Revamping Vacuum Heaters

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Revamping Vacuum Heaters Powered By Docstoc
					Originally appeared in:                                                       1998 issue, Pgs 119-125

PTQ Autumn                                                                    Reprinted with publisher’s permission.

                          HEATING THE PROCESS AND FLUID FLOW



    Revamping Vacuum Heaters
  With crudes becoming heavier, a number of refineries are carrying out deep cut
 revamps to improve cutpoint. In most cases vacuum heaters can be revamped to
 handle increased capacity, with the bonus of improved efficiency and longer life.
                                                Ashutosh Garg
                                             Furnace improvements

Vacuum units are the backbone           tubes are laid out on both                The industry is slowly moving
of any refinery. Next to the            sidewalls     in    a    horizontal   towards vertical tube designs of
atmospheric unit, the vacuum            manner. The flow of fluid is          vacuum heaters. Vertical tube
unit is the most common unit in         generally from top to bottom.         heaters are compact, require
the refining industry. In the past      The burners are provided on the       less plot space, and are more
decade, it has acquired extra           floor and are spaced for              economical than horizontal tube
significance due to the fact that       providing        uniform      heat    heaters. The vertical tube
crudes are becoming heavier.            distribution.                         radiant box could be a cylinder
There      is     a      continued          Some of the older designs of      or a box with tubes design
requirement      world-wide     for     heaters have burners firing           allows the use of larger and
heavy distillate feedstock, which       horizontally on both end walls.       fewer burners.
places much emphasis on the             The arch of the heater is either          The draught available in
need to cut deeper into                 flat or sloped. Sometimes,            vertical tube designs is mostly
atmospheric residue. Attention          radiant tubes are also installed      higher than box heaters. Vertical
is therefore focused on the             in the arch section. The              tube designs are generally
operation of vacuum distillation        convection section is mounted         limited to 60 ft. tube length due
unit.                                   on top of the radiant section. It     to tube handling limitations. The
    With heavier crudes now             consists of bare and extended         size of vertical tube cylindrical
being processed, the vacuum             surface tubes.                        heaters is limited to 150
units are becoming overloaded.              Finned tubes are used in the      MMBtu/hr        although    larger
The price differential in crudes is     gas-fired heaters and studded         heaters have been built using a
also forcing refiners to switch to      tubes are used in the oil fired       multi-cellular concept.
heavier crudes, and to this end         heaters. The convection section
a number of refiners are                absorbs 20-30 percent of the          Heater Outlet
revamping their vacuum units.           total heat duty. Most modern          One of the distinct parameters is
The two most important items of         vacuum heaters have provisions        the operating condition at the
equipment in the vacuum unit            for adding two extra rows in the      vacuum heater outlet. The
are the vacuum heater and the           convection section, which are         vacuum tower operates under
vacuum tower.                           useful during debottlenecking.        the vacuum conditions in order
    This article concentrates on        The flue gases are cooled within      to recover the high boiling
                                                  o
vacuum      heater     revamping,       150-250 F of the inlet fluid          components          from     the
which in most of the cases              temperature. A number of              atmospheric distilled oil.
results in increased capacity,          vacuum heaters are provided
improved efficiency and longer          with steam superheating coils in      While the thermal instability of
life.                                   the convection section for            the oil limits the operating
                                        superheating column steam.            temperature in the system, the
Vacuum Heaters                          The stack is mounted on top of        distillates yield is maximized by
The typical vacuum heater is a          the convection section to             injection of live steam to reduce
single cell or twin cell horizontal     provide draught and dispose of        the       hydrocarbon       partial
tube cabin heater. The radiant          the flue gas safely.                  pressure. A part of this steam is

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injected into vacuum heater             induces vaporization at the            fluid achieves the critical
tubes. However, this is achieved        lower       temperatures      and      velocity. If the transfer line is too
at the expense of considerably          therefore      the    peak    film     small, a high pressure drop will
higher volumetric flow rates,           temperatures       are   reduced.      be     created,      causing      the
which require a larger flow area        Steam injection in the vacuum          temperature of the stock to drop
of the tubes in order to limit the      heater gives rise to extra             in the transfer line as it
pressure drop.                          vaporization of the charge.            vaporizes. Consequently, the
    Heat absorption rate and the            Injecting steam often means        charge outlet temperature will
pressure determine the process          that the pressure drop across          have to be increased to
temperature profile inside the          the         heater      increases      maintain the desired design
tubes. A lower pressure drop            substantially. Steam can be            flash zone temperature and cut
will help in lowering the               injected at the heater inlet, at       points for the downstream
operating temperature.                  the radiant inlet, at the point of     processing column. Coking and
    A fine balance of these             vaporization or at the point           cracking the furnace will also
parameters is essential for the         where charge reaches the               increase.       The       maximum
optimum design of a vacuum              cracking temperature. Steam            allowable pressure drop in the
heater.                                 injection is also beneficial in        transfer line should be chosen
    The     radiant      coil    is     reducing coking of the tubes in a      so that the required flash zone
characterized by the presence           limited way. Some vacuum               conditions will be obtained
of increasing tube size towards         heaters are designed and               without excessively high furnace
the outlet. Typically the last          operated without any steam             outlet temperatures.
three or four tubes in each pass        injection and are known as dry
are gradually increased in size         vacuum heaters.                        Design Considerations
to keep the fluid velocity below                                               Appropriate      changes       and
the sonic velocity. Fluid velocity      Transfer Line                          modifications can be made to a
at the furnace outlet should be         Transfer line is used to connect       vacuum heater to allow heavier
as high as possible, without            the furnace outlets with the           residues to be satisfactorily
exceeding 80 per cent of critical       distillation tower. For most           processed.        The      heater
velocity. The tube size at the          furnaces, the transfer line            debottlenecking basis must be
outlet is generally limited to 10-      design is based on reasonable          such as to fully and adequately
12 in. diameter. Larger tube size       pressure       drop,    mechanical     exploit degrees of freedom
present problems of supporting          flexibility, and consistency with      available. The two main degrees
and handling inside the heater.         the process design. However,           of freedom, which can be
                                        vacuum heater transfer line            exploited in a vacuum heater
Steam Injection                         requires special considerations        revamp       are    the      outlet
Another distinct feature of the         due to the high specific volume        temperature and pressure.
vacuum heater is the steam              of the fluid at the tower              Outlet      Temperature:      The
injection in the heater coils. The      conditions.                            maximum temperature to which
use of live steam helps to                  Even with a very low fraction      the oil can be subjected is
maximize the distillates yield in       vaporized,       dispersed     flow    limited by the need to control
the vacuum distillation unit.           conditions exist in the outlet         the extent of thermal cracking.
However, excessive use of               tubes and transfer line due to         The cracking of oil can be
steam will lead to higher motive        the low-pressure level and high        regarded as being a first order
steam for the ejectors and              specific volume of the vapor.          reaction and the degree of
cooling water in the condensers.        Working back from the vacuum           reaction, or thermal breakdown,
    Generally, optimization of the      tower towards the furnace,             is a direct function of time and
vacuum level and the steam              charge pressure increases,             temperature.              Another
rate will ensure a maximum              thereby       reducing      specific   phenomenon occurring in the
yield of distillates at the highest     volume. This permits the use of        vacuum heater outlet tubes is
allowable                 operating     a progressively smaller diameter       the       peaking     of      fluid
temperature. As mentioned               transfer line without exceeding        temperatures.
above, a part of this steam is          the critical velocity.                      This   happens      due     to
injected into the heater tubes.             Fluid velocity in the transfer     pressure drop in the tubes and
    The presence of steam in the        line is also limited to 80 per cent    fittings and it suppresses
heater serves to reduce the             of critical velocity, although at      vaporization of charge and
hydrocarbon partial pressure. It        the outlet of transfer line the        results in fluid temperature
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increase. Therefore, in most of         Hence,        before        the      cracking and it will lead to
the cases, the maximum bulk             debottlenecking    scheme     is     coking in the column. The
temperature in the heater is            finalized      the      current      transfer line size may have to be
generally higher than the outlet        performance of the vacuum            increased if the pressure drop
temperature.                            heater must be thoroughly            across the transfer line is
    This differential needs to be       evaluated and analyzed.              leading to very high outlet
controlled, especially during the                                            temperature.
revamps. It is necessary to limit       Revamp Considerations                Case Study
oil degradation by adhering to a        Any revamp of a vacuum heater        Frequent coking was happening
suitable         time/temperature       must start with flash zone           in a vacuum heater. Analysis of
relationship. In the vacuum             conditions. Typically the column     the problem revealed the
heaters, a restriction on the           inlet temperature is in the range    following:        flash         zone
                                                     o                                             o
maximum, bulk oil temperature           of 700-750 F and the pressure        temperature 725 F; vacuum
and heater outlet temperature is        is in the range of 2-4 in Hg. For    heater      outlet      temperature
                                                                                  o
often placed to control the             heaters used in lube oil             777 F.
cracking. Excess cracking will          manufacture, colour and stability         There was a temperature
                                                                                                       o
increase the tendency for coke          of the charge also come into         drop of more than 50 F across
laydown.                                play.     These     heaters    are   the transfer line. In this case,
    Maximum permissible bulk            designed with lower heat fluxes      increasing the transfer line size
oil temperature is very much a          and with steam injection to the      was the first step towards
function of crude composition.          coils to reduce the outlet           debottlenecking.
For revamp purposes, it is              temperature.                              The     client     ended      up
frequently necessary to ensure               In the area of deep cut,        increasing the transfer line from
that the temperature limit is fully     where refiners are trying to         12 to 24 in. This reduced the
exploited. Full benefit from such       extract the maximum from the         temperature drop across the
                                                                                                 o
temperature increase results            atmospheric tower bottoms, the       transfer line to 15 F.
only when the appropriate               designers are shooting for                One of the important criteria
vaporization can be handled in          highest flash zone temperature       in vacuum heater design is the
the heater. High heater outlet          and     lowest     pressure.    An   radiant heat flux. It is usually
                                                                o
temperatures, with transfer line        increase of 25-70 F in the           limited to 10,000-11,000 Btu/hr-
                                                                               2
permitting gives higher column          heater outlet temperature is not     ft . Radiant heat flux is one of
temperature.                            unusual.                             the important parameters. It is
Outlet Pressure: For a given                 Once      the    flash   zone   defined as heat transferred per
maximum temperature limit, the          conditions are firmed up the         unit area. Vacuum heaters are
production        of     distillates    engineer         should      move    generally designed with lower
becomes a direct function of            backwards towards the vacuum         heat flux as compared to
pressure.       There      is      a    heater. Among the first items        atmospheric heater. Lower heat
considerable incentive to ensure        that need to be checked are the      flux keeps fluid temperature
that the column pressure is as          pressure drop and temperature        below the cracking temperature
low as can be economically              drop across the transfer line to     of the fluid. In dry vacuum
justified. This gets carried on to      fix the vacuum heater outlet         heaters, heat fluxes are even
the vacuum heater outlet.               conditions. This check will also     lower than the wet vacuum
     The transfer line pressure         reveal the adequacy of the           heaters.
drop becomes very important. It         transfer line and any need to             Flow distribution in a
is essential to have the pressure       revamp it.                           multipass      heater      is    very
drop in the transfer line as low             What is the acceptable          important. Maldistribution of flow
as possible. This will result in a      pressure drop across the             can cause coking in some
low pressure at the heater              transfer line? It varies according   passes. The number of passes
outlet. Using the right size of         to the operating conditions and      in a vacuum heater is designed
tubes at the heater outlet will         the cracking temperature. A          considering       the       available
provide the correct vaporization        good recommendation is to            pressure drop across the heater
without exceeding the maximum           have the pressure drop across        and the outlet velocity. The
temperatures.                           the transfer line equal to the       numbers       of      passes      are
     Debottlenecking                    flash zone pressure.                 generally kept even to ensure
concentrates       on   improving            A high furnace outlet           uniform heat transfer.
existing      vacuum      heaters.      temperature        would     cause
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Vacuum           heaters     are       reduce the flue gas
conservatively designed with           temperature leaving
low H/D ratio. The height of the       the        convection
radiant section is generally           section.
limited to between 2.0 and 2.5         Flue                gas
times the radiant section width,       temperature leaving
or diameter in the case of a           the        convection
vertical cylindrical heater.           section     can       be
     Lower     flux    rates are       reduced      by     the
desirable but these can be             addition               of
regarded as a reasonable               convective        heat-
maximum when debottlenecking           transfer surface. The
a heater.                              flue gas temperature
     Another parameter used in         approach can be reduced to               The client was experiencing
the industry is the residence
                                            o
                                       150 F of the charge inlet                short run lengths of the vacuum
time in the coil. Although no          temperature. This can be done            heater.
specific numbers are available,        in several ways, some of which                In the redesigned convection
the recommendation is to heat          are as follows:                          section, heat absorption in the
the fluid as fast as possible,               Adding tubes                      convection         section      was
while minimizing the residence               Replacing bare tubes              maximized. This minimized the
time of fluid in the zone where                 with extended surface           heat load in the radiant section,
the film temperatures are above                 tubes                           kept tubes cool and improved
     o
800 F, to minimize cracking.                 Substituting            finned    the overall operation and
     Other factors which are                    tubes for studded tubes         efficiency of the system.
equally important in a revamp                Change of service                      The     original    convection
are flow distribution, mass            Case Study                               section consisted of studded
velocity and flame impingement,        The convection section of a              tubes suitable for oil firing. Since
among others.                          vacuum heater was designed to            oil was no longer fired, studded
                                       minimize the convection section          tubes were replaced with finned
Debottlenecking Schemes                surface. Since the original              tubes, which are suitable for gas
Several          debottlenecking       design, the vacuum feed                  firing. Finned tubes are cheaper,
schemes can increase vacuum            preheat       temperature         had    offer more efficient heat transfer
heater capacity. The major ones
                                                               o
                                       increased from 650 F to 700 F.
                                                                          o     and lower flue gas pressure
are:                                   Currently, the flue gas leaving          drop.
                                                                                     The     original    convection
  Increasing the heat transfer        the convection section was
                                       more than 1100 F.
                                                         o                      section consisted of two bare
   area in the convection
                                           High flue gas temperature            rows and four studded rows.
   section
                                                                                Two additional rows of tubes
  Changing the number of              was loading the air preheater
                                       and ID fan. It reduced overall           can be installed in the
   passes.
                                                                                convection section of most
  Increasing the radiant coil         system thermal efficiency by
                                       about 5 per cent. The flue gas           vacuum heaters without making
   tube size.
                                                                                a major change. Most of the
  Installing     forced-draught       temperature leaving the air
                                                                                vacuum        heaters     have     a
   burners.                            preheater was higher by almost
                                            o
                                       150 F.      High         flue     gas    provision for the addition of two
  Installing an air preheating                                                 future rows.
   system.                             temperature to air preheater
                                       was also leading to higher air                The inlet-piping terminals
                                       preheat temperature. Higher              need to be relocated. In this
Increasing Convection                                                           case, space for the installation
Surface                                preheat     meant         more     air
                                       pressure drop across APH,                of sootblowers was used to
It is much easier to add a heat                                                 install an extra row of tubes and
transfer     surface      in   the     ducting and burners; and the FD
                                       fan started limiting.                    three additional rows were
convection section than in the                                                  added to the existing convection
radiant section. Convection                Higher        air         preheat
                                       temperature led to an increase           section layout without extending
section absorbs 20-30 per cent                                                  the current convection section.
of the total heat duty. Increasing     in the radiant heat flux. The high
                                       rate of radiant heat transfer was             The new convection section
the heat transfer surface can                                                   consisted of two bare and seven
                                       accelerating coking in the tubes.
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finned     rows,     which      were    the             downtime
accommodated in the existing            significantly.
convection box. While changing          Prefabrication        also
stud tubes with fin tubes, it is        eliminates     constraints
important to match the outer            associated            with
dimensions of finned and                tubesheets,     refractory
studded tubes. The flue gas             etc. Fieldwork is also
temperature exiting the new             minimized.
convection         dropped         to
                         o
approximately       800 F.       The    Changing Number of
performance before and after            Passes
the modification is compared in         Generally, the feedstock to the
Table 1.                                vacuum heater increases in
    Additional     heat      transfer   quantity substantially as heavier   Changing Radiant Tube Size
surface increases the flue gas          crudes are processed in the         In some cases, to handle
pressure drop. Lower stack gas          units. This often results in a      increased     throughput      and
temperature        reduces        the   high-pressure drop in the           vaporization, the outlet tubes in
draught       availability.      This   existing coil configuration and     the vacuum heater need to be
requires rerating the stack for         high mass velocities. The           increased in size to avoid
new operating conditions. One           vaporization in the vacuum          choking of flow. In such cases,
alternative is to make the stack        heater also changes due to the      the designer needs to make
taller or add an induced-draught        properties of the reduced crude.    sure the critical velocity is not
fan at the top of convection            Where heavy crude is being          exceeded in any of the tubes.
section.                                processed, vaporization at the          In one instance, the client
    A check of the heater’s             heater outlet is much less than     had 5 in, 6 in and 8 in tubes at
existing       foundation        and    as compared to a lighter            the heater outlet, and 6 in, 8 in
structure is required to ensure         feedstock. However, subjecting      and 10 in tubes were used to
that the additional loading can         the      charge      at    higher   replace them. In this case,
be safely borne by the                  temperature and lower pressure      additional heat transfer surface
foundation.             Sometimes       restores       some     of    the   was also provided in the radiant
foundation load limitations may         vaporization.                       section. Two tubes per pass
not allow either option. In such a          In most of these cases, the     were added towards the inlet of
case, one possibility would be to       radiant coil needs to be checked    each coil.
install a grade-mounted stack or        and rerated. If the number of
to place the convection section         heater passes is doubled as has     Installing FD Burners
and stack on a separate                 been done in some revamps,          Replacing natural draught with
foundation.                             the pressure drop in the            forced draught burners is of
    More space will be taken up         revamped coil becomes almost        immense benefit for vacuum
if an outboard convection bank          one-eighth of the original          heaters, which have very tight
(one       mounted         on      an   pressure drop.                      fireboxes.     Increasing     the
independent external structure)             The change in the number        capacity will require harder
is added to the heater. In              of passes needs to be done          firing. This generally results in
addition, the fluid pressure drop       carefully. In one case, a vacuum    flame impingement. In such
across the vacuum heater will           heater was revamped from four       cases, forced-draught burners
go up. Sometimes a new feed             passes to eight passes, to          provide     short    flame    and
pump may be required to offset          handle      50    percent   more    increased capacity. Flame size
the     additional    tube       side   throughput. This reduced the        is reduced and the firebox
pressure drop.                          pressure drop across the heater     temperature becomes uniform.
    In most of the cases                significantly.    Simultaneously,        Before   replacing    natural
involving convection section            the mass velocity was reduced.      draught burners, check the
revamp, it has been found that          This, combined with an increase     heater floor elevations, because
prefabricating the convection           in outlet temperature, led to       forced-draught burners require
section and replacing part or           reduction in heater run lengths     ductwork        and        deeper
complete convection section is          and       high     tube     metal   windboxes. Space should also
economical on an installed costs        temperatures. The heater had to     be available for installation of
basis. Prefabrication reduces           be completely rerated.              ducts and fans.

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Case Study                              the main burners
A horizontal tube cabin type            can     help   the
vacuum heater was plagued               problem. Forced-
with short run lengths due to           draught firing can
flame impingement and tube              also     help    in
failures. On inspection, it was         improving
found that the heater had a very        combustion of sour
narrow box width and the burner         gas.
                                                                               drop in the exit flue gas
flames were touching the arch
                                        Installing Air Preheating              temperature boosts thermal
of the heater. Natural draught
                                                                               efficiency by 1 per cent. Total
burners     have     long      flame    A vacuum heater built in 1956
                                                                               fuel savings in this case were
lengths.                                was revamped to increase the
                                                                               about 14-15 per cent.
     The air pressure drop              capacity. An air preheating
                                                                                    Installing an air-preheating
available across the burners            system was added and the heat
                                                                               system is a major revamp. It
was limited to 0.3 WC. The              duty of the vacuum heater was
                                                                               entails installing forced-draught
combustion air into the burners         increased by 16 per cent. The
                                                                               burners, forced-draught, and
was being induced at very low           charge         throughput       was
                                                                               induced-draught fans, hot and
velocities. Good mixing of air          increased by 30 per cent due to
                                                                               cold air and flue gas ducts, and
and fuel oil was not taking             change in operating conditions
                                                                               the air preheater. Space must
place. Flame lengths of these           and improved preheat of the
                                                                               be available for the air
fuel oil burners were almost 13-        charge.
                                                                               preheater, fan, ducts and
14 ft, and excess air was                    Adding an air preheater has
                                                                               dampers.
running at 40 per cent.                 remained one of the most
                                                                                    The performance of vacuum
     The        client           was    popular ways of debottlenecking
                                                                               heater before and after revamp
recommended to change the               vacuum heaters. Typically the
                                                                               is compared in Table 3.
burners to the force-draught            flue gas temperature leaving the
                                                                                    To conclude, exploitation of
type. Force-draught burners             vacuum        heater    convection
                                                                      o        vacuum heater potential will
used air pressure to induce high        section ranges from 650 F to
                                             o                                 become vital in the optimization
air velocity. High air pressure         850 F, which makes vacuum
                                                                               of refinery operation. Vacuum
created a lot of turbulence in the      heater ideal candidates for
                                                                               heaters offer a great potential
firebox. This led to a uniform          installing air preheaters.
                                                                               for    debottleneck     if    done
heating of tubes in the firebox.             The installation of air
                                                                                                   properly.
     The flames were short and          preheater recovers the heat
stable.     In     forced-draught       from the stack and introduces it
burners, air pressure energy            into the radiant section in the
promotes intimate mixing of fuel        form of the preheated air. The
and air, with excess air limited        preheated air combined with
to 10 per cent for fuel oil firing.     forced        draught       burners
     Table    2     shows         the   introduces turbulence in the
                                                                                                  Ashutosh
comparison of heater operating          firebox. At the same time, the
                                                                               Garg is a thermal engineer at
conditions before and after the         flame size is reduced.
                                                                               Furnace     Improvements,     Sugar
revamp.                                      With the addition of a            Land TX, USA, and has over 24
      Sour gas burning in a             preheater, the heater must be          years’ experience in design,
vacuum heater results in poor           rerated because air preheating         engineering and troubleshooting of
firing conditions. It is generally      boosts      the     radiant     heat   furnaces and systems for refining
available at a very low pressure        absorption. This raises the            and petrochemical industries. He
and has a high calorific value.         radiant heat flux and tubewall         holds a degree in chemical
This often results in flame             temperatures.                          engineering from the Indian institute
impingement. It can become a                 Besides       the     capacity    of technology, Kanpur.
limit in achieving the maximum          increase, the air preheating
heater capacity. Installation of a      system offers a huge energy
                                                                          o
correct design of sour gas tips in      saving potential. Every 35 F




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                                              *Reproduced with the permission of PTQ Autumn.

				
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Description: In most cases vacuum heaters can be revamped to handle increased capacity, with the bonus of improved efficiency and longer life.