Acid runaway

Levy, L. B., and J. D. Penrod (April 1989). "The Anatomy of an Acrylic Acid Runaway Polymerization." Plant/Operations Progress 8, 2, 105-8. 

In this study, the acid concentration was not explicitly taken into account. At concentrations below 85%, olefins react preferentially with olefins, to produce polymers. Due to their unsaturated nature, these polymers are soluble in sulfuric acid, which leads to a further concentration decrease. Additionally, polymers are oxidized, while sulfuric acid is reduced to water and sulfur dioxide. These positive-feedback effects are known as acid runaway [17]. Acid runaway is a major concern in alkylation plants, as the acid strength can no longer be controlled and the olefin feed must be cut out. 

Sulfuric acid alkylation units have excellent safety reports [18], The main concerns are the H2S04 and the large inventories of light hydrocarbons. The incidents that have been reported include emulsion-separation difficulties leading to acid carryover, acid runaway resulting in low-quality alkylate and requiring shut down of the olefin feed, or tight emulsion in the net effluent water wash with the consequence of carryover of the alkaline water into the separation section. 

A relatively sharp transition between oligomerization and alkylation activity has been measured with sulfuric acid at Hq values between — 8.0 and — 8.5 (109). If such low-acidity values occur in an alkylation reactor, oligomerization reactions become so predominant that the acid strength cannot be maintained and the plant is said to be in an acid runaway condition. 

This seems to indicate an "acid runaway" condition and more will... 

Acid runaway" is a term commonly used in commercial alkylation operations and is characterized by a condition of low acid strength and rapid build up of diluents. In certain situations an acid runaway may force olefin feed to be cut out since fresh acid usually cannot be added fast enough to counteract the rapid diluent formation. 

The alkylation acid penalty for ethylene is so large it makes close control of the feed deethanizer absolutely mandatory for economical overall operation. The ethane and lighter content of the C3-C4 cut should be maintained nil as measured by continuous chromatography. Even a trace amount of C2 s may represent several barrels of ethylene per day. In addition, if a slug of ethylene is accidentally charged to the alkylation reactor it can also have the effect of suddenly reducing the strength of the acid catalyst to the point that alkylation ceases, olefin absorption rapidly dilutes the acid, and chain polymerization of olefins ("acid runaways") can occur. 

Liquid-liquid Typically the reactor is a CSTR followed by a decanter to separate the phases and recycle the catalyst phase to the reactor. See decanter. Section 5.3.1. For alkylation Alkylate is purple [stable emulsion formation] /[density difference decrease] /[drops don t setde] /[acid runaway], "zip across the alkylate cooler> design [stable emulsion formation]- /[density difference decrease] / [drops don t settle] /[acid runaway] /acid recirculation rate too fast. 

Temperature of the recycled acid is > 1.7°C hotter than feed entering the reactor [acid runaway] /alkyl sulfates polymerize in the decanter/acid redrulation rate too fast. 

Commercial reactors using sulfuric acid as the catalysts currently employ residence times in the 15- to 30-min range. Times as little as 2-5 min have been used in the laboratory to produce high quality alkylates (8,29). With HF as a catalyst, times as little as 5-10 s are used (30). Yet, information is still needed on how the kinetics of reactions varies as CPs and water build up in the acid phase. As the acidity of sulfuric acid decreases, the rates of formation of isoalkyl acid sulfates eventually surpass the rates of decomposition (or reaction) of these sulfates in which case, the acidity of the acid drops rapidly, and simultaneously the rate of alkylate production decreases precipitously. Such a highly undesired phenomenon is referred to as an acid runaway. A similar phenomenon can occur with HF. 

D. C. Graves, Acid Consumption and Acid Runaway in Sulfuric Acid Alkylation Unit, Stratco Inc., Leawood, Kans., 1999. 

An acid runaway on a sulfuric acid alkylation unit is an unforgettable experience. The alkylate turns purple, numerous hydrocarbon I leaks appear in (he reactor effluent piping, and acid strength declines at an uncontrollable rate. Most disturbing of all are the irritating fumes emitted from the spent acid tanks as the low-strength runaway acid is pumped out of the alkylation unit. 

The operating engineer should Ije able to distinguish between the two types of acid carry-overs. Strong acid spillover from the settler is a physical acid carry-over. Weak acid, containing a high concentration of acid esters, is the well-known phenomenon of acid runaway. It may also be called a chemical acid carry-over. 

A positive response to these questions indicates the unit has experienced a chemical acid carry-over or an acid runaway. Indications of a physical acid carry-over are acid found in the settler is still strong (88+ wt%) and the alkylate properties are normal. 

Most large alky units have several reactors operating in parallel. Poor distribution of recycled isobutane between reactors is a common cause of acid runaways. 

On one unit faulty flow meters caused the iso in effluent to be 32% and 54% from two parallel reactors. The reactor with the low iso in effluent experienced an acid runaway. This could have been prevented by sampling individual reactor effleunts. In this case the operators had fooled themselves by only sampling the combined effluent for isobutane. 

During an acid carry-over crisis, operators too often cut acid recycle rales. If the unit is in an incipient stale of chemical acid carry-over, this rriay precipitate a full-blown acid runaway. The operating engineer can make a quick field decision whether to cut acid recycle by determining... 

The acid recycle is warmer than the reactor effluent. This is an indication that esters are continuing to react in the acid settler. The resulting heat of reaction increases the temperature of the acid recycle, while the hydrocarbon reactor effluent remains cool. The reaction of acid esters in an isobutane deficient environment increases the rate of acid consumption and lowers acid strength. Reduced acid strength produces more esters to fuel the after-reaction in the acid settler. This self-escalating production of esters is an acid runaway situation and is indicative of a chemical acid carryover requiring increased acid circulation. A 15°F temperature difference between the acid recycle and hydrocarbon effluent is an indication of an incipient acid runaway situation a S F difference, while undesirable, is normal. 

Alkylation quality is adversely affected by poor mixing or unbalanced olefin dispersion in the reactor. However, field observations have shown mixing and feed dispersion failures typically do not initiate acid runaways. 

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