Process and apparatus for removing soluble contaminants from hydrocarbon streams

A method for removing soluble contaminants, such as acetonitrile, from a non-polar hydrocarbon stream into a polar water stream by countercurrent flow between an electrostatic field generated by a pair of parallel electrodes. The electrostatic field is modulated in strength to produce a dispersing, mixing, coalescing, and settling cycle that is effective to mix and separate the fluids.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to electrostatic separation process and apparatus 
for the removal of acetonitrile from C.sub.4 and C.sub.5 hydrocarbon 
streams by passing a stream of water in counterflow to the direction of 
flow of the hydrocarbon through an electric field of varying electric 
field gradient established by a plurality of composite electrodes. 
2. Description of the Prior Art 
The Clean Air Act Amendments of 1990 have forced refiners to search for 
ways to introduce oxygen into gasoline to produce cleaner burning 
reformulated fuels. The leading component to satisfy these needs is Methyl 
Tertiary Butyl Ether (MTBE). MTBE has a high blending octane number and 
relatively low vapor pressure and is an excellent blending component. 
Other ethers presently to enter this market are tertiary amyl methyl ether 
(TAME) and ethyl tertiary butyl ether (ETBE). 
MTBE is formed by the reaction of isobutylene and methanol at mild 
operating conditions (100.degree.-180.degree. F., 100 psig) over a 
catalyst. The high selectivity of the reaction at these conditions allows 
94-95% of the reactive hydrocarbon to be converted to MTBE as limited by 
equilibrium constraints. By using a catalytic distillation column, 
essentially complete conversion is attainable. TAME and ETBE are formed in 
comparable processes by the reaction of isoamylene with methanol and the 
reaction of isobutylene with methanol, respectively. 
The etherification processes utilize strongly acidic ion exchange resins as 
etherification catalysts. These are strongly acidic organic polymers. As 
an isobutylene or isoamylene molecule meets alcohol at an active site, the 
reaction takes place rapidly forming ether. 
The activity of the catalyst for the etherification reaction is a function 
of the acid loading or capacity of the resin. This functionality is not 
linear; a loss of 20% of acid sites on the catalyst gives approximately 
50% loss of activity for conversion to MTBE. It is therefore important to 
minimize the deactivation of the catalyst with effective feed pretreatment 
to maintain peak performance and long catalyst life. 
The loss of catalytic activity may be caused by the adsorption of basic 
compounds or metal ions, the blockage of the active sites by polymeric 
products, or by the splitting off the functional groups due to long term 
operation at temperatures above 240.degree. F. The latter two causes are 
affected by the operating conditions of the MTBE unit. The major source of 
lost activity is typically from poisons entering with the feedstocks to 
the unit. Poisons to the catalyst include basic compounds such as ammonia, 
amines, caustic soda, and acetonitrile (ACN). 
In refinery applications, the largest source of hydrocarbon feedstock 
containing isobutylene is the C.sub.4 stream from the cat cracking unit 
(FCCU). Some C.sub.4 's are also obtained from fluid or delayed cokers. 
ACN is formed in these units that enters the etherification process with 
the hydrocarbon feed stream. The amount of ACN in the feed varies with the 
severity of the cat cracker operation, crude source, and catalyst used in 
the FCCU. The ACN level of refinery based MTBE unit feeds may range from 
&lt;10 ppm to &gt;550 ppm. Unlike all the other feed poisons which deactivate 
the catalyst in a plug flow fashion through the catalyst bed, ACN's 
deactivation mechanism is not immediate and results in a diffused 
deactivation throughout the entire bed. Catalyst deactivation by ACN 
occurs through the catalyzed hydrolysis of ACN to acetic acid and ammonia 
and the subsequent neutralization of the acid sites by the ammonia. 
In order to obtain adequate run lengths with the catalyst and optimum 
performance, the first step in the etherification process is a feed 
pretreatment step designed to remove the poisons to very low levels (&lt;1 
ppm). Since the poisons are much more soluble in water than hydrocarbon, 
the common treatment is a multistage water wash. The water and hydrocarbon 
streams are contacted utilizing trays or packing. In the tower the 
continuous water phase flows down the column as the liquid hydrocarbon 
droplets are dispersed upwards. Of the many poisons to the catalyst, ACN 
is the most troublesome. The tower design is based on ACN removal to 1 
ppm. The design variables include the number of theoretical contact stages 
and the flow rate of water. In most typical refinery MTBE units, a minimum 
of three contacting stages and at relative flow rate of 30 weight percent 
water to hydrocarbon is required to reduce ACN levels to the 1 ppm 
specification. This results in a tower containing at least three beds of 
at least 8 feet of packing in each bed, or 12-16 trays. The column must 
also contain sufficient height to allow the less dense hydrocarbon phase 
to separate from the water phase. This is important as free water can have 
an adverse effect on the catalyst. 
The amount of wash water required is also an important design variable. 
Wash water flow at 20 weight percent of the hydrocarbon is a minimum 
amount based on the efficiency of the liquid-liquid contacting. In many 
cases, much higher rates are used. This in turn results in a large flow of 
waste water extract leaving the column which must be handled either by 
reusing it in other refinery processes or more commonly, discharging it to 
the effluent treating plant. 
In summary, an important part of any refinery based etherification process 
is feed pretreatment to remove catalyst poisons so that economical 
catalyst life and high ether production rates are achieved. Early MTBE 
plants have water wash systems designed before the importance of ACN 
removal was recognized. Inadequate removal of ACN in those units gave 
catalyst bed life as short as six months. Water wash systems designed to 
effectively remove ACN has demonstrated catalyst life from 12 to 24 
months. Optimization of this step to make it more efficient resulting in 
reduced capital investment, operating expense, and water usage is 
extremely attractive. 
A liquid-liquid extraction process has three steps: 
1. Intimate contact between the two phases 
2. Coalescence of dispersed phase drops 
3. Separation of the phases 
Conventional liquid-liquid extraction devices use mechanical energy to 
create drops. The rate of mass transfer is proportional to the interfacial 
area, so one strives to create dispersed phase drops as small as 
practical. If the drop size is too small, residence time required for 
phase separation makes the contactor too large and too costly. 
Conventional phase contact devices generally use minimum dispersed phase 
drop diameters of approximately 0.5-1.0 millimeter. 
Extraction processes are often used when distillation is difficult or 
ineffective. Extraction utilizes differences in the solubilities of the 
components rather than differences in their volatilites. Extraction takes 
advantages of chemical differences between components rather than vapor 
pressure differences as in distillation. 
In liquid-liquid extraction two phases must be brought into good contact to 
permit transfer of material and then be separated. In extraction, since 
the two phases have comparable densities, the energy available for mixing 
and separation is small. The two phases are often hard to mix and harder 
to separate. The viscosities of both phases, also, are relatively high, 
and linear velocities through most extraction equipment are low. 
Therefore, in some types of extractors, energy for mixing and separation 
is supplied mechanically. This requires additional expense in equipment, 
maintenance, and operating costs. 
U.S. Pat. No. 4,702,815 discloses a system for removing brine from oil well 
production. A fresh water or less saline water is passed in counterflow to 
the oil well production through electric fields established by composite 
electrodes. 
U.S. Pat. No. 4,804,553 discloses a countercurrent dilution water flow 
system coupled with the electrostatic mixing of the dilution water with 
the brine inherent in oil well production. A plurality of parallel 
conductive electrode plates in which the voltage applied to the electrode 
plates is modulated becoming the equivalent of a multi-stage 
mixer/coalescer/separator. 
U.S. Pat. No. 4,606,801 discloses a method and apparatus for dispersing or 
mixing relatively polar fluids in a relatively non-polar fluid. The fluids 
are passed between electrostatic fields that are modulated to effectively 
mix and separate these fluids. 
The present invention is an improvement over conventional extraction 
techniques. It is an advantage that conventional type mixing and 
separation equipment are not needed. Generally, the electrostatic 
separation systems have been applied to the removal of connate insolubles 
in oil streams and other solid/liquid dispersions, no procedure has 
addressed the removal of hydrocarbon soluble materials into water by 
liquid-liquid extraction. 
SUMMARY OF THE INVENTION 
Broadly the present method of soluble contaminant removal comprises 
electrostatically separating a contaminant soluble in a non-polar liquid 
stream from said non-polar liquid stream into a polar liquid stream, the 
polarity of said streams being relative between said streams, comprising 
the steps of: 
(a) first flowing the non-polar stream between at least a pair of 
electrodes, 
(b) flowing the polar stream between the pair of electrodes, the streams 
preferably flowing countercurrent, 
(c) applying a voltage to the electrodes to establish an electrostatic 
field having the strength to shear and disperse the polar liquid into the 
non-polar liquid, 
(d) maintaining a strength of the electrostatic field to accomplish mixing 
the polar liquid with the non-polar liquid in order to extract the 
contaminant from the non-polar liquid into the polar liquid, 
(e) reducing the voltage applied to the electrodes to coalesce the 
dispersed polar liquid, 
(f) maintaining a low voltage on the electrodes for a predetermined period 
to allow coalesced droplets of polar liquid to settle and separate from 
the non-polar liquid. 
More preferably present invention relates to a method for removing 
acrylonitrile impurities from a C.sub.4 -C.sub.5 hydrocarbon stream by an 
electrostatic extraction and the apparatus for carrying out the method. 
Briefly a fresh water or less acrylonitrile contaminated stream is passed 
counterflow to the C.sub.4 -C.sub.5 hydrocarbon stream through electric 
fields established by electrodes. The power to the electrostatic field is 
modulated for the purpose of first mixing and then separating immiscible 
fluids in the electrostatic field. The acrylonitrile impurities are 
removed from the hydrocarbon stream by extraction into the water phase, 
without a dilution effect on the hydrocarbon stream. This extraction 
process differs significantly from the electrostatic methods used to 
dilute brine in oil well production. 
The apparatus of the present invention is that described for carrying out 
the process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Electrostatic phase contacting and separation can create much smaller 
dispersed phase droplets (0.01-0.05 millimeter) while maintaining the 
ability to separate phases in a relatively compact package. 
After creating interfacial area for mass transfer, dispersed phase drops 
must coalesce before separation. Coalescence occurs in two steps. First 
the drops must coagulate: a surface-chemical process that reduces forces 
stabilizing the dispersion such that drops can approach close enough for 
the attractive force between droplets to overcome electrostatic repulsion. 
Flocculation is the prelude to coalescence. First the drops collide. If the 
drops are properly coagulated, they stick together as a flocculated 
aggregate. This aggregate may coalesce into a single larger droplet 
provided there are no physical barriers (suspended solids, surfactant 
films) bound at the droplet interface. 
Flocculation can be promoted chemically, although adding chemicals to a 
closed loop process may cause problems. Flocculation by electric fields is 
preferred if the continuous phase is non-conductive relative to the 
dispersed phase. Electroflocculation has been commercially applied for 
several decades for water washing crude petroleum for salt removal. 
Phase separation in a solvent extraction process relies upon retention time 
in a quiescent zone to allow the flocculated drops to settle. The droplet 
size dominates behavior of a solvent extraction process since the drops 
must be large enough for practical phase disengagement. 
Direct current (DC) fields for flocculation have several beneficial 
effects. The dispersed water drops experience the attraction in a steady, 
unidirectional field so that the attractive force causes droplets to move 
toward each other. Therefore, coalescence effects dependent on drop 
proximity are enhanced. Migration ultimately results in movement of drops 
to one of the charged electrodes where they acquire a net charge from the 
electrode. In an array of oppositely charged electrodes, the charged drops 
then immediately accelerated toward the oppositely charged electrode. This 
results in drops of opposite charge flowing in opposite directions. The 
net result is a large increase in flocculation between droplets. 
Corrosion is a potential problem in any metal/electrolyte system in the 
presence of an electrical current. In an AC system, the rapid directional 
change in the current reverses the electrolytic reactions before diffusion 
of the reaction products makes these reactions irreversible. In a DC 
system, the electrolytic reactions are continuous, and corrosion can be a 
serious problem. 
The benefits of both AC and DC fields can be obtained by an electrical 
arrangement that places a DC field across adjacent electrodes while 
maintaining an AC field between these electrodes and electrical ground. 
The containment vessel and the water layer are at ground potential, so 
that corrosion is virtually eliminated. And AC field induced coalescence 
at the oil/water interface is maintained. Between the electrodes, DC 
induced migration of water droplets and enhanced electroflocculation 
result in greater droplet growth and better performance than an AC field. 
An equilibrium drop size is reached in an electroflocculation device that 
depends on field strength, with smaller drops occurring at higher field 
strength. Small drops require a high electric field gradient to achieve 
significant coalescence. Thus the field strength necessary to reduce the 
remnant water content of an organic stream to low levels (&lt;0.1 wt %) may 
produce a small equilibrium drop size that produces problems for phase 
separation. 
There are two approaches for avoiding this compromise. One approach varies 
the field strength as a function of time, and the other is to vary the 
electric field strength as a function of position in creating space. In 
the first, the field strength is subjected to a periodic variation that 
coalesces small droplets in a strong field, and then allows further drop 
growth in a declining field. FIG. 2 illustrates the concept of field 
strength variation as a function of time. Since the drops are settling as 
this occurs, the small droplets that settle slowly are subjected to 
repeated cycles while large drops fall out of the affected zone during 
periods of low field strength. 
The field strength can also be varied along its vertical axis by using 
resistive electrodes to generate the field. Again, the larger drops 
migrate downward into the zone of lower field strength which results in 
further growth. In practice, both methods may be applied simultaneously. 
Just as declining field strength can enhance drop growth, increasing field 
strength can decrease drop size by electrostatic mixing. Drop diameters of 
1-5 microns can be produced by electrostatic mixing. Several mechanisms 
contribute to this effect: 
(a) Increasing migration velocity at high field strength leads to increased 
hydrodynamic shear resulting in drop deformation and division. 
(b) If the field strength oscillates near the resonant frequency of the 
drops, oscillations resulting in drop shatter are produced. 
(c) Drop charge of sufficient magnitude has been shown to produce 
instability leading to drop shattering. 
It should be noted in each of these mechanisms that the forces leading to 
dispersion are largely confined to the dispersed phase with minimum power 
used to accelerate the continuous phase. Conventional extraction devices 
spend most of the power on accelerating the continuous phase. 
The rate of mass transfer in an extraction process is proportional to the 
interfacial area. Generally, one would like to operate with the smallest 
dispersed phase droplet size to obtain the highest mass transfer rate 
consistent with the intrinsic reaction rate of any chemical reaction. For 
a conventional extraction process that uses mechanical energy to mix 
phases, the dispersed droplet size is limited by the gravity phase 
separation process incorporated in the equipment. Since electrostatic 
energy can be used to control dispersed droplet size, one can build a 
countercurrent extractor that allows mass transfer to occur when the drops 
are small and disengages phases when the drops are larger. 
FIG. 1 shows the electrostatic separator having a pair of electrodes 14 
that are supported by high voltage bushings 16 disposed within a vessel 
22. The contaminant is contained within the continuous (non-polar) phase 
18 and enters the vessel 22 flowing up through the modulated electrostatic 
field generated between electrodes 14. The electrostatic field is 
modulated to effect a dispersing A, mixing B, coalescing C, and settling D 
sequence shown in FIG. 2. Although vertical flow for the contaminated 
fluid is preferred, the invention is not limited to this configuration, as 
the contaminated fluid may be flowed in any direction to enter the 
electrostatic field. At the same time the continuous phase 18 is 
introduced, the dispersed (polar) phase 12 is introduced, flowing downward 
through the electrodes 14. 
FIG. 3 shows a detailed view of the electrostatic separation process 
between the electrodes 14. The continuous (non-polar) phase 18 and enters 
from the continuous phase header 32 flowing up through the modulated 
electrostatic field generated between electrodes 14. At the same time the 
dispersed phase 12 enters from the dispersed phase header 30 and proceeds 
downward into the electrostatic field, which is modulated to first shear 
the polar fluid into small droplets and disperse them into the relatively 
non-polar fluid where they are mixed with the contaminant. These dispersed 
droplets then contact and unite with the contaminant in the non-polar 
fluid and are coalesced into droplets large enough to gravitate through 
the electrostatic field between the electrodes. This sequence is repeated 
many times as the fluid moves through the electrostatic field, allowing 
the polar fluid to gravitate downward and producing numerous 
countercurrent mixing stages. FIG. 2 shows a graph of the dispersing A, 
mixing B, coalescing C, and settling cycle D that is repeated as the fluid 
moves through the electrodes. 
FIG. 4 shows an alternative embodiment of a multistage electrostatic 
separator (ELECTRO-DYNAMIC.TM. Contactor). The dispersed phase 12 enters 
the vessel through a header placed above the electrode array 28 if this 
phase has higher density than the other phase. The continuous phase enters 
below the electrodes. The continuous phase 18 must be a relatively 
non-conductive organic phase to maintain the electrostatic field. 
As the electrostatic field is modulated as shown on FIG. 2, the dispersed 
phase 12 shatters to fine droplets (10-50 micron) for efficient mass 
transfer during the dispersing and mixing part of the cycle. During the 
coalescing and settling part of the cycle, the dispersed phase drops grow 
large enough to settle in the continuous phase to a slightly lower 
position in the electrode array before the next mixing and dispersing 
cycle begins. Thus many mixing and coalescing cycles occur while the 
dispersed phase is held between the electrodes. Many theoretical stages of 
extraction can be obtained in a single pass through the electrodes. 
EXAMPLE 1 
This work was done using a pilot vessel as shown in FIG. 1, constructed as 
a single channel (4".times.4") to simulate the exact geometry of a full 
scale commercial unit. The electrode length used for pilot work was 24". 
Commercial units can be built with up to 6' long electrodes. The pilot 
configuration has been proven to correlate with operation of full scale 
electrostatic contacting systems for other applications. TABLE I 
illustrates some typical results. The C.sub.4 feedstock obtained from a 
commercial MTBE plant. 
TABLE I 
______________________________________ 
Flow In C.sub.4 
Out C.sub.4 
C.sub.4 
Water ACN ACN Water/ Stages 
lb/hr lb/hr ppm ppm C.sub.4 (%) 
(Kd = 7) 
______________________________________ 
120 13 49 18 11 3 
100 13 24 6 13 4 
90 15 42 4 17 5 
144 13 40 3 9 8 
80 15 13 1 19 6 
______________________________________ 
The pilot operating data shows that for a range of typical operating 
conditions, the electrostatic contactor operates with 6" or less for one 
theoretical stage of contact. This contrasts with a typical packed tower 
or sieve tray contactor for this application which operates with HETP in 
the range of 6 to 8 feet. 
EXAMPLE 2 
Table II illustrates the performance for a commercial scale apparatus of 
the present process washing the C.sub.4 feed to an MTBE Unit. The results 
are based on a distribution coefficient K.sub.d =7 (mass concentration of 
ACN in water/ACN in C.sub.4 phase). The contactor design is based on 
electrode length to give 10 equilibrium stages. 
TABLE II 
______________________________________ 
Feed C.sub.4 Wash Water as 
Outlet C.sub.4 
ACN (wppm) mass % of C.sub.4 
ACN (wppm) 
______________________________________ 
100 25 0.2 
100 20 1.0 
100 15 7.0 
50 25 0.1 
50 20 0.5 
50 15 3.5 
25 25 0.1 
25 20 0.3 
25 15 1.8 
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A conventional feed pretreatment tower designed for 13000 BPD at 20% or 30% 
wash water rate requires a relatively tall vertical tower. The present 
process requires a horizontal vessel much shorter than the height of the 
conventional tower. Additionally, the water consumption could be reduced 
to 15 mass % or less. The savings in total installed cost for the new 
process can be up to 40% of the total installed cost for a conventional 
system. Reducing ACN levels to 1 ppm or less can increase both the 
catalyst life and MTBE production.