Coil Visbreaking

Coil visbreaking. The conversion is achieved by high-temperature cracking for a predetermined, relatively short period of time in the heater. 

Soaker visbreaking. It is a low-temperature/high-residence-time process the majority of conversion occurs in a reaction vessel or soaker drum, where the two-phase heater effluent is held at a lower temperature for a longer period of time. 

Coil visbreaking is characterized by high temperature and short residence time operation. In the coil visbreaking process, the residue feed passes through a furnace tube 

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Fuel consumption in soaker visbreaking is about 70% of that for the coil visbreaking. 

The coil visbreaking uses a two-zone fired heater, which provides high degree of flexibility in heat input and better control of the material being heated. 

Decoking of the heater tubes in the coil visbreaking is easier and done with a steam-air. 

The operation at higher temperature of the coil visbreaking produces more recovery of the heavy visbroken gasoil, which cannot be obtained with soaker visbreaking without the addition of a vacuum flasher. 

In coil visbreaking, coke is deposited in the furnace tubes with further fouling of blocking, obstrnction of heat transfer, throughput reduction, and diminution of efficiency and productivity of the process, which is compensated by burning additional fnel or reducing feed. 

Decoking is more freqnent in coil visbreaking, but for soaker visbreaking it is necessary to shut down the operation. 

Run time for coil visbreaking is 3-6 months and 6-18 months for soaker visbreaking. 

Fuel consumption is 1-1.5 wt% on feed for coil visbreaking, while for soaker visbreaking it is about 30%-35% lower. 

In coil visbreaking, the feed enters the furnace at 300°C-330°C, and the reactions occur in the coil in the temperature range of460°C-480°C. The product is quenched to 350°C. 

The heater outlet tanperature in the coil visbreaking process ( 480 C) is higher than that in the soaker visbreaking process ( 450°C). 

For coil visbreaking, pressure of a few bars is sufficient to keep the feed from vaporizing. For soaker visbreaking, pressure is chosen so that the desired products are vaporized and quickly leave the reaction zone, while the heavy products soak in the liquid phase. Five to eight bars are used for vacuum residue, and 10-12 bars for atmospheric residue. 

In coil visbreaking, the inlet pressure is about 2.5 MPa, which is maintained at higher values to facilitate the transport of the feed. Due to the two-phase frictional pressure drop, the inlet pressure along the reactor length is reduced in about 1.7 MPa. 

In coil visbreaking, the residence time of the feed in the furnace is of 2-5 min. 

The coil visbreaker is operated at high temperature and low residence time and, thus, is also termed as a high-temperature, short-residence-time (HTST) process. The feed enters the furnace at 300°C-330°C and undergoes thermal cracking in the coil at 460°C-480°C for 2-5 min. The cracking reactions occur in the liquid phase and are slightly endothermic (192kcal/kg of the 204 C-product). 

As can be observed, both coil and soaker reactors have been modeled in the literature by following different approaches, and their validation has been performed with experimental data (batch reactor and pilot-plant data) and connnercial data either for coil visbreaking or for coil-soaker visbreaker. 

Visbreaking by coil-type visbreaker Foster Wheeler/UOP bpd 6,000 15,000 0.278 0.48... 

Visbreaking is a relatively mild thermal (noncatalytic) cracking process that is used to reduce the viscosity of residua. A visbreaker reactor may be similar to a delayed coker with a furnace tube followed by a soaker drum. However, the drum is much smaller in volume to limit the residence time with the entire liquid product flowing overhead. Alternatively, the entire visbreaker may be a long tube coiled within a furnace. Coke formation can occur and the coke accumulates on visbreaker walls periodic decoking (cleaning) is necessary. 

This situation can be clearly seen when observing the time evolution of the tube metal temperature of the pyrolysis coils there is a fast initial increase and then a reduced asymptotic slope. Note that although the initial slope is initially related to the catalytic rate, it is also due to the relatively low thermal conductivity of the initial fibrous material as a result of the large void fraction. The thickness of this layer is in the order of 20-40 pm. The evolution of the fluid temperature over time either at the TLE outlet or in visbreaking processes and in delayed coking furnaces shows a very similar behaviour. 

As shown in Fig. 31a, a visbreaking reactor consists of very long coils inserted into a radiating furnace. The effluent often passes into the extra adiabatic... 

Fig. 36. External tube skin temperature at coil outlet in a visbreaking furnace. Prediction (line) vs. experimental points.

BSD of gas oil will be recycled back through the coke drums or visbreaker soaker coils. At 15 (U.S.) a barrel downgrading cost, that s 6000 (U.S.) a day. All to no purpose. Just remember to turn the seal flush back on before restarting the pump. 

Aqua-conversion. Catalytic process that uses catalyst-activated transfer of hydrogen from water added to the feedstock in slurry mode. The homogeneous catalyst is added in the presence of steam, which allows the hydrogen from the water to be transferred to the heavy oil when contacted in a coil-soaker system normally used for the visbreaking process. Reactions that lead to coke formation are suppressed and there is no separation of asphaltene-type material (Houde et al., 1998). 

Typical process schemes of coil and soaker visbreaking technologies are illustrated in Figures 3.2 and 3.3, respectively. 

The two types of visbreaking are similar and only differ in their temperatures and residence times. Product quality and yields from the coil and soaker visbreaking processes are essentially the same at a specifled severity. 

The typical profiles of temperature, pressure, and conversion (expressed as weight percent of gases plus naphtha with 165°C of end boiling point) along the reaction system for coil and soaker visbreaking are depicted in Figure 3.4. 

The values of kinetic parameters (reaction rate coefficients, activation energies, pre-exponential factors) reported by the authors were obtained from experiments at different temperature, pressure, residence time, type of operation (continuous or batch reactor, coil or soaker visbreaking), and type of feed (atmospheric residue, vacuum residue, asphaltenes separated from crude oils from different sources). 

These correlations require as input the residue conversion, and sulfur content and API gravity of the feed. Equations 3.10 through 3.21 were derived from the information collected by Maples (1993), which is based on 30 sets of data with conversion ranging from 3.8% to 15.69%. That is, conversions used to derive correlations are relatively lower than those reported in commercial visbreaking using coil and soaker in series reactors. In other words, the previous correlations can be used only for quick calculations and are limited to the range of application, particularly conversion. 

Coil and soaker reactors can be classified as gas-liquid systems wherein the dispersed phase is composed by the produced gas, the vaporized low molecular weight hydrocarbons, and superheated steam, while the continuous phase comprises the visbroken residue. Based on this, the coil reactor can be represented as a two-phase plug-flow reactor (PER) or as a series of equal-volume continuous-stirred tank reactor (CSTR), while the soaker reactor can be modeled as a bubble-column reactor or as CSTR. Visbreaking reactions proceed via free radical mechanism and take place in the liquid phase, being the feed flowrate that decides the liquid-phase residence time in both reactors. 

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