The application of NASTRAN to the design of a fluid solids unit plenum/cyclone/dipleg assembly is described. The major loads considered are thermal, pressure, and gravity. Such applications are of interest in the petroleum industry since the equipment described is historically critical

Nelson, N. W.

English

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APPLICATION OF NASTRAN TO A FLUID SOLIDS UNIT
IN THE PETROLEUM INDUSTRY
Norman W. Nelson
Exxon Research and Engineering Company
Florham Park, New Jersey
SUMMARY
The application of NASTRAN to the design of a fluid solids unit plenum/
cyclone/dipleg assembly is described. The major loads considered are thermal,
pressure and gravity. Such applications are of interest in the petroleum
industry since the equipment described is historically critical, and,
to the best knowledge of the author, has not previously been fully analyzed.
INTRODUCTION
Traditionally, mechanical equipment engineers at Exxon design and specify
certain pressure vessel internals which are deemed critical to good process
unit performance. Of the many unit operations carried out in a refinery, the
design of the internals for the fluid solids process presents perhaps one of
the most difficult challenges to the designer. Until recently, such designs
were based upon many years of cumulative company experience, and whatever
strength of materials and pressure vessel code approximations were available.
This paper deals with the relatively new application at Exxon of design by
use of numerical analysis computer codes. This change in approach has occurred
for a number of reasons, including continually increasing thermal and pressure
loadings for greater processing efficiency, higher cost of labor and materials
and the availability of numerical computer codes such as NASTRAN. This paper
describes the first plenum chamber/cyclone/dipleg assembly numerically
analyzed at Exxon.
121
https://ntrs.nasa.gov/search.jsp?R=19750023420 2020-03-19T21:23:21+00:00Z
STRUCTURALLOADINGSIN THE
FLUID SOLIDSREGENERATOR
A brief description of the fluid so]ids process is essential to under-
standing the nature of the loadings on fluid solids unit internals. Figure
i is a schematic drawing of the process flow. In the reactor, oil is
"cracked" at high temperature in the presence of a finely divided fluidized
catalyst. This cracked gaseous hydrocarbon passes through several stages of
cyclones to recover the catalyst, through the reactor plenum and out for
further processing. The cracking process leaves a carbon deposit on the
catalyst, which is continuously transferred to a regenerator where the car-
bon is burnt off. The combustion gases pass through several stages of cy-
clones, a plenum and from there out of the regenerator.
Both vessels are internally lined with an insulating refractory. How-
ever, since air is injected to cause combustion, the regenerator plenum runs
at a higher temperature (usually 650 to 750°C) and is, therefore, structurally
muchmore critical than the reactor plenum. Occasionally, combustion occurs
for short periods in the dilute phase above the fluid bed_ and_ in the
cyclones. This can raise local cyclone temperatures by several hundred °C
during an upset.
In addition to the thermal loadings described above, another major
loading occurs due to the pressure differential across the cyclone stages
separating the plenum from the reactor. This differential is usually about
14 kPa but may double during unit upsets.
The final major loading is the gravity loading. In addition to the
structural massesof the diplegs, cyclones and plenum, there exist substantial
nonstructural mass gravity loadings due to erosion-resistant refractory
linings in the cyclones and, in someapplications, the mass of
catalyst in a plugged cyclone.
PHYSICALLAYOUTAND
MODELLINGASSUMPTIONS
Figures 2 and 3 give the plenum planform and elevation for the particular
regenerator plenumwhich was analyzed. In this application_ the plenum isjoined to the head by an inner and outer skirt. There are ten primary/secondary
cyclone pairs which are arranged with the primaries alternately inside and
outside the secondaries. The secondaries are supported directly from the
plenum floor, whereas the outer and inner primaries are partly supporte_ by
rods. Determination of rod loads under the various loadings was one of the
objectives of the analysis.
122
As shownin Figure 2, a 36 ° segment of the planform was modelled.
Bince only symmetrical loadings were applied, symmetry boundary conditions were
enforced along these meridional planes. Study of Figure 2 will show that
rtrue symmetry is not quite satisfied along these boundaries. However, the
iapproximation is close, and, furthermore, the usual compromise with cost
must be made. Future NASTRAN levels which permit periodic symmetrlcity will
be of great value in such applications.
i
r The model is delineated circumferentially as shown in Figure 3.
iDisplacement boundary conditions for the skirt cylinders were calculated
iby hand.
i In setting up the load cases for the NASTRAN analysis, three general
load categories were recognized, i.e., gravity, pressure, and thermal.
Since the thermal displacement boundary conditions along the circumferential
cuts were at least one order greater than pressure displacement boundary
conditions, thermal displacement boundary conditions were assumed for all
loadings in the region_ and a separate load case was run for the thermal
displacement boundary conditions alone. This permitted all solutions to be
developed with only one decomposition and the use of the NASTRAN LOAD and
SUBCOM cards.
Figure 4 shows the coordinate systems utilized in mesh generation.
The ability of NASTRAN to accept multiple coordinate systems worked to great
advantage in applying in-house computer codes for mesh generation.
THE NASTRAN MODELS
The comDlete model is shown in Figure 5. Figure 6 shows the upper portion
of the model in more detail. CQUAD2 and CTRIA2 bending/membrane elements
were used to model the cylindrical skirts, the head, the floor and part
of the secondary cyclone barrels. These cyclone barrels were terminated
one cylinder decay length below the floor by a CBAR beam element "spider"
at which point a transition was made to stepped CBAR beam elements to model
the remainder of the cy_lone_ the dipleg, and the dipleg bracing. The. primary
cyclones, their diplegs, and the connecting ducts between primary and secondary
cyclones were all modelled using CBAR beam elements. The hanger rods were
modelled using CROD rod elements. CQDMEM membrane elements were used for the
floor beams.
As mentioned earlier, separate models were developed for the outer cylinder
skirt to head intersection. In addition to a high meridional thermal gradient
in this area, there is a change in material properties where the stainless
steel skirt joins the cooler mild steel shell. By studying stress response
to thermal fields arising from various insulation and structural arrangements
of the skirt cylinder, it was possible to optimize this detail.
123
The model in Figure 7 is a symmetry wedge of this skirt to head intersect£
which utilizes a pad plate. The "tails" are sufficiently long such that
temperature boundary conditions calculated from one-dimensional solutions
could be applied at the ends. For the plate elements, film coefficients
and environmental temperatures on either side were combined to form an
equivalent one-slded film coefficient and environmental temperature. In
the solid elements film coefficients and environmental temperatures were
applied in the usual way, except for the gap behind the pad plate. Here_
the environmental temperature was guessed, and an iterative approach adopted.
Symmetry conditions were enforced on the meridional planes for both
thermal and stress solutions. In both cases, the number of unknowns was
hulved by using multipoint constraints_ NASTRAN MPCs_ to tie like meridional
p6ints. NASTRAN, CHEXA2, and CWEDGE solid elements were used in the junction
zone and CQUAD2 plate-membrane elements were used in the "tails".
Figure 8 shows an alternative model studied wherein the junction between
stainless and mild steel components is located 5 cm below the intersection
point. Boundary conditions were developed as described previously for the
solid model. This model consists entirely of CQUAD2 elemeQts.
The large overall model contained 6787 degrees of freedom. It was
solved on the IBM 360-168 computer, with high speed multiplier, at a region
size of 2048K bytes. The semlbandwidth was 279 and there were 258 active
columns. The large semibandwidth was due to the lack of a bandwidth opti-
mizer being operational at Exxon at that time. Decomposition time was
2458 CPU seconds.
DISCUSSION OF RESULTS
In assessing the results, much use was made of MASTRAN deformed plots.
_cause of the different _ature of thermal and sustained loads, three general
load case categories were established. These are mechanical loads (which
consist of pressure and gravity loadings), thermal loads, and combined
loads (which are the sum of the first two).
Figure 9 is a meridional cut near the center of the wedge showing
underlayed deformations due to mechanical loads. The effects of pressure
on the outer skirt and head, as well as the effect of the hanger rod on
the head are clearly evident. The effect of the gravity load of the cyclone/
dipleg assembly on the floor deformations is also clear.
iS4
i Figure I0 shows the effect of combined loads along the same meridian.
ince the thermal load induced displacement field is at least an order
•eater than the mechanical 10ad induced displacement field, essentially only
ermal displacements can be seen. High local bending at the outer skirt
shell Junction can be clearly identified in this plot.
i A study of the stress printout showed that the structure as
_signed, with a few minor modifications, was indeed good for all the intended
_ads. The design pressure differential across the plenum was found to be
|sily accommodated within allowable stresses. This was found to be true
spite of the fact that it was twice that whlch would have been allowed
previously developed empirical procedures. These procedures limit the
_essure differential over the planform area to the equivalent gravity ef-
_ct of the head, floor, cyclones and diplegs.
I The one point of very high stress was the Junction area between the outer
_lindrical skirt and the head under thermal load. Figure ii shows the
_derlayed deformed plot of the design utilizing a mild steel pad plate.
gure 12 shows deformation vectors for the design employing a 5 cm long
Id steel skirt stub, at which point the junction to the stainless steel
_irt is made. Of the two designs, the latter shows lower stresses, and is
he one which was finally adopted.
CONCLUSIONS
This analysis has shown that modern applied mechanics computer codes
Uch as NASTRAN Can be effectively used in the petroleum industry to remove
verly conservatlve empirical restrictions which have developed over the years.
n this instance, not only was the empirical restriction on differential
lenum pressure removed, but the analysis also focused attention upon the
eal area of high stress; namely, the skirt to head intersection. Using
ASTRAN heat transfer and structural capabilities, a junction design was developed
hich brought stresses within allowable limits.
Various improvements planned for future NASTRAN _evels would be most
aluable now. Specifically, the ability to treat periodic symmetricity, as
tell as the ability to handle constitutive nonlinearities under thermal loadings
rould be most welcome.
125
FLOW DIAGRAM OF TYPICAL FLUID SOLIDS UNIT
Combustion
Air
R_egenerator
Reactor
d2
I
Figure i
126
PLANFORM OF PLENUM CHAMBER AND CYCLONES
36 ° Model
Seg ment
Floor
A
Transfer
Line
j.
A
//
/
I
I \I
"-. i I I ill / _1
\ /
Hea_ _ Load
Point
Outer
Skirt
Figure 2
127
TELEVATION VIEW OF PLENUM CHAMBER AND CYCLONES
------- T
T
P
THERMAL LOADING
PRESSURE LOADING
Figure 3
128
COORDINATE SYSTEM
IDENTIFICATION
Notes:
0 =
1 =
2 =
Implicit Cartesian System
Cylinder System For Skirts, Floor And Head
Cylinder System For Full Cyclone Barrel and Dipleg
Cylinder System For One-Half Cyclone Barrel and Dipleg
Figure 4
129
PLENUMCHAMBER
OVERALLVIEW
Figure 5
13o
OVERALL VIEW
Figure 6
i3i
CARBONSTEELPLATEMOUNTINGOF SKIRTTO HEAD
Figure 7
132
_,. STUB MOUNTING OF SKIRT TO HEAD
13-D MODEL)
T
r---"-
t
/
....._.
'2.
Figure 8
133
PROFILE VIEW
(UNDEFORMED AND DEFORMED)
MECHANICAL LOADS
MODEL PROFILE
(UNDEFORMED AND DEFORMED)
COMBINED THERMAL AND MECHANICAL LOADS
Z
i
Figure 9 Figure i0
134
CARBONSTEEL PLATE MOUNTINGOF SKIRT TO HEAD
[THERMALSTRESS SOLUTIONI
\
--t-'t-"1 I-
Figure ii
135
CARBONSTEEL STUB MOUNTINGOF SKIRT TO HEAD
(THERMAL STRESS SOLUTION)
Figure 12
136


Application of NASTRAN to a fluid solids unit in the petroleum industry

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