Integrated process for producing ethanol, methanol and butyl ethers

A methanol synthesis and an ethanol synthesis are integrated into a single continuous process with the by-product carbon dioxide generated in the ethanol synthesis being utilized in the methanol synthesis. The methanol synthesis and ethanol synthesis can be further integrated with isobutylene synthesis with by-product hydrogen formed during isobutylene synthesis being used as a raw material in the methanol synthesis. In the preferred embodiments the ethanol synthesis utilizes Zymomonas mobilis bacteria in anaerobic fermentation in order to maximize the amount of carbon dioxide produced in a form which can be utilized in the methanol synthesis, to reduce carbon dioxide emissions and to provide an ethnaol product which is highly suitable for reaction with the isobutylene to form ethyl tertiary butyl ether.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a novel integrated process for production 
of oxygenated compounds (alcohols and ethers) for motor fuel use. 
2. Background of the Prior Art 
In order to reduce air pollution, public policy dictates reductions in 
pollutants arising from the operation of motor vehicles powered by 
petroleum derivatives, especially motor fuel gasoline. The first major act 
was the virtual elimination of tetraethyl lead as a motor gasoline octane 
improver, which has resulted in definite, and measurable, reductions in 
the amount of poisonous lead emitted into the atmosphere. At the same 
time, refiners were obliged to replace lost performance by increasing 
motor gasoline octane levels by other means. Refiners placed primary 
emphasis on increasing the severity of reforming and also added 
octane-enhancing chemicals, including more butanes; more aromatics 
(benzene, toluene and xylene); and alcohols (especially ethanol) and 
ethers (especially methyl tertiary butyl ether, or MTBE). 
The outcome has been to successfully replace the lost anti-knock octane 
value from lead's disappearance, but concurrently, largely due to the 
increased reforming severity and higher levels of volatile butanes, the 
result has been to increase the evaporation of organic materials; to 
increase emissions of ozone-forming materials which cause urban smog; and 
to increase the aromatic content of motor gasoline, which leads to 
increased emissions of benzene, a known carcinogen. There is also the 
problem of increased emissions of poisonous carbon monoxide caused by the 
incomplete combustion of motor gasoline. 
Legislation has been enacted in the U.S. (and other industrialized 
countries) which mandates certain fuel characteristics in order to control 
deleterious emissions. Such legislation characteristically stipulates 
reductions in fuel volatility; carbon monoxide emissions; ozone-forming 
chemicals; aromatic content; and toxic emissions. One of the primary means 
of achieving these reductions has been to specify minimum oxygen levels in 
all motor gasoline sold in certain locations and at certain times of the 
year (in some cases, year-round). 
These mandated changes in the composition of motor gasoline will require 
significant increases in the amounts of oxygenated fuel materials being 
produced, especially: 
methanol as a component of MTBE; 
methanol for use in an 85:15 methanol: gasoline blend (primarily in 
California); 
ethanol as a highly oxygenated material to be added to finished gasoline at 
the downstream end of the distribution chain; and 
ethanol as a component of ethyl tertiary-butyl ether (ETBE). 
Large increases in the production of isobutylene to react with ethanol and 
methanol in producing ETBE and MTBE will also be required. 
Ethanol Production 
Fuel ethanol is currently commercially produced by yeast fermentation of 
fermentable sugars produced from starches, principally from corn. The 
carbon dioxide is often recovered and processed by third parties to be 
used in freezing poultry, making dry ice, in fire extinguishers, in 
recovering petroleum from "played out" wells by pressurizing them, etc. 
The value of crude carbon dioxide is low, normally $5.00 to $8.00 per ton, 
and in some locations there is no market for it at all. 
Furthermore, the inherent characteristics of the yeasts used in producing 
ethanol require that they go through an aerobic stage to multiply followed 
by an anaerobic stage to produce ethanol. The carbon dioxide from the 
first state thus contains considerable amounts of air, which is 
uneconomical to remove in most cases. See, for example, column 1, lines 
18-64 of U.S. Pat. No. 4,731,329 and column 1, lines 10-22 of U.S. Pat. 
No. 4,885,241. In addition, the metabolism of the yeast produces materials 
other than ethanol in significant amounts, including "fusel oils", 
"aldehydes", and especially "glycerol", which markedly reduce the amount 
of ethanol obtained from a given amount of sugar. 
Methanol Production 
Methanol is typically produced in essentially self-contained production 
facilities, which typically use natural gas (usually predominantly 
methane) and water as the basic raw materials. The process includes two 
basic steps: formation of "syngas" from methane and water, and synthesis 
of the methanol by reacting the syngas in the methanol reactor. The basic 
reactions are: 
##STR1## 
Natural gas is used not only as a reactant, but is also burned as a fuel to 
heat the reformer stage. In the reactor stage only a portion of the 
methanol synthesis takes place in one pass, so methanol is removed and 
unreacted materials are recycled. To prevent build-up of inert materials, 
a portion of the recycle gas is purged and its combustible content is also 
burned to heat the reformer. 
In large U.S. plants, which are generally highly efficient, the natural 
gas-to-methanol yield on a carbon basis is about 67%, and the heat value 
of the methanol (HHV) is only about 71% of the natural gas consumed. 
Furthermore, the cost of the natural gas represents a significant portion 
of the total plant cost. 
In the methanol synthesis shown above, it should be noted that the carbon, 
oxygen, and hydrogen are not in stoichiometric balance for methanol, there 
being an excess of hydrogen. When a supply of carbon dioxide is available, 
it is often added to the natural gas feed to correct the imbalance: 
##STR2## 
The addition of carbon dioxide to react with the excess hydrogen has thus 
reduced the amount of natural gas used per molecule of methanol from one 
to 0.75. However, this also reduces the hydrogen purge to heat the 
reformer, increasing its use of natural gas fuel somewhat. However, there 
is a net reduction in total cost. 
Isobutylene Production 
Isobutylene may be obtained in different ways. In petroleum refineries, 
off-gas from the fluid cat-cracker contains a mixture of butanes, normal, 
iso-, and iso-butylene. If this mixture is passed over an acid catalyst 
with methanol, the methanol reacts with the isobutylene to form MTBE, 
while the remaining butanes are unreacted and are returned to the 
refinery. The reaction is a simple addition: 
##STR3## 
Isobutylene may also be obtained as a byproduct of propylene oxide 
manufacture. However, the increased demand for isobutylene as a feedstock 
for tertiary butyl ethers has used up most of the easily available 
quantities, and it is now necessary to produce it from isobutane by 
dehydrogenation: 
##STR4## 
In practice, the hydrogen stream from this reaction contains, depending 
upon the specific process, additional quantities of hydrocarbons, ranging 
from C.sub.1 to C.sub.4 including normal, iso, alkane, or alkene forms. 
The stream is usually used as fuel in the refinery or chemical plant. 
The iso-butane discussed above normally is recovered from natural gas 
liquids, which contain both normal and isobutane forms, by firms that 
specialize in these separations. However, if isobutane is not available, 
mixed iso- and normal butanes may be purchased and converted to all iso- 
in a special unit preceding dehydrogenation. This step requires very large 
amounts of steam energy. 
ETBE 
ETBE (ethyl tertiary butyl ether) is produced by reacting ethanol, instead 
of methanol, with isobutylene over an acid catalyst. At present there are 
no dedicated units, but the product has been made in MTBE plants and in 
special pilot plants. 
MTBE 
MTBE, methyl tertiary butyl ether, being produced today is generally 
produced in refineries which combine their own isobutylene production with 
purchased methanol, or by companies with other sources of isbutylene who 
react it with purchased methanol. In any event, the raw materials have 
historically come from discrete, non-integrated sources. However, 
integration of methanol and isobutylene manufacture has recently been 
suggested by "Now, MTBE from Butane", by Muddarris et al, Hydrocarbon 
Processing. October 1980. 
Although MTBE has preempted the market for refinery-added oxygenates, there 
is a real need for others such as ETBE (or TAME, ETAE) to supplant or to 
be blended with MTBE. The following table highlights some of the more 
critical physical characteristics of MTBE and ETBE: 
______________________________________ 
MTBE ETBE 
______________________________________ 
Oxygen - wt. % 18.2 15.7 
Boiling Point - F. 133 163 
Blending RVP - psi 8.0-9.0 3.5-4.5 
Blending Octane - (R + M)/2 
107 112 
______________________________________ 
Although ETBE has a lower oxygen content than MTBE, it has a desirably 
higher octane value and lower vapor pressure. ETBE also has the advantage 
of being less reactive with automotive plastics and pipeline gasket 
material (e.g., less swelling of elastomers, etc.). Blends of the two 
ethers give better distillation curves than either separately. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to integrate the production of 
ethanol and methanol, and, optionally, tertiary butyl ethers, in a novel 
way that reduces the manufacturing costs of methanol and products derived 
therefrom by using by-product carbon dioxide from the production of 
ethanol. 
Another objective of the present invention is to replace a non-renewable 
raw material, i.e. natural gas methane, with a renewable carbon oxide 
source, i.e. by-product carbon dioxide resulting from the manufacture of 
ethanol from renewable fermentable vegetable materials. 
Another objective of the present invention is to replace yeast fermentation 
conventionally used in the production of ethanol with fermentation by 
bacteria such as Zymomonas mobilis in order to produce a by-product carbon 
dioxide uniquely suited to the production of methanol. 
A further object of the present invention is to reduce the level of carbon 
dioxide emissions often associated with the production of ethanol and 
thereby provide a process for ethanol production more compatible with 
public policy concerns regarding the so-called "greenhouse effect." 
Yet another object of the present invention is to provide an integrated 
process for the production of methyl and ethyl tertiary butyl ethers 
suitable for use as fuel additives. 
At its broadest aspect, the present invention involves the integration of 
ethanol production with methanol production in such a way as to achieve 
economies in both processes heretofore unattainable. More specifically, 
the present invention utilizes by-product carbon dioxide formed in the 
production of ethanol as a raw material for producing methanol. Further, 
the excess process heat from the methanol synthesis is utilized to 
generate steam which provides at least a portion of the energy required 
for ethanol production by fermentation. The present invention also 
involves the discovery that bacterial fermentation for the production of 
ethanol is uniquely suited to integration with methanol production and 
with the production of an ethanol product suitable for use in reaction 
with isobutylene for the production of ethyl tertiary butyl ether (ETBE). 
Accordingly, the present invention provides an integrated process for the 
simultaneous, separate production of ethanol and methanol. The process 
includes fermenting a vegetable material anaerobically in an aqueous 
medium to produce ethanol in the medium and by-product carbon dioxide 
containing less than 1% by volume air. The ethanol is recovered from the 
aqueous medium and, optionally, further reacted with isobutylene to form 
ETBE. The by-product carbon dioxide containing less than 1% by volume air 
is reacted with hydrogen resulting in the formation of methanol. The 
exothermic heat of reaction of the methanol synthesis, as well as heat 
from other process streams, is recovered by heat exchange between the 
intermediate products from the methanol synthesis and water, resulting in 
a generation of steam which is then utilized in the production of ethanol. 
In the preferred embodiments, the ethanol fermentation is effected 
utilizing the bacterium Zymomonas mobilis. This preferred fermentation has 
been found to be uniquely suited to applicants' integrated process both 
from the point of view of producing a by-product carbon dioxide suitable 
for use in the production of methanol, with minimum purge of carbon 
dioxide fractions containing excessive amounts of air, and from the point 
of view of producing an ethanol product containing less by-products having 
a deleterious effect in the production of MTBE by reaction with 
isobutylene. 
In one preferred embodiment the present invention is integrated with a 
process for the production of isobutylene from n-butane. In this preferred 
embodiment a further savings is achieved by utilization of the by-product 
hydrogen produced in isobutylene manufacture for reaction with the 
by-product carbon dioxide in the methanol synthesis. Prior to utilizing 
this hydrogen stream in the production of methanol, quantities of higher 
molecular weight organics, particularly olefinic compounds, must be 
removed. The cleaning of the hydrogen can be accomplished by one of two 
methods: (1) the deleterious compounds can be removed for sale or utilized 
in supplementing fuel gas or (2) the olefinic compounds can be converted 
to aliphatics, eliminating the related problems and thereby further 
reducing the demand for methane feed to the methanol synthesis reactor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 10 represents the methanol synthesis plant which is illustrated 
in detail in FIG. 2 and, as shown in FIG. 2, consists of a number of 
different units. Natural gas is introduced at 2 and water at 4 for steam 
reforming to form a synthesis gas (hereinafter "syngas") which, along with 
by-product carbon dioxide introduced at 6 and by-product hydrogen 
introduced at 8, is converted to methanol which exits at 12. The ethanol 
unit 20 is preferably multiple fermenters containing the bacteria 
Zymomonas mobilis. The by-product carbon dioxide produced by fermentation 
exits at 22 and is compressed from about two inches H.sub.2 O gauge to 
about 350 psig in a carbon dioxide recovery unit 24. From recovery unit 24 
the carbon dioxide is routed to the methanol unit 10 through a sulfur 
removal treatment 26 (sulfur containing gases are removed from the 
by-product carbon dioxide in a conventional manner, e.g. by pressure swing 
adsorption (psa) or by membrane separation). An ethanol product exits at 
28. Heat from the reforming and/or methanol synthesis reactions is 
recovered, e.g. in heat recovery unit 30, and utilized, for example, in 
the ethanol production units schematically illustrated at 20. Thus, the 
ethanol production process and the methanol production process are 
integrated with the ethanol production process supplying by-product carbon 
dioxide for methanol synthesis and the methanol synthesis providing heat, 
typically in the form of steam, for the milling and mashing, distillation, 
evaporation and/or distillers dried grains with solubles (DDGS) drying 
steps involved in ethanol production and DDGS recovery by fermentation. 
FIG. 1 also illustrates the further integration of ethanol synthesis units 
20 and the methanol synthesis units 10 into a process for the production 
of tertiary butyl ethers. N-butanes are introduced at 32 into an 
isomerization unit 34 wherein the n-butane is converted into isobutane 
exiting at 36. The isobutane is fed to a dehydrogenation unit 38 which 
produces isobutylene which exits at 40 and a by-product hydrogen gas which 
exits at 42. The by-product hydrogen gas is fed, after clean up, via 42, 
to the isomerization unit 34, and via 8 to the methanol production units 
10, thus integrating the isobutylene synthesis with the methanol 
synthesis. In the event isomerization is not integrated into the complex, 
more by-product hydrogen is available for purification and use in the 
methanol production unit 10. The isobutylene product 40 is then reacted 
with methanol in the MTBE unit 44 and/or with ethanol in the ETBE unit 46. 
Both MTBE and ETBE find utility as gasoline additives. 
FIG. 2 illustrates the methanol synthesis 10 in greater detail. As seen in 
FIG. 2 water 4 and natural gas 2 are introduced into a steam reformer 50. 
The result is a syngas product which is introduced into flash tank 52 to 
separate water and then into the suction side of a compressor 54, along 
with recycled gas 55 from the crude methanol separator 60 and by-product 
carbon dioxide from ethanol synthesis and purified hydrogen gas from the 
dehydrogenation unit 38. The compressor 54 compresses the admixture to 
approximately 1500 psia and feeds it into the methanol reactor wherein a 
catalytic conversion to methanol is effected. The methanol product is then 
routed through a heat recovery unit 58 which generates steam for use in 
the ethanol synthesis or elsewhere in the integrated process. The methanol 
product stream is subsequently fed to a separator 60 which serves to 
separate hydrous crude methanol from the unreacted raw materials. 
Several of the key unit processes shown in FIGS. 1 and 2 will now be 
discussed in greater detail below. 
Methanol Synthesis 
Compressed natural gas feed (primarily methane) is desulfurized, saturated 
with steam and catalytically reformed over a nickel reforming catalyst to 
convert the methane to synthesis gas (primarily carbon monoxide, carbon 
dioxide and hydrogen) utilizing conventional technology. The reforming 
reaction occurs at about 250-300 psig and 1600.degree. F. in the steam 
reforming furnace. The reformer-furnace stack gases pass through thermal 
recovery facilities where high pressure steam is generated which in turn 
is utilized to drive large syngas compressors. 
The methanol synthesis of the present invention departs from the prior art 
in that it utilizes by-product carbon dioxide, containing less than 1% 
air, derived from ethanol synthesis by fermentation. The by-product carbon 
dioxide may be fed to the reformer, along with the steam and natural gas. 
Preferably, the by-product carbon dioxide is fed to the suction side of 
the methanol synthesis gas compressor. The syngas exiting the reformer 
must be cooled from about 1600.degree. F. to about 100.degree. F. (at 
250-300 psig) prior to compression to 1,100 psig. Blending of the 
synthesis gas with the by-product carbon dioxide, and optionally 
by-product hydrogen, offers a number of advantages as compared to the 
feeding of these by-product gases to the reformer. Firstly, the blending 
downstream of the reformer allows use of a smaller reformer with 
consequent savings in capital costs. Secondly, the blending may be used to 
furnish a part of the requisite cooling of the synthesis gas. Thirdly, 
blending downstream of the reformer increases the concentration of the 
carbon dioxide in the synthesis gas stream with a related increase in 
reaction rate which serves to reduce the recycle rate for the methanol 
reactor. Fourthly, this preferred approach allows use of higher 
reliability centrifugal compressors in contrast to use of reciprocal 
compressors. Fifthly, if the flow of by-product gas is interrupted, the 
stable operation of the reformer is unaffected. 
The by-product carbon dioxide should be air free, preferably 1% air or 
less, in order to minimize: (1) the likelihood of side reactions, (2) 
poisoning of the catalyst, and (3) purge gas volumes. 
The capacity of the methanol synthesis reactor in terms of utilization of 
by-product carbon dioxide can be increased, if desired, by co-blending 
with hydrogen. In a preferred embodiment of the present invention 
by-product hydrogen, produced by conversion of isobutane to isobutylene, 
is desulfurized and then blended with the by-product carbon dioxide and 
synthesis gas at the suction side of the methanol reactor compressor. 
Desulfurization is effected in a conventional manner, e.g. by pressure 
swing adsorption (PSA) or by membrane separation. Thus, in this preferred 
approach, methanol, ethanol and isobutylene synthesis are integrated in 
such a way to produce substantial raw material savings (primarily methane) 
and substantial reductions of emissions of carbon dioxide. 
The synthesis gas, carbon dioxide and/or hydrogen, mixture is compressed to 
about 1,035 psig and enters the methanol synthesis gas recycle compressor 
where the pressure is increased to about 1,100 psig. Depending upon the 
exact design of the methanol unit, these pressures could vary slightly. 
This gas is now preheated and fed to the methanol synthesis reactor where 
the synthesis gas and by-product carbon dioxide are converted to methanol 
over a copper based catalyst utilizing conventional technology developed 
by ICI. See "The Methanol Synthesis: How Does it Work?", by Chinchen et 
al, Chentech (November 1990), the teachings of which are incorporated 
herein by reference. This conventional technology utilizes a copper/zinc 
oxide/alumina or a copper/zinc oxide/chromia catalyst. Most preferably, 
the exothermic methanol synthesis reaction occurs at about 1,100 psig and 
250.degree. C. over a copper/zinc oxide/alumina catalyst. Methanol 
synthesis reactor effluent is cooled in thermal recovery facilities and 
sent to distillation facilities to produce the desired methanol product, 
e.g. for use in MTBE synthesis. Unconverted synthesis gas is recycled to 
the methanol synthesis reactor. 
The reactions of by-product CO.sub.2 forming the methanol are: 
EQU CH.sub.4 +CO.sub.2 +2H.sub.2 .fwdarw.2CH.sub.3 OH+.DELTA. (I) 
EQU 3H.sub.2 +CO.sub.2 .fwdarw.CH.sub.3 OH+H.sub.2 O+.DELTA. (II) 
In the integrated process of the present invention the combined "hydrogen 
stream" from isomerization and dehydrogenation, when combined with 
by-product CO.sub.2, replace up to 100% (in terms of heating value) the 
amount of natural gas required for methanol production. 
In conventional processes methane is the primary raw material and, as a 
consequence, constitutes the primary source of the carbon oxides for 
methanol formation. In contradistinction, in the process of the present 
invention by-product CO.sub.2, e.g. from an ethanol fermentation unit, 
will constitute at least 50%, and preferably more than 60% of the carbon 
oxides to be reacted with the hydrogen from the dehydrogenation unit over 
a catalyst. As a consequence, the feed to the methanol catalytic converter 
will contain a higher ratio of CO.sub.2 to CO as compared with synthesis 
gas. The higher ratio of CO.sub.2 to CO will increase the reaction rate of 
methanol synthesis. Potential benefits include lower recycle rates with a 
related savings in energy, and possible downsizing of the methanol 
converter. 
Ethanol Synthesis 
The ethanol synthesis unit 20 employs a fermentation process which 
preferably contains the bacterium Zymomonas mobilis and serves to produce 
ethanol by continuous cascade fermentation. A suitable continuous process 
for the production of ethanol by Zymomonas mobilis fermentation is 
described in U.S. Pat. No. 4,731,329 issued to Lawford, the teachings of 
which are incorporated herein by reference. All phases of fermentation 
with Zymomonas mobilis are conducted anaerobically and, accordingly, the 
carbon dioxide produced by fermentation will contain less than 1% by 
volume air. It is possible, at least in theory, to derive a by-product 
carbon dioxide stream containing less than 1% by volume air from the 
second stage of a two-stage yeast fermentation process as described at 
column 1 of U.S. Pat. No. 4,731,329 and column 1 of U.S. Par. No. 
4,885,241. In such conventional two-stage yeast fermentation processes, 
the first stage involves propagation of the yeast under aerobic conditions 
and is referred to as the growth stage. In the second stage fermentation 
is conducted under anaerobic conditions to produce ethanol. However, in 
practice, a small amount of air or oxygen is typically added to the second 
stage in order to encourage yeast growth. Even if it were practical to 
conduct the second stage of a yeast fermentation under total anaerobic 
conditions, the result would still be a considerable loss of carbon 
dioxide from the first stage. In a conventional batch yeast fermentation, 
only 60-75% of the CO.sub.2 is recoverable (depending on the plant) since 
the initial volumes of carbon dioxide are contaminated with air and must 
be purged. Of course, the purging of such volumes of carbon dioxide is 
undesirable both from the viewpoint of environmental pollution and from 
the viewpoint of conservation of resources, which conservation translates 
to lower production costs in the present invention where the by-product 
carbon dioxide is used as a raw material in the synthesis of methanol. In 
continuous yeast fermentation processes the initial (first stage) vessels 
require air to promote yeast growth and vent gases are not recoverable 
from these fermenters. Again, only approximately 60-75% of the carbon 
dioxide is recoverable. 
Another advantage which accrues from use of Zymomonas fermentation in the 
integrated process of the present invention is that the ethanol produced 
by Zymomonas mobilis is superior to yeast derived ethanol because of its 
lower content of undesirable by-products and impurities such as fusel 
oils, aldehydes, etc., which are transformed during the etherification 
processes for the production of ETBE into undesirable by-products such as 
diethyl ethers. 
Isobutylene from n-butanes 
A mixed butane feedstock is first fed to a deisobutanisation 
(fractionation) column wherein the supply is separated into isobutane and 
n-butane and the n-butane split is then isomerized over a platinum on 
alumina catalyst as described, for example, in "Now, MTBE from Butane" by 
Muddarris et al, Hydrocarbon Processing, October 1980, pages 91-91, in UK 
Patent Application 2,080,297 by Rashid and Muddarris and in U.S. Pat. No. 
4,816,607 issued to Vora et al, the teachings of all three of which 
references are incorporated herein by reference. 
The combined isobutane streams are then fed to the dehydrogenation unit for 
conversion to isobutylene. The preferred butane dehydrogenation catalyst 
is a chromium oxide/alumina catalyst operated at 540.degree. C. to 
640.degree. C. as described by Rashid and Muddarris in UK 2,080,297A. 
However, a platinum/tin/alkali metal dehydrogenation catalyst, as 
described by Vora et al in U.S. Pat. No. 4,816,607 is a viable 
alternative, as is almost any conventional dehydrogenation catalyst. The 
conventional HOUDRY.TM. processes, specifically, the CATOFIN.TM. and 
CATADIENE.TM. process may also be employed to convert isobutane to 
isobutylene. HOUDRY.TM., CATOFIN.TM. and CATADIENE.TM. are trademarks of 
United Catalysts, Inc. 
Etherification 
The alcohol feed (methanol and/or ethanol) is reacted with the isobutylene 
over an acid bed catalyst in a low temperature/low pressure process that 
yields a high octane, clean burning oxygenate. The use of an acid catalyst 
for the production of MTBE in general, and the use of sulfonated solid 
resin catalysts in particular, is described by Vora et al in U.S. Pat. No. 
4,816,607. 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all changes 
which come within the meaning and range of equivalency of the claims are 
therefore intended to be embraced therein.