Field of the Invention
[0001] This invention relates to the treatment of petroleum refinery sludges, and particularly
to a process for the treatment of such sludges to produce a coke-like product useful
as a petroleum coke.
Background of the Invention
[0002] In petroleum refinery operations, oily sludges are produced as a waste product. Such
sludges are generally characterized as three-phase, multi-component, organic sludges
containing water, oil, and solids. The oil component is typically one or more liquid
hydrocarbon fractions of petroleum ranging from gasoline components to heavy oils.
The solids component may comprise high boiling organic materials, or carbonaceous
solids, and may contain metallic and other inorganic materials. The water is present
as a result of the use of water for process cooling, cleaning, or the like. Because
of the presence of hazardous materials in such sludges, their disposal presents a
problem for the petroleum refiner.
[0003] Various methods have been proposed for disposing of oil sludges, but to date none
have proven to be entirely satisfactory. One such proposed method involves incineration,
but this presents environmental problems, the processing cost is very high, and time
and money is required to obtain the necessary permits to operate the process. Another
disposal technique is land disposal, involving land fill and surface impoundment.
[0004] As pointed out in the paper "Evaluation of Treatment Technologies for Petroleum Refinery
Hazardous wastes" presented at the conference on Performance and Costs of Alternatives
to Land Disposal of Hazardous Waste, sponsored by the Air Pollution Control Association
on December 8-12, 1986, The Hazardous and Solid Waste Amendments of 1984 (HSWA) state
that reliance on land disposal should be minimized or eliminated, and land disposal,
particularly land fill and surface impoundment, should be the least favorite method
of managing hazardous wastes. Five types of petroleum refinery sludges are identified
as being hazardous. These are:
1. Dissolved air flotation (DAF)
2. Slop oil emulsion solids
3. Heat exchange bundle cleaning sludge
4. API separator sludge
5. Tank bottoms (leaded)
[0005] Such sludges are considered hazardous because of the presence of metals such as lead
and chromium, or organic materials such as benzene, cresols, phenol and the like.
As an alternative to land disposal methods, various alternative technologies have
been proposed. These include solvent extraction, mechanical dewatering, thermal treatment,
chemical fixation and stabilization (mixing the oily sludge with a solidifying agent
such as cement). There is, however, a continuing search for a technique of disposing
of refinery sludges which both meets environmental regulations and is economic to
install and operate.
[0006] The United States Environmental Protection Agency, by its 1984 Coke Exemption, has
exempted from its standards for fuels to which hazardous wastes have been added petroleum
refinery wastes containing oil, provided such wastes are converted into petroleum
coke at the same facility at which those wastes were generated, unless the resulting
coke product exceeds one or more hazardous waste characteristics.
[0007] Due to the severe restriction by the HSWA amendments on land disposal techniques
of hazardous waste, various other disposal methods have been proposed. For example,
U.S. Patent No. 3,917,564 discloses a processing technique wherein refinery sludges
are used as a quench medium in a delayed coking process wherein the sludge is converted
into petroleum coke at a pressure and temperature which cause the hydrocarbons in
the sludge to crack and/or polymerize to form coke. While this latter technique may
be sometime satisfactory, not all petroleum refineries include a delayed coxer. Further,
if the sludge is, for example, very high in undesirable metals and ash, the resulting
coke product may not be satisfactory for use as coke.
[0008] Thus, while there have been various methods proposed for disposing of petroleum refinery
sludges, there is still a need for a simple and inexpensive process to convert such
sludges to a coke-like product which is useable as a non-hazardous, non-regulated
fuel.
[0009] It is also known to distill hydrocarbons from contaminated oil to leave a residue.
For example, U.S. Patent No. 4.101,414 discloses a process wherein used lubricating
oil is predistilled in a steam-stripping still and the predistilled used oil is then
subjected to vacuum distillation at a temperature below the cracking temperature of
the oil to effect a separation of the used lubricating oil from a residue concentrate
byproduct of heavy lube oil hydrocarbons and additives. The concentrate product formed
during the vacuum distillation step includes heavy lube hydrocarbons, additives, metals,
metal compounds and the like and is stated to be useful as a lubrication grease.
[0010] It is an object of the present invention to provide a process for converting petroleum
refinery sludges to a coke-like product which is free, or substantially free, from
environmentally undesirable hydrocarbons, e.g. volatile organic compounds, such
as benzene, toluene and xylene.
[0011] Another object of the invention is such a process which permits treatment of petroleum
refinery sludges on-site and leaves non-hazardous waste.
[0012] A further object of the present invention is such a process which is simple in operation,
does not require substantial and elaborate equipment for carrying out the process,
which equipment may be portable, if desired.
[0013] An advantage of the present invention is that it does not require extraneous steam
for processing and, additionally, a cooling medium, such as cooling water, is not
required to cool the coke-like product produced by the process. A further advantage
of the invention is that the resulting coke-like product, while meeting environmental
requirements, has sufficient BTU content for it to qualify as a low grade petroleum
coke.
SUMMARY OF THE INVENTION
[0014] To achieve the objects and in accordance with the purpose of the inventicn as embodied
and broadly described herein, there is provided a process for treating petroleum refinery
sludge to produce a coke-like product which process comprises forming a feed stream
of an oily petroleum refinery sludge containing organic solid material boiling above
1000°F and a preselected amount of water; introducing said feed stream into a sludge
drier; heating the feed stream in said drier to a temperature above the boiling point
of water and below the temperature at which thermal cracking of hydrocarbons in the
feed stream occurs; maintaining the heated feed stream in a non-oxidative atmosphere
for a time sufficient for the water in the feed stream to form sream and for the steam
to strip any light hydrocarbons from said solid material in the feed stream; separating
a hydrocarbon/water vapor stream from said solid material; and recovering a solid,
coke-like residue product.
[0015] A preferred embodiment of the invention includes the step of pre-treating the feed-stream
prior to the heating step to adjust the water content to a level providing sufficient
steam for stripping light hydrocarbons from the solids while minimizing the heat required
to vaporize the water in the feed-stream.
[0016] It is also preferred to preheat the feed-stream, prior to introducing it to the sludge
drier, using the hot flue gases generated in the combustion of fuel used to heat
the sludge drier. Such hot flue gases may be used to produce steam for indirect heating
of the feed-stream in equipment similar to that used as the sludge drier.
[0017] The accompanying drawings, which are incorporated in and constitute part of this
specification, illustrate one embodiment of the invention, and, together with the
description, serve to explain the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018]
Figure 1 is a schematic representation of the preferred equipment employed in the
practice of the process of the present invention; and
Figure 2 is a schematic diagram of a feed hopper employed in the equipment of Figure
1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0019] The feed-stream for the present process is one or more petroleum refinery sludges
collected at the petroleum refinery at which the process is to be practiced. The petroluem
refinery sludges, typically are three-phase, multi-component sludges, usually containing
about 20-90 wt.% water, about 5-40 wt.% oil and about 5-35 wt.% solids. The oil portion
may include volatile hydrocarbons such as various aliphatic, heterocyclic and aromatic
compounds, as well as other organic compounds used or produced in the refinery operation.
Such organic compounds may be, for example, organic acids, such as phenols and cresols.
The solids portion of the sludge may include inorganic particulates, such as silica,
alumina and the like, metals and metallic compounds, and high boiling organic compounds,
e.g., those boiling above about 1000°F, which are normally solid in ambient temperatures.
In the case where the solid residue product is to be burned to utilize its BTU content,
the sludge should contain a sufficient amount of such high boiling organic solids
to make it feasible to operate the process for this purpose. Preferably, the sludge
does not contain large amounts of hazardous constituents such as lead, chromium, or
the like.
[0020] It is usually preferred that the sludge does not contain significant amounts of light
hydrocarbons, such as gasoline, naphtha, or kerosene, since they will not usually
provide enough residual energy and they are quickly volatilized because of their low
boiling points. Concentration of such light materials that will not recondense with
typical cooling water at standard pressure may present an explosion hazard and, therefore,
they are not desirable. Although crude petroleum is not preferred in the feed-stream
when it contains a high concentration of non-condensable hydrocarbons, it may be included
if appropriate equipment is provided.
[0021] The moisture content of the feed-stream is very important for the practice of the
present process. The feed-stream should contain sufficient water to provide steam
for adequately stripping light hydrocarbons and the like from the solids contained
in the feed-stream. If, however, the feed-stream contains water in an amount greater
than that required for the desired stripping, additional heat will be required to
vaporize the water and this will reduce the predicted capacity of the equipment. For
example, it was found that a sludge containing 15% water, 30% oil, and 55% solids
could be processed at a predicted throughput of 6 tons per hour; however, another
sludge containing 40% water, 9% oil and 51% solids could be processed at a capacity
of only 3.84 tons per hour.
[0022] If the sludge to be treated has an inadequate moisture content, of course, water
can be added to the sludge to adjust the water content of the feed-stream to a satisfactory
level. Usually, however, it is desirable to reduce the water content of the sludge
in order to form a satisfactory feed-stream for use in the process. Therefore, it
is usually preferred to pretreat the sludge by dewatering it prior to introducing
the feed-stream into the sludge drier, as hereinafter described. The sludge may be
dewatered by the use of centrifuges, filter presses or the like. It may be desirable
to use chemicals, such as demulsifiers and/or polymers, all of which may be used to
reduce the heat demand and increase the throughput so as to reduce operating costs.
[0023] As shown in Figure 1, a feed-stream 10 is formed from one or more of the petroleum
refinery sludges described above and passed into a solids concentration unit 12 such
as a filter press wherein oil and water are separated from the solids in the feed-stream
to provide a solids concentrate having a water content between about 5% and about
25%, based upon the total weight of the pretreated sludge.
[0024] The solids concentrate is passed by way of line 14 to a pre-drier unit 16 which preferrably
is a screw flight drier of the type well known in the art. A screw flight drier is
a hollow-screw jacketed-trough thermal drier wherein a solid material to be processed
is moved through the drier by a rotating screw mechanism and a heat transfer fluid
is circulated through the hollow screw and the jacket trough to heat the material
passing through the drier. The pre-heated solids are passed by line 18 to a thermal
desorber, or sludge drier 20, which also is preferrably a screw flight drier of the
type described above. A suitable heat transfer fluid, such as Dowtherm, or another
oil satisfactory for this purpose is heated to the desired temperature by heater
22 and circulated by a circulating pump (not shown) through thermal desorber 20 by
lines 24 and 26. Typically, heater 22 is fired by a refinery fuel, for example, refinery
gas, and the combustion thereof produces a hot flue gas which is passed via line 28
through heat exchanger 30 to vaporize water therein. The steam is then passed by line
32 to the predrier 16 and circulated therethrough as the heat medium to preheat the
solids concentrate.
[0025] In sludge drier 20 the solids concentrate is heared to a temperature above the boiling
point of water and maintained in a non-oxidative atmosphere. The residence time in
sludge drier 20 should be long enough for the water contained in the solids concentrate
to vaporize and permit the resulting steam to strip the remaining light hydrocarbons
from the solids in the feed stream to leave a solid residue.
[0026] Vapors containing light hydrocarbons and water are removed from sludge drier 20 via
line 34. The sludge drier 20 may be operated under a slight vacuum sufficient to
draw off the vapors, or it may be operated under a slight pressure sufficient to force
the vapors out of the system through line 34. These vapors are passed through a condenser
35 and the condensed water and hydrocarbons are collecte and separated in an accumulator
(not shown) and recovered. Non-condensable vapors exit through relief line 37 provided
with a relief valve 37a and are routed to the refinery flare (not shown)
[0027] It is important that the feed-stream passed through sludge drier 20 is not heated
to a temperature at which thermal cracking of hydrocarbons in the feed-stream occurs.
First, this would tend to produce coke which has a tendency to foul the system. Secondly,
at temperatures below the thermal cracking temperature no free radicals are formed,
preventing any non-condensables from being formed. Additionally, processing below
the thermal cracking temperature level, e.g., less than about 660°F, prevents the
same free radicals from, in a secondary reaction, joining together to form larger
molecules.
[0028] Advantageously, a rotary valve 36 of the type well known in the art is used in the
line 38 connected to the solid discharge port of sludge drier 20. The valve 36 is
divided into several segments so that solid material may be transported out of the
vessel without a release of vapors to the atmosphere. As the valve rotates, it seals
the following and leading vanes so that at no time is there an uninterrupted path
for discharging vapors. The valve 36 is located far enough from the bottom of the
sludge drier 20 to prevent liquids from leaving with the produced solid residue. An
outlet port on the bottom of the discharge end of sludge drier 20 preferably is fitted
with a fine mesh screen to permit the draining off of liquids that, for any reason,
fail to be vaporized or converted to a solid. Such liquid can be recycled back to
the inlet line 18 ahead of the feed-stream flowing into the sludge drier 20. The
volume of any recycled liquids can be measured by any convenient means of liquid flow
measurement. When flow is detected, the operator can be notified and the liquid can
then be fed back to the feed line 18.
[0029] The solid residue exits the system after passing through valve 36 and is collected
as it drops into a collection bin 39, a 20-25 cubic yard container having a closed
top.
[0030] In a typical operation, based upon a solids content of feedstock ranging from 30
to 60 wt.% (of the feed-stream passed through line 18 to thermal desorber 20) and
a throughput of from 3.84 to 6.0 tons per hour, from about 1.19 to about 3.744 tons
per hour of the coke-like residue can be produced in a continuous operation.
[0031] It is preferred to use a feed hopper, or bin, 40 located in line 14 ahead of the
pre-drier 16 to provide a surge vessel providing a continuous feed-stream to the pre-drier
16. The de-watered material can be placed into a bin, ideally between about 5 and
about 7 cubic yard capacity, by any well known conveying device, such as an articulated
conveyor belt (not shown). The bin, or hopper, should be located so that material
from the bin gravity feeds into the pre-drier 16, is then conveyed through the drier,
and exits at the end away from the bin. Depending upon the conveyance properties of
the feed material to the pre-drier 16, it may be necessary to attach a vibrating mechanism
41, typically similar to an oil field shaker screen, to prevent bridging and plugging
of the feed hopper.
[0032] The heated solids exiting pre-drier 16 will typically be at a temperature about 100°F,
but not usually above 212°F. Therefore, line 18 should be vapor tight from pre-drier
16 to the inlet of sludge drier 20 to prevent emission of hydrocarbon vapors to the
atmosphere.
[0033] Thermocouples should be placed at the exit of both pre-drier 16 and sludge drier
20, as well as in the lines 24, 26 and 32, transporting the heat transfer fluids to
the sludge drier 20 and pre-drier 16, respectively. This permits a record of the temperatures
so that a continuous heat balance can be maintained.
[0034] The above described technique for the thermal processing of the petroluem refinery
sludges at low temperature to produce a coke like product suitable for use as low
grade petroluem coke provides a number of advantages relative to other known forms
of disposing of such sludges. Among these advantages, the process is cheaper, particularly
in a service mode, since the cost of a compressor to capture, liquify and contain
produced non-condensables is eliminated. The equipment used in the practice of the
process can be portable and does not require additional complicated and expensive
equipment.
[0035] Another important advantage is that, since the process operates below the thermal
cracking temperature, coking is avoided, preventing a fouling problem, usually caused
by the feed stock being passed directly over a heated surface. As explained above,
this prevents the formation of free radicals leading to the formation of coke deposits
on the interior of the processing equipment and eliminates the need for breaker bars
to scrape off accumulated hydrocarbons from the interior walls of the processing
equipment. Also, expensive equipment for collecting and condensing gases is not required.
[0036] Having described a preferred embodiment of the present invention, variations and
modifications thereof, falling within the spirit of the present invention, will become
apparent to those skilled in the art and it is intended that the scope of the present
invention be limited only by the appended claims.
1. A process for treating petroleum refinery sludge to produce a coke-like product
which process comprises:
a) forming a feed-stream of an oily petroleum refinery sludge containing organic
solid material boiling above 1000°F and a preselected amount of water;
b) introducing said feed stream into a sludge drier;
c) heating said feed-stream in said drier to a temperature above the boiling point
of water and below the temperature at which thermal cracking of said feed-stream
occurs;
d) maintaining the heated feed-stream in a non-oxidative atmosphere for a time sufficient
for said water in said feed-stream to form steam and for said steam to strip any light
hydrocarbons from the solid material in said feed-stream fo form a solid residue;
e) separating a hydrocarbon/water vapor stream from said solid material; and
f) recovering a coke-like solid residue product.
2. The process of claim 1, further including pretreating said feed-stream prior to
said heating step to adjust the water content of said feed-stream to said pre-selected
level.
3. The process of claim 1, wherein said feed-stream is API separator sludge, heat
exchange bundle cleaning sludge, tank bottoms sludge, slop oil emulsion solids, dissolved
air flotation float or any mixture thereof.
4. The process of claim 1, wherein the temperature of said heating step is between
about 250°F and about 660°F,
5. The process of claim 1, wherein the water content of said pretreated feed-stream
is between about 5% and about 25%, based upon the total weight of the pretreated feed
stream.
6. The process of claim 2, wherein the solid material content of said pretreated
feed-stream is from about 20% to about 75%, based upon the total weight of the pretreated
feed stream.
7. The process of claim 2, wherein said pretreatment comprises passing said feed
stream through a centrifuge, filter press or chemical treatment step effective to
dewater said feed stream.
8. The process of claim 1, wherein said feed stream is indirectly heated by a heated
oil heat transfer medium in a hollow-screw jacketed-trough thermal drier.
9. The process of claim 7, further comprising preheating a dewatered feed stream with
hot flue gas generated in the combustion of fuel used to heat said feed stream, prior
to introducing the dewatered and pre-heated feed stream into said sludge drier.
10. The process of claim 8, wherein said heat transfer medium is heated by a hydrocarbon
fuel, said feed stream is dewatered to a pre-selected water level, the dewatered feed
stream is preheated in a hollow-screw, jacketed-trough thermal preheater, and hot
flue gas generated in the combustion of fuel used to heat said heat transfer medium
is used to indirectly heat said dewatered feed stream in said preheater.