Pretreatment of alkylation feedstock

For raw material processing, the current methods mainly include washing and hydrogenation

When hydrogen is available, the water-washing and hydrogenation mode should be chosen. Due to the high cost of hydrogen, as well as the significant expenses associated with hydrogen production and handling, many manufacturers opt for a water-washing tower and a depropanizer instead. Additionally, since many manufacturers fail to consider that methanol isn’t listed in the provided material list, design institutes often omit the water-washing tower from their designs. During the water-washing process, methanol can be removed as much as possible, taking advantage of the fact that water and methanol are completely miscible with each other； During water washing, a small amount of dimethyl ether can also be removed; dimethyl ether is soluble in water. However, this proportion of dimethyl ether is very minimal and must be removed using a depropanizer. Water washing also helps to remove certain heavy metal impurities. If the wash water isn’t changed for an extended period, its pH level decreases, leading to corrosion of the equipment’s inner walls and the formation of additional metal impurities, which in turn affects subsequent dehydration coalescers. The depropanizer can also remove propane and small amounts of C1 and C2 compounds, as well as methanol. However, this process results in some loss of liquefied gas. Nevertheless, the energy consumption and construction costs associated with using a depropanizer are significantly lower than those of water washing. It can be utilized in the short term to extract methanol based on the principle of azeotropy. Butadiene can only be removed through hydrogenation; although water can remove a portion of it, the amount is so small that it can be ignored. A dehydration coalescer must be installed before the light-end tower, and it needs to be effective, otherwise the catalyst may become deactivated or poisoned. It is also possible to bypass the hydrogenation reactor and use the light-end tower directly. The propane removal tower also has the capability to carry out initial dehydration; the gas-phase water in the material is condensed into free water in the subsequent heat exchangers, and then the free water is removed by the dehydration coalescer. However, the recycled isobutane used in the DuPont process contains a large amount of free water, which poses a problem with dehydration. That’s all for now; take some time to digest this information before we continue

"If the water isn’t changed for a long time, the pH value of the wash water will decrease. “What causes this?” To what extent can a dehydration coalescer remove water? One in a thousand, one in ten thousand? ““The depropanizer... can also remove methanol” – is this because it can form an azeotrope with propane? How much can be removed by butadiene hydrogenation? Can the free water in the cyclic isobutane band not be removed by a coalescer? I look forward to your reply.

Typically, there are water washing recovery and hydrogenation depropanization units installed; however, many plants do not operate them. Only the depropanization tower is utilized, which can remove some light components such as C3 hydrocarbons, dimethyl ether, and methanol. The feedstock and recycled isobutane are dried using coalescer dehydrators

The raw material specifications require no free water; methanol levels must be less than 0.002, dibutylene at 0.002, and dimethyl ether also at 0.002

It’s simple to wash, requires no significant energy consumption, and the water only needs to be changed once a month

Water washing not only removes methanol, but also impurities in the liquefied gas. I’ve opened water washing towers that had been in operation for over half a year; there was a layer of sludge on the trays

May I ask if you are a design institute or a manufacturer, a state-owned enterprise or a private one? P

It’s a state-owned enterprise, but it’s not a design institute nor a manufacturer :)