Automatic adsorption tower switching system

In a continuous adsorption process using dual towers for separation of a multi-component feed with one tower active and the other on regeneration, automatic switching of the tower feed based on loading of a hydrogen fluoride (HF) component in the active tower is achieved by inferring HF loading based on concentration measurement of acid soluble oil (ASO). An optimum switching point, which switches tower feed near full HF capacity of the active tower but before HF breakthrough occurs, is based on calculating a second derivative for a concentration vs. time curve of ASO in the active bed effluent.

This invention relates to process control. More specifically it relates to 
switching a feedstream between parallel arranged adsorption beds used in a 
separation process which removes a hydrogen halide compound from 
sulfolane. 
Fixed adsorption bed contactors are used in many chemical separation 
applications for the selective collection and concentration, onto solid 
surfaces, of particular types of molecules contained in a liquid or a gas. 
When the adsorptive capacity of the bed is reached, it must be regenerated 
to enable its reuse. Therefore to achieve essentially continuous operation 
from an adsorption process, a particular bed used in the separation 
process must cycle from adsorption to desorption. 
A parallel arrangement of adsorption beds is often utilized in a separation 
process to achieve continuous operation such that one adsorption bed is 
actively separating chemical components while the second bed is being 
regenerated. Regeneration of beds used in a multi-component separation 
system to adsorb hydrogen halide contaminates from a liquid process stream 
which is made-up of sulfolane and which also contains acid soluble oil 
(ASO), is accomplished by first passing a solvent for the ASO through the 
exhausted bed followed by passing a stripping fluid through the bed under 
conditions so as to regenerate the exhausted bed. Alkylation catalyst 
regeneration has been disclosed and claimed in patent application Ser. No. 
08/077,142 of Eastman, et al, filed Jun. 16, 1993, now U.S. Pat. No. 
5,306,859. 
Ideally, an adsorption bed would be utilized to its full capacity before 
the flow of liquid to be separated is switched to a fresh bed. However, in 
many operations a large margin of error must be maintained so as to insure 
that a hazardous hydrogen halide component such as hydrogen fluoride (HF) 
will not break through the adsorption bed. This large margin of error 
required in switching beds on a predetermined time cycle results in 
inefficient operation. 
Ideally, switching of the adsorption beds would be controlled on the basis 
of their loading so as to achieve their true capacity rather than operate 
on a predetermined time cycle. Since measurements based on HF are 
extremely difficult, attempts have been made to incorporate an inferential 
analyzer into a control scheme which would switch between adsorption beds 
based on HF loading of the exhausted bed. For hydrogen halide compounds 
these attempts include measuring the pH of the bed effluent and also 
calculation of the amount of material passed through the bed based on 
measurement of hydrogen halide concentration and flow rate. The 
aforementioned attempts, however, have not generally proven reliable 
enough for field applications. 
It is thus an object of this invention to provide a method and an apparatus 
for automatically switching the flow of a liquid feed stream comprising 
sulfolane, a hydrogen halide compound and ASO between parallel arranged 
adsorption beds. 
It is a further object of the invention to utilize the concept of control 
based on HF loading of adsorbent material for automatically switching 
feedstream flow between two adsorption beds. 
It is a still further object of this invention to eliminate reliability 
problems which occur when an adsorption bed switching is based on process 
measurements involving HF. 
SUMMARY OF THE INVENTION 
In accordance with this invention I have discovered that a suitable 
adsorbent material for a hydrogen halide compound also adsorbs ASO to some 
proportional extent, whereby the amount of the hydrogen halide compound 
captured in the bed during an adsorption cycle is inferred from the easily 
measured parameter of ASO concentration of the bed effluent. Switching of 
the flow of a process stream from a first tower containing an exhausted 
bed to a second tower containing a fresh bed is accomplished responsive to 
a signal representative of ASO concentration measurements, and which 
infers that an exhaustive amount of the hydrogen halide compound has been 
captured in the active bed. 
In a preferred embodiment the concentration of ASO in the effluent of an 
active bed is measured on line with the aid of UV-VIS 
(ultraviolet-visible) spectroscopy and this measurement yields an S shaped 
curve of ASO concentration as a function of time. Bed switching is 
triggered by a digital type signal based on the second derivative of the S 
shaped curve for ASO concentration reaching a maximum value, whereby the 
bed is switched when it is near its full capacity but before breakthrough 
of the hydrogen halide compound occurs. 
Other objects and advantages of the invention will be apparent from the 
foregoing brief description of the invention and the claims as well as a 
detailed description of the drawings which are briefly described as 
follows.

DETAILED DESCRIPTION OF THE INVENTION 
The invention is described in terms of a multi-component separation wherein 
contaminants of HF and ASO are removed from a liquid sulfolane stream. 
However, the invention is generally applicable to the separation of 
multi-component systems wherein an adsorbent material proportionally 
collects more than one type of molecule on its surface. 
Acid soluble oil is produced as a reaction by-product in an alkylation 
process which comprises the step of contacting an olefins/isoparaffin 
hydrocarbon mixture with an alkylation catalyst, which comprises a 
hydrogen halide component and sulfolane. As used within this description 
and in the claims, the term "acid soluble oil" or "ASO", means those 
conjunct polymers which are highly olefinic oils produced in an acid 
catalyzed reaction of hydrocarbons and which are soluble in the liquid 
catalyst. An extensive description and characterization of certain types 
of conjunct polymers oils is provided in the Journal of Chemical and 
Engineering Data Article entitled "Molecular Structure of Conjunct 
Polymers", pages 150-160, Vol. 8, No. 1, January 1963 by Miron and Lee. 
The hydrogen halide component of the catalyst composition or catalyst 
mixture can be selected from the group of compounds consisting of hydrogen 
fluoride (HF), hydrogen chloride (HCl), hydrogen bromide (HBr), and 
mixtures of two or more thereof. The preferred hydrogen halide component, 
however, is hydrogen fluoride, which can be utilized in the catalyst 
composition in anhydrous form; but, generally, the hydrogen fluoride 
component utilized can have a small amount of water. 
A specific control system configuration is set forth in FIG. 1 for the sake 
of illustration. However, the invention extends to different types of 
control system configurations which accomplish the purpose of the 
invention. Lines designated as signal lines in the drawings are electrical 
or pneumatic in this preferred embodiment. Generally, the signals provided 
from any analyzer transducer are electrical in form. 
The invention is also applicable to mechanical, hydraulic or other signal 
means for transmitting information. In almost all control systems some 
combination of electrical, pneumatic, mechanical or hydraulic signals will 
be used. However, use of any other type of signal transmission, compatible 
with the process and equipment in use, is within the scope of the 
invention. 
A digital computer is used in the preferred embodiment of this invention to 
calculate the required control signal based on measured process 
parameters. Any digital computer having software that allows operation in 
a real time environment for reading values of external variables and 
transmitting signals to external devices is suitable for use in this 
invention. Preferably a computer controlled spectrometer having sufficient 
excess computing capacity to calculate the required control signal is 
utilized. 
Signal lines are also utilized herein to represent the results of 
calculations carried out in a digital computer and the term "signal" is 
utilized to refer to such results. Thus, the term signal is used not only 
to refer to electrical currents or pneumatic pressures but is also used to 
refer to binary representations of a calculated or measured value. 
The control elements of the system can be implemented using electrical 
analog, digital electronic, pneumatic, hydraulic, mechanical or other 
similar types of equipment or combinations of one or more such equipment 
types. While the presently preferred embodiment of the invention 
preferably utilizes a combination of pneumatic final control elements in 
conjunction with electrical analog signal handling and translation 
apparatus, the apparatus and method of the invention can be implemented 
using a variety of specific equipment available to and understood by those 
skilled in the process control art. Likewise, the format of the various 
signals can be modified substantially in order to accommodate signal 
format requirements of the particular installation, safety factors, the 
physical characteristics of the measuring or control instruments and other 
similar factors. For example, a raw flow measurement signal produced by a 
differential pressure orifice flow meter would ordinarily exhibit a 
generally proportional relationship to the square of the actual flow rate. 
Other measuring instruments might produce a signal which is proportional 
to the measured parameter, and still other transducing means may produce a 
signal which bears a more complicated, but known, relationship to the 
measured parameter. Regardless of the signal format or the exact 
relationship of the signal to the parameter which it represents, each 
signal representative of a measured process parameter or representative of 
a desired process value will bear a relationship to the measured parameter 
or desired value which permits designation of a specific measured or 
desired value by a specific signal value. A signal which is representative 
of a process measurement or desired process value is therefore one from 
which the information regarding the measured or desired value can be 
readily retrieved regardless of the exact mathematical relationship 
between the signal units and the measured or desired process units. 
Referring now to FIG. 1, the feedstream to be separated is provided through 
the combination of conduits 11 and 12 to the adsorption tower 14 and 
through the combination of conduits 11 and 15 to the adsorption tower 16. 
The adsorption towers 14 and 16 will generally contain an adsorption bed 
preferably made up of materials such as gamma-alumina or reversible bases. 
Treated liquid sulfolane still containing a substantially amount of ASO is 
removed from the adsorption tower 14 through the combination of conduits 
21 and 22. In like manner the treated sulfolane is removed from the 
adsorption tower 16 through the combination of conduits 23 and 22. 
An analysis signal 28, which will be described more fully hereinafter, is 
provided from a UV-analyzer 29 to computer 100 and is utilized to monitor 
the concentration of ASO of the effluent streams from the adsorption 
towers 14 or 16 depending on the position of valves 43 and 44. Valve 
control box 27 provides control signals 36-39 to valves 41-44 
respectively, which are operably located in conduits 12, 15, 47 and 48 
respectively. When an adsorption cycle for adsorption tower 14 is 
completed as indicated by a change in signal 31, the valve control will 
operate to close valves 41 and 43 and open valves 42 and 44. In like 
manner when a change in signal 31 indicates that the adsorption cycle is 
completed for adsorption tower 16 the valve control 27 will operate to 
close valves 42 and 44 and open valves 41 and 43. The manner in which the 
switching is accomplished will be described more fully hereinafter. 
It is noted that the adsorption tower which is not being utilized for 
separating compounds will be placed on a regeneration cycle using conduits 
21, 23 and 50-55 and valves 60-63 to supply regenerating agents. However, 
since the regeneration plays no part in the present invention, for the 
sake of simplicity, regeneration will not be described in detail. Also 
other conventional equipment which would normally be associated with an 
adsorption process has not been illustrated for the same reason. 
As previously stated signal 28 is provided from UV-analyzer 29 to computer 
100. Any suitable spectrum analyzer having capacity for transmitting and 
detecting radiation in the UV and VIS wavelength (i.e. 375 nm to 670 nm) 
can be utilized. Preferably the selected spectrometer will embody a 
microprocessor as an integral component of the analytical instrument and 
wherein the microprocessor has capacity for real time computing tasks 
other than the required task for upgrading the instrument operation. 
A sample of the effluent stream flowing from adsorption towers 14 or 16 is 
provided to the UV spectrometer 29 through conduits 47 and 48 
respectively. The magnitude of signal 28, which typifies ASO 
concentration, will follow the generally S shaped curve illustrated in 
FIG. 2, wherein the maximum rate of concentration change as determined 
mathematically by a second derivative, occurs at the inflection point 
marked "A" in FIG. 2. Breakthrough of HF from the adsorption tower occurs 
sometime later at the point marked. "B" in FIG. 2. Switching adsorption 
towers essentially at the point "A" in FIG. 2 can be achieved by 
determining when the second derivative of the concentration curve equals 
zero. 
Any suitable means for determining the derivative of the continuous signal 
illustrated in FIG. 2 may be used in this invention. However, for digital 
computation of control algorithms such as determining on-line the 
derivative of a continuous signal, difference equations are generally 
preferred since they are easily implemented in digital systems. Noting 
that noise is accentuated in determining a first difference (corresponding 
to d/dt in the analog case) and even more so in the second difference 
(corresponding to d.sup.2 /dt.sup.2), some smoothing must be accomplished 
before the derivative is calculated. 
A preferred technique for obtaining derivatives is the use of interpolation 
formulas, wherein taking values of several equally spaced points, an 
analytical differentiation can be performed giving a much smoother 
derivative signal. One formula that has been successfully employed for 
obtaining derivatives is the four point central difference technique 
wherein one determines four points E.sub.n to E.sub.n-3 equally spaced at 
the sampling interval AT for the variable curve E. The derivative is 
calculated according to the equation: 
##EQU1## 
where: 
E=concentration variable, 
n-1, n-2, and n-3 denote times previous to time n, 
.DELTA.T=sample interval. 
In use the first four points (i.e. points 1 through 4) would be used for 
the first calculation and points 2 through 5 for the second calculation, 
and so forth. 
Referring now to FIG. 3 there is illustrated a computer program flowchart 
for computer 100 illustrated in FIG. 1. In a preferred embodiment a real 
time interrupt illustrated in step 101, periodically initializes a program 
at any desired time interval, for example once every second. On program 
initialization in step 103, the computer determines which tower is active 
and then proceeds to read in a series of data points for the active tower 
based on signal 28 as shown in step 105. 
In calculation step 107 the second derivative of the data input in step 105 
is determined, preferably by the four point central difference technique 
previously explained. 
The computer next determines in step 109 whether or not the second 
derivative with respect to time of the input data is equal to zero, and if 
so a digital type control signal is output on signal line 31 as 
illustrated in step 110. If not the program continues. 
Signal 31, which is a digital type signal, will have a first logic level 
when inactive and a second logic level when it is activated. Signal 31 is 
a control signal provided from computer 100 as an input to the valve 
control 27. 
Many different circuits could be utilized for the valve control 27 and one 
simplified circuit is shown in FIG. 4. Referring to FIG. 4 signal 31 from 
computer 100 is provided to the input of the toggle flip flop of 202. The 
Q output from the toggle flip flop 202 is supplied as the switch control 
input to the switch 203. Terminal 204 of switch 203 is tied to a power 
line. Terminal 205 supplies signals 37 and 39 which have been previously 
described. In like manner terminal 206 supplies signals 36 and 38 which 
have also been previously described. 
In operation a change, for example, to a high logic level from a low logic 
level by signal 31 will cause the toggle flip flop 202 to change states. 
This change will cause the switch 203 to change positions which will have 
the desired effect of changing the position of valves 41-44. 
In summary switching control of the two parallel arranged adsorption towers 
based on a UV-analyzer 29 provides switching between the adsorption towers 
14 and 16 based on HF loading of the exhausted tower. Such control 
significantly improves the efficiency of the adsorption separation 
process. 
The invention has been described in terms of a preferred embodiment as 
illustrated in FIGS. 1-4. Specific components which can be used in the 
practice of this invention as illustrated in FIG. 1 such as the 
UV-analyzer 29 and control valves 41-44 are each well known, commercially 
available control components such as are described at length in Perry's 
Chemical Handbook, 5th Edition, Chapter 22, McGraw-Hill 1984. 
While the invention has been described in terms of the presently preferred 
embodiment, reasonable variations and modifications are possible by those 
skilled in the art and such modifications and variations are within the 
scope of the described invention and the appended claims.