1.1 Background of the Study
Distillation is a unit operation that has been around for a long time and continues to be the primary method of separation in processing plants, in spite of its inherently low thermodynamic efficiency. The pre-eminence of distillation for the separation of fluid mixtures is not accidental, but fundamental, and therefore unlikely to be displaced (kister, 1992)
From a kinetic standpoint (kister, 1992) mass transfer per unit volume in distillation is limited only by the diffusional resistances on either side of the vapour-liquid interface in turbulent phases, with no inert present. In almost every other separation process, there are inert solvents or solid matrices, and these lower mass fluxes. Distillation has the potential for high mass transfer rates.
From a thermodynamic viewpoint, a typical thermodynamic efficiency of a distillation system is about 10 per cent (kister, 1992). This can be enhanced if inter-condensers and inter-reboilers are used.
In fact, it has been shown that conceptually, a distillation system can be devised, which requires only the minimum work of separation (kister, 1992).
Although, a thermodynamic efficiency of 10 per cent appears low, not many other processes are more efficient (Haselden, 1981)
Distillation in general provides the cheapest and best method for separating a liquid mixture from its components.
Distillation has evolved from simple laboratory procedure to prime separation in the crude oil refining processes.
The world crude oil refining capacity is approximately 82 million barrels per day equivalent to about 4,200 million tonnes (Mt) per annum. This capacity is currently provided by a total of 720 refineries. (Oche, November, 2014)
Nigeria has a total number of four (4) crude oil refineries located in Port-Harcourt, Warri and Kaduna, with a total refining capacity of 445,000BPD. Port-Harcourt has two refineries, one with a capacity of 60000BPD and the other 150000BPD, Warri refinery has a capacity of 125000BPD and Kaduna refinery-110000BPD. Despite the large amount of crude oil deposit, Nigeria continues to experience perpetual shortages of products due to poor configurations and inefficient operations of the refineries, resulting in frequent breakdowns occasioned by poor or lack of turn around maintenance. (Oche, November, 2014)
1.2 Statement of Problem
Distillation still remains the most important separation method in the pharmaceutical, fine chemical and petroleum industries (Ukeje-Eloagu, 1998). Throughout the chemical industry, the demand for purer products, coupled with a relentless pursuit of greater efficiency, has necessitated continued research into the techniques of distillation (Ukeje-Eloagu, 1998).
The industrial application of distillation is widespread. It is used commercially in the purification of many chemical compounds such glycerol, hydrogen peroxide, turpentine, and fatty acids. Distillation has also been employed for a number of years by the petroleum and coal tar industries for the separation of many products.
Despite the wide use of distillation, there has been a little limitation in separating the bottoms of the atmospheric distillation unit. These bottoms contain useful products which cannot be separated under atmospheric conditions. Continual heating of these bottom products will lead to their cracking (breaking) and therefore cause plugin of the transfer line of the column. This limitation impedes the distillation process and recovery of the important constituents of the bottom of the atmospheric distillation unit.
Vacuum distillation (distillation at reduced pressure) can often be used to separate substances which decompose at atmospheric conditions or to obtain an increased relative volatility. It is of particular value in separating materials which decompose or polymerise at their atmospheric boiling point temperature.
1.3 Aim and Objectives
The aim of this research work is to “design a vacuum distillation unit for the efficient processing of 10 barrels per day of atmospheric residue to produce light vacuum gas oil, heavy gas oil and vacuum residue”
The objectives of this research work are:
- Specify the degree of separation (i.e., product specification)
- Determination of the number of stages.
- Determination of reflux requirements.
- Select the type of contacting device (trays or packing)
- Column sizing (height, diameter, and number of real stages).
- Select the operating conditions for the column: temperature and vacuum pressure.
- Check flooding and weeping.
1.4 Significance of Study
Research work will present a simplified design of the vacuum distillation unit. Distillation processes have a determinant role in refining processes as it separates the crude oil into fractions, which are subsequently processed in other units or sent into storage.
This research work will increase appreciation and knowledge in distillation design. It can also be applied in the development of modular crude refinery to solve the problem of product shortages in Nigeria and Africa as a whole.
This research work when completed would help not only scale up but in the optimization of the entire petroleum refining process.
1.5 Scope of Study
The scope of this research work will be to give a study of the design of a vacuum distillation unit using the Liebman decomposition model. This research work shall be limited to the recovery of a binary product; vacuum gas oil and vacuum residue. Material and energy balance calculation will be carried out to ascertain the amount of feed and energy required by the unit. Determination of column dimensions and parameters using the McCabe-Thiele graphical method. Microsoft Excel spread sheet shall be used to plot the vapour-liquid equilibrium curve and also to obtain the number of stages.