李軍，朱慶山，李洪鐘

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基于甲烷化反應(yīng)的催化劑顆粒設(shè)計(jì)與過程強(qiáng)化

李軍，朱慶山，李洪鐘

（中國科學(xué)院過程工程研究所多相復(fù)雜系統(tǒng)國家重點(diǎn)實(shí)驗(yàn)室，北京100190）

甲烷化反應(yīng)過程的主要問題是“燒結(jié)”和“積炭”?；诩淄榛磻?yīng)的強(qiáng)放熱、減分子特性和對(duì)反應(yīng)機(jī)理的認(rèn)識(shí)，從催化劑與反應(yīng)器的匹配性角度，論述了當(dāng)前的主要甲烷化工藝、甲烷化催化劑、甲烷化反應(yīng)及過程強(qiáng)化方法。流化床技術(shù)可有效防止催化劑的積炭和燒結(jié)，從與流化床反應(yīng)器匹配的催化劑結(jié)構(gòu)設(shè)計(jì)源頭出發(fā)，制備具有耐磨損、易流化、低密度的高活性甲烷化催化劑，是流化床甲烷化發(fā)展的一個(gè)重要途徑。

甲烷化；流化床反應(yīng)器；強(qiáng)放熱；減分子；鎳催化劑；積炭

引 言

我國“富煤、貧油、少氣”的能源結(jié)構(gòu)特點(diǎn)決定了煤炭在能源利用中占主導(dǎo)地位，并且在今后相當(dāng)長的時(shí)期內(nèi)不會(huì)改變。然而，持續(xù)增加的天然氣需求及日益嚴(yán)格的環(huán)保要求促使人們尋求新的煤炭利用途徑和天然氣來源。這使得煤制天然氣技術(shù)的迅速發(fā)展成為煤炭潔凈利用的選擇之一。

煤制天然氣是煤經(jīng)過煤氣化、合成氣變換、凈化、甲烷化等化學(xué)反應(yīng)最終獲得清潔燃料甲烷的過程[1]。其中，煤氣化和合成氣甲烷化是煤制天然氣技術(shù)體系的核心。到目前為止，作為煤炭三大利用途徑之一的煤氣化技術(shù)已經(jīng)成熟。然而，甲烷化技術(shù)僅有國外少數(shù)幾個(gè)公司掌握，尚未實(shí)現(xiàn)國產(chǎn)化。甲烷化技術(shù)的核心是甲烷化催化劑和甲烷化反應(yīng)器的研制和開發(fā)，其關(guān)鍵在于如何有效控制催化劑床層溫度，避免因反應(yīng)的強(qiáng)放熱導(dǎo)致床層局部飛溫現(xiàn)象[2-3]。

本文系統(tǒng)地論述了當(dāng)前主要的甲烷化工藝、甲烷化催化劑和甲烷化反應(yīng)機(jī)理的主要進(jìn)展，提出了基于甲烷化反應(yīng)機(jī)理的流化床反應(yīng)器和催化劑顆粒設(shè)計(jì)是未來流化床甲烷化工藝的發(fā)展方向，以期為甲烷化工藝的進(jìn)一步應(yīng)用開發(fā)提供指導(dǎo)。

1 甲烷化工藝研究現(xiàn)狀

甲烷化反應(yīng)的一個(gè)重要工業(yè)應(yīng)用是合成氨、燃料電池等富氫氣體中痕量CO的脫除[4-9]，但更加引人關(guān)注的應(yīng)用是煤/生物質(zhì)氣化甲烷化制天然氣工藝[10-17]。煤制天然氣工藝大致包括煤氣化、合成氣變換、凈化和甲烷化等，如圖1所示。首先煤氣化使煤顆粒與水蒸氣和氧氣在高溫下反應(yīng)得到粗合成氣，主要成分包括H2、CO、CO2、H2O、CH4和少量碳?xì)浠衔?，含S、Cl雜質(zhì)，其組分含量與氣化工藝條件、反應(yīng)器類型、氣化劑等密切相關(guān)；由于粗合成氣中焦油、含S/Cl等微量雜質(zhì)氣體對(duì)后續(xù)的反應(yīng)器甲烷化催化劑有損害，需要經(jīng)過氣體凈化裝置處理；凈化處理后的氣體經(jīng)水煤氣變換反應(yīng)調(diào)整H2和CO比例為3左右；進(jìn)入甲烷化反應(yīng)裝置和提純裝置得到甲烷（>95%）。

表1概括了甲烷化過程主要反應(yīng)，反應(yīng)（1）為甲烷化反應(yīng)的主反應(yīng)，但實(shí)際反應(yīng)過程中，存在CO歧化（2）、甲烷分解（3）等副反應(yīng)，使催化劑表面積炭，降低催化劑的活性和使用壽命。同時(shí)甲烷也可能通過CO2甲烷化反應(yīng)（4）和逆CO2-CH4重整反應(yīng)（5）獲得。其中反應(yīng)（5）可以認(rèn)為是甲烷化反應(yīng)和水蒸氣變換反應(yīng)（6）的疊加。從熱力學(xué)上分析，甲烷化反應(yīng)為減分子的強(qiáng)放熱反應(yīng)，增加壓力和低溫對(duì)甲烷化反應(yīng)有利。但在動(dòng)力學(xué)上高溫有利于提高甲烷化反應(yīng)速率，同時(shí)從能量利用角度考慮，高溫甲烷化有利于提高能量利用率，近年來受到研究者們的高度關(guān)注[18-19]。然而，在高溫條件下催化劑積炭和活性金屬燒結(jié)等問題是嚴(yán)重影響高溫甲烷化的不利因素。因此，甲烷化工藝的難點(diǎn)在于如何有效控制反應(yīng)區(qū)域的溫度，防止催化劑積炭、燒結(jié)失活。其關(guān)鍵是甲烷化反應(yīng)器和甲烷化催化劑的開發(fā)。

表1 甲烷化工藝過程主要反應(yīng)和副反應(yīng)[20-21]

1.1 固定床甲烷化工藝

甲烷化反應(yīng)器與甲烷化催化劑并列為甲烷化技術(shù)的兩大核心[22]。自20世紀(jì)50年代，研究者就開始致力于甲烷化反應(yīng)器的開發(fā)，如固定床、流化床和漿態(tài)床甲烷化反應(yīng)器[23-25]。由于固定床具有反應(yīng)速率高、催化劑用量少、催化劑不易磨損等優(yōu)點(diǎn)，已經(jīng)成熟的工業(yè)化甲烷化技術(shù)普遍采用絕熱多段固定床甲烷化反應(yīng)器，包括丹麥托普索公司（Topsoe）的TREMPTM技術(shù)[26]、英國戴維公司（Davy）的CRG技術(shù)[27]和德國魯奇公司（Lurgi）的甲烷化技術(shù)[28]，均采用了固定床甲烷化反應(yīng)器。但由于固定床反應(yīng)器傳熱性能差，如何移出甲烷化反應(yīng)大量放熱是固定床甲烷化技術(shù)的關(guān)鍵。通常采用多段絕熱式固定床反應(yīng)器的串聯(lián)方式，通過控制各段反應(yīng)器的轉(zhuǎn)化率、部分產(chǎn)品氣體循環(huán)和內(nèi)置或外置預(yù)熱器等方法實(shí)現(xiàn)反應(yīng)過程的溫度控制。根據(jù)催化劑的耐受溫度范圍不同，其甲烷化工藝的操作溫度和回收熱量的方式有所不同。比如，丹麥的TREMPTM技術(shù)的特點(diǎn)是采用MCR-2X催化劑具有寬的溫度窗口（250～700℃），在較高溫度下（600℃）運(yùn)行，可減少氣體循環(huán)量和回收高壓蒸汽熱量，能量利用率高[29]。Davy的CRG甲烷化技術(shù)的特點(diǎn)是采用CRG催化劑具有變換功能，不需要調(diào)節(jié)合成氣的H/C比，并且在250～700℃具有較高活性[30]。魯奇甲烷化工藝的特點(diǎn)是甲烷化反應(yīng)溫度較低（450℃），采用氣體循環(huán)限制原料氣的進(jìn)口溫度（<300℃），防止催化劑積炭[31]。最近報(bào)道顯示，魯奇甲烷化工藝為了提高競爭力，開發(fā)了高溫甲烷化催化劑，提高了甲烷化反應(yīng)溫度[32]。3種甲烷化技術(shù)各有特色，魯奇和戴維的甲烷化技術(shù)得到了美國大平原項(xiàng)目的長期驗(yàn)證[33-34]，引進(jìn)托普索甲烷化技術(shù)的新疆慶華年產(chǎn)55億立方米煤制天然氣項(xiàng)目一期已于2013年8月竣工投產(chǎn)，產(chǎn)出的煤制天然氣已送入西氣東輸管線[35]。而采用戴維甲烷化工藝的大唐內(nèi)蒙古克什克騰旗年產(chǎn)40億立方米煤制天然氣一期示范項(xiàng)目已于2013 年12月投運(yùn)，正式向北京供氣[36]。

甲烷化反應(yīng)器的設(shè)計(jì)通常與甲烷化工藝和催化劑配套，其反應(yīng)器結(jié)構(gòu)中很多經(jīng)驗(yàn)取值與其配套工藝和催化劑密切相關(guān)，是甲烷化技術(shù)的關(guān)鍵技術(shù)之一。由于甲烷化反應(yīng)具有反應(yīng)迅速、放熱量大、易積炭等特點(diǎn)，在反應(yīng)器設(shè)計(jì)中，除了防止催化劑床層飛溫、積炭失活問題外，還需要考慮諸如床層熱點(diǎn)穿出、水浸入催化劑結(jié)構(gòu)性破壞和反應(yīng)器冷熱位移等問題[37]。由于甲烷化工藝與催化劑高度匹配，目前只有丹麥托普索公司、英國戴維公司和德國魯奇公司等少數(shù)公司掌握固定床甲烷化技術(shù)，國內(nèi)鮮有關(guān)于固定床甲烷化反應(yīng)器結(jié)構(gòu)的文獻(xiàn)報(bào)道。

多級(jí)串聯(lián)的固定床反應(yīng)器結(jié)構(gòu)使得整體設(shè)備和流程相對(duì)復(fù)雜，工藝參數(shù)控制相對(duì)較難，同時(shí)需要返回大量的產(chǎn)品氣稀釋原料氣，限制了生產(chǎn)能力，并且增加了動(dòng)力消耗，因而操作成本較高，影響了工藝的整體經(jīng)濟(jì)性。為克服工業(yè)固定床工藝中的缺點(diǎn)，許多研究機(jī)構(gòu)對(duì)甲烷化工藝及其設(shè)備進(jìn)行改進(jìn)，開發(fā)了流化床工藝和漿態(tài)床工藝。

1.2 流化床甲烷化工藝

與固定床反應(yīng)器比較，流化床反應(yīng)器具有相間接觸良好、床層溫度均勻的特點(diǎn)，易于規(guī)?；B續(xù)化操作的優(yōu)勢，特別適合于應(yīng)用于強(qiáng)放熱的甲烷化反應(yīng)。1950～1980年間，先后有多個(gè)國家參與開發(fā)流化床甲烷化工藝，主要有美國礦務(wù)局（Bureau of Mines）建立的多段流化床甲烷化工藝[38]、美國Bituminous Coal Research Inc. (BCR, United States)公司的Bi-Gas流化床甲烷化工藝[39]和德國卡爾斯魯厄大學(xué)（University of Karlsruhe）與Thyssenga公司合作開發(fā)的Comflux甲烷化工藝[40]。其工藝參數(shù)和運(yùn)行狀況列于表2中[38-42]。

表2 典型的流化床甲烷化工藝參數(shù)

從表2中看出，美國礦務(wù)局（Bureau of Mines）建立的煤氣化甲烷化制天然氣流化床工藝的規(guī)模較小，其流化床直徑僅為1.9～2.54 cm。催化劑采用Fe基或Ni基催化劑（p=63～180mm），運(yùn)行結(jié)果顯示鎳基催化劑優(yōu)于Fe基催化劑，床層溫度控制較好，CO和H2轉(zhuǎn)化率95%～98%，但該工藝自1956年后未見有報(bào)道[38]。

Bi-Gas工藝流化床反應(yīng)器直徑15 cm，反應(yīng)區(qū)高度2.5 m。催化劑采用NiCoMo/Al2O3催化劑，具有水煤氣變換和甲烷化功能[41]。但運(yùn)行結(jié)果顯示，在催化劑量23～27 kg，H2/CO比1.4～3，表觀氣速2.4～5.5 cm·s-1（8～18倍mf）條件下，CO和H2轉(zhuǎn)化率70%～95%，還需要進(jìn)一步提高轉(zhuǎn)化率。但在運(yùn)行過程中發(fā)現(xiàn)，在甲烷化反應(yīng)初期催化劑顆粒的磨損較為嚴(yán)重。自1979年Cobb等[39]利用其運(yùn)行數(shù)據(jù)計(jì)算了CO反應(yīng)動(dòng)力學(xué)和建立了兩相流數(shù)學(xué)模型之后，未見與Bi-Gas甲烷化工藝及流化床反應(yīng)器的文獻(xiàn)報(bào)道。

Comflux工藝的突出特點(diǎn)是水煤氣變換反應(yīng)和甲烷化反應(yīng)集中在一個(gè)流化床中進(jìn)行。與前兩個(gè)流化床甲烷化工藝比較，Comflux工藝規(guī)模顯著提高，流化床反應(yīng)器直徑為40～100 cm，能容納1000～3000 kg催化劑(p=10～400mm)，SNG 生產(chǎn)規(guī)模達(dá)到2000 m3·h-1?？紤]到省去了變換單元和產(chǎn)品循環(huán)氣壓縮機(jī)，該工藝降低了投資運(yùn)行的成本，比固定床工藝減少了將近10%的成本。該工藝通過了中試和半商業(yè)運(yùn)營，尚無商業(yè)化規(guī)模的運(yùn)營。受石油價(jià)格影響，該裝置自20世紀(jì)80年代中期終止運(yùn)行。

除了以上3種煤基甲烷化流化床工藝外，自20世紀(jì)90年代，瑞士PSI (Paul-Scherrer Institut, Switzerland)公司開始致力于生物質(zhì)轉(zhuǎn)化制SNG技術(shù)開發(fā)，稱為PSI流化床甲烷化工藝[43-46]。其核心技術(shù)源于Comflux流化床甲烷化技術(shù)，催化劑同時(shí)具有水煤氣變換反應(yīng)和甲烷化反應(yīng)的功能。該工藝于2007年在10 kW SNG中試規(guī)模的裝置上運(yùn)行了1000 h，結(jié)果顯示產(chǎn)品氣含有高達(dá)40%的CH4和極少量的CO，并于2009年在1 MW SNG PDU規(guī)模裝置上完成驗(yàn)證。2010年初，PSI工藝與奧地利的快速內(nèi)循環(huán)流化床氣化工藝（FICFB gasifier）嫁接形成具有競爭力的甲烷化技術(shù)，預(yù)計(jì)2016年建成100 MW SNG的甲烷化工藝。

1.3 漿態(tài)床甲烷化工藝

漿態(tài)床反應(yīng)器以液態(tài)惰性烴為反應(yīng)介質(zhì)，涉及氣、液、固三相反應(yīng)器，由于其反應(yīng)系統(tǒng)的熱穩(wěn)定性高，系統(tǒng)溫度可以達(dá)到瞬間平衡的特點(diǎn)，非常適用于甲烷化反應(yīng)。其基本工藝原理是反應(yīng)器下部通入原料氣和流化用液體，與流化床中懸浮的Ni 催化劑作用進(jìn)行甲烷化反應(yīng)。反應(yīng)熱被液體吸收。由于液體熱容量大，反應(yīng)基本是在等溫條件下進(jìn)行。氣化的流化液體與產(chǎn)品氣體在反應(yīng)器外部用熱交換器進(jìn)行冷卻分離，液體進(jìn)行循環(huán)再利用。美國化學(xué)系統(tǒng)研究所(Chem. System)開發(fā)了LPM（liquid phase methanation）工藝，該工藝在bench-scale unit (BSU)、process development unit（PDU）、pilot plant（PP）3種規(guī)模的實(shí)驗(yàn)裝置進(jìn)行了驗(yàn)證，其工藝條件和裝置規(guī)模見表3[46-48]。在PP裝置上運(yùn)行300 h結(jié)果顯示，該工藝存在甲烷合成效率較低、催化劑損失嚴(yán)重的問題，于1981年被終止。

國內(nèi)太原理工大學(xué)、中國礦業(yè)大學(xué)（北京）等科研機(jī)構(gòu)也對(duì)漿態(tài)床甲烷化進(jìn)行了研究[49-51]。太原理工大學(xué)的研究表明，漿態(tài)床CO甲烷化在280℃的反應(yīng)溫度下，CO的轉(zhuǎn)化率保持在96%以上，取得了很好的反應(yīng)結(jié)果[49-50]。目前，該工藝還在研究開發(fā)階段，未見到工業(yè)化項(xiàng)目相關(guān)報(bào)道。

表3 漿態(tài)床甲烷化工藝參數(shù)

1.4 甲烷化反應(yīng)器性能對(duì)比分析

從以上甲烷化工藝及甲烷化反應(yīng)器的研究結(jié)果可以看出，甲烷化反應(yīng)器的設(shè)計(jì)是整個(gè)甲烷化工藝的關(guān)鍵技術(shù)。一個(gè)理想的甲烷化反應(yīng)器應(yīng)具有高效的傳熱性能、防止催化劑積炭失活和減少催化劑損失等優(yōu)點(diǎn)。

表4列出了固定床、流化床和漿態(tài)床甲烷化工藝的性能，可以看出，漿態(tài)床反應(yīng)器的CO轉(zhuǎn)化率低、催化劑磨損嚴(yán)重，仍然處于實(shí)驗(yàn)室研究階段，距離工業(yè)化較遠(yuǎn)。固定床具有CO轉(zhuǎn)化率高、催化劑用量少、催化劑無磨損等優(yōu)點(diǎn)。但固定床工藝流程和結(jié)構(gòu)復(fù)雜，運(yùn)行成本高。流化床甲烷化工藝CO轉(zhuǎn)化率高，具有流程結(jié)構(gòu)簡單、生產(chǎn)能力大等優(yōu)勢，操作成本較固定床低，但存在催化劑磨損嚴(yán)重問題，制約了其工業(yè)化進(jìn)展。因此，未來流化床甲烷化工藝的研發(fā)重點(diǎn)在于如何防止催化劑磨損和研制抗磨損的甲烷化催化劑。

表4 固定床、流化床和漿態(tài)床甲烷化工藝對(duì)比[35,46]

2 甲烷化催化劑研究現(xiàn)狀

自1902年Sabatier等[52]發(fā)現(xiàn)在Ni及其他金屬(Ru、Rh、Pt、Fe、Co)催化劑能催化CO甲烷化反應(yīng)以來，甲烷化催化劑的研究一直是該技術(shù)關(guān)注的焦點(diǎn)，涉及了甲烷化反應(yīng)熱力學(xué)、反應(yīng)動(dòng)力學(xué)、催化反應(yīng)機(jī)理、失活機(jī)理等多個(gè)方面[53-60]。大量的研究表明第Ⅷ族金屬及Ag和Mo均有甲烷化活性，其單位金屬表面的甲烷化催化活性順序依次為Ru>Ir>Rh>Ni>Co>Os>Pt>Fe>Mo>Pd> Ag[20]。在眾多的金屬催化劑中，具有高甲烷化催化活性的有貴金屬Ru、Rh及過渡金屬元素Ni、Co、Fe、Mo等[61-66]。Fe、Co作為甲烷化催化劑的選擇性較差，且易積炭失活[67-68]。具有工業(yè)化應(yīng)用前景的催化劑主要是Ru基和Ni基催化劑，Ru基催化劑比Ni基催化劑的催化活性高，為最理想的甲烷化催化劑，但因其價(jià)格昂貴，限制了它的工業(yè)使用[69-71]。鎳基催化劑由于其較好的甲烷化催化活性、選擇性高且價(jià)格相對(duì)低廉，是工業(yè)化甲烷化催化劑的主要選擇[72-76]。從目前已經(jīng)工業(yè)化的甲烷化催化劑看，如托普索公司的MCR-2X[19,77]、戴維的CRG[78-79]以及魯奇公司的Cl-85[46,80]，均是鎳基催化劑，其典型的催化劑特點(diǎn)見表5?？梢钥闯?，高溫甲烷化技術(shù)和高溫甲烷化催化劑是未來甲烷化工藝的重點(diǎn)發(fā)展方向。

表5 商業(yè)化甲烷化催化劑的特點(diǎn)

到目前為止，國內(nèi)研究機(jī)構(gòu)開發(fā)的甲烷化催化劑主要是應(yīng)用于中低溫微量CO脫除方面（部分甲烷化），而針對(duì)完全甲烷化的高溫甲烷化催化劑，由于沒有配套的甲烷化工藝的支撐，其甲烷化催化劑正處于研發(fā)階段，主要以實(shí)驗(yàn)室研究為主，缺乏中試示范裝置和工業(yè)裝置驗(yàn)證。國內(nèi)中國科學(xué)技術(shù)大學(xué)研制的KD-306催化劑的CO轉(zhuǎn)化率僅40%～60%，甲烷選擇性>70%，上海煤氣公司研發(fā)的SG-100鎳基催化劑的CO轉(zhuǎn)化率為60%～74%，甲烷選擇性50%～63%[81]，遠(yuǎn)低于工業(yè)化的甲烷化催化劑[82-83]。近年來，中科院過程所在催化劑載體、鎳粒子調(diào)控及甲烷化催化劑抗積炭方面進(jìn)行詳細(xì)的研究，開發(fā)出多種活性高的Ni基催化劑[25,61,84-86]。北京低碳清潔能源研究所在耐硫型催化劑和寬溫型（250～700℃）Ni基甲烷化催化劑開發(fā)方面取得了進(jìn)展，通過添加助劑MgO提高了NiO熱穩(wěn)定性和甲烷選擇性[87-91]。但缺乏長時(shí)間的穩(wěn)定性實(shí)驗(yàn)驗(yàn)證該催化劑的高溫穩(wěn)定性。中科院大連化物所開發(fā)了高溫和低溫甲烷化催化劑，完成的5000 m3·d-1煤制天然氣甲烷化工業(yè)中試裝置已連續(xù)穩(wěn)定運(yùn)行超過1000 h[24]，為中國煤制天然氣技術(shù)的產(chǎn)業(yè)化發(fā)展向前邁出了關(guān)鍵一步。

在高溫甲烷化反應(yīng)過程中，原料氣與甲烷化催化劑（鎳、助劑和載體組成）顆粒表面的Ni原子接觸并反應(yīng)，其甲烷化催化劑活性決定于CO解離能和主要中間體在金屬催化劑表面的穩(wěn)定性[58-60]，理想的催化劑是在兩個(gè)因素之間取得平衡。要求甲烷化催化劑具有高比表面積、高鎳分散性及與載體的強(qiáng)相互作用[92-94]。大量研究表明，導(dǎo)致甲烷化催化劑失活的原因有：(1)積炭[95-97]；(2)燒結(jié)[19,56,98-100]；(3)鎳流失[67,101]；(4)硫中毒[44,102-103]。針對(duì)硫中毒和鎳的流失，工業(yè)上一般采用對(duì)原料氣體進(jìn)行深度預(yù)脫硫，高于生成Ni(CO)4溫度操作，使硫中毒和鎳流失問題得以解決。然而，高溫下積炭和鎳的燒結(jié)仍然是鎳基催化劑甲烷化工藝面臨的兩個(gè)技術(shù)難題。工業(yè)上，通常以犧牲生產(chǎn)能力和耗費(fèi)能量來減少催化劑的積炭和燒結(jié)，如魯奇甲烷化工藝采用產(chǎn)品氣循環(huán)以稀釋原料氣控制反應(yīng)器溫升，托普索公司就是采用從第二反應(yīng)器出來的部分氣體循環(huán)到第一反應(yīng)器入口來控制反應(yīng)器溫度[31,104]。

催化劑積炭主要來源于CO的歧化反應(yīng)和甲烷分解反應(yīng)，積炭通常發(fā)生在催化劑床層上部和固定床反應(yīng)器入口處[105]。生成的碳晶須或聚合炭會(huì)沉積在催化劑表面而覆蓋其金屬活性位，阻塞催化劑載體的孔道，使活性組分與載體分離，不僅造成催化劑的失活，縮短催化劑壽命[106-107]，還會(huì)增加催化床層阻力。Czekaj等[43]給出了積炭機(jī)理，該機(jī)理認(rèn)為催化劑表面上的NiO 和Ni(OH)2不具催化活性，只有被H2還原后的金屬態(tài)Ni才具有甲烷化催化活性。催化劑活性降低的原因是金屬態(tài)Ni 晶格和-Al2O3晶格不匹配而形成了由Ni和NiC或Ni3C組成的一個(gè)薄層界面，造成活性組分鎳與載體間作用力弱，從而導(dǎo)致具有活性的Ni粒子從載體上脫落。另一方面，積炭會(huì)形成惰性炭層或低反應(yīng)活性的碳化物覆蓋在催化劑表面，阻止反應(yīng)進(jìn)行。因此，選擇與鎳兼容性好的催化劑載體材料，以增強(qiáng)活性鎳與載體之間的作用力，是防止積炭的有效方法。

高溫或低溫高CO濃度甲烷化過程均會(huì)導(dǎo)致鎳催化劑燒結(jié)失活，鎳基催化劑的燒結(jié)失活存在兩種燒結(jié)機(jī)理[108]：一種是粒子遷移機(jī)理，即金屬晶粒在催化劑表面上遷移、碰撞、聚并長大；另一種是原子遷移機(jī)理，認(rèn)為金屬原子從金屬晶粒上脫離開，在催化劑表面遷移，并被另一個(gè)晶粒捕獲。無論是哪一種燒結(jié)機(jī)理，都與催化劑載體結(jié)構(gòu)、活性金屬含量、鎳與載體的相互作用密切相關(guān)[99-100]。由于甲烷化反應(yīng)的強(qiáng)放熱特性，高溫下引起床層局部過熱是導(dǎo)致催化劑燒結(jié)失活的另一個(gè)主要原因。

由此可見，催化劑積炭和活性金屬燒結(jié)是甲烷化催化劑失活的兩個(gè)主要原因，并且受催化劑結(jié)構(gòu)和操作條件影響較大。無論是積炭，還是鎳粒子燒結(jié)都與甲烷化的強(qiáng)放熱特性引起的床層過熱有關(guān)。一方面，從甲烷化催化劑本身出發(fā)，研制具有高熱穩(wěn)定性的新型抗積炭抗燒結(jié)催化劑；另一方面從甲烷化工藝入手，比如產(chǎn)品氣循環(huán)以稀釋原料氣、通入水蒸氣以調(diào)節(jié)CO分壓等手段穩(wěn)定床層溫度。另外，通過流化床反應(yīng)器強(qiáng)化傳熱是甲烷化一個(gè)重要途徑。

3 甲烷化反應(yīng)過程強(qiáng)化

關(guān)于CO甲烷化的動(dòng)力學(xué)和反應(yīng)機(jī)理的研究很多。早期的研究認(rèn)為，氧中間體（CHO）是甲烷化反應(yīng)的中間體[20]。但在甲烷化過程的紅外研究中沒有發(fā)現(xiàn)CHO物種的存在。Wise[109-111]提出了表面碳中間體機(jī)理并給出了甲烷化反應(yīng)路徑，即CO 在催化劑表面解離得到表面碳原子（Cs），部分加氫（CH）后通過CO 的不斷插入和部分氫解作用使鏈增長，最終加氫使鏈終止。表面碳機(jī)理得到了大多數(shù)實(shí)驗(yàn)的證實(shí)，但對(duì)氫氣在甲烷化中的作用和控速步驟目前尚未達(dá)成共識(shí)，最大分歧在于是CO直接解離還是氫助解離[112-114]，以及速控步驟是CO解離還是表面碳加氫[115-117]。甲烷化反應(yīng)機(jī)理的爭論也反映了甲烷化反應(yīng)的復(fù)雜性，但無論是何種加氫機(jī)理，CO在催化劑表面吸附都是關(guān)鍵。

大量研究表明，CO在催化劑表面解離生成的吸附碳是甲烷化反應(yīng)的前驅(qū)體。催化劑表面的碳可以分為Cα、Cβ、石墨碳以及碳須[118]。其中，Cα是原子狀態(tài)的碳，低溫下易于加氫，Cβ在較高溫度下才能加氫，活性約為Cα的1/100[119]；溫度在600 K以上，Cα可以緩慢轉(zhuǎn)變?yōu)镃β，Cβ在高溫下轉(zhuǎn)變?yōu)槭?。碳須由吸附碳原子在金屬表面進(jìn)行擴(kuò)散，并在金屬和載體界面處成核并生長而成[120]。碳須不占據(jù)金屬表面，對(duì)催化活性沒有太大影響。但是，由于具有很高機(jī)械強(qiáng)度，大量碳須生成會(huì)使催化劑強(qiáng)度嚴(yán)重下降，甚至粉化。因此，從催化劑結(jié)構(gòu)和反應(yīng)器出發(fā)，降低催化劑積炭和燒結(jié)仍然是甲烷化反應(yīng)研究的前沿領(lǐng)域。

基于以上認(rèn)識(shí)，從甲烷化反應(yīng)過程強(qiáng)化擴(kuò)散、傳熱角度認(rèn)識(shí)甲烷化反應(yīng)和降低催化劑積炭和燒結(jié)速率引起研究者的關(guān)注[121-122]。流化床反應(yīng)器具有高效的氣固傳質(zhì)傳熱效率，反應(yīng)床層內(nèi)溫度和催化劑顆粒均勻分布等優(yōu)點(diǎn)，有利于實(shí)現(xiàn)對(duì)甲烷化反應(yīng)溫度的控制，抑制床層溫度過熱和防止燒結(jié)。研究表明，流化床甲烷化反應(yīng)可以分為富CO區(qū)和貧CO區(qū)，高流化氣速有利于強(qiáng)化傳熱，避免出現(xiàn)局部熱點(diǎn)，但高流化氣速也會(huì)帶來氣泡快速的穿越催化劑床層，降低反應(yīng)轉(zhuǎn)化率[123-124]。同時(shí)，催化劑顆粒在流化床中循環(huán)流動(dòng)使得流化床成為積炭的緩沖器，能夠有效防止催化劑積炭，使得流化床甲烷化重新引起重視[45,105,121]。許光文課題組[125-128]研究表明，流化床具有比固定床高的CO轉(zhuǎn)化率和CH4選擇性，并進(jìn)一步證實(shí)了流化床的強(qiáng)化傳熱效果。

早期的流化床甲烷化工藝研究中發(fā)現(xiàn)催化劑顆粒的磨損嚴(yán)重[39]，但對(duì)流化床甲烷化催化劑的流化行為及催化劑顆粒的流化質(zhì)量對(duì)甲烷化反應(yīng)的影響很少有文獻(xiàn)報(bào)道。近年來，Kai等[129-131]研究表明，甲烷化減分子反應(yīng)造成催化劑流化質(zhì)量降低，甚至失流。在其他反應(yīng)中也發(fā)現(xiàn)了減分子反應(yīng)對(duì)流化床反應(yīng)和反應(yīng)器放大的不利影響[132-133]。而且，催化劑的磨損使得催化劑顆粒變細(xì)，也會(huì)導(dǎo)致催化劑的黏結(jié)失流，造成CO轉(zhuǎn)化率和甲烷選擇性降低。Li等[134]也通過添加B類顆粒，改善了催化劑的流化質(zhì)量，提高了CO轉(zhuǎn)化率和甲烷選擇性。納米催化劑顆粒具有比普通負(fù)載催化劑更高的甲烷化活性，但由于其強(qiáng)的黏結(jié)性，極易團(tuán)聚而導(dǎo)致失流[135-136]，通過外加磁場[137]和預(yù)燒結(jié)造粒[138]，顯著提高了納米催化劑的流化質(zhì)量，抑制催化劑燒結(jié)和積炭。宗保寧等[139-143]利用磁場強(qiáng)化受傳熱和傳質(zhì)限制的反應(yīng)過程，如己內(nèi)酰胺加氫精制，顯示出良好的工業(yè)應(yīng)用前景。

以往的研究中，忽略了催化劑與反應(yīng)器的匹配性及甲烷化反應(yīng)的強(qiáng)放熱和減分子反應(yīng)特性，是導(dǎo)致流化床甲烷化失敗的一個(gè)重要因素。通過顆粒及反應(yīng)器結(jié)構(gòu)設(shè)計(jì)，強(qiáng)化傳遞和反應(yīng)的研究成為新的研究方向[144-156]。微反應(yīng)器（微通道）具有狹窄的通道、大的比表面積和體積比，大大強(qiáng)化了傳熱和傳質(zhì)速率，明顯優(yōu)于傳統(tǒng)的反應(yīng)器，特別是在甲烷化反應(yīng)中起到了獨(dú)特的作用[144-148]。程易等[121]考慮到選擇適于流化床的甲烷化催化劑顆粒，以易于流化的耐磨損的-Al2O3為載體，通過浸漬法制備了鎳基催化劑。從與流化床反應(yīng)器匹配的催化劑結(jié)構(gòu)設(shè)計(jì)源頭出發(fā)，采用非常規(guī)制備技術(shù)手段（如超重力）[153-154]制備具有耐磨損、易流化、低密度的高活性甲烷化催化劑，可能是流化床甲烷化發(fā)展的一個(gè)重要途徑。

4 結(jié)論和展望

甲烷化反應(yīng)過程的主要問題是“燒結(jié)”和“積炭”。現(xiàn)有固定床甲烷化工藝以犧牲生產(chǎn)能力和耗費(fèi)能量來減少催化劑的積炭和燒結(jié)。這為流化床甲烷化反應(yīng)器及配套催化劑設(shè)計(jì)帶來了機(jī)遇和挑戰(zhàn)。

基于甲烷化反應(yīng)特點(diǎn)和對(duì)甲烷化反應(yīng)機(jī)理的認(rèn)識(shí)，從與流化床反應(yīng)器-催化劑結(jié)構(gòu)的匹配角度，制備具有耐磨損、易流化、低密度的高活性甲烷化催化劑，可能是流化床甲烷化發(fā)展方向。

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Process intensification and catalysts particle design for CO methanation

LI Jun，ZHU Qingshan，LI Hongzhong

State Key Laboratory of Multiphase Complex SystemsInstitute of Process EngineeringChinese Academic SciencesBeijingChina

Carbon deposition and sintering of metal particles are the two dominating reasons for deactivation of the methanation catalyst. Based on the strong exothermic reaction accompanied by a large decrease in mole number and methanation mechanism,from the perspective of the matching of catalyst and reactor, this paper summarizes the development of main CO methanation techniques, CO methanation catalysts, reaction mechanism of CO methanation and its process intensifications. Fluidized bed reactors have the advantages in preventing the carbon deposition and sintering of Ni catalysts. Thus, the design of wear-resistant, easy fluidized and low density catalyst structure particles that applicable to fluidized bed reactors should be a feasible way and the new direction for the development of methanation techniquesfluidized bed reactors.

methanation; fluidized bed reactor; strong exothermic; molecular reduction; Ni catalyst; carbon deposition

2015-05-29.

Prof. ZHU Qingshan, qszhu@ipe.ac.cn

10.11949/j.issn.0438-1157.20150748

TQ 032.4

A

0438—1157（2015）08—2773—11

朱慶山。

李軍（1979—），男，博士，副研究員。

國家自然科學(xué)基金項(xiàng)目（91334108）；國家重大科學(xué)儀器設(shè)備開發(fā)專項(xiàng)項(xiàng)目（2011YQ12003908）。

2015-05-29收到初稿，2015-06-10收到修改稿。

supported by the National Natural Science Foundation of China (91334108) and the National Special Project for Development of Major Scientific Equipment (2011YQ12003908).