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Nov-2019

Removing acid droplets produced by alkylation reaction

Studies lead to a fuller understanding of how sulphuric acid droplets form in alkylation reactions and how to combat resulting downstream corrosion.


DIWAKAR RANA, J RANDALL PETERSON and ROB EWING
DuPont Clean Technologies
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Article Summary
Sulphuric acid alkylation technologies typically incorporate intense mixing of sulphuric acid (catalyst) with the hydrocarbon feeds to produce alkylate. Real world and laboratory experiments on Stratco alkylation equipment show that additional incremental mixing improves product quality and lowers catalyst consumption. But does more mixing in the reaction zone correlate to an increase in small droplets of entrained acid? Does reduced mixing intensity in the reaction zone correlate to a decrease in micron-sized droplets of entrained acid?

Recent research shows these same droplets exist regardless of the intensity of mixing, so effluent treating in addition to coalescing is necessary for their removal. Unit upsets, contaminants, increased throughput, propylene and high isobutylene feeds increase the need for effective effluent treating.

In this article, we provide a fresh perspective on the fundamental principles of acid droplet generation from the sulphuric acid alkylation reaction. Research by the University of Kansas shows that very small (~3 µm) and stable sulphuric acid droplets are formed during the actual reaction regardless of the mixing intensity.1 The research team also found that intense reaction zone mixing does not increase the production of stable acid droplets nor the load on well-designed downstream separation and effluent treating equipment.1

For the past several decades, common wisdom in sulphuric acid alkylation mostly blamed hydrocarbon-soluble esters in the net effluent for downstream corrosion and fouling of equipment. DuPont Clean Technologies now believes that much of the trouble is caused by tiny droplets of acid that get past many effluent treating systems. All sulphuric acid alkylation reactors, no matter which technology, create 2-5 µm droplets of partially spent acid that entrains out of the reactor with the effluent. Since the droplets are very tiny, coalescers are not effective in their removal. It is therefore critical to treat the effluent to protect downstream equipment. The acid droplets contain acid soluble esters, while the clear hydrocarbon phase typically contains very few hydrocarbon soluble esters.

If the hydrocarbon phase is left untreated, these sulphuric acid droplets will degrade and turn into acid sludge and SO2 within the fractionation section, thereby causing corrosion and fouling in the deisobutaniser overhead equipment and reboiler. There is also a potential for these acid droplets to exit the unit with the alkylate product. Proper effluent treating can provide protection against corrosion and fouling in downstream equipment and vehicles, ensuring quality alkylate free from micron-sized emulsified acid droplets.

To take a deeper dive into how effluent treating serves its purpose, first and foremost it is important to understand what makes up the sulphur compounds in the reactor effluent and to examine the behaviour of the acid droplets in it. To this end, the University of Kansas employed a combination of state-of-the-art microscopic and analytical techniques to characterise weathered effluent (alkylate) as described below.

Reactor effluent analysis
Reactor effluent from the alkylation pilot plant in DuPont’s Alkylation Technology Center was collected, then allowed to weather at ~90°F (32°C) to evaporate the butanes and other light hydrocarbons. The alkylate was otherwise untreated. The University of Kansas analysed these samples using dynamic light scattering (DLS) and optical microscopy. DLS identifies droplets in emulsion by measuring the diffraction of light as a laser passes through the sample (see Figure 1). DLS showed the presence of emulsified droplets which could not be separated from the alkylate even after days or months of gravity settling.

The results showed the presence of 2-5 µm spherical droplets (see Figure 2).
A sulphur mass balance was performed on several samples. Knowing the total sulphur content of the untreated alkylate (both hydrocarbon and water soluble), it was water washed to extract the water soluble sulphur species. The water phase was analysed using the total acid number (TAN) titration method and the washed alkylate was analysed for total sulphur content. The sulphur mass balance typically closed within 3%. The sulphur species in the alkylate were analysed using mass spectrometry and a soft ionisation technique called CI-HEX. Secondary butyl sulphate was found in alkylate produced from butylene, and isopropyl sulphate was found in propylene alkylate. We suspect that these components act as surfactants and contribute to the stable acid emulsions in the hydrocarbon phase.

Regardless of the exact mechanism, these droplets are very stable and do not separate out from the alkylate phase even when left to settle for months. They are also very acidic with a sulphur to oxygen ratio close to that expected for SO4.

On the basis of these results, sulphuric acid exists in the alkylate phase as part of a tight emulsion stabilised by the presence of alkyl sulphate.
The acid droplets, approximately 3 µm in size (see Figure 3), do not coalesce but rather form larger agglomerate droplets when the alkylate is vacuum flashed (see Figure 4). These agglomerated droplets can easily be broken apart to their original disperse state by sonication for a few seconds. We believe that turbulent flow through piping in a commercial unit will keep the droplets from agglomerating.

Another interesting discovery made by the University of Kansas research team was that, when untreated alkylate is pressured through a 0.2 µm syringe filter, the acid droplets are not coalesced nor strained out but rather squeeze through the pores and are reduced to ~0.8 µm immediately afterwards. Within 18 hours, however, the droplets return to their preferred ~3 µm size.

Testing the hypothesis
Experiments were conducted at various mixing speeds: intense mixing – 25% more mixing speed, and mild mixing – 50% less mixing. The results from these experiments indicated that the size of these acid droplets was also 2-5 µm when analysed using DLS techniques. Other experiments were conducted using a propylene feed which produces a more stable (tighter) emulsion compared to butylene. The sulphur content and quantity of droplets was greater but the size of the acid droplets was still found to be mostly within the 2-5 µm range. We also collected untreated net effluent from a commercial Stratco alkylation unit which contained similar tiny acid droplets.

A new set of experiments was designed to synthetically create these acid droplets by intensely mixing sulphuric acid (either fresh or spent) with treated alkylate (containing no acid droplets) for 48 hours. However, no stable acid droplets were observed by DLS after 60 minutes of settling. Also, the resulting sulphur content was below the level of detection (<0.3 ppm) which proves that sulphuric acid is not soluble in alkylate.

To summarise the findings, when alkylate is produced in a reaction, stable acid droplets of approximately 3 µm in size are formed, no matter the mixing intensity or type of olefin feed. But when the University of Kansas team tried to create similar droplets within treated alkylate under intense mixing but with no chemical reaction occurring, they were unsuccessful. Past work published by Koch-Glitsch2 reports that chemical reactions typically 
produce fine droplets (<10 µm), whereas intense mechanical agitation creates droplets that are much bigger in size (see Figure 5).3
Intense mixing is beneficial to the alkylation reaction by increasing the surface area and mass transfer between the two phases. The above-mentioned experiments demonstrate that intense mixing alone is unable to create stable emulsified droplets. We deduce that chemical reactions involving the olefin, iso-paraffin and acid play an important role in the formation of stable droplets within the alkylate phase.
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