Combination alkylation-etherification process

In a combination alkylation-methyltertiary butyl ether (MTBE) operation, isobutane vapor side-draw and bottoms product yield from the alkylation fractionation, respectively, are used to indirectly heat the mid-section and reboil section of the methyltertiary butyl ether fractionator for heat conservation, beneficiating both the alkylation operation and the MTBE operation.

BACKGROUND OF THE INVENTION 
This invention relates to an alkylation process and the recovery of the 
produced hydrocarbon phase in a more efficient manner and to the 
preparation of tert-alkyl ethers. in accordance with another aspect, this 
invention relates to the exchange of heat in a combination 
alkylation-etherification process. In one of its aspects, it relates to a 
process for conserving the heat produced in an alkylation process and 
utilizing same in the recovery of tert-alkyl ethers in the effluent of the 
etherification process. In accordance with another aspect, it relates to a 
process for supplying heat to a fractionation zone in a combination 
alkylation-etherification process. 
Alkylation of isoparaffinic hydrocarbons with olefinic hydrocarbons is well 
known as a commercially important process for producing gasoline boiling 
range hydrocarbons. Also, it is known that tert-alkyl ethers can be 
prepared by reacting a primary alcohol with an olefin having a double bond 
on a tertiary carbon atom, such as the reaction of methanol with 
isobutylene and isoamylenes to form methyl tert-butyl ether, when using 
isobutylene. The energy requirements and costs for the recovery of 
tert-alkyl ethers can be great, and it is, therefore, necessary to 
maintain the energy requirements for an etherification process at a low 
level. This is particularly important where energy is valuable and the 
products for generating energy are in relatively short supply and 
expensive. It has been found that by maximizing the use of available waste 
heat and an alkylation fractionation system and an etherification 
fractionation system that the energy requirements of an etherification can 
be reduced thereby resulting in great energy savings. 
Accordingly, it is an object of this invention to provide an improved 
etherification process wherein the outside energy requirements are 
reduced. 
Another object of this invention is to utilize excess energy available in 
an alkylation process as energy for an etherification process recovery 
system. 
Still another object of this invention is to provide an efficient 
alkylation-etherification process which maximizes the possible use of 
available waste heat in the system. 
Other aspects, objects and advantages of the present invention will become 
apparent from a study of the disclosure, the appended claims and the 
drawing. 
SUMMARY OF THE INVENTION 
In accordance with the invention, a combination alkylation-etherification 
process that is energy efficient is provided which comprises utilizing at 
least two high energy containing streams recovered from the alkylation 
process fractionation recovery system as the source of heat energy for the 
operation of the primary fractionation zone following etherification. 
In a specific embodiment of the invention in a combination 
alkylation-etherification operation isobutane vapor side-draw and bottoms 
alkylate product yield from the alkylation fractionation, respectively, 
are used to indirectly heat an intermediate portion and the reboil or 
bottoms portions of the etherification fractionation for heat 
conservation. 
Thus, the invention relates to the utilization of at least two high energy 
containing streams obtained from an alkylation unit product fractionation 
system in order to supply the energy needed in the principal tert-alkyl 
ether fractionation unit which has a high heat duty requirement. 
In a specific embodiment of the invention in a combination 
alkylation-etherification process and product recovery systems improvement 
comprises reboiling the tert-alkyl ether fractionation unit with hot 
alkylation kettle product removed from the alkylation unit product 
fractionation and a stream of vaporous, primarily isobutane, containing 
side-draw removed from the alkylation product fractionation which is 
condensed to provide heat through an intermediate heat exchange zone 
positioned in the tert-alkyl ether fractionation unit. Thus, according to 
the present invention the main fractionation of the alkylation unit 
supplies two heat sources at different temperatures for the tert-alkyl 
fractionation unit.

DETAILED DESCRIPTION OF THE INVENTION 
The instant invention is primarily directed to an improved fractionation 
system following an alkylation process in which energy is used efficiently 
and effectively in a fractionation system following the etherification 
process thereby reducing energy costs for the recovery of tert-alkyl ether 
and the recovery of alkylate following alkylation. 
A better understanding of the invention will be obtained by reference to 
the accompanying drawing which shows an arrangement of an apparatus 
representing a preferred embodiment of the invention. 
The drawing depicts schematically the relationship of the invention taking 
two streams of differing temperatures from an alkylation product 
fractionation unit and applying that heat to a fractionation unit 
recovering tert-alkyl ether, specifically methyl tertiary butyl ether 
(MTBE), at two different temperatures and physical location levels and two 
indirect heat exchangers in the MTBE fractionation unit. 
Various stream flow arrangements are illustrated on the drawing to show the 
flexibility of the operation for producing gasoline and gasoline blending 
components in an energy efficient manner. 
Referring now to the drawing, a mixed butylenes-containing stream 4 is 
passed by way of line 5 and introduced into etherification unit 3 wherein 
it is mixed with methanol or other conventionally used alcohols introduced 
by way of line 2. Conditions within unit 3 are such that the reactants 
methanol and isobutylene are converted to methyl tertiary-butyl ether 
which is removed along with unreacted butylenes from unit 3 by way of line 
6 and introduced into an intermediate portion of fractionation zone 20. 
The reaction between the butylenes cut and methanol is conventionally 
performed in the presence of an acid catalyst. The usual operating 
conditions are a temperature from about 0.degree. to about 65.degree. C., 
more often from about 10.degree. to about 40.degree. C. Etherification 
reaction is well known. 
Fractionation unit 20 is operated under conditions such that an overhead 
stream 8 comprises unreacted butenes and some methanol, and a bottom 
stream 26 comprising MTBE product and methanol. 
Fractionation 20 is operated generally at a kettle temperature of about 
250.degree. to about 280.degree. F. and a pressure of about 100 to about 
140 psia. The top temperature in column 20 will range from about 
120.degree. to about 170.degree. F. and a pressure of about 90 to about 
130 psia. 
Fractionation zone 20 is provided with heat exchange zone 22 positioned at 
an intermediate portion of the fractionator and a reboiler exchange unit 
24 located near the bottom of fractionator 20. Intermediate heater 22 is 
ordinarily operated at a temperature of about 220.degree. F. and 100 psia 
and is used to condense isobutane vapor recovered from a fractionation 
unit to be described herein below. 
Fractionator 20 kettle product comprises MTBE contaminated with methanol 
and is passed by way of conduit 26 to conventional methanol removal (not 
shown). 
Overhead stream 8 comprising straight chain butenes contaminated with 
methanol is passed through condenser 9 and passed by way of conduit 10 to 
overhead accumulator 21 wherein the condensate is collected. Condensate is 
removed by line 11 from accumulator 21 and a portion is passed as reflux 
by way of line 12 and introduced in an upper portion of fractionator 20. 
The remainder or yield portion of the butenes-methanol condensate removed 
from accumulator 21 is passed by way of line 13 to a water wash column 60 
wherein it is contacted countercurrently with water introduced by way of 
line 61, recovered from stripper 63. Unit 60 is operated under conditions 
such that methanol is substantially removed from the butenes stream by the 
water which is removed from a lower portion of column 60 by way of line 62 
and passed to stripper 63 wherein methanol and water are separated. The 
water is recycled by way of a cooler (not numbered) and line 61 to column 
60, and the methanol is condensed and cooled and passed by way of line 64 
to MTBE unit 3, or for other use as desired. 
The water washed butenes steam 15 removed from unit 60 is passed after 
drying (not shown) to alkylation 30 wherein it is contacted with isobutane 
introduced by way of line 15' and alkylation catalyst, e.g. HF acid, by 
way of line 16. 
The alkylation reaction is conducted under conventional conditions for 
aliphatic alkylation. The alkylation is suitably carried out by the 
reaction of the mixture of hydrocarbons comprising isoparaffins containing 
from 4 to 8 carbon atoms and olefins containing 3 to 8 carbon atoms. The 
isoparaffins most commonly used as feedstock for motor gasoline alkylate 
are isobutane and isopentane. The olefins most commonly used are propylene 
and butenes. Preferred feedstocks currently are isobutane and a butylenes 
mixture. In this specific example, isobutane is reacted with the mainly 
straight chain butylenes remaining from the MTBE plant. 
The hydrocarbon phase comprising alkylate, isobutane, normal butane and 
C.sub.3 and lighter hydrocarbons is removed from unit 30 by way of line 17 
and passed to fractionation column 40. 
Column 40 is operated under conditions such that propane and lighter 
hydrocarbon vapors are taken overhead by way of line 18, a vaporous 
sidestream 41 comprising isobutane, a vaporous hydrocarbon stream 42 
comprising normal butane, and a bottoms liquid stream comprising alkylate 
by way of line 48. Conventionally, column 40 is operated in this example 
at an upper temperature of about 156.degree. F. and 225 psia and a bottoms 
temperature of about 420.degree. F. and a pressure of about 230 psia. The 
temperature of the column near the isobutane withdrawal is normally about 
250.degree. F. 
The overhead stream 18 is passed through condenser 70 line 71 and 
introduced into overhead accumulator 72. The hydrocarbon condensate is 
removed by line 73 and a portion is passed as reflux 19 to an upper 
portion of column 40. The remainder or yield is removed as product for 
further use as desired by way of line 74. In this example, this stream 
comprises propane with some HF and is charged to an HF stripper (not 
shown) to yield stripper bottoms product of LPG quality propane. 
A bottom stream comprising alkylate is removed from tower 40 by way of line 
48 and thereafter is split into two streams, one passing by way of pump 49 
line 50, heater 51, and line 52 for return to a lower portion of 
fractionation unit 40 to provide reboiler heat. 
The other bottom stream comprising alkylate removed from tower 40 is passed 
by way of line 45 to reboiler 24 positioned in the lower portion of 
fractionation unit 20 whereupon this alkylate product is cooled in the 
reboiler and exits reboiler 24 by way of conduit means 47. The alkylate 
stream 45 cycled through reboiler 24 provides one source of heat for 
column 20. 
Vaporous isobutane removed from column 40 by way of line 41 is passed 
through inner heater 22 positioned at an intermediate point in column 20 
wherein the isobutane vapor is condensed and removed from heater 22 by way 
of line 44 and reintroduced after additional cooling, not numbered, into 
alkylation 30. 
Heat exchangers 43 and 46 are used only in the event etherification 10 is 
shut down so as to cool the vaporous isobutane stream and alkylate stream, 
respectively. Similarly, if MTBE unit 10 is shut down, the olefinic 
butenes can be passed directly by way of conduit 14 to HF alkylation unit 
30. It is also within the scope of the invention to pass mixed butenes to 
both units 3 and 30 when both are in operation. 
As can be seen from the above description of the drawing the invention 
comprises reboiling the MTBE-butylene fractionator 20 with the hot 
alkylation kettle product 45 removed from the alkylation unit product 
fractionator 40, and a stream of vaporous, primarily isobutane, containing 
side-draw 41 removed from the alkylation product fractionator 40 is 
condensed to provide through a second inner heater or reboiler 22 in the 
MTBE-butylene fractionator. Thus, maximum heat removal is made of hot 
alkylation kettle product with remaining heat duty requirements of the 
MTBE butene fractionator 20 being provided by the condensing side-draw of 
essentially isobutane vapor from the alkylate product fractionator 40. 
Thus, in accordance with the invention, a main fractionator of the 
alkylation unit supplies two heat sources with different temperatures for 
another fractionation unit, specifically the MTBE fractionator 20. 
EXAMPLE I 
A calculated example as herewith given in order to illustrate one set of 
possible operating conditions in accordance with the invention. 
______________________________________ 
Calculated Operation 
1. Operating conditions (Specific Operation) 
Range 
______________________________________ 
(20) MTBE-n Butenes Fractionator 
(8) Top Zone 
Pressure psia 90-130 
Temperature .degree.F. 
120-170 
(22) Interheater Zone 
Pressure psia 100-140 
Temperature .degree.F. 
200-220 
(24) Kettle Reboiler Zone 
Pressure psia 100-140 
Temperature .degree.F. 
250-280 
(21) Accumulator 
Pressure psia 70-100 
Temperature .degree.F. 
100-140 
(40) Alkylation Product Fractionator 
(18) Top Zone 
Pressure psia 225-245 
Temperature .degree.F. 
156-164 
(41) Isobutane Side draw 
Pressure psia 227-247 
Temperature .degree.F. 
203-250 
(42) Normal Butane Side Draw 
Pressure psia 228-248 
Temperature .degree.F. 
243-290 
(48) Kettle Bottom Zone 
Pressure psia 230-250 
Temperature .degree.F. 
423-448 
______________________________________ 
Estimated heat duty available from heat in the alkylate product (45) is 6 
to 15 million Btu/HR, while MTBE kettle reboiler (24) duty is 8-12 
million Btu/HR. Isobutane side draw (41) is 2-5 million Btu/HR available 
to heat interheater (22). 
Various valves, pumps, coolers, etc., are not shown on FIG. 1 in order to 
simplify the drawing. 
This invention has intercooperation between and mutual beneficiation of two 
dissimilar units. The MTBE unit prepares the normal butylenes-rich stream 
(having reacted out isobutene therefrom to make MTBE by reaction with 
methanol) for the HF alkylation of isobutane and, at the same time, 
receives the needed heating of the MTBE fractionation at two different 
temperature levels from two separate streams issuing from the HF 
alkylation fractionator, while, at the same time, cooling these two 
streams from the alkylation, this cooling of one stream (recycle 
isobutane) prepares this stream so that it can be processed at its proper 
temperature for recycle to the alkylation.