CN1098447C - 从液化天然气中除去氮气的方法 - Google Patents

从液化天然气中除去氮气的方法 Download PDF

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CN1098447C
CN1098447C CN96106235A CN96106235A CN1098447C CN 1098447 C CN1098447 C CN 1098447C CN 96106235 A CN96106235 A CN 96106235A CN 96106235 A CN96106235 A CN 96106235A CN 1098447 C CN1098447 C CN 1098447C
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D·A·科义耳
F·J·F·迪拉韦拉
C·A·德尔
A·拉斯托吉
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MW Kellogg Co
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Abstract

本发明公开了使用散热片式回流热交换器从液化天然气(LNG)中去除氮的方法。一个较暖热的高压LNG流和一冷的低压LNG流在热交换器中被逆流地导向，从而在回流热交换器中使高压流冷却并使低压LNG流部分气化。由此产生的蒸气将低压LNG流中的氮气提。冷的低压LNG流通过经冷却的高压LNG流的膨胀产生。由膨胀产生的气体和在交换器产生的蒸气相混合并在顶部回收。贫氮的产品液化天然气从交换器的底部回收。

Description

从液化天然气中除去氮气的方法
本发明涉及用回流或散热片式热交换器来从液化天然气(LNG)中除去氮气的方法。
大家已知多种处理液化天然气中氮气的方法和技术。其例子包括下列的美国专利：Laverty等的2500129；Eakin等的2823523；Hoffman的3559418；Harper等的3874184；Bailey等的4225329和Nelson等的5036671。其大多数涉及分馏和/或部分冷凝的天然气流中的富氮气流的分离。
在散热片式热交换器的生产中的最新进展已使这种设备在包括空气分离；氢气、乙烯、天然气液体和液化石油气的回收；和二氧化碳的纯化等一些低温方法中代替常规的蒸馏塔使用。该设备也称为回流交换器，传热和传质操作均可同时高效地实现。由于其轻便、设计紧凑，回流热交器一般具有高的表面积/体积比，优选仅以2至3℃的最低温度驱动力来操作。
回流交换器包括相邻通道用于导入原料和传热流体。最好液体原料流通过重力向下流过原料通道而加热的流体通过相邻的传热通道向上流动，因此它们是彼此逆流的。热传递到向下流动的流体中使其至少部分气化。如此形成的蒸气通过同一原料流通道上升而将最轻组分的液相汽提。然后原料蒸气相从原料通道的顶部收回。
在这种布置中，回流交换器相当于蒸馏塔的汽提部分。但是其明显存在重大的区别。沿单元全线的同时热交换和分离由于增大了热力效率而允许传热和传质的驱动力维持在小的水平。由于驱动力小，气液相间的温度和组成差异差更近于代表可逆的热力学过程。因此回流交换器相当于每级均有再沸器的多级汽提塔。
作为多级汽提塔的回流交换器也提供了超过常规蒸馏塔一些其它优点。在常规的部分气化(汽提)过程中，原料被加热到足够高的温度以确保大部分较轻组分汽化和回收。这可能导致较大量不想要的较重组分被气化进入气相。相反，用较低平均再沸温度的回流交换器具有较少量的被气化的较重组分。结果，由于重沸用的热负载降低而使加热负荷下降。换句话说，对同样的重沸负荷，可获得更好的回收率。
可以看出对于气态原料流，类似的交换器也同样地用作多级精馏器。在每级同时发生的致冷源将原料冷凝、将气体回流。
有关散热片式热交换器的综述及其在天然气加工中的使用由Finn，A公开于Chemical Engineering，1994年5月出版的101卷第5期142至147页中。
Costain Oil，Gas & Process，Ltd.Plate Fin Exchanger Bulletin of1989，P5-9描述了用于设计散热片式热交换器的尺寸计算方法。
French的美国专利3203191描述了使用骤冷器来降低需能的气体液化方法。
Paradowski的美国专利4334902描述了通过用来自在液态条件下其膨胀后低温冷却的液体致冷剂的蒸气冷却气体而液化天然气的方法，其中蒸气同时也将液化的致冷剂低温冷却。低温冷却后的高压液体致冷剂在水轮机中膨胀。
通过用回流散热片式交换器替代常规氮分离塔完成从液化天然气(LNG)中去除氮气，达到了节能和减少了基本投资。
作为一个实施方案，本发明提供了用于天然气液化工厂的除氮方法，以从含至少80％(摩尔)甲烷和至多20％(摩尔)氮气的较温的高压液流中去除氮气。作为步骤(a)，较温的高压液流在增强的表面热交换器中逆着较低压的液化天然气流冷却而形成较冷的高压液流并将低压液化的天然气流部分气化。作为步骤(b)，步骤(a)的较冷的高压液流被膨胀而形成深冷却的液气混合物。作为步骤(c)，当步骤(b)的混合物被送到分离器形成液流和气流。作为步骤(d)，步骤(c)的液流作为较低压流供应到步骤(a)的热交换器中，其被部分气化而形成增浓了氮含量的流体和贫氮的液态产物流。作为步骤(e)，低压液化的天然气流在热交换器中和在其中气化的流体逆流接触而从中汽提氮。作为步骤(f)，在热交换器中气化的流体被供到步骤(c)中的分离器。作为步骤(g)，增浓了氮含量的蒸气流从分离器回收。
在优选的实施方案中，在步.骤(a)、(d)和(e)中的热交换器包含一散热片式交换器。较温的高压液流具有约-165至约-130℃的温度和约1MPa至5MPa的压力，来自分离器的液态产物流蒸气流具有约0.1MPa至0.5MPa的压力。液态产物流被收集于贮存罐中。低压液化天然气流靠重力向下流过处于经调整便于气化流体向上流动的通道中的热交换器。
在一种安排中，膨胀步骤(b)优选用Joule-Thomson阀来完成。在另一种安排中，膨胀步骤(b)优选用液体骤冷器来完成。
附图是使用回流热交换器的本发明的LNG氮去除方法的示意图。
散热片式/回流热交换器在从液态天然气中去除氮气的处理中可有利地代替常规的蒸馏塔使用，由于氮和甲烷间的相对挥发性存在足够大的差异，可避免需要太多的级和太大的重沸率。
参照附图，氮分离单元10包含强化表面热交换器12，它最好由用作多级汽提塔的垂直向的散热片式交换器组成。所述散热片式交换器12包括具有用于导入较暖热的高压液流的管线16的第一通道14。所述较暖热的高压流体16最好包括液化天然气，其组成为至少80摩尔％的甲烷和至多20摩尔％的氮，温度在约-165℃和-130℃之间，压力在约1MPa和约5MPa之间。
较暖热的高压LNG流16在通过散热片式交换器12的第一通道14向上流动时，由于和通过管线18导入一般靠重力通过散热片式交换器12的相邻的第二通道20向下流动的较冷低压LNG流的热交换而逐渐被冷却。
在本发明的实践中，从较温的高压上行液流16至较冷的低压下行液流18的连续交换的热使低压液流18部分地气化。富含轻组分如氮的流18的气相向上流动，并与向下流动的流18的液相紧密接触而将液相中的另外存在的轻组分如氮汽提。贫含轻组分象氮的液体产品流通过管线22从交换器12中排除。
热被传递到在第二通道20中的低压液流18而使在第一通道14中的温的高压液流16不断冷却，从而较冷的高压液流通过管线24导出。然后所述的高压液流24通过膨胀减压，一般是通过Joule-Thomson阀26来进一步冷却流24并部分地气化最轻组分。
一低压多相流在管线28被送到分离器转鼓30以分离液相和气相。所分离的液相作为冷的低压液流通过管线18被导至上述的交换器12。与此同时，通过第二通道20向上流动的气流也通过管线18通入到分离转鼓30并和从多相流28分离的气相相混合。富含最轻组分诸如氮的混合后的气流通过管线32导出。
在从LNG中进行氮分离的过程中，贫氮LNG产品流通过管线22导出，富氮气流通过管线32导出。LNG产品流22可保存在供应具有高压卸出线38和泵36的存贮鼓34中。富氮气流32可用作燃料气体。
在另一实施方案中，可用液体骤冷器(未画出)代替膨胀阀26来从液流24的膨胀回收功并节约在该过程中其它处所花的压缩能。
散热片式热交换器的设计和制造是在本领域为人熟悉的。这种交换器一般是用钎焊铝制造，但也可用其它材料如不锈钢制造。散热片式热交换器一般以逆流方式操作，较温和较冷的液流16和18通过第一和第二流动通道14和20逆流流动。
本发明方法将通过下列实施例来进一步说明：
                     实施例
用ASPENPLUS软件对附图所示的LNG氮去除方法进行计算机模拟。最初模拟设置包括100、102、104、106和106五级的RADFRAC块，每级均有一内重沸器。第一通道14的每级压降设在11KPa。其它输入参数列在表1中。
                表1
    入进流    ：     属性
    流速  (mol/hr)     18511.1
    温度  (℃)     -149.0
    压力  (MPa(a))     1.990
                组成(mol％)：
    He     0.060
    N2     4.212
    C1     87.788
    C2     5.241
    C3     1.733
    异C4     0.352
    正C4     0.550
    异C5     0.055
    正C5     0.009
          温度分布第一通道14(℃)
    第5级    108     -161.0
    第4级    106     -159.0
    第3级    104     -157.0
    第2级    102     -156.0
    第1级    100     -154.0
    压力鼓  (MPa(a))     0.125
来自进行天然气液化的主交换器的较温的高压LNG被通过管线16导入到气提回流交换器12的第一通道14，在那里较温的LNG流被冷却。所述温的高压LNG流具有约4.212mol％N2和87.788mol％C1的组成。温度为-161℃的经冷却的高压LNG流通过管线24从交换器12导出。这个LNG流被膨胀到0.125MPa(a)并具有-165.8℃的相应温度。气相分离后，经冷却的低压液态LNG流通过管线18被再导入到交换器12的第二通道20。在交换器12中，经冷却的低压LNG流18被再加热并部分气化。再加热后，经其中产生的蒸气气提氮后的液态低压LNG流作为产品LNG流在-158.5℃通过管线22输出到交换器。所述LNG产品流22包括约0.391mol％N2、90.814mol％C1和8.795mol％C2-C5。包括排放产生的蒸气28和在交换器12中产生的蒸气18’的富氮气流32包含约39.750mol％N2和59.628mo1％C1。
结果总结于表2中。此外，结果表明在处理端和致冷剂端之间没有扭点出现。包括两端面积之和的截面面积计算值约为1.4m2。
                         表2
     LNG产品流22     蒸气流32
    流速    (mol/hr)     16714.3     1796.8
    温度    (℃)     -158.5     -164.3
    压力    (MPa(a))     0.133     0.125
                        组成  ：
    He     0     0.618
    N2     0.391     39.750
    C1     90.814     59.628
    C2     5.804     0.004
    C3     1.920     0
    iC4     0.390     0
    nC4     0.610     0
    iC5     0.061     0
    nC5     0.010     0
                温度分布第二通道20(℃)
    第5级  108     -164.3
    第4级  106     -162.6
    第3级  104     -161.2
    第2级  102     -159.8
    第1级  100     -158.5
                  每级热输入(Q)(kw)
    第5级  108     555
    第4级  106     568
    第3级  104     289
    第2级  102     584
    第1级  100     1505
通过前面描述和实施例的方式说明了本发明的氮去除方法。前面描述并不是限定性说明。因为鉴于此本领域技术人员很容易做出多种改变。而所有在所附权利要求书的范围和精神内的这种改变均在本发明范围内。

Claims (6)

1.用于从含至少80mol％甲烷和至多20mol％氮的较暖热的高压液化天然气流中去除氮的天然气液化工厂的除氮方法，它包括下列步骤：
(a)将较暖热的高压液化天然气流在散热片式热交换器中逆
   着较低压的液化天然气流冷却而形成较冷的高压液化天
   然气流并将较低压的液化天然气流部分气化；
(b)将步骤(a)的较冷的高压液流膨胀以进一步冷却该液流并
   形成液体和蒸气的冷的混合物；
(c)将步骤(b)的液体和蒸气的冷的混合物送到分离器将该冷
   的混合物分离为液流和蒸气流，该液流含有较低压的液化
   天然气流；
(d)将步骤(c)的液流作为较低压液化天然气流供应到步骤(a)
   的热交换器中，由此使所述的液流被部分气化而逆着较暖
   热的高压液化天然气流而形成气化的富氮的流体和贫氮
   的液态产物流；
(e)使低压液化的天然气流逆流流动在(d)步骤供应到热交换
   器中与在热交换器中(d)步骤形成的气化的流体接触而从
   所述的低压气流汽提氮并由此产生贫氮的液态产物流；
(f)将(d)步骤的气化的富氮的流体供应到步骤(c)的分离器
   中；和在其中合并气化的流体与步骤(c)中分离的气流；以
   及
(g)从分离器回收合并的气化的流体和气流，其中合并的气化
   的流体和气流是富含氮含量的。
2.权利要求1的氮去除方法，其中在步骤(a)、(d)和(e)中的热交换器包括散热片式交换器。
3.权利要求1的氮去除方法，其中较暖热的高压液流具有-165℃至-130℃的温度、1MPa至5MPa的压力，来自分离器的液体产物流和蒸气流具有0.1MPa至0.5MPa的压力。
4.权利要求1的氮去除方法，其中膨胀步骤(b)用Joule-Thomson阀来完成的。
5.权利要求1的氮去除方法，其中所述膨胀步骤(b)用液体骤冷器来完成。
6.权利要求1的氮去除方法，还包括将步骤(d)的贫氮的液态产物流收集在贮罐中。
CN96106235A 1995-05-09 1996-05-09 从液化天然气中除去氮气的方法 Expired - Lifetime CN1098447C (zh)

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