The most effective and energy-efficient
way of drying pressed beet pulp, DDGS, wood
chips and miscellaneous types of biomass

Advantages

BMA's fluidised-bed steam dryers offer
the following benefits:

• More than 90% of the energy input for drying can be utilised in  downstream processes
• No air pollution from dust and odours
• Improved product quality, because oxidation and exhaust gas contamination are avoided
• No fire risk, because drying takes place in superheated water vapour
• High level of availability
• No loss of product as a result of overdried edges
• Small dryer footprint, owing to a compact and entirely cylindrical design
• Low noise level

CSD™ during installation

Characteristic features and sizes
CSD™ series

With its revised range of CSD™ sizes, BMA offers its customers various steam dryers for a broad  spectrum of requirements (see above). The electric power consumption of the standard CSD™  sizes has been optimised. For special requirements, the water evaporation rates of the standard sizes can be increased by raising the volume of the circulation steam. For this purpose, a more powerful motor with a specially designed fan impeller has to be installed. 

Water evaporation rates of the CSD™, using the example of beet pulp

Drying with the fluidised-bed steam dryer

The photo below is a view of a smokeless stack, seen through the semiellipsoid head of the fluidised-bed steam dryer before it was closed. The photo was taken while conversion work was underway at Nordzucker AG's Uelzen factory and it is quite symbolic. Certainly, even this modern dryer cannot ensure that the plant will operate without any emissions, but the CSD™ does provide for - almost complete - reduction of primary fuel input for drying beet pulp, DDGS and wood chips (collectively and briefly referred to as biomass below). The CSD™ therefore makes a very essential contribution to reducing overall CO2 emissions.

View through the still open semi-ellipsoid head at the top of the dryer in Uelzen

Technological principles

Example of a CSD™ integrated into the steam system of a sugar factory

When biomass - such as exhausted beet pulp, stillage that is obtained as a byproduct of ethanol production, or wood chips - is dried in conventional dryers, considerable amounts of primary energy have to be generated. During the drying process, most of this energy is converted together with the evaporated water into drying vapour. Very little of this energy can be put to further economic use with any of the conventional types of dryers. This is one very decisive advantage of the CSD™. Since this special fluidised-bed steam dryer makes the drying vapour available at a pressure of up to 4 barA, the energy contained in the vapour can be used in other steps of the production process. In this way, up to 90% of the energy input for drying can be put to several uses. The example shown above of how a CSD™ can be integrated into the steam system of a sugar factory illustrates this quite clearly. The CSD™ fluidised-bed steam dryer therefore plays an essential role in reducing the energy requirements of the plant as a wole, cutting CO2 emissions and enhancing the overall plant economy.

Schematic representation, with prefeed system for DDGS

Owing to the BMA technology for material conditioning, the advantages of the CSD™ drying process can be used even if the particles of the input material are too small for optimal configuration of the fluidised-bed steam dryer. This may, in particular, be the case with the drying of brewery stillage or stillage obtained as a by-product of bioethanol production from maize or other cereals. US patent 7,578,073 B2 has already been granted for the CSD™.

Functional characteristics

The bed (item 3) with the fluidised product forms in an annular space around the central superheater (item 9). The steam required for product fluidisation is generated by a fan (item 10) below the superheater, from where it circulates upwards through the distributor plate (item 12) and into the fluidised bed, which is subdivided into a number of interconnected cells. The input material is fed by the product feeder system into the first dryer cell, and then passes all other cells in the fluidised state. In the last cell, the product and the dust from the rotary dust separator (item 6) are collected and a screw conveyor removes them from the dryer. To evaporate the water contained in the product, the necessary amount of energy has to be transferred to the product. Most of the energy is transferred to the circulation steam in the superheater and enters the fluidised bed together with the steam. As a second means of energy transfer, steam-heated panels are provided in the fluidised bed. Because of direct contact between the product and the heating panels, this kind of heat transfer is particularly efficient. The velocity of the circulation steam is rated so that large particles, too, are kept in motion. Comparatively large and heavy particles primarily travel along the bottom of the fluidised bed and use openings in the cell walls immediately above the distributor plate for entering the next cells. As a result of the highly intensive motion inside the fluidised bed, smaller and lighter particles are forced upwards in the partitioned cells. The curvature at the top end of the cell partitions has two effects. Firstly, the particles are deflected so that they drop back into the fluidised bed in the next cell or the one after the next. The second effect is that the steam is forced to perform a rotary motion as it flows upwards. In developing the CSD™, extensive fluid-dynamic calculations were made to be able to achieve a maximum separation effect while keeping pressure losses as low as possible. The top part of the CSD™ form a cyclone, in which dust and very fine particles are effectively separated in a centrifugal force field. From here the particles enter the ejector (item 7); they are then blown directly into the last cell of the fluidised bed from where they are removed from the CSD™ together with the dried product. The circulation steam contains almost no dust when it enters the superheater for reheating with high-pressure steam. The superheated circulation steam is sucked in by the fan (item 10) and forced again through the distributor plate and into the fluidised bed.  

Bottom section being placed on its feet	View of the inside of the dryer during assembly

Fluid dynamic calculations

Excess steam is produced in the fluidised bed as the water contained in the product evaporates. This steam leaves the CSD™ at the centre of the rotary separator through the top-end vapour pipe (item 8). Its energy content can be put to further use, for instance in the evaporator station. The product feed system comprises the feed lock (item 1) and the feed screw conveyor (item 2). This system feeds the particles into the CSD™ without any loss of steam. The feed lock is a special-purpose device that separates the pressurised CSD™ from the environment. The feed screw conveyor feeds the particles into the dryer against the direction of steam flow, and the particles are heated as the steam condenses.

Example of a feed lock

The product discharge system removes the dried product from the CSD™ and transports it to the downstream conveyor system. It comprises the discharge screw conveyor (item 4), the discharge lock (item 5), and the expansion cyclone (item 11) with a separate rotary lock. The discharge screw conveyor conveys the product from the last cell of the CSD™ into the discharge lock, from where it enters the downstream flash tank.

Example of a discharge lock

Design

Since the outer shell of the CSD™, the integrated superheater and the different heating panels in the  fluidised bed have to be dimensioned as pressure vessels, the materials used have to be selected very  carefully. BMA has obtained the required licenses for strength design calculations in compliance with TÜV, ASME and other international regulations, and can therefore manufacture the apparatus in its own workshop. While the outer shell may be made from mild steel, high-temperature steel is used for the superheater. For the inside elements of the CSD™, both stainless steel and mild steel are used, depending on the mechanical and thermal requirements these elements have to meet. As well as performing a number of monitoring functions for continuous operation, the measuring and control system for the CSD™ has to ensure above all that the dried product has a constant residual moisture for downstream processes and subsequent storing. The temperatures recorded in all the fluidised bed cells and the differential pressure across the fan are also used for process control purposes. These parameters serve as criteria for assessing stable drying conditions and adequate fluidisation of the particles in the fluidised bed. For the feed and discharge locks there is a separate control system with various monitoring and alarm functions. A central lubrication system supplies the locks as well as the feed and discharge screw conveyors with the required amounts of lubricant.