT)大至少2K。更具体地讲，WEDT与PHX窄点DT之差将超过3K，最优选超过4K。本发明需要的过量冷冻通过使加工气体通过涡轮膨胀而产生。许多情况下，通过减少冷凝空气流的压缩能而实现的节能超过了弥补与过量冷冻生产相关的不利。在较高氧沸腾压下特别如此。
[0020]将参考附图对本发明进行详细描述。现在参考图1，压缩、冷冻、预纯化进料空气1(其已在主空气压缩机中压缩)分流成两股流；流2进入主换热器101的热端而流3进入增压压缩机109。在增压压缩机109中将这部分进料空气提高到足够高的压力，从而其能逆着沸腾氧产品冷凝。高压空气流4通过冷却器110，冷却的高压气流5进入主换热器的热端。中压空气6从换热器101出来，已冷却到接近露点。冷空气6随后进入高压精馏塔102的底部，精馏塔102与低压塔104形成双塔。高压空气流5在主换热器中逆着沸腾高压氧液化并作为过冷液体从主换热器出来。过冷液体空气流7在液体涡轮111中膨胀以提供低温空气分离装置所需的一部分冷冻。液体空气流膨胀到接近塔102的工作压力。液体空气流8分流成3股流；流9在比流6进入该塔的位置高几级的位置进料至塔102，流10在底部以上几级的位置进料至中压塔103，流11进料至换热器108。在换热器108中，流11逆着热氮蒸气进一步冷却，在此过冷液体空气流27在顶部以下几级的位置进料至低压塔104。
[0021]在塔102中将空气分离成富氧和富氮部分。将富氧液体12从所述塔底移走，引入换热器108，逆着加热氮蒸气冷却，作为过冷液体21出来，并进料至塔103中间点，低于流10的进料点但高于塔底。氮蒸气13从中压塔102顶部离开。将蒸气流14的一部分作为中压氮产品移走，并进料至主换热器101的冷端。流14在主换热器101中逆着冷空气流升温并作为热中压氮气流39离开热端。流13的剩余部分15进入冷凝器/再沸器105的冷凝侧。流15在塔104中逆着蒸发的塔底液体液化。离开冷凝器/再沸器的液氮16分流成两股流；将流17送入换热器108，流18作为回流返回塔102。流17逆着加热氮蒸气过冷，所得过冷液氮流28在或靠近顶部进入低压塔104。富氮蒸气流19在塔102顶部下方至少一级的位置被移走并进入冷凝器/再沸器106的冷凝侧。流19在塔103中逆着蒸发的塔底液体被液化并作为液流20返回塔102。流20在流19的回收点或以上的位置进入塔102。
[0022]中压塔103用于进一步补充送入低压塔104的氮回流。氮蒸气23从中压塔103的顶部出来并进入冷凝器/再沸器107的冷凝侧。流23在塔104中间逆着蒸发液体液化。离开冷凝器/再沸器107的液氮24分流成两股流；流25返回塔103顶部，流26进料至换热器108。流26逆着加热氮蒸气被过冷，所得过冷液氮流29进料至低压塔104顶部或附近。将富氧液体22从塔103底部移走并在冷凝器/再沸器107以上几级的位置进料至低压蒸馏塔104的中间点。
[0023]低压蒸馏塔104进一步将其进料流分离成富氧液体和富氮蒸气。将富氧液流30从塔104的下部移走，送到低温氧气泵112并提高到稍微超过最终氧气输送压力。高压液流32进料至主换热器101的冷端，在此使之升温并逆着冷凝高压进料空气流沸腾。热的高压氧蒸气产品42离开主换热器101的热端。富氮蒸气31从低压塔104的上部出来，进料至换热器108，逆着冷却液体升温并作为过热氮蒸气流33离开。
[0024]流33进入主换热器101的冷端，在此它逆着冷却空气流部分升温并分流成两股流。将完成氮产品所不需的那部分流从主换热器101中间点移走，该流34进料至废气涡轮113并膨胀到较低压力。废气涡轮113与液体涡轮111一起用于产生低温空气分离装置的冷冻。低压氮气流35离开废气涡轮113，进料至主换热器101并作为热的低压废氮气36离开热端。流37作为热的低压产物氮离开换热器101的热端并进料至氮气压缩机114的第一级并在那些级的级间冷却器115中冷却。使冷却的压缩氮气流38与相同压力的氮气流39混合以形成流40。将氮气流40进料至氮气压缩机116的剩余级并在那些级的级间冷却器117中冷却。所得高压氮气流经过冷却(未显示后冷却器)以形成输送到最终用户的产品氮气流41。
[0025]就此给定实施例而言，所需的氧气输送压力为1115磅/平方英寸(绝对值)(psia)而所需的氮气输送压力为335psia。理想地，高压空气流5将提高至少2300psia，以提供在1115psia以上沸腾的氧气。然而，铜焊铝换热器(BAHX)能承受的压力有限。这种情况下，我们根据经济性和BAHX的压力限制将流5限定到1215psia。稍微高的压力对于BAHX是可能，但可能不经济。可能需要供选技术如螺旋形换热器用来处理2300psia的气流压力。然而，这非常昂贵。
[0026]在常规示范的情况下，当将塔上部压力提高到刚好足以使所有废氮气的膨胀提供了所需的主换热器热端温差时，功率最低化。如果将压力提高到超过如此，废气膨胀机会提供超过所需的冷冻。当按照常规示范采用废气膨胀时，塔102的压力仅为约95psia，塔104的压力为约25psia。
[0027]由于主换热器中氧的高沸腾压和冷凝高压空气流的允许压力上限，装置进料的绝大部分必须进入增压压缩机109。在本实施例中，流5的流速为流1流速的大约35％。这样高的流速加上高的排气压力意味着增压压缩机109消耗了所述装置总能量消耗的大部分。这种情况下，超过25％的装置能量消耗来自增压压缩机109。图2显示了压力最低化使得废氮膨胀机冷冻产生3.0K的主换热器温差(WEDT)的***的主换热器的冷却曲线。2.0K的内部窄点(PHX窄点DT)是由于超临界(1115psia)氧气逆着冷却超临界空气(1215psia)升温。大量高压空气流在主换热器冷端提供了过量冷冻，如冷端的大温差所证实。WEDT和PHX窄点DT之差为1.0K。
[0028]通过提高整个装置压力将本发明用于这种循环中。当将塔102的压力从95psia提高到180psia和将塔104的压力从25psia提高到57psia时，通过废气膨胀涡轮产生过量冷冻，因为不必作为产品的所有氮气都仍然通过废气膨胀机。结果，PHX相当宽的冷却曲线如图3中所示。WEDT与PHX窄点DT之差现在超过7K。结果是，就相同主换热器101而言，需要少许多的来自增压空气压缩机109的高压空气5以适当地使所有高压氧沸腾。通过该过量冷冻，流5的流速为进料流1的35％-25％而被增压压缩机109消耗的装置能量的分数为约25％-12.5％。将本发明应用于这种循环的另一益处是产生的涡轮能量明显提高，这通过废气膨胀涡轮113实现。此外，因为将整个装置的压力提高以产生过量冷冻，氮气产物流37和39在较高压力下离开所述装置，从而氮气压缩机的能量要求下降。
[0029]在该具体实施例中，通过应用本发明还可实现非常大基建投资益处。通过优选在较高压力下运行所述装置，允许所述装置各件设备的尺寸小许多，从而避免需要建立两个独立空气分离装置列，如在低压运行的如此大生产量装置将需的。能通过该较高压力运行而变小的设备有所有的BAHX、蒸馏塔和管道及所述装置的预纯化器。此外，在较高压力下运行该装置提供了与气体涡轮空气压缩机(GTAC)的有效、直接结合；在较高压力运行该装置允许最佳使用GTAC萃取空气。
[0030]尽管主进料空气压缩机需要较高能量，本发明实践提供了超过常规实践的优势。这显示于表1中，该表显示了常规实践(A)和采用本发明实践(B)的图1中所示循环的归一化能量消耗。表1中的数字编号与图1相同。所述实施例用于说明和对比而非限定，氧产品流42以1115psia离开所述装置，氮产物流41被压缩到335psia。此外，本发明实践能有效地生产适量的液体产品。一部分过量涡轮冷冻可用于制备液体产物且相关装置的能量会非常低。
表1
[0031]当氧产物的压力为至少250psia时，本发明实践的益处将特别有益。通常通过本发明实践，氧产品的压力将为200-1500psia。
[0032]尽管已参考某些实施方案和低温空气分离循环对本发明进行描述，本领域技术人员会认识到：在权利要求书精神和范围内，本发明存在其它实施方案和其它低温空气分离。

Claims (10)

1.一种将进料空气低温分离的方法，其中进料空气在主换热器中冷却，通过在至少一个塔中低温精馏分离而产生富氧液体和富氮蒸气，将富氧液体压力提高，通过与至少部分进料空气进行间接热交换使所述加压的富氧液体蒸发，产生产品氧，改进之处包括：产生超过进行低温精馏所需的足够过量冷冻，使得所述方法的总热端温差超过主换热器的最低内部温差至少2K。
2.权利要求1的方法，其中至少部分冷冻通过富氮蒸气的涡轮膨胀产生。
3.权利要求1的方法，其中至少部分冷冻通过进料空气的涡轮膨胀产生。
4.权利要求3的方法，其中所述进行涡轮膨胀的进料空气为液化的进料空气。
5.权利要求1的方法，其中所述总热端温差超过所述最低内部温差至少3K。
6.权利要求1的方法，其中所述总热端温差超过所述最低内部温差至少4K。
7.权利要求1的方法，其中所述氧产品的压力为至少200psia。
8.权利要求1的方法，其中所述富氧液体在所述主换热器中蒸发。
9.权利要求1的方法，其中所述富氧液体在与主换热器分离的产物再沸器中蒸发。
10.权利要求1的方法，所述方法还包括回收富氮蒸气作为产品氮。
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