- Email: [email protected]
Delayed coking: Industrial and laboratory aspects
Carbon Vol. 36, Nos. 1, pp. 105~116,1998 0 1997ElsevierScienceLtd Printed in Great Britain. All rights reserved OOOS-6223/97 $17.00+ 0.00
Pergamon PII: SOOOS-6223(97)00154-l
DELAYED
COKING: INDUSTRIAL ASPECTS
F. RODRIGUEZ-REINOSO,~,*
AND LABORATORY
P. SANTANA,~ E. ROMERO PALAZON,~ M.-A.
DIEZ~ and
H. MARSHA “Department of Inorganic Chemistry, University of Alicante, 03080 Alicante, Spain bREPSOLpPETROLEO, Centro de Investigacibn de Cartagena, Cartagena, Spain ‘Institute National de1 Carbon, La Corredoria, Apartado 73, 33080 Oviedo, Spain (Received 13 March 1997; accepted in revisedform
21 July 1997)
Abstract-Delayed
coking is a thermal process to convert petroleum residues to a solid coke material. Processes occurring in a delayed coker are complicated and attempts have been made, at the laboratory level, to simulate industrial delayed coking. Although the latter studies are useful, it is impossible to scale-down to the laboratory level. Industrial delayed coking is a turbulent process and such movements cannot be simulated easily in the laboratory. Of industrial importance are the multiphase systems, i.e. volumes of unreacted isotropic pitch residue, transported through the bulk, fluid anisotropic mesophase, so creating ordering into acicular structures in the vicinity of the multiphase systems. Four petroleum residues were analysed chemically. Pyrolyses were carried out under pressures of up to 1.0 MPa. Complete mass balances were obtained and the semicokes examined by optical microscopy. Feedstocks for delayed cokers can be blends of petroleum residues, some of which can produce considerable amounts of volatile materials. Volatile evolution, at the optimum operating condition of the delayed coker, can bring about improvements in resultant coke quality. In industrial delayed coking it is important not only to consider the chemistry of the feedstocks, but attention must also be given to the physico-chemical aspects of coker operation. 0 1997 Elsevier Science Ltd Key
Words-A.
Mesophase,
A. petroleum
pitch,
B. carbonization,
C. delayed
coking,
C. optical
microscopy
1. INTRODUCTION
a simplified flow diagram of the coking operation. The petroleum residue, or a blend, (termed feedstock) is processed in the fractionation tower. Such products as kerosene, light gas oil (LGO) and heavy gas oil (HGO) are removed. The residue is then pumped into a furnace which preheats the feedstock, prior to its entry into the coke drums. The furnace heats the feedstock in a time interval of about three minutes, the outlet temperature being slightly below 500°C. Higher temperatures would result in coking occurring within the furnace and in the pipes connecting the furnace to the coking drums. Figure 3 describes stages in the coking of feedstocks within the coke drums. The coke drums do not function by first filling them completely with liquid feedstock, and when full, all of the liquid converting to mesophase and then to coke in two sequential stages. Rather, the processes of pyrolysis, mesophase growth, mesophase coalescence with development of anisotropic structures, and finally of coke formation occur progressively upwards in the drum at the same time as the drum is being filled. Figure 3 illustrates the channels or “pipes” existing in the coke of the drum as a result of the pressurised entry of the feedstock into the drum. Figure 3 also gives an idea of the turbulent flow which can exist in a coke drum. A typical coking unit can produce about 4000 tonnes per day. This has to be compared with the carbonization of no more than several kilograms in laboratory coking
The objective of this paper, in this area of coking of petroleum residues, is to discuss models to explain the behaviour of different feedstocks during their carbonization, possibly in delayed cokers, to obtain correlations between feedstock properties and carbonization variables. Although several theoretical models of carbonization have been published [ l- 151, their application to the industrial situation is still very limited. A principal reason for this is the difficulty of scaling-up results from the laboratory to those of the industrial scene which are designed for large volumes of material associated with rather complex flow systems, including distillations, fractionations and gas production. This paper attempts to relate that which can be learnt from laboratory simulations to the industrial scene. Recently, Mochida et al. [ 161 discussed the chemistry of the production and utilisation of needle coke, and Adams [ 171 discussed the practice and theory of delayed coking, with special emphasis given to the industrial point-of-view. Figure 1 is a simplified flow diagram of a petroleum refinery to relate the coking drums of the delayed coker to the operations of the refinery itself. About 50 000 000 tonnes of delayed coke are produced annually, worldwide. Figure 2 is *Corresponding author. Fax: + 34 6 5903454; e-mail: [email protected] 105
F. Rou~~ti~lir-RIil~osO
106
et rd.
FCC Fractionation Tower
Coke Drums
Fractionation Tower Fig.
I, A simplified flow diagram
of a petroleum
refinery
[ 171 (FCC =Auidised-bed
catalytic
cracker).
Fractionation
Fig. 2. A simplified
flow diagram
of a coker operation
[ 171 (Kero = kerosene; LGO = light gas oil; HGO = heavy gas oil ).
simulators. Figure 2 shows the line connecting the top of the drums to the base of the feedstock fractionation tower, whereby recycle oils t-e-enter the system. Such arrangements are not available in all laboratory simulators. This paper is part of a study to design, construct and operate a pressurised tubular reactor (PTR) for use in the laboratory [ 181. Results obtained can then be compared with experience in the use of the industrial delayed coker (IDC). One of the most significant differences is the result of limitations in the reproduction of the industrial
feedstock system at the laboratory level. For example, any changes or improvements to the petroleum residue being carbonised in the PTR must be made before the start of the thermal treatment. On the other hand, within the IDC some changes can be brought about which improve feedstock quality. Improvements may include enrichment in compounds which are aromatic, by the continuous addition to the feedstock, prior to entry into the furnace (Fig. 2) of a heavy fraction of the distillations and fractionations (recycle oil) which are collected from the head
Delayed
Initial
Coking
coking:
industrial
and laboratory
1
aspects
Initial
107
Coking
2
TOP
TC
:ad
overhead vapors
Condensation of vapors
c/I
Liquid
Solid coke oil inlet +--
HOT TO5
Initial
Coking
Initial
3
Fig. 3. Four stages in the initial coking
Hot oil inlet
of petroleum
of the IDC. In laboratory experiments, under pressure, in a tubular reactor, the gases are purged from the system ahead of the coking stage (in order to keep the pressure in the system constant), but are not returned to the reacting system. In addition, the composition of the feedstock, the relationship with the recycled material and the generation of gases are variable within the IDC, but are not within the PTR of the laboratory. But, when considering these differences (for a given feedstock), it is still possible to compare and establish useful correlations between the PTR of the laboratory and the processes in the IDC. However, should there exist, for the given feedstock, an acceptable, quantitative correlation between the PTR and IDC approaches (in terms of rate of heating, time and temperature of carbonization, etc.) there does not necessarily occur the same
residues
Coking 4
in the coking
drum of a delayed
coker
[ 171
in terms of the behaviour of coking in the laboratory of other, different feedstocks. Care has to be taken in relating directly, for a given coke quality, the carbonization mechanisms. It does not follow, logically, that the mechanisms of carbonization have to be identical in both situations (PTR and IDC). However, the stages of carbonization and the physice-chemical changes occurring within the feedstock, must be more or less identical both in the PTR and in the IDC, even when the operating conditions and the design are different. The objective of this study is to describe and justify a model to explain mechanisms of carbonization of petroleum residues, on a laboratory scale, in a pressurised tubular reactor (PTR). The model attempts to reflect, with close simulation, the separate phenomena which occur in the IDC in order to
F. RODRIGUEZ-REINOSO et (11.
108
Table 1. Characteristics Residue
of the feedstocks
‘H NMR
VR DCA DCB MZ
‘“C NMR
H,
H,
H,
H,,
0.08 0.15 0.13 0.16
0.71 0.58 .048 0.47
0.19 0.13 0.12 0.19
0.04 0.14 0.25 0.17
C,,
C,,
M,
C/H
0.22 0.50 0.58 0.45
0.78 0.50 0.42 0.55
1007 303 306 313
0.624 0.777 0.838 0.738
H, = alpha hydrogen (3.5~ 2.0 ppm). Ha= beta hydrogen (2.0-1.0 ppm). H,=gamma hydrogen (1.0-0.5 ppm). C,, = fraction of aromatic carbon atoms (by ‘%I NMR) C,,= fraction of aliphatic carbon atoms (by 13C NMR).
predict behaviour during different feedstocks when experimental conditions.
2. MATERIALS
the carbonization of submitted to different
AND METHODS
The characteristics of four typical petroleum residues (feedstocks) are listed in Table 1 (VR: vacuum residue; DCA: decant oil of medium aromaticity; DCB: decant oil of high aromaticity; and MZ: a blend of VR/DCA). The feedstocks were characterised by ‘H NMR and 13C NMR, osmometry, elemental analysis, and thermogravimetric analysis, and coefficients of thermal expansion (CT_!?) were measured for the cokes [ 181. Carbonizations were carried out in the PTR, designed in the Centro de Investigation de Repsol (Cartagena). The flow diagram of the PTR is as in Fig. 4. It took a minimum of thirty minutes for the pitch residue sample to reach the final heat treatment temperature (HTT). Slower rates of heating were possible. Samples of feedstock were carbonised under nitrogen, controlling the rate of heating, the final HTT. soak time at the final HTT, and the pressure within the system, which could be between 0.1 and 4.5 MPa. Complete mass balances could be obtained, including knowing the quantities of gases evolved and the carbonization stage (usually temperature)
when they were evolved. Optical textures of resultant cokes were characterised by reflected light optical microscopy (Vickers M14), in association with scanning electron microscopy (Jeol T20). CTE measurements were made using cylindrical rods, 10 mm diameter and 100 mm in length, produced at high temperature and extruded hot. Measurements were made in the longitudinal direction.
3. MECHANISM
OF CARBONIZATION
Models of delayed coking, describing the chemistry of carbonization of aromatic compounds, with special reference to the origin and development of anisotropic structures have been developed [ 16,191. A scheme for the formation of the anisotropic structures of cokes, based on the analysis of intermediate stages and on microscopy studies of the development 01 different optical textures, is set out in the following four stages: ( 1) Formation of spheres of mesophase (2) Growth in sphere size of mesophase spheres (3) Coalescence of the mesophase spheres (4) Formation and development of bulk mesophase This scheme of carbonization explains the route for formation of the anisotropic, isochromatic areas of the optical anisotropy of the coke. Without doubt, when the starting material is a petroleum residue
Reduction
valve
Gas meter
Fig. 4. A simplified
flow diagram
of the laboratory
pressurised
tubular
reactor
(PTR)
Delayed
coking:
industrial
(with more than a thousand different compounds) and the carbonization process is complex for the production of coke in a PTR or in an IDC, then both parallel mechanisms and some additional concepts have to be formulated to explain the processes of carbonization. For the VR feedstock, the initial phenomenon, observed microscopically, after the loss of 30-50 wt%, is the formation of spheres of mesophase (0.5-1.5 mm diameter) dispersed homogeneously within the PTR. The precursors of this initial mesophase are compounds of high “reactivity” of the asphaltene fraction of the parent VR feedstock. The VR residue has > 6 wt% asphaltenes and in MZ with DCA there is 3.8 wt% and in the DCA there is 2.6 wt%. The origin of these small initial spheres of mesophase in the first stages of carbonization results from radical chain reactions in the separate phases. These spheres grow, and without coalescence, settle to the bottom of the reactor (PTR). The appearance of two phases from the homogeneous mixture of many compounds is possible through: 1) a decrease in the solubility of a component, or 2) the loss of solvent which could maintain the two phases as a single phase, and 3) the creation of mesogens (i.e. molecules which are large enough under the experimental conditions, mainly viscosity and temperature, to form mesophase). Explanation 3) is thought to be the most probable [ 201. For example, aromatic hydrocarbons of high molecular weight of the asphaltene fraction of the feedstock, at the initiation of carbonization, lose their alkyl chains rapidly as volatiles (Fig. 5(a) and (b)) so increasing the concentration of reactive free radicals left in the bulk of the system.
g k-nlo-so%
0
0
Oo
t
0 a
d
0 0 OO b
e
f
Fig. 5. Scheme describing carbonization in a pressurised tubular reactor (PTR). (a) Filling of the PTR; (b) volatihsation in the initial stages; (c) formation of initial mesophase (collection at the bottom of the coker); (d) formation, growth and coalescence of typical mesophase; (e) movements within the multiphase system; (f) evolution of gases through the length of the PTR, and reordering of the fluid domains.
and laboratory
aspects
109
Thus, the naphthenic groups of the asphaltene portion initiate radical-forming reactions much earlier than the rest of the reaction system, so forming mesogenic molecules and the small spheres of mesophase. As a result of the dispersion of these small spheres and the high viscosity of the system, there is no possibility for their coalescence. The initial mesophase spheres are more dense than the isotropic medium and hence they settle to the bottom of the coker (Fig. 5(c)) where they accumulate and partially coalesce so forming a band of material of optical texture of fine mosaics (Figs 6 and 7) of variable thickness dependent on the quality of the feedstock and the final temperature of the system (Fig. 8). For the PTR, the thickness of the band of mosaics (Fig. 8) is a function of 1) the percentage of VR in the feedstock mixture and responsible for the content of asphaltenes, and 2) the temperature of the carbonization. When there is 30-35% of VR in the DCA, then the thicker is the band of fine mosaics (Fig. 8). For mixtures with more than 50 wt% of VR, the band of fine mosaics is not formed, because all of the coke is made up of fine mosaics. When the maximum temperature of carbonization (T,) is below 475°C e.g. 46O”C, then the time gap between the formation of the initial mesophase spheres (mosaics) and the more typical mesophase (formed from the more stable aromatic compounds) is wider, and the band is not obvious (Fig. 8). A little above 475°C the initial mesophase spheres, which are formed independently at a temperature of 475°C are not now formed independently, but occur at the same time as the typical bulk mesophase. At this temperature, the viscosity of the system is suitably low and allows coalescence of all the mesophase into a single phase. If the temperature is much higher, e.g. 5OO”C, a more violent reaction occurs in the system. The asphaltene fraction produces an initial mesophase at the same moment as the more stable fraction; but as a result of a sudden increase in the viscosity of the system because of the high chemical reactivity of the system, no coalescence occurs, and the fine mosaics of the initial mesophase are included in the bulk of the domains produced from the stable fraction of the system. Before discussing the second stage of carbonization, it is appropriate to mention that the origin of coke with an optical texture of mosaics and with a high density (including “shot coke”), characteristic of the lower sections of the industrial coker (IDC), must be attributed to the phenomena described above. During the filling of the IDC, which originally is cold, the first mesophase is formed as a band of mosaics from the small spheres, from the asphaltic content of the feedstock [lo]. Two pieces of evidence strengthen this concept: in the first place, the section of coke of lowest quality (with mosaics) in a second or third coking drum, heated previously with the gases from the first drum, is much less in size than that of the cold first drum; in the second place, there
F. RODRIGUEZ-REINOSOct NI
1 IO
Fig. 6. Optical
Fig. 7. Optical
micrograph
micrograph
of the initial mesophase
of the initial mesophase
formed
formed
is the absence of fine mosaic coke in the central zones of the coker. Recently, the possibility has been discovered that recycling oil, incorporated into the petroleum residue is able to reduce the formation of this band of mosaic coke. The reason for this is the stabilisation of the asphaltene fraction of the original residue by means of the transferable hydrogen carried within the recycling oil. In the second stage of the mechanism, the most stable fraction of the feedstock reacts forming mesogens which, with suitable arrangement and order,
from DCA: 475 C. 1.0 MPa.
in the bottom
of the coker,
t,= 120minutes
showing
typical
mosaics.
form spheres of mesophase. The growth, coalesce :nce and development of bulk mesophase (Fig. 5(d)) are stages controlled by the viscosity of the medium. The influence of viscosity on the final properties ot the coke is discussed below.
4. EVOLUTION
OF GASES
During the process of formation, growth and coalescence of the mesophase, in the body of the reactor, gases are released in different quantities and
Delayed
Thickness
coking: industrial and laboratory aspects
of the band of mosaics (mm)
Weiaht
loss
111
I%\
4 .Ol
Temperature Weight
0
10 Content
20
30
40
of VR in DCA
50
loss
(“C)
(%)
40
(9%) 30
Fig. 8. Variation of the band thickness of mosaic coke formed in the coker. for mixtures of VR with DCA.
at different stages, dependent on the residue used, the pressure (P) in the system and the temperature of the carbonization (T,). These gases leave the bulk mesophase during reaction in the form of small bubbles (Fig. 5(f )). The evolution of these gas bubbles moves and orders the fluid anisotropic domains parallel to the principal axis of the PTR. This concept to explain the ordering of the fluid domains has been generally accepted as an essential need to obtain cokes of high quality, the so-called needle cokes [3,5,10,19,21]. Thermogravimetric analyses of the different feedstocks studied confirm, in general, results described in the literature to explain the uniaxial ordering of the fluid domains, as a consequence of the evolution of gases. Without doubt, some of the characteristics of the feedstocks reflect a behaviour which asks if the production of gases in itself is sufficient and necessary to obtain cokes with uniaxially ordered optical texture (needle cokes) and, in general, if it is necessary to formulate a mechanism involving a prior formation of the domains which could be capable of maintaining a later reorientation. In this case, the said reorientation could be attributed to the physical displacement or pushing by the bubbles of gas produced during the process of carbonization, into the fluid domains previously formed. Figure 9(a) and Fig. 9(b) illustrate the evolution of gases for the different feedstocks tested, and represents how the loss in weight during carbonization relates to the heating of the feedstock, at constant rate, to an HTT of 550°C. In Fig. 9(a) and Fig. 9(b) is seen the behaviour of the typical feedstocks: VR, MZ, DCA and DCB (in order of increasing reactivity). The first point to note from Fig. 9(a) and Fig. 9(b) is that, although the profile of VR describes clearly the generation of gases towards the end of the process (320-4OO”C), DCA releases the major volume of gas at the much lower temperatures of 200-320°C.
20 10 0 200
300
400
Temperature
500
(“C)
Fig. 9. Variation of loss in weight (%) with carbonization temperature (“C) during heating of the feedstocks at a constant rate of 5°C min-‘: (a) VR and DCA; (b) MZ and DCB
It is emphasised that the evolution of gases through the bulk mesophase must produce effects just before the point of solidification in order to be effective. If effects are produced earlier, then the anisotropic system cannot be developed completely. Any advantageous effects produced early in the coking cycle can be annealed out by bulk movement of mesophase under minimum viscosity conditions. The growth of the domains (fluid or not) will give non-ordered textures, since the generation of gases has already occurred. However, on the other hand, if gases are produced too near to the final stages of the carbonization, where the viscosity has increased significantly and solidification is starting, then the gas bubbles will be trapped in the system so giving rise to cokes which are porous and fragile. The feedstock DCA is a typical example of decant oil with gas evolution simultaneous to the development and growth of mesophase and producing very small quantities of volatile material (l-4% of the total gases generated) in the critical moment of creating and ordering the fluid domains. The gases produced by the feedstock DCA originate from the methyl groups of the aromatic molecules. These methyl groups separate from the aromatic systems at the beginning of the reaction, so allowing the growth of macromolecular systems (eventually into mesogens) and ordering to give more complete aromatic structures. On the other hand, from the corresponding profile
112
F. RODRIGUEZ-RHNOSO et (11
for the feedstock VR (Fig. 9(a)), it is seen that the gases are produced in the stages very close to solidification, when the viscosity reaches limiting values and where the bubbles pass through the developed bulk mesophase. Cokes produced from the feedstock VR (microscopically they resemble Gruybre cheese) always have high values of porosity and are fragile. VR is a feedstock with the most effective potential for gas production compared with the decant oils. The VR type of feedstock has alkyl and paraffinic molecular substituents which produce, at the beginning of the carbonization process through the disproportionation and fractionation of the aromatic macro-structures, large quantities of hydrogen, methane and ethane, through cracking. From the microscopy study of the cokes from the VR, it can be seen that these potential sources of gas production are not successful in terms of coke amelioration at the time of carbonization because the viscosity is too high and the process of solidification is too well advanced. The feedstock MZ (mixture of DCA/VR) has a behaviour which approximates to an “ideal” behaviour as far as the timing of gas production is concerned; it occurs immediately after the formation of the fluid domains. The addition of VR causes a shift in the evolution of the last of the gases to higher temperatures (Fig. 9(a) and Fig. 9(b)) although most of the gases are produced in the lower temperature region of 200-320°C when the high percentage of DCA controls the process. In the carbonization of MZ, any significant decrease in the size of the optical texture is not seen, as would be expected with the addition of VR; consequently, a major aligning of the fluid domains occurs through the optimisation of the quantity and of the moment of production of gases during the carbonization. The feedstock DCB, of higher reactivity, exhibits an earlier stage of gas evolution, which has little effect and may be prejudicial to coke quality by creating turbulence which makes difficult the growth and coalescence of the still-developing mesophase. Results of this study show that additions of VR to a feedstock essentially aromatic (decant oil) cause a major increase in gas production in the stages of the system prior to solidification so causing a major uniaxial ordering of the fluid domains. Without doubt, due to the special characteristics of the VR (high percentage of asphaltenes, high reactivity), there is an increase in the production of coke of low quality (band of mosaic coke) in the bottom of the coker. On some occasions, additions of VR at high percentages could notably modify the expected behaviour of a stable and aromatic feedstock such as DCA, producing turbulence during the growth of the mesophase, which impedes the coalescence and development of the mesophase, and/or an increase in the minimum viscosity reached in the carbonization of the decant oils. Further, the co-carbonization of blends of feedstocks of different properties allows a better control over the formation of mesophase
and the production of gases, so obtaining cokes of a higher quality than those from the carbonization of each feedstock, singly. The relationship between the composition of the mixture and the temperature of carbonization are variables which require a rigorous and precise control with the objective of producing the desired phenomena; that is to say, coalescence of the mesophase in the conditions of minimum viscosity and evolution of gases at the later stages of the carbonization process. In this study, feedstocks have been analysed which, even inside the general guidelines, differ from the rest in that they do not indicate production of gases in the final moments of the carbonization. Moreover, the coke yields of the feedstocks remain constant from the moment of formation of the mosaic mesophase. To monitor the carbonization of these feedstocks, there were obtained cokes with low values of CTE ( 1.O x 10mh C ‘) and with ordered optical textures. Based on the microscopy evidence it can be expected that the evolution of gases (in the form of bubbles) through the bulk coke, moments before the solidification stage, is the decisive factor for the orientation of the fluid domains, to obtain cokes of quality (CTE= 1.0 x 10e6 C-i) from the feedstocks. However, these do not generate gases in those moments which are considered to be suitable. For this, although the evolution of gases must be considered as a stage of the mechanism of carbonization, it cannot occur in all the feedstocks in the same manner and moment. The practical deduction of most interest is that it does not appear to be essential to have cokes with high order in the fluid domains to have low values of CTE, characteristic of cokes of high quality. Cokes with close values of optical texture index, 26.6627.3 have CTE values of 0.99-1.10 x 10m6 C’. However, when examined by polarized light optical microscopy the fluid domains show very different orientations. Thus, although flow domains are absolutely necessary to have low CTE values, they do not need to be well orientated (aligned) along any given axis [IS].
5. MULTIPHASE SYSTEMS In the microscopy examination of cokes formed from VR, DCA, DVB and MZ, at defined soaktimes of carbonization (t,), it is noted that the molecular ordering of the fluid domains does not occur spontaneously. It would appear that other forces are operating within the coking system, probably of a mechanical nature. As described in Fig. 10 and Fig. ll(a,b), it is considered that the observed movement of isotropic volumes of feedstock material (not reacted) is responsible for the elongation of the domains at the position of the interface of the domain with the isotropic volume. Figure 10 and Fig. 11 (a,b) show, in position A, the fluid domains, in positions B and E the formation of mesophase, in position C the isotropic/anisotropic interface, and in position
Delayed
No. 1
No.
coking:
industrial
2
Fig. 10. Diagram describing the multiphase carbonization system. (A) Fluid domains; (B) mesophase at the stage of formation; (C) interface between the isotropic and anisotropic phases; (D) pocket of unreacted feedstock.
D, a volume of the feedstock which is still unreacted. Such a system is defined here as a multiphase system. A period of minimum viscosity, of sufficiently long duration, has to be maintained to favour the production of such multiphase systems, which by their movement, produce the observed acicular structures, characteristic of the needle cokes. Very reactive feedstocks such as VR or drastic conditions of carbonization (7’,>49O”C) shorten the period of minimum viscosity so preventing the movement of the multiphase systems. The cokes produced under these conditions possess an optical texture of intermediate size as a consequence of the impossibility of the multiphase system to enlarge these structures.
6. ACCELERATED DEVELOPMENT MESOPHASE
OF
After studying the mechanisms of carbonization, and being acquainted with the effects of temperature, soak time, rate of heating and pressure [ 181, and knowing the several stages of the process [22], then certain exceptional situations can be commented upon. When the feedstocks are carbonised, quite independent of their previous history, at temperatures higher than 490-5OO”C, a quite fast reaction results, which is called accelerated development of mesophase. The formation and propagation of free radicals in the system occurs very rapidly. These free radicals cannot be stabilised on this time scale by transfer of hydrogen, and condensation reactions occur rapidly without the time necessary for the growth of mesogen molecules of the required size. The viscosity of the system, likewise, does not have time to reach the minimum necessary for the growth and development of mesophase. These temperatures of 490%500°C bring about cracking of the paraffinic fractions and the volatilisation of those substances responsible for maintaining a fluid system. As the viscosity cannot reach the minimum values required, so coalescence and development of mesophase increase rapidly at this time.
and laboratory
aspects
113
As a result of this phenomenon, carbonization occurs without the formation of multiphase systems. In these conditions, evolution of gases occurs during the solidification of the coke from the very aromatic feedstocks (DCA and DCB) so giving porous cokes (Fig. 12) which are fragile and contain large quantities of volatiles. When the feedstock is reactive (VR) the gases are released violently during the formation of the mesophase so creating turbulence. A microscopy analysis of the resultant cokes shows the texture to be smaller in size and more disorganised than that of cokes produced from the same feedstock, but at lower temperatures. Explanations are as follows: 1. At elevated temperatures, the coalescence of the mesophase to give large sized optical textures is restricted by the conditions of high viscosity. 2. When multiphase systems are not produced, the transition of material, isotropic to mesophase to coalescence, is very rapid and accordingly, the enlargement and parallel ordering of the domains is impossible.
7. CHANGES IN THE VISCOSITY DURING CARBONIZATIONS Comparative studies of different cokes show variations in optical texture, associated with differences in the feedstocks used, and the extremes reached for the operating conditions. A detailed analysis of the mechanism of carbonization can relate such differences to a single variable, that is the changes in viscosity of the system when it is treated thermally [23]. Variations in the viscosity of the mesophase are controlled by both the chemical and physical phenomena of the system. On the one hand, there is the typical physical phenomenon of a decrease in viscosity of a heated liquid, and on the other hand, there is a chemical phenomenon sufficiently more complicated due to those reactions which take place within the system, causing an increase in viscosity. In this way, structures of small size and compounds which separate from the aromatic systems help to reduce the viscosity in the initial period of the carbonization [24,25]. But, at the same time, light molecules and paraffinic groups responsible to maintain the fluidity of the medium are volatilized. As a consequence of all this, the viscosity increases drastically. Figure 13 describes the variation of viscosity for three possible systems during the course of a carbonization. Curve A corresponds to a mesophase system with a relatively high minimum viscosity. The high reactivity of some feedstocks studied (VR and similar materials) follow this model of behaviour; condensation and cross-linking reactions occur rapidly. Consequently, the growth and coalescence of the mesophase is not produced because of the lack of a fluid phase. All the carbonizations of VR have this type of behaviour giving cokes with an optical texture of fine-grained mosaics with the high values of CTE of 2.1-3.0 x 1.0 x 10m6 C-i. Curve B describes the
114
F. ROURIGUtL-REINoX) (‘t LII.
Fig. I I, Optical micrograph of the multiphase carbonization system showing: (A) fluid domains; (B) mesophase of formation; (C) interface between the isotropic and anisotropic phases: (D) pocket of unreacted feedstock: development of mesophase.
behaviour of any aromatic residues (e.g. DCA and DCB) when they are carbonised to temperatures above 490°C. Independently of the characteristics of these feedstocks, of pressure and of duration of carbonization, all these cokes produced at temperature > 490°C (500 and 5 15°C) have optical textures of mosaics or domains with values of CTE much higher than those obtained from the same feedstocks at lower temperatures. Without doubt, the reactions of condensation occur very rapidly, and the transition from isotropy to anisotropy is almost instantaneous. Although coalescence occurs at low viscosities, it is not possible to enlarge and align the domains when the multiphase systems are not present. It is vital to have a period of time of minimum viscosity to
at the stage (E) further
produce multiphase systems and for the multiphase systems to be effective. Curve C represents the changes in viscosity in “normal” conditions for the carbonization of any decant oil (460-485°C P=O.5-3.0 MPa). The viscosity reaches minimum values in the range where the coalescence and development of mesophase is possible, during time intervals sufficiently long (the transition isotropy/anisotropy is slow) which allows the activity of the multiphase systems. Resultant cokes have fluid domains and CTE values 01 0.991.2 x 1.0 x 10mh C’. The time for the influence of the multiphase systems is optimised just when the viscosity/HTT curve is beginning to rise rapidly (vertically)
as in Fig.
13.
Delayed
coking:
industrial
and laboratory
aspects
Fig. 12. Optical micrograph of coke from the carbonization of DCA: 500°C 1.0 MPa, t,=360 minutes. Viscosity
A*
t
I
A
A
I
A AAAA Time of Carbonization
Fig. 13. Variation of viscosity within a carbonising system [24,25]. 8. THEORIES FOR THE OPTIMISATION OF THE CARBONIZATION OF PETROLEUM RESIDUES The design and description of mechanism models on the laboratory scale is limited in scope if they do not permit some level of application on the industrial scale. In agreement with the model described above, the requirements for the optimisation of carbonization are intimately related to the characteristics of the feedstock. The IDC, not following the behaviour patterns of the PTR (which is more flexible in its operation), is unable to convert any petroleum residue into a coke of good quality. In fact, very few residues possess the necessary physical chemistry to behave appropriately during all the stages of the carbonization to produce good cokes. Certainly, there exists the possibility to improve the quality of some cokes, by e.g. hydrotreatments, purification from
inerts, pre-distillation and possible variations of the variables of carbonization systems. The question can be asked as to what is the ideal behaviour of a feedstock, during a carbonization, to produce a coke of high quality. Three quite independent stages are required: (1) Growth in the isotropic area, following the formation, coalescence and development of the mesophase. (2) Movement of the multiphase systems providing elongation of the domains and fluid domains. (3) Gases must be generated, with the evolution of bubbles through the bulk of the coking system, to order, uniaxially, the fluid domains formed prior to the stage of solidification of the coke, Fig. 14. Of these three stages, the first two are absolutely indispensable to formation of cokes of adequate specifications. With regard to the third stage, gas
Viscosity
Viscosity
Time of Carbonization
Fig. 14. Diagram showing the variation of viscosity and the positions of gas evolution within a carbonising system. (A) Ideal period for the development of the multiphase system; (B) time of generation of useful gases; (C) early generation of gases; (D) the late production of gases.
F. R~~RIGUEZ-REIN~SO~~UI
116
evolution is not a necessary requirement in all systems. But, it can be beneficial where gas evolution (a function of feedstock chemical composition) occurs after the formation of fluid domains and prior to solidification. Some level of control of delayed coking is possible in terms of control of feedstock quality, rate of heating, final HTT, soak time and pressure. Thus, during a daily cycle in the operation of the delayed coker, there exists a “window of opportunity” when a combination of synergistic conditions occurs for a limited period of time, and which are just right to establish the needle coke structure. The task of the coking manager is to create this “window of opportunity” by controlling such variables as he has at his disposal.
7
8.
9
10 II
9. CONCLUSIONS This study attempts to model mechanisms of carbonization of petroleum residues, on a laboratory scale, in a pressurised tubular reactor. The principal requirements of the model are summarised as follows: ( I) The system should provide a minimum in the viscosity/HTT curve, and this should be maintained over a sufficiently long length of time. (2) The multiphase systems must be produced during the period of minimum viscosity. (3) Gas generation must occur after the formation of domains and coinciding with the formation and operation of the multiphase systems prior to solidification. (4) It must not be assumed that knowledge of the chemistry of the feedstock, the rate of heating, final HTT and pressure, and resultant coke quality, as obtained in the PTR can be transferred immediately to the IDC. But the concepts developed in the PTR certainly help understand industrial delayed coking. support given for this project by Repsol Petroleo, Spain, is appreciated. H. M. thanks the Spanish DGICYT (SAB95-0086).
Acknowledgements-The
I?
13 14 IS 16.
17
18.
19. 20.
21. 22.
REFERENCES 1. White, J. L., in Petroleum
Derived
Carbons.
ed. M. L.
23. 24.
Marvin and T. M. O’Grady. ACS Symposium Series, Washington, DC, U.S.A., 1976, p. 440. 2. Romero Palazon, E., Marsh, H., Rodriguez-Reinoso, F. and Menendez, R., Extended Abstracts, 18th Biennial Conference on Carbon. American Carbon Society, Newcastle upon Tyne, U.K., September 1988, p, 237.
25.
Mochida, I., Korai, Y. and Fujitsu, H., Carbon, 1987, 25, 259. Marsh, H., Calvert, C. and Bacha, J., J. Mater. Sci., 1985, 20, 289. Mochida. I., Korai, Y., Oyama, T. and Qiug Fei, Y., Fuel, 1988. 67, 1171. Brooks, J. D. and Taylor, G. H., in Chemistry ond Physics of Carbon, Vol. 4, ed. P. L. Walker Jr. Marcel Dekker Inc., New York, 1968, p. 243. Marsh, H. and Walker, Jr, P. L., in Chrmisrry und Physics ofCarbon, Vol. 15, ed. P. L. Walker Jr. Marcel Dekker inc., New York, 1979, p, 228. Marsh, H. and Menendez, R., in Introduction to Curhorr Science, ed. H. Marsh. Butterworths, London, 1989, p. 59. Buechler, M. and White, J. L., 15th Biennial Conjtirence on Carbon. American Carbon Society, Philadelphia. U.S.A., 1981, p. 182. Mochida, I., Korai, Y., Oyama, T., Neshumi. Y. and Todo, Y., Carbon, 1989, 2?, 359. Mochida. I.. Matshuoka. H.. Fuiitsu. H. and Korai. ” Y., Curhon, 1981, 19, 218. Diefendorf, R. J.. 16th Biennial Conference on Curbon. American Carbon Society, San Diego, U.S.A.. 1983. p. 26. Mochida, I., Korai, Y. and Oyama, T.. C&on, 1987, 25, 213. White. J. L., Buechler, M. and Ng. C. B., C’arhotl, 1982. 20, 536. Lewis. R. T.. Lewis, I. C., Greinke. R. D. and Strong. L. S.. Curbon, 1987. 25, 289. Mochida. I., Fujimoto, K. and Oyama, T.. in Chemi.w~ und Ph~~.sic.vof Carbon, Vol. 24, ed. P. A. Thrower, Marcel Dekker Inc.. New York. 1994. n. I I I. Adams, H. A., in Introduction to C’urhon Technologies. ed. H. Marsh. E. A. Heintz and F. Rodriguez-Reinoso, Secretariado de Publicaciones, Universidad de Alicante. Spain, 1997, p, 492. Romero Palazon, E., Carbonization de residuos de petroleo: mecanrsmo y control. Ph.D. thesis, University of Alicante. Spain. 1990. Mochida. I., Oyama, T, and Korai, Y., C’urbo~~. 1988, 26, 51. Marsh. H., Rodriguez-Reinoso. F., Martinez Escandell. M. and Torregrosa Rodriguez, P., in The kinetics of pitch pyrolysis a review, to be submitted to C&on, 1997. Dubois, J., Agache, C. and White. J. L., Merullogruphy, 1970, 3, 337. Romero Palazon. E., Marsh, H., Rodriguez-Reinoso. F. and Santana, P., Extended Abstracts, IYrh Bienniul Conjerence on C&on. American Carbon Society, University Park, PA, U.S.A.. 1989. D. 178. White, J.-L. and Price, R. J., Carbon, 1974, 12, 321. Marsh, H. and Latham, C. S., in Petroleum Derived Carbons, No. 303, ed. M. L. Marvin and T. M. O’Grady. ACS Symposium Series, Washington, DC, U.S.A., 1986, pp. l-28. Rand, B., in Petroleum Derived Carbons, No. 303, ed. M. L. Marvin and T. M. O’Grady. ACS Symposium Series, Washington, DC. U.S.A., 1986, p. 45.
Pergamon PII: SOOOS-6223(97)00154-l
DELAYED
COKING: INDUSTRIAL ASPECTS
F. RODRIGUEZ-REINOSO,~,*
AND LABORATORY
P. SANTANA,~ E. ROMERO PALAZON,~ M.-A.
DIEZ~ and
H. MARSHA “Department of Inorganic Chemistry, University of Alicante, 03080 Alicante, Spain bREPSOLpPETROLEO, Centro de Investigacibn de Cartagena, Cartagena, Spain ‘Institute National de1 Carbon, La Corredoria, Apartado 73, 33080 Oviedo, Spain (Received 13 March 1997; accepted in revisedform
21 July 1997)
Abstract-Delayed
coking is a thermal process to convert petroleum residues to a solid coke material. Processes occurring in a delayed coker are complicated and attempts have been made, at the laboratory level, to simulate industrial delayed coking. Although the latter studies are useful, it is impossible to scale-down to the laboratory level. Industrial delayed coking is a turbulent process and such movements cannot be simulated easily in the laboratory. Of industrial importance are the multiphase systems, i.e. volumes of unreacted isotropic pitch residue, transported through the bulk, fluid anisotropic mesophase, so creating ordering into acicular structures in the vicinity of the multiphase systems. Four petroleum residues were analysed chemically. Pyrolyses were carried out under pressures of up to 1.0 MPa. Complete mass balances were obtained and the semicokes examined by optical microscopy. Feedstocks for delayed cokers can be blends of petroleum residues, some of which can produce considerable amounts of volatile materials. Volatile evolution, at the optimum operating condition of the delayed coker, can bring about improvements in resultant coke quality. In industrial delayed coking it is important not only to consider the chemistry of the feedstocks, but attention must also be given to the physico-chemical aspects of coker operation. 0 1997 Elsevier Science Ltd Key
Words-A.
Mesophase,
A. petroleum
pitch,
B. carbonization,
C. delayed
coking,
C. optical
microscopy
1. INTRODUCTION
a simplified flow diagram of the coking operation. The petroleum residue, or a blend, (termed feedstock) is processed in the fractionation tower. Such products as kerosene, light gas oil (LGO) and heavy gas oil (HGO) are removed. The residue is then pumped into a furnace which preheats the feedstock, prior to its entry into the coke drums. The furnace heats the feedstock in a time interval of about three minutes, the outlet temperature being slightly below 500°C. Higher temperatures would result in coking occurring within the furnace and in the pipes connecting the furnace to the coking drums. Figure 3 describes stages in the coking of feedstocks within the coke drums. The coke drums do not function by first filling them completely with liquid feedstock, and when full, all of the liquid converting to mesophase and then to coke in two sequential stages. Rather, the processes of pyrolysis, mesophase growth, mesophase coalescence with development of anisotropic structures, and finally of coke formation occur progressively upwards in the drum at the same time as the drum is being filled. Figure 3 illustrates the channels or “pipes” existing in the coke of the drum as a result of the pressurised entry of the feedstock into the drum. Figure 3 also gives an idea of the turbulent flow which can exist in a coke drum. A typical coking unit can produce about 4000 tonnes per day. This has to be compared with the carbonization of no more than several kilograms in laboratory coking
The objective of this paper, in this area of coking of petroleum residues, is to discuss models to explain the behaviour of different feedstocks during their carbonization, possibly in delayed cokers, to obtain correlations between feedstock properties and carbonization variables. Although several theoretical models of carbonization have been published [ l- 151, their application to the industrial situation is still very limited. A principal reason for this is the difficulty of scaling-up results from the laboratory to those of the industrial scene which are designed for large volumes of material associated with rather complex flow systems, including distillations, fractionations and gas production. This paper attempts to relate that which can be learnt from laboratory simulations to the industrial scene. Recently, Mochida et al. [ 161 discussed the chemistry of the production and utilisation of needle coke, and Adams [ 171 discussed the practice and theory of delayed coking, with special emphasis given to the industrial point-of-view. Figure 1 is a simplified flow diagram of a petroleum refinery to relate the coking drums of the delayed coker to the operations of the refinery itself. About 50 000 000 tonnes of delayed coke are produced annually, worldwide. Figure 2 is *Corresponding author. Fax: + 34 6 5903454; e-mail: [email protected] 105
F. Rou~~ti~lir-RIil~osO
106
et rd.
FCC Fractionation Tower
Coke Drums
Fractionation Tower Fig.
I, A simplified flow diagram
of a petroleum
refinery
[ 171 (FCC =Auidised-bed
catalytic
cracker).
Fractionation
Fig. 2. A simplified
flow diagram
of a coker operation
[ 171 (Kero = kerosene; LGO = light gas oil; HGO = heavy gas oil ).
simulators. Figure 2 shows the line connecting the top of the drums to the base of the feedstock fractionation tower, whereby recycle oils t-e-enter the system. Such arrangements are not available in all laboratory simulators. This paper is part of a study to design, construct and operate a pressurised tubular reactor (PTR) for use in the laboratory [ 181. Results obtained can then be compared with experience in the use of the industrial delayed coker (IDC). One of the most significant differences is the result of limitations in the reproduction of the industrial
feedstock system at the laboratory level. For example, any changes or improvements to the petroleum residue being carbonised in the PTR must be made before the start of the thermal treatment. On the other hand, within the IDC some changes can be brought about which improve feedstock quality. Improvements may include enrichment in compounds which are aromatic, by the continuous addition to the feedstock, prior to entry into the furnace (Fig. 2) of a heavy fraction of the distillations and fractionations (recycle oil) which are collected from the head
Delayed
Initial
Coking
coking:
industrial
and laboratory
1
aspects
Initial
107
Coking
2
TOP
TC
:ad
overhead vapors
Condensation of vapors
c/I
Liquid
Solid coke oil inlet +--
HOT TO5
Initial
Coking
Initial
3
Fig. 3. Four stages in the initial coking
Hot oil inlet
of petroleum
of the IDC. In laboratory experiments, under pressure, in a tubular reactor, the gases are purged from the system ahead of the coking stage (in order to keep the pressure in the system constant), but are not returned to the reacting system. In addition, the composition of the feedstock, the relationship with the recycled material and the generation of gases are variable within the IDC, but are not within the PTR of the laboratory. But, when considering these differences (for a given feedstock), it is still possible to compare and establish useful correlations between the PTR of the laboratory and the processes in the IDC. However, should there exist, for the given feedstock, an acceptable, quantitative correlation between the PTR and IDC approaches (in terms of rate of heating, time and temperature of carbonization, etc.) there does not necessarily occur the same
residues
Coking 4
in the coking
drum of a delayed
coker
[ 171
in terms of the behaviour of coking in the laboratory of other, different feedstocks. Care has to be taken in relating directly, for a given coke quality, the carbonization mechanisms. It does not follow, logically, that the mechanisms of carbonization have to be identical in both situations (PTR and IDC). However, the stages of carbonization and the physice-chemical changes occurring within the feedstock, must be more or less identical both in the PTR and in the IDC, even when the operating conditions and the design are different. The objective of this study is to describe and justify a model to explain mechanisms of carbonization of petroleum residues, on a laboratory scale, in a pressurised tubular reactor (PTR). The model attempts to reflect, with close simulation, the separate phenomena which occur in the IDC in order to
F. RODRIGUEZ-REINOSO et (11.
108
Table 1. Characteristics Residue
of the feedstocks
‘H NMR
VR DCA DCB MZ
‘“C NMR
H,
H,
H,
H,,
0.08 0.15 0.13 0.16
0.71 0.58 .048 0.47
0.19 0.13 0.12 0.19
0.04 0.14 0.25 0.17
C,,
C,,
M,
C/H
0.22 0.50 0.58 0.45
0.78 0.50 0.42 0.55
1007 303 306 313
0.624 0.777 0.838 0.738
H, = alpha hydrogen (3.5~ 2.0 ppm). Ha= beta hydrogen (2.0-1.0 ppm). H,=gamma hydrogen (1.0-0.5 ppm). C,, = fraction of aromatic carbon atoms (by ‘%I NMR) C,,= fraction of aliphatic carbon atoms (by 13C NMR).
predict behaviour during different feedstocks when experimental conditions.
2. MATERIALS
the carbonization of submitted to different
AND METHODS
The characteristics of four typical petroleum residues (feedstocks) are listed in Table 1 (VR: vacuum residue; DCA: decant oil of medium aromaticity; DCB: decant oil of high aromaticity; and MZ: a blend of VR/DCA). The feedstocks were characterised by ‘H NMR and 13C NMR, osmometry, elemental analysis, and thermogravimetric analysis, and coefficients of thermal expansion (CT_!?) were measured for the cokes [ 181. Carbonizations were carried out in the PTR, designed in the Centro de Investigation de Repsol (Cartagena). The flow diagram of the PTR is as in Fig. 4. It took a minimum of thirty minutes for the pitch residue sample to reach the final heat treatment temperature (HTT). Slower rates of heating were possible. Samples of feedstock were carbonised under nitrogen, controlling the rate of heating, the final HTT. soak time at the final HTT, and the pressure within the system, which could be between 0.1 and 4.5 MPa. Complete mass balances could be obtained, including knowing the quantities of gases evolved and the carbonization stage (usually temperature)
when they were evolved. Optical textures of resultant cokes were characterised by reflected light optical microscopy (Vickers M14), in association with scanning electron microscopy (Jeol T20). CTE measurements were made using cylindrical rods, 10 mm diameter and 100 mm in length, produced at high temperature and extruded hot. Measurements were made in the longitudinal direction.
3. MECHANISM
OF CARBONIZATION
Models of delayed coking, describing the chemistry of carbonization of aromatic compounds, with special reference to the origin and development of anisotropic structures have been developed [ 16,191. A scheme for the formation of the anisotropic structures of cokes, based on the analysis of intermediate stages and on microscopy studies of the development 01 different optical textures, is set out in the following four stages: ( 1) Formation of spheres of mesophase (2) Growth in sphere size of mesophase spheres (3) Coalescence of the mesophase spheres (4) Formation and development of bulk mesophase This scheme of carbonization explains the route for formation of the anisotropic, isochromatic areas of the optical anisotropy of the coke. Without doubt, when the starting material is a petroleum residue
Reduction
valve
Gas meter
Fig. 4. A simplified
flow diagram
of the laboratory
pressurised
tubular
reactor
(PTR)
Delayed
coking:
industrial
(with more than a thousand different compounds) and the carbonization process is complex for the production of coke in a PTR or in an IDC, then both parallel mechanisms and some additional concepts have to be formulated to explain the processes of carbonization. For the VR feedstock, the initial phenomenon, observed microscopically, after the loss of 30-50 wt%, is the formation of spheres of mesophase (0.5-1.5 mm diameter) dispersed homogeneously within the PTR. The precursors of this initial mesophase are compounds of high “reactivity” of the asphaltene fraction of the parent VR feedstock. The VR residue has > 6 wt% asphaltenes and in MZ with DCA there is 3.8 wt% and in the DCA there is 2.6 wt%. The origin of these small initial spheres of mesophase in the first stages of carbonization results from radical chain reactions in the separate phases. These spheres grow, and without coalescence, settle to the bottom of the reactor (PTR). The appearance of two phases from the homogeneous mixture of many compounds is possible through: 1) a decrease in the solubility of a component, or 2) the loss of solvent which could maintain the two phases as a single phase, and 3) the creation of mesogens (i.e. molecules which are large enough under the experimental conditions, mainly viscosity and temperature, to form mesophase). Explanation 3) is thought to be the most probable [ 201. For example, aromatic hydrocarbons of high molecular weight of the asphaltene fraction of the feedstock, at the initiation of carbonization, lose their alkyl chains rapidly as volatiles (Fig. 5(a) and (b)) so increasing the concentration of reactive free radicals left in the bulk of the system.
g k-nlo-so%
0
0
Oo
t
0 a
d
0 0 OO b
e
f
Fig. 5. Scheme describing carbonization in a pressurised tubular reactor (PTR). (a) Filling of the PTR; (b) volatihsation in the initial stages; (c) formation of initial mesophase (collection at the bottom of the coker); (d) formation, growth and coalescence of typical mesophase; (e) movements within the multiphase system; (f) evolution of gases through the length of the PTR, and reordering of the fluid domains.
and laboratory
aspects
109
Thus, the naphthenic groups of the asphaltene portion initiate radical-forming reactions much earlier than the rest of the reaction system, so forming mesogenic molecules and the small spheres of mesophase. As a result of the dispersion of these small spheres and the high viscosity of the system, there is no possibility for their coalescence. The initial mesophase spheres are more dense than the isotropic medium and hence they settle to the bottom of the coker (Fig. 5(c)) where they accumulate and partially coalesce so forming a band of material of optical texture of fine mosaics (Figs 6 and 7) of variable thickness dependent on the quality of the feedstock and the final temperature of the system (Fig. 8). For the PTR, the thickness of the band of mosaics (Fig. 8) is a function of 1) the percentage of VR in the feedstock mixture and responsible for the content of asphaltenes, and 2) the temperature of the carbonization. When there is 30-35% of VR in the DCA, then the thicker is the band of fine mosaics (Fig. 8). For mixtures with more than 50 wt% of VR, the band of fine mosaics is not formed, because all of the coke is made up of fine mosaics. When the maximum temperature of carbonization (T,) is below 475°C e.g. 46O”C, then the time gap between the formation of the initial mesophase spheres (mosaics) and the more typical mesophase (formed from the more stable aromatic compounds) is wider, and the band is not obvious (Fig. 8). A little above 475°C the initial mesophase spheres, which are formed independently at a temperature of 475°C are not now formed independently, but occur at the same time as the typical bulk mesophase. At this temperature, the viscosity of the system is suitably low and allows coalescence of all the mesophase into a single phase. If the temperature is much higher, e.g. 5OO”C, a more violent reaction occurs in the system. The asphaltene fraction produces an initial mesophase at the same moment as the more stable fraction; but as a result of a sudden increase in the viscosity of the system because of the high chemical reactivity of the system, no coalescence occurs, and the fine mosaics of the initial mesophase are included in the bulk of the domains produced from the stable fraction of the system. Before discussing the second stage of carbonization, it is appropriate to mention that the origin of coke with an optical texture of mosaics and with a high density (including “shot coke”), characteristic of the lower sections of the industrial coker (IDC), must be attributed to the phenomena described above. During the filling of the IDC, which originally is cold, the first mesophase is formed as a band of mosaics from the small spheres, from the asphaltic content of the feedstock [lo]. Two pieces of evidence strengthen this concept: in the first place, the section of coke of lowest quality (with mosaics) in a second or third coking drum, heated previously with the gases from the first drum, is much less in size than that of the cold first drum; in the second place, there
F. RODRIGUEZ-REINOSOct NI
1 IO
Fig. 6. Optical
Fig. 7. Optical
micrograph
micrograph
of the initial mesophase
of the initial mesophase
formed
formed
is the absence of fine mosaic coke in the central zones of the coker. Recently, the possibility has been discovered that recycling oil, incorporated into the petroleum residue is able to reduce the formation of this band of mosaic coke. The reason for this is the stabilisation of the asphaltene fraction of the original residue by means of the transferable hydrogen carried within the recycling oil. In the second stage of the mechanism, the most stable fraction of the feedstock reacts forming mesogens which, with suitable arrangement and order,
from DCA: 475 C. 1.0 MPa.
in the bottom
of the coker,
t,= 120minutes
showing
typical
mosaics.
form spheres of mesophase. The growth, coalesce :nce and development of bulk mesophase (Fig. 5(d)) are stages controlled by the viscosity of the medium. The influence of viscosity on the final properties ot the coke is discussed below.
4. EVOLUTION
OF GASES
During the process of formation, growth and coalescence of the mesophase, in the body of the reactor, gases are released in different quantities and
Delayed
Thickness
coking: industrial and laboratory aspects
of the band of mosaics (mm)
Weiaht
loss
111
I%\
4 .Ol
Temperature Weight
0
10 Content
20
30
40
of VR in DCA
50
loss
(“C)
(%)
40
(9%) 30
Fig. 8. Variation of the band thickness of mosaic coke formed in the coker. for mixtures of VR with DCA.
at different stages, dependent on the residue used, the pressure (P) in the system and the temperature of the carbonization (T,). These gases leave the bulk mesophase during reaction in the form of small bubbles (Fig. 5(f )). The evolution of these gas bubbles moves and orders the fluid anisotropic domains parallel to the principal axis of the PTR. This concept to explain the ordering of the fluid domains has been generally accepted as an essential need to obtain cokes of high quality, the so-called needle cokes [3,5,10,19,21]. Thermogravimetric analyses of the different feedstocks studied confirm, in general, results described in the literature to explain the uniaxial ordering of the fluid domains, as a consequence of the evolution of gases. Without doubt, some of the characteristics of the feedstocks reflect a behaviour which asks if the production of gases in itself is sufficient and necessary to obtain cokes with uniaxially ordered optical texture (needle cokes) and, in general, if it is necessary to formulate a mechanism involving a prior formation of the domains which could be capable of maintaining a later reorientation. In this case, the said reorientation could be attributed to the physical displacement or pushing by the bubbles of gas produced during the process of carbonization, into the fluid domains previously formed. Figure 9(a) and Fig. 9(b) illustrate the evolution of gases for the different feedstocks tested, and represents how the loss in weight during carbonization relates to the heating of the feedstock, at constant rate, to an HTT of 550°C. In Fig. 9(a) and Fig. 9(b) is seen the behaviour of the typical feedstocks: VR, MZ, DCA and DCB (in order of increasing reactivity). The first point to note from Fig. 9(a) and Fig. 9(b) is that, although the profile of VR describes clearly the generation of gases towards the end of the process (320-4OO”C), DCA releases the major volume of gas at the much lower temperatures of 200-320°C.
20 10 0 200
300
400
Temperature
500
(“C)
Fig. 9. Variation of loss in weight (%) with carbonization temperature (“C) during heating of the feedstocks at a constant rate of 5°C min-‘: (a) VR and DCA; (b) MZ and DCB
It is emphasised that the evolution of gases through the bulk mesophase must produce effects just before the point of solidification in order to be effective. If effects are produced earlier, then the anisotropic system cannot be developed completely. Any advantageous effects produced early in the coking cycle can be annealed out by bulk movement of mesophase under minimum viscosity conditions. The growth of the domains (fluid or not) will give non-ordered textures, since the generation of gases has already occurred. However, on the other hand, if gases are produced too near to the final stages of the carbonization, where the viscosity has increased significantly and solidification is starting, then the gas bubbles will be trapped in the system so giving rise to cokes which are porous and fragile. The feedstock DCA is a typical example of decant oil with gas evolution simultaneous to the development and growth of mesophase and producing very small quantities of volatile material (l-4% of the total gases generated) in the critical moment of creating and ordering the fluid domains. The gases produced by the feedstock DCA originate from the methyl groups of the aromatic molecules. These methyl groups separate from the aromatic systems at the beginning of the reaction, so allowing the growth of macromolecular systems (eventually into mesogens) and ordering to give more complete aromatic structures. On the other hand, from the corresponding profile
112
F. RODRIGUEZ-RHNOSO et (11
for the feedstock VR (Fig. 9(a)), it is seen that the gases are produced in the stages very close to solidification, when the viscosity reaches limiting values and where the bubbles pass through the developed bulk mesophase. Cokes produced from the feedstock VR (microscopically they resemble Gruybre cheese) always have high values of porosity and are fragile. VR is a feedstock with the most effective potential for gas production compared with the decant oils. The VR type of feedstock has alkyl and paraffinic molecular substituents which produce, at the beginning of the carbonization process through the disproportionation and fractionation of the aromatic macro-structures, large quantities of hydrogen, methane and ethane, through cracking. From the microscopy study of the cokes from the VR, it can be seen that these potential sources of gas production are not successful in terms of coke amelioration at the time of carbonization because the viscosity is too high and the process of solidification is too well advanced. The feedstock MZ (mixture of DCA/VR) has a behaviour which approximates to an “ideal” behaviour as far as the timing of gas production is concerned; it occurs immediately after the formation of the fluid domains. The addition of VR causes a shift in the evolution of the last of the gases to higher temperatures (Fig. 9(a) and Fig. 9(b)) although most of the gases are produced in the lower temperature region of 200-320°C when the high percentage of DCA controls the process. In the carbonization of MZ, any significant decrease in the size of the optical texture is not seen, as would be expected with the addition of VR; consequently, a major aligning of the fluid domains occurs through the optimisation of the quantity and of the moment of production of gases during the carbonization. The feedstock DCB, of higher reactivity, exhibits an earlier stage of gas evolution, which has little effect and may be prejudicial to coke quality by creating turbulence which makes difficult the growth and coalescence of the still-developing mesophase. Results of this study show that additions of VR to a feedstock essentially aromatic (decant oil) cause a major increase in gas production in the stages of the system prior to solidification so causing a major uniaxial ordering of the fluid domains. Without doubt, due to the special characteristics of the VR (high percentage of asphaltenes, high reactivity), there is an increase in the production of coke of low quality (band of mosaic coke) in the bottom of the coker. On some occasions, additions of VR at high percentages could notably modify the expected behaviour of a stable and aromatic feedstock such as DCA, producing turbulence during the growth of the mesophase, which impedes the coalescence and development of the mesophase, and/or an increase in the minimum viscosity reached in the carbonization of the decant oils. Further, the co-carbonization of blends of feedstocks of different properties allows a better control over the formation of mesophase
and the production of gases, so obtaining cokes of a higher quality than those from the carbonization of each feedstock, singly. The relationship between the composition of the mixture and the temperature of carbonization are variables which require a rigorous and precise control with the objective of producing the desired phenomena; that is to say, coalescence of the mesophase in the conditions of minimum viscosity and evolution of gases at the later stages of the carbonization process. In this study, feedstocks have been analysed which, even inside the general guidelines, differ from the rest in that they do not indicate production of gases in the final moments of the carbonization. Moreover, the coke yields of the feedstocks remain constant from the moment of formation of the mosaic mesophase. To monitor the carbonization of these feedstocks, there were obtained cokes with low values of CTE ( 1.O x 10mh C ‘) and with ordered optical textures. Based on the microscopy evidence it can be expected that the evolution of gases (in the form of bubbles) through the bulk coke, moments before the solidification stage, is the decisive factor for the orientation of the fluid domains, to obtain cokes of quality (CTE= 1.0 x 10e6 C-i) from the feedstocks. However, these do not generate gases in those moments which are considered to be suitable. For this, although the evolution of gases must be considered as a stage of the mechanism of carbonization, it cannot occur in all the feedstocks in the same manner and moment. The practical deduction of most interest is that it does not appear to be essential to have cokes with high order in the fluid domains to have low values of CTE, characteristic of cokes of high quality. Cokes with close values of optical texture index, 26.6627.3 have CTE values of 0.99-1.10 x 10m6 C’. However, when examined by polarized light optical microscopy the fluid domains show very different orientations. Thus, although flow domains are absolutely necessary to have low CTE values, they do not need to be well orientated (aligned) along any given axis [IS].
5. MULTIPHASE SYSTEMS In the microscopy examination of cokes formed from VR, DCA, DVB and MZ, at defined soaktimes of carbonization (t,), it is noted that the molecular ordering of the fluid domains does not occur spontaneously. It would appear that other forces are operating within the coking system, probably of a mechanical nature. As described in Fig. 10 and Fig. ll(a,b), it is considered that the observed movement of isotropic volumes of feedstock material (not reacted) is responsible for the elongation of the domains at the position of the interface of the domain with the isotropic volume. Figure 10 and Fig. 11 (a,b) show, in position A, the fluid domains, in positions B and E the formation of mesophase, in position C the isotropic/anisotropic interface, and in position
Delayed
No. 1
No.
coking:
industrial
2
Fig. 10. Diagram describing the multiphase carbonization system. (A) Fluid domains; (B) mesophase at the stage of formation; (C) interface between the isotropic and anisotropic phases; (D) pocket of unreacted feedstock.
D, a volume of the feedstock which is still unreacted. Such a system is defined here as a multiphase system. A period of minimum viscosity, of sufficiently long duration, has to be maintained to favour the production of such multiphase systems, which by their movement, produce the observed acicular structures, characteristic of the needle cokes. Very reactive feedstocks such as VR or drastic conditions of carbonization (7’,>49O”C) shorten the period of minimum viscosity so preventing the movement of the multiphase systems. The cokes produced under these conditions possess an optical texture of intermediate size as a consequence of the impossibility of the multiphase system to enlarge these structures.
6. ACCELERATED DEVELOPMENT MESOPHASE
OF
After studying the mechanisms of carbonization, and being acquainted with the effects of temperature, soak time, rate of heating and pressure [ 181, and knowing the several stages of the process [22], then certain exceptional situations can be commented upon. When the feedstocks are carbonised, quite independent of their previous history, at temperatures higher than 490-5OO”C, a quite fast reaction results, which is called accelerated development of mesophase. The formation and propagation of free radicals in the system occurs very rapidly. These free radicals cannot be stabilised on this time scale by transfer of hydrogen, and condensation reactions occur rapidly without the time necessary for the growth of mesogen molecules of the required size. The viscosity of the system, likewise, does not have time to reach the minimum necessary for the growth and development of mesophase. These temperatures of 490%500°C bring about cracking of the paraffinic fractions and the volatilisation of those substances responsible for maintaining a fluid system. As the viscosity cannot reach the minimum values required, so coalescence and development of mesophase increase rapidly at this time.
and laboratory
aspects
113
As a result of this phenomenon, carbonization occurs without the formation of multiphase systems. In these conditions, evolution of gases occurs during the solidification of the coke from the very aromatic feedstocks (DCA and DCB) so giving porous cokes (Fig. 12) which are fragile and contain large quantities of volatiles. When the feedstock is reactive (VR) the gases are released violently during the formation of the mesophase so creating turbulence. A microscopy analysis of the resultant cokes shows the texture to be smaller in size and more disorganised than that of cokes produced from the same feedstock, but at lower temperatures. Explanations are as follows: 1. At elevated temperatures, the coalescence of the mesophase to give large sized optical textures is restricted by the conditions of high viscosity. 2. When multiphase systems are not produced, the transition of material, isotropic to mesophase to coalescence, is very rapid and accordingly, the enlargement and parallel ordering of the domains is impossible.
7. CHANGES IN THE VISCOSITY DURING CARBONIZATIONS Comparative studies of different cokes show variations in optical texture, associated with differences in the feedstocks used, and the extremes reached for the operating conditions. A detailed analysis of the mechanism of carbonization can relate such differences to a single variable, that is the changes in viscosity of the system when it is treated thermally [23]. Variations in the viscosity of the mesophase are controlled by both the chemical and physical phenomena of the system. On the one hand, there is the typical physical phenomenon of a decrease in viscosity of a heated liquid, and on the other hand, there is a chemical phenomenon sufficiently more complicated due to those reactions which take place within the system, causing an increase in viscosity. In this way, structures of small size and compounds which separate from the aromatic systems help to reduce the viscosity in the initial period of the carbonization [24,25]. But, at the same time, light molecules and paraffinic groups responsible to maintain the fluidity of the medium are volatilized. As a consequence of all this, the viscosity increases drastically. Figure 13 describes the variation of viscosity for three possible systems during the course of a carbonization. Curve A corresponds to a mesophase system with a relatively high minimum viscosity. The high reactivity of some feedstocks studied (VR and similar materials) follow this model of behaviour; condensation and cross-linking reactions occur rapidly. Consequently, the growth and coalescence of the mesophase is not produced because of the lack of a fluid phase. All the carbonizations of VR have this type of behaviour giving cokes with an optical texture of fine-grained mosaics with the high values of CTE of 2.1-3.0 x 1.0 x 10m6 C-i. Curve B describes the
114
F. ROURIGUtL-REINoX) (‘t LII.
Fig. I I, Optical micrograph of the multiphase carbonization system showing: (A) fluid domains; (B) mesophase of formation; (C) interface between the isotropic and anisotropic phases: (D) pocket of unreacted feedstock: development of mesophase.
behaviour of any aromatic residues (e.g. DCA and DCB) when they are carbonised to temperatures above 490°C. Independently of the characteristics of these feedstocks, of pressure and of duration of carbonization, all these cokes produced at temperature > 490°C (500 and 5 15°C) have optical textures of mosaics or domains with values of CTE much higher than those obtained from the same feedstocks at lower temperatures. Without doubt, the reactions of condensation occur very rapidly, and the transition from isotropy to anisotropy is almost instantaneous. Although coalescence occurs at low viscosities, it is not possible to enlarge and align the domains when the multiphase systems are not present. It is vital to have a period of time of minimum viscosity to
at the stage (E) further
produce multiphase systems and for the multiphase systems to be effective. Curve C represents the changes in viscosity in “normal” conditions for the carbonization of any decant oil (460-485°C P=O.5-3.0 MPa). The viscosity reaches minimum values in the range where the coalescence and development of mesophase is possible, during time intervals sufficiently long (the transition isotropy/anisotropy is slow) which allows the activity of the multiphase systems. Resultant cokes have fluid domains and CTE values 01 0.991.2 x 1.0 x 10mh C’. The time for the influence of the multiphase systems is optimised just when the viscosity/HTT curve is beginning to rise rapidly (vertically)
as in Fig.
13.
Delayed
coking:
industrial
and laboratory
aspects
Fig. 12. Optical micrograph of coke from the carbonization of DCA: 500°C 1.0 MPa, t,=360 minutes. Viscosity
A*
t
I
A
A
I
A AAAA Time of Carbonization
Fig. 13. Variation of viscosity within a carbonising system [24,25]. 8. THEORIES FOR THE OPTIMISATION OF THE CARBONIZATION OF PETROLEUM RESIDUES The design and description of mechanism models on the laboratory scale is limited in scope if they do not permit some level of application on the industrial scale. In agreement with the model described above, the requirements for the optimisation of carbonization are intimately related to the characteristics of the feedstock. The IDC, not following the behaviour patterns of the PTR (which is more flexible in its operation), is unable to convert any petroleum residue into a coke of good quality. In fact, very few residues possess the necessary physical chemistry to behave appropriately during all the stages of the carbonization to produce good cokes. Certainly, there exists the possibility to improve the quality of some cokes, by e.g. hydrotreatments, purification from
inerts, pre-distillation and possible variations of the variables of carbonization systems. The question can be asked as to what is the ideal behaviour of a feedstock, during a carbonization, to produce a coke of high quality. Three quite independent stages are required: (1) Growth in the isotropic area, following the formation, coalescence and development of the mesophase. (2) Movement of the multiphase systems providing elongation of the domains and fluid domains. (3) Gases must be generated, with the evolution of bubbles through the bulk of the coking system, to order, uniaxially, the fluid domains formed prior to the stage of solidification of the coke, Fig. 14. Of these three stages, the first two are absolutely indispensable to formation of cokes of adequate specifications. With regard to the third stage, gas
Viscosity
Viscosity
Time of Carbonization
Fig. 14. Diagram showing the variation of viscosity and the positions of gas evolution within a carbonising system. (A) Ideal period for the development of the multiphase system; (B) time of generation of useful gases; (C) early generation of gases; (D) the late production of gases.
F. R~~RIGUEZ-REIN~SO~~UI
116
evolution is not a necessary requirement in all systems. But, it can be beneficial where gas evolution (a function of feedstock chemical composition) occurs after the formation of fluid domains and prior to solidification. Some level of control of delayed coking is possible in terms of control of feedstock quality, rate of heating, final HTT, soak time and pressure. Thus, during a daily cycle in the operation of the delayed coker, there exists a “window of opportunity” when a combination of synergistic conditions occurs for a limited period of time, and which are just right to establish the needle coke structure. The task of the coking manager is to create this “window of opportunity” by controlling such variables as he has at his disposal.
7
8.
9
10 II
9. CONCLUSIONS This study attempts to model mechanisms of carbonization of petroleum residues, on a laboratory scale, in a pressurised tubular reactor. The principal requirements of the model are summarised as follows: ( I) The system should provide a minimum in the viscosity/HTT curve, and this should be maintained over a sufficiently long length of time. (2) The multiphase systems must be produced during the period of minimum viscosity. (3) Gas generation must occur after the formation of domains and coinciding with the formation and operation of the multiphase systems prior to solidification. (4) It must not be assumed that knowledge of the chemistry of the feedstock, the rate of heating, final HTT and pressure, and resultant coke quality, as obtained in the PTR can be transferred immediately to the IDC. But the concepts developed in the PTR certainly help understand industrial delayed coking. support given for this project by Repsol Petroleo, Spain, is appreciated. H. M. thanks the Spanish DGICYT (SAB95-0086).
Acknowledgements-The
I?
13 14 IS 16.
17
18.
19. 20.
21. 22.
REFERENCES 1. White, J. L., in Petroleum
Derived
Carbons.
ed. M. L.
23. 24.
Marvin and T. M. O’Grady. ACS Symposium Series, Washington, DC, U.S.A., 1976, p. 440. 2. Romero Palazon, E., Marsh, H., Rodriguez-Reinoso, F. and Menendez, R., Extended Abstracts, 18th Biennial Conference on Carbon. American Carbon Society, Newcastle upon Tyne, U.K., September 1988, p, 237.
25.
Mochida, I., Korai, Y. and Fujitsu, H., Carbon, 1987, 25, 259. Marsh, H., Calvert, C. and Bacha, J., J. Mater. Sci., 1985, 20, 289. Mochida. I., Korai, Y., Oyama, T. and Qiug Fei, Y., Fuel, 1988. 67, 1171. Brooks, J. D. and Taylor, G. H., in Chemistry ond Physics of Carbon, Vol. 4, ed. P. L. Walker Jr. Marcel Dekker Inc., New York, 1968, p. 243. Marsh, H. and Walker, Jr, P. L., in Chrmisrry und Physics ofCarbon, Vol. 15, ed. P. L. Walker Jr. Marcel Dekker inc., New York, 1979, p, 228. Marsh, H. and Menendez, R., in Introduction to Curhorr Science, ed. H. Marsh. Butterworths, London, 1989, p. 59. Buechler, M. and White, J. L., 15th Biennial Conjtirence on Carbon. American Carbon Society, Philadelphia. U.S.A., 1981, p. 182. Mochida, I., Korai, Y., Oyama, T., Neshumi. Y. and Todo, Y., Carbon, 1989, 2?, 359. Mochida. I.. Matshuoka. H.. Fuiitsu. H. and Korai. ” Y., Curhon, 1981, 19, 218. Diefendorf, R. J.. 16th Biennial Conference on Curbon. American Carbon Society, San Diego, U.S.A.. 1983. p. 26. Mochida, I., Korai, Y. and Oyama, T.. C&on, 1987, 25, 213. White. J. L., Buechler, M. and Ng. C. B., C’arhotl, 1982. 20, 536. Lewis. R. T.. Lewis, I. C., Greinke. R. D. and Strong. L. S.. Curbon, 1987. 25, 289. Mochida. I., Fujimoto, K. and Oyama, T.. in Chemi.w~ und Ph~~.sic.vof Carbon, Vol. 24, ed. P. A. Thrower, Marcel Dekker Inc.. New York. 1994. n. I I I. Adams, H. A., in Introduction to C’urhon Technologies. ed. H. Marsh. E. A. Heintz and F. Rodriguez-Reinoso, Secretariado de Publicaciones, Universidad de Alicante. Spain, 1997, p, 492. Romero Palazon, E., Carbonization de residuos de petroleo: mecanrsmo y control. Ph.D. thesis, University of Alicante. Spain. 1990. Mochida. I., Oyama, T, and Korai, Y., C’urbo~~. 1988, 26, 51. Marsh. H., Rodriguez-Reinoso. F., Martinez Escandell. M. and Torregrosa Rodriguez, P., in The kinetics of pitch pyrolysis a review, to be submitted to C&on, 1997. Dubois, J., Agache, C. and White. J. L., Merullogruphy, 1970, 3, 337. Romero Palazon. E., Marsh, H., Rodriguez-Reinoso. F. and Santana, P., Extended Abstracts, IYrh Bienniul Conjerence on C&on. American Carbon Society, University Park, PA, U.S.A.. 1989. D. 178. White, J.-L. and Price, R. J., Carbon, 1974, 12, 321. Marsh, H. and Latham, C. S., in Petroleum Derived Carbons, No. 303, ed. M. L. Marvin and T. M. O’Grady. ACS Symposium Series, Washington, DC, U.S.A., 1986, pp. l-28. Rand, B., in Petroleum Derived Carbons, No. 303, ed. M. L. Marvin and T. M. O’Grady. ACS Symposium Series, Washington, DC. U.S.A., 1986, p. 45.