精馏塔提馏段串级控制系统设计1111111

目录引言··································································································1第一章精馏塔的自动控制········································································31.1控制目的·······················································································31.2精馏塔的扰动分析··········································································41.3精馏过程·······················································································5第二章精馏塔的特性··············································································92.1静态影响的分析·············································································92.2动态影响分析···············································································142.3补偿方法·····················································································152.4精馏塔的质量指标控制··································································17第三章串级控制理论············································································203.1串级控制系统的结构·····································································203.2串级控制系统的基本原理·······························································213.3串级控制系统的分析·····································································233.4串级控制系统的设计和控制器的选择················································243.4.1主、副回路的设计···································································243.4.2主、副控制器规律的选择··························································253.4.3主、副控制器正反作用的确定····················································263.4.4串级控制系统PID参数的整定····················································26第四章精馏塔提馏段串级控制系统的设计·················································324.1控制方案的确定···········································································324.2提馏段温度控制系统的设计····························································33第五章仿真研究··················································································365.1提馏段温度串级控制系统仿真模型···················································365.1.1测量变送环节单元···································································365.1.2执行器/控制阀········································································375.1.3被控对象···············································································375.2控制器的参数整定········································································375.2.1副回路的整定·········································································385.2.2主回路的整定·········································································405.3蒸汽压力扰动的仿真分析·······························································425.4进料量扰动的仿真分析··································································435.5单回路控制系统的MATLAB仿真及扰动分析·····································44结论································································································471引言“过程控制”是一门与工业生产过程联系十分密切的课程。随着科学技术的快速发展，过程控制也在日新月异的发展。它不仅在传统工业改造中，起到了提高质量、节约原材料和能源，而且在减少环境污染上也起到了十分重要的作用，所以，它在规模大和结构复杂的工业生产过程中是必要的组成部分[1]。精馏操作是炼油、化工生产过程中的一个十分重要的环节。精馏塔的控制直接影响到产品的产量、质量和能量的消耗，同时各塔工艺结构特点各不相同，这就需要对其特性进行深入分析，针对具体塔的特点，进行自动控制方案的设计和研究。精馏塔的控制最终目标是：在保证产品质量的前提下，使精馏塔釜液为主要产品时，经常按提馏段的指标来进行控制。如果是按照液相进收率最高、能耗最小，或使总收益最大。在这种情况下，为了能够更好实现精馏的目标就有了提馏段温度控制系统的产生。按提馏段指标的控制方案：当进料量为液相时，也常常采用这种方案。这是因为在液相进料时，对于进料量的变化，首先影响到精馏塔塔底产品的浓度，由于塔顶或精馏段塔板上的温度不能够很好的反映出浓度的变化，所以采用提馏段温度控制。另外，提馏段温度是衡量质量指标的间接指标，以改变再沸器加热量作为温控手段的方案，就是所说的提馏段温控。精馏是化工、炼油生产领域中极为广泛的传质传热过程，目的是将混合物中的各组分分离，达到规定标准的纯度。比如说，石油化工生产中的中间产品裂解气，需要通过精馏操作进一步分离成要求很高纯度的乙烯、丙烯、丁二烯及芳烃等化工原科。精馏过程的实质，是利用混合物中各组分具有不同的沸点，也就是说，使液相中的轻组分转移到气相中，而气相中的重组分转移到液相中，从而达到分离的目的。此外，对于精密精馏，如果没有相应的自动控制和它配合，就很难达到预想的效果。所以，精馏塔的自动控制极为重要，同时也很受人们的关注。精馏过程中的主要设备就是精馏塔。有关精馏塔的结构设计有各种各样的类型，但一个精馏塔一旦投入使用，塔板的结构和数量就不容易改变，在其精馏塔的整个运行过程中，只能通过操作条件的不断变换来保证产品的质量。从工程控制的角度出发，希望能更好地控制精馏塔来保证产品质量，提高产品产量和降低能量消耗。目前一般的精馏塔和它的附属设备中，大多数工艺参数均已得到了测量和控制。如2果仅仅是从每个单独的控制回路着手来改进一个塔的性能，都会遇到难以克服的困难。之所以控制方案多，是因为在精馏塔的操作过程当中，被控的变量比较多，可以选用的操纵变量也很多，同时又可有各种不同组合。由于精馏塔是一个高阶对象，动态响应缓慢，所以较难控制，剧烈的波动易出不合格产品。因此工艺对精馏塔操作的基本要求是能达到规定的产品分离度，并且能量的消耗要小。常规上，采用了串级控制系统。3第一章精馏塔的自动控制1.1控制目的要对精馏塔实施有效的控制，必须首先清楚精馏塔的控制目的，也就是对控制的要求。一般来说，对于精馏塔的控制系统必须能够保证产品的质量指标、物料能量平衡和约束条件，这些是一个精馏过程能平稳安全生产出合格产品的前提[2]。1、质量指标塔顶或塔底产品之一应该保证合乎规定的纯度，另一产品成分也应该维持在规定的范围之内，或者塔顶和塔底产品均应保证一定的纯度要求。对于二元精馏来说，质量指标就是为塔顶产品中的轻组分（或重组分）含量和塔底产品中的重组分（轻组分）含量；但是，对于多元精馏，通常以关键组分的含量来描述质量指标。所谓关键组分，是指对产品质量影响较大的组分，挥发度较大而由塔顶馏出的关键组分，称为轻关键组分；而挥发度较小由塔底馏出的关键组分，称为重关键组分。图1-1精馏塔示意图2、物料能量平衡馏出液和釜液的平均采出量之和，应等于平均进料量，并且这两个采出量的变动应该比较和缓，有利于上下工序的平稳操作，塔内及顶、底容器的蓄液量应介于规定的上、下限之间；此外，精馏塔的输入输出能量应平衡，以保证塔内操作压力的稳定不变。3、约束条件4为了能够让精馏塔工作正常、操作安全，我们规定它的约束条件在一定范围内，约束条件为液泛限、压力限和边缘温差限等等。液泛限，又名气相速度限，就是精馏塔里德气相速度比较高时，雾沫夹带会比较严重，事实上，液相是从下面塔板倒流到上面塔板上，产生液泛破坏正常操作。漏液限也称最小气相速度限，当气相速度小于某一值时，将产生塔板漏液，板效率下降。最好是在稍低于液泛的流速下操作。流速的控制还要考虑塔的工作弹性。对于浮阀塔来说，由于工作范围较宽，通常很易满足条件。但对于某些工作范围较狭窄的筛板塔和乳化填料塔就必须要很好地注意。防止液泛和漏液，我们可以用塔压降或者是压差来监视气相速度。压力限是塔的操作压力的限制，一般来说，塔操作压力不能过大，否则会影响塔内的气液平衡，严重越限有可能导致安全生产。临界温差限主要是指再沸器两侧间的温差，当这一温差低于临界温差时，给热系数将会急剧下降，传热量也会随之下降，不能保证精馏塔的正常传热的需要。4、能量平衡和经济平衡性指标在保证精馏塔产品质量、产量的同时，考虑降低能量的消耗，使能量平衡，用来实现较好的经济性。1.2精馏塔的扰动分析影响物料平衡的因素包括进料量和进料成分的变化、塔顶馏出物及塔底部出料量的变化。影响能量平衡的因素主要包括进料温度或釜温的变化、再沸器加热量