4立方米的酯化釜及儲罐設(shè)計（全套CAD圖+說明書+開題報告+翻譯）
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英語翻譯
Fouling resistance model for prediction of CaCO3 scaling in AISI 316 tubes
M. Sultan Khan, S. M. Zubair, M. O. Budair, A. K. Sheikh, A. Quddus
Abstract：The term fouling is generally used to describe the deposition of unwanted (initially fluid) particles, which increases both resistance to heat transfer and pressure drop through the heat exchanger. CaCO3 which is predominantly present in the cooling water, has inverse solubility characteristics i.e., it is less soluble in warm water, resulting in deposition of scales in heat transfer equipment. An experimental program is described in this paper to study the growth of fouling as a function of tube surface temperature, Reynolds number, tube diameter and the time for which the tube has been subjected to the scale forming solution. The data collected from the experiments are used to develop a fouling resistance model. In addition, the results obtained from the present study are also compared with those discussed earlier by several investigators with regard to CaCO3 fouling.
Keywords：scaling，the heat exchanger，CaCO3
1 .Introduction
The deposition of unwanted particles on the surfaces of heat exchangers is defined as fouling. The presence of these deposits represents an additional thermal resistance to heat transfer which reduces the thermal-hydraulic performance of the heat transfer equipment. The deposition may be crystalline, biological material, the products of chemical reactions including corrosion, or particulate matter [1]. The growth of deposits depends upon a number of parameters such as fluid composition (pH value, concentration, etc.), temperature, geometric dimensions, and Reynolds number (Re) of the flowing fluid. Different aspects of the deposition process and its characterization are discussed in the pioneering paper of Taborek et al. [2]. It should be noted that an understanding of the economic penalties associated with fouling is one of the primary reasons for greater interest in the fouling-related research. Pritchard [3] presented cost estimates associated with fouling in Britain. Thackery [4] estimated the overall annual cost of fouling in U.K. to about 0.3 of GNP for the year 1978 (approximately $1 billion). The order of magnitudes of these estimates is confirmed by Van Nostrand et al. [5] while investigating the fouling related costs for the U.S. specificrefinery units. Steinhagen et al. [6] estimated the foulingrelated costs for New Zealand at about $30 to $46 million which is about 0.1-0.17% of the annual GNP for the year 1988.
A project has been initiated at the King Fahd University of Petroleum and Minerals, Dhahran to study the impact of fouling in heat exchangers. The objective of this paper, which is partially based on the above mentioned project, is to present a fouling resistance model of CaCO3 scaling in AISI 316 stainless steel tubes. In this regard, a literature survey on precipitation fouling with emphasis on CaCO3 fouling is discussed in the next section which is followed by details of test equipment design and procedure to obtain parametric experimental fouling data. The data thus obtained from the test equipment is presented in the form of a dimensionless regression model.
2
Background of CaCO3 fouling
The problem of fouling is encountered in industrial operations and processes with natural water or aqueous solutions containing dissolved inorganic salts. Some of these salts or their combinations have inverse solubility characteristics, so that they are less soluble in the hot fluid adjacent to the heattransfer surface. Examples of such salts are CaCO3 and CaSO4. Figure 1 shows the behavior of normal and inverse solubility salt solutions [1]. For normal solubility salt solution, at point A, solution is undersaturated but on cooling to point B it is just saturated. On further cooling, the solution becomes supersaturatedand crystal nucleation occurs at point C. As crystallization and cooling proceeds solution concentration falls and moves in the direction of D. Now for an inverse solubility salt solution it is undersaturated at point A, as it is heated it reaches the solubility limit at point B at temperature 1 and then under continued heating the solution becomes supersaturated reaching point C at temperature 2 where precipitation starts.
The formation of scale on heat transfer surfaces is a common phenomenon where aqueous solutions are involved, e.g. the use of natural waters for cooling purposes or evaporative desalination. Unless suitable measures are taken, the problem of scale formation can give rise to serious consequences. In steam boilers, for instance, the presence of scale on water side can give rise to high metal temperatures that may result in mechanical failure of heat-transfer equipment. Hanlon, as mentioned in reference [1], commented on the potential of scale formation in industrial equipment is very high. As an example, he observed that for a 1 million gallon/day desalination plant under normal concentration conditions,a maximum of about 1400 kg of CaCO3 could be precipitated each day. In terms of scale thickness, it would represent a build up of 0.1 mm per day on the total heat exchanger surfaces within a typical plant. Although this may be regarded as an extreme example it does illustrate the magnitude of scaling or crystallization fouling problems in industrial plants.
A systematic study of scaling characteristics of cooling tower was conducted by Morse and Knudsen [7]. Effect of surface temperature on the scaling behavior was discussed by Story and Knudsen [8]. Lee and Knudsen [9] designed an experimental apparatus to simulate the operating conditions of a cooling tower. This is a somewhat extensive investigation to determine the effect of flow velocity, surface temperature and water quality on scaling of exchanger tubes. Coates and Knudsen [10] have discussed results of their experiments conducted for obtaining data regarding CaCO3 scaling. Watkinson and Martinez [11] studied scaling due to CaCO3 in copper tubes under conditions that promote rapid and severe scaling. In this regard, artificially hardened water of high dissolved and suspended solids was circulated through a heated test section. Effects of flow velocity, tube diameter and bulk temperature on asymptotic fouling resistance have been determined. Manzoor [12] conducted fouling related experiments and statistically analysed CaCO3 fouling data. The objective of Manzoor’s study was to demonstrate that fouling resistance varies from point to point along a horizontal tube and also for the same point it varies from replicate to replicate. The operating parameters were temperature, pressure, solution concentration and velocity which were kept constant during the experiments. Konings [13] on the basis of experimental work with cooling water, treated by different methods to eliminate scaling, presented a table of guide values for the fouling resistance. An experimental study of tube-side fouling resistance in water chilled evaporator was carried out by Haider et al. [14] in which 12.6 ft long evaporator tubes were used and fouling data were taken for four tube geometries. No data were taken at different sections of the tube. The fouling characteristics of cooling water for precipitation and particulate fouling are also discussed by Knudsen [15] where he emphasized serious problems when heat exchangers are over designed due to the use of incorrect design fouling allowance.
Practical and fundamental aspects of precipitation fouling(CaCO3 scaling) were reviewed by Hasson [16]. He considered the problem of defining precipitation fouling tendency by reviewing principles of solution equilibria and precipitation kinetics for salt systems frequently encountered in heat exchanger applications. Branch and Muller-Steinhagen [17] developed a model for fouling in shell and tube heat exchangers by considering Hasson’s ionic diffusion model for CaCO3 scaling. Hesselgreaves [18] discussed the effect of system parameters on the fouling performance of heat exchangers. A model for CaCO3 scale formation, which gives reliable prediction of the fouling rate with alteration of feed water chemistry, was developed by Tretyakov et al. [19]. It should be noted from the above studies that so far no fouling resistance (f ) model has been developed that may predict f as a function of , tube surface temperature and tube diameter.
3Test equipment design
The test apparatus was a double-pipe counter-flow heat exchanger as shown in Fig. 2. The working fluid was passed through the inner tube comprised of six test sections, each 0.1524 m long. To heat up the surface of the inner tube, hot water was circulated in the outer tube using a Constant- Temperature Water-Circultor Bath
(CTWCB), which had a provision of variable temperature settings. Three heat
exchangers were fabricated with inner tube sizes of 1/4 in (0.00635 m), 3/8 in (0.00952 m) and 1/2 in (0.0127 m). The outer to inner radii ratio of tubes was set at four for all three heat exchangers. All fittings were made of AISI 316.
To simulate the conditions encountered in cooling water systems, Na2CO3 and CaCl2 solutions, prepared in de-mineralized (distilled) water were used to produce
CaCO3 as a product of chemical reaction. The product solution, when passed through the inner tube resulted in the deposition of CaCO3 scale on the inner side of the tube. The chemical reaction to produce CaCO3 scale is given by [20]
The chemical solutions were pre-heated, separately, using pre-heaters and heating tapes to achieve a temperature of 50C before the solution entered the heat exchanger. The system is a once through type and a Back Pressure Regulator (BPR) was used to maintain a pressure of 689 kPa at the end of the heat exchanger. Figure 3 shows the scaling apparatus which consists of two high pressure variable stroke pumps, storage tanks for the chemical solutions, pre-heaters, heating tapes, CTWCB, thermocouples, temperature controllers and a BPR. Additional details and description of the test apparatus are presented in reference [21].
4
Experimental procedure
The concentration of the product solution was kept constant at 0.0006 mol/l. This required 2.543 and 3.528 g of Na2CO3 and CaCl2, respectively, to be mixed in 40 l of distilled water. The parameters that were varied during the experiments are Reynolds number, surface temperature and diameter of the inner tube. It should be noted that experiments were conducted for all possible combinations of the parameter values as shown in Table 1, in which Reynolds number is based on the inner diameter of the test sections.
As already mentioned three heat exchangers were designed and fabricated for the three tube sizes. For a particular size, Reynolds number was fixed and various experiments were conducted by varying the surface temperatures of the inner tube by the help of the CTWCB. Two hours continuous operation of the test is referred to as a test run. One experiment consisted of five 2 h runs for a particular set of parameters. At the end of each run, the heat exchanger was dismantled and the test sections were dried in the oven. Mass gain of the test pieces due to scaling was then measured using an analytical weighing scale which had an accuracy of^1 mg. The heat exchanger was then re-assembled for the next experimental run. It was observed that 0.25 in (0.0127 m) tube blocked due to scaling after 10 h of operation thus restricting the duration of the experiments to a maximum of 10 hours. Periodic measurements of the flow rates were carried out to maintain a constant Reynolds number during the entire experiments. For the next set of experiment, new set of tubes were used.
Using the mass gain method, fouling resistance (f ) wasdetermined as follows:
where 2 is the inside radius of the tube, 1 is the average value of radius due to the
deposit for a particular test section which can be calculated by using the relation
5
Concluding remarks
The fouling resistance data of CaCO3 scaling were presented to study the influence of tube surface temperature, Reynolds number and tube diameter. It was observed that the influence of Reynolds number in the range investigated (\9001700) was almost negligible, which was also noticed by Lee and Knudsen [9]
who have presented the same conclusion for their experimental data on asymptotic fouling resistance. They had observed that by varying the fluid velocities from 3 to 10 ft/s (0.91 to 3.05 m/s) there was no profound effect on the amount of CaCO3 fouling resistance. However, the influence of tube surface temperature and tube diameter on the fouling growth was found to be appreciable for the range investigated. The reasons for the increased fouling resistance as a function of surface temperature and diameter were explained by considering the inverse solubility characteristics of CaCO3 and tube surface effects.
The data obtained from experiments are presented in the form of a dimensionless fouling resistance model for estimation and prediction purpose. In this regard, all the variables in the model are non-dimensionalized with respect to the respective maximum values considered in this study. The model thus developed has been investigated in somewhat more detail by observing the normal probability plot of residuals. In addition, several other statistical checks are also made to assess the suitability of the model. No apparent model defects are noticed. It is thus concluded that the fouling resistance model may be considered as a reliable model within the range of experimental parameters investigated in the present study.
References
1. Bott, T.R.: Fouling of Heat Exchangers. Elsevier, Netherlands(1995)
2. Taborek, J.; Aoki, T.; Knudsen, J.G.: Fouling, the major unresolved problem in heat transfer. Chem. Eng. Progress 68-2 (1972) 5967
3. Pritchard, A.M.: Fouling-Science or art. In: E.F.C. Somerscales and J.G. Knudsen (Eds.). Heat Transfer Equipment. Hemisphere, Washington, D.C. (1981)
4. Thackery, P.A.: The cost of fouling in heat exchanger plant. In: A.M. Pritcard (Ed.), Fouling-Science or Art.Guildford, United Kingdom (1979)
5. Van Nostrand, W.L.; Leach, S.H.; Haluska, J.L.: Economic penalties associated with the fouling of refinery heat transfer equipment. In: E.F.C. Somerscales and J.G. Knudsen, (Eds.),Fouling of Heat Transfer Equipment, Hemisphere, Washington,DC (1981)
6. Steinhagen, R.; Steinhagen, H.M.; Maagni, K.: Problems and costs due to heat exchanger fouling in New Zealand Industries. Heat Transfer Eng. 11-7 (1993) 1930
7. Morse, R.W.; Knudsen, J.G.: Effect of alkalanity on the scaling of simulated cooling tower water. Canad J. Chem. Eng. 55 (1977)272278
8. Story, M.; Knudsen, J.G.: The effect of heat transfer surface temperature on the scaling behaviour of simulated cooling tower water. AIChE Symp. Ser. 74-1124 (1978) 2530
9. Lee, S.H.; Knudsen, J.G.: Scaling characteristics of cooling tower water. ASHRAE Trans. 85-1 (1979) 281302
10. Coates, K.E.; Knudsen, J.G.: Calcium carbonate scaling characteristics of cooling tower water. ASHRAE Trans. 86-2 (1980) 6891
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12. Haq, M.U.: Reliability-based maintenance stratigies for heatexchangers subject to fouling. Masters Thesis, King Fahd University of Petroleum and Minerals, Saudi Arabia (1995)
13. Konings, A.M.: Guide Values for the fouling resistances of cooling water with different types of treatment for design of shell-andtube heat exchangers. Heat Transfer Eng. 10-4 (1989) 5461
14. Haider, S.I.; Meitz, A.K.; Webb, R.L.: An experimental study of tube-side fouling resistance in water-chiller-flooded evaporators. ASHRAE Trans. 98-2 (1992) 86103.
15. Knudsen, J.G.: Coping with cooling water fouling in tubular heat exchangers. AIChE Symp. Ser. 85-267 (1989) 112
16. Hasson, D.: Precipitation fouling. In: E.F.C. Somerscales ad J.G. Knudsen, (Eds), Fouling of Heat Transfer Equipment. Hemisphere, Washington, DC (1981)
17. Branch, C.A.; Steinhagen, H.M.M.: Influence of scaling on the performance of shell-and-tube heat exchangers. Heat Transfer Eng. 12-2 (1991) 3785
18. Hesselgreaves, J.E.: The effect of system parameters on the fouling performance of heat exchangers. ICHEME Symp. Ser. 129 (1992) 9951006
19. Tretyakov, O.V.; Kristskiy, V.G.; Styazhkin, P.S.: Improved prediction of the formation of calcium carbonate scale in heat exchangers of secondary loops of conventional thermal and nuclear power plants. Heat Transfer - Sov. Res. 23 ((1991) 532538
20. Masterten, W.L.; Hurley, C.N.: Chemistry, Principles and Reactions. Saunders, Philadelphia (1989)
21. Khan, M.S.: Effect of thermal-hydraulic parameters on CaCO scaling in heat exchangers. Masters Thesis, King Fahd Univer3sity of Petroleum and Minerals, Saudi Arabia (1996)
22. Parry, D.J.; Hawthorn, D.; Rantell, A.: Fouling of Power Station Condensers within the Midlands Region of the C.E.G.B., In: Somerscales, E.F.C. and Knudsen, J.G. (Eds.) Fouling of Heat .Transfer Equipment. Hemisphere, Washington, DC
(1979)
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碳酸鈣在AISI316管內(nèi)污垢熱阻模型的預(yù)測
摘要：結(jié)垢此術(shù)語常常用來描述不期望的會加大熱交換器的壓力降和熱阻的顆粒(最早在液體中)的沉降?？芍?，碳酸鈣主要存于冷卻水當(dāng)中，有難溶的特點，因為其難溶于熱水，導(dǎo)致它在熱交換設(shè)備中的結(jié)垢沉降。此篇論文目的在于研究有關(guān)管子表面的溶液，雷諾數(shù)，溫度和在管子中結(jié)垢時間和污垢增長關(guān)系，將描述一個的實驗項目。從當(dāng)今研究中得到的結(jié)果也被用來與早些時候部分調(diào)研人員關(guān)于碳酸鈣結(jié)垢的討論進(jìn)行比較。。另外，來自實驗中的數(shù)據(jù)被用來建立一個污垢熱阻模型
關(guān)鍵詞：結(jié)垢，熱交換設(shè)備，碳酸鈣
1.緒論
這些沉積的存在會表現(xiàn)出附加的熱阻，將會降低熱交換設(shè)備的水傳熱性能。沉積的增加取決與包括液體成分（pH值，濃度等），溫度，幾何尺寸和流體雷諾數(shù)在內(nèi)的一系列參數(shù)。結(jié)垢是指非期望顆粒在熱交換器表面的沉積這些沉降物可能是包括腐蝕、顆粒物質(zhì)在內(nèi)的化學(xué)反應(yīng)產(chǎn)物或者結(jié)晶生物物質(zhì)。在Taborek等人早期的論文中對沉積過程的不同方面及其特性進(jìn)行了討論。應(yīng)該注意到結(jié)垢所造成的經(jīng)濟(jì)損失是結(jié)垢相關(guān)研究的一個主要原因。Pritchard提出了英國在結(jié)垢方面的大概成本。Van Nostrand等人在調(diào)查了美國精煉廠之后確認(rèn)了這些估計。Steinhagen在1985年在新西蘭，污垢造成的損失約占當(dāng)年國民生產(chǎn)總值的0.1-0.17%，約為3000-4600萬美元。Thackery在1973年英國污垢所造成的全部損失大約是當(dāng)年國民生產(chǎn)總值的0.3（大約為10億美元）。
在這方面，一篇有關(guān)碳酸鈣結(jié)垢的調(diào)查將在下一個有相關(guān)實驗設(shè)計細(xì)節(jié)和獲得實驗性的結(jié)垢數(shù)據(jù)的程序的部分被討論。本篇論文部分是根據(jù)此項目，其目的是提出碳酸鈣在AISI316不銹鋼管子內(nèi)沉積的污垢熱阻模型。因此，這些數(shù)據(jù)將以一個無量綱衰退模型的形式來提出。為了研究結(jié)垢對熱交換器的影響，Dhahran的King Fahd University of Petroleum and Minerals發(fā)起了一個項目。
溫度 溫度2.碳酸鈣結(jié)垢的背景
在有無機鹽水溶液或天然水的工業(yè)過程和操作中，常常會遇到結(jié)垢的問題。一些鹽或它們的組成具有溶解度低的特點，所以較難溶解在與熱交換器表面相鄰的熱流體內(nèi)，例如硫酸鈣和碳酸鈣。圖1表示不溶性鹽和普通的鹽的表現(xiàn)。對于一般的可溶性鹽的解決方法：觀點一，在較進(jìn)一步的冷卻上，解決變成使過度飽和而且水晶成核。觀點三，集中解決結(jié)晶和冷卻的問題。觀點二，解決不溶解但是在冷卻上指出它是飽和的。觀點四，現(xiàn)在對于相反的方向移動的可溶性鹽解決，如同它是一樣加熱溫度到點B到達(dá)可溶性界限，隨后在繼續(xù)的加熱解決之下變成過度飽和，到達(dá)溫度點C，開始墜落。
換熱器表面水垢的形成是十分常見的現(xiàn)象（包括水溶液），例如，使用天然水來冷卻或者蒸發(fā)脫鹽。除非采用適當(dāng)?shù)拇胧?，不然水垢將會引起?yán)重的后果。例如，蒸發(fā)器里水垢引起的高金屬溫度可能導(dǎo)致?lián)Q熱設(shè)備機械失靈。上文提到的Hanlon認(rèn)為工業(yè)設(shè)備中水垢的位勢事非常高的。轉(zhuǎn)化成水垢厚度相當(dāng)于換熱器表面全面積的0.1毫米。雖然這可能事一個極端的例子，但是它確實說明了工業(yè)設(shè)備水垢的大小或者析晶結(jié)垢的問題。
Story和Knudsen討論了水垢表面溫度的影響。Lee和Knudsen設(shè)計了一個冷卻塔試驗裝置模擬操作情況。這項調(diào)查略帶廣泛性，目的是研究熱交換器管道上流速，表面溫度個水質(zhì)對結(jié)垢的影響。Coates和Knudsen已經(jīng)討論他們?yōu)榱双@得與碳酸鈣結(jié)垢有關(guān)的的數(shù)據(jù)而進(jìn)行的的試驗。Watkinson和 Martinez 研究了碳酸鈣在銅管內(nèi)快速又劇烈的結(jié)垢作用。在這方面，高度溶解的人工硬水和懸浮固體在一段熱測試部分循環(huán)，然后測定出流速，管直徑和溫度對結(jié)垢的影響。Manzoor做了與結(jié)垢相關(guān)的試驗并從統(tǒng)計學(xué)上來分析碳酸鈣結(jié)垢地數(shù)據(jù)。Manzoor的目的是證明直管上不同點的熱阻事不一樣的，同樣的，相同點在不同的彎曲條件下熱阻也事不同的。操作參數(shù)如溫度、壓力、溶液密度和流速在試驗時都保持恒定。Konings在冷卻水的試驗基礎(chǔ)上，用不同的方法減少水垢，并制定了一張熱阻數(shù)據(jù)的表格。Haider對水冷蒸發(fā)器內(nèi)管端的水垢熱阻的做了一個實驗性研究。實驗中使用了四根長12.6英尺不同幾何規(guī)格的的蒸發(fā)管。沒有數(shù)據(jù)來自管的不同區(qū)域段流動。Knudsen還討論了冷卻水的沉淀結(jié)垢特性和微粒子結(jié)垢，他強調(diào)當(dāng)熱交換器是由于不正確的結(jié)垢設(shè)計將引起的嚴(yán)重問題。
Hasson檢驗了污垢（碳酸鈣結(jié)垢）實際和基本的方面。通過對含鹽系統(tǒng)溶解平衡和沉淀動力學(xué)的檢驗，他認(rèn)為換熱設(shè)備經(jīng)常碰到定義沉淀結(jié)構(gòu)趨勢的問題。在考慮了Hasson的碳酸鈣污垢離子散布的模型后，Branch 和Muller-Steinhagen 設(shè)計了一個在殼體和管熱交換器中的結(jié)垢模型。Hesselgreaves討論了關(guān)于結(jié)垢在熱交換器上系統(tǒng)參數(shù)的影響。Tretyakov改進(jìn)了碳酸鈣的結(jié)垢模型，給出了在不同水溶液里結(jié)垢速率變化的可靠性預(yù)測。值得注意的是，到目前為止上述所說的研究中還沒有一個結(jié)垢熱阻模型能夠用包含雷諾數(shù)，管表面溫度和管徑的公式來預(yù)測結(jié)垢熱阻。
3.測試儀器設(shè)計
測試裝置是如圖2所顯示的雙管對流換熱器。工作液體通過內(nèi)置換熱管的六個測試區(qū)段，每個區(qū)段長0.1524米。為了加熱內(nèi)置管的表面，在外部使用水循環(huán)恒溫裝置在管外用熱水循環(huán)，該裝置可以改變溫度設(shè)置。三個熱交換器內(nèi)置管徑分別是四分之一（0.00635m），八分之三（0.00952m），二分之一（0.0127m）。所有換熱器的外管半徑都是四，所有管子的材料都是316不銹鋼。
工作液體出口
為了模擬在冷卻水中碰到的情況，碳酸鈉和氯化鈣問題。圖3表示組成的污垢裝置高的壓力變數(shù)筆劃泵，儲藏箱 為化學(xué)的解決，前加熱器，暖氣音帶，熱點偶，溫度控制器和調(diào)整器背壓。另外的測試裝置的細(xì)節(jié)和描述被呈現(xiàn)出。
4. 試驗的程序
使用碳酸鈉和氯化鈣水溶液化學(xué)反應(yīng)產(chǎn)生碳酸鈣。水溶液流過內(nèi)管時在內(nèi)管壁上產(chǎn)生碳酸鈣水垢。水溶液進(jìn)入換熱器前是單獨預(yù)熱的，通過預(yù)熱器加熱到50℃。
在產(chǎn)品生產(chǎn)時往往被保持常數(shù)在0.0006摩爾每立方分米。此分別需要3.528克的氯化鈣和2.543克碳酸鈉，在蒸餾水40里混合。那在試驗期間雷偌數(shù)參數(shù)被改變，升至水面溫度和直徑內(nèi)胎。在資訊科技應(yīng)該是著名的，試驗是為所有的參數(shù)的可能組合引導(dǎo)評價。連續(xù)二小時的測試操作被稱為一個測試階段。如表
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