32978579 cracking xuc tac

8/6/2019 32978579 Cracking Xuc Tac

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GS. TS. Nguyn Hu Ph

Cracking xc tc

Ho hc

Cht xc tc Qu trnh cng ngh

Nh xut bn Khoa hc v K thutH Ni


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Tc gi: GS. TS. Nguyn Hu Ph

Chu trch nhim xut bn: PGS. TS. T ng HiBin tp v sa bi: ThS.Nguyn Huy Tin

Ngc Linh Trnh by ba: hng Lan

Nh xut bn khoa hc v k thut

70 Trn Hng o H Ni

54 541

KHKT 05546 17 05

In 700 cun, kh 19 27 cm, ti

Giy php xut bn s: 546 17 20/5/2005

In xong v np lu chiu thng 6 nm 2005.


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Li ni u

Vit Nam ang bt u xy dng cc nh my lc du v lc ho du.

Trong mt nh my lc du, qu trnh cracking xc tc chim mt v tr

kh quan trng. Do , cc kin thc c bn v ho hc qu trnh cracking xc

tc, v bn cht v cc phng php tng hp cht xc tc v v cng ngh qu

trnh cracking l ht sc quan trng i vi nhng ai , ang v s lm vic

(hc tp, nghin cu v vn hnh sn xut) trong lnh vc lc ho du.

Ti vit cun sch ny vi s chn thnh mun ng gp phn kin thc

t i ca mnh cho s pht trin ngnh Du kh cn non tr ca Vit Nam.

Hy vng rng, cun sch s em li nhng iu c ch cho cc sinh vin,

bn b v ng nghip ca ti ang hc tp, nghin cu v cng tc trong cc

trng i hc, vin nghin cu v cc c s sn xut.

Ti xin thnh thc cm n cc li nhn xt, gp ca c gi v cun

sch ny.

GS. TS. Nguyn Hu Ph

Phng Ho l B mt

Vin Ho hc, Vin Khoa hc v Cng ngh Vit Nam

18 Hong Quc Vit, Cu Giy, H Ni

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4


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Bng cc ch vit tt v k hiu

Cc ch vit tt v k hiu c ch thch ngay ln u tin xut hin trong cun

sch. Tuy nhin, cho thun tin, nht l i vi cc c gi ch c tng phn, chng tinhc li tt c cc ch vit tt v k hiu trong bng ny d dng theo di.

eM~

Chuyn dch nhm metyl (alkyl)

H~

Chuyn dch hydro (hydrua)

%kl Phn trm khi lng

%LV Phn trm th tch lng

%tt Phn trm th tch

G* Bin thin th ng p (nng lng t do) ca giai on chuyn dch ( , )eM~

H~

Gpu Bin thin th ng p ca phn ng

Hpu Bin thin entanpy ca phn ng

Hht Bin thin entanpy hnh thnh (nhit sinh)

Spu Bit thin entropy ca phn ng

ABD Mt khi trung bnh (average bulk density)

AFS Fluosilicat amoni

AFSY Zeolit Y c x l vi (NH4)2SiF6

Al (E) Nhm ngoi mng tinh th

Al (T) Nhm trong mng li tinh th

AP im anilin, aniline point

ASTM U ban Th nghim v Vt liu ca M (American Society for Testing and

Materials)ATB Sn phm giu aromat y thp

B Tm Brnsted

BPSD Thng du/ngy hot ng (barrels per calendar day)

C3

Hydrocacbon c mch cacbon nh hn 3

C3= Hydrocacbon olefin c mch 3 cacbon (propylen)

C5+ Hydrocacbon c mch cacbon ln hn 5

CA Phn trm trng lng cacbon ca aromat; phn trm trng lng aromat

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CCR Cn cacbon Condrason

CD Mt ln cht (compacted density)

CI Ch s cetan, cetane index

CN Phn trm trng lng cacbon ca naphten, phn trm trng lng naphten

Cn

Phn hydrocacbon c mch cacbon nh hn n

Cn+

Phn hydrocacbon c mch cacbon ln hn nCO Kh oxit cacbon

CO Du giu hydrocacbon vng thm (cycle oil)

CO Phn trm trng lng cacbon ca olefin, phn trm trng lng olefin

COC M hnh suy gim hot tnh xc tc do s hnh thnh cc (coke on catalyst)

CP Phn trm trng lng cacbon ca parafin, phn trm trng lng parafin

cSt n v o nht, xenti stoke (St)

D T trng

DCC Cracking xc tc su (deep catalytic cracking)

DHA Hydrodealkyl ho

DMO Du x l kim loi (demetallized oil)

DO Du gn (decanted oil)

EBP im si cui (end boiling point)

EDTA Etylen diamin tetra axetic (ethylenediaminetetraacetic acid)

ESP My lc kh tnh in (electrostatic precipitaion)

EXT Chit tch aromat

FAU Faujazit (Faujasite): cu trc tinh th ca zeolit Y

FCC Cracking xc tc pha lu th (fluid catalytic cracking)

FGSV Van iu chnh kh bung hon nguyn

GCRON RON xc nh bng phng php sc k kh

HCO Du giu hydrocacbon vng thm nng (heavy cycle oil)HSY Zeolit Y c hm lng silic cao (high silic Y zeolite)

IBP im si u (initial boiling point)

IUPAC Union International of the pure and applied chemistry

Kpu Hng s cn bng ca phn ng

L Tm Lewis

LCO Du giu hydrocacbon vng thm nh (light cycle oil)

LHSV Tc khng gian theo th tch cht lng theo n v thi gian gi (liquid

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hourly space velocity)

LPG Kh du m ho lng (liquified petroleum gas)

MAS NMR Phng php cng hng t ht nhn vt liu rn (magic angle spinning nuclear

magnetic resonance)

MAT Php o hot tnh xc tc trong thit b MAT (th hot tnh dng vi lng,

micro activity test)MON Tr s octan ng c

MONC Tr s MON c xc nh vi xng khng pha ch

MTBE Metyl tectiary butyl ete, methyl tertiary buthyl ether

MTC H hn hp khng ch nht

N&TN Nhit v thu nhit ( bn nhit v thu nhit)

NSTB Nhit si trung bnh

NOx Cc oxit nit (NO2, NO...)oAPI i lng o mt ca du moR Nhit theo thang Rankine

P Poise, n v o nht

PONA Parafin, olefin, naphten v aromat

PP Polypropylen

R Nhm alkyl

RCC Cracking xc tc nguyn liu du cn (residue catalytic cracking)

RE Cation t him

RFCC Cracking xc tc pha lu th du cn

RON Tr s octan nghin cu

RONC Tr s RON c xc nh vi xng pha ch

ROT Nhit u ra ca react (raiz)

RPT Ry phn t (vt liu ry phn t)SA Aluminosilicat

SAP Hydro ho chn lc

SBU n v cu trc th cp (secondary unit building)

SC Hi nc - cracking (steam cracking)

SCR Kh xc tc chn lc (selective catalytic reduction)

SFV Viscomet Furol Saybolt

SIMDIS Phng php chng ct m phng nhit thpSIMS Phng php ph khi lng ion th cp (secondary ion mass spectroscopy)

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SUV Viscomet vn nng Saybolt

TBP im si thc (true boiling point)

TO4 T din gm 1 tm T v 4 oxy lin kt

TOS M hnh suy gim hot tnh xc tc theo thi gian phn ng trong dng

USY Zeolit Y dng siu bn (ultra stabilized Y zeolite)

VGO Vacuum gas oil, gas oil chn khngV/V T s th tch/th tch

VPI-5 Tn ca mt vt liu h aluminophosphat

VRDS Du cn chn khng c kh lu hunh

VTB Sn phm y thp chng ct chn khng

WHSV Tc khng gian trng lng tnh vi n v thi gian gi (weigh hourly

space velocity)

xt/d T s cht xc tc/du

Y Zeolit kiu Y dng faujazit (Faujasite)

ZSM-11 Zeolit ZSM-11 c m cu trc quc t l MEL

ZSM-5 Zeolit ZSM-5 c m cu trc quc t l MFI

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Chng 1

M u

Hu ht cc phn ng ho hc c p dng quy m cng nghip u l cc phn

ng xc tc. Rt nhiu qu trnh cng ngh c ci tin, hon thin l nh nhng pht

minh v cht xc tc mi. Mt trong nhng qu trnh c quy m cng nghip ln nht l qu

trnh cracking xc tc.Cracking l s chuyn ho cc phn t ln ca du m thnh cc

phn t hydrocacbon nh hn thuc phn on gasolin (xng).

1.1. Gii thiu tng qut v cng ngh cracking

Cc qu trnh cracking, thot tin,

c thc hin khng c mt cht xc tc, nh

ngv sau, trong 4-5 thp k gn y, nhiu cht xc tc cracking lin tc xut hin v ci tin.

Hu ht cht xc tc cracking l xc tc axit. Thnh tu quan trng nht trong cng ngh

cracking xc tc trong hn 4 thp k qua l s pht minh v s pht trin lin tc ca xc

tc zeolit.

Cc zeolit (dng axit, H-zeolit) xc tc cho phn ng cracking du m nhanh hn,

hiu qu hn rt nhiu so vi cht xc tc dng aluminosilicat v nh hnh trc kia, n

mc ngi ta phi thay i c thit k ca cc thit b cracking c, dng lp xc tc n nh,

hoc dng lp xc tc ng (tng si n nh) thnh cc react ng nh thng ng (reactor

- riser). Trong react riser, cc ht xc tc c kch thc nh c chuyn qua react rt

nhanh nh dng hydrocacbon ho hi trong trng thi lu th (fluid), cht xc tc v

hydrocacbon c tip xc nhau trong khong thi gian rt ngn, khong 5 - 10 giy*.

C th ni, xt v mt ho hc ca nhiu qu trnh lc - ho du (cracking, reforming,

izome ho...) th qu trnh cracking c nghin cu nhiu nht v t c nhiu thnh

tu nht. l ho hc v axit mnh, hydrocacbon, cacbocation v v zeolit.

Zeolit l vt liu aluminosilicat tinh th, bn trong n cha nhng h mao qun ngnht c kch thc c phn t. Cu trc tinh th v tnh cht b mt ca zeolit c xc nh

kh chnh xc v r rng, trong khi cc tham s cu trc ca cc cht xc tc rn khc

dng v nh hnh li hay thay i v kh xc nh.

Ho hc ca qu trnh cracking xc tc s c trnh by t m trong mt chng

ring. Tuy nhin, hnh dung ho hc cracking xc tc trong ng cnh ca thc t cng

nghip, chng ta c th theo di mt s khi qut ca mt qu trnh cng nghip nh sau:

* V qu trnh cracking xy ra trong trng thi lu th (fluid) ca cht xc tcv hydrocacbon, nn qutrnh xc tc ny thng c gi l Fluid Catalytic Cracking vit tt l FCC (cracking xc tc pha lu th)

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Thit b hon nguyn

(200 - 340oC)

Kh chy n bnh t CO675oC; 2,1 atm

Naphta(25 - 200oC)

H2 - C4Kht

Gas oil

675oC2,4 atm

Khng kh

Hi nc

Du cn(>425oC)

Du nhiuhydrocacbonaromat nh

(Light Cycle Oil, LCO)Du nhiuhydrocacbonaromat nng

(340 - 429oC)

(Heavy Cycle Oil, HCO)

Thit b phn ng xc tcThit b phn ring

(chng ct phn on)

Khng kh

Hnh 1.1. S nguyn tc ca qu trnh cracking xc tc cng nghip.

S gm mt react cha lp xc tc ng, cng vi mt b phn tch ht xc tc

v sn phm (thit b phn ng xc tc); mt thit b hon nguyn xc tc, trong cc -

sn phm cacbon phn t lng cao, c t chy phc hi hot tnh xc tc v mt

thit b chng ct tch sn phm cracking thnh cc phn on c nhit si khc nhau

v mt phn du nng c hon lu tr li react cracking. Cc tham s cng ngh ca qu

trnh cracking xc tc th

ng xy ra trong react ng ng nh

sau (bng 1.1):Bng 1.1. Cc tham s qu trnh cracking xc tc

React ng ng

Nhit oC

- y 550

- nh 510

p sut, atm 3

T s cht xc tc/du 6

Thi gian lu, s 5 7Thit b hon nguyn xc tc

Nhit trong xyclon oC 650 760

T s CO/CO2 (mol/mol) 0,7 1,3 : 1

p sut y ca tng xc tc ng, atm 3,5

Tc dng pha kh, m/s 60

Thi gian lu ca cht xc tc rn, s 30

Hm lng cc ca cht xc tc (%kl)

- Li vo 0,8- Li ra < 0,1

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Bng 1.2 trnh by hiu sut sn phm ca mt qu trnh cracking xc tc vi cc chtxc tc khc nhau: zeolit v aluminosilicat.

Bng 1.2. Cc tham s cng ngh v hiu sut sn phm cracking xc tc

Tham s cng ngh Durabead5(a) Durabead1(b)

Nhit hi nguyn liu vo, oC 476 476Nhit xc tc vo, oC 548 549

Nhit sn phm ra,o

C 474 471Tc khng gian th tch, h1 1,0 0,9

T s cht xc tc/du (%tt) 1,9 2,0

T s hon nguynmitcxcchttchth

nnguyhontcxctchth

0,84 0,82

Hm lng hi nc trong nguyn liu, %kl 3,6 3,5Cng sut thit b xc tc bbl/ngy* 12900 13400Tc tun hon cht xc tc, kg/h 136065 136065Tc t chy cc, kg/h 2267 1542

Khong nhit si ca dng nguyn liu hon lu,o

C 215-332 232-327 chuyn ho, % th tch 73,4 49,5

Hiu sut cracking, % 100hochuynddutchth

gasolintchth 77,6 77,3

Hiu sut%tt %kl %tt %kl

Sn phm y thp chng ct 13,7 15,2 21,3 22,3Du t 12,9 13,3 29,2 29,4Gasolin (khng c C4) 56,9 48,7 38,3 32,4

Cc butan 13,4 8,5 8,5 5,4Kh kh (C3 v kh nh khc) - 8,9 - 6,6Cc - 5,4 - 3,4Tng cng - 100 - 100n-butan 2,1 1,3 1,1 0,6

Isobutan 6,5 4,0 2,9 1,8Buten 4,8 3,2 4,5 3,0Tng C4 13,4 8,5 8,5 5,4T si-C4/C4 1,35 - 0,64 -

Propan 3,8 2,1 2,4 1,3Propylen 4,1 2,4 3,7 2,1Tng C3 7,9 4,5 6,1 3,4Etan - 1,3 - 1,0Etylen - 0,6 - 0,4Metan - 1,8 - 1,2Hydro - 0,1 - 0,1Sulfua hydro - 0,6 - 0,5Tng C2 v nh hn - 4,4 - 3,2

* bbl: barrel, n v o th tch du m thng dng trong giao dch thng mi, 1bbl = 165 lit.(a): xc tc zeolit REHY trong pha nn aluminosilicat v nh hnh; (b): xc tc aluminosilicat vnh hnh; %tt: phn trm tnh theo th tch; %kl: phn trm tnh theo khi lng

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Hin nay cc thit b cracking xc tc c nhiu hng ch to v ci tin (s trnh

by phn qu trnh cng ngh). Tuy nhin, hnh dung nguyn tc ca react dng ng

ng (reactor - riser) chng ta kho st mt s react c in nh trn hnh 1-2.

Gas oil c a vo phn y ca reactor - riser vi dng hi nc rt phn tn, c

ho trn vi cht xc tc dng ht mn n t phn y thit b hon nguyn xc tc. ng

knh ca react tng dn theo chiu cao gi cho tc lu chuyn cht xc tc hu nh

khng thay i, bi v p sut thu tnh trong ng ng gim dn n u ra.

Hi nc

Gas oil

React(reactor)

Thit b tchhydrocacbon strip

Hi nc

Thit b hon nguyn cht xctc, regenerat (regenerator)

Thi

Cht xc tc b sung

Khng kh

Hi nc

Hnh 1.2.Thit b craking xc tc FCC.

Sau react, cht xc tc c tch ra khi sn phm nh thit b tch xyclon. Hydro-

cacbon c tch bng hi nc khi cht xc tc b cc ho trong vng trn ca thit b

tch xyclon. Trong thit b hon nguyn xc tc, cc

c t chy, gii phng cht xc tccho chu trnh chuyn ho tip theo.

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1.2. Gii thiu s chung ca cc phn ng cracking

Cc phn ng cracking hydrocacbon c thc hin bi cc cht xc tc axit v xy

ra theo c ch cc hp cht trung gian cacbocation. V ho hc, cc phn ng cacbocation

chng ta s xt sau, cn v mt cng ngh qu trnh, c th hnh dung trong mt thit b

phn ng xc tc, phn ng cracking gas oil c miu t nh s n gin sau y:

k1 k2 XGO

k3

Gas oil O, Gasolin G v sn phm ph X.

Qua s , nhn thy rng, tng hiu sut gasolin cn hn ch cc phn ng

cracking su, chuyn ho G thnh X. Vic s dng react - riser, chnh l gim thiu s

cracking su ca gasolin. Mt khc, cng c th nhn thy rng, cc sn phm ph X nhnc khng ch t gasolin m cn trc tip t gas oil.

1.3. Gii thiu chung v cht xc tc cracking cng nghip

Cc xc tc cng nghip thng c iu ch t 3 - 25% (kl) ca zeolit tinh th

(ng knh ht tinh th zeolit c 1 m) trong mt cht nn (matrix) l aluminosilicat v

nh hnh v/hoc khong st. bo m ch lm vic trng thi lu th (fluid) trong

dng hi hydrocacbon, kch thc ht xc tc phi nm trong khong 20 n 60 m (ng

knh ht). Zeolit phi c phn tn vo trong pha nn aluminosilicat v nh hnh trnh

cc hiu ng nhit cc b, n nh hot tnh xc tc ca zeolit, nh cu trc xp v

axit khc nhau gia zeolit v pha v nh hnh.

Nh s khc nhau m xc tc zeolit c hot tnh cracking gas oil v chn lc

gasolin cao hn nhiu so vi xc tc aluminosilicat v nh hnh (xem bng 1.2). Ngy nay ,

trong cht xc tc FCC, ngoi cc hp phn c bn l zeolit Y(Faujasite) dng USY v pha

nn aluminosilicat v nh hnh, ngi ta cn thm vo cc zeolit ph gia (vi hm lng t

1 n 10% khi lng) H-ZSM-5, HZSM-11, H-Bta... gia tng ch s octan ca gasolinhoc gia tng hm lng olefin nh trong thnh phn kh cracking, v thm mt s ph gia

th ng ho kim loi (xem mc cht xc tc cracking).

1.4. Cc c trng v nguyn liu cho cracking xc tc (FCC)

Cc nh lc du phi ch bin nhiu loi du th khc nhau. Cht lng ca du th

thng b bin i theo tng vng khc nhau, do , ngi ta phi xc nh r cc tnh cht

c trng ca tng loi nguyn liu m bo s vn hnh n nh ca cc cng on ch

bin (cracking, reforming...) trong mt nh my lc du.

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c trng nguyn liu FCC l mt nhim v rt quan trng cho s hot ng ca cng

on FCC. Nh , ngi ta c th chn cht xc tc, x l cc s c, ti u ho qu trnh

cracking xc tc.

Hai yu t quan trng nht nh hng n cht lng ca nguyn liu FCC l:

- cc hydrocacbon;- cc tp cht;

cha trong nguyn liu FCC.

1.4.1. Phn loi cc hydrocacbon

Cc hydrocacbon trong nguyn liu FCC thng c phn thnh parafin, olefin,

naphten v cc aromat (hydrocacbon cha vng thm) vit tt l PONA.

Parafin:parafin l cc hydrocacbon mch thng hoc mch nhnh c cng thc ho

hc l CnH2n+2.

Ni chung, nguyn liu FCC cha ch yu cc hydrocacbon parafin. Hm lng cacacbon trong parafin chim khong 50 n 65% khi lng ca nguyn liu. Parafin d b

cracking v to ra lng sn phm lng nhiu nht. Parafin to ra gasolin nhiu nht, kh

nhin liu t nht, nhng cng c gi tr octan thp nht.

Olefin: olefin l cc hydrocacbon cha bo ho, c cng thc l CnH2n. So vi

parafin, olefin l hp cht km bn hn v c th phn ng vi nhau hoc vi cc cht khc

nh oxy, v vi dung dch brom. Olefin khng tn ti trong t nhin, n c mt trong

nguyn liu FCC l do cc qu trnh x l trc , v d nh, do cracking nhit, hoc do

cracking xc tc...

Olefin khng phi l hp phn mong mun trong nguyn liu FCC v olefin thng b

polyme ho to ra cc sn phm nh cc v nha. Hm lng olefin ti a trong nguyn liu

FCC l 5%kl.

Naphten: Naphten c cng thc CnH2n cng nh cng thc ho hc ca olefin.

Olefin c cu trc mch thng (chnh xc hn l mch h), cn naphten l parafin c cu

trc mch vng. Naphten l hp cht bo ho. V d, cc naphten nh xyclopentan,

xylcohexan, metyl-xyclohexan c cu trc vng nh sau:

CH3

Metyl xyclo hexanXyclo hexanXyclo pentan C6H12C5H10 C7H14

Naphten cng l hp phn mong mun trong nguyn liu FCC v chng to ra gasolin

c gi tr octan cao. Gasolin c to ra t naphten c nhiu aromat hn, v l gasolin nng

hn so vi gasolin c sn xut t cracking parafin.

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Aromat: aromat (CnH2n-6) tng t nh naphten, nhng cha vng cacbon cha bo

ho v kh n nh. Aromat l cc hp cht cha t nht mt vng benzen. Vng benzen rt

bn v khng b cracking. Cc aromat l hp phn mong mun cho nguyn liu FCC hin

nay, v s to ra gasolin c gi tr octan cao.

Cracking cc aromat thng xy ra s phn ct cc mch nhnh v to ra cc hydro-

cacbon phn t nh. Ngoi ra, mt s t hp cht aromat a vng c th to ra nhng mng

cacbon nm li trn b mt cht xc tc (cc), hoc cc sn phm a phn t khc (nha).

Trong tng lai, khi hm lng benzen trong gasolin b hn ch ngt ngho th c l

aromat khng phi l hp phn a chung cho nguyn liu FCC.

1.4.2. Cc tnh cht vt l ca nguyn liu FCC

c trng nguyn liu FCC bao gm vic xc nh cc tnh cht ho hc v vt l ca

nguyn liu. Song, cc k thut phn tch qu nhy v chnh xc nh ph khi l khng

thch hp cho cc hot ng cng ngh thng ngy, do ngi ta thng phi s dng cctnh cht vt l nh gi nhanh cht lng nguyn liu. Cc phng th nghim ca cc

nh my lc du thng c trang b cc dng c, thit b cn thit thc hin vic th

nghim cc tnh cht vt l thng thng ca nguyn liu nh:

- T trng, oAPI;- Chng ct nguyn liu;- im anilin;- Ch s khc x (RI);- S brom (BN) v ch s brom (BI);- nht;- Cacbon Conradson, Ramsbottom, Micro-cacbon v cacbon khng tan trong heptan.T trng, oAPI

T trng (density, D) l mt i lng o mt cht lng: D ca cht lng l t s ca

trng lng mt th tch cht lng vi trng lng ca th tch nc tng ng, ti mt nhit

cho, v d nhit 15,5oC. oAPI l i lng o mt ca hydrocacbon lng.

So vi t trng, oAPI xc nh cc thay i mt ca cht lng mc nh hn

(nhy hn). V d, t 24oAPI n 26oAPI tng ng vi s thay i ca t trng l 0,011.

Vi s thay i 2oAPI c th nh hng ng k n hiu sut sn phm cracking.

Gia D15,5 voAPI c mi lin h sau y:

API5,131

5,141D

oC5,15 o+

= (1.1)

5,131D5,141

APIC5,15

o

o= (1.2)

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V mt tnh theo oAPI t l nghch vi t trng D, nn oAPI cng cao th cht lng

cng nh. Trong ch bin du m, oAPI thng c o hng ngy i vi mi nguyn liu

v sn phm. Phng php tiu chun ASTM D-287 l php th c thc hin hng ngy

bi mt k thut vin ca phng th nghim hoc bi mt cng nhn vn hnh phn xng.

Ngi ta dng mt t trng k (hydrometer) khc oAPI cm vo mt ng ong ng hn hp

cn o, c gi tr oAPI v nhit ca cht lng. Sau cn c vo cc bng chun quy

i v oAPI 15,5oC.

i vi nguyn liu giu parafin (sp nn), ngi ta phi gia nhit n ~49oC trc

khi nhng t trng k. Gia nhit lm cho sp nng chy v c chnh xc. Theo dioAPI hng ngy gip cho cng nhn vn hnh hiu c tnh hnh hot ng ca phn xng

cracking. Cng mt khong nhit chng ct nh nhau, phn on c 26oAPI d b

cracking hn so vi phn on 24oAPI, v nguyn liu vi 26oAPI nhiu parafin mch thng

di hn. Khi tip xc vi cht xc tc nhit ~700oC, chng d b phn ct thnh cc sn

phm c gi tr.Parafin mch thng di rt c li cho cracking v n d b cracking, to ra nhiu

gasolin v LPG (kh du m, liquified petroleum gas), to ra ti thiu cc sn phm nha v

kh kh.

Cc gi tr oAPI cung cp nhng thng tin quan trng v phm cht nguyn liu. Tuy

nhin s dch chuyn cc gi tr oAPI cng cn lin quan n nhiu tnh cht khc ca

nguyn liu nh dng cacbon, im anilin, do , ngi ta phi o cc tham s khc c

trng y tnh cht ca nguyn liu.

Chng ct nguyn liu

Cc s liu v khong nhit chng ct cung cp cc thng tin quan trng v phm

cht v thnh phn ca nguyn liu. Nguyn liu cho cng on cracking xc tc l mt hn

hp c pha trn t nhiu gas oil ca cc cng on chng ct du th, chng ct chn

khng, x l nha ng bng dung mi, v t l cc. Mt s nh lc du cn mua thm

nguyn liu FCC bn ngoi nh my m bo thnh phn nguyn liu t cht lng

thch hp. Mt s nh lc du ch bin du cn t chng ct kh quyn v chng ct chnkhng. Trong nhng nm gn y, ngi ta c khuynh hng ch bin gas oil nng v du

cn. Du cn l phn du ng vi im si trn 565oC. Mi mt nguyn liu FCC c nhng

c tnh ring ca mnh.

S lng v phng php th nghim i vi tng m nguyn liu tu thuc vo cc

nh lc du. Mt s nh lc du tin hnh phn tch hng ngy, mt s khc t mt n 3

ln mt tun v mt s khc na mt ln mt tun. S ln phn tch ph thuc vo cc kt

qu ch

ng ct, s bin i ngun du th, v tnh hnh nhn s ca phng th nghim nhmy.

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Chng ct phn on du th trong phng th nghim c tin hnh bng cch o

nhit ca hi chng ct im si ban u (IBP, initial boiling point) ng vi cc phn

on, % th tch: 5, 10, 20, 30, 40, 50, 60, 70, 80, 90 v 95; v im si cui cng (EBP,

end boiling point). Ba phng php tiu chun ASTM thng c s dng o im si

D-86, D-1160 v D-2887.

D-86 l phng php ph bin nht trong cc nh my lc du. Chng ct c thc

hin p sut kh quyn, khi mu c EBP thp hn 400oC. Trn nhit , mu bt u b

cracking nhit. S cracking nhit c biu hin bi s st gim nhit ca hi chng ct,

c khi nu xut hin v p sut ca h tng. Hu ht cc nguyn liu cho FCC hin nay u

nng, khng thch hp vi phng php D-86, phng php ny c s dng cho cc sn

phm nh.

Phng php D-1160 c thc hin iu kin chn khng (1mmHg). Cc kt qu

o c chuyn i v iu kin p sut thng bng bng cho sn. Mt s thit b mi sn

xut c phn mm chuyn i t ng. D-1160 c gii hn cc i EBP khong 538oC psut kh quyn. Cao hn nhit , mu bt u b cracking nhit.

D-2887 l phng php chng ct m phng nhit thp (SIMDIS. Simulated

distillation), ngi ta o % th tch ca phn on c nhit si thc (TBP, true boiling

point) bng phng php sc k kh (GC, gas chromatography). Tng t nh D-1160,

phng php D-2887 b gii hn nhit si cc i ~538oC. Tuy nhin, vi cc h GC

mi, ngi ta c th o c cc nhit si cao n 750oC. Nh phng php ny, ngi

ta c th o

c nhit si ca cc nguyn liu nng cha du cn v c tr

ng cc duth. So vi phng php D-1160, phng php SIMDIS c thao tc n gin hn, lp li

tt hn v o im IBP v nhit si ca im 5% v 10% chnh xc hn.

Cc s liu chng ct cung cp cc thng tin v phn on nguyn liu nh c nhit

si thp hn 343oC. Cc nguyn liu nh ny thng cho hiu sut LCO cao v

chuyn ho thp. Cc ngun nguyn liu nh thng l cc gas oil ca cc cng on chng

ct kh quyn, chng ct chn khng nh v sn phm nh ca l cc. Nguyn liu nh c

chuyn ho thp v cc l do sau y:

- Cc sn phm hydrocacbon nh thng kh b cracking hn.- Gas oil nh t l cc cha rt nhiu aromat.- Aromat nh c mch bn th phn mch bn d b cracking.Ni chung, nguyn liu nh c s dng rt hn ch. Tuy nhin gas oil nh trong

nguyn liu FCC lm gim lng cc trn cht xc tc, c ngha l lm gim nhit hon

nguyn xc tc.

Cc s liu chng ct cho bit thng tin v phn nguyn liu c nhit si trn

482oC. Phn nguyn liu ny c khuynh hng to cc b mt v cha nhiu tp cht nhcc hp cht cha kim loi v nit, to ra sn phm lng t hn, cn cc v kh th nhiu hn.

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im anilin

Anilin l mt amin thm (C6H5NH2). Khi c s dng lm dung mi, anilin ho tan

chn lc cc phn t aromat nhit thp, cn parafin v naphten nhit cao hn.

Anilin c s dng xc nh hm lng cc hydrocacbon thm ( aromat) ca cc sn

phm du m, trong c nguyn liu cho FCC. im anilin (AP) l nhit ti thiu

ho tan hon ton mt mu du vo anilin.

Phng php ASTM D-611 c thc hin bng cch gia nhit mt hn hp 50/50

mu v anilin cho n khi thnh mt pha ng th. Sau , hn hp c lm lnh; nhit

ti hn hp bt u xut hin mu c, l im anilin. Php th c tin hnh nh mt

ngun sng chiu qua mu.

AP tng vi s tng hm lng parafin ( parafin) v gim vi aromat. im

anilin cng tng theo trng lng phn t. Naphten v olefin c gi tr nm gia parafin v

aromat. im anilin cao hn 93oC c trng cho nguyn liu parafin, thp hn 65oC c

trng cho aromat.

im anilin c dng xc nh aromat ca gas oil v cc nguyn liu nh.

Gia AP v ch s khc x c mi quan h tng h. Do , ngi ta cn s dng ch s

khc x c trng nguyn liu FCC.

Ch s khc x

Tng t nh im anilin, ch s khc x (RI, refractive index) cho bit hm lng

aromat ca mu. RI cng cao, aromat cng nhiu v mu cng kh b cracking. Mt mu c

RI bng 1,5105 b cracking kh hn mu RI 1,4990. Ch s khc x c o bi phng

php thc nghim (ASTM D-1218) hoc c xc nh theo quan h chuyn i theo h

thc TOTAL t AP. Trong phng th nghim , ngi ta o RI bng khc x k. i vi

nhng mu du c mu ti v nht cao, c AP v RI u b hn ch v chnh xc v

kh nng s dng.

S brom v ch s brom

S brom (ASTM D-1159) v ch s brom (ASTM D-2710) l cc phng php nh

tnh xc nh s tm phn ng ca mu. Brom phn ng khng ch vi cc lin kt olefin m

cn vi cc phn t baz cha nit v vi mt s dn xut aromat cha sulfua. Tuy nhin,

olefin l cc tm phn ng ch yu nht, nn s Brom c dng nh gi hm lng

olefin ( olefin) trong nguyn liu.

S brom l s gam brom c dng tc dng vi 100 g mu. Cc s brom in

hnh l:

- Nh hn 5 i vi nguyn liu qua hydro - x l.- 10 i vi gas oil nng t cng on chng ct chn khng.- 50 i vi gas oil t l cc.

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Theo kinh nghim thc t, phn olefin ca mu bng mt na s brom.

Ch s brom l s mg ca brom c dng tc dng vi 100 g mu. Ch s ny c

dng trong cng ngh ho hc i vi cc ngun nguyn liu c hm lng olefin rt thp.

nht

nht biu th thnh phn ho hc ca mt mu du. Khi nht ca mu tng,

hm lng parafin tng, hm lng hydro tng v hm lng aromat gim. nht c o 2 nhit khc nhau, c bit 38oC v 99oC. i vi nhiu

nguyn liu FCC, mu du kh c v st 38oC nn mu c gia nhit khong 50oC. Cc

s liu v nht c tm thy trn biu "nhit - nht" (trong cc s tay lc - ho

du). nht khng bin i tuyn tnh vi nhit , nhng trn cc biu , ngi ta

hiu chnh c quan h tuyn tnh.

nht l mt thc o bn chy ca mt cht lng. n v o nht tuyt i

l poise (P). Song vic xc nh nht tuyt i thng khng n gin, do , ngi tathng s dng nht ng(kinematic (flowing) viscosity). nht ng c xc nh

bng cch o thi gian cht lng chy qua mt ng dng mao qun c ng knh v di

quy nh. n v o nht ng l stoke (st).

Gia nht tuyt i v nht ng c quan h:

dPSt = (1-3)

trong , P = 0,1 Ns/m2 = 1 dyn.s/cm2 = 1 g/cm.s;

St = 1 cm2/s;d = khi lng ring, g/cm3.

nht Saybolt (ASTM D-88) l i lng c s dng ph bin nht o nht

ca du m, ca nguyn liu FCC. Phng php Saybolt c hai dng:

- Viscomet vn nng Saybolt (SUV, Saybolt universal viscometer) p dng cho du nh.- Viscomet furol Saybolt (SFV) dng cho du nng.C hai cch u da trn nguyn tc o thi gian chy ca mt th tch mu c nh

qua mt ng mao qun mt nhit cho.S khc nhau gia hai dng c o ni trn l ng knh bn trong ca ng chy khc

nhau. SUV c ng knh trong ca ng l 0,176 cm, ca SFV l 0,315 cm. SUV c dng

cho cc mu c thi gian chy ln hn 600 sec. i vi cc gas oil thng thng nht, thi

gian chy l ngn nn c o bng SUV. Kch thc ca ng trong hai phng php o

c chn sao cho nht Furol ca du bng 1/10 nht Universal cng mt nhit .

Cacbon: Conradson, Ramsbottom, Micro-cacbon v cacbon khng tan trong heptan

Mt vn hin nay vn cha c nghin cu mt cch y trong cng nghcracking xc tc l cn cacbon ca nguyn liu. Ngi ta cha c cch xc nh chnh xc

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v nh hng ca n n vic to cc.

Cn cacbon c nh ngha l cn c to ra sau khi mu b phn hu nhit.

Cc thit b cracking xc tc thng c kh nng gii hn t chy cc, do s c

mt ca cn cacbon trong nguyn liu s to ra lng cc nhiu hn v lm gim r rt nng

sut ca thit b FCC. Gas oil thng thng dng lm nguyn liu FCC c t hn 0,5% khi

l

ng cn cacbon, nhng nguyn liu cha du cn, hm l

ng cn cacbon ln n 15%khi lng.

Hin nay c 4 phng php c s dng xc nh cn cacbon trong nguyn liu

FCC:

- Conradson- Ramsbottom- Micro-cacbon-

Khng tan trong heptanCc phng php ch ra c trng s hnh thnh cc dng cc ca nguyn liu. Mi

mt phng php c nhng u im v nhc im ca mnh, v khng mt phng php

no c th cung cp mt nh ngha chnh xc v cn cacbon hoc v cn asphalten.

Xc nh cn Conradson theo phng php ASTM D-189 bng cch o cn cacbon

sau khi bay hi mu bng phn hu nhit. Mu c cho vo a cn sn. Sau c gia

nhit bng n ga cho n khi hi ca mu ngng chy v khng thy khi xm bc ra. Sau

khi lm ngui, a ng mu c cn li tnh ton % cn cacbon. Php th mc d

kh n gin v ph bin, song khng cho kt qu tt v c im hnh thnh cc ca

nguyn liu FCC, bi v n ch cho bit cn to ra do hiu ng nhit ch khng phi do hiu

ng xc tc. Ngoi ra, phng php ny c lp li khng cao, tn cng v kh ph thuc

vo ngi thc hin.

Phng php Ramsbottom ASTM D-524 c dng xc nh cn cacbon. Mu

c cho vo mt bnh cu thu tinh cn trc, sau lp bnh cu vo bp gia nhit

trong 20 pht. Nhit ca bp gi 553oC. Sau 20 pht, bnh cu c lm ngui v cn.

So vi phng php Conradson, th phng php Ramsbottom chnh xc hn v lp li tthn.

C hai phng php u cho cc kt qu tng t nhau v c mi tng quan vi nhau

(hnh 1.3).

Phng php xc nh microcacbon hin nay cn gi l phng php micro s dng

mt dng c o cacbon Conradson. Phng php micro (ASTM D-4530) cho cc kt qu

cng tng t nh phng php Conradson (ASTM D-189).

Phng php khng tan trong heptan (ASTM D-3279) c s dng ph bin xcnh hm lng asphalten trong nguyn liu. Asphalten l cc cm vt liu c to ra t

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1001010,10,010,01

100

CncacbonRamsbottom,

%kl

10

1

0,1

Cn cacbon Conradson, %kl

Hnh 1.3. Mi quan h gia cn cacbon Ramsbottom v Conradson.

cc mng aromat a nhn, nhng cho n nay ngi ta vn cha hiu c mt cch r rng

v cu trc phn t ca chng. Cc asphalten khng tan trong cc parafin C5 - C7. Lng cc

asphalten b kt lng lun thay i t dung mi ny n dung mi khc. Do , iu quan

trng l: gi tr asphalten o c phi tng ng vi mt dung mi tng ng. Ngi ta

thng dng phng php khng tan trong heptan v pentan xc nh asphalten. Mc d

c hai cch ny khng cho mt nh ngha r rng v asphalten, nhng chng to ra mt gii

php thc t nh gi cc cht khi u to ra cc trong cc ngun nguyn liu cho FCC.

Xin lu rng, theo nh ngha truyn thng th lng cn khng tan trong heptan

c gi l asphalten v nha (resin). Hiu s ca lng cn khng tan trong pentan v

heptan l resin. Resin l tp hp cc phn t ln hn aromat v nh hn asphalten.

1.4.3. Cc tp cht

Trong nhng nm gn y, cc nh lc du phi ch bin cc ngun du th nng hn,

lm cho li nhun t lc du tr nn t hn. Nhng nh ngi ta ni: t cn hn khng, do

cc thit b cracking - mt trong nhng cng on ch yu trong nh my lc du, c

thit k a nng p ng mi ngun nguyn liu khc nhau. Cc ngun nguyn liu FCC

hin nay u l nguyn liu nng v cha nhiu tp cht lu hunh, nit v kim loi. Cc tp

cht nh hng xu n s hot ng ca thit b. Hiu r bn cht v tc hi ca cc tp

cht l iu rt quan trng x l cc s c trong vn hnh nh my.

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Hu ht cc tp cht trong nguyn liu FCC u nm trong thnh phn cc cht hu

c phn t ln, di dng cc hp cht cha:

- Nit;- Lu hunh;- Niken;- Vanadi;- Natri.Cc tp cht trn gy ra nhim c cht xc tc FCC, lm mt hot tnh i vi cc

phn ng to ra cc sn phm c gi tr. Lu hunh trong nguyn liu lm tng gi thnh

ch bin v cn phi c cc phng tin x l nguyn liu v sn phm p ng tiu

chun mi trng.

Nit

Nit trong nguyn liu FCC tn ti di dng cc hp cht hu c cha nit. Hmlng nit trong nguyn liu FCC thng c nh gi di hai dng: nit baz v nit

tng. Nit tng l tng nit baz v nit khng baz. Nit baz thng bng 1/4 n 1/2

nit tng. T baz ng cho cc phn t c kh nng phn ng vi axit. Cc hp cht nit

baz s trung ho cc tm axit ca cht xc tc v lm gim chuyn ho. Tuy nhin, hp

cht nit l cht ng c xc tc tm thi. Trong thit b hon nguyn, hp cht nit b t

chy, hot tnh cht xc tc c hi phc, khong 70 - 90% nit trong cc c chuyn

thnh N2, phn cn li c chuyn thnh oxyt nit (NOx). NOx thot ra mi trng cng

vi kh thi.

S ng c xc tc do cc hp cht nit khc trong nguyn liu FCC l kh ln. Tuy

nhin, s quan tm nhm gim thiu tc hi ny l cha thch ng. Ngi ta bit rng, 100

ppm nit dng baz trong nguyn liu lm gim khong 1% hiu sut gasolin. khc phc

tc hi ca hp cht nit baz, ngi ta phi nng cao nhit phn ng v s dng cc cht

xc tc c hm lng zeolit cao v pha nn hot tnh.

Trong mt s nh my lc du, nguyn liu c hydro-x l (hydrotreating) t

hiu qu kinh t cao hn. Ni chung, cc nguyn liu c hm lng nit cao u cha thmcc tp cht khc. Do , kh m nh gi ring r hiu ng tc hi ca nit i vi cht

xc tc. Hydro-x l nguyn liu lm gim khng ch hm lng nit m cn gim hu ht

cc nhim khc.

Ngoi tc hi ng c cht xc tc, hp cht nit cn c tc dng xu i vi mt s b

phn khc. Chng hn, trong react ng ng (riser), hp cht nit c th chuyn thnh amoniac

v xyanid (HCN). Xyanid tng nhanh qu trnh n mn ca cc thit b trong phn xng kh,

s n mn to ra hydro nguyn t v do dn n s n mn vy cc chi tit thit bkim loi. S hnh thnh xyanid c khuynh hng tng theo khc nghit ca qu trnh.

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Ngoi ra, mt s hp cht nit cng c mt trong du nh (LCO) nh pyrol (pyrolle)

v piridin. Nhng hp cht ny d b oxy ho v s lm i mu ca sn phm. Lng cc

hp cht nit trong LCO ph thuc vo chuyn ho. chuyn ho tng, hm lng nit

trong LCO gim.

Xut x v mt ca ngun du th nh hng kh ln n hm lng nit trong

nguyn liu FCC (bng 1.3)

Bng 1.3. oAPI, hm lng nit v du cn ca mt s ngun du m

Xut x oAPIPhn du y chng ct

chn khng %LV

Hm lng nit ca

gas oil chn khng, ppm*

Maga 21,6 33,5 2498Cao nguyn bc Alaska 28,4 20,4 1845Du trung bnh rp 28,7 23,4 829Forcados 29,5 7,6 1746

Cabinda 32,5 23,1 1504Du nh rp 32,7 17,2 1047Du trung bnh Bonny 35,1 5,3 1964Brent 38,4 11,4 1450Min trung nguyn ty Texas 38,7 10,6 957Forties 39,0 10,1 1407

* Hm lng nit thay i theo ngun du th v hm lng du cn.

Ni chung, du th nng cha nhiu nit hn du th nh. Ngoi ra, hm l ng nit

c khuynh hng gia tng theo phn cn trong du th.

Mt s hp cht nit trong du th:

A. Hp cht trung tnh

N

H

N

H

Indol Cacbazol

B. Hp cht nit baz

N

PhenanthridinNNN

Pyridin AcridinQuinolinC. Hp cht nit baz yu

N OH N OH

Hydroxyl quinolinHydroxyl pyridin

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Phng php ca UOP 313 thng c s dng ph bin xc nh hm lng nit

baz trong nguyn liu FCC. Mu nguyn liu, trc ht c trn vi axit axetic theo t l

50/50 sau chun vi axit percloric.

Phng php ASTM D-3228 (hoc detect hunh quang ho hc) c s dng ph

bin o nit tng, v bng cch chuyn tt c cc dng nit trong nguyn liu thnh

amoniac v sau chun amoniac vi axit sulfuric. Mt phng php khc ASTM

D-4629 c s dng o nit tng thng qua t chy oxy ho v detect hunh quang

ho hc.

Lu hunh

Nguyn liu FCC cha lu hunh di dng cc hp cht hu c - sulfua nh

mercaptan, sulfua v thiofen. Thng thng khi hm lng cn trong du th tng ln th

hm lng lu hunh cng tng. Hm lng lu hunh tng trong nguyn liu FCC c

xc nh bng phng php ph hunh quang tia X sng di (ASTM D-2622). Cc kt qu

c tnh i sang hm lng lu hunh nguyn t.

Mc d desulfua ho khng phi l mc ch ca qu trnh cracking xc tc, song

trong khi cracking, khong 50% lu hunh trong nguyn liu c chuyn ho thnh H2S.

Ngoi ra, cc hp cht lu hunh cn li trong cc sn phm FCC c loi b trong qu

trnh x l hydro-desulfua ho p sut thng.

Trong qu trnh cracking FCC, H2S c hnh thnh ch yu t phn ng phn hu

xc tc ca cc hp cht khng cha vng thm.

S phn b lu hunh trong cc sn phm FCC khc nhau ph thuc vo nhiu yu t:

ngun nguyn liu, kiu cht xc tc, chuyn ho v ch vn hnh.

i vi nguyn liu khng c hydro-x l, ng vi chuyn ho 78%, khong

50% khi lng lu hunh trong nguyn liu c chuyn ho thnh H2S. Phn cn li c

phn b nh sau:

-6% trong gasolin;-23% trong du nh (LCO);-15% trong du gn (DO);-6% trong cc.Cc hp cht lu hunh dng thiofen (dng vng) b cracking chm, v cc thiofen

khng b cracking chuyn vo trong gasolin, du nh v du gn.

Hydro-x l lm gim lng lu hunh trong mi sn phm. Vi nguyn liu c

hydro-x l, lng lu hunh trong nguyn liu c chuyn vo cc v sn phm nng

nhiu hn. Lng lu hunh l ra c chuyn thnh H2S trong cracking FCC th nh

hydro-x l m c loi b. Phn cn li kh loi b hn. Nguyn liu cng nng, cnggiu aromat th hm lng lu hunh trong cc cng cao.

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Mc du hydro-x l lm tng hm lng lu hunh trong cc v trong cn nha,

nhng lng lu hunh thc t t hn ng k so vi lng lu hunh trong nguyn liu

cha x l. Lu hunh khng nh hng n nhit chuyn ho v hiu sut sn phm.

Tc ng xu ca lu hunh l rt nh trong qu trnh ch bin du m. Mt s hp cht

hu c cha lu hunh khng b chuyn ho cng tng t nh nhiu hp cht aromat khc.

Chng s lm gim phn no chuyn ho v hiu sut sn phm cc i.

Cc kim loi

Cc kim loi nh niken, vanadi v natri c trong du th.

Cc kim loi l cht xc tc v tr xc tc cho nhiu phn ng khng mong mun,

nh dehydro ho v ngng t, lm cho hiu sut hydro v cc tng ln v hiu sut gasolin

gim. Kim loi lm gim hot tnh xc tc i vi cc sn phm mong mun.

Hu nh tt c cc kim loi trong nguyn liu FCC u b gi li trn cc cht xc tc

cracking. Nguyn liu giu parafin cha nhiu niken hn vanadi. Mi mt kim loi c mt

tc hi nht nh.

Niken (Ni)

Nh mc 1.3, cht xc tc cracking cng nghip c hai phn ch yu:

- Pha hot ng xc tc l zeolit Y.- Cht nn: ch yu l khong st, aluminosilicat v nh hnh.Khi tip xc vi cht xc tc, niken lng ng trn pha nn. Niken tr xc tc cho

phn ng dehydro ho, tch hydro t cc hp cht bn v to ra cc olefin khng bn. Ccolefin ny c th polyme ho to ra cc hydrocacbon nng. Cc phn ng ny dn n

hiu sut cao ca hydro v cc. Lng cc trn b mt cht xc tc tng lm cho nhit

hon nguyn cao hn. Nh vy, cn gim t s cht xc tc/du v do gim chuyn ho.

Hm lng niken cao l iu thng thy khi ch bin nguyn liu nng. To ra nhiu

hydro hoc nhit hon nguyn cao l iu khng mong mun i vi cc nh lc du.

Ngi ta nhn thy rng, da vo s liu xc nh lng kim loi trn cht xc tc

lm vic n nh (cht xc tc cn bng) c th tnh ton lng kim loi trong nguyn liu

mt cch chnh xc hn so vi s phn tch trc tip t nguyn liu. Nu niken l kim loi

thng xuyn c mt trong nguyn liu, th c th s dng cht th ng hn ch nh

hng ca niken. Thng thng, ngi ta s dng antimon th ng ho niken. Bin php

ny l kh d nu hm lng niken trong cht xc tc cn bng ln hn 1.000 ppm.

Vanadi (V)

Vanadi cng l kim loi cho cc phn ng dehydro ho, nhng yu hn so vi niken.

Phn ng gp to ra hydro ca vanadi ch bng 20% n 50% so vi niken. Khng ging

niken, vanadi khng b gi li trn b mt cht xc tc . Thc vy, n di chuyn vo bn

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trong cht xc tc (zeolit) v ph hoi cu trc tinh th ca zeolit. B mt ring ca cht xc

tc v hot tnh xc tc gim dn.

Vanadi c mt trong cc phn t hu c kim loi (c kim) c phn t lng cao, khi

nhng phn t nng ny b cracking, cn cc cha vanadi b gi li trn cht xc tc. Trong

qu trnh hon nguyn, cc b chy v vanadi c chuyn thnh oxyt vanadi V2O5.

V2O5 nng chy 690

o

C v ph v cu trc tinh th zeolit. V2O5 kh linh ng nnc th di chuyn t ht tinh th ny n ht tinh th khc.

Hin nay, c mt vi l thuyt v ho hc ng c ca vanadi. Trong , l thuyt ph

bin nht cho rng, V2O5 chuyn ho thnh axit vanadic (H3VO4) trong iu kin hon

nguyn xc tc. Axit vanadic tch nhm t din (Al) trong mng cu trc tinh th zeolit v

do lm sp khung tinh th.

Tc dng ng c ca vanadi ph thuc vo cc yu t sau y:

-Nng vanadi: ni chung, nng vanadi trn 2.000 ppm trong cht xc tc cn

bng cn p dng bin php th ng ho hn ch tc hi ca vanadi.

-Nhit thit b hon nguyn xc tc: nhit thit b hon nguyn cht xc tc(>677oC) vt qu im nng chy ca oxyt vanadi lm gia tng kh nng linh ng ca

V2O5 di chuyn n cc tinh th zeolit.

-Cch t chy cc: ch t chy cc hon ton to ra cht xc tc "sch" lmtng s hnh thnh V2O5 v nh c mt oxy d.

-Natri: natri v vanadi tc dng vi nhau to ra vanadat natri. Hp cht ny c imnng chy thp (649oC) v lm tng kh nng linh ng ca vanadi.-Hi nc: hi nc tc dng vi V2O5 to ra axit vanadic bay hi. Axit vanadic

thng qua s thu phn, gy ra s sp khung tinh th zeolit.

-Kiu cht xc tc: hm lng nhm, lng nguyn t t him, kiu v lng zeolitnh hng n bn ng c ca cht xc tc i vi vanadi.

-Ch b sung cht xc tc: vic b sung cht xc tc cng nhiu cng lm gimnng kim loi trong cht xc tc v lm rt ngn thi gian vanadi kp oxy ho hon ton.

Kim loi kim: natri

Natri l kim loi c hi cho cht xc tc FCC. Natri lm mt hot tnh xc tc v n

trung ho cc tm axit. Trong thit b hon nguyn, natri kt hp vi vanadi lm sp khung

tinh th ca zeolit. Natri c xut x t hai ngun chnh:

-natri trong cht xc tc ban u;-natri trong nguyn liu.Cht xc tc ban u cha natri v do qu trnh sn xut cht xc tc (xem chng

cht xc tc FCC).

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Natri trong nguyn liu c gi l natri b sung. nh hng bt li ca cc dng

natri u nh nhau bt k t u.

1.4.4. Cc gi tr v h thc kinh nghim

Phng th nghim ca nh my lc du khng th trang b mt cch y cc thit

b tin hnh phn tch hm lng ca PONA (parafin/olefin/naphten/aromat) v cc phn

tch ho hc khc ca nguyn liu mt cch tc thi v th

ng xuyn.Tuy nhin, cc tnh cht vt l nhoAPI, s liu chng ct c th o c d dng. Do

, ngi ta tm ra cc i lng kinh nghim p dng cho cng nghip nhm xc nh

nhanh cc tnh cht ho hc t cc tham s vt l.

c trng y nguyn liu FCC cho php chng ta nh gi mt cch nh tnh v

nh lng v tnh hnh hot ng ca nh my lc du. M hnh ho qu trnh c thc

hin da vo cc tnh cht ca nguyn liu, tin on hiu sut sn phm FCC v cht

lng ca cc sn phm. M hnh cng ngh c s dng hng ngy theo di hot ng

ca phn xng, nh gi cht xc tc, ti u ho qu trnh.

Hin nay, khng c mt i lng kinh nghim tiu chun no. Tuy nhin, mt s t

hng ra cc i lng kinh nghim ring ca mnh, iu cng rt c li cho vic nh

gi cht lng nguyn liu v hiu sut sn phm. Ni chung, cc i lng kinh nghim

u bao gm hu ht hoc mt s tnh cht vt l nh nhau. Hin nay, ngi ta thng hay

dng cc i lng kinh nghim: tha s K, h thc TOTAL, phng php n-d-M, phng

php API.

Tha s KTha s K l mt ch s rt hu ch xt on kh nng cracking ca mt nguyn

liu. K lin h vi hm lng hydro ca nguyn liu: K c tnh ton da vo cc s liu

chng ct v mt ca nguyn liu. Tha s K cng cao, cng th hin parafin cao v

kh nng cracking tt. Gi tr K hn 12,0 chng t nguyn liu l parafin, thp hn 11,0,

nguyn liu l aromat.

Tng t nh im anilin, tha s K thay i t nguyn liu giu parafin n nguyn

liu giu aromat. Tuy nhin, trong mt khong hp (K = 11,5 - 12,0) K khng c mi quan

h no gia aromat v naphten. Thc vy, trong khong , K bin i rt tuyn tnh vi

hm lng parafin. Tha s K khng cho bit cc thng tin v t s hm lng gia naphten

v parafin. T s c th thay i ng k i vi mt gi tr K no .

K c tnh ton t nhit si v mt ca nguyn liu. Hin nay, ng i ta s

dng hai phng php kinh nghim tnh K: KW v KUOP.

D)460MeABP(

K3/1

W+

= (1.4)

D)460CABP(K 3/1UOP

+= (1.5)

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trong :

MeABP: im si trung bnh, oF;

D: t trng 60oF (15,5oC);

CABP = (Fvi x TB i1/3)3;

vi:

Fvi: phn th tch ca cu t i;

TBi: nhit si tiu chun ca cu t i,oF.

Cc h thc TOTAL

Cc h thc TOTAL cho php tnh ton hm lng cc cacbon aromat, trng lng

phn t v ch s khc x da trn cc th nghim thng thng phng th nghim. Cc h

thc ny nh sau:

Hm l

ng cacbon aromat (%kl):CA = 814,136 + 635,192.RI(20) 129,266.D + 0,1013.MW 0,304.S 0,872.ln()

Hm lng hydro (%kl):

H2 = 52,825 14,26.RI(20) 21,329.D 0,0024.MW 0,052.S + 0,757.ln() (1.6)

Trng lng phn t (MW):

MW = 7,8312.103.D

0,0976.AP0,1238 (1-7)

Ch s khc x 20oC RI(20):

RI(20) = 1 + 0,8447.D1,2392.(VABPoC + 273,16) 0,0557.MW0,0044 (1-8)Ch s khc x 60oC RI(60):

RI(60) = 1 + 0,8156.D1,2392x(VABPoC + 273,16)0,0576.MW

0,0007 (1-9)

trong :

S : hm lng ca lu hunh % khi lng;

: nht 100oC;

AP,oC: im anilin oC;

D: t trng 20oC;

VABP: nhit si trung bnh theo th tch chng ct, oC;

5%)90(T%)50(T%)10(T

VABP++

=

vi T(a%): nhit si ca phn th tch chng ct ng vi a%, oC.

H thc n-d-M*

H thc n-d-M l mt phng php (ASTM D-3238) nh gi s phn b cacbon

* Cc k hiu n, d, M c gi theo tn gi truyn thng ca phng php.

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tng loi hydrocacbon trong cacbon tng s. H thc kinh nghim ny da trn cc ch s

khc x (n), t trng (d) v trng lng phn t trung bnh (MW) v hm lng lu hunh

(S). Ch s khc x v t trng u c o 20oC. Lu rng h thc n-d-M cho php tnh

ton phn trm cacbon trong cu trc ca vng aromat. V d, nu c toluen trong nguyn

liu th phng php n-d-M cho s liu cacbon aromat l 85%, ngha l tng ng vi 6

cacbon trong vng benzen, khng k 1 cacbon mch bn: 6/7 = 0,85 (85%).

Phng php n-d-M rt nhy vi ch s khc x v mt . Phng php da vo

cc php o ch s khc x 20oC. Song, hu ht cc nguyn liu FCC 20oC u trng

thi rn, do , khc x k khng th o ch s khc x v mt nhit . s dng

phng php n-d-M, ch s khc x 20oC cn c xc nh bng cc h thc khc

c cng b. V th phng php n-d-M thng c s dng kt hp vi cc h thc khc,

nh h thc TOTAL.

Phng php API

Phng php API l mt phng php tng qut nhm nh gi phn mol ca cc cht

parafin, naphten hoc aromat i vi mt nguyn liu khng cha olefin. S thit lp cc

phng php tnh da trn c s chia hydrocacbon thnh hai dng phn t: phn hydrocacbon

nng, trng lng phn t MW < 600 v phn hydrocacbon nh 70 < MW < 200. Cc cng

thc API c cng b trong cc s tay lc ho du.

Nh vy, nguyn liu cho FCC bao gm cc dng hydrocacbon khc nhau: parafin,

olefin, naphten v aromat (PONA); v mt s tp cht cha N, S, cc kim loi (Ni, V,

Na...). Vic xc nh chnh xc cc hp phn PONA v cc tp cht trong nguyn liu FCC

l iu ht sc quan trng khng nhng cho cc tnh ton thit k thit b, la chn cht xc

tc, xc nh hiu sut sn phm m cn cn thit nhm x l kp thi cc s c trong qu

trnh vn hnh sn xut.

Thnh phn ho hc ca nguyn liu thng lin quan cht ch vi cc tnh cht vt

l: nhit si, ch s khc x, mt nguyn liu... Do , nhiu khi khng th v khng

cn xc nh trc tip thnh phn ho hc, ngi ta c th s dng cc tham s vt l v

thng qua cc h thc kinh nghim: K, TOTAL, n-d-M c trng nguyn liu mt cchthun tin, nhanh chng.

1.5. Cc sn phm ca FCC

Cracking xc tc l mt qu trnh nhm chuyn ho gas oil thnh cc sn phm c gi

tr hn. Mc ch chnh ca cc phn xng FCC l chuyn ho cc i gas oil thnh

gasolin v LPG.

Cc sn phm ch yu ca cng on FCC l: kh kh, LPG, gasolin, LCO, HCO, du

gn v cc.

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Kh kh

Cc kh nh (C2 v nh hn) thot ra t thp hp th du c gi l kh kh. Cc cu

t chnh ca kh kh l hydro, metan, etan, etylen v H2S. Ngay sau khi c x l vi amin

tch H2S v cc kh axit khc, kh c trn vo h kh nhin liu (fuel gas) ca nh my

lc du. Tu thuc vo thnh phn ca hydro trong kh kh, mt s nh lc du thu hi

hydro bng cc k thut nh lm lnh, hp th di p sut thay i (pressure-swing

absorption) hoc mng phn ring. Hydro thu hi c s dng trong cc qu trnh hydro -

x l.

Kh kh l mt sn phm khng mong mun ca phn xng FCC, kh kh nhiu qu

gy ra qu ti my nn kht (WGC, wet gas compressor) v thng gy bt li. Kh kh

c to ra ch yu do cracking nhit, dehydro ho trn tp cht kim loi, v do cracking

xc tc khng chn lc (cht xc tc c lc axit qu cao ...).

LPG

Dng sn phm thot ra t phn trn thp tch butan hoc thp n nh l mt hn hp

cc hydrocacbon C3 v C4 thng c gi l kh ho lng t du m LPG (liquefied

petroleum gas). LPG cha nhiu olefin, propylen v butylen. Cc olefin nh ny ng vai

tr quan trng trong vic sn xut gasolin ci bin, RFG (reformulated gasoline). Tu thuc

vo c cu ca nh my lc du, LPG ca FCC c s dng trong cc dng sau y :

-Sn phm ho cht: LPG c tch thnh hydrocacbon C3 v C4. C3 c bn didng propylen c sch ho cht hoc sch cng nghip. Cc olefin C4 c s dng

polyme ho hoc ankyl ho.

-Pha trn trc tip: Cc hydrocacbon C4 c pha trn vo gasolin iu chnh psut hi v thay i gi tr octan. Tuy nhin, cc tiu chun cht lng mi v gasolin yu

cu phi gim p sut hi bo ho, do mt lng ln C4 c s dng vo cc mc ch

khc thch hp hn.

-Ankyl ho: Olefin tc dng vi isobutan to ra cc sn phm rt tt cho pha trngasolin. Cc ankylat (sn phm ankyl ho) l cc hp phn pha trn gasolin rt a chung v

chng khng cha aromat v sulfua, v c p sut hi thp, im si thp v c gi tr octancao.

-MTBE : isobutylen tc dng vi metanol to ra mt oxygenat lm ph gia rt ttcho gasolin thng c gi l MTBE (methyl tertiary buthyl ether). MTBE c thm vo

gasolin p ng nhu cu ti thiu cho gasolin ci bin.

Gasolin

Gasolin FCC l mt sn phm ch yu v c gi tr ca phn xng FCC. Hiu sut

gasolin c th gia tng nh:

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- Tng t s cht xc tc/ du bng cch gim nhit gia nhit nguyn liu.- Tng hot tnh xc tc bng cch tng lng cht xc tc mi hoc bng cht xc

tc c hot tnh cao.

- Tng nhit si cui ca gasolin bng cch gim tc hi lu nh thp chngct chnh.

- Tng nhit react (nu khng dn n s cracking su).Cht lng gasolin ch yu c nh gi bi gi tr octan.

-RON v MON: gi tr octan ca gasolin l thc o c tnh kch n ca gasolin vc xc nh theo phng php tiu chun ASTM. Gi tr octan cng cao kh nng chng

kch n cng tt, ngha l n ca ng c cng gim.

Ngi ta o gi tr octan thng qua cc php th n ca cc ng c n xylanh

trong phng th nghim vi s vng quay khc nhau nhit khng kh 49oC. Vi ng c

600 vng/pht, gi tr octan o c gi l RON (tr s octan nghin cu, research octanenumber), vi 900 vng/pht gi l MON (tr s octan ng c, motor octane number).

RON c o theo tiu chun ASTM-2669, cn MON theo ASTM-2700. Nh vy, s khc

nhau gia RON v MON thc cht l m hnh hot ng ca ng c trong hai iu kin

khc nghit khc nhau.

Tr s octan ca mt gasolin c th c xc nh bng cch pha trn n-heptan (c tr

s octan bng 0) v iso-octan (2,2,4 trimetyl pentan, c tr s octan bng 100) theo mt t l

no ri th n vi ng c. V d, mt gasolin c n ng c t

ng

ng vi n ca hn hp cha 85% iso-octan v 15% n-heptan, th mu gasolin c tr s octan

l 85%.

- nhy octan: tr mt vi ngoi l, ni chung gi tr RON cao hn gi tr MON.i vi cc hydrocacbon sch, s chnh lch l t 0 n 15 n v. Hiu s gia RON v

MON c gi l nhy octan. nhy octan cng thp cng tt.

-Octan xa l (road octane) l gi tr trung bnh ca MON v RON, (R+M)/2. Gi trtrung bnh ny c s dng ti cc trm bm xng cnh ng, nn gi l octan xa l. Cc

ng c t c t s nn cao i hi nhin liu c ch s octan xa l cao.

-GC-RON: l RON gasolin c xc nh bng phng php sc k kh, GC-RON(gas chromatographic method). Phng php ny c xy dng bi Anderson (P.C.

Anderson et al, J. Inst. Petr., 58, 83, 1972) v sau c ci tin trong nhiu phng th

nghim khc nhau. Theo phng php ny, gi tr octan c th c tnh ton t hm lng

hydrocacbon ca sn phm lng, theo cc s liu sc k kh.

-Octan - barrel: octan - barrel l tch s gia phn gasolin tnh theo barrel nhn ctrong qu trnh cracking v gi tr (M+R)/2 ca gasolin . Thng thng, cc nh lc du

quan tm n vic t ti a octan - barrel hn l i vi tr s octan ca gasolin.

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LCO (light cycle oil)

Nhiu khi, do quan tm n sn phm gasolin m ngi ta thng khng ch n

cc sn phm khc ca FCC, c th l i vi LCO.

LCO l phn sn phm thu c thp chng ct sn phm FCC. N c khong nhit

si cao hn gasolin, t 220 - 350oC. LCO l ch vit tt ca nhm t light cycle oil, sn

phm ny cha cc vng thm nh. LCO c hm l

ng aromat cao, c t 50 - 75%kl. Trong, 30 - 50% aromat tn ti di dng cc phn t cha 2 v 3 vng thm.

LCO c s dng rng ri nh l mt sn phm pha trn vo du t v nhin liu

diezen, nht l trong ma ng, gi bn LCO nhiu khi li cao hn gasolin. Do , cc nh

lc du thng iu chnh hot ng ca cng on FCC to ra hiu sut gasolin v LCO

hp l, p ng nhu cu th trng.

Cht lng ca LCO c nh gi qua gi tr xetan v hm lng lu hunh.

Cn c vo hai ch s ny, ngi ta mi bit c lng LCO c php pha vo dut v diezen m bo tiu chun cht lng t ra.

Xetan

Tr s xetan l gi tr bng s c trng cho tnh bt la ca nhin liu. Octan v xetan

l 2 i lng ngc nhau c trng cho tnh cht chy ca nhin liu. Cc cht lm tng tr

s octan s lm gim tr s xetan. V d, hydrocacbon parafin mch thng c tr s octan

thp nhng li c tr s xetan cao; aromat c tr s octan cao nhng c tr s xetan thp.

Tr s xetan c o bng ng c n xylanh trong phng th nghim. Trong thc t,ngi ta hay dng i lng gi l ch s xetan CI (cetane index). Ch s xetan CI c tnh

ton t cc phng trnh kinh nghim. Hu ht cc nh lc du u dng cch tnh ca

ASTM D-976-80 nh sau:

CI = 65,011(logT)2 + [0,192(oAPI).logT] + 0,16 (oAPI)2 0,0001809(T)2 420,34 (1-10)

trong , T l nhit ti im si 50%, oF.oAPI c xc nh 60oF (15,5oC).

V d: T = 550oF (288oC), oAPI = 19Tnh ra CI = 28,9

LCO thng c ch s xetan thp, CI = 20 - 30.

Hydro-x l LCO c th tng tr s xetan. Mc ci thin tr s xetan ca LCO tu

thuc vo iu kin khc nghit ca qu trnh hydro-x l.

HCO v du gn (DO)

HCO l sn phm nhn c t thp chng ct sn phm FCC, c khong nhit si

nm gia LCO v DO (du gn, decanted oil). HCO l tn vit tt ca heavy cylce oil, du

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(cha) nhiu vng thm nng. HCO thng c s dng nh dng hi lu ca thp chng

ct nhm truyn nhit cho nguyn liu mi v/hoc cho b phn gia nhit ca thp tch C4.

HCO c th c x l tip trong cng on hydrocracking, hoc c dng pha trn vi

du gn.

Du gn (DO) l sn phm nng nht ca cng on cracking xc tc. DO cn c

gi l du st, du y v du cn FCC.

DO l sn phm c gi tr kinh t thp nht, nn ngi ta c gng hn ch hiu sut

DO. Hiu sut DO ph thuc nhiu vo cht lng nguyn liu cracking v chuyn ho.

Nguyn liu giu naphten v aromat c xu hng to ra nhiu DO hn nguyn liu giu

parafin. Nu chuyn ho thp n trung bnh, khi tng t s cht xc tc/du hoc s

dng cht xc tc c pha nn hot ng th c th dn n gim hiu sut DO. Tng

chuyn ho, lm gim hiu sut sn phm y.

Cc (coke)

Trong qu trnh cracking, mt phn nguyn liu b chuyn ho thnh cc do cc phn

ng cracking th cp, polyme ho, ngng t... Cc c hnh thnh trn b mt cht xc tc

dn n s suy gim hot tnh cht xc tc. Do vy, hon nguyn hot tnh ca cht xc

tc, ngi ta phi t chy cc trong thit b hon nguyn (regenerator). Tuy nhin, cc l

mt sn phm ph cn thit cho s hot ng bnh thng cho cng on FCC, bi v nhit

thot ra t phn ng t chy cc li bo m ch nhit (nhit ) cho react cracking.

S hnh thnh, bn cht, nh hng ca cc n hot tnh xc tc c xem xt k

trong chng 2 v 3.

Sau khi cracking cc sn phm ca cng on FCC c phn ring theo khong

nhit si: kh kh, LPG, gasolin, LCO, HCO, DO v cc. Trong , gasolin l sn phm

quan trng v c gi tr nht. Hiu sut cc sn phm FCC ph thuc vo nhiu yu t:

ngun nguyn liu, bn cht v kiu loi cht xc tc, iu kin vn hnh v kinh t

cng ngh ca nh lc du.

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Chng 2

Ho hc qu trnh cracking xc tc

2.1. M u

Cc phn ng cracking thc hin cc phn ct lin kt C C, v v l cc phn ng

thu nhit nn xt v mt nhit ng hc, chng c tin hnh thun li nhit cao.

Qu trnh cracking hydrocacbon c th bao gm cc phn ng c th sau y:

1. Parafin b cracking cho olefin v cc parafin nh hn:

CnH2n+2 CmH2m + CpH2p+2 (2-1)Olefin

Vi n = m + p

2. Olefin b cracking cho olefin nh hn

CnH2n CmH2m + CpH2p (2-2)

Vi n = m + p

3. Cc alkyl hydrocacbon aromat (cc hydrocacbon vng thm vit tt ArCnH2n+1,

trong Ar: gc hydrocacbon aromat, CnH2n+1 gc alkyl)

ArCnH2n+1 ArH + CnH2n (2-3)

Aromat Olefin

4. Thay v phn ng dealkyl ho, phn ng cracking mch nhnh ca vng thm c

th xy ra:

ArCnH2n+1 ArCmH2m1 + CpH2p+2 (2-4)

Hydrocacbon vng thm Parafin

c mch nhnh olefin

Vi n = m + p

Cc hydrocacbon aromat khng c nhm th tng i kh b cracking iu kin

cng nghip v s bn vng ca cc vng thm.

5. Cracking naphten (xycloparafin) to ra cc olefin:

CnH2n CmH2m + CpH2p (2-5)

Olefin Olefin

Vi n = m + p

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Nu parafin mch c cha mt vng xyclo hexan th vng khng b ph v:

CnH2n C6H12 + CmH2m + CpH2p (2-6)

Xyclohexan Olefin Olefin

Vi n = m + p

Cc phn ng th cp xy ra tip sau cc phn ng cracking s cp ng mt vai tr

quan trng trong vic xc lp thnh phn sn phm cui cng trong qu trnh cracking.

Cc phn ng th cp thng l:

6. Chuyn dch hydro*

Naphten + Olefin Aromat + Parafin (2-7)

Tin cht cc aromat + Olefin Cc + Parafin (2-8)

7. Isome ho

Olefin Isoolefin (2-9)8. Chuyn dch nhm alkyl

C6H4(CH3)2 + C6H6 C6H5(CH3) + C6H5(CH3)

9. Phn ng ngng t (condensation reactions):

CH CH2R1CH CHR2+

R2R1

2H+ (2-10)

10. Phn ng ti phn b cc olefin phn t lng thp

2H2C=CHCH2CH3 H2C=CHCH3 + H2C=CHCH2CH2CH3 (2-11)

Trong iu kin cng nghip, cc phn ng cracking chnh khng b hn ch bi cn

bng nhit ng hc, ti cn bng cc hydrocacbon u c th phn hu hon ton thnh

cacbon graphit (C) v hydro (bng 2.1). Ngc li, cc phn ng ph nh isome ho, ti

phn b nhm alkyl, v dealkyl ho cc aromat c th xy ra ch mc nh trong iu

kin cracking cng nghip. Cc phn ng alkyl ho parafin - olefin, hydro ho aromat vpolyme ho olefin (ngoi tr polyme ho etylen) u hon ton khng xy ra.

Cc phn ng cracking thu nhit mnh, isome ho c hiu ng nhit nh, cn phn

ng chuyn dch hydro to nhit. Trong qu trnh cracking, cc phn ng thu nhit lun lun

chim u th, hiu ng nhit ca qu trnh ph thuc vo ngun nguyn liu, cht xc tc v

iu kin phn ng.

* Cn gi l s chuyn dch hydrua, v thc cht l s chuyn i H

trong v gia cc phn tphn ng.

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Bng 2.1. Hng s cn bng ho hc ca cc phn ng cracking hydrocacbon

Phn ng Hng s cn bng

CnHm nC + m/2H2 (tr khi n = 1)

(Graphit)Rt ln

CnHm CH4 + Cn-1Hm-4 (tr khi n = 1) Rt ln

Parafin ln Parafin + Olefin Rt lnOlefin ln 2 Olefin Rt ln

Parafin Aromatic + H2 Rt ln

Parafin + H2 2 Parafin nh hn Rt ln

Hydroaromatic + Olefin Aromatic + Parafin Rt ln

ng vng olefin thnh naphten (1-hexen xyclohexan) Trung bnh (15,2)

Isome ho olefin (n-buten isobuten) Trung bnh

Isome ho parafin (n-butan isobutan) Nh (0,51)

Dehydro ho ng vng parafin (n-hexan xyclohexan + H2) Nh (0,07)

Cracking nhitTrc khi pht hin ra cc cht xc tc cracking, ngi ta tin hnh cracking cc

hydrocacbon khng c mt cht xc tc - cracking nhit. Qu trnh n nay vn cn ng

vai tr quan trng trong vic chuyn ho naphta* thnh cc olefin nh v x l cc

nguyn liu nng. Khi khng c mt cht xc tc, nhit cao, cc hydrocacbon b

cracking nhit theo c ch gc t do.Giai on khi mo ca cracking nhit mt parafin l s phn ct ng ly (homolysis)

ca lin kt cacbon - cacbon:

(2-13)+R1 C C R2H

H

H

H

R1 C

H

H

C R2

H

H

Gc va c hnh thnh c th b phn ct cho etylen v mt gc bc mt c s

nguyn t cacbon t hn 2 (so vi gc ban u). Quy tc kinh nghim phn ct cho rng,s phn ct lin kt CC v tr i vi cacbon mang electron khng cp i:

RCH2CH2CH2 RCH2 + CH2=CH2 (2-14)

Gc t do bc mt va mi hnh thnh c th tip tc b phn ct theo quy tc

cho etylen v cc gc mi nh hn cho n khi to ra mt gc metyl. Gc metyl kt hp vi

mt gc H ca mt phn t hydrocacbon khc to thnh mt metan v mt gc th cp:

* Naphta: phn on du nh ca xng, khong C5 - C11, l ngun nguyn liu cho qu trnh hi nc -cracking (steam cracking).

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H3C + RCH2CH2CH2CH2CH2CH2CH3 CH4 + RCH2CH2CH2CH2CHCH2CH3 (2-15)

Gc ny li b phn ct theo quy tc to thnh mt -olefin v mt gc t do bc

mt.

RCH2CH2CH2 CH2CHCH2CH3 RCH2CH2CH2 + H2C=CHCH2CH3 (2-16)

S lp li phn ng ny v cc phn ng (2-14) v (2-15) dn n s hnh thnh mt

lng ln etylen, mt t metan v olefin.

Tng t nh gc metyl, gc RCH2 c th kt hp vi gc H

tch t mt parafin

khc to ra mt gc t do bc 2 v mt parafin nh hn, nhng phn ng xy ra vi

tc chm hn so vi CH3 v tnh n nh ca RCH2 cao hn. Ch khong 10% ca cc

chui gc b bin i theo kiu nh th trc khi b phn ct n CH3. Thc vy, lng

parafin c to ra trong cracking nhit rt t, khi nng hydrocacbon cao, phn ng

chuyn chui gc tr nn quan trng hn, v sn phm cracking nhit c nng cao ca

cc parafin phn t ln v cc olefin, c bit, t C3=

, n C7=

.

R1CH2 + RCH2CH2CH2CH2CH2CH2CH3

R2CH3 + RCH2CH2CH2CH2CH2CHCH3 (2-17)

Cc gc t do khng b isome ho bng cch chuyn dch cc nhm alkyl hoc

chuyn dch cc tm gc t cacbon ny n cacbon khc trong chui. V cc gc t do bc 1

km bn nhit ng hc hn so vi gc bc 2 v gc bc 3, nn cc gc bc 1 chui di c

th t cun li chim ly gc hydro H t cc v tr bc 2 hoc bc 3 ngay trong bn

thn chui gc :

(2-18)CH2

CH2 CH2

CH2CH2 CH2

RCHCH3 CH2

CH2CH2 CH2

RCCH3

CH2

CH3

Phn ng v phn ng (2-17) l rt quan trng bi v chng to ra cc sn phm t

etylen hn v giu gasolin hn.

Phn ng kt hp gia cc gc t do (2-13) xy ra vi xc sut khng ln lm, bi v

nng ca cc gc t do thp. Cc phn ng ng vng v phn ng ng ng t xy ra vi

mc rt khng ng k trong cracking nhit, tng ng vi lng nha aromat thu c

rt t.

T l tng i v kh nng tch gc hydro t cc nguyn t cacbon khc nhau ca

mt parafin c d on theo l thuyt ca Rice v ng s nh sau: i vi nguyn t

cacbon bc mt: 1; i vi nguyn cacbon bc hai: 3,66 v i vi nguyn t cacbon bc ba:

13,4. T l tng i ca tc cracking cc parafin mch thng v mch nhnh c th don theo l thuyt , vi gi thuyt l giai on tch nguyn t hydro l chm nht. T

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cc nghin cu thc nghim v cracking nhit ca nhiu parafin c th nhn thy rng, t l

tch nguyn t hydro l 1 : 3,66 : 14,4. Kt qu kh ph hp vi l thuyt.

S phn b sn phm cracking nhit ca naphta v ca n-hexadecan c trnh by

trong bng 2.2. ng nh d on, hiu sut etylen ca hai trng hp u cao. i vi

n-hexadecan, ngi ta thy c mt mt lng tng t nhau ca cc sn phm C5 v cc hp

cht aromat c phn t lng cao hn.

Bng 2.2. Cracking nhit hydrocacbon

Cracking nhit naphta thit bcng nghip (a)

Cracking nhit n-hexadecan(b) quy m phng th nghim

(mol/100 mol cracking)

Sn phm %mol Sn phm Thc nghim Tnh ton

Hydro 16,2 C1 53 61

Metan 25,7 C2 130 139

Axetylen 1,4 C3 60 50

Etylen 30,7 C4 23 27

Propadien 2,5 C5 9 15

Propylen 0,8 C6 24 17

Propan 7,5 C7 16 14

Butadien 0,2 C8 13 12

Butylen 2,4 C9 10 11

Butan 1,0 C10 11 10

Lng C5+ 11,5(c) C11 9 9

Tng cng 100 C12 7 8

C13 8 7

C14 5 7

H/C ca C5+ =1,0

iC4/C4 = 0,07

(a): Thi gian tip xc trong l ng = 0,05 - 0,1 giy, nhit ~900oC.

(b) Tc khng gian cht lng LHSV = 0,05. React cha lp m thch anh, chuyn ho

31,5% khi lng.

(c) Xc nh trn c s C5H5.

Tm li, nhng c im quan trng ca cc phn ng cracking nhit l: s phn ct

lin kt cacbon - cacbon xy ra v tr i vi nguyn t cacbon c electron cha cp

i; xc sut chuyn i gc t do t hydrocacbon ny sang hydrocacbon khc l khng

ng k; v s chuyn dch electron cha cp i t nguyn t cacbon ny sang nguyn t

cacbon khc l hu nh khng th, ngha l gc t do khng th thc hin cc phn ng

isome ho bng cch chuyn dch nhm metyl. L thuyt cracking nhit rt ph hp vi cc

d on v hiu sut cao ca etylen, hiu sut thp ca metan, ca -olefin, s vng mt ca

cc sn phm isome ho v t s cao ca olefin so vi parafin trong sn phm cracking nhit.

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2.2. Cracking xc tc

Cc phn ng cracking xy ra nh cc cht xc tc axit theo c ch cacbocation. C

ch ny c xut u tin bi Gayer (F.H. Gayer, Ind. Eng. Chem, 75, 1122, 1935) nm

1933, sau c Hansford (R.C. Hansford, Ind. Eng. Chem. 39:849, 1947), Olah v

Thomas (C. L. Thomas, Ind. Eng. Chem., 41:2004, 1949) ng h, cng c v pht trin. n

nay, c ch cacbocation trong cracking xc tc hu nh c hon ton tha nhn. Do

, nhng hiu bit v bn cht, v s hnh thnh ca cc cacbocation l ht sc cn thit

nghin cu cc phn ng xy ra trong qu trnh cracking xc tc.

2.2.1. Cc kin thc cs v cacbocation

(i)Danh php

Theo danh php ca UIPAC th thut ng cacbocation c dng biu th cho cc

cation hu c c hnh thnh nh thm mt proton (H+) vo mt phn t hu c hoc tch

ra mt ion hydrua (H

) t mt phn t hu c.Cacbocation gm hai loi ion:

- Ion cacbeni*: l cc cacbocation, trong , cacbon mang in tch dng c s phi

tr ba, v d nh: CH3+, C2H5

+, C3H7+, (CH3)3C

+...

- Ion cacboni: l cc cabocation, trong , cacbon mang in tch dng c s phi tr

nm, v d nh: CH5+, C6H7

+, R1-CH3+-CH2-R2,...

Cc ion cacbeni khc nhau c gi tn theo tn ca gc alkyl tng ng v thm mt

tcation pha trc tn gc alkyl. V d:+ CH3

+: cation metyl, C2H5+ cation etyl, C3H7

+ cation propyl, (CH3)3C+ cation tert-

butyl,...

Hoc i khi ngi ta c th gi tn theo mt cch khc: gi CH3+ l ion cacbeni, i

vi cc ion khc th chn tn nhm alkyl thay th H ca CH 3+ vo gia 2 tion v cacbeni.

V d:

CH3+: ion cacbeni

CH3CH2+: ion metyl cacbeni

C2H5CH2+: ion etyl cacbeni

CH3C+HCH3: ion dimetyl cacbeni.

(ii)C ch hnh thnh cc cacbocation

C nhiu cch hnh thnh cacbocation. Sau y l mt s trng hp thng gp:

a)Thm mt proton vo mt phn t cha bo ho

* Xin lu rng, hin nay cc ti liu (sch gio khoa, gio trnh...) Vit Nam vn ang dngdanh php c.

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41/208

+

H+

+C C H+

CC C H C +

R C H+

+ R CH

H+

+R C H2+

Nh vy, tnh kim ca cc lin kt CH v CC ca cc alkan s quyt nh kh

nng tn cng ca proton ln cc lin kt .

c) Chuyn dch electron

Cc axit Lewis, ni chung, c kh nng to ra cc ion cacbeni xut pht t cc alkan.

C ch hnh thnh c th nh sau:

Chuyn dch mt electron t alkan n axit Lewis (L)

RH + L RH+ + L

Tip n, phn hu gc t do cation RH+ thnh R+

RH+

+ L

R+ + HL

d)Phn ct d ly mt phn t bo ho

S phn ct d ly ny c thc hin lin kt cacbon-cacbon ca alkan. S phn ct

thc hin do hiu ng nhit thng to ra 2 gc t do, tuy nhin, khi c mt mt axit th c

th to ra mt cp ion, trong c mt ion l cacbeni.

R1R2 R1+R2

(iii) bn nhit ng ca cc ion cacbeni

Nng lng hnh thnh (Hht) cc ion cacbeni tng theo s nguyn t hydro lin kt

vi nguyn t cacbon mang in tch dng. Cc gi tr nng lng hnh thnh tng i

ca cc ion cacbon bc 3, bc 2 v bc 1 nh sau:

Ion (CH3)3C+ (CH3)2CH

+ CH3CH2CH2+

Gi tr tng i ca Hht (kcal/mol) 0 14 21

T cc s liu trn, c th nhn thy rng, bn nhit ng ca cc ion cacbeni gim

theo trt t sau y:

R3C+ >> R2CH

+ > RCH2+ >> CH3

+

V d, phn ng

CH3CH2C+HCH3 (CH3)3C

+

c bin thin hiu ng nhit phn ng Hpu = 14 kcal/molNh chng ta bit, bin thin nng lng t do ca phn ng l:

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Gpu = Hpu TSpu

Nu gi thit bin thin entropy ca phn ng l khng ng k th:

GpuHpu

V hng s cn bng Kpu ca phn ng 25oC l:

Gpu = RTlnKpu

lnKpu = RT

G =298987,1

14000

= 23,644

Kpu = 1,85.1010

Ngha l, khi cc cation cha cng mt s nguyn t cacbon nh nhau th hiu sut

chuyn ho t cation bc 3 sang cation bc 2 l rt ln.

Hng s ca phn ng:

R1H + R2+ R1

+ + R2H

bng 1 nu cc cation R1+ v R2

+ u l 2 ion cacbeni bc 3 (hoc bc 2). Nu R1+ l ion

bc 3 v R2+ l ion bc 2 th hng s cn bng ca phn ng ~1010, v d phn ng:

(CH3)3CH + CH3CH2C+HCH3 nC4H10 + (CH3)3C

+

(iv) Cc phn ng ca cacbocation

Tt c cc cacbocation c hnh thnh u c th tham gia cc chuyn ho sao

cho t c trng thi bn vng nhit ng nht, vi tc chuyn ho nhanh nht. Cc

chuyn ho l:- Chuyn dch hydrua v chuyn dch nhm alkyl.

- Hnh thnh hoc ph v cc lin kt cacbon - cacbon.

Tt c cc chuyn dch c th c thc hin ngay trong bn thn mt phn t hoc

gia cc phn t. Chnh nhng chuyn dch ny l cc giai on cha kho ca cc phn

ng isome ho, polyme ho, ng vng, alkyl ho v cracking ca cc hydrocacbon.

1.Chuyn dch hydrua v chuyn dch nhm alkyl

S chuyn dch ny xy ra bi mt qu trnh chuyn i 1~2, ngha l bi mt s ich ca 2 in tch dng ti 2 cacbon cnh nhau. Nu cc ion cacbeni tham gia chuyn i

c bn nhit ng xp x nhau, th bin thin nng lng t do G* nh, thng thng

thp hn 3 kcal/mol v do tc chuyn dch ln*.

V d, i vi ion cacbeni bc 3

Chuyn dch H

* Da theo l thuyt phc cht hot ng (xem trang 256, Gio trnh Ho l - Ho keo, Nguyn Hu Ph,Nh xut bn KHKT, 2003) th hng s tc phn ng k =

hkTe

G/RT, nu G nh th k ln.

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Chuyn dch nhm alkyl (Me: nhm metyl)

~

G* = 3 - 3,5 kcal/mol

k = 107 - 108 s1

(T = 120oC)

~

Me

C C C C

C C

+C C C C+

C C

C C

H~

C C C C

H

C C

+C C C C

+

H

C C

Nh vy, phn ng chuyn dch hydrua v nhm alkyl gia 2 cacbeni bc 3 xy ra vi

tc rt ln.

i vi cc ion cacbeni bc 2 cng xy ra tng t

Chuyn dch H

HC C C C

H

H H

+C C C C

+

H

H H

G* < 3 kcal/kmol

Chuyn dch nhm alkyl

G* ~ 2 kcal/kmol

~Me

C C C C

H H

+C C C C

+

H H

C C

Ngc li, khi chuyn t mt ion bc 3 sang mt ion cacbeni bc 2 phi v t qua mtthm nng lng l 13 kcal/mol. Do , tc chuyn dch l rt nh. V d, phn ng

isome ho cation 2,4,4-trimetylpentyl thnh cation 2,3,4-trimetylpentyl xy ra vi tc rt

nh mc d bn nhit ng hc ca 2 cation xp x nhau. Tuy nhin, thc hin s

isome ho , cn phi vt qua giai on chuyn dch hydrua to ra mt ion cacbeni bc 2

trung gian, chnh giai on ny l giai on chm, quyt nh tc chung ca qu trnh

isome ho.

2.Sp xp li mch cacbon

~ ~Me

C C C C C+

CH

C C C C C

C C

+

HC

H C

C C C C C

C H C C

H HC C C C C

C

C H

+

C

H

Giai on chmG* = 13,5 kcal/mol

k = 5.104 s1 (ti 80oC)

S sp xp li mch cacbon ca cc hydrocacbon theo 2 cch: khng phn nhnh v

phn nhnh.

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Khng phn nhnh

C ch ca s sp xp li ny da trn cc chuyn dch 1~2 ca cc hydrua v ca cc

nhm alkyl v c mt giai on chuyn qua ion cacbeni bc 2.

Giai on chuyn dch cation bc 3 thnh cation bc 2 lun lm cho tc chung ca

phn ng khng ln

~ ~ ~

Cation bc 3 Cation bc 2

C C C C

H

H C

C C C C

H

C

H

C C C C

HH

C

C C C C+

H

C HH

+

MeH + +

Giai on chm

Phn ng:

Vi G* = 14,8 kcal/mol

Xy ra vi tc khng ln, k = 10 s1 ( 0oC).

Tng t, phn ng

C C C C+

H

C H

C C C C

H

H C

+

C C C C C+

H

C H

C C C C C

H

H C

+

Cng c tc nh k = 8.104 s

1 (78oC).

Phn nhnh

Cc sp xp li mch cacbon c phn nhnh xy ra vi tc nh hn so vi tc

ca cc phn ng sp xp li mch cacbon khng phn nhnh v theo mt c ch khc.

V d:

Phn ng

C C C C C

C

+C C C C

+

C C

C hng s tc phn ng

k = 0,08 s1 0oC

k = 4.10

4

s

1

-45

o

CPhn ng:

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CC+

CCCC C C+

C C C

CC

H+C C CC C

CC+

CCCC

C C+

C CCC

C

H+C CC C

C

C

k = 600 s1 120oCk = 600 s1 120oC

C C C C+

C

C C C C C+

Phn ngPhn ng

++

k = 900 s1 100oCk = 900 s1 100oC

Thot tin, c ch ca cc phn ng sp xp li mch cacbon c phn nhnh c

ngh da vo cc chuyn dch 1~2 ca hydrua v ca alkyl.

Thot tin, c ch ca cc phn ng sp xp li mch cacbon c phn nhnh c

ngh da vo cc chuyn dch 1~2 ca hydrua v ca alkyl.

C C C C C+

C C C C C+

C

C C C C C+

C

Theo s trn, th s chuyn ho phi thng qua mt ion cacbeni bc 1, nn tc

ca qu trnh trn rt chm.

Theo s trn, th s chuyn ho phi thng qua mt ion cacbeni bc 1, nn tc

ca qu trnh trn rt chm.

Tuy nhin, c ch ny, v sau, b loi b v ngay c trong mi trng super axit phn

ng isome ho n-butan hu nh khng xy ra, trong khi , n-pentan li b oxy ho rt

nhanh. Nh vy, s chuyn i cation n-butyl thnh cation tert-butyl l kh khn hn rt

nhiu so vi s chuyn i cation n-pentyl thnh cation tert-pentyl. iu mu thun vi

c ch c in v chuyn i 1~2 ca hydrua v ca alkyl, ngha l tc ca 2 qu trnh

trn phi tng t nhau.

Tuy nhin, c ch ny, v sau, b loi b v ngay c trong mi trng super axit phn

ng isome ho n-butan hu nh khng xy ra, trong khi , n-pentan li b oxy ho rt

nhanh. Nh vy, s chuyn i cation n-butyl thnh cation tert-butyl l kh khn hn rt

nhiu so vi s chuyn i cation n-pentyl thnh cation tert-pentyl. iu mu thun vi

c ch c in v chuyn i 1~2 ca hydrua v ca alkyl, ngha l tc ca 2 qu trnh

trn phi tng t nhau.Do , mt c ch khc c ngh nh da vo mt hp cht trung gian

xyclopropan proton ho:

Do , mt c ch khc c ngh nh da vo mt hp cht trung gian

xyclopropan proton ho:

R CH2 C

CH2

CH2H+

R CH2 C+

CH3

CH3

R CH2 CH

CH2

C H

H

H

+

Trong c ch ny, cacbon v tr ca C+

trong mch cacbocation kt hp vi C+

tora mt vng xyclopropan proton ho. M vng xyclopropan s to ra mt ion cacbeni

mi c cu trc mch cacbon khc vi cu trc mch cacbon ca ion cacbeni ban u: mt

nhnh thnh 2 nhnh mch thng thnh mch nhnh.

V d:

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Da vo c ch , ngi ta c th gii thch c c ch sp xp li mch cacbon

khng phi tri qua hp cht trung gian ion cacbeni bc 1, v cc kt qu thc nghim ca

tc isome ho kh ln ca n-pentan thnh isopentan trong mi trng axit.

3.Phn ng ct mch cacbon v tr (phn ct)Cc cacbocation c th tham gia phn ng ct mch cacbon CC v tr i vi

cacbon mang in tch d

ng to ra mt olefin v mt ion cacbeni nh hn.H

R1CHCH2CH2R2 R1C=CH2 + CH2R2

++

Tc phn ng ny ph thuc vo bn nhit ng ca ion cacbeni u v ion

cacbeni cui.

Nu cc ion cacbeni u v cui u l bc 3, phn ng ct xy ra rt nhanh. V d,

cation 2,4,4 trimetylpentyl d dng thc hin phn ng:

C CH3 C CCCCC+C CC+C + C=CC

C

Tc phn ng phn ct ln hn tc phn ng sp xp li c phn nhnh v nh

hn tc phn ng sp xp li khng phn nhnh ca mch hydrocacbon. Do , ion

trimetylpentyl s cn bng vi cc cation trimetylpentyl khc, ch mt trong cc ion thc

hin phn ng ct .

C C C C C+

C C C C C+

CC C

C C

C

ct C C C C C

+C C

C

C C C

C+

C C C

C

C C C C C+

C CC

C C C C C+

C CC

Phn ng ct ca ion cacbeni 2 nhnh xy ra chm hn so vi phn ng ct ca

ion cacbeni 3 nhnh, v s phn ct ny to ra mt ion cacbeni bc 2 t mt ion cacbeni bc

3 (khng thun li v mt nhit ng hc).

~

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Qu trnh phn ct ca ion cacbeni mch thng xy ra vi tc cn chm hn, kh

khn hn, v n to ra mt ion cacbeni bc mt.

Di y l s biu din cc giai on khc nhau xy ra khi cracking n-octan.

Qua s trn thy r c im ca cc qu trnh chuyn dch hydrua, nhm alkyl v

phn ct : chiu hng v tc cc qu trnh u ph thuc vo bn nhit ng ca

cc cation cacbeni v gi tr bin thin nng lng ca qu trnh.

S phn ct cc lin kt CC trong mch vng ca cc cation xycloalkyl l mt phn

ng kh khn hn so vi lin kt CC trong mch thng.

V d, cation trimetylxyclopentyl bc 3 tng i bn 0oC, trong khi cation

2,2,4-trimetylpentyl bc 3 li b phn ct 73oC.

G* < 3 kcal/kmol

C C C+

C

C C C C C+

C

+ C C Cnhanh

C

C C C C C+

C C

+C C C

C

C C C+chm

Cc cation bc 2

n-octyl

Phn ng phn ct rt chm

rt nhanh

n

-octan

Cc cation metylheptylv etyl hexyl

Phn ng phn ct cc k chmCc metylheptan

3-etylhexan

chm

Cc cation dimetylhexylv metyl etyl pentyl

Phn ng phn ct rt chmCc dimetylhexanmetyl etyl pentan

chm

Cc cation trimetylpentyltrong c

Phn ng phn ct nhanh

+C C C C C

CC

C

+C C C

C

C C C

C

+

Cc trimetylpentan

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+ +

++

+

k = 104s

1 0oC

k = 5.104s

1 73oC

Tnh cht xut pht t bn cht ca cu hnh cc orbital trong khng gian. Trong

ion xyclopentyl cc lin kt CC ca mch vng nm trong mt mt phng vung gc vi

orbital trng pz ca cacbon mang in tch dng. Do , s m vng cng vi s hnh

thnh mt lin kt i i hi mt s che ph cc i ca 2 orbital p i vi orbitalpz v do

, i hi mt nng lng hot ho cao hn.

Phn ng cracking xc tc xy ra theo c ch cacbocation. Cacbocation bao gm hai

loi: ion cacbeni (cha C+ phi tr 3) v ion cacboni (cha C+ phi tr 5).

Ion cacbeni chuyn ho theo quy tc phn ct lin kti vi tm C+ to ra mt

olefin v mt ion cacbeni mi, nh hn.

Cc chuyn ho ca cc cacbocation u tun theo quy tc to ra cacbocation bn

vng nhit ng nht (R3C+ >> R2CH

+ >> RCH2+ >> CH3

+) v vt qua thm nng lng

hot ng nh nht.

2.2.2. Cracking xc tc cc hydrocacbon n l

Nhng cng trnh nghin cu u tin v cracking xc tc l cracking cc hydrocacbonn l nhm xc nh cc giai on phn ng v c ch xc tc. Mc du trong nhng iu

kin thc nghim ban u cha hin i, nhng nhng nghin cu cng dn n cc kt

qu v ang c tha nhn, v d nh, quy tc cracking , s hnh thnh cc ion cacbeni

bc 1, 2 v 3,...

Tuy nhin, cc nghin cu v cracking xc tc ca hydrocacbon n l, v sau, t

c thc hin v ngi ta nhn thy rng, cht xc tc cracking rt nhanh chng b suy

thoi do s to cc trn b mt cht xc tc.

Trong nhng nm 1970 nh s pht trin ca my tnh v cc l thuyt mi cp

n s suy gim hot tnh xc tc (l thuyt to cc trn b mt cht xc tc, l thuyt phn

ng trong dng) m ngi ta c th p dng cc s liu phng th nghim cho quy m

cng nghip. Chnh v vy, cc nghin cu v cracking hydrocacbon n l li c hi

phc nhm xc nh mt m hnh ng hc v cc tm hot ng ca cht xc tc v do ,

cho php ch to cc cht xc tc cracking tt nht.

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2.2.2.1. Cracking xc tc parafin

Parafin l mt cu t quan trng ca gas oil, ngun nguyn liu ca nhiu nh my

lc du, v l sn phm cracking s cp ca nh my .

V th, chng ta s tng quan li cc kt qu trc y t c v cp n cc

kt qu mi nht lin quan n cracking xc tc cc hydrocacbon parafin.

Tnh cht cracking

Mt trong nhng nghin cu u tin v cracking xc tc hydrocacbon n l l

cracking n-hexadecan. Bi v kch thc ca phn t ny i din cho nhiu phn t trong

nguyn liu lc du. Tuy nhin cracking xc tc ca cc phn t mch ngn li c nghin

cu mt cch chi tit hn.

T thc nghim, ngi ta nhn thy rng, trong cc iu kin phn ng nh nhau,

di ca chui cacbon nh h

ng r rt n nng l

ng hot ho ca phn ng cracking nh

bng 2.3.

Bng 2.3. nh hng ca di chui cacbon n nng lng hot ho cracking

Hydrocacbon Nng lng hot ho (kcal/g.mol)

n-C6H14 36,6

n-C7H16 29,4

n-C8H

1824,9

T bng 2.3, c th nhn thy rng, s hnh thnh cc cacbocation cng d dng trn

cc nguyn t cacbon cng su bn trong chui. Cc kt qu trn cng t ng t nh kt

qu ca Voge khi nghin cu chuyn ho ca cc parafin khc nhau trn xc tc

aluminosilicat - zirconi, 500oC, 1 atm (bng 2.4).

Bng 2.4. nh hng ca di chui cacbon n chuyn ho cracking

Parafin chuyn ho (%)

n-C5H12 1

n-C7H16 3

n-C12H26 18

n-C16H34 42

Mt khc, ngi ta nhn thy rng, mch ca parafin c nh hng rt ln n ng

hc phn ng cracking (bng 2.5).

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T bng 2.6 thy rng, t 100 mol C16H34 chuyn ho 24% ngi ta thu c 359

mol hydrocacbon (cha k 12 mol H2, khng lit k trong bng 2.6). Nh vy, mt s sn

phm trong bng 2.6 khng phi l sn phm s cp, m cn l sn phm c to ra t cc

sn phm phn ng th cp v phn ng cracking nhit. Thc vy, nu l sn phm s cp

th s phn b s tun theo quy lut:

15

1

C

C

= 14

2

C

C

= 13

3

C

C

= 12

4

C

C

= 11

5

C

C

= 10

6

C

C

= = 1

Ch nhng thc nghim c tin hnh chuyn ho rt thp v thi gian phn

ng (thi gian tip xc gia hydrocacbon v cht xc tc) rt ngn th mi hn ch c s

sai lch do phn ng th cp xy ra. Da vo mt s gi thit (tc hnh thnh cc ion

cacbeni th cp, s phn ct , s cracking ca olefin va c to ra...), Voge tnh ton s

phn b sn phm khi cracking 100 mol hexadecan to ra ~359 mol sn phm v cho kt

qu nh bng 2.7.

Bng 2.7. Phn b sn phm cracking 100 mol hexadecan theo tnh ton ca Voge

Hydrocacbon C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14

Mol 0 0 95 97 72 41 7 6 5 4 4 4 0 0

Cc kt qu ca bng 2.7 gn tng t nh bng 2.6. iu chng t rng, cc kt

qu thc nghim cracking hexandecan bao gm cc sn phm s cp v th cp.

Hin nay ngi ta dng cc k thut thc nghim hin i nh ph khi, cacbon

phng x... nghin cu phn ng cracking mt cch chnh xc hn v phn b cc snphm. n-Hexan l mt hydrocacbon thng c chn lm cht th (test) cho phn ng

cracking quy m phng th nghim.

Cc kt qu cracking n-hexan ca Good v cng s (G. M. Good et al, Ind. Eng.

Chem. 39:1032, 1947) 550oC, 60 pht trong dng, trn aluminosilicat.

Bng 2.8. Phn b sn phm cracking n-hexan trn aluminosilicat v nh hnh

H2 CH4 C2 tng C3tng C4 tng C5 tng

% mol 9,4 12,5 19,5 9,5 8,5 1,0

T bng 2.8 c th c nhng nhn xt:

- Lng hydro kh ln.

- T s C1/C4 kh ln chng t y l khng phi l mt phn ng cracking n gin.

- T s C1/C5 cng kh cao.

Thc vy, lng hydro, metan v hydrocacbon C2 kh ln chng t trong thc nghim

ny c s tham gia ca phn ng cracking nhit. Hn na, mu c ly 60 pht phn ng,ti thi im , cht xc tc tri qua bao nhiu bin i (suy gim hot tnh), do , t

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cc kt qu tc thi kh m a ra mt kt lun chnh xc v bn cht vn c ca cht

xc tc.

C ch phn ng cracking xc tc ca parafin

iu tin quyt cho mt phn ng cracking xc tc l s hnh thnh cacbocation. Sau

, cacbocation b cracking theo quy tc phn ct , to ra mt cacbocation nh hn b

hp ph trn cht xc tc v mt -olefin pha kh.V d:

RCH2CH2CH2CH2R

Xc tc

RCH2C+HCH2CH2R RCH2CH=CH2 + RC

+H2

Ion cacbeni mi hnh thnh rt khng bn vng nhit ng, nn phi tip nhn H

t

tm xc tc v kh hp ph hoc phi isome ho thnh cc ion cacbeni c ion mang in

tch dng nm pha bn trong mch cacbon nhn c trng thi n nh (bn vng

hn).

C ch cracking xc tc parafin nh trn hu nh u c mi ngi tha nhn. Vn

cn li l nguyn nhn to thnh cacbocation. C kin cho rng, trong mi trng supe

axit cc alkan thng to ra cc ion cacboni; trn cht xc tc d th cc ion cacbeni c

hnh thnh t cc tm axit Lewis bng cch nhng li ion hydrua H

ca alkan. Trn xc

tc zeolit, c mt s gi thit v s hnh thnh cacbocation nh sau:1. Ion cacbeni c hnh thnh bng cch tch ion H

t alkan trn tm Lewis.

2. Ion cacbeni c hnh thnh bng cch tch ion H

ca alkan trn tm axit mnh

Brnsted, ng thi to ra hydro.

3. Ion hnh thnh trc tin l cacboni phi tr nm, do alkan ly thm proton (H +) t

cc tm axit mnh Brnsted.

4. Ion cacbeni c to ra nh hp ph olefin trn tm axit Brnsted. Cc olefin ny

l tp cht trong nguyn liu hoc do cracking nhit.

5. Cacbocation c hnh thnh nh s phn cc ca phn t nguyn liu trong trng

tnh in mnh ca mao qun zeolit.

Ngi ta tin hnh rt nhiu nghin cu thc nghim xc nh chnh xc cc c

ch hnh thnh cacbocation trong giai on khi mo ca phn ng cracking. Trong s

gi thit 4 c tha nhn rng ri nht. Song, c th ni, cho n nay cha c mt gi thit

no c hon ton khng nh, v vn cn mt s kt qu thc nghim cha c sng t

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khi da vo cc gi thit . Nhng ni chung, ngi ta thng gii thch s hnh thnh

cacbocation bng mt s gi thit ni trn.

Alkan

Ion cacboni

Ion cacbeni

H2, Alkan (sn phm)(1)

Olefin (sn phm)(2)

Alkan (sn phm)

Olefin (sn phm)

Alkan (sn phm)

Cacking protolyt

Phn ct

Ion cacbeni+H-PhchitmLewis

-H+

Phc

hitm

Bronsted

+Alkan

(nguynliu)

+H+

Tm

Brons

tedm

nh

-H-TmLewismnh

-

Cc phn ng khi mo Cc phn ng pht trin mch

G

iaion

khim

ophnng

Giaion

phttrinphnng

Giaion

ktthcphnng

Hnh 2.1. S c ch phn ng cracking alkan trn zeolit

Gn y nht, bng tnh ton orbital phn t khi nghin cu tng tc gia proton ca

nhm OH vi n-butan, ng

i ta nhn thy rng, phn ng cracking vn xy ra qua cc ioncacboni (thng c gi l cracking protolyt) rt thun li v mt nng lng. Do , mt

s c ch phn ng cracking ca alkan trn zeolit c ngh nh trn hnh 2.1.

S ny cho php gii thch c s hnh thnh H2 v cc sn phm alkan, olefin

trong phn ng cracking xc tc: S phn b sn phm ph thuc vo bn cht, s tm v

lc axit ca cc cht xc tc.

Giai on quyt nh ca phn ng cracking parafin

Hin nay, rt nhiu ngi cho rng, giai on hnh thnh cc cacbocation l giai onquyt nh tc phn ng cracking parafin. S d nh th l v, thc nghim xc nhn

rng, s hnh thnh cacbocation t olefin d dng hn t parafin, ng vi tc phn ng

cracking olefin ln hn tc cracking parafin.

Tuy nhin, suy lun nh trn l khng hon ton chnh xc, bi v cch hnh thnh

cacbocation t olefin v t parafin trn hai loi tm axit khc nhau. Thc vy, trn cht xc

tc axit rn, parafin c th b cracking trn c hai tm Lewis v Brnsted, trong khi , ch

tm Brnsted mi xc tc cho phn ng cracking olefin.

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C kin cho rng, s hnh thnh cacbocation khng phi l giai on quyt nh

(giai on c tc chm nht) trong phn ng cracking m l giai on chuyn ho ca

cc cacbocation ( to ra mt trng thi hot ng nht). Thc vy, khi nghin cu phn

ng cracking heptan ngi ta nhn thy rng, nng lng hot ho khi cracking n-heptan

thnh C2 + C5 v thnh C3 + C4 khc bit nhau 11 kcal/mol, nhng nng lng hnh thnh

cc cacbocation to ra C2 + C5 v C3 + C4 u xp x nhau ~ 1kcal/mol. Do , r rng l,

giai on quyt nh phn ng cracking n-heptan khng phi l giai on hnh thnh

cacbocation.

Nh vy, cho n nay, cc vn v s nh hng ca cu trc phn t parafin n

tc phn ng cracking, v nguyn nhn v bn cht s phn b sn phm phn ng

cracking l hu nh c tha nhn v sng t. Song, cc vn v bn cht ca cc tm

xc tc khi mo cho s hnh thnh cc cacbocation v giai on no l giai on quyt

nh trong phn ng cracking th vn cn c tip tc tho lun v nghin cu.

2.2.2.2. Cracking xc tc olefin

Trong phn ng cracking xc tc olefin, mi ngi u tha nhn rng, s hnh thnh

cacboca

32978579 cracking xuc tac

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