8.EN_CN_真空制盐蒸发结晶器的设计与实践-re

8.EN_CN_真空制盐蒸发结晶器的设计与实践-re 第九届世界盐业大会 真空制盐蒸发结晶器的设计与实践 罗大忠 (自贡市轻工业设计研究院，四川自贡 643000) 摘 要:真空蒸发制盐外热式强制逆循环抽向出料蒸发结晶器，经多个厂家生产应用 实践证明是成功的，具有生命力的。这种新型结构，作为一项新技术新设备应加强研究，总 结提高，推广应用，不断完善。文章从流体力学、结晶机理角度要求，到具体工程设计参数 和材质选用。论述了该罐的特点。 关健词:真空制盆;蒸发结晶器;结晶机理;罐型结构;设计参数;材质选用 中图分类号:TS3 文献标识码:A 文章编号:1001-0335(2003)05-0009-06 Design and Practice of Evaporation Crystallizer in Vacuum Pan Salt Making Luo Dazhotig (Zigong Light Industry Design and Research Institute Zigong, Sichuan 643000) Abstract: In the application by many factories, the backward forced circulation and axial discharge evaporation crystallizer with external heater has been proved to be successful and vigorous. This new equipment should be further researched, improved, popularized and perfected. This article elaborates the characteristics of this equipment on the bases of hydromechanics, crystallization mechanism, specific design parameters and the selection of materials. Key words: Vacuum pan salt making; evaporation crystallizer; crystallization mechanism; structure of evaporator; design parameters; selection of materials. 1 前 言 蒸发和结晶是重要的化工单元操作过程，在真空制盐行业中处于关键地位并起主导 作用。目前我们所采用的蒸发结晶器是在原始蒸发装置的基础上发展起来的，它不再是仅仅 为了强化传热及蒸发能力而获得产品，同时更主要的是以提高结晶产品的质量和粒度为目 的。所以说传热及蒸发是为结晶产出合格的产品创造传热、传质的条件和环境。在传热蒸发 过程中，严格控制料液的过饱和度以及晶核的形成和成长环境，产出合格的结晶产品，这是 蒸发与结晶相结合的原理方面向前迈进了一大步。 2 蒸发结晶器的沿革 盐的生产主要是通过对卤水进行加热，使其蒸发浓缩结晶析出固体NaCl的过程。 随着社会发展和科学技术进步，盐作为人们食用所占比例越来越小，而是大量作为基础化学 工业和其它工业部门的原料。盐的品种由古老的雪花盐、筒盐、锅巴盐，发展到今天的各种 特殊要求用途的特种盐。制盐设备也由古老的作坊式手工操作的园锅、镶锅、小方锅、小平 锅、大平锅，至近代制盐工业用的内热式强制循环(标准式)蒸发结晶器和现代外热式强制 正循环(又分为切向进料和轴向进料两种)蒸发结晶器及外热式强制逆循环(分为径向出料 和轴向出料两种)蒸发结晶器。这也是目前国内制盐企业应用最多的蒸发结晶器(如图1 所示)。若为了获得粒径更大的结晶盐可在上述蒸发结晶器上增设奥斯陆(OsLo)育晶器。 D?T?B型育晶器或倒园锥型育晶器，这样可获得粒径在Imm至数毫米的结晶盐产品。 1 a.外热式强制正循环切向进料蒸发结晶器;b.外热式强制正循环轴向进料蒸发结晶器; c.外热式强制逆循环径向出料蒸发结晶器;d..外热式强制逆循环轴向出料蒸发结晶器。 3 NaCl结晶机理简介 3.1 NaCl结晶的环境和条件，NaCl结晶要从盐卤料液中结晶析出，料液必须从外部不断地获得热能，使料液中的水分不断蒸发浓缩，使其达到饱和和过饱和(如图2所示)。 3.1.1 当卤水未达到饱和时NaCl不会产生结晶，当放入NaCI晶体时则会溶解。如图2 AB线下方的不饱和区域(稳定区)。 3.1.2 当卤水继续蒸发NaCl达到饱和，如图2中的AB线即平衡溶解度曲线进人介稳区，此时NaCl结晶和溶解处于动态平衡，溶质NaCl不会自发成核析出结晶。若有NaCl晶核进人就能生长成晶体，即图2中AB线和CD线之间的介稳区; 3.1.3 当卤水继续蒸发溶质NaCl含量超过过饱和线CD线进人过饱和区(不稳区)，则会自发地产生较多NaCl晶核。 三个区域以介稳区为最重要(当料液中有晶体存在的条件下即使在介稳区中也会有晶核发生，而介稳区极易受外界影响即有无晶种、晶种大小、多少，有无搅拌、振动及杂质等等因索)，晶体的成长应控制在此区域内进行。而NaCl与其它盐类比较，其介稳区范围非常窄。所以要获得较大粒径的晶体较难。而溶液的过饱和度ΔC是结晶成长的推动力，是关键因素，其关系式如下: ΔC=C,C (g/L)„„„(1) 21 式中:ΔC——溶液的过饱和度(g/L); C——溶液在同一温度下的平衡饱和浓度(g/L); 1 2 C——溶液的实际过饱和浓度(g/L)。 2 要使结晶成长，必须使溶液达到过饱和，并控制在介稳区内，溶液的过饱和度完全用于晶种成长而消失。在实际的运行过程中溶液的实际过饱和度远比其最大的过饱和浓度低。有资料讲:最大允许过饱和度又取决于系统的性质通常为0.5~5g/L，一般情况下溶液的实际过饱和浓度 ΔC值宜控制在1.50g/L左右，为最大过饱和浓度ΔC的10~30% max 3.2 NaCl晶核——晶体的成长 根据化工单元操作普遍扩散理论分析，晶体成长与以下几个因素密切相关。 3.2.1 晶体成长的推动力是溶液的过饱和浓度差和传质速度。过饱和溶液中溶质扩散到晶核附近的相对静止液层并穿过相对静止溶液层到达晶体表面结晶生长在其表面上，使其晶体长大，并放出结晶热，热量再依靠扩散传递到溶液中去。如图3所示。溶液的过饱和度亦可用下式求得: 过饱和度=产盐量(g/h)/循环量(L/h)„„„(g/L)(2) 3.2.2 溶液的温度:在相同的时间和相同的溶液过饱和浓度差条件下，溶液的温度越高，溶液的粘度越小，溶质的扩散速度越快，晶体的成长速度也快。因此，溶液温度高时容易得到粒径较大的产品，如图4。 3.2.3溶液中的杂质浓度及悬浮物的变化:在相同的温度条件下，溶液中杂质含量及悬浮物增加，则溶液浓度增高。溶液粘度上升，溶液的扩散速度下降，晶体的成长速度也减小。 3.2.4 晶体在蒸发结晶器内停留时间:根据溶液中NaCl的成核速率与产品排出速率基本一致,NaCl晶体的成长速率和产品粒径的要求，从而确定晶体在蒸发结晶器内的停留时间。 3 据资料介绍，产品平均粒径~0.4mm时其停留时间应在1小时以上。晶体生长速率公式如下(按球形计): 2Ra=(6α/β)ρ V (kg/m?s)„„„„(3) 2式中:Ra——晶体生长速率(kg/m?s); α——晶体容积系数，按球形计α=1/6; R——表面积换算系数，按球形计:β=π; 3 ρ——晶体密度(kg/m); V——晶体平均成长系数(m/s)。 3.2.5 循环溶液流量:当加热蒸汽量一定时循环溶液流量和溶液的过饱和度成反比的函数关系，而循环流量又确定了蒸发结晶器各部位的流速大小，速度大又引起晶体之间、晶体与器壁之间的碰撞加剧，致使晶体破碎成二次晶核的可能性增大，对产品粒径影响也很大，因此要有适当的流量。 3.2.6 盐浆浓度:指参加循环料液中的晶体浓度，又叫固液比，在其它条件一定的前提下，盐浆浓度高则蒸发结晶器内晶体的保有量多。晶体停留时间增长有利于料液过饱和度的消除和晶体成长。但盐浆浓度过高，晶体之间、晶体与器壁之间碰撞机率增多，晶体被破碎成二次晶核的机率也多，对晶体成长也不利，所以应控制适当的晶体浓度才行，一般的固液比控制在20%左右为宜。 4 现代蒸发结晶器的设计与实践 设计是科研实验和生产实践的桥梁和纽带。工程设计不能是简单地照抄照搬前人原有的图纸、资料和成果，盲目、机械的加以缩小或放大。设计要结合国情，是一项切合实际的创新性劳动。创新是设计的灵魂与推动力。要创新必须迎接风险和挑战，必须实事求是，善于总结前人的经验、有所发现、有所改进、有所提高，设计才能做到技术先进、经济合理、安全适用、达到资源合理利用、清洁生产、节能降耗、提高经济效益之目的。 我国制盐行业目前普遍采用的蒸发结晶器型型如图1所示的四种为主。它们主要由蒸发室、加热室、上下循环管、循环泵及盐脚组成一个功能完善的罐型整体。要求结构合理、符合流体力学原理，做到系统阻力小，动力消耗省，传热效率高，蒸发强度大，汽液分离效果好，能满足盐晶成长所需的条件和环境，产出合格的产品。根据系统物料平衡和热量平衡计算结果及相关经验数据来确定蒸发结晶器各组成部分的相关尺寸和参数。现分述于下。 4.1 蒸发室直径及分离室空间高度 蒸发室相关尺寸设计应满足下述三点:一是能有效地减少和消除料液过饱和度，使晶体有一个良好的成长条件和环境。二是减少料液的短路温度损失，有利于闪发和汽液分离，尽量减少液沫带出。三是尽量使蒸发室内表面平整光洁，防止结盐垢成块成疤，确保生产正常连续运行。 4.1.1 蒸发体积强度法——即每一秒钟从每一立方米蒸发空间排出的二次蒸汽体积，当分离空间高度确定时，其蒸发室直径按下式计算。 D=?W/V?π?H (m)„„„„„„(4) 式中:D——蒸发室直径(m); 3 W——二次蒸汽体积流量(m/s); 3333 V一一允许蒸发体积强度 1.1~1.5m/m? s(有学者建议取0.8~1.3 m/m? s); H——汽液分离空间高度1.8~2.5m(另有建议2.5~3.0m); π——圆周率。 4.1.2 质量速度法——单位时间内单位蒸发表面积允许蒸发水量计算出蒸发室直径D D=?4W/π?V (m)„„„„„„(5) 式中:D——蒸发室直径(m); 4 W——蒸发室蒸发水分量(kg/h); 2 V——允许质量速度2500~800kg/m?h。末效取下限; π——圆周率。 4.1.3 近似比例法——将蒸发室分离空间看作汽液分离器，分离器直径D按下式计算。 D=?U/0.541R=?U/0.54×0.44 =6.47?U(m) „„(6) 负荷负荷负荷d 式中:D——蒸发室直径(m); 3 U=W?r/r-r(m/s); 负荷秒汽液汽 3 W——二次蒸汽体积(m/s); 秒 3 R——二次蒸汽重度(kg/m); 汽 r——料液重度(kg/m); 液 R——实际汽体速度与基础速度之比值(一般取0.44，若设有捕沫器可取d 1.15)。 4.1.4 二次蒸汽断面流速法——通过蒸发室的二次蒸汽流速(即空塔速度)设计确定为4~7m/s，笔者认为在多效蒸发时末效二次蒸汽速度宜控制在4m/s左右，其直径D按下式计算: D=?4GV/πW (m)„„„„„„(7) 式中:D——蒸发室直径(m); G——二次蒸汽量(kg/h); 3V——二次蒸汽比容(m/kg); W——二次蒸汽流速(m/s); π——圆周率。 4.1.5 二次蒸汽分离空间高度——指蒸发室液面上汽液分离段的有效高度。料液经加热进入蒸发室沸腾，汽泡不断产生，穿过料液层到达液面汽泡破裂，逸出二次蒸汽时所带出的液滴和液沫，绝大部分能沉降回落的有效高度。文献资料上大多采用2.0~3.0。 我国现生产的蒸发结晶器蒸发室有效分离空间高度(在没有捕沫设施的情况下)大多在3m以上。尚有进一步增高的趋势。 4.1.6 二次蒸汽管径——二次蒸汽比容随压力而变化，为了减小二次蒸汽管道压力和温度损失，目前设计中二次蒸汽管内流速大多控制在10~40m/s进行计算。 4.2 加热室面积、加热管直径、长度的确定 4.2.1加热面积的确定:根据物料平衡和热量平衡计算结果及选用的参数，按下式计算: 2 F=Q/K?Δt (m) ??????????????????(8) 2式中:F——加热面积(m); Q——单位时间内传热量(kcal/h); 2 K——总括传热系数(kcal/m?h??); Δt——传热有效温度差(?)。 4.2.2 加热管直径与长度的确定 在一定的热负荷条件下，确定了加热面积，这就基本确定了进出加热室料液温度升高Δt和料液的过饱和浓度Δc。根据Δt与直控d、长度L和管内料液流速W之间的关系，我cc 们可用下式计算。 Q=K?F?Δt=k?n?π?d?L?Δt (kcal/h)???????(9) 2Q=π/4?d?n?w?rc?Δt?3600 (kcal/h)???????(10) c 将(9)代入(10)化简得: Δt=K?Δt?L/900?r?w?d (?)„„„(11) c 式中:Q——传热量(kca/h); 5 2 K——总括传热系数(kcal/m?h??); Δt——传热有效温度差(?); Δt一一进出加热室料液温度升高(?); c d——加热管计算管径(m); L——加热管有效换热长度(m); W——加热管内料液流速(m/s); 3 R——循环料液重度(kg/m); C——料液比热(kcal /kg??);, n——加热管根数; π——圆周率。 上式说明当热负荷和加热面积确定后，Δt与L成正比，与d和W成反比。Δtcc是设计时必须予以重视的一个重要参数。因为Δt与料液的过饱和度，晶体的成长速度及盐c 产品的产量和质量密切相关。一般应先确定Δt值的情况下再决定加热管直径d、长度 L及c 料液在加热管内流速W。 根据文献资料和实践经验，Δt大多在3?左右,最高达5?以上，管径d大多在,外c 32~,45mm，管长L大多在5000~7000mm，最长达12000m以上，加热管内料液流速一般在1.2~2.5m/s范围内选取，大多控制在1.8m/s左右。 4.2.3 加热管根数的确定 当加热面积确定之后按下式计算 n=F/π?d?L (12) 式中:n——换热管根数; 2 F——加热面积(m); d——加热管计箕直径(m); L一一加热管有效换热长度(m); π——圆周率。 上式关键在于加热管计算直径的选取。我国对于列管式换热器计算的有关标准规范规定，加热管计算直径取外径。我国制盐行业传统习惯采用加热管内径与外径的平均值——中径作为计算直径。根据笔者的设计实践与资料介绍，因为总括传热系数K值直接受管外蒸汽冷的给热系数a和管内料液对流给热系数α所左右。当α< α时计算直径取外径。1212 仪α= α时计算直径取中径。α> α时，计算直径取内径。在我们的制盐加热室中，管1212 外蒸汽侧的α远远高于管内料液侧的α，起码α是α的2~3倍。所以笔者认为在计算用1212 于制盐的加热室加热管根数时应取加热管内径作为计算直径为宜。 4.2.4 加热室筒体直径的确定 在加热管直径和根数确定之后，制盐用加热室内加热管多用等边三角形方式排列，其筒体直径可用以下经验公式计算: D=1.15Pt?N (m) „„„„„„(13) 式中:D——加热室筒体直径m; Pt——管间距mm。 N——加热管根数。 管间距——加热管与加热管的中心距离，采用胀管法Pt=1.3~1.5d。我国制盐行业外在较长时期内，一套多效装的加热室均采用相同的管间距，这样各效加热室直径也均相等。笔者认为这是欠妥的，从首效到末效进人加热室的蒸汽比容相差很大，为降低压力及温度损失有利于传热，应依据蒸汽比容的不同，也应采用不同的管间距。在较高真空度的状态下末效的管间距应进一步扩大。所以Pt=1.3~1.8d来选取为好。 外 6 4.2.5 加热室蒸汽进口的设置 为了防止高速蒸汽流对加热管的冲刷而引起侵蚀和振动，应在蒸汽进口处设防冲挡板或导流器。 支架 挡板 设防冲挡板时，防冲挡板与筒体内侧的距离h应大于加热蒸汽进口管内径的0.25d，挡板直径D大于进口管直径d，其蒸汽通道切面积，必须大于蒸汽管进口横切面积。如图5a所示。 支架 设导流器，是一种较为理想的蒸汽进口防冲结构，也是制盐行业用得较多的一种。它将流速较高的蒸汽送入夹套的环型槽内进行再分配，其环型通道宽度h与蒸汽进口管直径d的关系是h?0.3d，如图5b所示。 4.2.6 加热室内加折流板 我国制盐行业早期在加热室内大多没有加折流板，到80年代因材质的改进，耐蚀 挡板 性能提高，更换加热管的周期延长和引进国外制盐技术装备开始，我国自己设计的加热室内开始装设折流板，以强化传热过程。大多采用圆块型折流板，其圆缺率基本按统一的0.25D进行设计。折流板间距采用上宽下窄方式，最大处亦应小于加热室直径。笔者认为，蒸汽冷凝型若设折流板，其圆缺率应扩大至0.45D左右，不应当是统一的，应该是上大下小。因为加热蒸汽进人加热室逐步冷凝放热，蒸汽体积流量逐步减少之故。而折流板的间距也是同样原因，应该是上高下矮。另外，笔者很赞同在好些文献资料上讲的:在换热器壳层空间为蒸汽冷凝时折流板对其给热系数α的大小几乎没有影响，所以不需装设折流板。但为了增加1 管束的刚度和防止管子振动和冷凝液的排除而装设折流板者是例外。 4.3 循环管直径的确定 上下循环管直径通常都是按经验数据作为计算依据。外热式强制循环蒸发结晶器下循环管切面积与加热管总切面积之比，一般文献资料为0.8~1.1。上循环管切面积与下循环管相同，或者上循环管直径为蒸发室直径的0.2~0.3倍。经长期实践，为了降低料液在循环管中流动阻力，降低循环泵扬程宜将循环管直径加大。从目前设计情况看循环管切面积与加热管总切面积之比控制在1.0~1.5之间。近期有进一步扩大的趋势，而上下循环管直径趋于一致。而加热室上锥体与上循环管连接的弯头，传统是采用等径弯头。笔者将其改为渐缩式弯头，实践证明效果良好。 4.4 加热室上花板至燕发室液面的距离 为了防止料液经加热在加热管内沸腾造成管壁结垢而降低总括传热系数，甚至发生堵实 7 心管而影响生产正常运行，所以在上花板以上必须保持足够的液柱高度。这个液柱高度大致可用下式计算。 H=(P,P)/r (m) „„„„„„(14) 12 式中:H——液柱高度(m); P——料液出加热管口的温度减去料液沸点升后对应温度下饱和蒸汽压力1 (at); P——蒸发室二次蒸汽压力(at); 23 r——为汽液固三相流的平均重度(kg/m). 4.5 循环泵 循环泵是外热式强制循环蒸发结晶器的重要组成部分。其性能的优劣直接影响装置的能耗、产品粒度、生产强度、运行周期及运行费用。从设计到生产不仅要求泵的流量、扬程，能满足装置所需，更要求泵的效率高，操作、维修方便、安全、运行时间长，对晶体的破碎率少。 制盐用循环泵属于大流量、低扬程、高比转速的轴流泵。泵的流量、扬程、功率分别和转速的一次方、二次方、三次方与泵叶直径的三次方、二次方、五次方成正比关系。而比转速又是确定泵叶形状及性能参数特性和汽蚀性能的主要参数。当前泵的转速常按汽蚀比转速公式计算。 3/4 C=5.62n?Q/NPSH (15) 3式中:Q一一泵的最佳工况流量(m/s); N——泵的规定转速(r/min); NPSH——设计或最佳工况时必须汽蚀余量(m); C——汽蚀比转速。 322 当泵的几何相似，运动相似时则Q=K?n?D，NPSH=kn?D，将Q与NPSH21 代人(15)式，整理得n?D=常数。 对于几何相似，运动相似的泵，n?D值相等，则汽蚀相似。目前国内轴流泵模型D=0.3m，n=1450r/min ，n?D=435。模型泵可在此值下运行可以无汽蚀。因而把n?D=435(国外较先进的大型轴流泵的n?D值为200)作为一项，来选择泵。不论泵的直径多大其n?D值应当小于435。此值越小，泵越不容易发生汽蚀。所以当流量一定时，宜选用泵叶直径较大、转速较低的泵。几何相似告诉我们，泵的流量和转速的一次方、泵叶直径的三次方成正比，因流速降低从而减小了水力损失，可以提高泵的效率，效果是非常明显的。 4.6装料容积 指蒸发结晶器循环系统间的所有料液容积。这是完成料液蒸发，使溶质NaCl达到过饱和析出晶核，并使晶体成长为所需产品粒度的地方。为确保其所需的环境和条件，其容积应满足料液每一次循环流经循环系统的时间，应大于30秒为宜。 4.7 盐脚直径与长度 盐脚是成品NaCl晶体沉降增稠储存中转的容器。外排盐浆量控制着循环料液中的固液比，确保蒸发结晶器正常运行。为了提高产品质量，降低汽耗，可加淘洗卤水对外排盐浆进行淘洗降温回溶晶体表面的可溶性杂质，浮选淘洗除去细小的盐晶和其它细小的固体微粒，起到降低排出盐浆温度，提高产品质量，降低汽耗的作用。 盐脚直径:根据科学试验和生产实践验证，按盐脚单位横切面积单位时间内沉降盐 2浆晶体量来确定，一般控制在14,20t/mh。若加淘洗卤水其上升流速视盐晶粒度与盐浆固液比而定。据文献资料介绍一般在1,10mm/s之间选用。 盐脚长度，根据国内外生产装置的实践经验大多控制在5,左右。也有控制在3m左右的。这些应视规模、工艺流程等条件而因地置宜选用才能取得良好效果。 8 4.8 材质选择 4.8.1 选用蒸发室及上下循环管和加热室上下锥体的材质，应从具体条件出发，选择表面光洁、加工、维修较易，耐蚀、耐磨、耐冲刷性能好的材料。上世纪80,90年代应用于生产的有B铜镍合金复合板、超低碳不锈钢(316L)复合板、双相不锈钢(18,5)复合,,,板、钛及钛合金复合板。经实践证明均是可行的。但超低碳不锈钢(316L)在浓,,存在的条件下易受pH值及硫化物等的影响而发生均匀点蚀、孔蚀、穿晶，甚至穿晶断裂，影响使用寿命。所以要充分注意使用环境条件，调整罐内料液pH值、除去硫化物(包括有机硫化物)等措施，从目前环境、条件，就耐蚀、耐磨、耐用、加工制造及经济合理综合考虑，以选用耐蚀性能优于(316L)的钛及钛合金或双相不锈钢(2205)复合板为宜。不管选用何种材质.对其所有与料液接触的器壁表面及焊缝必须进行打磨抛光、酸洗、钝化处理。使其表面形成光滑、致密的钝化膜以防止腐蚀和诱发晶核的附着，进而形成结晶堆积成盐块之目的。 4.8.2 加热室加热管、管板及筒体 加热室是蒸发结晶器的心脏，选取更应仔细认真地加以比较确定。上世纪80、90年代常用的有普通无缝碳钢管低合金(E2)钢管、紫铜管、B铜镍合金管、钛合金及纯钛,, 管。并匹配相应的管板或复合管板。筒体采用碳钢板或不锈钢复合板加工制作。目前从经济合理考虑，加热管宜选用钛合金及纯钛管。因为它具有表面光洁粗糙度低与水无亲合力，传热性能好，耐蚀、蚀磨、耐冲刷能力强，比重轻、管壁薄，使用寿命长等特点。管板配用相应的复合板。加热室筒体宜选用不锈钢或双相不锈钢复合板加工制作。 5 结束语 蒸发结晶器是真空制盐生产装置的关键设备，历来是设计和生产单位十分关注的焦点。特别是最近几年在《中国井矿盐》期刊上发表了不少文章，对我国普遍采用和推荐采用的罐型(如图1所示)，从蒸发结晶机理和流体力学原理角度进行探讨;从各生产厂家提供的生产数据进行技术经济和产品质量的分析对比。多数认为图1中的罐型;、,优于,、,认为;、,罐型基本克服了,、,罐型的缺点，保留其优点并充分地运用了结晶机理，料液加热升温后，在蒸发室下锥体，切向进入蒸发室，逐步形成汽、液、固三相流呈螺旋状上升， 穿过盐浆层到达液面蒸发。有利于料液过饱和度的消除和晶体成长，并对盐晶进行浮洗，到达液面料液的过饱和度低，器壁不易结盐块的优势，因而具有块盐堵管概率少，运转周期长，料液短路温度损失小，有效传热温度差增大，循环系统阻力降低，循环泵扬程下降，动力消耗减小，单位面积产量增加，从而达到节能降耗之目的。而图1中c，d两种罐型相比，又更倾向d罐型。正如江苏井神盐电厂葛总所言:“这种罐型目前正如一股流行风在井矿盐企业推广”。 虽然我国真空制盐起步与国外相比晚了半多个世纪，但是经过几代盐业科技工作者的艰苦努力，求实探索，技术创新，差距正在缩小。笔者设计的“外热式强制逆循环轴向出料熬发结晶器”(见图l，d)，并多次与国内外同行交流探讨，均受到制盐专家赞许。自1993年推出至今已有数十套在各种不同原料不同规模的大中小型真空制盐装置上推广应用。无论是新设计的厂还是在老装置上改造经生产实践验证均达到了预期目的，证明是成功的。作为一项新技术、新设备，还要继续与国内外同行交流，加强研发工作，深入到生产实践中去，努力学习、总结提高、不断完善、开拓创新。争取在更大规模的真空制盐工程设计中应用得更好。力争将此工程建成一座独具特色的一流水平的真空制盐厂，成为亚洲第一，屹立于世界。 ※本文在写作过程中得到信息中心周伯琦先生、孔志远先生的大力支持，还得到了本文的英译者吴基泰先生的顶力支助才使此文较好地完成，在此深表感谢。 参考文献 9 [1]化学工程手册编辑委员会.化学工程手册第9篇“蒸发及结晶”[S].北京，化学工业出版杜，1985. [2]上海化工学院，基础化学工程上册[S].上海，上海科学技术出版社，1978. [3]化工设备设计全书编辑委员会，换热器设计[S].上海，上海科学技术出版杜，1989. [4]中国井矿盆编辑部.井矿盐技术1970-1989年精选本[C].自贡全国井矿盐工业科技情报站，1991. [5]日本海水学会志，第44卷:第一号，平成元年二月. [6]葛永喜.真空制盐结晶机理及控制[M]，中国井矿盐，2001(2). [7]水泵技术编辑部.水泵技术——农用泵专辑[C].1975(1,2期合刊). [8]陈坚，杨树雄，朱丽楠.水泵nD值的意义及其合理取值范围. [M].水泵技术，2002(1). [9]中华人民共和国行业标准——真空制盐盆厂设计规范[S].中国轻工业出版杜，1995. [10]制盐工业手册编辑委员会.制盐工业手册[S].1994. [11]关醒凡等.南水北调工程大型轴流泵选型中值得注意的几个问[J].水泵技术，2002(2). (于2008年9月修改) (收稿日期:2003-05-15) (编辑 周伯琦) 10 Design and Practice of Evaporation Crystallizer in Vacuum Pan Salt Making Luo Dazhong (Zigong Light Industry Design and Research Institute Zigong, Sichuan 643000) Abstract: In the application by many factories, the backward forced circulation and axial discharge evaporation crystallizer with external heater has been proved to be successful and vigorous. This new equipment should be further researched, improved, popularized and perfected. This article elaborates the characteristics of this equipment on the bases of hydromechanics, crystallization mechanism, specific design parameters and the selection of materials. Key words: Vacuum pan salt making; evaporation crystallizer; crystallization mechanism; structure of evaporator; design parameters; selection of materials. 1. INTRODUCTION Evaporation and crystallization are transfer and mass transfer in which qualified important unit operation in the chemical product is made. In the process of heat process, which play a dominant role in transfer and evaporation, qualified crystal vacuum pan salt making. The evaporation product can only be made with strict control crystallizers being used are developed on the of supersaturation of feed, formation of of the primitive evaporation facilities. The nuclei and the environment of crystal growth. use of the present evaporators is not only to This means a big step taken to combine the aquire product by intensifying the heat principles of evaporation and crystallization. transfer and evaporation but to enhance the quality and grain size of the crystal product 2. EVOLUTION OF EVAPORATION as the principal purpose. That is to say, the CRYSTALLIZERS heat transfer and evaporation are the preconditions and environment for heat Salt is made by means of a process in various uses. Salt making facilities are also which brine is concentrated for developed from manual-operated kitchen crystallization of solid NaCl through heating round pan, inlaid pan, small square pan, and evaporation. With the social small flat pan and large flat pan to development and the progress of science and internally-heated forced circulation technology, the percentage of salt for human (Calandria) evaporator, externally-heated consumption is getting smaller and smaller forced circulation (with tangential inlet or while the large percentage of salt is used as axial inlet) evaporator and externally-heated raw material for basic chemical industry and reversed circulation (with radial discharge or other industries. Salt products have axial discharge) evaporator. (See Figure 1). experienced a development from ancient Oslo, DTB or inverted cone crystallizers can flake salt, tube salt, cake salt to special salt be added to the above-mentioned that meets the special requirements of evaporators if larger crystal size (?1mm) of 11 salt is required. Figure 1 Schematic drawing of evaporation crystallizer a. Evaporator with external heating, normal forced-circulation and tangential inlet b. Evaporator with external heating, forced-circulation and axial inlet c. Evaporator with external heating, reversed forced-circulation and radial outlet d. Evaporator with external heating, reversed forced-circulation and axial outlet 3. BRIEF INTRODUCTION OF MECHANISM OF NACL CRYSTALLIZATION 3.1 Crystallization of NaCl from feed brine requires ceaseless heat which evaporates the 3.1.2 Evaporation of brine continues and water content so that the feed brine becomes NaCl reaches saturation, as indicated by the saturated or supersaturated. See Figure 2. curve A-B which shows that the equilibrium solubility enters metastable zone. At this stage, the crystallization and dissolution of NaCl are in a state of dynamic balance and the solute NaCl will not crystallize out as nuclei. If NaCl nuclei enter, they will grow by themselves, as is shown in the metastable zone between curve A-B and curve C-D in the figure. 3.1.3 More NaCl nuclei will spontaneously form when NaCl content exceeds supersaturation curve C-D and enter Figure 2 Solution temperature vs solubility supersaturation zone (unstable zone) with curve continuous evaporation of brine. 3.1.1 NaCl will not crystallize when brine is Among the three zones, the most not saturated and NaCl crystals will dissolve important is the metastable zone. Under the in such unsaturated brine, as indicated by the conditions of crystal existence in feed, the unsaturated zone (stable zone) at the lower formation of nuclei will take place in the part under curve A-B in the figure. metastable zone. The metastable zone is apt 12 to be affected by such factors as crystal heat is liberated and transferred by diffusion existence, size of crystal, number of crystals, into the solution. See Figure 3. The agitation, vibration and impurities. The supersaturation of the solution can be found growth of crystals should be controlled in by the following equation. this zone. Compared to other salts, the range Supersaturation=salt production of NaCl metastable zone is rather narrow. So, (g/h)/circulation amount (l/h) (g/L) (2) it is difficult to get larger crystal size. Supersaturation ΔC of the solution is the key factor and the driving force for the crystal to grow, which is defined by ΔC=C–C (g/L) (1) 21 Where: C—equilibrium saturation of 1 solution at the same temperature (g/L) C—actual saturation of solution (g/L) 2 The growth of crystals requires supersaturation of the solution and must be Figure 3 Concentration driving forces in controlled in the metastable zone. crystallization from solution according to the Supersaturation is consumed by crystal simple diffusion-reaction model growth. During the actual operation, the actual supersaturation of the solution is far lower than the maximum supersaturation. 3.2.2 Temperature of solution According to literature, the maximum allowed supersaturation depends upon the Under the conditions of the same time nature of the system, which is normally and the same supersaturation difference, the 0.5~5 g/L. Generally, the actual higher temperature of the solution is, the supersaturation ΔC is controlled at 1.50g/L, smaller the viscosity of the solution is, the which is 10~30% of the maximum faster the velocity of the solute diffusion is supersaturation ΔC. and the quicker the crystal growth is. max Therefore, it is easier to get crystals of larger size when temperature of the solution is 3.2 NaCl crystal nucleation—crystal higher. See Figure 4. growthing In accordance with the analysis of the general diffusion theory of unit operation, crystal growth is related to the following factors. 3.2.1 The driving force for crystal growth is the difference of supersaturation of the solution and velocity of mass transfer. The solute in the supersaturated solution diffuses Figure 4 Salt crystal growth rate vs to the static liquid layer adjacent to the supersaturation nuclei, pierces the liquid layer and reaches the surface of the crystals in such a way that the crystals grow larger, and crystallization 3.2.3 Concentration of impurities in solution 13 3.2.6 Concentration of salt slurry and suspended substance Concentration of salt slurry means the At the same temperature, concentration concentration of crystals in circulating feed, of the solution increases with the increasing or the ratio of solid to liquid. Under the same of the content of impurities and suspended condition, high concentration of salt slurry is substance in solution. The diffusion velocity higher in the crystallizer, the amount of of the solution and the velocity of crystal crystals are larger. Longer residence time is growthing decrease with increasing of better for the elimination of supersaturation solution viscosity. and the growth of crystals. However, if the concentration of salt slurry is too high, 3.2.4 Crystal residence time in crystallizer collision among crystals and between crystals and wall of vessel tends to increase. Crystal residence time is determined by Hence the possibility of formation of the requirement of NaCl crystal growth rate secondary nucei due to breakage increases, and crystal size, when NaCl crystal which is disadvantageous to crystal growth.. nucleation rate is basically the same as the So, concentration of crystals must be product discharge rate. It is described in controlled at a proper level. Ideal ratio of literature that residence time should be solid to liquid is controlled at about 20%. maintained at more than one hour when the 4.DESIGN AND PRACTICE OF average crystal size ca.0.4mm . CONTEMPORARY EVAPORATION CRYSTALLIZERS 6, ,RaV (3) ,,Design is the bridge and the link between the scientific experiment and productive practice. Engineering design is Where: R— crystal growth rate, by no means a blind scale-up or scale-down kg/(m2?s) and copy of drawings, data and results of the α― crystal volumetric conversion factor, predecessors. Design must be combined with calculated as a sphere: α=1/6 the actual conditions. It is a creational job. β― surface area conversion factor, Creation is the soul and driving force for calculated as a sphere: β=π design. Creation requires one to meet the 3ρ― crystal density, kg/m challenge and risks. Designers must seek V― average crystal growth coefficient, m/s truth from facts and be good at discovery, 3.2.5 Circulation flow improvement and enhancement by summarizing the experiences of the Circulation flow, which determines the predecessors so that the designed project can local flow rate in the crystallizer, is inversely be of advanced technology with good proportional to the supersaturation of utilization of resources, clean operation, solution when steam is fixed. High speed energy saving, reduction of consumption, will lead to fierce collision among crystals, reasonable economy and safety. between crystals and wall of the vessel, Most commonly used types of which will result in the formation of evaporators in China’s salt making industry secondary nuclei because of crystal breakage, are shown in Figure 1. They are composed and eventually influence the crystal size. of evaporator, heat exchanger, upper and Therefore, the flow rate must be properly lower circulation pipes, circulation pump controlled. and salt leg. Such evaporation systems are required to be rational in structure, comply 14 33with the fundamentals of hydrodynamics /m?s) (Others suggest 1.8~2.5 mwith small resistance, low energy (consumption, high efficiency of heat transfer, Ohigh evaporation intensity, good separation tof steam and water, and meet the e requirement of crystal growth with qualified H― height of steam and water products. Dimensions and parameters of the separation space: 1.8~2.5m components of the crystallizer are (Others suggest 2.5~33.0m) determined by results of the calculation of material balance , heat balance and related 4.1.2 Mass and speed method empiric data, which are described as follows. This means water allowed to be 4.1 Diameter of evaporator and height of evaporated per unit surface area per unit separation chamber time. DWV,4/(), (5) The design of relevant dimensions of Where: W― water evaporated kg/h the heater must meet the following three V― allowed mass speed: 2requirements: 2500~800kg/m?h ( lower limit for last a. Supersaturation of the feed can be effect) effectively reduced and eliminated so that crystals can grow under fine condition and 4.13 Approximate proportion method in excellent environment b. The reduction of short-circuit loss of Taking the separation space as a steam feed favors flash and separation of vapor and water separator, its diameter is from liquid so that entrainment is minimized calculated as c. The internal surface of the evaporator DURUU,,，,/(0.541)/(0.5410.44)6.47loaddloadloadmust be polished and clean so that salt where: U— W?r/r–r loadsecvaporliquidvaporincrustation is prevented and normal 3(m/s) continuous operation is ensured 3 W― vapor volume (m/s) sec 3 r― vapor gravity (kg/m) vapor4.1.1 Evaporation volumetric intensity 3 r― feed gravity (kg/m) liquidmethod R―ratio of actual steam speed d to basic speed ( normally 0.44, but 1.15 This means the volume of the vapor without demister) discharged from each cubic meter of evaporating space per second. Given the 4.1.4 Cross section speed of vapor method height of separation space, diameter of the Speed of vapor (or speed in empty evaporator is calculated as the following vessel) in evaporator is specified as 4~7m/s. equation. The author suggests that it should be controlled at 4m/s in the last effect of a multiple-effect evaporation system. Its W(m) (4) ,D,VH，，diameter is calculated as Where: D― diameter of evaporator (m) W― volumetric flow of vapor 4/()GVW,D= (m) (7) 3(m/s) Where: G―vapor flow (kg/h) V― allowed evaporation V―specific volume of vapor 33volumetric intensity: 1.1~1.5 m/m?s (m3/kg) 15 W―flow rate of vapor (m/s) Given heat load is set, the correlation of π―ratio of circumference of ?t with d, L and W can basically be a circle to its diameter determined when heating area is set. We can calculate as Height of vapor separation space 4.1.5 Q=k?F??t =k?n?π?d?L??t (Kcal/h) (9) Q=π/4?d2?n?w?γ?c??t?3600(Kcal/h) (10) Height of vapor separation space means Substituting (9) into (10), it follows the effective height of the steam and liquid =k??t?L/900?γ?w?d(?C) (11) ?tc separation section above the feed level in Where:k―total heat transfer coefficient 2evaporator. Feed which enters into the (Kcal/m?h??C) evaporator boils by heating. Bubbles go ?t―effective temperature difference upward and break at the surface of the feed (?C) and forms vapor. The effective height is that ?t―feed temperature rise (?C) c at which most of the droplets and mist d―calculated diametr of heating tube entrained in vapor can fall and settle down. (m) This height stated in literature is 2.0~3.0 m L―effective length of heat transfer of in most cases. However, the height in China heating tube (m) is more than 3 m without demister, and even w―feed flow rate in heating tube (m/s) 3tends to increasing. γ―gravity of circulating feed (kg/m) c―specific heat of feed (kJ/kg??C) 4.1.6 Diameter of secondary steam pipe n―number of heating tubes It can be seen from the equation above Specific volume of vapor varies with that ?t is proportional to L and inversely c pressure. In order to reduce the pressure proportional to d and w when heat load and heating area are determined. ?t is an drop in secondary steam pipe and c temperature loss, vapor speed in the important parameter for design because it is secondary steam pipe is controlled at closely related to the superaturation of feed, speed of crystal growth and quantity and 10~40m/s for calculation. quality of the salt product. Generally 4.2 Determination of heating area, speaking, the value of ?tis determined c diameter and length of heating tube before d, L and w are determined. According to literature and experiences, in most cases, ?t=3?C, 5?C is highest, d 4.2.1 Heating area c (outside)=32~45 mm, L=5000~7000mm, Based on the calculation of material 12000mm is the longest, w=1.2~2.5m/s (1.8m/s is preferable). balance and heat balance and the parameters chosen, heating area is calculated as F=Q/(k??t) 4.2.3 Number of heating tubes 2(m) (8) 2Where: F― heating area (m) Number of tubes is calculated as the Q― heat transferred in unit following when heating area is determined. time (Kcal/h) N=F/(π?d?L) (12) k― total heat transfer Where: F― heating area (m2) 2coefficient (Kcal/m?h??C) d― calculated diameter of heating ?t― effective temperature tube (m) difference (?C) L― effective heat transfer length of heating tube (m) 4.2.2 Diameter and length of heating tube π― ratio of circumference of a circle to its diameter 16 It is specified in the Chinese standards quite a long period of time, same clearance governing the calculation of shell-and-tube was applied to the exchangers of the heat exchanger that outside diameter is taken multiple effect facilities, hence the same for calculation of diameter of heating tubes. diameter of the shell body of exchangers. In salt industry, average value of inside and The author does not think it’s a proper outside diameters is conventionally applied. practice. We know that there is large According to the author’s design practice difference between specific volume of steam and data from literature, the value of total entering the first effect and that in the last heat transfer coefficient is influenced by αeffect. In order to reduce the loss of pressure 1 (heat supply coefficient outside the tube) and and temperature to favor heat transfer, α (heat supply coefficient inside the tube). different clearances must be applied 2 When α<α, outside diameter is taken for according to different specific volume of 12 calculation. When α=α, average value is steam. At a higher vacuum, the clearance for 12 taken and when α>α, inisde diameter is the last effect should be larger. So, 12 taken for calculation. In the exchangers used Pt=1.3~1.8d(outside) is a better choice. in Chinese salt plants, α is far higher than 1 α2, at least by 2 to 3 times. Therefore, the 4.2.5 Steam inlet arrangement author suggests that taking the value of inside diameter is suitable for calculation of In order to prevent tubes from erosion numbers of heating tubes. and vibration caused by steam, dam board or guide vane should be used at steam inlet. 4.2.4 Diameter of shell body of heat When dam board is used, the distance h exchanger between the board and inside of the shell should be 0.25 larger than inside diameter d When diameter and number of the tubes of steam inlet pipe, diameter D of the board are set, tubes are commonly arranged in an should be larger than diameter d of steam equilateral triangle pattern. The diameter of inlet pipe and cross-sectional area of the the shell of the exchanger can be calculated steam passage must be larger than that of with the following empiric equation. steam inlet pipe, as is shown by a in Figure D=1.15Pt?N (m) (13) 5. Where D― diameter of shell of Guide vane is another ideal exchanger (m) arrangement, which is commonly used in Pt― clearance betwwen tubes salt making industry. It leads the steam of (mm) comparatively high flow rate into the N― number of tubes jacketed annular channel for redistribution. Clearance between tubes―center to The correlation of width h of annular center distance between tubes―Pt=1.3~1.5d passage to diameter d of steam inlet pipe is (outside) when expansion method is used. In h?0.3d, as is shown by b in Figure 5. 17 Figure 5 Schematic drawing of anti-surge device at steam inlet 4.2.6 Baffle arrangement Empiric data is normally the basis for calculation of diameters of the upper and Baffles were not used in most of the lower circulation pipes. Data in literature has Chinese salt plants in early times until in it that the ratio of cross-sectional area of 1980s when material was improved with lower circulation pipe to the total enhanced corrosion resistance, cycle of cross-sectional area of heating tubes is changing tubes was elongated and complete 0.8~1.1 for externally-heated forced set of salt plant was imported from abroad, circulation evaporation crystallizer. the Chinese designers began to use them for Cross-sectional area of upper circulation the purpose of intensifying the process of pipe is the same as that of lower pipe or the heat transfer. In most cases, notched round diameter of upper circulation pipe is 0.2~0.3 plate are used. The notching rate of 0.25D is times of diameter of exchanger. In the long normally used in design. Distance between period of practice, the head of circulation baffles at the upper section is larger than that pump is reduced by enlarging the diameter at lower section with the largest distance less of circulation pipe in an attempt to reduce than the diameter of the exchanger shell. For feed resistance in circulation pipe. In the baffles of condensing type, the author design in recent years, the ratio of suggests that the notching rate be increased cross-sectional area of circulation pipe to the to about 0.45D. The reason for this is the total cross-sectional area of the heating tubes gradual condensation of steam and gradual has been controlled at 1.0~1.5, which tends decrease of the steam volumetric flow. It is to increase at present while the diameters of the same for the distance between baffles. upper and lower circulation pipes tend to be Distance at the upper section should be the same. However, the elbow connecting larger than that at the lower section. upper circulation pipe to the upper cone of However, it is stated in some literature that the vessel is of the same diameter. The there is no need to use baffles because author changed it into a reducing type, baffles have little impact on heat supply which has been proved to be effective in coefficient when the space of the shell body practice. is used for condensation of steam. But the installation of baffles for increasing rigidity 4.4 Distance between the upper tube plate of tube string, preventing tube vibration and and the level in evaporator discharging condensate is an exception. In order to prevent the decrease of total 4.3 Diameter of circulation pipe heat transfer coefficient caused by feed boiling and scale plugging in tubes, enough 18 n― rated speed of pump (r/min) height of liquid column must be maintained NPSH― required net positive suction head above the upper tube plate. This height of designed or at optimum working condition liquid colunm can be roughly calculated as C― cavitation specific speed Given similar geometry and movement PP,312 (m) (14) ?n?D, of the pump, it follows Q=KH,222rNPSH=k?n?D. Substituting Q and NPSH 1 Where: into Equation (15), it follows n?D=constant. H―height of liquid colunm (m) For pumps with similar geometry and P1―pressure (at) of saturated steam at the movement, the value of n?D is the same and temperature of feed outlet temperature cavitation is similar. At present in China a minus feed boiling point rise model axial flow pump with D=300mm, P2―pressure (at) of vapor from n=1450r/min and n?D=435 can operate evaporatorr― average gravity of steam, without cavitation. Therefore, n?D=435 (for liquid and solid (kg/m3) large capacity axial flow pump made in D=200) is taken as a foreign countries, n?4.5 Circulation pump principle to select pump. The value of n?D should be less than 435 no matter how large Circulation pump is an important the diameter of the pump is. The less the component of an evaporation crystallizer value is, the less the possibility of cavitation with externally-heated forced circulation. Its is. Accordingly, pump with larger diameter quality directly influences the energy of impeller and lower speed should be consumption of the system, product grain selected when flow is set. The similar size, smooth operation of the system and geometry tells us that pump flow is operation cost. The flow, the head and the proportional to the first power of speed and efficiency of the pump must meet the the third power of diameter of impeller. The requirement of the easy, long time and safe reduction of flow rate decreases the operation of the system. hydraulic loss and raises the pump Circulation pump used in salt plant is of efficiency. axial flow type with large flow, low head and high specific speed. Its flow, head and 4.6 Feed volume efficiency are proportional to the first power, This means the volume of feed filling second power and third power of speed the whole system of evaporation, respectively, and to the third power, second crystallization and circulation, where feed is power and fifth power of the diameter of evaporated, the solute NaCl reaches pump impeller respectively. And the specific saturation and crystallizes, and crystals grow speed is a principal parameter which to the required size. In order to ensure the determines the form of impeller, required environment and conditions, the characteristics of performance parameters volume should meet the requirement of and cavitation performance. At present, about 30 seconds for which feed finishes a cycle in the the circulation system. pump speed is calculated as 4.7 Diameter and length of salt leg 5.62nQ (15) C,3Salt leg is a vessel where NaCl crystals 4NPSHsettle down and become thickened. The Where: Q― optimum working flow amount of slurry discharge controls the ratio 3(m/s) of solid to liquid in the circulating feed and 19 ensures normal operation of the evaporation select duplex stainless steel-2205 which is crystallizer. In order to enhance the quality superior to 316L in terms of anti-corrosion, of product and reduce steam consumption, anti-erosion, easy fabrication and economic elutriation brine can be used to remove fine aspects. No matter what material is selected, salt crystals and dissolvable particles of the surface of the vessel wall and welded impurities. joints contacting feed must be ground, Diameter of salt leg polished and treated with acid washing and According to scientific experiment and passivation so that a smooth and dense film productive practice, the diameter of salt leg is formed to prevent salting up by the is determined by the settled amount of salt adherence of the induced nuclei. crystals per unit cross-sectional area per unit 2time, normally 14~20t/mh.. When 4.8.2 Tube, tube plate and shell of heat elutriation brine is used, its rising speed is exchanger determined by the required crystal size and the ratio of solid to liquid in slurry. Heat exchanger is a critical component Literature suggests 1~10mm/s. of the evaporation crystallizer. Selection Length of salt leg must be based on careful comparison. In In most cases according to the 1980s and 1990s, seamless carbon steel experiences of both at home and abroad, 5 tubes, low alloy (E2) steel tubes, copper meters or so is a common practice, and tubes, B30 copper-nickel alloy tubes, length of 3 meters is also seen. Better results titanium alloy tubes and full titanium tubes can only be obtained when correct length is were commonly used with coordinated tube taken in accordance with the actual plate or clad tube plate. Carbon steel plate or conditions such as the scale of the plant and stainless steel clad plate was used to process applied. fabricate the shell. In consideration of reasonable economy, titanium alloy or full 4.8 Selection of material titanium are the best choice for making tubes because they are characteristic of smooth 4.8.1 Material that is smooth on surface, surface without hydrophilicity, and of high easy to fabricate and maintain, corrosion and rate of heat transfer, anti-corrosion, erosion resistant can be selected for anti-erosion, light specific gravity, small evaporator, upper and lower circulation wall thickness and long life of operation. pipes and upper and lower cones of Tube plate is made of plate clad with the exchanger. In 1980s and 1990s, B same material as that of tube. For the shell, 30 copper-nickel-clad plate, 316L-clad plate, stainless steel or duplex stainless steel clad duplex stainless steel (18-5)clad plate, full plate is used. titanium and titanium alloy clad plate were used in salt plants. Practice has proved that 5 CONCLUSION all these plates are feasible in use. However, under the conditions of dense concentration Evaporation crystallizer is a key unit of –of Cl, pit corrosion and transcrystallization vacuum pan salt making system, to which and even transcrystalline rupture occur on great attention has always been paid by the 316L influenced by pH value and sulfides. designers and salt producers. In this paper, Therefore, when 316L is used, great approaches are made on the crystallizing attention must be paid to the environment mechanism and hydrodynamics. Based on and conditions, such as adjusting the pH the data provided by some salt producers, value of the feed and removing sulfides the selected technology and economy and including organic sulfides. At present the product quality are analyzed. Advantages environment and conditions, it is better to and disadvantages of different types of 20 evaporation crystallizers are discussed. The I wish to thank Mr. Zhou Boqi and Mr. crystallizer designed by the author, which is Kong Zhiyuan of China National indicated by d in Figure 1, has been used in Information Center for Well and Rock Salt over 10 newly-built or modified salt plants Industry for their great assistance in making with great success in China. the writing of this article. I do, however, wish to say a special word of appreciation to ACKNOWLEDGEMENTS Mr. Wu Jitai for his English translation. 21