Misting 101: How to maximize shaft retorting

yields"

30th Oil Shale Symposium October 17-20, 2010

Larry M. Southwick, P.E. Cincinnati, Ohio

Outline"  Introduction   Gas Combustion Process   Misting   Benefits and detractions   Conversion to “hard driving”   Other units   The solution

Gas Combustion Process"  US Congress in 1944 enacted Synthetic Fuels Act

authorizing construction of demo plants

  Oil Shale Experiment Station at Rifle, Colorado

  Retorting processes studied depended on method of heat application:

  Thru wall - Pumperston

  Combustion in retort - Gas Combustion

  Heated gases or liquids - Royster

  Hot solids - TOSCO

  USBM Bulletin 635 reported on Gas Combustion

  Other studies by oil companies (6 Company, 17 Co)

Gas Combustion"  Crushed and sized shale   Rising hot gases retort shale

and vaporize oil   Carbonaceous residue is

burned in combustion zone   Oil product removed from

gas, which is recycled back to retort

  Process works efficiently because of mist formed in product cooling zone

Oil Misting Basics"  “From the onset of experimental work, it was observed that

the gas streams from the retorts usually contained shale-oil mist” (not droplets, but fine mist)

  “This fundamentally new concept led to the development of Gas Combustion Process”

  “If the oil is to leave the retort as a mist in the offgas stream, the droplets must be formed in the spaces between the shale particles and must be small enough so that inertial separation does not occur”

  “A refluxing problem occurs when the amount of oil on the shale is great enough to drip or flow down through the bed of shale”

  Entrainment of droplets off of shale does not occur here because gas velocity is too low - thus oil collected on shale will descend with the bed = REFLUXING

Misting Section"  Mist forms just above

retorting zone   Retort operates as a

countercurrent heat exchanger

  No sharp demarcation between retorting and product cooling

  Assume 700°F shale temperature as dividing point

Refluxing"  Refluxing of condensed mist

causes oil cracking

  Alters heat distribution in misting section due to revaporization and secondary cracking

  Equilibrium is stable under refluxing and not-refluxing

  Often depends upon conditions at start of run

  Cracking produces lighter, less viscous oil, but loss of production is severe

Mist Formation"  Mist is formed if:

  Oil vapor cools until gas becomes saturated

  Nucleation occurs

  Supersaturation, S, favors mist formation

  S is oil partial pressure in gas divided by its vapor pressure at shale temperature

  Mist forms when heat transfer to shale exceeds mass transfer of oil to shale

  Mass transfer depends on diffusion and on impaction

  Nucleation sites help form mists

Mist Dynamics"  Mist flooding rate obtained by

drawing mist from retort and feeding to external bed

  Raise cooling rate by lowering temp. of bed until refluxing

  When MassMeanDia = 2.5µ

  Oil rate is 8-10 lb oil/MSCF

  Flooding mist MMD = 3.0µ   5-6 lb oil/MSCF   Thus lb/MSCF 3-4 lost   Thus there is a maximum carrying

capacity to gas

  Flooding vel ¼ x 1” = 2.7 ft/sec   For 1” x 3” = 3.3 ft/sec

Mist Measurements"  Refluxing caused by collision between

mist and bed particle was incomplete explanation of refluxing – also unstable mist, mist growth and coagulation

  Mist impactor is standard test   Stages, 16, 8, 4, 2, 1, 0.5 µ

  Considerable (50%) collected in piping and elbows off retort

  High dilution gas = oil loss from gas carrying capacity

  Collection efficiency increases   Mist particle size goes up   Gas velocity increases   Mist loading increases   Small shale particles   High bed packing fraction (wide particle

size range)

Mist Profile"Distance above air inlet, ft. 8 6 4 2

Droplet, mass mean dia., µ 2.36 2.28 1.82 Plugged with fines

Loading, lb oil/MSCF 9.36 8.04 5.61

Temperature, ºF 140 300 470 800

  Once nuclei occurs, no new nuclei form   Mass balance confirms growth since larger diameter =

more oil per particle = loading rate   So oil is growing on existing nuclei   Tests using injected nuclei did not resolve

Removal of Liquid"  Refluxing liquid would cause accretions to form just above

air distributor, blocking retort operation

  Use drawoff systems to collect refluxing liquid   Worked well on small lab retorts, 1”, 2”, and 3.6”   Variable results when applied to 150 TPD retort   Drew off at zone where shale temperature is 600 ºF

  Two other options to eliminate refluxing   Draw off unmisted hot, dry gas   Draw off hot misted gas but above refluxing zone   These point the way to “hard driving” of retort

Challenges"  Minimize losses from impaction of mist on shale

particles - ergo no small particles

  Maximize evolution of oil - ergo, the smaller the particles the faster the net retorting rate

  Testing found that particles as small as 1/8 inch could be used, but particles smaller than that caused significant oil yield losses

  Limiting the minimum size to greater than 1/8 inch provided no great advantage

  But retort still limited by oil refluxing, not easily controlled nor readily amenable to design

  The challenge was also what to do with the fines from crushing - ergo TOSCO process

Hard Driving Paraho"  Capacity of iron ore blast furnaces were increased 150

years ago by “hard driving”

  Hard driving meant just feeding more and more ore

  They found blast furnace could handle ~30X feed

  So if remove shale retort bottleneck of oil refluxing, should be able to “hard drive”

  Thus eliminate mist formation or oil condensation

  6 Company (1966) solution was an oil drawoff pan, which did not work (a typical “boiling oil” solution)

  Rather try one of the other two solutions not picked (pull off oil before it cools enough to begin refluxing)

  The oil would be cooled and condensed externally to retort in equipment similar to that used before

  This now oil-free gas can be reheated, re-injected above pull-off, and provide mist-free shale heating

Demo Plant Example"  One shaft retort converted to this

concept to make it operable

  Original Hytort scheme ran under conditions where normal mists did not form (high pressure, H2 gas, small particles), and which enhanced condensation of oil vapor on solids

  Thus extract fumes before they condense, inoperable otherwise

  Scheme studied in cold flow model, had good zone isolation

  Hot tests were always just with retort zone, which worked well

Union B Retort"

  Spent shale gasifier had moving bed, rising vapors   Hot shale still evolving gases - cools and condenses   Mist or otherwise, refluxing became a problem

Zinc Fuming/Misting"

  Zinc ore (oxide) reduced and volatilized from retort

  Zinc metal fume will condense upon contacting cold downward flowing solids

  These vertical shaft retorts extracted hot fume or mist

  Process on left used splash condenser, right had labyrinth

Imperial Smelting"

  Shaft furnace, briquette feed (carbon + zinc oxide)   Keep top of shaft hot (1000°C), so no mist forms   Splash condensers inefficient, use four in series

Processing EAF Dust"

  Steel made in electric arc furnaces (EAF) by melting scrap

  Volatilizes zinc from galvanized steel, dust is hazardous

  EAF dust processed by heating to remove zinc

  Shaft furnace has similar zinc condensing problems

The Problem"  Nature of oil shale retorting leads to shaft retorts

  Counterflow of oil vapor and cold solids leads to formation of very fine oil mists

  Mists can lead to refluxing of oil, net yields suffer

  Retort operation also suffers - accretions, flow blockage, channeling of gas and shale

  Fines WILL lead to oil losses, lighter oil and more gas and more coke

  Low top temperature can also cause yield losses

  The bottleneck to capacity is oil refluxing down the retort

  The higher the shale rate, the more likely refluxing will occur, which sets and limits the feed rate

The Solution - DryTop"  Eliminate the oil refluxing bottleneck by removing mist or

hot vapor before oil condenses onto shale

  Collect oil in devices similar to WetTop operation, then reheat and re-inject gases above drawoff

  Blast furnaces, zinc retorting and distillation, EAF dust processing, even modified Hytort concept provide examples of hard-driving operation

  Further, if shale feed is wet, the heat required to vaporize the water can be supplied by the re-injected gas, eliminating high temperatures in the retorting zone

  Shale feeding and withdrawal devices may have to be modified for the greater throughput