趙　巖，任冬梅，夏云生，李新華，包德才

(渤海大學化學化工學院，遼寧 錦州 121013)

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有機化工與催化

混合二甲苯系統(tǒng)熱力學平衡組成的計算

趙巖*，任冬梅，夏云生，李新華，包德才

(渤海大學化學化工學院，遼寧 錦州 121013)

摘要：研究混合二甲苯模型中3種組分在不同溫度下的熱力學平衡組成，依據(jù)狀態(tài)函數(shù)特點，分別計算各平衡的標準摩爾反應熵變、標準摩爾反應焓變及標準摩爾吉布斯自由能變，進而計算平衡組成。結(jié)果表明，在該混合二甲苯理想氣體模型中，反應溫度由298.15 K升至1 023.15 K，混合二甲苯中3個平衡組成隨著反應溫度升高呈如下變化規(guī)律：間二甲苯含量y(MX)由0.599 0逐漸降至0.496 3，降幅達17.15%；鄰二甲苯含量y(OX)由0.162 6逐漸升至0.275 9，升幅達69.68%；對二甲苯含量y(PX)由0.238 4緩慢升至0.242 6，而后逐漸降至0.227 7，最大值出現(xiàn)在423.15 K，最大降幅僅為6.14%，表明升高溫度不利于提高對二甲苯的平衡組成。

關(guān)鍵詞：化學熱力學；混合二甲苯；平衡組成；對二甲苯；異構(gòu)化

CLC number:TQ241.1+3;O643.12Document code: AArticle ID: 1008-1143(2016)05-0075-06

混合二甲苯是間二甲苯(MX)、鄰二甲苯(OX)和對二甲苯(PX)構(gòu)成的混合體系，如指定了某一溫度和壓力，則該系統(tǒng)的狀態(tài)函數(shù)不再變化，即此時系統(tǒng)處于熱力學動態(tài)平衡，其組分含量是定值。當混合二甲苯看作理想氣體或理想液體混合物時，可以對系統(tǒng)熱力學性質(zhì)進行演繹推理，為深入開發(fā)混合二甲苯轉(zhuǎn)化工藝提供重要的熱力學基礎(chǔ)數(shù)據(jù)。工業(yè)合成對二甲苯的工藝(如甲苯歧化、烷基轉(zhuǎn)移和汽油裂解等)主要產(chǎn)物為混合二甲苯[1-3]，因此要設(shè)計高效分離設(shè)備，對生產(chǎn)操作(如冷凝、氣化、閃蒸、精餾、吸收、萃取、結(jié)晶和吸附)進行優(yōu)化。本文對混合二甲苯系統(tǒng)熱力學平衡組成的計算進行研究。

1模型建立

圖1　理想二甲苯混合系統(tǒng)模型Figure 1　Ideal model of xylene mixed system

2計算過程

計算平衡組成熱力學方程式及計算路線：

3結(jié)果與討論

表1　二甲苯的標準摩爾恒壓熱容與溫度的關(guān)系[4-5]

表2　不同反應溫度下二甲苯平衡體系的標準摩爾反應熵變(T)

表3　不同反應溫度下二甲苯平衡體系的標準摩爾反應焓變(T)

表4　不同反應溫度下二甲苯平衡體系的標準摩爾反應吉布斯自由能變(T)

從表4可以看出，反應溫度低于673.15 K，僅有鄰二甲苯向?qū)Χ妆降漠悩?gòu)化反應可以自發(fā)進行，且隨著反應溫度升高，自發(fā)進行的趨勢越來越弱；反應溫度高于673.15 K，上述3個異構(gòu)化反應均不能自發(fā)進行，且隨著反應溫度升高，自發(fā)程度越來越難。

3.5平衡常數(shù)及平衡組成

幅為17.15%；鄰二甲苯含量隨著反應溫度升高逐漸提高，增幅為69.68%；對二甲苯含量隨著反應溫度升高先增后降，降幅為6.14%。從熱力學角度看，反應溫度變化對二甲苯含量影響較小，而升高溫度更有利于間二甲苯向鄰二甲苯異構(gòu)。因此，若要提高對二甲苯選擇性，需要從反應動力學著手[9-11]。選擇合適的擇形催化劑并適當改性[12-23]以及設(shè)計新的分離工藝[24]或合成路線[25]均至關(guān)重要。

表5　不同反應溫度下二甲苯平衡體系的氣相組成

4結(jié)論

(1) 通過對混合二甲苯熱力學平衡組成的計算可知，間二甲苯含量隨著反應溫度升高逐漸降低，鄰二甲苯含量隨著反應溫度升高逐漸提高，對二甲苯含量隨著反應溫度升高先增加后降低，但降幅較小。升高溫度有利于促進間二甲苯向鄰二甲苯異構(gòu)，而對二甲苯含量變化不大。若要提高對二甲苯選擇性，需從選擇催化劑、分離工藝和新合成路線入手。

(2) 混合二甲苯系統(tǒng)熱力學平衡組成的計算結(jié)果將為產(chǎn)品質(zhì)量控制提供至關(guān)重要的熱力學參數(shù)和多元相平衡數(shù)據(jù)，經(jīng)過對某些參數(shù)的適當修正，該方法適用于其他理想混合體系的熱力學平衡計算。

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Calculation of thermodynamic equilibrium compositions of mixed xylenes

Zhao Yan*, Ren Dongmei, Xia Yunsheng, Li Xinhua, Bao Decai

(College of Chemistry and Chemical Engineering, Bohai University, Jinzhou 121013, Liaoning, China)

Abstract:The thermodynamic equilibriums of three components of mixed xylenes model under different temperatures were researched.Based on the characteristics of the state function,three thermodynamic functions (T) and (T) of three equilibriums components under different temperatures were calculated,respectively.And then the equilibrium compositions were calculated.The results showed that in the ideal gas model of mixed xylene,the change rules of the three equilibrium compositions with the increase of reaction temperatures from 298.15 K to 1 023.15 K were as follows:the y(MX)of m-xylene contents reduced gradually from 0.599 0 to 0.496 3 with the drop of 17.15%;the y(OX)of o-xylene contents enhanced gradually from 0.162 6 to 0.275 9 with the increase of 69.68%;the y(PX)of p-xylene contents increased slowly from 0.238 4 to 0.242 6,and then decreased gradually to 0.227 7.Its maximum of 0.242 6 at 423.15 K with the biggest drop of 6.14% was obtained,which indicated that increasing temperature was unfavorable to improving balance composition of p-xylene.

Key words:chemical thermodynamics; mixed xylenes; equilibrium composition; p-xylene; isomerization

收稿日期：2016-03-16

基金項目：渤海大學博士啟動基金(BSQD201416，BSQD201417)資助項目；國家自然科學基金項目(21076026)；遼寧省教育廳項目(L2013430)

作者簡介：趙巖，1976年生，男，遼寧省錦州市人，博士，講師，研究方向為多相催化。

doi:10.3969/j.issn.1008-1143.2016.05.015 10.3969/j.issn.1008-1143.2016.05.015

中圖分類號：TQ241.1+3;O643.12

文獻標識碼：A

文章編號：1008-1143(2016)05-0075-06

通訊聯(lián)系人：趙巖。