OLEFINS
Olefins are the basic building blocks for many chemical syntheses. These
unsaturated materials enter into polymers, rubbers, and plastics, and react to
form a wide variety of chemical compounds such as alcohols, amines, chlorides
and oxides.
Steam Cracking is the thermal crachg and reforming of hydrocarbons in
the presence of steam at high temperature, short contact time, and rather low
pressure in a fired tubular furnace. From the standpoint of both the amount and
variety of compounds produced, steam cracking of gas oils and naphthas is one
of the most important petroleum process for producing a wide range of chemical
raw materials. Ethane and propane cracking are used widely by others but
relatively few products other than ethylene result.
In a typical gas oil design, the lighter products overhead from the quench
tower/primary fractionator are compressed to 210 psi, and cooled to about
100°F. Some C, plus material is recovered from the compressor knockout
drums. The gases are ethanolamine and caustic washed to remove acid gases:
sulfur compounds and carbon dioxide, and then desiccant dried to remove last
traces of water. This is to prevent ice and hydrate formation in the low
temperature section downstream.
In the depropanizer tower the propane and lighter gases are taken overhead
to become feed to the ethylene and propylene recovery facilities. Separation is
accomplished at a relatively low overhead temperature of -25 "F to minimize
reboiler fouling by olefin polymerization.
Butane and heavier bottoms from the depropanizer flow to the debutanizer
where the C, stream (almost entirely olefins and diolefins) is taken overhead and
sent to butadiene and isobutylene recovery facilities.
Depending upon the refinery needs, the raw C, plus steam cracked naphtha
may be sent to isoprene extraction, treated to remove gum forming diolefins and
sent to the refinery gasoline pool, or else completely hydrogenated and then fed
to an aromatics extraction unit.
The principal function of the ethylene recovery facilities is to recover high
purity ethylene (Figure 2). Ethylene recovery consists basically of a low
temperature, relatively high pressure distillation process to separate ethylene
from other hydrocarbons and hydrogen. In addition, acetylene conversion and
caustic treating steps are employed to reduce contaminants which would not be
adequately removed by the distillation process.
The depropanizer overhead, C, and lighter feed is compressed to about 300
psi and then passed over a fixed bed of acetylene removal catalyst, generally
palladium on alumina. Because of the very large amount of hydrogen contained
in this stream, the operating conditions are critical to selectively hydrogenate the
acetylene without degrading the valuable ethylene to ethane.
The gases are again dried and then further compressed to about 550 psi.
Separation of hydrogen and methane take place in the demethanizer and in its
preflash system. Three successive Golder preflash steps are used in this
separation, with propylene as refrigerant, then ethylene, and finally a selfgenerated
methane refrigerant at -200°F.
A high purity hydrogen and a low purity methane stream result. The 95%
hydrogen may be used directly to hydrogenate steam cracked naphtha or directly
consumed elsewhere in the refinery. The methane stream goes to fuel.
The C, plus bottoms from the demethanizer then go to the deethanizer. A
propylene-propane bottoms product containing 90-92 % propylene is obtained
which may either be sold, used directly as propylene- 90, or further purified. The
ethylene-ethane overhead from the deethanizer is separated in the splitter tower
yielding a 99.8% overhead ethylene product at -25°F. The ethane bottoms at + 18°F may either be sent to fuel gas or used as feed to an ethane craclung
furnace. Overall ethylene recovery in these facilities is about 98 % . The product
is of very high purity with less than 50 parts per million of non-hydrocarbon
contaminants and a methane plus ethane level below 250 ppm.
Propylene Recovery
The propylene-90 bottoms product from the deethanizer may be upgraded to high
polymer grade 99.8 % purity by superfractionation. Propane bottoms are used
elsewhere in the refinery.
Butenes
N o d butenes and isobutylene are separated by a selective reaction-extraction
process which takes advantage of differences in reactivity with dilute sulfuric
acid to form butyl alcohols. Because of differences in olefin structure,
isobutylene reacts much more rapidly than normal butenes with weak acid. In
fact, reaction of normal butenes in acids weaker than 65% is negligible at
commercial conditions. Reaction products are soluble in dilute acid. The unreacted
feed is only slightly soluble.
The acid extract phase is separated, diluted with water, and heated to
regenerate isobutylene. The isobutylene is then caustic and water washed to
remove traces of acid, distillation dried, and rerun. The unreacted C, stream,
containing normal butenes, is also caustic washed before further processing.
C, cuts, after extraction of butadiene, are preferred as feed to isobutylene
extraction units because the isobutylene concentration (about 30-40%) is higher
than in C, streams from catalytic cracking. The basic reaction in isobutylene
extraction is the reversible hydration of isobutylene to tertiary butyl alcohol in the
presence of sulfuric acid.
Polymerization to C, and C;, olefins is the chief side reaction.
Polymerization increases with extraction temperature and with the hold-up time
in the extraction section. It limits the temperature used to obtain high extraction
rates.
The extraction is carried out in a staged countercurrent system for good
recovery of isobutylene. Temperature is maintained by refrigeration, since heat
is evolved in the hydration. Normal (2,'s are rejected as the raffinate from the
lean stage. The stream, typically containing 70 mol% normal butenes, can be
used as feedstock for dehydrogenation to butadiene. The rich acid extract is
flashed to about 2 psig and blown with a small amount of steam to remove
butylenes and butanes physically dissolved in the extract. Isobutylene is then
recovered from the acid extract by direct injection of steam in the regenerator
tower.
Enough steam is used to reduce the acid concentration from 65 % to 45 % .
The heat supplied by the steam is used in: (a) regeneration of isobutylene from
t-butyl alcohol (an endothermic reaction) (b) raising the acid temperature to
250°F and (c) distilling out isobutylene, polymer, and residual tertiary butyl
alcohol. High temperature and low acid strength allow regeneration of the
isobutylene with minimal polymerization. Acid strength in the regenerator tower
is critical. Too low values result in separation of unwanted alcohol while high
concentrations increase polymerization rates.
The regenerator overhead is caustic and water washed, yielding a 95-96%
isobutylene product. The 45% acid taken as bottoms from the regenerator is
concentrated to 57% for steam cracked C, cuts (65% for cat cracked C,'s) and
recycled to the lean stage of the extraction section.
High purity 99+ % isobutylene can be made by rerunning, with a recovery
of over 85% of the isobutylene in the feed.
When hgh purity isobutylene is not required, the acid extract from the rich
stage may be heated for a few minutes to 250-300"F, and then quickly cooled.
Under these conditions the isobutylene dimerizes to form largely 2,4,4, trimethyl
pentene- 1. This is known as the dimer process and may be used to concentrate
1-butenes for dehydrogenation feed, the isobutylene dimer being added to the
motor gasoline pool. Trimers, as well as codimers with normal butenes are also
produced.
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