Vo l. 33. N o . 286. Oc t o b e r 1947.
THE EFFECT OF SULPHUR AND PHOSPHORUS ON AVIATION FUEL PERFORMANCE,
B y T. K . H a n s o n * (Fellow) and K . F . C o l e s * Su m m a r y.
T h e effect o f sm all q u a n titie s o f s u lp h u r a n d p h o sp h o ru s a n d th e ir co m p o u n d s on th e p erfo rm an ce o f fuels is discussed b o th for o ctan e (m o to r m eth o d ) ra tin g s a n d rich m ix tu re perfo rm an ce (3-C) ra tin g s. I t is show n t h a t th e a d v erse effect o f s u lp h u r o n engine p erfo rm an ce o f lead ed fuels is v e ry m u ch g re a te r b y rich m ix tu re ra tin g th a n b y m o to r m e th o d ra tin g , b u t* th a t ph o sp h o ru s show s th e rev erse effect, h a v in g fo r v e ry low c o n ce n tra tio n s a slig h t p o sitiv e effect o n rich m ix tu re ra tin g . T h e effect on fuel perfo rm an ce o f d ifferen t classes o f c o m p o u n d s co n ta in in g th ese elem en ts is discussed.
In t r o d u c t i o n.
T h e effect of sulphur com pounds on m otor fuel has been very fully studied, and some of th e earliest refining processes were directed to modifica
tio n or rem oval of these com pounds. This refining was, however, directed to prevention of corrosion and odour, and it is only more recently th a t the effect of sulphur com pounds on th e perform ance of a m otor fuel has been system atically studied. I n 1942, J . G. R yan published his results 1 on
“ The Influence of S ulphur Compounds on O ctane N um ber and Lead Susceptibility of Gasolines.” H e was able to show th a t there was a reduc
tio n of octane num ber of clear fuels and also a reduction of lead suscepti
bility. The reduction of octane num ber for a given sulphur content in clear fuel was dependent both on th e ty p e and molecular size of th e sulphur com pound, whereas th e effect on lead susceptibility was shown to be dependent alm ost entirely on th e ty p e of compound. F u rth e r results on th e effect of sulphur com pounds and also of organic chlorine compounds on octane num ber have recently been published.4
In an aviation fuel a property of m ajor significance is its perform ance under m axim um power in a supercharged engine—and this is norm ally m easured experim entally as th e “ rich m ixture perform ance nu m b er.”
Prelim inary experim ents showed th a t sulphur compounds in a fuel had a very m arked effect on rich m ixture perform ance rating. I t was therefore decided to stu d y this effect for a num ber of sulphur compounds. As th e high perform ance required of aviation fuels can norm ally only be obtained by th e addition of te tra e th y l le a d ,f th e effect of sulphur com pounds on th e rich m ixture perform ance rating of a clear fuel was not determ ined. A typical leaded fuel was therefore selected and rich m ixture perform ance num bers determ ined by th e oxygen m ethod reported elsewhere.2
The effect of phosphorus on th e perform ance of aviation fuels was studied in this laboratory in 1940 in connexion w ith th e developm ent of m ethods for sabotaging fuel storage. A great reduction of th e octane num ber of leaded fuels by very small additions of phosphorus was noted. A lthough
* T rin id a d L easeh o ld s L td ., C e n tra l L ab o ra to ry , K in g ’s L an g ley , H e rts.
t A d d itio n s o f te tr a e th y l le a d to a fuel will here be ex p ressed as M .T .I.G .— m illi
litre s te tr a e th y l lead p e r Im p e ria l gallon.
, T T
p it
th is m ethod o f sabotage was n o t used in practice, it was recently decided to m ake a few te sts on th e rich m ix tu re perform ance of a v ia tio n fuels con
taining sm all ad d itio n s of phosphorus. The unexpected n a tu re of th e results has p ro m p ted p ublication here in th e hope th a t th e y m ay ev en tu ally co n trib u te to a consistent ex p lan atio n of th e behaviour of fuels containing sm all additions of com pounds containing sulphur, phosphorus, or nitrogen.
Ex p e r i m e n t a l.
Sulphur Compounds.
A pproxim ately 1 p er cent solutions of th e various sulphur com pounds were p rep ared an d th e su lp h u r c o n ten t checked by th e lam p m ethod.
These were th e n add ed in th e req u ired am o u n ts to th e clear fuel an d the te tra e th y l lead ad d ed ju s t prior to te st.
Fuel.
The fuels used for te s t were blends of s tra ig h t-ru n com ponents and alk y late w ith rich m ix tu re perform ances of 98-0 to 99-0 per cent of th e Air M inistry 10 0-octane reference fuel (B.A.M. 100) w hen leaded a t 4-8 M.T.I.G.
The rich m ix tu re perform ance num bers of th e series of fuels are shown in Tables I and I I .
Ta b l e I .
E ffect o f S u lp h u r C om pounds on R ic h M ix tu r e P erform ance.
F u e l C o m p o sitio n : 3 6 % a lk y la te .
6 4 % s tr a ig h t- r u n c o m p o n e n t.
0-005% S.
4-8 M .T .I.G .
5 9 0 H A N S O N A N D C O L E S ! T H E E F F E C T O F S U L P H U R A N D
C om pound a d d e d .
% t>y w t o f a d d e d s u lp h u r.
R ich m ix tu re p e rfo rm a n ce
(% B.A.M . 100).
D e p ressio n o f R .M .P .
E q u iv a le n t d ro p in le a d c o n te n t, M .T .I.G .
P b /S m ol ra tio re a c te d .
— — 98-0 0 — —
E th y l 0 0003 97-5 0-5 _ ._
m e rc a p ta n 0 0031 95-1 2-9 0-8 1-3
0 0 1 2 4 87-0 11-0 1-9 ■8
D i-e th y l 0 0015 97-4 0-6 _ _
d isu lp h id e 0-0034 96-0 2-0 0-6 0-9
0-0066 92-7 5-3 1-0 0-8
0-0072 90-7 7-3 1-4 * 1-0
0-0145 84-6 13-4 2-2 0-8
E le m e n ta ry 0-0004 97-7 0-3 _ _
s u lp h u r 0-0035 95-5 2-2 0-7 1-0
0-0140 89-5 8-2 1-6 0-6
T h io p h e n 0-0124 97-4 0-6 ' _
0-0248 96-1 1-3 0-6 0-01
0-0496 91-2 6-2 i-3 0-01
C arb o n 0-0124 97-0 1-0 0-2 0-008
d isu lp h id e 0-0248 96-7 1-3 0-3 0-006
0-0496 94-3 3-7 0-8 0-008
PH O SPH O RU S ON AVIATION F U E L PER FORM ANCE.
Ta b l e I I .
Lead Response o f F u el used in S u lp h u r A d d itio n Tests.
M .T .I.G . R .M .P .
(% B.A .M . 100).
4-8 9 8 0
• 4-0 95-0
3 0 87-5
Phosphorus and Phosphorus Compounds.
W hite phosphorus was weighed into th e clear fuel a t 0-1 per cent w t concentrations and dissolved after several hours’ shaking. This solution was used to prepare th e required blends, which were leaded ju s t prior to engine te st. M aster solutions were also prepared by weighing th e esters into th e clear fuel.
Engine Tests.
The rich m ixture perform ance ratings were determ ined by th e oxygen injection m ethod reported elsewhere 2 for th e sulphur com pound additions and by th e sta n d a rd 3-C m ethod for th e phosphorus additions. The excellent correlation of results obtained by th e tw o m ethods for different types of fuel m akes it highly probable th a t th e ir direct com parison is permissible.
The octane ratings were m ade under “ m otor m ethod ” conditions in th e C.F.R. engine.
The Effect of Added Compounds on Rich Mixture Performance of Leaded Fuels.
\ (a) Sulphur Compounds.
One obvious explanation for th e effect of sulphur com pounds on lead susceptibility would be an assum ed reaction of th e lead w ith th e sulphur, possibly (as suggested by J . G. R yan) in th e form of H2S. I f th is were so, th en th e perform ance num ber should be lowered th e sam e am ount by th e rem oval of th e chemically equivalent q u a n tity of te tra e th y l lead as b y th e addition of a sulphur com pound. I f it be assum ed th a t one ato m of lead is chemically equivalent to one atom of sulphur in these reactions^—
for example, w ith an end product of P b S 0 4—th e n in th e fuel used, 4-8 M .T.I.G. is chemically equivalent to 0-0241 per cent by w t sulphur. The rich m ixture perform ance num ber of th e fuel w ith varying am ounts of lead is shown in Table II.
I f th e results of Table I and Table I I are considered together, it will be no ted th a t for th e reactive types of sulphur com pound (m ercaptan, disulphide, elem entary sulphur) th e rich m ixture perform ance is lowered approxim ately nine num bers for an addition of 0-01 per cent S. On th e assum ption th a t th e depression of rich m ixture perform ance from addition of sulphur is due to reaction of th e sulphur w ith th e lead, th e lead /su lp h u r mol ratio reacted is shown in th e last column of Table I.
5 9 2 H A N S O N A N D C O L E S : T H E E F F E C T O F S U L P H U R A N D
The results are also shown graphically in Fig. 1 to illu strate th is p o in t by showing th e response to sulphur additions an d to lead su b tractio n s on chem ically e quivalent scales.
The results also su p p o rt th e view p u t by J . G. R y a n t h a t ease of pyrolysis of th e sulphur com pounds is relate d to th e ir effect on engine perform ance, as carbon disulphide an d th io p h en have only ab o u t one-tw elfth th e effect of th e o th er com pounds studied.
J . G. R y a n ’s in terp re ta tio n of his results has been developed recently 5 by A. Adirovich, who has shown t h a t th e relatio n betw een decrease in
9 6
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8 4 _________
O % SULPHUR ADDED O-OOS 0*01 0*015
4^0 3 3 M .T.I.C . 2
f F i g . 1.
THE EFFECT OF SULPHUR ADDITIONS ON RICH MIXTURE. THE SCALE FOR LEAD ADDITIONS IS DRAWN ON A CHEMICALLY EQUIVALENT SCALE ON THE ASSUMPTION THAT ONE ATOM OF LEAD REACTS WITH ONE ATOM OF SULPHUR. THE SULPHUR ADDITIONS ARE EACH MADE TO A FUEL CONTAINING 4 '8 M.T.I.G
lead susceptibility a n d sulphur c o n ten t can be derived from kinetic considerations.
I n order to com pare th e effect of th e ad d itio n of su lp h u r com pounds on th e rich m ix tu re perform ance a n d m otor m ethod octane ratin g s of leaded fuels, some of th e results presented by J . G. R y a n 1 have been recalculated (Table I I I ) to show th e decreases in octane num ber resulting from sulphur com pound additions as th e corresponding decreases in lead content. F rom th is rearrangem ent of th e results it can be seen th a t if th e m ajor effect of th e sulphur is to rem ove lead as an effective anti-knock agent, th e n only less th a n 40 per cent of th e chem ical equivalent of th e su lp h u r presen t is
P H O S P H O R U S O N A V I A T I O N F U E L P E R F O R M A N C E . 5 9 3
Ta b l e I I I .
T h e figures in th is ta b le a re c alc u la te d fro m th e re su lts p re se n te d b y J . G. R y a n 1 (T ab le I I I , p. 827). R e fo rm e d gasoline lead ed a t 3-0 cc/U .S . gal.
S u lp h u r co m p o u n d ad d ed .
% w t S.
A .S.T.M . o c ta n e no.
depression.
* E q u iv a le n t d ro p
in lead c o n te n t P b /S mol ra tio re ac te d . cc/U .S . gal. % w t P b .
T h io p h e n 0-053 1-2 0-6 0-2 0-06
0-292 3-8 1-75 0-63 0 0 3
n -P ro p y l su lp h id e 0-053 1-5 0-9 0-3 0 0 9
0-103 2 1 1-15 0-41 0-00
n -P ro p y l m e rc a p ta n 0 053 3-7 1-7 0-61 0-18
n -P ro p y l d isu lp h id e 0-053 3-7 1-7 0-61 0-18
0-103 4-7 2-0 0-72 0-11
T eri-b u ty l te tra s u lp h id e 0-033 » 5 - 8 2-25 0-81 0-38
* T hese figures a re ta k e n fro m a lea d s u s ce p tib ility g ra p h fo r th e fuel d ra w n to J . G. R y a n ’s figures.
rem oved, even for th e m ost active compounds. J . G. R y a n ’s results for disulphide additions have been confirmed in this laboratory.
Com parable figures are also obtained by calculations m ade on th e results presented by H . B ottom ley 3 for th e reduction of octane num ber by known sulphur com pound additions.
I f th e tw o sets of results for rich m ixture perform ance, and for octane ratings, be com pared, it is clear th a t sulphur com pounds have th e same general effect on leaded blends, h u t th a t in th e rich m ixture te sts th is effect is very m uch greater. The m ajor differences in engine conditions are in th e increased speed, m ixture strength, and inlet pressure for th e rich m ixture te st. The overall effect would be a higher tem p eratu re in th e m ixture ju st before com bustion, an d th is supports th e view th a t th e reaction of th e sulphur w ith th e lead is increased w ith tem perature, b u t th a t th e reaction is of th e same ty p e in b o th te sts and is n o t affected appreciably by th e g reatly increased m ixture strength in th e rich m ixture test.
(b) Phosphorus.
The general effect of white phosphorus on octane num ber an d on rich m ixture perform ance is evident from Figs. 2 and 3 respectively. These results can be considered in th e sam e m anner as those for th e effect of sulphur com pounds by expressing th e depression of ratin g num ber in term s of th e equivalent reduction of lead. These equivalents are shown in Tables V I an d V II, and from these it can be seen th a t th e m axim um P b /P mol ratio observed is for low concentrations (0-005 p er cent w t) of phosphorus, either as white phosphorus or as trib u ty l phosphate, an d th a t it agrees w ith th e equivalent ratio for form ation of P b3( P 04)2 (P b /P = 1-5).
The residts for th e rich m ixture perform ance of leaded fuels containing sm all am ounts of phosphorus indicate a very m uch sm aller reduction in ratin g th a n for th e octane rating of a corresponding blend. W hite phos
phorus also shows a sm all increase in perform ance w hen ad d ed in am o u n ts up to 0-006 per cen t w t. This unexpected resu lt is su p p o rted b y five sep arate ratin g s by th e 3-C m ethod, so th a t th e re seems little d o u b t th a t it is a real effect. The ad d itio n of phosphorus as an ester does n o t show th is increase in perform ance an d th e effect is n o t shown in th e clear blend w ith w hite phosphorus.
C alculation of th e P b /P ra tio shown in T able Y II indicates t h a t a t concentrations below 0-01 per cent P th e reaction of phosphorus w ith th e lead cannot be m ore th a n 30 per cent com plete a t m ost, a n d it th u s con
tra s ts sharply w ith octane ratin g s of corresponding blends, where the reaction is ap p a re n tly alm ost com plete.
5 9 4 H A N S O N A N D C O L E S : T H E E F F E C T O F S U L P H U R A N D
97f
1»
*OI * 0 2 *03 - 0 4 - 0 5 O -l ° /Q P H O S PH O R U S
Fi g. 2 .
No rigid conclusions can be draw n from this consideration of th e effects of sulphur an d phosphorus on th e engine perform ance of fuels under different te s t conditions, b u t certain general tendencies are clear.
I t appears highly probable t h a t u n d er a p p ro p ria te conditions either sulphur or phosphorus com pounds will react in th e com bustion cham ber w ith lead a t ratio s approaching th e th eo retical P b S 04 an d (P b )3( P 04)2 respectively. The reaction betw een sulphur an d lead is g reatly accelerated when going from weak to rich te s t conditions, b u t th e p h o sp h o ru s-lead reaction is reta rd e d w hen going from w eak to rich te s t conditions. P h o s
phorus itself can a c t as an anti-knock agent w ithin a narrow range of concentration, an d u n d er rich m ixture test'co n d itio n s.
P H O S P H O R U S O N A V I A T I O N F U E L P E R F O R M A N C E . 5 9 5
P E R C E N T P H O S P H O R U S Fi g. 3 .
Ta b l e IV .
E ffect o f Phosphorus on Octane N u m b er (M otor M ethod).
F u e l. L ead
M .T .I.G .
% P h o s p h o ru s b y w t.
F o rm in w hich a d d ed .
O ctane n u m b er (M.M.).
D ep res
sion o f o ctan e n u m b er.
A v ia tio n b len d w ith 60% aro- m atics
C lear C lear Clear C lear C lear
Nil 0-01 0 0 5 0-10 0-20
E le m e n ta ry (whiljg)
>> ii
yy yy
81-9 81-3 80-5 79-1 77-5
0 6 1-4 2-8 4-4 4^8
4-8 4-8 4-8 4-8 4-8 4-8
N il 0-0025 0-005 0-01 0-02 0-05 0-10
E le m e n ta ry (w hite)
yy a
yy a
yy a
yy ys
yy yy
96-6 94-9 93-5 92-0 82-2 81-1 79-7
1-7 3-1 4-6 14-4 15-5 16-9
2-5 N il — 91-8 —
A v ia tio n b len d — isoparaffins -f s tra ig h t-ru n co m p o n e n ts
4-8 4-8 4-8 3 0 1-5
N il 0-005 0-01
N il Nil
T rib u ty l p h o sp h a te
>> >>
yy a
97-2 95-1 92-4 94-5 91-5
2-1 4-8
Ta b l e V .
5 9 6 H A N S O N A N D C O L E S : T H E E F F E C T O F S U L P H U R A N D
E ffect o f P h osphorus on R ic h M ix tu r e P erform ance (% B .A .M . 100).
F u e l.
L ea d a d d itio n , M .T .I.G .
% P h o s p h o ru s b y w t.
P h o s p h o ru s co m p o u n d a d d e d .
R .M .P (% B .A .M .
100).
D e p re s sio n o f R .M .P . A v ia tio n b le n d — 4-8 0-001 E le m e n ta ry (w hite) 99-0 - 2 - 0
i.soparaffin + 4-8 0 0015 99 99 99-0 —2-0
s tr a ig h t- r u n c o m 4-8 0-002 99 99 98-9 - 1 - 9
p o n e n ts 4-8 0-005 99 99 97-5 —0-5
4-8 0-01 99 99 95-8 1-2
4-8 0-015 99 99 82-9 14-1
4-8 0-001 T r ib u ty l p h o s p h a te
99 99
97-0 N il
4-8 0-0015 96-1 0-9
4-8 0-006 99 99 94-2 2-8
4-8 0-015 99 99 91-1 5-9
4-8 N il — 97-0,
96-9 N il
3-0 99 -- 86-6 10-4
1-5 99 --- • 75-5 21-5
B len d 2 4-8 N il E le m e n ta r y (w hite) 99-3 _
4-8 0-0025 99 99 100-2 - 0 - 9
S4 referen ce fuel C lear N il _ 83-0 _
99 0-002 E le m e n ta r y (w hite) 82-5 0-5
99 0-01 99 99 81-1 1-9
Ta b l e V I.
L ea d reacted w ith P hosp h o ru s i n Octane R a tin g s (M otor M ethod) (Calculated fr o m Table I V ) .
A d d ed c o m p o u n d .
°//o P h o s p h o ru s
in blen d .
M .T .I.G . e q u iv a le n t to o c ta n e no.
dep ressio n .
P b / P m o l r a tio
re a c te d .
E le m e n ta ry (w hite) 0-005 1-6 1-6
0-01 2-3 1-1
0-02 4-8 1-2
T rib u ty l p h o s p h a te 0-005 1-6 1-6
0-01 2-7 1-3
Ta b l e V II.
L ea d reacted w ith P hosphorus i n R .M .P . (% B .A .M . 100 R a tin g s) (Calculated fr o m Table V ).
A d d ed co m p o u n d .
°//o P h o s p h o ru s
in b len d .
M .T .I.G . e q u iv a le n t
to R .M .P . d e p ressio n .
P b / P m o l r a tio
r e a c te d .
E le m e n ta r y (w hite) 0-01 0-4 0-2
0-015 2-3 0-8
T r ib u ty l p h o s p h a te 0-0015 0-2 0-6
0-006 0-5 0-4
0-015 1-0 0-3
P H O S P H O R U S O N A V I A T I O N F U E L P E R F O R M A N C E . 5 9 7
Ac k n o w l e d g m e n t s .
The au th o rs wish to th a n k th e Directors of T rinidad Leaseholds L td . for perm ission to publish th is rep o rt of experim ental w ork carried o u t in th e C entral L aboratory, K ing’s Langley.
References.
1 “ In flu en ce o f S u lp h u r C om pounds o n O ctan e N u m b e r a n d L e a d S u s ce p tib ility o f G asoline.” J . G. R y a n . in d u str. Engng Chem., 1942, 34, 824.
* “ T h e O x y g en B o o st M e th o d o f R a tin g th e E n g in e P e rfo rm an c e o f A v ia tio n F u e ls .”
W . B . H e a to n , T . K . H a n so n a n d R . C. M orris. J . In st. P etrol.,1947, 33, 256.
3 “ S u lp h u r C om pounds in P e tro le u m a n d its D istilla te s.” Oil and Gas J ., 23.10.41, 40 (24), 37.
4 “ O ctan e N u m b e r a n d L e a d S u s ce p tib ility o f G asoline— E ffec t o f O rganic C hlorine a n d S u lp h u r.” C. H o llerw ay , J r . , a n d W . S. B o n n ell. In du str. Engng Chem., 1945, 37, 1089.
5 “ M echanism o f th e In flu en c e o f S u lp h u r C om pounds o n th e K n o c k V alu e o f E th y la te d G asoline.” A . A d iro v ich . A cta Physicochemica U .S .S .R ., 1946, 21, 283. (E n g lish T ra n s la tio n : U .O .P . Co. S u rv ey o f F o re ig n P e tro le u m L ite ra tu re , S ep tem b er 6—13, 1946.)
5 9 8
AUTOMATIC CONTROL OF R EFIN ER Y OPERATIONS.
B y W . S. Au l t, B.A. (Fellow).
Sy n o p s i s.
I n s tr u m e n ts fo r th e a u to m a tic c o n tro l o f re fin e ry o p e ra tio n s a re co n sid ered fro m th e v ie w p o in t o f th e u ser. T h e ir d esig n o r h o w th e y o p e ra te is n o t . co n sid ered , b u t stre ss is la id o n th e c a p a b iltie s o f th e in s tru m e n ts a n d th e m ea n s b y w h ich th e y c a n b e m a d e to giv e th e b e s t re su lts .
Fo r th e best to be got o u t of an y in stru m e n ta tio n it is desirable th a t th ere should be very close co-operation betw een th e in stru m e n t engineer and all levels of operating personnel. A lthough it is essential to avoid any unskilled interference on th e p a rt o f th e o perating personnel w ith control instrum ents, it is equally essential, if th e y are to co-operate intelligently w ith th e in stru m e n t d e p a rtm e n t, for th em to have a certain minimum knowledge of th e th eo ry of au to m atic control. This p ap er attem p ts to review th is necessary m inim um of th eo ry , an d also considers certain exam ples of in stru m en ta tio n w hich are com m on to m an y refinery plants, tre a tin g th em from th e view point of th e p la n t o p erato r ra th e r than the in stru m en t engineer.
The extensive use of au to m atic control is essentially a result of the developm ent of continuous processes in petroleum refining, as compared w ith th e b atch processes originally em ployed. I t is perfectly possible to control a b atch still w ith th e assistance of only tw o or th re e therm om eters a n d to produce m aterials m eeting close specifications. M odern continuous u nits, however, present problem s in control which are essentially dynamic ra th e r th a n static, an d can only be solved by ad vanced ty p es of autom atic control instrum ents. I t is no exaggeration to say t h a t th e m odern large continuous u n it could n o t have been developed w ith o u t th e parallel develop
m e n t of th e present control instrum ents.
Before proceeding fu rth er, it is necessary to define certain variables inherent in th e design an d settings of all control in stru m en ts, an d which directly affect th e ir perform ance.
Load.
The load on a controller m ay be defined as th e q u a n tity o f m aterial which has to pass th ro u g h th e control valve in order to m a in ta in th e system a t th e required control pojnt. This definition o f load should be understood as independent of th e ty p e of controller, flow, te m p e ra tu re, liquid level, etc, which is concerned.
Throttling Range.
M ay be defined generally as th e p roportional change in control valve position corresponding to a given p roportional d ev iatio n o f th e controlled variable from th e required control point. A pplying th is to air-pperated controllers, w ith which ty p e alone th is pap er deals, th ro ttlin g range m ay be defined as th e change in a ir pressure to th e control valve corresponding
A U L T : A U T O M A T I C C O N T R O L O F R E F I N E R Y O P E R A T I O N S . 5 9 9
to a given d ep artu re of th e indicating or recording pen from th e control point. To sim plify discussion, th e pen connected to th e m easuring device, w hether for flow, tem p eratu re, or pressure, etc., will be referred to th ro u g h ou t as th e recording pen, irrespective of w hether th e in stru m en t is of the recording or m erely indicating t y p e ; and th e pen attach ed to th e m echanism by which th e control point is set, as th e index pen.
T h ro ttlin g *range is frequently expressed as a percentage of th e full-scale reading of th e in stru m en t. This is illu strated in Fig. 1, for pressure- controllers set to 10 per cent an d 50 per cent th ro ttlin g ranges.
100
a-. C O N T R O L V A L V E FU LLY OPEN b :C O N T R O L V A L V E SHUT
10% T H R O T T L I N G RANGE ' 5 0 % T H R O T T L I N G R A N G E
FlO. 1.
Droop.
The term droop will be used to describe th e perm anent d ep artu re from th e control po in t brought ab o u t by a change in load, when using instrum ents of the simple th ro ttlin g ty p e (Fig. 2). I t will be evident th a t th e degree of droop will be dependent on th e e x te n t of th e load change, and also on the th ro ttlin g range to which th e in stru m en t is set. The recording pen can
C O N T R O L P O I N T 2
C O N T R O L P O I N T I A N D I N D E X S E T T Î N G I
I N D E X S E T T I N G 2 Fi g. 2.
be b rought back to th e original control point, and th e load change com pen
sated, by m anual resetting of th e index pen, b u t th is will n o t p rev en t a fu rth er d ep artu re from th e control po in t if a fresh change in load occurs.
In certain processes load changes are infrequent, and it is therefore possible to use simple th ro ttlin g controllers and reset th em by h an d as necessary.
In th e petroleum in d u stry , however, departures from th e control p o in t calling for m anual resetting of th e in stru m en t cannot generally be tolerated, and it is necessary to provide controllers w ith au to m atic reset.
Automatic Reset.
F o r th e present purpose th is can be regarded sim ply as an autom atic m eans of doing w hat th e operator does w hen he resets th e index pen of a
6 0 0 A U L T : A U T O M A T I C C O N T R O L O F R E F I N E R Y O P E R A T I O N S .
controller after a load change. There is no need to consider th e m echanism by w hich th is resettin g is effected. I t should, how ever, be understood th a t it is capable of re-ad ju stin g th e air pressure to th e control valve to the lim it in either direction in order to restore th e recording pen to th e control point. T he ra te a t w hich th is re-ad ju stm en t is effected can be modified by altering th e settin g of th e reset m echanism .
Au t o m a t i c Co n t r o l l e r s.
A utom atic controllers can be divided in to th ree m ain groups, in so far as th eir operation is concerned. T he first o f these is o n-off controllers.
On-off Controllers.
I n th is group th ere are only tw o possible positions for th e control valve, fully open or fully sh u t, dep en d en t on w hether th e variable under control is abpve or below th e control p o in t, or vice versa. C ontrollers of th is type can only be applied successfully w hen th e re is considerable storage capacity for th e variable under control, an d find v irtu ally no applications in continu
ous refining processes, where in m ost cases storage cap acity is very small.
The second group is th ro ttlin g controllers, w ith variable th ro ttlin g range.
Throttling Controllers.
In stru m e n ts of th is group are so designed th a t th e control valve is held in a position interm ediate betw een fully open an d com pletely sh u t, th e exact position being d ependent on th e position o f th e recording pen relative to th e index pen. T hey can be used successfully in applications where appreci
able p erm an en t d ep artu re from th e original control p o in t w ith change in load can be to lerated . Such applications are, how ever, com paratively rare in petroleum refining, an d m ost o f th e in stru m en ts used in modern p la n t fall in to th e th ird group, th ro ttlin g controllers w ith au to m atic reset.
Throttling Controllers with Automatic Reset.
These in stru m en ts combine th ro ttlin g control, hav in g th e w idth of th ro ttlin g range ad ju stab le over wide lim its, w ith a u to m a tic com pensation for load changes th ro u g h th e au to m atic reset m echanism . I n th e ir stan d ard form th e y are capable of dealing w ith all norm al process lags, an d are the ty p e inv ariab ly chosen for' an y critical control service.
In s t r u m e n t a t i o n i n Pr a c t i c e.
H aving outlined such th eo ry of au to m atic controllers as concerns th e p la n t operator, and divided th e range of in stru m en ts available in to three m ain groups, we will now consider how th e a c tu a l user of such in stru m en ts, th e p la n t operator, can assist th e in stru m e n t engineer. H e can do so m aterially b o th a t th e design stage of a new p la n t, an d also during its subsequent operation. The d eterm ination from th e flow-sheet of those portions of th e p la n t which are inh eren tly stable will avoid th e in stallatio n of in stru m en ts which subsequently prove to be unnecessary. T he in s tru m en tatio n scheme should be studied for possible unfavourable reactions
A U L T : A U T O M A T I C C O N T R O L O F R E F I N E R Y O P E R A T I O N S 6 0 1
of one in stru m en t upon another, or upon another p a rt of the plant. Such reactions will probably have been anticipated by th e instrum ent engineer, b u t th e p la n t operator should carefully check th e proposed instrum entation from his own view point. Special a tte n tio n should be paid to th e action chosen for each in stru m en t on air failure, n o t only as regards the im m ediate effect on th e variable controlled by th e p articu lar instrum ent, b u t also as regards th e effect on th e p la n t as a whole. Control valves will have been sized by th e in stru m en t engineer in accordance w ith th e quantities shown on th e design flow-sheet for norm al operation. N orm ally th e operating range of th e m odern control valve is sufficient to tak e care of th e variations in load th a t will be encountered, an d leave adequate m argin for unavoidable errors in th e initial sizing. The p la n t operator can, however, m aterially assist th e in stru m en t engineer by draw ing his a tte n tio n to any valves iii which under-sizing, or alternatively over-sizing, will prove particularly troublesom e.
M odern practice is to group th e m ajo rity of th e control instrum ents in a central control room , an d this arrangem ent has m any obvious advantages.
H owever, in th e case of flow an d pressure control and m easuring instrum ents, it entails th e use of long in stru m en t lines from th e orifice or pressure gapping point. This n o t only m akes for sluggish response of th e instrum ents, b u t also increases th e risk of fire in th e control room in case of leakage from in stru m en t lines which m ay be under high pressure. B oth these disad v an tages can be avoided by th e use of rem ote recording instrum ents using electric or pneum atic transm ission from th e m easuring elem ents located out on th e p lan t. Such instrum ents are necessarily more expensive th a n the norm al type, b u t against th e increased cost should be offset th e saving in not having to ru n long in stru m en t lines, th e b etter response obtained in the case of flow instrum ents, and th e fact th a t th e control room need no longer be regarded as a hazardous area. I t would appear th a t th e rem ote recording ty p e of in stru m e n t m ight be more widely employed th a n it is a t present. W ith th e foregoing in m ind, th e p lan t operator can m aterially assist th e in stru m en t engineer by indicating those instrum ents which m ust be located in th e control room, and those which can equally conveniently be placed out on th e p lan t w ith a considerable saving in cost.
As regards plants which are in operation, th e user of control instrum ents—
th a t is, th e p la n t opera^pr— can greatly assist th e in stru m en t d ep artm en t in their w ork by inform ed co-operation a t all levels. To this end, it seems desirable th a t all plant-opersfting personnel down to th e sta tu s of charge- hand should have some instruction in th e elem entary th eo ry and practice of autom atic control. Too often reports of faults are very laconic, and there is an im pression am ongst th e operating personnel th a t th ey can m ake no useful contribution to th e work of th e instrum ent departm ent. The rep o rt o f a fa u lt should include a t least a precise statem en t of th e sym ptom s, exactly when th e trouble occurred, and w hether its onset coincided w ith any know n change in operating procedure or th e m aterial being processed.
A ny steps tak en to overcome th e trouble should be given in detail. The p lan t charge-hand and th e in stru m en t m echanic sent to rectify th e fault m ust collaborate closely if th e fa u lt is to be rectified w ith m inim um delay, a n d th e in stru m en t p u t back into operation w ith th e least disturbance to th e p lan t. The p lan t charge-hand will be considerably assisted in this if
6 0 2 A U L T : A U T O M A T I C C O N T R O L O F R E F I N E R Y O P E R A T I O N S .
he has sufficient theoretical knowledge to u n d ersta n d th e steps t h a t th e in stru m e n t m echanic is tak in g , a t a n y ra te in principle.
As far as recording in stru m en ts a re concerned, an d m ost im p o rta n t control in stru m en ts in a refinery p la n t are of th is ty p e, certain common fau lts can be readily diagnosed from a stu d y o f th e trac e produced on th e ch art. I t m ay be of in tere st to consider these in some detail.
Hunting.
True hu n tin g m u st be cyclic (Fig. 3), w ith a regular periodicity. I t m ay, however, s ta r t a t a considerable a m plitude an d th e n gradually reduce its am plitude to zero, afte r which satisfacto ry control is obtained un til some fresh disturbance to th e system re sta rts th e h u n ting. I n general, h u n tin g is due to process lags, a n d th e rem edy is th e use of a w ider th ro ttlin g range. I n itself a w ider th ro ttlin g range will increase th e tendency
C O N T R O L POINT
A L T E R N A T I V E L Y
CO N TR O L PO IN T
Fi g. 3.
for droop, or d e p artu re from th e control p o in t, on changes in load.
N orm ally, th e droop will be corrected by th e a u to m atic reset mechanism, b u t in order to reduce th e am o u n t of w ork w hich th e la tte r has to do, it is undesirable to em ploy a th ro ttlin g range m uch w ider th a n is strictly necessary to cope w ith th e process lags. I f a lte ra tio n to th e th ro ttlin g range fails to m ake an y appreciable im pression on th e h u n tin g , it is almost certain th a t it is a reflection of h u n tin g in some o th er in stru m en t, and the cure m u st be looked for in locating an d resettin g th e in stru m e n t concerned.
F o r exam ple, ap p a re n t h u n tin g of th e pressure controller on a distillation colum n m ay actually be due to h u n tin g of th e to p tem p e ra tu re controller, which is causing a cyclic v a riatio n in th e am o u n t of reflux.
Droop.
This has been described already as characteristic of th ro ttlin g controllers n o t provided w ith au to m a tic reset. Assum ing t h a t th e in stru m e n t con
cerned is provided w ith a u to m atic reset, an d t h a t th e case is n o t one of the
Fi g. 4.
incorrect application of a simple th ro ttlin g controller, th is fa u lt can only be due to th e au to m atic reset m echanism becom ing inoperative (Fig. 4).
Wandering.
As will be seen (Fig. 5) th is consists of a som ew hat irregular d e p a rtu re from th e control p o in t for an appreciable period, followed b y a g rad u al
re tu rn to it, w ith possible overshooting and fu rth er wandering on th e other side of th e control point. This fa u lt is norm ally only m et w ith in applica
tions involving considerable process lags, and is due to th e use of too wide a th ro ttlin g range setting, or too slow operation of th e autom atic reset.
A U L T : A U T O M A T I C C O N T R O L O F R E F I N E R Y O P E R A T I O N S . b l M
Fi g. 5.
The rem edy consists in narrow ing th e th ro ttlin g range, or in speeding up th e operation of th e au to m atic reset m echanism by cutting o u t some of th e p n eu m atic resistance.
Sticking of Control Valve.
This fa u lt m ay be im m ediately recognized by th e characteristic “ stepped ” trace on th e c h art (Fig. 6), an d th e rem edy is to reduce th e friction in th e control valve. In some cases, such as high pressure an d high tem perature
Fi g. 6.
operation where th ere is bound to be appreciable friction in th e valve gland, it m ay be essential to em ploy a valve positioner. The m anner in which this is connected is shown diagram m atically in Fig. 7. The device consists essentially of a pressure relay supphed w ith air a t th e sta n d ard working
Fi g. 7.
pressure, usually 17 p.s.i., from an independent source. I ts operation can best be illu strated b y an exam ple. Supposing th e controller increases th e air pressure on th e o u tp u t side from 8 p.s.i. to 9 p.s.i. The control valve should th e n ta k e up a position appropriate to 9 p.s.i. If, however, there is appreciable friction in th e valve system , th e valve m ay only move to a position app ro p riate to 8| p.s.i. The linkage betw een th e valve spindle and th e positioner m echanism will indicate to th e la tte r th a t th e spindle has n o t m oved th e correct am ount, whereupon th e positioner will apply an increasing air pressure to th e diaphragm of th e valve u ntil th e la tte r does eventually m ove. If, as is probable, th e valve overshoots th e correct
position, th e positioner will au to m atically reduce th e d iaphragm pressure, u n til by a series of successive overshootings th e valve spindle is b ro u g h t to re st in th e correct position. To- ob tain th e best perform ance from any controller, th e use of valve positioners is desirable in all cases w here there is th e possibility of considerable friction in th e valve gland, also w here the distance betw een th e control in stru m e n t a n d th e control valve is great.
The co n trib u tio n w hich th e operating personnel can m ake to th e im proved operation of th e control in stru m en ts of an y p la n t will largely consist of the correct and careful reporting o f fau lts, carried to its u ltim a te conclusion, so th a t eventually no fa u lt rem ain to be elim inated. F o r th is to be acheived th e y m u st n o t only have a high s ta n d a rd of perform ance to aim a t but m u st also realize th e lim itations of th e ir control in stru m en ts, as well as having a clear idea o f w h a t irregularities in th e various factors under m easurem ent or control can ju stifiab ly be laid a t th e door of th e control in stru m en ts, an d w h at are in h eren t in th e o peration of th e process and the design of th e p lan t. F o r exam ple, w here th e to p tem p e ra tu re of a distil
latio n colum n is controlled by varying th e q u a n tity of reflux returned, and a t th e sam e tim e a flo'w-recorder is provided to m easure th e q u an tity of reflux, th e p la n t operator m ay reasonably expect a sm ooth trace on the Chart from th e tem p e ra tu re controller, b u t cannot a t th e sam e tim e expect a sm ooth c h a rt from th e reflux flow-recorder.
A p o in t w hich m ay easily be overlooked w hen a p la n t has been in opera
tio n for a considerable period is th e correctness of control valve sizes. I t is qu ite freq u en tly found necessary to o p erate w ith th e by-pass round a control valve cracked o p e n ; alte rn a tiv e ly it m a y be necessary to throttle one of th e isolating valves on one side of a control valve to lim it th e m axi
m um q u a n tity passed by th e la tte r. I n n eith er of these cases, in general, can th e controller give its b est perform ance, a n d th e a tte n tio n of th e in stru m en t d e p a rtm e n t should be draw n to th e fact, so t h a t th e control valve m ay be exchanged for one of th e correct size. T he only case in which it m ay be necessary to re ta in th e p artia lly cracked by-pass is one in which th e variations in load are too g re at to be ta k e n care o f by th e range of the norm al control valve. E ven in such a case, th e preferable course is to install tw o control valves in parallel, of different si^es w hich betw een them will cover th e load range required, a n d to select th e control valve appro
p ria te in size to th e operating conditions a t th e tim e.
To illu strate th e principles d ealt w ith in th e foregoing, th e in stru m e n ta tio n of a simple distillation u n it will be considered. A simplified flow-sheet is show n in Fig. 8, and it is assum ed t h a t th e u n it charges a reduced crude and produces gas oil as overhead, lubricating oil d istillates from tw o side stream s, an d b itu m en as bottom s. I n th is case th e colum n will operate u n d er vacuum , an d it will be necessary to pum p aw ay all products. For th e sake of sim plicity all h e a t exchangers an d p ro d u c t coolers h av e been o m itted from th e flow-sheet, since th e y will hav e no bearing on the in stru m e n ta tio n .
The feed to th e p la n t will be controlled by th e recording flow-controller F .R .C .l. W e have assum ed t h a t th e feed p u m p will be of th e ro ta ry positive-displacem ent ty p e, consequently th e control valve has been placed in a by-pass betw een th e delivery a n d suction of th e pum p. F o r sm ooth operation of th e p la n t it will be essential th a t th e ra te of feed be k ep t
6 0 4 A U L T : A U T O M A T I C C O N T R O L O F R E F I N E R Y O P E R A T I O N S .
constant, independent of load changes brought ab o u t by changes in te m p era tu re or viscosity of th e feed. Consequently, controller F .R .C .l m ust be a fu ll-th ro ttlin g controller equipped w ith au tom atic reset.
The furnace ou tlet tem p eratu re will be a critical factor in th e operation
* of th e p lan t, since th e penetration of th e bitum en produced will largely depend on th is tem p eratu re. There will be a considerable process lag in th e passage of th e feed through th e furnace, an d ,th ere m ay also be ap p re
ciable load changes. The o u tlet tem p eratu re controller m u st therefore be of th e full-throttling, autom atic reset type. I t has been assum ed th a t th e h eater is oil fired, using pressure-atom izing burners. Disregarding for th e present th e special relay shown betw een th e tem p eratu re controller
A U L T : A U T O M A T I C C O N T R O L O F R E F I N E R Y O P E R A T I O N S . 6 0 5
and the pressure controller, th e tem p eratu re controller is arranged to reset pneum atically th e control p o in t of th e indicating pressure-controller I.P .C .l. This pressure-controller in its tu rn regulates th e control valve in the retu rn fuel oil line from th e burners to m aintain th e required control pressure a t th e burners. W ith this ty p e of burner th e q u a n tity o f fuel passed, and consequently th e h e at in p u t to th e furnace, is proportional to th e fuel oil pressure a t th e burners. The object of using a pneum atically reset pressure-controller instead of connecting th e o u tp u t side of th e tem - perature-controller directly to th e control valve, is to enable incidental variations in th e tem p eratu re or viscosity of th e fuel oil, or in th e o u tp u t of th e fuel pum p, which would affect th e burner pressure, to be im m ediately corrected by th e pressure controller before th e y have h a d tim e to affect th e o u tlet tem perature. The pneum atic setting device on th e pressure-
u u
controller, w hich shold n o t be confused w ith th e norm al a u to m atic reset m echanism , consists sim ply o f a bellows connected to th e o u tp u t side of th e te m p era tu re controller an d linked to th e norm al settin g arran g em en t of th e pressure-controller so t h a t th e control p o in t of th e la tte r can be set over th e required range by variations in th e o u tp u t pressure from th e * te m p eratu re controller. W ith rise in te m p eratu re th e control p o in t of th e pressure-controller is lowered, an d vice versa.
W ith m ost refinery h eaters th e process lags are such th a t even th e m ost advanced ty p e of te m p eratu re controller cannot deal w ith th em w ithout occasional d ep artu res from th e control p o in t for a longer period th a n can be to lerated . I n such cases it is necessary to em ploy a special relay between th e tem perature-controller an d its controlled valve, or in th is case between th e tem perature-controller a n d th e pressure-controller. This special relay has th e pow er of analysing th e ra te a t w hich th e tem p e ra tu re is departing from th e control p o in t before a n y serious d ep artu re has ta k e n place. I t th e n passes on to th e control valve, or in th is case to th e pressure-controller, a g reater change in air pressure th a n th e change w hich has been fed into it, th e degree of m agnification being p roportional to th e ra te of change. This m agnification, which can be a d ju sted by varying th e settings o f th e relay m echanism , applies a drastic correction for ra p id rates o f change, and effectively overcomes th e process lags.
The to p tem p eratu re of th e colum n will require careful control, and this has been arranged b y recording tem p eratu re-co n tro ller R .T.C.2, which should preferably be of th e p o ten tio m eter ty p e , a n d m u st in an y case be equipped w ith full th ro ttlin g range a d ju stm e n t a n d au to m atic reset. The use of th e p otentiom eter ty p e in preference to th e th erm o m eter type is advisable b o th to give a quicker response a n d also to avoid th e use of long a n d vulnerable capilhary connexions betw een th e tem p e ra tu re point in th e v ap o u r line a n d th e in stru m en t, w hich will be lo cated in th e control room . Since th e control valve will pro b ab ly be located some considerable distance from th e in stru m en t, it will be advisable to equip it w ith a valve positioner.
P a r t of th e gas oil overhead p ro d u ct will be re tu rn e d to th e column as reflux, under th e control of R.T.C.2 as alread y m entioned, a n d p a r t will have to be d iv erted to storage. The la tte r q u a n tity is controlled by liquid level controller L .L .C .l, of th e ex te rn al float-cage ty p e , w hich regulates th e control valve shown on th e line to storage. A lthough th is level con
troller m u st be of th e fu ll-th ro ttlin g ty p e , v ariatio n s in th e e x act level held in th e accum ulator ta n k w ith variations in load will n o t m a tte r, and th e in stru m e n t need n o t be equipped w ith a u to m atic reset. F o r the correct functioning of R .T .C.2 a n d L .L .C .l it is ev id en t t h a t th e pressure u p stream of th eir respective control valves m u st be held as c o n stan t as possible. This is achieved by th e d irect-acting pressure controller P .C .l, w hich regulates th e steam supply to th e gas oil pum p.
As regards sidestream control, th e sim plest a n d generally th e m ost satisfacto ry arrangem ent is to w ithdraw th e sidestream a t a constant ra te , which is re-ad ju sted only w hen a change in th e specification of th e sidestream is required. U sually th ere is a c o n stan t head betw een th e draw -off tr a y level an d th e sidestream strip p er, a n d all t h a t is req u ired is th e provision of a rem ote operated valve, as shown, w hich can be m anually
6 0 6 A U L T : A U T O M A T I C C O N T E O L O F R E F I N E R Y O P E R A T I O N S .
a u x t : A u t o m a t i c c o n t r o l o f r e f i n e r y o p e r a t i o n s . 6 0 7
set from th e control room. I t will be seen from th e flow-sheet th a t rem ote operated valve R .O .l regulates th e first sidestream , and R .0 .2 th e second sidestream . The corresponding flow recorders, F .R .l an d F .R .2, will be located in th e control room, w ith their orifices installed in th e lines to storage.
L iquid level controllers L.L.C.2 and L.L.C.3 regulate th e bottom level in th e tw o sidestream strippers by control of th e steam supply to th e corresponding pum ps. U sually variation in level in th e strippers over th e range of th e norm al outside float-cage level controller, which is th e ty p e th a t will m ost probably be em ployed, can be to lerated consequent on changes in load. If, however, th e design of th e strip p er is such th a t the level m ust be held very closely to th e same control point, irrespective of changes in load, th en full-throttling controllers fitted w ith autom atic reset m ust be used for th is service. I n order to o b tain a readable chart on flow-recorders F .R .l an d F .R .2 it is essential th a t there should be Hfo hunting of level controllers L.L.C.2 and L.L.C.3. Since th e w idth of th ro ttlin g range th a t can be em ployed in any in stru m en t w ithout autom atic reset is lim ited, it is preferable th a t these level controllers should in any case be provided w ith autom atic reset.
The bottom level in th e m ain column has been shown as controlled by liquid level controller L.L.C.4, of in tern al float ty p e, regulating a control valve in a by-pass betw een th e delivery and suction o f th e positive dis
placem ent ro ta ry bottom s pum p. To reduce to th e m inim um variations in th e bottom level, w ith variations in load, which m ay be considerable, L.L.C.4 should be fitted w ith au to m atic reset mechanism . Since th e control valve will be handling bitum en, w ith th e strong probability of th e valve sticking on occasions, it should be equipped w ith a valve positioner.
Even w ith th e simple exam ple of refinery p la n t which has been chosen as an exam ple th ere are several alterativ e instrum entations which could be p u t fore w ard as w orkable propositions. The one illustrated should not be regarded as th e only one, or necessarily th e best, b u t m erely as a typical example.
6 0 8
THE FLUID SOLIDS TECHNIQUE: APPLICATIONS IN THE PETROLEUM INDUSTRY.*
B y E . V . M u r p h r e e , ! E J . G o rn q f a n d . A. F . K a u l a k i s . |
A n ew chem ical engineering technique involving th e use of finely-divided solids is receiving w idespread application in th e petroleum in d u stry , and its use in o th er in d u strial fields, such as th e chem ical, coal processing, and m etallurgical, is under active consideration. This technique, a n outgrow th of th e S ta n d a rd Oil D evelopm ent C om pany’s w ork in developing th e widely accepted fluid cataly tic cracking process, p o te n tially has application to an y process in w hich large q u an tities of h e a t are tran sferred , or in which
\fèry in tim a te co n tact betw een gases a n d solids is desired. T he petroleum in d u s try has alread y applied th e fluid solids technique on either a com
mercial or sem i-com m ercial scale to b o th cataly tic a n d non-catalytic fuel production processes. I t is th e object of th is p ap er to explain th e hasic elem ents of th e F lu id Solids Technique, an d to describe th ree specific applications o f th is developm ent in th e petroleum field, nam ely, the cataly tic processing o f gas oils, th e ex tra c tio n of liquid fuels from shales, an d th e synthesis of th e higher hydrocarbon hom ologues from natural gas.
Ba s i c El e m e n t s o f t h e Fl u i d So l i d s Te c h n i q u e.
F u n d am en tally , th e fluid solids technique represents a useful application of tw o basic properties or characteristics of finely-divided solids. These properties m ay be described sim ply as follows :
(1) P roperly sized solids, w hen m ixed w ith a gas, will form a hom o
geneous solids-gas m ix tu re or “ fluid,” th is “ fluid ” having flow properties sim ilar to o rd in ary liquids b u t its den sity being subject to v a riatio n th ro u g h sim ple m an ip u latio n of eith er th e solids to gas ratio or th e flowing conditions.
(2) A pow dered solid, w hen suspended in a gas stream flowing upw ards a t relatively low velocities, null form , c o n trary to Stokes’
law, a continuous dense phase which in m an y aspects resem bles a boiling liquid a n d w hich assum es a relativ ely well defined level.
T he first of these properties m akes possible th e circulation of vast qu an tities of solids w ith o u t benefit of m echanical devices, such as pum ps, elevators, etc, an d it also allows th e use of m ore or less conventionally designed 'equipm ent, such as pipes, valves, a n d exchangers, to handle and to control th e solids. The second p ro p erty provides m eans for carrying o u t such functions as h e a t tran sfer u n d er excellent conditions, a n d also for providing process reaction tim e requirem ents in reasonably sized equipm ent.
* P a p e r re ad b efore 11th I n te r n a tio n a l C ongress o f P u r e a n d A p p lied C h e m istry , L o n d o n , J u l y 17-24, 1947.
f S ta n d a rd Oil D e v elo p m en t C o m p an y .
F L U I D S O L I D S T E C H N I Q U E . 6 0 9 *
The fluid solids technique as applied to m anufacturing processes consists essentially of either one or both of tw o basic elem ents, nam ely, a circulating system and a zone in which a concentrated or “ dense phase ” of solid is b uilt up or m aintained for th e basic purpose of carrying out a desired reaction. An illustration of a simple com bination of these tw o elem ents is given in Fig. 1.
Circulation of Solids.
The circulation system consists of a standpipe in which th e sta tic head of th e flowing solid itself builds up th e pressure requisite for its circulation, and a carrier line which contains a gas-flowing stream into which th e solids
Fi g. 1.
BASIC ELEMENTS OF THE FLUID SOLIDS TECHNIQUE.
are dispersed an d are carried to a reaction zone. The fluid solids in th e standpipe are m aintained in a relatively high density sta te in order t h a t reasonable build-up of static pressure per u n it length of standpipe will be obtained. However, th e solids m ixture, being compressible, can a tta in excessively high densities such th a t th e m ixture loses its “ fluid ” charac
teristics; gas is accordingly added to th e solids m ixture a t points along th e length of th e standpipe to m aintain its density in a “ fluid ” range.
Control of cataly st flow in th e standpipe is m aintained b y m eans of a modified gate ty p e valve located a t th e base of th e standpipe. P ressure having been built up in this m anner, th e solids m ixture can th e n be injected into a suitable gas stream for conveyance to an y desired p o int, norm ally a reaction zone. The density of th e solids-gas m ixture in th e carrier line is m aintained low in order to facilitate circulation of th e solids. Low density m ixture is obtained sim ply b y operating a t relatively high gas velocities an d m aintaining a low solids/gas ratio.
