CA1069063A - Purification of natural gas streams containing oxygen - Google Patents

Abstract

PURIFICATION OF NATURAL GAS
STREAMS CONTAINING OXYGEN

ABSTRACT OF DISCLOSURE

Hydrocarbon gas streams containing small quantities of molecular oxygen are found to adversely affect the adsorptive characteristics of zeolite adsor-bent beds at temperatures above 350°F. Problem is avoided by converting the oxygen to readily sorbable compounds prior to contacting the bed with the gas stream.

Description

~ D-9968 ~ 3 The present invention relates in general to the purification o hydrocarbon fluid streams and more particularly to the purification of hydrocarbon fluid streams which contain from 10 ppm to 10,000 ppm oxygen impurity in addition to one or more other impurities such as water, carbon dioxide and sulfur com~ounds.
The puri~ication o a wide variety o~
hydrocarbon ~eedstocks using zeolitic molecular sieves to selectively adsorb the impurity has in recent years become a common practice. Most petroleum crudes contain more than tolerable amounts of sul~ur impurities whîch mwst be removed in conjunc~ion with one or more refining operation before the refined product is ultimately consumed. Natural gas, in addi~io~ to sulfur compound impuri~ies can also contain unacceptably high amounts of water vapor and carbon dioxide. Thus whether the eedstock is in the liquid phase or the gas phase, selective adsorption processes have been developed to reduce the impurlty content to levels compatible with the intended end use o ~he produc~
Mo6t commonly the adsorp~ion process utilizes one or more fixed beds of molecular sieve adsorbent - through which the feedstock is passed and the impuri~y is retained. Flow-through of the ~eedstock is ~ermi-nated before breakthrough of the adsorbable impurit~
and thereafter the bed is regenerated by countercurrent

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~L069~i3 hot purge desorption and subsequent cool-down with a minor portion of the purified product or some other available purge fluid essentially free of sorb'able constituents undesirable in the purified f~edstock pro~uct.
Although not ordinarily considered a significant impurity~ oxygen is frequently found in ~elatively small concentrat~ons in either the hydrocarbon feedstocks bcing purified or in the hydrocarbon purge fluid or in both~ Being non-condensi-bl~.at temperature and pressure conditions used to l~qui~y hydrocarbon gas stream and being essentially non-sorbable on molecular sieves under the conditions prevailing in adsorption purification processes for hydrocarbons, the oxygen present in most hydrocarbons has been lar~ely ignored. It is found, however, that a number of problems can b~ created by the presence of oxygen in hydrocarbon fluids treated in contact with molecular ~leves, even ~f present in amounts as low as 10 ppm.
For a variety of reasons, natural ~as quite~~~~~~
~rëquently contains somë gaseous oxygen, and can contain as much as 10,000 parts per million (volume~. Usually :
amounts grea~er than 500 ppm are found in natural gas obtained from low pressure or sub atmospheric pressure gas fields. In pipeline natural gas some ox~gen is doubtlessl~ introducted during pipeline hydrotesting, :

069~

during in ground, i.e., cavern, storage and during periodic compression along ~he pipeline.
In treating oxygen-containing hydrocarbon feed-stocks, e.g. natural gas, to remove other impurities, the oxygen can interfere wlth the adsorption-puri~ication process in a number of ways, depending on the roneentration of the oxygen, the temperature o the adsorption system and the presence of sulfur compo~mds. At temperatures above 150F. oxygen reacts appreciably with sulfur com-pounds such as H2S and mercaptans to produce sulfur and water as principal reaction products. These substances are strongly held on the zeolite surfaces and seriously af~ect the capacity of the adsorbent bed to retain the impurities desired to be removed from the feedstock being treated. Sulfur is especially harmul in this regard. In the absence of sulfur compounds, at operating temperatures of 350F. and higher, oxygen is still a p~ob~em in hydro~
carbon feed~tocks since it reacts appreciably with hydro-carbons to form water, oxygenated organic compounds and/or carbon dioxide. These reaction products are, in part, formed in the adsorption bed ahead of the impurities mass transfer zones and are thus to some extent purged from the bed into the purified product stream beore the normal termina~ion of the adsorption stroke in the bed. Purity specifications for the product stream are thereby adversely affected.
It is requently the case that adsorption puri-fication processes employ temperature~ below 150F. during D-g968 ~ ~ 9 ~ ~ 3 the adsorption step, and hence most of the above-mentioned problems are not encoun~ered in tha~ stage of the overall process. In regenerating the bed in preparation for the next adsorption step, however, the purge-desorption step must be accomplished at te~peratures at least higher than 150F. and preferably at temperatures higher than 350F.
in order to avoid the need for unduly large quantities of purge gas. Accordingly, whsre the purge gas is a non-sorbable hydrocarbon, such as purifled natural gas, and contains from 10 to 10,000 ppm (vol~une) f 2~ the afore-said hanm~ul effects due to the presence of oxygen are encountered. For example, when a molecular sieve adsorp-tion purification is employed to remove carbon dioxide from a 1uid which is to be treated cryogenically below the freezing point o~ carbon dioxide for liquefaction of some or all of the purified stream, the water generated by oxygen impurity in the hot purge fluid and deposited on the adsorbent reduces the adsorbent'~ capacity for carbon dioxide. Thus deleterious levels of both water and carbon dio$ide pass into the cryogenic unit causlng plugging problems.
Having recogni~ed the problems and their source, we have di~covered a method for solving same without the need for supplemental adsorption appara~us and withou~
removing all sorbable impurities from the hydrocarbon purge gas stream.
Xn accordance with a generic emb~dimen~ of the process of the present i~vention a hyd~ocarbon feedstock :

D~9968 '~613~3 containing at least one sorbable impurity selected from the group consisting of water, carbon dioxide, hydrogen sul~
fide and alkyl mercaptan is passed through a first fixed bed of activated zeolitic molecular sieve having pore dia~e-ters large enough to adsorb the said impurity, with the proviso that th2 said fixed bed is at a temperature of less tha~ 150F. when said hydrocarbon feedstock also contains ~rom 10 to 10,000 ppm (volume) of gaseous oxygen, termin~ing the passage of the feeds~oek through the b~d prior to break-through of the impurity adsorbed therein, thereafter desorb~
ing and removing the adsorbed impurity from said first bed by purging same counter-currently with a non-sorbable hydrocarbon purge gas previously containing from 10 to 10,000 ppm by volume of molecular o~ygen~ said oxygen-containing hydrocarbon purge gas stream having been treated by the steps o~ (a) reducing the elemental oxygen content thereof b~ reacting the oxygen with hydrocarbon molecules comprising the said gas stream, preferably by heterogeneous catalysis in contact with an oxygenation catalyst mass in the soLid state, and (b) passing the oxygen-depleted hydro-carbon gas stream together with at least some o~ the oxygén-containing reaction products produced in situ therein through a second fixed bed o activated zeolitic molecular sieve adsorbent, the temperature of the gas stream being less than 350F., preferably less than 150F. and said molecular sieve bed being at a tempera~ure of greater.than 350F.

~ 3 The specie~ o~ molecular sieve adsorbent employed in the adsorbent beds of the present process are well knownin the art and are not critical factors. I~ is necessary o~ly that the pores of the adsorbent are large enough to adsorb the impurity components of the feedstock hydrocar~on being treated and the oxygen-containing reac- :
tion products produced in the purge gas stream. The calcium form of ze~lite A, described in detail in U.S.P.
2~88Z,243~ has a hi~h capacity for the adsorption of water and carbon dioxide and is advantageously employed.
The hydrocarbon feedstock treated can be any of those commonly involved in petroleum refining operations and in some aspects of petroleum production. Natural gas streams are ideally suited for treatment by the present proces~. In the purification-adsorption step of the pro~
cess the feeds~ock can be in the liquid or in the vapor ~tate.
The hydrocarbon gas stream which is ~reated so that it can be used to hot purg~ de~or~ the impurity-laden -adsorbent can be any oxygen-con~aining hydrocarbon stream in which the principal hydrocarbons are non-sorbable, ~.e.
are less strongly adsorbed in the in~er adsorption cavities of the moLecular sieve adsorbent than tha least strongly adsorbed lmp~lrity to ~e removed from the feedstock being purified. It ls ~o be under~tood that molecules which are excluded ~rom the inner adsorption cavities o~ a molecular ~ieve species by virtue of the pore diameters o~ thereof - 7 ~
.

9g68 c ~ :)69~)~3 are considered to be less strongly sorbable on that zeolite species than smaller molecules which can pass through the zeolite pores even though the larger molecules may be more strongly held than the smaller ones in zeoli~es having pores large enough to adsorb both molecular species. Thus methane, ethane and n-butane can be used to purge C02 impurity from a zeolite adsorbent having pore diameters not greater than 4 Angstroms, whereas methane, ethane and iso-butane can be used to purge CO2 from a molecular sieve hav~ng pore di-ameters o~ 5 Angstroms or less. Large concentrations of hydrogen nitrogen and inert gases can be tolerated in the purge gas stream Most commonly when the feedstock is nat-ural gas, the purge gas will be natural gas from which the water, carbon dioxide and sulfur compounds have been removed, or a comparable gas stream consisting essentially of methane.
In reacting the oxygen of the purge gas stream precursor with hydrocarbon constituents thereof, the precise means employed are not critical to the present process. El-evated temperatures alone are sufficient to accomplish the desired results, but a more efficient method is the use of any of the numerous oxygenation catalyst material commer-cially available. Especially effective are the copper, manganese and iron compound catalyst systems described in detail in U.S.P. 3,361,531, and simil~r oxide compositions described in Boreskov, G.K. "Mechanism of Catalytic Oxidation Reactions on Solid Oxide Catalysts" Kinetica i Kataliz, Vol.
14, No. 1, p. 7, Jan-Feb 1973, issued January 2, 1968.

` D-9968 ~q~69~3 In its generic aspect, the present invention not only conver~s an unsui~able purge gas s~ream to an : .;
e~tirely ~atisfac~ory one~ bu~ also in the treatmen~ of the gas stre~m th~re is.provided the added ad~antage that a hot previously regenerated adsorbent bed is cooled down to adsorption stroke temperature and much of ~he heat ~9 ~ ~ 3 energy therefrom is transferred to the purge desorbing of another bed using the newly purified purge gas stream.
These advantages are realized to a high degree in a more specific process embodiment of this invention in which at least three fixed adsorption beds are used cyclically for the purification of natural gas streams. In such an em-bodiment a natural gas feedstock (a) containing at least one sorbable impurity selected from water3 carbon dioxide, hydrogen sulfide and alkyl mercap~an, and containlng as a non-sorba~le impurity from 10 to 10,000 ppm (volume) of entrained oxygen is passed at a temperature balow 150F.
~hrough a first fixed bed of activated zeolitic molecular sieve having pore diameters large enough to adsorb ~he said sorbable impurity of said feedstock and recover a purified feedstock product containing a~ least 10 ppm (volume) of oxygen, terminating ~he passage of the fee~- :
stock (a) through the bed prior to breakthrough of the impurity adsorbed therein, reacting the oxygen in a portion of the recovered purified feedstock wlth hydro~
carbon molecules comprising same to form carbon dioxide and water and to reduce the oxygen concentration thereof, preferably to 12ss than 10 ppm (volume) J thereafter passing at a temperature below 350F. the resulting oxygen-deple~ed hydrocarbon gas stream (b) containing reaction products formed in situ therein through a second fixed adsorbent bed con~aining zeolitic molecular sieve adsorbent having pore diameter of at least 4 Angstroms, said second fixed ~ 9~3 bed being at a temperature higher than 350F. as a result of being hot purgëd with natural gas stream substantially free of C02, H2S and H20, recovering the heated and sub-stantially C02 and H20~free hydrocarbon e~fluent (c) from said second fix~d bad and passing same as a purge gas at a temperature of greater than 350F. through a third ixed adsorbent bed containing zeolitic molecular sieve adsorbent having adsorbed thereon lmpurity constituents as a result of passage therethrough of the said natural gas feeds~ock (a) the direction of ~low of said purge gas (c) from the said second bed through said third bed being counter-current to the direction of passage of natural gas feed-stock (a) through said third bed, and thereafter passing said natural gas feedstock (a) through said second bed in a direction co-current to the passage of oxygen-depleted hydrocarbon gas stream (b) therethrough.
The present invention is illustrated by the follawing description taken in conjunction with the draw-ings.
In the drawings the figure is a schematic ~low diagram of a three-bed adsorption purification system in which each of the three beds cyclically u~dergoes the steps of adsorption,countercurrent hot purge desorption and co- -current cool down. Operation of the process i~ such that at any given time all three steps are in progress with each step being carried out in a differe~t bed. The conventional valving and conduit connections which enables cycling of the process steps in each bed are not shown in the drawings.

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~59~ 3 Natural gas which contains 1.5 volume-% carbon dioxide and 150 ppm (volume) H20 and 50 ppm (volume) oxygen is purified in an adsorption system comprising three fixed adsorption beds, each con~aining 35,000 pounds of type 4A
molecular sieve. With reference to the drawing, the natural gas feedstock is passed at the rate 32.5 million standard cubic feet per day through line 10 at a temperature of 85F, and at a pressure of 600 psi. In passage through bed 12 carbon dioxide and water are adsorbed and ~he effluent product gas stream through line 14 contains less ~-than 50 ppm C02, less than 1 ppm H20 and essentially the same concentration entrained oxygen present in the feed-stock. A slipstream of product gas is removed from line 14 via line 16 at the rate of 17.4 million standard cubic feet per day, heated to 400~F. in ~urnace 18 and passed through line 20 to catalytic oxida~ion unit 22. The cat-alyst mass in uni~ 22 consists of cuprous oxide dispersed on synthetic mordenite having an SiO~/A1203 molar ratio of llo 2/ and converts sufficient oxygen of the gas stream to C02 and H20 to lower the entrained ox~gen content to less than 10 ppm (volume). The efluent gas stream carrying the product C02 and ~2 is passed via line 24 through coolex 26 wherein the temperature of the effluent is re-duced to about 100F. and thereafter is fed through lin~
28 into adsorption bed 30. Pr~viously bed 30 had been utilized to purify a portion of tha same feedstock as currently is being treated i~ bed 12. Bed 30 has also been ~06~ 3 hot purge desorbed at a temperature o 500F. in a direction counter-current to the flow of the feedstock stream and the cooling oxygen-depleted gas stream currently.
flowing through line 28. In its passage through bed 30, the gas stream from cooler 26 through line 28 deposits C2 and H20 as adsorbates on ~he ingress end o the bed in a well defined adsorption zone, cools the bed 30 along an advancing cold ~ront, and is itself heated to approximately 500F. This effluent hot, dry and essentially C02-free gas stream is passed via line 32 to urnace 34, wherein it is heated to 600F. and thereafter fed through line 36 through adsorp~ion bed 40. Bed 40 has previously been employed to purify, in a direction counter-curre~t to the direction of flow of the present gas stream, a portion of the same feeds~ock as is currently being treated in bed 12, and is loaded with adsorbed C02 and H2O impurities. Bed 40 is regener ted and heated by the passage ther~hrough of ~he purging gas stream from line 36 and the desorbed C2 and H20 is passed through li~e 42 for disposal.

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Claims (5)

WHAT IS CLAIMED IS:

1. Process for purifying a hydrocarbon feedstock containing at least one sorbable impurity selected from the group consisting of water, carbon dioxide hydrogen sulfide and alkyl mercaptan which comprises passing said feedstock through a first fixed bed of activated zeolitic molecular sieve having pore diameters large enough to adsorb the said impurity, with the proviso that the said fixed bed is at a temperature of less than 150°F. when said hydrocarbon feedstock also contains from 10 to 10,000 ppm by volume of gaseous oxygen, terminating the passage of the feedstock through the bed prior to breakthrough of the impurity adsorbed therein, thereafter desorbing and remov-ing the adsorbed impurity from said first bed by purging same counter-currently with a non-sorbable hydrocarbon purge gas previously containing from 10 to 10,000 ppm by volume of molecular oxygen, said oxygen-containing hydro-carbon purge gas stream having been treated by the steps of (a) reducing the said oxygen content thereof by reacting the oxygen with hydrocarbon molecules comprising the said gas stream, and (b) passing the oxygen-depleted hydrocarbon gas stream together with at least some of the oxygen-containing reaction products produced in situ therein through a second fixed bed of activated zeolitic molecular sieve adsorbent, the temperature of the gas stream being less than 350°F., and said molecular sieve bed being at a tem-perature of greater than 350°F.

2. Process according to Claim 1 wherein the initial oxygen content of the non-sorbable hydrocarbon purge gas stream is reduced by reacting said oxygen with hydrocarbon molecules in contact with an oxidation catalyst mass in the solid state.

3. Process according to Claim 1 wherein the hydro-carbon feedstock being purified is natural gas.

4. Process according to Claim 3 wherein a portion of the feedstock after passage thereof through the first adsorption bed, is utilized as the purge gas stream after treatment to decrease etc. oxygen content.

5. Process for purifying a natural gas feedstock, (a) containing at least one sorbable impurity selected from water, carbon dioxide, hydrogen sulfide and alkyl mercaptan, and containing as a non-sorbable impurity from 10 to 10,000 ppm by volume of gaseous oxygen, which comprises passing said stream (a) at a temperature below 150°F. through a first fixed bed of activated zeolitic molecular sieve, having pore diameters large enough to adsorb the said sorb-able impurity of said feedstock (a) terminating the passage of the feedstock (a) through the bed prior to breakthrough of the impurity adsorbed therein, reacting the oxygen in a portion of the recovered purified feedstock with hydro-carbon molecules comprising same to form carbon dioxide and water and to reduce the oxygen concentration thereof, thereafter passing at a temperature below 350°F. the resulting oxygen-depleted hydrocarbon gas stream (b) con-taining reaction products formed in situ therein through a second fixed adsorbent bed containing zeolitic molecular sieve adsorbent having pore diameter of at least 4 Angstroms, said second fixed bed being at a temperature higher than 350°F. as a result of being hot purged with natural gas stream substantially free of CO2, H2S AND H2O, recovering the heated and substantially CO2 and H2O-free hydrocarbon effluents (c) from said second fixed bed and passing same as a purge gas at a temperature of greater than 350°F, through a third fixed adsorbent bed containing zeolitic molecular sieve adsorbent having adsorbed thereon impurity constituents as a result of passage therethrough of the said natural gas feedstock (a) the direction of flow of said purge gas (c) from the said second bed through said third bed being countercurrent to the direction of passage of natural gas feedstock (a) through said third bed, and thereafter passing said natural gas feedstock (a) through said second bed in a direction co-current to the passage of oxygen-depleted hydrocarbon gas stream (b) therethrough.