不同气氛下流化床菱镁矿轻烧产物特性研究

利用实验室小型流化床装置,通过改变CO₂、H₂O浓度得到不同品位菱镁矿的轻烧产物,通过分解率、吸碘值、XRD、BET、SEM对轻烧产物的理化性质进行表征,研究了轻烧气氛中CO₂、H₂O含量对产物活性的影响。结果表明,在原料分解完全时,产物的活性由微观结构决定;增大CO₂浓度,轻烧产物的吸碘值逐渐降低,晶粒尺寸由小逐渐变大,表面结构由多孔变为少孔,说明CO₂的存在降低了产物活性;增大H₂O浓度,轻烧产物的吸碘值会逐渐降低,比表面积由大变小,表面结构由疏松多孔变得致密光滑,H₂O通过促进氧化镁的烧结来抑制产物的活性;相同工况下,高品位的菱镁矿轻烧产物活性高于低品位。     

微型流化床内菱镁矿轻烧反应特性及动力学

通过微型流化床反应分析仪(MFBRA)和热重分析仪(TGA)对比研究了菱镁矿粉在氮气和空气气氛中的煅烧反应特性和反应动力学特性。结果表明:菱镁矿在氮气和空气中测得的表观活化能相似,说明空气中的氧气对菱镁矿的煅烧没有影响。MFBRA中菱镁矿粉在800℃以上完全分解的时间只有短短数秒,显著少于在TGA中完全分解的时间,揭示了TGA和MFBRA在测定反应动力学方面的差异。MFBRA测定菱镁矿在空气和氮气气氛中分解的表观活化能(约125 kJ/mol)和指前因子(10~5s~(-1))显著低于TGA所得结果 (约200 kJ/mol,10~(15)s~(-1)),进一步测试表明这与TGA中的反应受气氛抑制有关。即菱镁矿在TGA坩埚内分解产生的CO_2不能及时随气流排出,抑制了轻烧反应的进行,而MFBRA可有效降低外扩散对反应的影响。但两种仪器测试的反应均遵循成核与生长控制模型。对于流化床反应,证明了MFBRA测试结果具有更高的准确性,为有气体生成的煅烧或分解反应(如菱镁矿轻烧)提供了可靠的反应测试方法和仪器。     

菱镁矿输送床轻烧过程反应与产物微观结构特性

利用高温烟气加热实验室规模输送床模拟菱镁矿闪速轻烧工业输送床反应器,建立基于热重分析计算菱镁矿中MgCO₃分解率方法,研究了煅烧条件和原料粒径对菱镁矿细粉输送床分解的转化率和产品活性的影响,揭示了煅烧过程中产物微观结构变化特性。菱镁矿粉（<150 μm）轻烧为秒级快速反应,仅需1~2 s菱镁矿中MgCO₃分解率即可达98%以上,验证了输送床闪速轻烧技术的可行性。输送床煅烧产物的柠檬酸显色时间17~55 s,活性显著高于固定床煅烧产物（显色时间294 s）,且产物活性由菱镁矿分解率和微观结构共同决定;煅烧过程中产物MgO晶粒尺寸逐渐增大,表面结构由疏松多孔变为致密光滑,该结构变化可在数秒内完成。     

新疆和静菱镁矿热分解特性及轻烧工艺研究

以新疆和静菱镁矿为原料,利用热重分析法研究了升温速率对菱镁矿分解的影响,据此确定最佳升温速率.利用正交实验法控制煅烧温度、保温时间和原料粒度得到不同轻烧氧化镁,采用柠檬酸活性法表征化学活性,分析烧后试样矿物相组成,研究其对轻烧氧化镁活性的影响.结果表明:在相同的温度条件下,菱镁矿的分解程度随着升温速率的增大而减小,热分解温度随着升温速率的增大而提高,升温速率过大会阻碍分解反应进行;煅烧时当菱镁矿转化率为30％,温度在550 ～ 600℃时,分解反应较难进行,最佳升温速率选择5℃/min;煅烧温度对轻烧氧化镁活性影响最大,原料粒度次之,保温时间影响相对较小,最佳轻烧工艺为:煅烧温度800℃、保温120 min、原料粒度为2 mm.     

轻烧菱镁矿制备高纯纳米氧化镁

以轻烧菱镁矿为原料,通过酸浸去除杂质得到纯净的含镁溶液,以草酸为沉淀剂,采用直接沉淀法制备纳米氧化镁粉体。考察了草酸和氧化镁的摩尔比、反应温度、前驱体草酸镁热分解温度及时间对纳米氧化镁粒径大小的影响。采用热重-差热分析仪、激光粒度仪、X射线衍射仪、扫描电子显微镜等对前驱体的热分解情况、产品的粒径及晶型结构进行检测。结果表明:合成的氧化镁粉体的粒径分布较窄,分散性良好,平均粒径在80nm左右,纯度达到99.1%。     

利用菱镁矿粉状尾矿制备氧化镁工艺研究

针对制备高纯镁砂的菱镁矿粉状尾矿,采用菱镁矿精制-压球-轻烧工艺,可直接得到合格的镁砂产品。在菱镁矿精制过程中,添加球团添加剂,使后续工艺轻烧过程中菱镁矿保持球团形貌特征,避免球团大量粉化造成竖窑堵塞。通过冲击测试得到的轻烧球团强度,即从0.5 m坠落次数可达10次,粉体活性＜230 s,烧失率＜0.1%。     

抑制性气氛中菱镁矿分解特性与动力学研究

利用热重分析仪研究了菱镁矿在含产物(CO₂)抑制性气氛中的分解反应特性,并根据Flynn-WallOzawa法和Coats-Redfern法计算获得了反应动力学参数。结果表明：随着反应气氛中CO₂浓度的增加,菱镁矿分解受到的抑制作用显著增强,初始分解温度与反应活化能呈对数型增加,最大反应速率对应温度则呈指数型增加;在含不同浓度的CO₂气氛中菱镁矿分解均遵循成核与生长控制模型G(x)=-ln(1-x)。该论文为实际工业生产中菱镁矿煅烧及资源利用提供了理论依据。     

利用菱镁矿制备高活性氧化镁

以菱镁矿为原料,先在850℃下煅烧2h获得轻烧氧化镁,然后水化成氢氧化镁,干燥后在不同温度(分别为450℃、500℃、550℃、600℃)和保温时间(分别为1h、1.5h、2h)下轻烧,通过分解氢氧化镁制备了高活性氧化镁。结果表明:氢氧化镁在500℃时完全分解成氧化镁;轻烧温度和保温时间对氧化镁活性(用吸碘值表征)有较大的影响,随着温度的升高,氧化镁活性逐渐降低,而在相同温度下,随着保温时间的延长,氧化镁的活性降低;在加热温度为500℃,保温时间为1h的条件下轻烧分解氢氧化镁,能得到吸碘值达191.42mg·g-1的活性氧化镁。     

凝胶-燃烧法由轻烧菱镁矿制备纳米氧化镁

采用凝胶-燃烧法,以菱镁矿、乙酸铵及尿素为原料,加入成胶剂柠檬酸并烘干,在一定温度下点燃并退火即得纳米氧化镁。考察了前驱体溶液p H值、水及分散剂用量、退火等工艺参数对反应过程及最终产物性质的影响。通过X-射线衍射(XRD)、扫描电镜(SEM)及化学分析等对纳米氧化镁粉体进行表征后得到在优化条件下,纳米氧化镁的晶粒尺寸100 nm,纯度为98.2%。     

凝胶-燃烧法由轻烧菱镁矿制备纳米氧化镁

采用凝胶-燃烧法,以菱镁矿、乙酸铵及尿素为原料,加入成胶剂柠檬酸并烘干,在一定温度下点燃并退火即得纳米氧化镁。考察了前驱体溶液p H值、水及分散剂用量、退火等工艺参数对反应过程及最终产物性质的影响。通过X-射线衍射(XRD)、扫描电镜(SEM)及化学分析等对纳米氧化镁粉体进行表征后得到在优化条件下,纳米氧化镁的晶粒尺寸<100 nm,纯度为98.2%。     

热分解气氛对流化床煤热解制油的影响

在循环流化床锅炉上耦合流化床热解反应器既可提供电力又副产热解油,明显提高煤的利用价值。在这个过程中,热解反应器通常利用自身产生的热解气作为流化介质。本文考察了模拟热解气反应气氛对流化床煤热解拔头制取热解油产率的影响,并利用TG-FTIR分析了焦油官能团组成及随TG温度的变化。针对锅炉用烟煤的实验结果表明：采用热解气作为反应气氛时焦油产率最大,相对无水无灰基煤达13%。反应气氛中H₂和CO₂的存在不利于焦油生成,但CO 和CH₄的加入提高了焦油产率;H₂的加入有利于焦油中酚羟基、羧基类化合物生成,同时也促进了脂肪族化合物的裂解;CH₄的存在可以提高焦油中单环芳烃、脂肪族及酚羟基类化合物的含量。     

轻烧氧化镁活性测定方法的研究

用柠檬酸法和三种不同水化法测定和研究了氧化镁的活性,研究了氧化镁活性与煅烧温度、煅烧时间之间的关系,结果表明,当煅烧温度为750℃时,由菱镁矿煅烧所得的氧化镁活性最高,在菱镁矿分解完全的情况下,煅烧时间以短为宜。本研究为氧化镁生产工艺控制提供了理论依据。     

Acquiring real kinetics of reactions in the inhibitory atmosphere containing product gases using micro fluidized bed

This study proposed an isotope-tagging method to investigate reactions under the atmosphere of product gas. To illustrate this method, the calcination kinetics of calcium carbonate Ca¹³CO₃ in CO₂ atmospheres were investigated by monitoring ¹³CO₂ produced using a micro fluidized bed reaction analyzer (MFBRA). The results demonstrated that the presence of CO₂ in reaction atmosphere increases the apparent activation energy. The increase in the apparent activation energy is, however, significantly overestimated by the thermogravimetric analyzer (TGA) because of the excessive suppression by stagnated product gas inside the sample crucible. Comparatively, the apparent activation energy increases with CO₂ from the MFBRA due primarily to the thermal equilibrium limitation, because the gas diffusion in the MFBRA is essentially eliminated. It is thus concluded that the MFBRA is quite capable of acquiring the real kinetics of reactions in such inhibitory atmospheres.     

旋流振动流化床干燥性能的研究

文章以聚四氟乙烯粒子和蔗糖为物料研究了旋流振动流化床的流体力学及干燥特性,并与振动流化床的各种特性进行了对比。实验在一个干燥室直径为240 mm的小型圆筒振动流化床中进行,采用斜孔分布板使床内产生旋流。实验结果表明:在空气分布板开孔率相差不多的情况下,物料流化以前,旋流振动流化床的床层压降要大于普通振动流化床,一旦物料达到正常流化状态以后,2种振动流化床的床层压降相差不多;旋流振动流化床可以降低物料的临界含湿质量分数。     

Calcium sulphide formation in fluidized bed boilers

Analyses of samples of bed ash from a stationary fluidized bed boiler show the presence of calcium sulphide. In some samples, half of the total sulphur was present as sulphide. The samples containing CaS were obtained under unstaged conditions and with a high excess air ratio, 1.3 to 1.4. The samples were taken after a stop in the limestone addition, i.e. at high SO₂ emissions of about 1000 mL/m³ (ppm). No CaS was found during limestone addition when the SO₂ emission was 300–400 mL/m³. This indicates that formation of large amounts of CaS may be initiated as the SO₂ concentration exceeds some critical level.     

Effect of ring promoter on ionic mass transfer in a fluidized bed

Mass transfer data were obtained at the wall of a batch fluidized bed in the presence of a ring promoter used for the reduction of ferricyanide ion. The ring promoter was a copper rod on which rings of uniform size were fixed at equal distances. In these studies, the cross‐sectional diameter of the ring, the spacing between the rings, the particle size and the void fraction were varied and the limiting current data were measured. It was observed that the mass transfer coefficients were augmented with increased particle diameter, decreased spacing between the rings, and increased cross‐sectional diameter of the ring. Copyright © 2003 Society of Chemical Industry     

Effect of temperature on slug properties in a gas fluidized bed

In order to understand the effect of temperature on slug properties, the onset velocity of slugging, slug rising velocity and slug frequency have been measured by a differential pressure method in an electrically heated gas fluidized bed of 0.1 m i.d. and height of 1.97 m and, with respect to variations in gas velocity (U_mf∼0.15 m/s) and temperature (25-400 °C). Air was used as fluidizing gas and fluid catalytic cracking (FCC) catalyst particle (Geldart's group-A particle, d_p=0.071 mm, ρ_p=1600 kg/m³) as bed material. The minimum slugging velocity was found to increase a little with bed temperature. The qualitative change in minimum slugging velocity was found to agree with the inverse of minimum fluidizing velocity as temperature was varied. As the bed temperature increased, slug frequency was found to decrease a little, whereas slug rising velocity increased. A correlation between slug rising velocity and bed temperature was proposed.     

Development of a fluidized bed thermogravimetric analyzer

A new fluidized bed thermogravimetric analyzer (FB‐TGA) was developed that introduces two major particularities: the pseudo variation of the weight of the reactor and the special strategy for gas flow rate adjustment according to temperature. A momentum balance was performed on the reactor and the pseudo variation of the reactor weight was evaluated by measuring the pressure drop through the gas distributor and filter. The real weight loss of the reactor was obtained by subtracting the pseudo variation of the weight from the total weight loss measured by the load cell. In addition, a special program for the gas flow rate as a function of temperature was developed and used; so the minimum fluidization regime is maintained throughout all of the experiments. The validation test of the FB‐TGA was carried out on calcium hydroxide decomposition, and the results were compared with those obtained from the conventional TGA. Diffusion control was suppressed by the application of the FB‐TGA, which was confirmed by the x‐ray diffraction analysis on the treated samples. © 2014 American Institute of Chemical Engineers AIChE J, 61: 84–89, 2015